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Ole Martin Løvvik

Professor II - Solid-state Physics and Quantum Technology

Email o.m.lovvik@fys.uio.no

Phone +47 22 84 06 89

Mobile phone 99018428

Username

Visiting address Forskningsparken Gaustadalleen 21 0349 Oslo

Postal address Postboks 1048 Blindern 0316 Oslo

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Academic Interests

Løvvik is using contemporary high-throughput techniques to identify novel functional materials with superior properties. This is based on a combination of first-principles atomistic modelling of materials using density functional theory (DFT) and related techniques and a variety of machine-learning methods to speed up the atomistic simulations, the screening procedure, or both. This is employed in several systems and technologies:

Thermoelectric materials for waste heat harvesting
Battery materials
Metals and alloys
Hydrogen technology
Semiconductors and solar cells
Oxides and fuel cells
Phase transformation materials with high reversibility

The screening aims for a variety of properties that may require special care or input:

Transport properties from Boltzmann theory (Seebeck coefficient, electrical conductivity, etc.) using first-principles input
Temperature-dependent properties (thermodynamics, thermal conductivity, etc.) from phonon calculations
Diffusivity and other mobility properties from (first principles) molecular dynamics

Teaching

Supervisor for 16 master students (13 finished) and 13 PhD students (9 finished)
FYS-MENA4111 Quantum mechanical modelling of nanomaterials, lecturer and co-responsible for developing the course (2010-).
FYS-MENA3110/4110 Quantum Nano Physics, lecturer and co-responsible for developing the course (2007-2009).
Energy and Environmental Science, Oslo Technical College, undergraduate course: guest lecturer (2005).
MEF3200 Energy Materials, University of Oslo, undergraduate course: guest lecturer (2004-2005).
MVT333 Materials Science, University of Oslo, undergraduate course: guest lecturer (2004).
National research course in neutron scattering, Institute for Energy Technology, national graduate course: guest lecturer (2004).
FYS240 Physics and Energy Resources, University of Oslo, undergraduate course: guest lecturer (2003-2005).
FYS310 Numerical Methods of Physics 3, University of Oslo, undergraduate course: lecturer and partly responsible for developing the course (2003).
The International Summer School, Centre for Development and Environment, University of Oslo, international course, lecturer (2002-2009).
TFY114 Thermophysics, Norwegian University of Life Sciences, undergraduate course: lecturer and responsible for the course (2001).
FYS270 Energy Physics, Norwegian University of Life Sciences, undergraduate course: guest lecturer (2001).
Environment and Energy Technology, Høgskolestiftelsen at Kjeller, undergraduate course: guest lecturer (1999-2001).
FYS403 Quantum Field Theory, University of Oslo, Graduate course: teacher (1994-1995).
FYS115 Experimental Physics 1, University of Oslo, Undergraduate lab course: lab supervisor (1993,1996).
FYS111 Mechanics, University of Oslo, Undergraduate course: teacher (1992).

Higher education and employment history

Chief Research Scientist, SINTEF Materials and Chemistry, May 2008 – Now
Adjunct Professor, Dept. of Phys., Univ. of Oslo, Jan. 2008 – Now
University Lecturer, Dept. of Phys., Univ. of Oslo, Aug. 2007 – Dec. 2007
Research Fellow, Institute for Energy Technology, Kjeller, Norway, July 2006 – Apr. 2008
Consultant, Norwegian Research Council, Feb. 2006 – Nov. 2006
Research Fellow, Centre for Materials Science and Nanotechnology, Univ. of Oslo, Jan. 2004 – Dec. 2007
Post Doc., Dept. of Phys., Univ. of Oslo and Institute for Energy Technology, Kjeller, Norway, Jan. 2002 – Dec. 2003
Associate Professor, Norwegian Univ. of Life Sciences, Aug. 2001 – Dec. 2001
Post Doc., Dept. of Phys., Univ. of Oslo, Aug. 1999 – Dec. 2001
Civil service as musician, Nordreisa, Norway, Aug. 1998 – Aug. 1999
Parental leaves, 1997, 2000, 2004
Doctoral scholarship, Dept. of Phys., Univ. of Oslo, Aug. 1993 – July 1998
Teaching beginner’s courses and PhD courses Dept. of Phys., Univ. of Oslo, Jan. 1992 – Dec. 1995
Freelance musician, several tours (altogether about 6 months), Rikskonsertene Norway, Jan. 1994 – Aug. 1996
Composer and text writer, Among other things, the musical «Vasshjulets herrer», Jan. 1996 and Oct. 2008

Appointments

Referee in >20 international scientific journals (Nature Materials, Phys. Rev. Lett., Nanotechnology, Angewandte Chemie, etc.)
Secretary writing the National Strategy on Nanoscience and -technology, the Norwegian Research Council (2006)
Member of various committees, boards, etc.

Cooperation (ongoing projects and joint publications)

University of Oslo: Solis state electronics, Solid State Ionics
NTNU: Department of Physics
Institute for Energy Technology (IFE), Kjeller: Physics Department
United Technologies Research Center, Hartford, USA
University of Leiden, the Netherlands: Theoretical Chemistry
Free University of Amsterdam, the Netherlands: Condensed Matter Physics
University of Vienna, Austria: Computational Materials Physics
University of Uppsala, Sweden: Condensed Matter Theory Group, Materials Physics
University of Osaka, Japan: Institute of Scientific and Industrial Research

Tags: SMN, Structure Physics

Grimenes, Øven Andreas; Løvvik, Ole Martin & Berland, Kristian (2026). Exceptional thermoelectric properties in Na2⁢TlSb enabled by quasi-one-dimensional band structure. Physical review B (PRB). ISSN 2469-9950. 113(15). doi: 10.1103/JNFJ-2N8P. Full text in Research Archive Show summary
Materials with reduced dimensionality offer beneficial density-of-states (DOS) profiles for thermoelectric energy conversion, but can be impractical in realistic devices. Encouragingly, bulk high-symmetry materials can also exhibit similar quasi-low-dimensional band structures. A striking example is the full-Heusler compound Na2⁢TlSb, whose valence-band energy isosurfaces can form intersecting two-dimensional pockets, i.e., a boxlike structure. The individual energy isosurface sheets resemble those of 1D quantum wires. The combination of high electron velocities (perpendicular to the pockets) and a rapidly increasing DOS with energy in the transport regime (due to the low dimensionality) makes Na2⁢TlSb a representative case where the band structure gives rise to attractive electronic transport properties. However, these beneficial features could be counteracted by high electronic scattering rates due to the large scattering space. In this first-principles study of Na2⁢TlSb, we find that the electronic scattering rates remain modest. This result is linked to the reduced matrix elements of large-momentum (𝐪) scattering across the delocalized energy isosurfaces. The enhanced free-carrier screening due to the large DOS also contributes to reducing scattering. In combination, the low-dimensional features and modest scattering result in excellent electronic transport properties. Combined with an ultralow lattice thermal conductivity of 𝜅ℓ<1W/mK reported in the literature, we predict a thermoelectric figure of merit ranging from 2.4 at 300 K to 4.4 at 600 K. The 𝑛-type properties are also favorable, with 𝑧⁢𝑇 values from 1.5 at 300 K to 3.0 at 600 K.
Stange, Marit Synnøve Sæverud; Bumke, Lars; Quandt, Eckhard; Kušnír, Jakub; Grabec, Tomáš & Seiner, Hanuš [Show all 11 contributors for this article] (2026). Screening for new elastocaloric materials in a compositionally graded NiCuTiZr thin film. Materialia. 50, p. 1–16. doi: 10.1016/j.mtla.2026.102896. Full text in Research Archive Show summary
NiTi-based shape memory alloys can be used as the active material in elastocaloric cooling devices. For optimal performance, they should simultaneously exhibit high reversibility and low temperature hysteresis, which can be achieved by ensuring that the middle eigenvalue of the primary stretch matrix λ2=1. Also, an adequate transformation temperature and a high enthalpy of transformation are required. In this study, a systematic screening for highly reversible shape memory alloys was performed, using compositionally graded Ni-Cu-Ti-Zr thin films generated by magnetron sputtering. Based on experimental data from the literature, a model predicting λ2 from the quaternary film compositions indicated a range of targeted compositions with λ2=1. The properties of the thin-film compositions were experimentally investigated to corroborate the theoretical possibilities. Martensitic phase transformations were observed with differential scanning calorimetry, demonstrating a systematic change in transformation temperatures with composition. X-ray diffraction confirmed a transition between B2 austenite and B19’ martensite structures, similar to binary NiTi compounds. The targeted compositions did not exhibit the expected temperature-driven phase transformations with low hysteresis. Tensile testing revealed superelastic behaviour, but the samples were intrinsically brittle with large hysteresis and mechanical instability. This was explained by phase separation and the formation of Ti2Ni- and Ni4Ti3-like minority phases. Despite the failure to demonstrate new shape memory alloys with high reversibility, the study suggests a path to efficient searches for new alloys with novel functional properties.
Grimenes, Øven Andreas; Løvvik, Ole M. & Berland, Kristian (2026). Exceptional thermoelectric properties in Na2TlSb enabled by quasi-one-dimensional band structure. Physical review B (PRB). ISSN 2469-9950. 113(15). doi: 10.1103/jnfj-2n8p. Full text in Research Archive Show summary
Materials with reduced dimensionality offer beneficial density-of-states (DOS) profiles for thermoelectric energy conversion, but can be impractical in realistic devices. Encouragingly, bulk high-symmetry materials can also exhibit similar quasi-low-dimensional band structures. A striking example is the full-Heusler compound Na2⁢TlSb, whose valence-band energy isosurfaces can form intersecting two-dimensional pockets, i.e., a boxlike structure. The individual energy isosurface sheets resemble those of 1D quantum wires. The combination of high electron velocities (perpendicular to the pockets) and a rapidly increasing DOS with energy in the transport regime (due to the low dimensionality) makes Na2⁢TlSb a representative case where the band structure gives rise to attractive electronic transport properties. However, these beneficial features could be counteracted by high electronic scattering rates due to the large scattering space. In this first-principles study of Na2⁢TlSb, we find that the electronic scattering rates remain modest. This result is linked to the reduced matrix elements of large-momentum (𝐪) scattering across the delocalized energy isosurfaces. The enhanced free-carrier screening due to the large DOS also contributes to reducing scattering. In combination, the low-dimensional features and modest scattering result in excellent electronic transport properties. Combined with an ultralow lattice thermal conductivity of 𝜅ℓ<1W/mK reported in the literature, we predict a thermoelectric figure of merit ranging from 2.4 at 300 K to 4.4 at 600 K. The 𝑛-type properties are also favorable, with 𝑧⁢𝑇 values from 1.5 at 300 K to 3.0 at 600 K.
Larsen, Aleksander Amble; Gorantla, Sandeep; Nguyen, Phuong Dan; Rosnes, Andreas; Xu, Yijiang & Iordanidou, Konstantina [Show all 10 contributors for this article] (2026). Atomic-Resolution EDS Characterization of Ordering in a β–Cu–Zn–Al Alloy. Metallurgical and Materials Transactions A. ISSN 1073-5623. doi: 10.1007/s11661-025-08093-9. Full text in Research Archive Show summary
Abstract Atomic ordering in single-crystal β –Cu–Zn–Al, relevant to the martensitic transformation and functional properties, is investigated using analytical STEM. Atomic-resolution EDS along the [110] zone axis, where all Wyckoff positions are spatially resolved, reveals three chemically distinct site occupancies. Correlation with HAADF-STEM contrast provides real-space evidence consistent with L2 1 ordering. In agreement with experiments, first-principles calculations show a slight energetic preference for L2 1 over D0 3 , with finite-temperature free-energy calculations indicating that this preference persists at elevated temperatures.
Rahman, Jamil Ur; Soltani, Nayereh; Sunding, Martin Fleissner; Løvvik, Ole Martin; Gunnæs, Anette Eleonora & Finstad, Terje Gunnar [Show all 7 contributors for this article] (2026). Deciphering the structure of a high entropy Heusler antimonide. Acta Materialia. ISSN 1359-6454. Full text in Research Archive
Karami, Mostafa; Carvalho, Patricia Almeida; Gunnæs, Anette Eleonora; Løvvik, Ole Martin & Chen, Xian (2025). Grain boundary compatibility and its impact on the reversibility of superelastic micropillars. Journal of the Mechanics and Physics of Solids. ISSN 0022-5096. 205, p. 1–12. doi: 10.1016/j.jmps.2025.106309. Full text in Research Archive Show summary
The mechanical reversibility of stress-induced martensitic transformations is critical for the functional durability of superelastic materials, particularly at small scale. This study investigates the impact of grain boundary (GB) compatibility on the mechanical reversibility of martensitic transformations in bicrystal Cu67Al24Mn9 micropillars. By combining the geometrically nonlinear theory with high-resolution transmission electron microscopy (TEM) and nanomechanical compression experiments, we demonstrate that the crystallographic orientation and geometric coherence of GBs play a decisive role in transformation reversibility. Micropillars with GBs satisfying a necessary condition for rank-one connections between twin laminates exhibit diffuse GB morphology and recoverable transformation strains over 10,000 cycles. The findings validate recent theoretical predictions on kinematic compatibility in polycrystalline systems with non-transforming defects. The rank-one criterion underlies a design strategy for polycrystalline superelastic alloys to achieve high fatigue-resistant without altering chemical composition. The theoretical framework opens new directions for advancing microstructural design in shape memory alloys.
Álvarez, José Luis Garrido; Lopez-Garcia, Javier; Dura, Oscar J.; Schrade, Matthias; Poulia, Anthoula & González, Carlos F. Gutiérrez [Show all 14 contributors for this article] (2025). Synergistic effects of Sb doping and nanostructuring on spark plasma-sintered ZrNiSn thermoelectrics. Materials Advances. p. 1–11. doi: 10.1039/d5ma01100k. Full text in Research Archive Show summary
Nanostructured ZrNiSn and ZrNiSn1−xSbx (x = 0.02 and 0.05) half-Heusler alloys were successfully synthesized via mechanical milling followed by spark plasma sintering. All samples crystallized in the expected MgAgAs-type crystal structure (space group 𝐹-43m) and retained their nanostructured powder morphology post-sintering. Thermoelectric measurements revealed that the mechanical milling effectively reduced lattice thermal conductivity, while Sb doping significantly enhances the electrical conductivity in the ZrNiSn system. However, extending the milling time beyond 5 hours led to reduced performance due to the increased structural disorder without further thermal conductivity decrease. Hall effect measurements yielded a relatively low effective carrier mass of around 0.7-1.3 me, which accounts for the observed reduction in the Seebeck coefficient. The optimal Sb doping level was found to be x = 0.02. At this doping level, a substantial improvement in thermoelectric performance is detected with a maximum thermoelectric figure of merit (ZT) of approximately 0.6 at 600 K (two folds that of the undoped sample) and a 60% improvement over the unmilled ZrNiSn0.98Sb0.02 alloy These results highlight the synergistic effects of controlled Sb doping and nanostructuring via mechanical milling on optimizing the thermoelectric properties of ZrNiSn-based half-Heuslers.
Grimnes, Øven Andreas; Snyder, G. Jeffrey; Løvvik, Ole Martin & Berland, Kristian (2025). Thermoelectric transport of strained CsK2 Sb: Role of electron velocities and scattering within extended Fermi surfaces. Physical review B (PRB). ISSN 2469-9950. 111(19), p. 1–12. doi: 10.1103/PhysRevB.111.195205. Full text in Research Archive Show summary
In this first-principles study, we investigated the thermoelectric properties of the full-Heusler compound CsK2⁢Sb at different compressive strains. This material exhibits a valence band structure with significant effective mass anisotropy, forming tubelike energy isosurfaces below the band edge, akin to that of two-dimensional (2D) systems. Such systems can have a large number of high-mobility charge carriers and a beneficial density of states profile. In the calculations, we predicted a maximum p-type figure of merit (𝑧⁢𝑇) of 2.6 at 800 K, in line with previous predictions of high 𝑧⁢𝑇. This high 𝑧⁢𝑇 arises from the low lattice thermal conductivity of 0.35 Wm−1K−1 and the beneficial electronic band structure. The high density of states significantly increased the electron-scattering space, but this effect was largely compensated by reduced scattering rates of electrons with large momentum 𝐪. We further explored the effect of enhancing the low-dimensionality through compressive strain. This increased the p-type power factor by up to 66%; partly due to more strongly pronounced 2D features of the valence band, but primarily due to increased Fermi velocities. However, compressive strain also increased phonon velocities and hence the lattice thermal conductivity. The maximum p-type 𝑧⁢𝑇 thus only increased slightly, to 2.7 at 1% compressive strain. In the conduction band, strain aligned the Γ- and X-centered valleys, resulting in the optimal n-type 𝑧⁢𝑇 increasing from 0.9 to 2.3 at 2% compressive strain. Thus highly strained CsK2⁢Sb has the potential for both good p- and n-type thermoelectricity.
Morton, William James; Melau, Jørgen; Olsen, Roar A.; Løvvik, Ole Martin; Hisdal, Jonny & Søvik, Signe (2025). Thermal physiology of open water wetsuited swimming: A cohort study. Temperature. ISSN 2332-8940. 12(3), p. 245–263. doi: 10.1080/23328940.2025.2479893. Full text in Research Archive Show summary
Swimming in cold water is intrinsically unsafe. One of the threats is a fall in deep-body temperature, which adversely affects all body systems and increases the risk of death. Wetsuits mitigate, but do not negate this threat. Environmental conditions may confound findings from laboratory studies or mathematical models. The study aimed to elucidate associations and patterns of deep-body temperature change in open water wetsuited swimmers, thus facilitating risk assessment and improving swimmer safety. In this exploratory cohort study, following collection of anthropomorphic data including a Dual-energy X-ray Absorptiometry scan, deep-body temperature was continuously measured in 76 open water swims by 31 wetsuited swimmers in water temperatures ranging from 8.4°C to 24.5°C. The temperature trajectories were analyzed by piece-wise and linear mixed model regression. Registered with clinicaltrials.gov NCT06685627. Following immersion, there was a tri-phasic deep-body temperature trajectory, although not all phases were present in all swims: an initial rise, then a plateau phase and then thermal decompensation. Water temperature was correlated with the duration of these phases, as was body habitus for the initial rise. Minimum deep-body temperature was correlated with pre-swim temperature. Despite environmental and individual variations, this paper identified a tri-phasic deep-body temperature trajectory for wetsuited swimmers and proposed physiological mechanisms that may account for them. The tri-phasic pattern of an initial rise, a plateau phase and then thermal decompensation, may be useful in assessing the risks of open water swimming and may facilitate development and validation of deep-body temperature models for wetsuited swimmers.
Grimnes, Øven Andreas; Snyder, G. Jeffrey; Løvvik, Ole Martin & Berland, Kristian (2024). Thermoelectric transport of strained CsK2Sb: Role of momentum-dependent scattering in low-dimensional Fermi surfaces. arXiv.org. ISSN 2331-8422. doi: 10.48550/arXiv.2412.01681. Full text in Research Archive
Morton, William James; Melau, Jørgen; Olsen, Roar A.; Løvvik, Ole Martin; Hisdal, Jonny & Søvik, Signe (2024). Manual Dexterity in Open-Water Wetsuited Swimmers: A Cohort Crossover Study. International Journal of Sports Physiology and Performance (IJSPP). ISSN 1555-0265. 20(2), p. 213–223. doi: 10.1123/ijspp.2024-0100. Full text in Research Archive Show summary
Purpose: Laboratory studies have demonstrated that manual dexterity decreases with increasing cold, which may adverselyaffect performance. Dexterity may be impaired by cooling of the hand, cooling of the lower motor neurons, and cognitiveimpairment. Wetsuits are commonly used in open-water swimming and are mandated in some situations. This study investigatesthe effects of cold-water wetsuited swimming on dexterity. Methods: Five male and 4 female trained swimmers were recruited forthis cohort crossover study. Following dual-energy X-ray absorptiometry scans to determine body composition, they swam in afreshwater lake on 7 occasions with water temperatures between 24.5 °C and 8.4 °C. Dexterity was measured preswim andpostswim with a “nut–washer–bolt assembly time test” and cognition with a Stroop test. Core and peripheral body temperatureswere continuously monitored. Effects were analyzed by linear mixed-model regression. Results: Pre–post swim difference intime to complete the nut–bolt assembly increased as water temperatures decreased (1.0 s, 95% CI, 0.5–1.5 per 1 °C, P < .0001;R2 = .456), to a maximum of 14.7 seconds (95% CI, 3.3–26.0). This represented a 47.5% increase in assembly time from 24.5 °Cto 8.4 °C, which we consider to be of practical significance. Decreased dexterity was associated with decreased forearm andscapular temperature and decreased cognitive function. Body composition did not affect dexterity, cognitive function, or bodytemperature during swims. Water temperature did not affect swim speed. Conclusions: Despite the use of wetsuits, manualdexterity decreased with cold-water swimming. Swimmers, triathletes, and event organizers should consider the implications forsafety, performance, and equipment utilization.
Tranås, Rasmus André; Løvvik, Ole Martin & Berland, Kristian (2023). Lattice Thermal Conductivity from First Principles and Active Learning with Gaussian Process Regression. arXiv.org. ISSN 2331-8422. doi: 10.48550/arXiv.2309.06786. Full text in Research Archive
Amici, Julia; Asinari, Pietro; Ayerbe, Elixabete; Barboux, Philippe; Bayle-Guillemaud, Pascale & Behm, R. Juergen [Show all 49 contributors for this article] (2022). A Roadmap for Transforming Research to Invent the Batteries of the Future Designed within the European Large Scale Research Initiative BATTERY 2030+. Advanced Energy Materials. ISSN 1614-6832. 12. doi: 10.1002/aenm.202102785. Full text in Research Archive Show summary
This roadmap presents the transformational research ideas proposed by “BATTERY 2030+,” the European large-scale research initiative for future battery chemistries. A “chemistry-neutral” roadmap to advance battery research, particularly at low technology readiness levels, is outlined, with a time horizon of more than ten years. The roadmap is centered around six themes: 1) accelerated materials discovery platform, 2) battery interface genome, with the integration of smart functionalities such as 3) sensing and 4) self-healing processes. Beyond chemistry related aspects also include crosscutting research regarding 5) manufacturability and 6) recyclability. This roadmap should be seen as an enabling complement to the global battery roadmaps which focus on expected ultrahigh battery performance, especially for the future of transport. Batteries are used in many applications and are considered to be one technology necessary to reach the climate goals. Currently the market is dominated by lithium-ion batteries, which perform well, but despite new generations coming in the near future, they will soon approach their performance limits. Without major breakthroughs, battery performance and production requirements will not be sufficient to enable the building of a climate-neutral society. Through this “chemistry neutral” approach a generic toolbox transforming the way batteries are developed, designed and manufactured, will be created. © 2022 The Authors. Advanced Energy Materials published by Wiley-VCH GmbH
Mayandi, Jeyanthinath; Schrade, Matthias; Ponniah, Vajeeston; Stange, Marit Synnøve Sæverud; Lind, Anna Maria & Sunding, Martin Fleissner [Show all 13 contributors for this article] (2022). High entropy alloy CrFeNiCoCu sputter deposited films: Structure, electrical properties, and oxidation. Journal of Vacuum Science & Technology. A. Vacuum, Surfaces, and Films. ISSN 0734-2101. 40(2). doi: 10.1116/6.0001394. Full text in Research Archive Show summary
High entropy alloy (HEA) films of CrFeCoNiCu were deposited by sputtering. Their structure was characterized and their electric transport properties were studied by temperature-dependent Hall and Seebeck measurements. The HEA films show a solid solution with an fcc structure. The residual electrical resistivity of the films is around 130 μΩ cm, which is higher than the Mott limit for a metal while the temperature dependence of the resistivity above 30 K is metal-like but with a small temperature coefficient of resistivity (2 ppm/K). The dominant scattering mechanism of charge carriers is alloy scattering due to chemical disorder in the HEA. The Hall coefficient is positive while the Seebeck coefficient is negative. This is interpreted as arising from an electronic structure where the Fermi level passes through band states having both holes and electrons as indicated by band structure calculations. Below 30 K, the conduction is discussed in terms of weak localization and Kondo effects. The HEA structure appears stable for annealing in vacuum, while annealing in an oxygen-containing atmosphere causes the surface to oxidize and grow a Cr-rich oxide on the surface. This is then accompanied by demixing of the HEA solid solution and a decrease in the effective resistance of the film.
Bazioti, Kalliopi; Løvvik, Ole Martin; Poulia, Anthoula; Carvalho, Patricia Almeida; Azar, Amin S. & Mikheenko, Pavlo [Show all 8 contributors for this article] (2022). Probing the structural evolution and its impact on magnetic properties of FeCoNi(AlMn)x high-entropy alloy at the nanoscale. Journal of Alloys and Compounds. ISSN 0925-8388. 910, p. 1–12. doi: 10.1016/j.jallcom.2022.164724. Full text in Research Archive Show summary
We report the first nanoscale investigation of FeCoNi(AlMn)x high-entropy alloys (HEAs) processed by laser metal deposition. The structural evolution of the alloy upon chemical composition variation (0.2 ≤ x ≤ 1.5) was investigated by combining imaging and spectroscopies in (scanning) transmission electron microscopy (S)TEM with density functional theory (DFT). A gradual change from a face-centered cubic (FCC) towards an ordered full-Heusler (L21) phase by increasing the Al and Mn contents was observed. Direct imaging and atomic-scale calculations revealed a nanoscale interplay between B2 and L21 ordered structures for x = 1.5, wherein the latter, Al and Mn occupy two different Wyckoff sites. By decreasing x, the FCC phase dominates exhibiting intense phase separation tendency, ordering phenomena, and nano-precipitation. Although not chemically discriminated, plasmon-peak splitting in low-loss electron energy loss spectra revealed the presence of two valence electron densities within the FCC phase. Lorentz TEM showed that the ordered nano-precipitates and nano-sized grains with a structure based on a tripled FCC unit cell are pinning-sites for magnetic domain walls and dislocations. All alloy compositions exhibited soft-magnetic behavior with coercivity (Hc) values< 1000 A/m. The FeCoNi(AlMn)1.5 alloy with L21/B2 nanostructure showed the highest magnetization (Ms) with relatively low Hc, attributed to the large magnetic moment of Mn and the synergistic effect of Mn-Al according to DFT, whilst ordering does not impose a negative effect. Phase separation trends within the FCC phase seem to decrease the Ms however, the overall impact on the magnetic behavior is not intense, opening up for new avenues for tuning FeCoNiAlMn properties through chemically-designed phase decomposition regimes.
Amundsen, Monika; Pike, Nicholas; Løvvik, Ole Martin; Carvalho, Patricia Almeida & Gunnæs, Anette Eleonora (2022). New structure and insight on the phase transition within the Cu-Pd-Sn system with 25 at. % Sn. Materialia. 24, p. 1–6. doi: 10.1016/j.mtla.2022.101461. Full text in Research Archive Show summary
The Cu-Pd-Sn material system has a (Cu, Pd) 3 Sn phase region that shows an interesting and unexplored marten- sitic phase transformation. Literature on this system is limited and the description of the phases present is incom- plete and partly contradictory. With focus on a specific Cu-rich composition in this phase region (Cu 33 Pd 42 Sn 25 ) and having employed a combination of differential scanning calorimetry, X-ray diffraction, optical microscopy and density functional theory (DFT) methods we analyze the phases involved and the phase transformation tem- peratures. The martensitic transformation was found to occur at approximately 378 K upon heating and at 328 K under cooling conditions. The crystal structure of the high temperature (HT) austenite phase was identified as face centered cubic with lattice constant 𝑎 = 6 . 2261(1) Åat 473 K. DFT simulations of plausible atomic arrange- ments indicated that the HT phase was an L2 1 ( 𝐹 𝑚 ̄3 𝑚 ) type structure. The martensitic LT phase was found to be orthorhombic with a D0 11 ( 𝑃 𝑛𝑚𝑎 ) type structure with lattice parameters a = 8.9860(3) Å, 𝑏 = 5 . 9347(2) Åand 𝑐 = 4 . 5328(2) Åat 298 K.
Tranås, Rasmus André; Løvvik, Ole Martin & Berland, Kristian (2022). Attaining Low Lattice Thermal Conductivity in Half-Heusler Sublattice Solid Solutions: Which Substitution Site Is Most Effective? Electronic Materials. 3(1), p. 1–14. doi: 10.3390/electronicmat3010001. Full text in Research Archive Show summary
Low thermal conductivity is an important materials property for thermoelectricity. The lattice thermal conductivity (LTC) can be reduced by introducing sublattice disorder through partial isovalent substitution. Yet, large-scale screening of materials has seldom taken this opportunity into account. The present study aims to investigate the effect of partial sublattice substitution on the LTC. The study relies on the temperature-dependent effective potential method based on forces obtained from density functional theory. Solid solutions are simulated within a virtual crystal approximation, and the effect of grain-boundary scattering is also included. This is done to systematically probe the effect of sublattice substitution on the LTC of 122 half-Heusler compounds. It is found that substitution on the three different crystallographic sites leads to a reduction of the LTC that varies significantly both between the sites and between the different compounds. Nevertheless, some common criteria are identified as most efficient for reduction of the LTC: The mass contrast should be large within the parent compound, and substitution should be performed on the heaviest atoms. It is also found that the combined effect of sublattice substitution and grain-boundary scattering can lead to a drastic reduction of the LTC. The lowest LTC of the current set of half-Heusler compounds is around 2 W/Km at 300 K for two of the parent compounds. Four additional compounds can reach similarly low LTC with the combined effect of sublattice disorder and grain boundaries. Two of these four compounds have an intrinsic LTC above ∼15 W/Km, underlining that materials with high intrinsic LTC could still be viable for thermoelectric applications.
Mayandi, Jeyanthinath; Finstad, Terje Gunnar; Dahl, Øystein; Ponniah, Vajeeston; Schrade, Matthias & Løvvik, Ole Martin [Show all 8 contributors for this article] (2022). Thin films made by reactive sputtering of high entropy alloy FeCoNiCuGe: Optical, electrical and structural properties. Thin Solid Films. ISSN 0040-6090. 744(139083). doi: 10.1016/j.tsf.2022.139083. Full text in Research Archive Show summary
Films were reactively sputtered from a high entropy alloy (HEA) FeCoNiCuGe target in an Ar/O2 plasma. The case of zero O2 gas flow yielded HEA films of FeCoNiCuGe. These as-deposited HEA films have a face centered cubic (FCC) structure, showing a texture with columnar grains containing many planar defects. The residual electrical resistivity of the films is around 225 μΩcm and the temperature dependence of the resistivity is metal-like. The temperature coefficient of resistivity is small (4.5 ppm/K). The Hall coefficient is positive while the Seebeck coefficient is negative. This is interpreted as arising from an electronic structure having both holes and electrons at the Fermi level as indicated by band structure calculations. The HEA FCC structure is unstable upon annealing in forming gas and showed demixing. Annealing in O2 also yielded inhomogeneous oxides, with a thick layer of CuO growing on the surface. The cases of reactive sputtering with an oxygen flow yielded oxides that are either nanocrystalline or amorphous dependent upon the sputter conditions. These can be classified as high entropy oxides (HEO) and have an optical bandgap around 1.9 eV and high transmission in the infrared region. The amorphous HEO has an electrical conduction interpreted as due to variable range hopping described by the Efros–Shklovskii theory. The HEO films were reduced in forming gas. For the amorphous HEO film, the reduction at 300–500 °C yielded hexagonal Ni5Ge2 and an FCC phase. For the nanocrystalline HEO, reduction resulted in creation of FCC and body centered cubic HEA metal phases.
Berland, Kristian; Løvvik, Ole Martin & Tranås, Rasmus André (2021). Discarded gems: Thermoelectric performance of materials with band gap emerging at the hybrid-functional level. Applied Physics Letters. ISSN 0003-6951. 119(8). doi: 10.1063/5.0058685. Full text in Research Archive Show summary
A finite electronic band gap is a standard filter in high-throughput screening of materials using density functional theory (DFT). However, because of the systematic underestimation of band gaps in standard DFT approximations, a number of compounds may be incorrectly predicted metallic. In a more accurate treatment, such materials may instead appear as low band gap materials and could have good thermoelectric properties if suitable doping is feasible. To explore this possibility, we performed hybrid functional calculations on 1093 cubic materials listed in the MATERIALS PROJECT database with four atoms in the primitive unit cell, spin-neutral ground state, and a formation energy within 0.3 eV of the convex hull. Out of these materials, we identified eight compounds for which a finite band gap emerges. Evaluating electronic and thermal transport properties of these compounds, we found the compositions MgSc2Hg and Li2CaSi to exhibit promising thermoelectric properties. These findings underline the potential of reassessing band gaps and band structures of compounds to identify additional potential thermoelectric materials.
Mayandi, Jeyanthinath; Finstad, Terje Gunnar; Stange, Marit Synnøve Sæverud; Vasquez, Geraldo Cristian; Sunding, Martin Fleissner & Løvvik, Ole Martin [Show all 8 contributors for this article] (2021). Controlling the Electrical Properties of Reactively Sputtered High Entropy Alloy CrFeNiCoCu Films. Journal of Electronic Materials. ISSN 0361-5235. 51, p. 803–812. doi: 10.1007/s11664-021-09343-3. Full text in Research Archive Show summary
Oxide-containing films were made by reactively sputtering a high-entropy alloy target of CrFeCoNiCu. We report on a wide range of changes to the electrical properties made by different heat treatments in oxidizing and reducing atmospheres, respectively. We combine temperature-dependent Hall effect measurements down to 10 K to study the transport mechanisms and correlate that with structural measurements by x-ray diffraction and scanning electron microscopy. The measured/effective resistivity could be varied between 1.3 × 10−4 Ω cm and 1.2 × 10−3 Ω cm by post-deposition processing. The temperature coefficient of resistivity could be varied between − 1.2 × 10−3 K−1 through 0 and to + 0.7 × 10−3 K−1. The key to the variation is controlling the morphology and topology of the film. The conduction of charge carriers is dominated by the relative contribution of weak localization and alloy scattering by varying the degree of disorder in the metallic high-entropy alloy and its topology.
Bernal, Fabian Leonardo Martinez; Lundvall, Fredrik; Kumar, Susmit; Hansen, Per-Anders S.; Wragg, David S. & Fjellvåg, Helmer [Show all 7 contributors for this article] (2021). Jahn-Teller active fluoroperovskites ACrF3 (A=Na+, K+): Magnetic and thermo-optical properties. PHYSICAL REVIEW MATERIALS. 5(6). doi: 10.1103/PhysRevMaterials.5.064420. Full text in Research Archive Show summary
Chromium (II) fluoroperovskites A CrF 3 ( A = Na + , K + ) are strongly correlated Jahn-Teller active materials at low temperatures. In this paper, we examine the role that the A -site ion plays in this family of fluoroperovskites using both experimental methods (x-ray diffraction, optical absorption spectroscopy, and magnetic fields) and density functional theory simulations. Temperature-dependent optical absorption experiments show that the spin-allowed transitions E 2 and E 3 only merge completely for A = Na at 2 K. Field-dependent optical absorption measurements at 2 K show that the oscillating strength of the spin-allowed transitions in NaCrF 3 increases with increasing applied field. Direct magneto-optical correlations suppress the spin-flip transitions for KCrF 3 below its Néel temperature. In NaCrF 3 the spin-flip transitions vanish abruptly below 9 K revealing magneto-optical correlations possibly linked to crystal structure changes. This suggests that as the long range ordering is reduced, local Jahn-Teller effects in the individual CrF 4 − 6 octahedra take control of the observed behavior. Our results show clear deviation from the pattern found for the isoelectronic A x MnF 3 + x system. The size of the A -site cation is shown to be central in dictating the physical properties and phase transitions in A CrF 3 , opening up the possibility of varying the composition to create novel states of matter with tunable properties.
Graff, Joachim Seland; Schuler, Raphael; Song, Xin; Castillo-Hernandez, Gustavo; Skomedal, Gunstein & Enebakk, Erik [Show all 11 contributors for this article] (2021). Fabrication of a Silicide Thermoelectric Module Employing Fractional Factorial Design Principles. Journal of Electronic Materials. ISSN 0361-5235. 50, p. 4041–4049. doi: 10.1007/s11664-021-08902-y. Full text in Research Archive Show summary
Thermoelectric modules can be used in waste heat harvesting, sensing, and cooling applications. Here, we report on the fabrication and performance of a four-leg module based on abundant silicide materials. While previously optimized Mg2Si0.3Sn0.675Bi0.025 is used as the n-type leg, we employ a fractional factorial design based on the Taguchi methods mapping out a four-dimensional parameter space among Mnx-εMoεSi1.75−δGeδ higher manganese silicide compositions for the p-type material. The module is assembled using a scalable fabrication process, using a Cu metallization layer and a Pb-based soldering paste. The maximum power output density of 53 μW cm–2 is achieved at a hot-side temperature of 250 °C and a temperature difference of 100 °C. This low thermoelectric output is related to the high contact resistance between the thermoelectric materials and the metallic contacts, underlining the importance of improved metallization schemes for thermoelectric module assembly.
Tranås, Rasmus André; Løvvik, Ole Martin; Tomic, Oliver & Berland, Kristian (2021). Lattice thermal conductivity of half-Heuslers with density functional theory and machine learning: Enhancing predictivity by active sampling with principal component analysis. Computational Materials Science. ISSN 0927-0256. 202. doi: 10.1016/j.commatsci.2021.110938. Full text in Research Archive Show summary
Low lattice thermal conductivity is essential for high thermoelectric performance of a material. Lattice thermal conductivity is often computed using density functional theory (DFT), typically at a high computational cost. Training machine learning models to predict lattice thermal conductivity could offer an effective procedure to identify low lattice thermal conductivity compounds. However, in doing so, we must face the fact that such compounds can be quite rare and distinct from those in a typical training set. This distinctness can be problematic as standard machine learning methods are inaccurate when predicting properties of compounds with features differing significantly from those in the training set. By computing the lattice thermal conductivity of 122 half-Heusler compounds, using the temperature-dependent effective potential method, we generate a data set to explore this issue. We first show how random forest regression can fail to identify low lattice thermal conductivity compounds with random selection of training data. Next, we show how active selection of training data using feature and principal component analysis can be used to improve model performance and the ability to identify low lattice thermal conductivity compounds. Lastly, we find that active learning without the use of DFT-based features can be viable as a quicker way of selecting samples.
Polfus, Jonathan; Peters, Thijs; Bredesen, Rune & Løvvik, Ole Martin (2021). Vacancy diffusion in palladium hydrides. Physical Chemistry, Chemical Physics - PCCP. ISSN 1463-9076. 23(24), p. 13680–13686. doi: 10.1039/d1cp01960k. Full text in Research Archive Show summary
The self-diffusion coefficients of palladium in PdHx (x = 0, 0.25, 0.5, 0.75, 1) were studied using density functional theory to obtain the required thermodynamic and kinetic parameters. The enthalpy of migration decreased from 0.95 eV for Pd to 0.78 eV for PdH. The enthalpy of vacancy formation exhibited a substantial decrease from about 1.1 eV in Pd to 0.4 eV in PdH, which was ascribed to successive filling of antibonding states weakening the Pd–Pd bonds. Concurrently, the Arrhenius pre-exponential was significantly reduced from 4.75 × 10−3 cm2 s−1 for Pd to 5.67 × 10−9 cm2 s−1 for PdH due to softening of the vibrational modes that determine the entropy of vacancy formation and initial/transition state frequencies. A linear correlation between the logarithm of the pre-exponential and the activation energy was interpreted as enthalpy–entropy compensation (Meyer–Neldel rule). The Pd self-diffusion coefficients in the hydrides were within 1 order of magnitude of that in pure palladium above 200 °C for hydrogen pressures up to at least 107 Pa.
Bernal, Fabian Leonardo Martinez; Sottmann, Jonas; Wragg, David; Fjellvåg, Helmer; Fjellvåg, Øystein & Drathen, Christina [Show all 8 contributors for this article] (2020). Structural and magnetic characterization of the elusive Jahn-Teller active NaCrF3. PHYSICAL REVIEW MATERIALS. 4(5). doi: 10.1103/PhysRevMaterials.4.054412. Full text in Research Archive Show summary
We report on the structural and magnetic properties of the elusive Jahn-Teller active compound NaCrF3, for first time synthesized in large quantities allowing detailed characterization. The crystal structure of NaCrF3 is initially described from a DFT model, which helped serve as the basis for indexing and structure determination confirmed by high-resolution synchrotron x-ray diffraction experiments. NaCrF3 adopts the triclinic space group P¯1 (isostructural with NaCuF3). Magnetometry studies at low temperature show that NaCrF3 is a weak antiferromagnet, Weiss temperature θ=−4 K. The Néel temperature is TN=21.3 K and the paramagnetic moment μ=4.47μB is in accordance with the theoretical S=2. Field-dependent measurements between 2 and 12 K unveil the onset of metamagnetic behavior. Our experiments revealed a weakly canted A-type magnetic structure observed by neutron powder diffraction, with a magnetic propagation vector (1/2,1/2,0) and a magnetic moment of 3.51μB at 1.5 K. Our results shed further light on the Jahn-Teller effects and strong correlations as a function of A-ion size in the family ACrF3.
Løvvik, Ole Martin; Flage-Larsen, Espen & Skomedal, Gunstein (2020). Screening of thermoelectric silicides with atomistic transport calculations. Journal of Applied Physics. ISSN 0021-8979. 128. doi: 10.1063/5.0008198. Full text in Research Archive Show summary
More than 1000 crystalline silicide materials have been screened for thermoelectric properties using first-principles atomistic calculations coupled with the semi-classical Boltzmann transport equation. Compounds that contain radioactive, toxic, rare, and expensive elements as well as oxides, hydrides, carbides, nitrides, and halides have been neglected in the study. The already well-known silicides with good thermoelectric properties, such as SiGe, Mg2Si, and MnSix, are successfully predicted to be promising compounds along with a number of other binary and ternary silicide compositions. Some of these materials have only been scarcely studied in the literature, with no thermoelectric properties being reported in experimental papers. These novel materials can be very interesting for thermoelectric applications provided that they can be heavily doped to give a sufficiently high charge carrier concentration and that they can be alloyed with isoelectronic elements to achieve adequately low phonon thermal conductivity. The study concludes with a list of the most promising silicide compounds that are recommended for further experimental and theoretical investigations.
Polfus, Jonathan Marc; Løvvik, Ole Martin; Bredesen, Rune & Peters, Thijs (2020). Hydrogen induced vacancy clustering and void formation mechanisms at grain boundaries in palladium. Acta Materialia. ISSN 1359-6454. 195, p. 708–719. doi: 10.1016/j.actamat.2020.06.007. Full text in Research Archive Show summary
Hydrogen has a significant impact on the formation of vacancies, clusters and voids in palladium and other metals. The formation of vacancy-hydrogen complexes in bulk Pd and at Σ3 and Σ5 grain boundaries was investigated using first-principles calculations and thermodynamic models. Equilibrium vacancy and cluster concentrations were evaluated as a function of temperature and hydrogen partial pressure based on the Gibbs energy of formation including vibrational and configurational entropies. Vacancies were found to be significantly stabilized by association with interstitial hydrogen, leading to enhanced concentrations by several orders of magnitude. Vacancy clusters were further stabilized at grain boundaries, with equilibrium concentrations reaching site saturation for clusters comprising up to three vacancies. Nanovoids were investigated based on Wulff constructions from calculated surface energies of the (0 0 1) and (1 1 1) terminations as a function of temperature and coverage of hydrogen adsorbates. The most stable termination changed from (1 1 1) in vacuum to (0 0 1) in H2, and the surface energies were lowered due to hydrogen adsorbates. Consequently, voids were also stabilized in the presence of hydrogen. Coalescing of vacancies into nanovoids was found to be thermodynamically unfavorable due to the loss of configurational entropy. It was therefore concluded that enhanced concentrations of vacancies and clusters does not directly cause the formation of voids. The formation of voids in Pd-based membranes was discussed in terms of microstructural characteristics, and strain due to chemical expansion and plastic deformation.
Larsson, Andreas; Tollefsen, Torleif Andre; Løvvik, Ole Martin & Aasmundtveit, Knut (2019). A Review of Eutectic Au-Ge Solder Joints. Metallurgical and Materials Transactions A. ISSN 1073-5623. 50(10), p. 4632–4641. doi: 10.1007/s11661-019-05356-0. Full text in Research Archive Show summary
Gold-germanium (Au-Ge) joints have been part of the electronics industry since the birth of the solid state transistor. Today they find their role as a reliable joining technology, especially for high-temperature applications. This article is a literature study reviewing Au-Ge joints: Their uses, properties, material compatibility, application techniques, and performance characteristics. The review concludes that it is possible to create high-quality and very strong Au-Ge joints with a shear strength up to 150 MPa. They are stable and reliable, showing limited degradation after thousands of hours at high temperature and thousands of thermal cycles. Joints may be used in low-stress applications up to 300 °C.
Berland, Kristian; Shulumba, Nina; Hellman, Olle; Persson, Clas & Løvvik, Ole Martin (2019). Thermoelectric transport trends in group 4 half-Heusler alloys. Journal of Applied Physics. ISSN 0021-8979. 126(14), p. 1–15. doi: 10.1063/1.5117288. Full text in Research Archive Show summary
The thermoelectric properties of 54 different group 4 half-Heusler (HH) alloys have been studied from first principles. Electronic transport was studied with density functional theory using hybrid functionals facilitated by the k⋅p method, while the temperature-dependent effective potential method was used for the phonon contributions to the figure of merit ZT. The phonon thermal conductivity was calculated including anharmonic phonon-phonon, isotope, alloy, and grain-boundary scattering. HH alloys have an XYZ composition, and those studied here are in the group 4-9-15 (Ti,Zr,Hf)(Co,Rh,Ir)(As,Sb,Bi) and group 4-10-14 (Ti,Zr,Hf)(Ni,Pd,Pt)(Ge,Sn,Pb). The electronic part of the thermal conductivity was found to significantly impact ZT and thus the optimal doping level. Furthermore, the choice of functional was found to significantly affect thermoelectric properties, particularly for structures exhibiting band alignment features. The intrinsic thermal conductivity was significantly reduced when alloy and grain-boundary scattering were accounted for, which also reduced the spread in thermal conductivity. It was found that sublattice disorder on the Z-site, i.e., the site occupied by group 14 or 15 elements, was more effective than X-site substitution, occupied by group 4 elements. The calculations confirmed that ZrNiSn, ZrCoSb, and ZrCoBi based alloys display promising thermoelectric properties. A few other n-type and p-type compounds were also predicted to be potentially excellent thermoelectric materials, given that sufficiently high charge carrier concentrations can be achieved. This study provides insight into the thermoelectric potential of HH alloys and casts light on strategies to optimize the thermoelectric performance of multicomponent alloys.
Pike, Nicholas; Matt, Amina & Løvvik, Ole Martin (2019). Determining the Optimal Phase-Change Material via High-Throughput Calculations. MRS Advances. doi: 10.1557/adv.2019.235. Full text in Research Archive
Pike, Nicholas & Løvvik, Ole Martin (2019). Calculation of the anisotropic coefficients of thermal expansion: A first-principles approach. Computational Materials Science. ISSN 0927-0256. 167, p. 257–263. doi: 10.1016/j.commatsci.2019.05.045. Full text in Research Archive Show summary
Predictions of the anisotropic coefficients of thermal expansion are needed to not only compare to experimental measurement, but also as input for macroscopic modeling of devices which operate over a large temperature range. While most current methods are limited to isotropic systems within the quasiharmonic approximation, our method uses first-principles calculations and includes anharmonic effects to determine the temperature-dependent properties of materials. These include the lattice parameters, anisotropic coefficients of thermal expansion, isotherm bulk modulus, and specific heat at constant pressure. Our method has been tested on two compounds (Cu and AlN) and predicts thermal properties which compare favorably to experimental measurement over a wide temperature range.
Zhang, H.; Hippalgaonkar, K.; Buonassisi, T.; Løvvik, Ole Martin; Sagvolden, Espen & Ding, D. (2019). Machine Learning for Novel Thermal-Materials Discovery: Early Successes, Opportunities, and Challenges. Engineered Science. ISSN 2576-988X. 2, p. 1–8. doi: 10.30919/esee8c209. Full text in Research Archive
Bondevik, Tarjei; Kuwabara, Akihide & Løvvik, Ole Martin (2019). Application of machine learning-based selective sampling to determine BaZrO3 grain boundary structures. Computational Materials Science. ISSN 0927-0256. 164, p. 57–65. doi: 10.1016/j.commatsci.2019.03.054. Full text in Research Archive Show summary
A selective sampling procedure is applied to reduce the number of density functional theory calculations needed to find energetically favorable grain boundary structures. The procedure is based on a machine learning algorithm involving a Gaussian process, and uses statistical modelling to map the energies of the all grain boundaries. Using the procedure, energetically favorable grain boundaries in BaZrO3 are identified with up to 85% lower computational cost than the brute force alternative of calculating all possible structures. Furthermore, our results suggest that using a grid size of 0.3 Å in each dimension is sufficient when creating grain boundary structures using such sampling procedures.
Remonato, Filippo; Løvvik, Ole Martin & Flage-Larsen, Espen (2019). Effectiveness of Neural Networks for Research on Novel Thermoelectric Materials. A Proof of Concept. Nordic Artificial Intelligence Research and Development: Third Symposium of the Norwegian AI Society, NAIS 2019. Springer. ISSN 9783030356644. p. 69–77. doi: 10.1007/978-3-030-35664-4_7. Full text in Research Archive Show summary
This paper describes the application of neural network approaches to the discovery of new materials exhibiting thermoelectric properties. Thermoelectricity is the ability of a material to convert energy from heat to electricity. At present, only few materials are known to have this property to a degree which is interesting for use in industrial applications like, for example, large-scale energy harvesting [3, 8]. We employ a standard neural network architecture with supervised learning on a training dataset representing materials and later predict the properties on a disjoint test set. At this proof of concept stage, both sets are synthetically generated with plausible values of the features. A substantial increase in performance is seen when utilising available physical knowledge in the machine learning model. The results show that this approach is feasible and ready for future tests with experimental laboratory data.
Bondevik, Tarjei; Ness, Heine Håland; Bazioti, Kalliopi; Norby, Truls Eivind; Løvvik, Ole Martin & Koch, Christoph T. [Show all 7 contributors for this article] (2019). Investigation of the electrostatic potential of a grain boundary in Y-substituted BaZrO3 using inline electron holography. Physical Chemistry, Chemical Physics - PCCP. ISSN 1463-9076. 21(32), p. 17662–17672. doi: 10.1039/c9cp02676b. Full text in Research Archive Show summary
We apply inline electron holography to investigate the electrostatic potential across an individual BaZr0.9Y0.1O3 grain boundary. With holography, we measure a grain boundary potential of −1.3 V. Electron energy loss spectroscopy analyses indicate that barium vacancies at the grain boundary are the main contributors to the potential well in this sample. Furthermore, geometric phase analysis and density functional theory calculations suggest that reduced atomic density at the grain boundary also contributes to the experimentally measured potential well.
Thøgersen, Annett; Jensen, Ingvild Julie Thue; Stange, Marit Synnøve Sæverud; Røyset, Arne Karstein; Løvvik, Ole Martin & Ulyashin, Alexander [Show all 7 contributors for this article] (2018). Valence charge distribution in homogenous silicon-aluminium thin-films. Journal of Physics: Condensed Matter. ISSN 0953-8984. 30:35502(33), p. 1–11. doi: 10.1088/1361-648X/aad216. Full text in Research Archive Show summary
Homogenous aSi1−xAlxHy alloyed thin films, made by magnetron sputtering, has been found to exhibit tunable band gap and dielectric constant depending on their composition. The optical properties of alloys are largely defined by their electronic structure, which is is strongly influenced by interatomic charge transfer. In this work we have quantified interatomic charge transfer between Si, Al and H in aSi1−xAlxHy thin-films, with and . Charge transfer was found experimentally using x-ray photoelectron spectroscopy, by incorporating Auger parameter data into the Thomas and Weightman model. Both the perfect and imperfect screening models were tested, and the results were compared to models calculated using density functional theory based molecular dynamics. Using imperfect screening properties of Si and Al resulted in an excellent agreement between the experimental and computational results. Alloying aSi with Al is associated with donation of electrons from Al to Si for y = 0. For y > 0 electrons are transferred away from both Al and Si. The change in Si valence charge increases linearly with increasing band gap and decreasing dielectric constant. These relationships can be used as a quick guide for the evaluation of the Si valence charge and subsequently optoelectronic properties, at specific Al/Si ratios.
Løvvik, Ole Martin & Berland, Kristian (2018). Predicting the thermoelectric figure-of-merit from first principles. Materials Today: Proceedings. 5(4), p. 10227–10234. doi: 10.1016/j.matpr.2017.12.269. Full text in Research Archive
Larsson, Andreas; Tollefsen, Torleif Andre; Aasmundtveit, Knut & Løvvik, Ole Martin (2018). Liquid Solid Diffusion (LSD) bonding: A novel joining technology, 2017 21st European Microelectronics and Packaging Conference (EMPC) & Exhibition. IEEE (Institute of Electrical and Electronics Engineers). ISSN 9781538623091. doi: 10.23919/EMPC.2017.8346886. Full text in Research Archive Show summary
Liquid Solid Diffusion (LSD) bonding is a novel joining technology forming solid and liquid two-phase field joints from simple binary systems. LSD creates joints that are applicable at temperatures above the melting temperature of the material composition of the initial bond. Liquid and solid inter-diffusion transform the joint composition and microstructure. The final joint is characterized by incongruent re-melting forming a coherent continuous porous solid solution phase with liquefied pores. This article briefly describes the methodology and presents experimental results based on the Au-Ge system. High quality joints were created. The effective melting point was increased by more than 200 °C above the initial eutectic melting point of the system.
Tabib, Mandar; Løvvik, Ole Martin; Johannessen, Kjetil Andre; Rasheed, Adil; Sagvolden, Espen & Rustad, Anne Marthine (2018). Discovering Thermoelectric Materials Using Machine Learning: Insights and Challenges. Lecture Notes in Computer Science (LNCS). ISSN 0302-9743. 11139 LNCS, p. 392–401. doi: 10.1007/978-3-030-01418-6_39. Full text in Research Archive Show summary
This work involves the use of combined forces of data-driven machine learning models and high fidelity density functional theory for the identification of new potential thermoelectric materials. The traditional method of thermoelectric material discovery from an almost limitless search space of chemical compounds involves expensive and time consuming experiments. In the current work, the density functional theory (DFT) simulations are used to compute the descriptors (features) and thermoelectric characteristics (labels) of a set of compounds. The DFT simulations are computationally very expensive and hence the database is not very exhaustive. With an anticipation that the important features can be learned by machine learning (ML) from the limited database and the knowledge could be used to predict the behavior of any new compound, the current work adds knowledge related to (a) understanding the impact of selection of influence of training/test data, (b) influence of complexity of ML algorithms, and (c) computational efficiency of combined DFT-ML methodology.
Zhao, Dongdong; Løvvik, Ole Martin; Marthinsen, Knut & Li, Yanjun (2018). Segregation of Mg, Cu and their effects on the strength of Al Σ5 (210)[001] symmetrical tilt grain boundary. Acta Materialia. ISSN 1359-6454. 145, p. 235–246. doi: 10.1016/j.actamat.2017.12.023. Full text in Research Archive Show summary
Alien elemental segregation can pronouncedly change the grain boundary properties. Systematic first-principles calculations were performed to investigate the Mg and Cu segregation behavior at Σ5 (210)[001] symmetrical tilt grain boundary (STGB) in Al. The mechanical properties of Mg or Cu containing Σ5 (210)[001] STGBs were probed by combining a canonical Griffith fracture model with an ab-initio tensile test method. It is found that both Mg and Cu have a large driving force to segregate to Al grain boundaries, with Mg preferentially segregating at symmetric substitutional core sites and Cu at interstitial hollow sites at the grain boundary. Interestingly, Al Σ5 (210)[001] is shown to possess a stronger sink strength of Cu impurities than Mg. Both Mg and Cu segregation leads to a grain boundary expansion and a significant decrease of the grain boundary energy. Calculations show that Mg segregation leads to embrittlement of the STGB, contrary to the cohesion enhancing effect of Cu solutes on Al grain boundaries. The Mg induced embrittlement is due to a combination of “structural effect” ‒ (grain boundary expansion) and “chemical effect” ‒ (charge density depletion). The strengthening effect of Cu solutes lies in the creation of new Cu-Al bonds across the grain boundary, which is considered as a strong contribution to the grain boundary cohesion, thereby increasing its resistance against intergranular cleavage. This work demonstrates how a fundamental theoretical understanding on the atomic and electronic level can rationalize mechanical properties of alloys at the macroscopic scale.
Carvalho, Patricia Almeida; Thøgersen, Annett; Ma, Quanbao; Wright, Daniel Nilsen; Diplas, Spyridon & Galeckas, Augustinas [Show all 12 contributors for this article] (2018). Boron-doping of cubic SiC for intermediate band solar cells: a scanning transmission electron microscopy study. SciPost Physics. 5(3). doi: 10.21468/SciPostPhys.5.3.021. Full text in Research Archive
Thøgersen, Annett; Jensen, Ingvild Julie Thue; Stange, Marit Synnøve Sæverud; Kjeldstad, Torunn; Martinez-Martinez, Diego & Løvvik, Ole Martin [Show all 8 contributors for this article] (2018). Formation of nanoporous Si upon self-organized growth of Al and Si nanostructures. Nanotechnology. ISSN 0957-4484. 29(31). doi: 10.1088/1361-6528/aac36a. Full text in Research Archive Show summary
Nanostructured materials offer unique electronic and optical properties compared to their bulk counterparts. The challenging part of the synthesis is to create a balance between the control of design, size limitations, up-scalability and contamination. In this work we show that self-organized Al nanowires in amorphous Si can be produced at room temperature by magnetron co-sputtering using two individual targets. Nanoporous Si, containing nanotunnels with dimensions within the quantum confinement regime, were then made by selective etching of Al. The material properties, film growth, and composition of the films were investigated for different compositions. In addition, the reflectance of the etched film has been measured.
Galeckas, Augustinas; Carvalho, Patricia Almeida; Ma, Quanbao; Azarov, Alexander; Hovden, Sigurd Slettemark & Thøgersen, Annett [Show all 13 contributors for this article] (2018). Optical and microstructural investigation of heavy B-doping effects in sublimation-grown 3C-SiC. Materials Science Forum. ISSN 0255-5476. 924, p. 221–224. doi: 10.4028/www.scientific.net/MSF.924.221. Full text in Research Archive Show summary
In this work, a complementary microstructural and optical approach is used to define processing conditions favorable for the formation of deep boron-related acceptor centers that may provide a pathway for achieving an intermediate band behavior in highly B-doped 3C-SiC. The crystallinity, boron solubility and precipitation mechanisms in sublimation-grown 3C-SiC crystals implanted to 1-3 at.% B concentrations were investigated by STEM. The revealed defect formation and boron precipitation trends upon thermal treatment in the range 1100-2000°C have been cross-correlated with the optical characterization results provided by imaging PL spectroscopy. We discuss optical activity of the implanted B ions in terms of both shallow acceptors and deep D-centers, a complex formed by a boron atom and a carbon vacancy, and associate the observed spectral developments upon annealing with the strong temperature dependence of the D-center formation efficiency, which is further enhanced by the presence of implantation-induced defects.
Jensen, Ingvild Julie Thue; Olden, Vigdis & Løvvik, Ole Martin (2018). Decohesion Energy of ? 5 (012) Grain Boundaries in Ni as Function of Hydrogen Content. Metallurgical and Materials Transactions A. ISSN 1073-5623. p. 1–6. doi: 10.1007/s11661-018-4982-8. Full text in Research Archive
Peters, Thijs; Carvalho, Patricia Almeida; Wees, J.F. van; Overbeek, J.P.; Sagvolden, Espen & Berkel, F.P.F. van [Show all 8 contributors for this article] (2018). Leakage evolution and atomic-scale changes in Pd-based membranes induced by long-term hydrogen permeation. Journal of Membrane Science. ISSN 0376-7388. 563, p. 398–404. doi: 10.1016/j.memsci.2018.06.008. Full text in Research Archive
Løvvik, Ole Martin; Zhao, Dongdong; Li, Yanjun; Bredesen, Rune & Peters, Thijs (2018). Grain boundary segregation in Pd-Cu-Ag alloys for high permeability hydrogen separation membranes. Membranes. 8(3). doi: 10.3390/membranes8030081. Full text in Research Archive Show summary
Dense metal membranes that are based on palladium (Pd) are promising for hydrogen separation and production due to their high selectivity and permeability. Optimization of alloy composition has normally focused on bulk properties, but there is growing evidence that grain boundaries (GBs) play a crucial role in the overall performance of membranes. The present study provides parameters and analyses of GBs in the ternary Pd-Ag-Cu system, based on first-principles electronic structure calculations. The segregation tendency of Cu, Ag, and vacancies towards 12 different coherent ∑ GBs in Pd was quantified using three different procedures for relaxation of supercell lattice constants, representing the outer bounds of infinitely elastic and stiff lattice around the GBs. This demonstrated a clear linear correlation between the excess volume and the GB energy when volume relaxation was allowed for. The point defects were attracted by most of the GBs that were investigated. Realistic atomic-scale models of binary Pd-Cu and ternary Pd-Cu-Ag alloys were created for the ∑ 5(210) boundary, in which the strong GB segregation tendency was affirmed. This is a starting point for more targeted engineering of alloys and grain structure in dense metal membranes and related systems.
Zhao, Dongdong; Løvvik, Ole Martin; Marthinsen, Knut & Li, Yanjun (2017). Twinnability of Al–Mg alloys: A first-principles interpretation. Transactions of Nonferrous Metals Society of China. ISSN 1003-6326. 27(6), p. 1313–1318. doi: 10.1016/S1003-6326(17)60152-3. Full text in Research Archive Show summary
Al–Mg alloys are considered to have potentials to form twins during deformation because Mg can reduce the intrinsic stacking fault energy γISFE of Al. Nevertheless, twinning has rarely been found in Al–Mg alloys even subjected to various severe plastic deformation (SPD) techniques. In order to probe the twinning propensity of Al–Mg alloys, first-principles calculations were carried out to investigate the effects of Mg and vacancies on the generalized planar fault energy (GPFE) of Al. It is found that both Mg and vacancies exhibit a Suzuki segregation feature to the stacking fault, and have the influence of decreasing the γISFE of Al. However, γISFE does not decrease and the twinnability parameter τa of Al does not increase monotonically with increasing Mg concentration in the alloy. On the basis of τa evaluated from the calculated GPFE of Al–Mg alloys, we conclude that deformation twinning is difficult for Al–Mg alloys even with a high content of Mg. Besides, the decrease of γISFE caused by the introduction of Mg and vacancies is supposed to have the effect of improving the work-hardening rate and facilitating the formation of band structures in Al–Mg alloys subjected to SPD.
Jones, Lewys; Wenner, Sigurd; Nord, Magnus Kristofer; Ninive, Per Harald; Løvvik, Ole Martin & Holmestad, Randi [Show all 7 contributors for this article] (2017). Optimising multi-frame ADF-STEM for high-precision atomic-resolution strain mapping. Ultramicroscopy. ISSN 0304-3991. 179, p. 57–62. doi: 10.1016/j.ultramic.2017.04.007. Full text in Research Archive Show summary
Annular dark-field scanning transmission electron microscopy is a powerful tool to study crystal defects at the atomic scale but historically single slow-scanned frames have been plagued by low-frequency scanning-distortions prohibiting accurate strain mapping at atomic resolution. Recently, multi-frame acquisition approaches combined with post-processing have demonstrated significant improvements in strain precision, but the optimum number of frames to record has not been explored. Here we use a non-rigid image registration procedure before applying established strain mapping methods. We determine how, for a fixed total electron-budget, the available dose should be fractionated for maximum strain mapping precision. We find that reductions in scanning-artefacts of more than 70% are achievable with image series of 20–30 frames in length. For our setup, series longer than 30 frames showed little further improvement. As an application, the strain field around an aluminium alloy precipitate was studied, from which our optimised approach yields data whos strain accuracy is verified using density functional theory.
Jones, Lewys; Wenner, Sigurd; Nord, Magnus Kristofer; Ninive, Per Harald; Løvvik, Ole Martin & Marioara, Calin Daniel [Show all 8 contributors for this article] (2017). Mapping the chemistry within, and the strain around, Al-alloy precipitates at atomic resolution by multi-frame scanning transmission electron microscopy. Microscopy and Microanalysis. ISSN 1431-9276. 23, p. 384–385. doi: 10.1017/S1431927617002604. Full text in Research Archive Show summary
Al-Mg-Si(-Cu)alloys, used primarily in the transportation sector, are strengthened by nano-sized precipitate particles. There is a plethora of possible phases, depending onwhich elements are added to the alloy and how the material is heat treated.Here we present results fromthe S, β’’, β’Geand Q’ phases,which allformrod-shaped precipitate with typical dimensions 7x7x50 nm. These phases are meta-stable, which means that they cannot exist outside the Al matrix.
Ma, Quanbao; Carvalho, Patricia; Galeckas, Augustinas; Azarov, Alexander; Hovden, Sigurd Slettemark & Thøgersen, Annett [Show all 13 contributors for this article] (2017). Characterization of B-implanted 3C-SiC for intermediate band solar cells. Materials Science Forum. ISSN 0255-5476. 897, p. 299–302. doi: 10.4028/www.scientific.net/MSF.897.299. Full text in Research Archive Show summary
Sublimation-grown 3C-SiC crystals were implanted with B ions at elevated temperature (400 °C) using multiple energies (100 to 575 keV) with a total dose of 1.3×1017 atoms/cm2 in order to form intermediate band (IB) in 3C-SiC. The samples were then annealed at 1400 °C for 60 min. An anomalous area in the center was observed in the PL emission pattern. The SIMS analysis indicated that the B concentration was the same both within and outside the anomalous area. The buried boron box-like concentration profile can reach ~3×1021 cm-3 in the plateau region. In the anomalous area a broad emission band (possible IB) emerges at around ~1.7-1.8 eV, which may be associated with B-precipitates having a sufficiently high density. Sublimation-grown 3C-SiC crystals were implanted with B ions at elevated temperature (400 °C) using multiple energies (100 to 575 keV) with a total dose of 1.3×1017 atoms/cm2 in order to form intermediate band (IB) in 3C-SiC. The samples were then annealed at 1400 °C for 60 min. An anomalous area in the center was observed in the PL emission pattern. The SIMS analysis indicated that the B concentration was the same both within and outside the anomalous area. The buried boron box-like concentration profile can reach ~3×1021 cm-3 in the plateau region. In the anomalous area a broad emission band (possible IB) emerges at around ~1.7-1.8 eV, which may be associated with B-precipitates having a sufficiently high density.
Jensen, Ingvild Julie Thue; Gorantla, Sandeep Madhukar; Løvvik, Ole Martin; Gan, Jiantuo; Nguyen, Phuong Dan & Monakhov, Edouard [Show all 9 contributors for this article] (2017). Interface phenomena in magnetron sputtered Cu2O/ZnO heterostructures. Journal of Physics: Condensed Matter. ISSN 0953-8984. 29(43). doi: 10.1088/1361-648X/aa8799. Full text in Research Archive Show summary
The interface between ZnO and Cu2O has been predicted to be a good candidate for use in thin film solar cells. However, the high predicted conversion efficiency has yet to be fully realized experimentally. To explore the underlying causes of this we investigate the interface between ZnO and Cu2O in magnetron sputtered samples. Two different sample geometries were made: In the first set thin layers of ZnO were deposited on Cu2O (type A), while in the second set the order was reversed (type B). Using x-ray photoelectron spectroscopy (XPS), an intermediate CuO layer was identified regardless of the order in which the Cu2O and ZnO layers were deposited. The presence of a CuO layer was supported by transmission electron microscopy (TEM) results. Changes in the electron hole screening conditions were observed in CuO near the interface with ZnO, manifested as changes in the relative peak-to-satellite ratio and the degree of asymmetric broadness in the Cu 2p peak. The suppression of the Cu 2p satellite characteristic of CuO may cause the CuO presence to be overlooked and cause errors in determinations of valence band offsets (VBOs). For the type A samples, we compare four different approaches to XPS-based determination of VBO and find that the most reliable results are obtained when the thin CuO layer and the altered screening conditions at the interface were taken into account. The VBOs were found to range between 2.5 eV and 2.8 eV. For the B type samples a reduction of the Cu 2p-LMM Auger parameter was found as compared to bulk Cu2O, indicative of quantum confinement in the Cu2O overlayer.
Jelle, Bjørn Petter; Tilset, Bente Gilbu; Gao, Tao; Grandcolas, Mathieu; Løvvik, Ole Martin & Bohne, Rolf André [Show all 9 contributors for this article] (2017). High-Performance Nano Insulation Materials for Energy-Efficient Buildings. In N, N (Eds.), Proceedings of TechConnect World Innovation Conference 2017. TechConnect. ISSN 9780997511765. p. 289–292. Full text in Research Archive

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Løvvik, Ole Martin (2026). Spille klarinett med helium. [Radio]. NRK P2. doi: https:/radio.nrk.no/podkast/abels_taarn/l_c5c3e976-8c65-4426-83e9-768c65142664. Full text in Research Archive
Løvvik, Ole Martin (2026). Hvordan låter en klarinett som spilles med helium? Full text in Research Archive
Løvvik, Ole Martin (2026). Accelerated discovery of functional materials at the atomistic scale. doi: https:/eu-mace.eu/event/5862:training-school-2026-accelmaps-accelerating-energy-materials-development-from-legacy-labs-to-material-accelerationplatforms.html. Full text in Research Archive
Løvvik, Ole Martin (2026). Solceller og strømstøtte. [TV]. NRK1. doi: https:/tv.nrk.no/serie/dagsnytt-atten-tv/sesong/202606/episode/NNFA56060926. Full text in Research Archive
Løvvik, Ole Martin (2026). Solcelletak er det nesten ingen som vil ha lenger. [Internet]. www.nrk.no. Full text in Research Archive Show summary
Interessen for solcellepanel på taket har stupt. Bransjen snakker om full krise. Strømstøtte og norgespris får skylda, men regjeringen er helt uenig.
Løvvik, Ole Martin (2026). Norgespris er aktiv dødshjelp. [TV]. NRK1. doi: https:/tv.nrk.no/se?v=NNFA19060726&t=791s. Full text in Research Archive
Løvvik, Ole Martin (2026). Na-ion batteries – what is their current status, and what is their future potential? Full text in Research Archive
Løvvik, Ole Martin (2026). Development of new materials with modern digital tools – the Emperor’s new clothes? Full text in Research Archive
Løvvik, Ole Martin (2026). Development of new materials with beneficial properties for thermoelectricity. [Internet]. ThermoBuild Brief. doi: https:/www.unav.edu/documents/d/departamento-de-construccion-instalaciones-estructuras/bites-newsletter-2. Full text in Research Archive
Løvvik, Ole Martin; Bratlie, Sigrid; Goodwin, Morten & Vidnes, Aksel Kjær (2026). Årets forskningsgjennombrudd - Abels tårn - NRK Radio. [Radio]. NRK P2. Full text in Research Archive Show summary
Torkild Jemterud og hans panel ser på 2025s forskningsresultater. Har datamaskiner bestått Turing-testen? Lever nakenrotter evig?
Jemterud, Torkild; Bjorå, Charlotte Sletten; Görbitz, Carl Henrik; Bøhn, Einar Duenger; Dietrichs, Erik Sveberg & Paus-Knudsen, Julie Sørlie [Show all 8 contributors for this article] (2025). Abels tårns ekstrasending. [Radio]. P2. doi: https:/radio.nrk.no/podkast/abels_taarn/sesong/2025. Full text in Research Archive
Løvvik, Ole Martin; Stange, Marit Synnøve Sæverud; Bumke, Lars; Quandt, Eckhard; Kušnír, Jakub & Grabek, Tomas [Show all 11 contributors for this article] (2025). Phase change materials for elastocaloric cooling. Full text in Research Archive
Løvvik, Ole Martin; Stange, Marit Synnøve Sæverud; Bumke, Lars; Quandt, Eckhardt; Kusnir, Jakub & Grabek, Tomas [Show all 10 contributors for this article] (2025). Martensitic phase transformations in a compositionally graded thin film. doi: https:/www.icomat2025.org/wp-content/uploads/2025/09/Full_program_ICOMAT2025_10092025.pdf. Full text in Research Archive
Løvvik, Ole Martin (2025). Kan vi hente verdifulle mineraler fra månen? - Abels tårn - NRK Radio. [Radio]. NRK P2. Full text in Research Archive
Løvvik, Ole Martin (2025). Tellur - Grunnstoffene. Full text in Research Archive Show summary
Tellur er en skikkelig luring. Dette grunnstoffet som ligger helt i skjæringspunktet mellom å være en halvleder og en hel-leder (eller metall som det jo også heter) gir oppgav til mange interessante fysiske fenomener, og er en av grunnene til at det er dagens gjest, Kristian Berland fra NMBU, har det som sitt favorittgrunnstoff. Ikke… Fortsett å lese Tellur
Uggerud, Hilde Thelle; Skomedal, Gunstein; Løvvik, Ole Martin & Runde, Birte (2025). Livepod på NILU - Tungmetaller på avveie, med Hilde Uggerud. doi: https:/youtu.be/8V63hTgz9B8?si=mb_1c-48wEoMWPNW. Full text in Research Archive
Larsen, Aleksander Amble; Huang, Xinyue; Carvalho, Patricia A.; Gunnæs, Anette Eleonora; Løvvik, Ole Martin & Chen, Xian (2025). Size and Orientation Effects in Cu-Zn-Al Shape Memory Micropillars. Full text in Research Archive Show summary
Introduction and objective Martensitic phase transformations in Cu-Zn-Al alloys give rise to functional behaviors such as superelasticity, the shape memory effect, and elastocaloric responses, making them attractive for various applications, including solid-state cooling. While the bulk behavior of these alloys is well-characterized, their response under small-scale mechanical loading is complex and remains an area of ongoing research. This study explores the effects of pillar size and crystallographic orientation on stress-induced martensitic transformation in Cu-Zn-Al single crystals using nanomechanical compression testing. Methods Two compositions of β-phase Cu-Zn-Al single crystals were selected for nanomechanical studies. Samples were cut and prepared with surface orientations corresponding to the (001), (011), and (111) planes of the austenitic phase, as confirmed by electron backscatter diffraction (EBSD). Micropillars with diameters ranging from 0.2 to 3 µm were fabricated using focused ion beam (FIB) milling. Mechanical uniaxial compression tests were performed at room temperature using a nanoindenter to induce martensitic transformation in the pillars. Results Micropillar compression experiments revealed a clear size effect on the martensitic phase transformation. Smaller pillars showed diminished or incomplete transformation compared to larger ones. In addition, crystal orientation significantly influenced the nature and extent of the transformation, with certain orientations exhibiting more substantial transformation strains during compression. For some orientations, no transformation was observed, indicating a strong dependence on crystallographic alignment. These observations highlight how geometric confinement and crystal orientation affect martensitic phase transformation behavior in Cu-Zn-Al.
Bjorå, Charlotte Sletten; Jemterud, Torkild; Løvvik, Ole Martin & Sletsjøe, Arne Bernhard (2024). Abels tårn. [Radio]. NRK P2. Full text in Research Archive
Sagvolden, Espen; Løvvik, Ole Martin; Fertitta, Edoardo; Roshaninejad, Parisa; Habicht, Klaus & Quintero-Castro, Diana Lucia [Show all 7 contributors for this article] (2024). Ab-initio calculations of anharmonicity in thermoelectric materials via the stochastic temperature-dependent effective potential (sTDEP) method. Full text in Research Archive
Løvvik, Ole Martin (2024). Lyddemping i tømmervegger, holdbarhet av hamptau og den høyeste mulige temperaturen. [Radio]. Intervju i Abels tårn, NRK P2. Full text in Research Archive
Quintero-Castro, Diana Lucia; Habicht, Klaus; kojda, Danny; Keller, Thomas; Bourdarot, Frederic & Løvvik, Ole Martin [Show all 10 contributors for this article] (2024). Probing Phonon Lifetimes in Strongly Anharmonic SrTiO3. Full text in Research Archive
Skomedal, Gunstein & Løvvik, Ole Martin (2024). Podcast Grunnstoffene - en vimsete reise gjennom det periodiske system. [Internet]. Podkast. Full text in Research Archive Show summary
En podcast om alle grunnstoffene i det periodiske system, hvert grunnstoff én episode, som publiserer 1-2 episoder per måned.
Vestre, Katharina; Mossige, Joachim; Løvvik, Ole Martin & Jemterud, Torkild (2024). Abels tårn 12.1.2024. [Radio]. Abels tårn NRK P2. Full text in Research Archive
Løvvik, Ole Martin (2024). Hvor tynt kan gull bli og hvorfor er jorda full av tunge grunnstoffer? [Radio]. Intervju i Abels tårn, NRK P2. Full text in Research Archive
Grimenes, Øven Andreas; Løvvik, Ole Martin & Berland, Kristian (2024). The Importance of Electronic Relaxation Times in Complex Fermi Surfaces. Full text in Research Archive
Løvvik, Ole Martin (2024). Sykling, radongass og oversikt over hva vi vet. [Radio]. Intervju i Abels tårn, NRK P2. Full text in Research Archive
Grimenes, Øven Andreas; Løvvik, Ole Martin; Snyder, G. Jeffrey & Berland, Kristian (2024). Electron scattering in materials with extended Fermi surfaces. Full text in Research Archive
Tranås, Rasmus André; Berland, Kristian; Grimenes, Øven Andreas & Løvvik, Ole Martin (2024). New materials with very low lattice conductivity from first principles and machine learning. Full text in Research Archive
Roshaninejad, Parisa; Quintero-Castro, Diana Lucia; Løvvik, Ole Martin; Habicht, Klaus; kojda, Danny & Fertitta, Edoardo [Show all 7 contributors for this article] (2024). Phonon lifetime studies in thermoelectric materials by combining inelastic neutron scattering and ab-initio calculations. Full text in Research Archive
Løvvik, Ole Martin (2023). Hvor dramatisk er det å la kjøleskapdøra stå åpen flere minutter? [TV]. Intervju i Praktisk Info med Jon Almaas, TV Norge. Full text in Research Archive
Delsett, Lene Liebe; Løvvik, Ole Martin; Strumke, Inga & Jemterud, Torkild (2023). Abels tårn - Den viktigste forskninga i 2022. [Radio]. NRK P2. Full text in Research Archive
Rahman, Jamil Ur; Gunnæs, Anette Eleonora; Finstad, Terje Gunnar; Løvvik, Ole Martin & Carvalho, Patricia Almeida (2023). Structure of a high entropy Heusler antimonide. Full text in Research Archive
Tranås, Rasmus André; Løvvik, Ole Martin & Berland, Kristian (2023). Screening of low lattice thermal conductivity materials using active learning. Full text in Research Archive
Marthinsen, Astrid; Marchand, Daniel; Ringdalen, Inga Gudem; Zhang, Kai; Løvvik, Ole Martin & Du, Qiang [Show all 11 contributors for this article] (2023). Transforming ideas into reality: exploring the power of numerical modelling & simulation tools for material and process development. Full text in Research Archive
Rahman, Jamil Ur; Sunding, Martin Fleissner; Løvvik, Ole Martin; Gunnæs, Anette Eleonora; Finstad, Terje Gunnar & Carvalho, Patricia Almeida (2023). Deciphering the structure of a high-entropy Heusler antimonide. Full text in Research Archive
Grimenes, Øven Andreas; Løvvik, Ole Martin & Berland, Kristian (2023). Complex Fermi surfaces for high ZT. Full text in Research Archive
Løvvik, Ole Martin (2023). Abels tårns årskavalkade – nye materialer og maskinlæring. [Radio]. Intervju i Abels tårn, NRK P2. Full text in Research Archive
Rønning, Sissel Beate; Uggerud, Einar & Løvvik, Ole Martin (2023). Abels tårn - Hvordan bør oppvaskkosten oppbevares? [Radio]. NRK Radio. Full text in Research Archive Show summary
- Er selv brød ultraprosessert, og gjør det noe? - Lading av powerbank på tur - Bør man la oppvaskkosten ligge i vasken eller henge den opp? - Hva er best: oppvaskkost eller svamp? - Hva ville en kjemiker kunne lage hvis hen ble kasta 2000 år tilbake i tid? - Hvordan ville det vært å bevege seg rundt inni jorda, dersom den var hul? - Er kværna rødt kjøtt mer usunt enn en hel biff? - Kan man lage en stjerne av vann? I panelet: Sissel Beate Rønning - Genteknolog og matforsker Einar Uggerud - Kjemiker Ole Martin Løvvik - Fysiker
Løvvik, Ole Martin (2023). Plast som leder strøm og andre nye materialer for det grønne skiftet. [Radio]. Intervju i Ekko, NRK P2. Full text in Research Archive
Løvvik, Ole Martin (2023). Storsatsing på utvikling av materialer med automatisering og kunstig intelligens. [Radio]. Intervju i Abels tårn. Full text in Research Archive
Løvvik, Ole Martin (2023). Om varmepumper, magnetisme og tyngdekraft. [Radio]. Intervju i Abels tårn. Full text in Research Archive
Finstad, Terje Gunnar; Mayandi, Jeyanthinath; Soltani, Nayereh; Pokle, Anuj; Løvvik, Ole Martin & Carvalho, Patricia Almeida (2023). TiFe2Si Thin Films. Full text in Research Archive
Løvvik, Ole Martin (2023). Norwegian efforts towards a materials acceleration platform: combining high-throughput experiments, modelling, and machine learning/artificial intelligence. Full text in Research Archive
Tranås, Rasmus André; Løvvik, Ole Martin & Berland, Kristian (2022). Finding Low Lattice Thermal Conductivity Compounds in Materials Space: Machine Learning with Active Sampling. Full text in Research Archive
Løvvik, Ole Martin (2022). Hvordan få strøm fra varme – uten bevegelige deler? Full text in Research Archive
Tranås, Rasmus André; Løvvik, Ole Martin & Berland, Kristian (2022). Alloying leads to drastic reduction of lattice thermal conductivity of half-Heusler compounds. Full text in Research Archive
Løvvik, Ole Martin (2022). Bør du lukke døra til komfyren når den er ferdig med å steke? [TV]. Intervju i Praktisk Info med Jon Almaas, TV Norge. Full text in Research Archive
Løvvik, Ole Martin (2022). Er virkelig norsk olje verdens reneste? [Radio]. Intervju i Abels bakgård, NRK P2. Full text in Research Archive
Løvvik, Ole Martin (2022). Vind, vann og sol kan stille hele verdens energibehov innen 2050. [Radio]. Intervju i Abels bakgård, NRK P2. Full text in Research Archive
Løvvik, Ole Martin (2022). Thermoelectrics of alloys and intermetallic materials: potential benefits and challenges of using SPS. Full text in Research Archive
Tranås, Rasmus André; Løvvik, Ole Martin & Berland, Kristian (2022). Material Informatics: Machine Learning with Active Sampling for Small Training Sets. Full text in Research Archive
Løvvik, Ole Martin; Schrade, Matthias; Grimenes, Øven Andreas; Tranås, Rasmus André & Berland, Kristian (2022). Predictive screening for thermoelectric properties using atomic-scale simulations and machine learning tools. Full text in Research Archive
Løvvik, Ole Martin; Carvalho, Patricia A.; Stange, Marit Synnøve Sæverud; Sunding, Martin Fleissner; Rajput, Kalpna & Larsen, Aleksander Amble [Show all 9 contributors for this article] (2022). Elastocaloric cooling with TiNiCuZr alloys? Full text in Research Archive
Grimenes, Øven Andreas; Løvvik, Ole Martin & Berland, Kristian (2022). Hybrid Functional Screening Identifies Previously Discarded Thermoelectric Materials. Full text in Research Archive
Grimenes, Øven Andreas; Løvvik, Ole Martin & Berland, Kristian (2022). More than 130 new low bandgap materials identified in a hybrid functional screening study – some with very promising thermoelectric properties. Full text in Research Archive
Løvvik, Ole Martin; Rahman, Jamil Ur; Finstad, Terje Gunnar; Gunnæs, Anette Eleonora & Carvalho, Patricia Almeida (2022). Thermoelectric properties of multiple component (high entropy) silicides and antimonides. Full text in Research Archive
Tranås, Rasmus André; Berland, Kristian; Tomic, Oliver & Løvvik, Ole Martin (2022). Discovering Energy Materials: Low Thermal Conductivity Thermoelectric Compounds and Barocaloric Compounds. Full text in Research Archive
Løvvik, Ole Martin (2022). Jordvarme, kjernekraft og batterier. [Radio]. Intervju i Abels tårn, NRK P2. Full text in Research Archive
Løvvik, Ole Martin (2022). Abels tårn: Artig underholdning eller nyttig kunnskap? Full text in Research Archive
Løvvik, Ole Martin (2021). Volum av Li-batterier og horisontale raketter. [Radio]. Intervju i Abels tårn, NRK P2. Full text in Research Archive
Løvvik, Ole Martin (2021). Magneter, solceller og isbiter. [Radio]. Abels tårn, NRK P2. Full text in Research Archive
Løvvik, Ole Martin (2021). Screening of materials for thermoelectric conversion of heat into electricity. Full text in Research Archive
Løvvik, Ole Martin (2021). Vi skal utvikle materialer for fornybar energi ti ganger raskere enn i dag. [Journal]. Apollon : Forskningsmagasin for Universitetet i Oslo. Full text in Research Archive
Løvvik, Ole Martin (2021). Casting Materials Acceleration Platform and the accelerated search for thermoelectric materials. Full text in Research Archive
Mayandi, Jeyanthinath; Finstad, Terje; Stange, Marit Synnøve Sæverud; Vasque, Giovanni; Sunding, Martin Fleissner & Løvvik, Ole Martin [Show all 8 contributors for this article] (2021). Engineering Electrical Properties of Reactively Sputtered High Entropy Alloy CrFeNiCoCu Films. Full text in Research Archive Show summary
Oxide containing films were deposited by reactive sputtering using a high entropy alloy (HEA) target made of equal concentrations of the five 3d-transion metals: Cr, Fe, Co, Ni, and Cu. We report on controlled alterations to the electrical properties made by post-deposition heat treatments in oxidizing and reducing atmospheres respectively. The temperature coefficient of resistivity of the films could be varied between -1.2×10-3 K-1 through 0 and to +0.7×10-3 K-1, while the measured effective resistivity could be varied between 1.3×10-4 Ωcm and 1.2×10-3 Ωcm by post deposition processing. To study the transport mechanisms, we performed temperature dependent Hall effect measurements down to 10 K. We correlated the electrical behavior with structural measurements by a combination of X-ray diffraction (XRD), scanning electron microscopy (SEM) with energy dispersive X-ray spectroscopy (EDS) for elemental mapping at various length scales and secondary ions mass spectroscopy (SIMS). The key to engineering the resistivity is controlling the topology of the film. The topology of the as- deposited thin films can be controlled by the oxygen flow during reactive sputtering as the most important parameter. For zero oxygen flow we have obtained a homogenous HEA film having a FCC structure with the CrFeNiCoCu target. For high oxygen flow we have observed a high entropy oxide with a rock salt crystal structure. Here we report on the case of intermediate flow of oxygen. The as-deposited structure is then complex. There are nano sized regions with oxides and regions with FCC HEA alloys with different compositions of all the 3d-metals but also supersaturated with oxygen. The topology of these regions, and also their concentration, can be altered by post-deposition heat treatment. The transport in the HEA regions are dominated by alloy scattering. The potential in the FCC HEA has disorder due to the random mixing of the elements, which also gives distortion of the individual atom sites. This gives resistivities above the Mott- Ioffe-Regel limit while still having a positive TCR. These disorder effects will be further enhanced by the uptake of oxygen atoms. For these cases one gets weak localization, yielding a negative TCR. The oxides may be insulating but may also conduct by variable range hopping of the Efros–Shklovskii-type or the Mott- type yielding characteristic negative TCR. The post-deposition yields oxide segregation, of for example of spinel type containing Cr. This reduce the entropy and stability of the metal alloy, creating FCC and BCC metal alloys of different compositions. In the case of reducing the heat treatment in a reducing atmosphere, the oxides are reduced. The experimental resistivities are in excellent agreement with the model for the conduction.
Bazioti, Kalliopi; Poulia, Anthoula; Løvvik, Ole Martin; Azar, Amin S.; Mikheenko, Pavlo & Diplas, Spyridon [Show all 7 contributors for this article] (2021). Advanced Scanning Transmission Electron Microscopy investigations of FeCoNi(AlMn)x high-entropy alloy: nanoscale structure and impact on magnetic properties. Full text in Research Archive
Bazioti, Kalliopi; Poulia, Anthoula; Løvvik, Ole Martin; Carvalho, Patricia Almeida; Azar, Amin S. & Mikheenko, Pavlo [Show all 8 contributors for this article] (2021). STEM-EELS investigations and Lorentz microscopy unravelling the structural evolution of FeCoNi(AlMn)x high-entropy alloy and its impact on magnetic properties. Full text in Research Archive
Tranås, Rasmus André; Løvvik, Ole Martin; Tomic, Oliver & Berland, Kristian (2021). Active sample selection for finding low lattice thermal conductivity materials using machine learning. Full text in Research Archive
Azar, Amin S.; Schrade, Matthias; Graff, Joachim Seland; Flage-Larsen, Espen; Løvvik, Ole Martin & Belle, Branson [Show all 8 contributors for this article] (2021). Structural and magnetic performance of soft magnets based on FeCoNiMnAl additively manufactured high entropy alloys. Full text in Research Archive
Bazioti, Kalliopi; Poulia, Anthoula; Løvvik, Ole Martin; Azar, Amin S.; Mikheenko, Pavlo & Diplas, Spyridon [Show all 7 contributors for this article] (2021). Structural evolution of FeCoNi(AlMn)x high-entropy alloy and impact on magnetic properties: nano-scale STEM-EELS investigations. Full text in Research Archive
Løvvik, Ole Martin (2021). Pepperkakehus, stearinlys og sprø ribbe. [Radio]. Intervju i Abels tårn, NRK P2. Full text in Research Archive
Løvvik, Ole Martin (2021). Effektivitet og pris av batterier i batteribanker og nettbrett. [Radio]. Intervju i Abels tårn, NRK P2. Full text in Research Archive
Tranås, Rasmus André; Berland, Kristian; Løvvik, Ole Martin & Tomic, Oliver (2021). Fingerprints of low lattice thermal conductivity compounds: half-Heusler case study and beyond. Full text in Research Archive
Jemterud, Torkild; Danielsen, Anne; Brodal, Per; Løvvik, Ole Martin; Goodwin, Morten & Knudsen, Pål Moddi (2020). Abels tårn: En følsom AI tester visesang. Full text in Research Archive Show summary
Kan en AI lage bedre tekster enn en visesanger? Og bedre drinker enn en bartender? Vi tester begge deler. Prøvekanin er Pål Moddi Knudsen. En AI har tygd seg gjennom de gamle tekstene hans, og spytta ut en splitter ny. Dessuten - Hvorfor hører man bare bassen fra festen ved siden av? - Konsentrer man seg bedre med musikk? - Løper man raskere med musikk? - Hvorfor har vi så ulik musikksmak? - Hvorfor er Moddis gitar alltid sur? - Hvor varmt blir det i en peisovn? I panelet Visesanger Pål Moddi Knudsen Datalog Morten Goodwin Musikkviter Anne Danielsen Fysiker Ole Martin Løvvik Hjerneforsker Per Brodal
Jeyanthinath, Mayandi; Stange, Marit Synnøve Sæverud; Dahl, Øystein; Sunding, Martin Fleissner; Sagvolden, Espen & Flage-Larsen, Espen [Show all 13 contributors for this article] (2020). Functional Properties of CrFeCoNiCu and GeFeCoNiCu oxides. Full text in Research Archive Show summary
The present work explores the concept of high-entropy alloys to design FeCoNi-based oxides suitable for functional applications. Fundamental changes in electronic behavior produced by metal substitution and variable oxygen content were screened to investigate the potential of these materials for specific applications, such as transparent conductors and thermoelectric materials. Elements with atomic radius similar to the average value expected for FeCoNi have been selected for substitutional replacement: (i) Cr and Cu as relatively abundant commodities with intensive industrial application and (ii) Ge due its semiconductor nature. CrFeCoNiCu and GeFeCoNiCu thin films with variable oxygen concentration were deposited by reactive DC magnetron sputtering onto optically transparent substrates and characterized by structural, spectroscopic and electrical methods. Transmission electron microscopy showed that for low oxygen content the materials adopted an fcc-type structure while the NaCl-structure was found for higher oxygen concentrations. X-ray-photoelectron spectroscopy was used to characterize the oxidation state of the metals. The resistivity measured at room temperature ranged from 10-4 to values above 104 Ω.cm. Hall measurements and Seebeck measurements show that both electrons and holes contribute to conduction and that at room temperature the Hall coefficient and Seebeck coefficient have different sign. This behaviour is discussed in terms of the structural analysis and suggested electronic model of the films.
Løvvik, Ole Martin (2020). Uranglassur og enorme vindmøller. [Radio]. Intervju i Abels tårn, NRK P2. Full text in Research Archive
Løvvik, Ole Martin (2020). Elektronikk i varme omgivelser og termoelektriske generatorer. [Radio]. Intervju i Abels tårn, NRK P2. Full text in Research Archive
Løvvik, Ole Martin; Graff, Joachim Seland; Schrade, Matthias; Sunding, Martin Fleissner; Wright, Daniel Nilsen & Song, Xin [Show all 10 contributors for this article] (2020). Silicide module produced in the TESil project. Full text in Research Archive
Løvvik, Ole Martin (2020). Palladium membranes for hydrogen separation - interesting for nanoparticles? Full text in Research Archive
Løvvik, Ole Martin (2020). Energy Conversion Using Phase Transformations. Full text in Research Archive
Løvvik, Ole Martin (2020). Mission Innovation – inspiration for new infrastructure in Norway. Full text in Research Archive
Løvvik, Ole Martin; Flage-Larsen, Espen & Skomedal, Gunstein (2019). Atomistic simulations and machine learning methods for development of thermoelectric materials. Full text in Research Archive
Pike, Nicholas; Matt, Amina & Løvvik, Ole Martin (2019). Determining the Optimal Phase-Change Material via High-Throughput Calculations. Full text in Research Archive
Jemterud, Torkild; Joner, Erik; Oort, Bob van; Dyrrdal, Anita Verpe & Løvvik, Ole Martin (2019). Abels tårn om klima. [Radio]. NRK P2. Full text in Research Archive
Løberg, Monika; Pike, Nicholas; Carvalho, Patricia Almeida; Karlsen, Ole Bjørn; Løvvik, Ole Martin & Gunnæs, Anette Eleonora (2019). Martensitic-like phase transformations within the Cu-Pd-Sn system. Full text in Research Archive
Løberg, Monika; Løvvik, Ole Martin; Carvalho, Patricia Almeida; Karlsen, Ole Bjørn & Gunnæs, Anette Eleonora (2019). Phase Transformations within the Cu-Pd-Sn system. Full text in Research Archive
Løvvik, Ole Martin (2019). Hvor blir kiloene av, grunnstoffer i periodesystemet. [TV]. Intervju i Abels tårn, NRK2. Full text in Research Archive
Løberg, Monika; Karlsen, Ole Bjørn; Løvvik, Ole Martin; Carvalho, Patricia Almeida & Gunnæs, Anette Eleonora (2019). Characterization of phase transformations within the Cu-Pd-Sn system - a possible shape memory alloy for energy conversion. Full text in Research Archive
Løvvik, Ole Martin (2019). Lys om vinteren, hvor blir kiloene av, grunnstoffer i periodesystemet. [Radio]. Intervju i Ekko, NRK P2. Full text in Research Archive
Løvvik, Ole Martin (2019). Introduction to the COMET project. Full text in Research Archive
Sunding, Martin Fleissner; Løberg, Monika; Azar, Amin S.; Stange, Marit Synnøve Sæverud; Carvalho, Patricia Almeida & Karlsen, Ole Bjørn [Show all 10 contributors for this article] (2019). The Need for a Multi-Technique Approach in the Search for New Phase-Transforming Materials. Full text in Research Archive Show summary
Introduction/Purpose Reversible diffusionless first-order phase transformations are a fundamental part of phase change materials and shape memory alloys [1]. Some of these materials even possess useful physical characteristics that show abrupt changes e.g. in their electrical[2] and magnetic [3] polarizations. These phase transformations can be studied using multiple techniques, enabling access to different types of information including length scales and time periods, but how should this characterisation be approached? Methods TEM studies give highly relevant data in this field of research, from detailed crystallographic structures, to relative orientations between grains and phases and to the phase transformation temperature. However, TEM analyses are time consuming and requires small sample sizes and ad hoc preparation. High-throughput measurement techniques are thus required in the search for improved compositions and new materials in order to rapidly assess the properties of many samples or samples with graded composition [4]. Optical methods (for example unpolarised and polarised microscopy and thermal imaging) and electric conductivity measurements can for example be used to map phase transformation temperatures much faster and on much larger areas than traditional TEM techniques. XRD and SEM-EDS-EBSD can then be applied to obtain initial crystallographic and microstructure information on an intermediate size scale, before TEM is applied for the in-depth study of the most relevant compositions. Results We applied this multi-pronged approach in the search for Heusler and half-Heusler alloys with a low temperature martensitic phase transformation and with low thermal hysteresis, with potential applications in waste heat energy harvesting. Conclusions Drawing advantage of the strengths of multiple analysis techniques – with respect to the obtainable information, the sample size and/or the time frame of the analyses – renders the search for new phase change materials more effective. It enables faster identification of potential target compositions and of sample areas optimal for in depth studies. Selected references [1] David Dye, Nature Materials14 (8), 760 (2015). [2] M. M. Vopson, Critical Reviews in Solid State and Materials Sciences40 (4), 223 (2015). [3] L. Huang, D. Y. Cong, L. Ma, Z. H. Nie, Z. L. Wang, H. L. Suo, Y. Ren, and Y. D. Wang, Applied Physics Letters108 (3) (2016). [4] M. L. Green, C. L. Choi, J. R. Hattrick-Simpers, A. M. Joshi, I. Takeuchi, S. C. Barron, E. Campo, T. Chiang, S. Empedocles, J. M. Gregoire, A. G. Kusne, J. Martin, A. Mehta, K. Persson, Z. Trautt, J. Van Duren, and A. Zakutayev, Applied Physics Reviews 4 (1), 011105 (2017).
Mayandi, Jeyanthinath; Stange, Marit Synnøve Sæverud; Dahl, Øystein; Sunding, Martin Fleissner; Sagvolden, Espen & Schrade, Matthias [Show all 12 contributors for this article] (2019). Exploring functional properties of high entropy oxides. Full text in Research Archive Show summary
The present work explores the concept of high-entropy alloys to design FeCoNi-based oxides suitable for functional applications. Fundamental changes in electronic behaviour produced by metal substitution and variable oxygen content were screened to investigate the potential of these materials for specific applications, such as transparent conductors and thermoelectric materials. Elements with atomic radius similar to the average value expected for FeCoNi have been selected for substitutional replacement: (i) Cr and Cu as relatively abundant commodities with intensive industrial application and (ii) Ge due its semiconductor nature. We have sputtered CrFeCoNiCu and of GeFeCoNiCu thin films onto insulating and optically transparent substrates in order to measure electrical and optical properties. The deposition was done by reactive DC magnetron sputtering from targets with equimolar proportions of the metallic elements using atmospheres ranging from 0 to 20 at.% O2 /Ar. The films where characterized by structural, spectroscopic and electrical methods. Transmission electron microscopy showed that for oxygen content up to 8 at.% the materials adopted an fcc-type structure while the NaCl-structure was found for higher oxygen concentrations. X-ray-photoelectron spectroscopy was used to characterize the oxidation state of the metals. Standard absorption analysis by UV-VIS spectrometry revealed that the films had bandgaps ranging from 0.8 to 2.8 eV. The resistivity measured at RT ranged from 10-4 to values above 104 Ω.cm. Hall measurements and Seebeck measurements indicated that both electrons and holes contribute to conduction and at RT the Hall coefficient and Seebeck coefficients have different sign. This behaviour is discussed in terms of the structural analysis and suggested electronic model of the films. In conclusion, the optical band gap and resistivity of the materials produced span over large ranges demonstrating potential for a variety of functional applications.
Mayandi, Jeyanthinath; Stange, Marit Synnøve Sæverud; Sagvolden, Espen; Sunding, Martin Fleissner; Dahl, Øystein & Schrade, Matthias [Show all 12 contributors for this article] (2019). Development of sputtered HEA thin films and their functional properties. Full text in Research Archive
Løvvik, Ole Martin (2019). Klimasending fra Eidsvolls plass om bærekraft og fornybar energi. [Radio]. Intervju i Abels tårn, NRK2. Full text in Research Archive
Løvvik, Ole Martin (2019). Hvor sikker er en hydrogenbil? [TV]. Intervju i Dagsrevyen, NRK1. Full text in Research Archive
Løvvik, Ole Martin (2019). Nye løsninger for å lage strøm av spillvarme kan gi muligheter for norsk industri. Kommentarer i artikkelen. Teknisk Ukeblad. ISSN 0040-2354. Full text in Research Archive
Løvvik, Ole Martin; Berland, Kristian; Remonato, Filippo; Sagvolden, Espen; Flage-Larsen, Espen & Schrade, Matthias [Show all 13 contributors for this article] (2019). Screening thermoelectric materials with ab initio atomistic modelling and machine learning techniques. Full text in Research Archive
Løvvik, Ole Martin (2019). Hvor miljøvennlig er en elbil? [Radio]. Intervju i Abels Tårn, NRK P2. Full text in Research Archive
Løvvik, Ole Martin (2019). Batteriteknologi. [Radio]. Intervju i Ekko, NRK P2. Full text in Research Archive
Løvvik, Ole Martin (2019). Batterier i kulda, luftmotstand hos skihoppere og sang i dusjen. [Radio]. Intervju i Ekko, NRK P2. Full text in Research Archive
Løvvik, Ole Martin (2019). Følelser redder miljøet? Om kobbergruven ved Repparfjorden. [Radio]. Intervju i Ekko, NRK P2. Full text in Research Archive
Løvvik, Ole Martin (2019). Hvor mye kobber trenger vi egentlig til fornybar energiteknologi? [Radio]. Intervju i Radio Nova. Full text in Research Archive
Mayandi, Jeyanthinath; Stange, Marit Synnøve Sæverud; Sunding, Martin Fleissner; Schrade, Matthias; Deuermeier, J. & Fortunato, E. [Show all 10 contributors for this article] (2019). Physical properties of sputtered CrFeCoNiCu thin films on quartz substrate. Full text in Research Archive Show summary
Equiatomic high entropy alloys (HEA) have attracted considerable interest due to their exceptional properties, which might be closely related to their extreme disorder induced by chemical complexity. We have sputtered thin films of CrFeCoNiCu onto insulating and optically transparent substrates in order to measure electrical and optical functional properties. After several optimizations desired sputtering conditions were obtained for the deposition of CrFeCoNiCu onto fused quartz wafers. All the samples were characterized by structural, chemical and electrical methods. The as prepared films had fcc-type structure. Hall measurements were performed from 10 K to 600K. The resistivity was a factor 104 higher than the elemental metals and the temperature dependence was different. The sign of the Hall coefficient indicates that the film was dominated by holes, while the Seebeck coefficient was negative and dominated by electrons. The electric properties will be discussed in terms of a model considering electronic structure and scattering.
Mayandi, Jeyanthinath; Stange, Marit Synnøve Sæverud; Sagvolden, Espen; Sunding, Martin Fleissner; Dahl, Øystein & Schrade, Matthias [Show all 12 contributors for this article] (2019). Structural and Electrical Properties of Sputtered HEA Thin Films of CrFeCoNiCu and their Oxidation Studies. Full text in Research Archive Show summary
High-entropy alloys (HEAs) represent a class of materials that is intensively investigated for a range of possible applications. They generally show a high degree of phase stability by the high entropy while the structure is a random atom position disorder unlike other alloys which can influence physical properties differently than regular alloys. In general, there are few studies on oxidation of HEA and studies on adding oxygen during the fabrication of HEA. In this study we have sputtered thin films of CrFeCoNiCu onto insulating and optically transparent substrates in order to measure structural, electrical and optical functional properties. We have varied the oxygen pressure in the sputtering environment as well as oxidizing the samples at elevated temperatures after deposition. Optical and electrical characterization was performed on films sputter deposited on fused quartz wafers. The films were characterized by TEM, XRD and XPS. The films with no intentional oxygen had an FCC structure with a texture showing strong (111) preferred orientation as seen by XRD. TEM analysis showed columnar morphology with twins parallel to (111) planes. Samples sputtered under high oxygen content showed a simple NaCl structure(FeO). The samples were annealed in air and O2 ambient in the temperature range of 300 to 500 °C. This caused an oxide layer growing on top of the FCC structure. XPS was utilized to find the atomic compositions and chemical stated of the elements. Hall measurements and Seebeck measurements were performed on the as prepared and oxidized films from 10 K to 600K. For the FCC structure the resistivity was a factor 104 higher than the elemental metals while can be satisfactory described by electron phonon scattering by the Bloch-Grüneisen description and the low temperature negative temperature effect by the Kondo effect. The sign of the Hall coefficient was positive while the Seebeck coefficient was negative, indicating the Fermi surface containing pockets of electrons and holes and an energy dependent scattering time. A detailed comparison of the as prepared and the oxidized thin films will be discussed in terms of the structural chemical and electrical properties of the grown films. In addition the electric properties will be discussed in terms of a model considering electronic structure and scattering.
Løberg, Monika; Pike, Nicholas; Karlsen, Ole Bjørn; Carvalho, Patricia Almeida; Løvvik, Ole Martin & Gunnæs, Anette Eleonora (2019). Characterization of phase transforming, Heusler based (Cu-Pd-Sn), alloys for energy harvesting. Full text in Research Archive
Løberg, Monika; Løvvik, Ole Martin & Gunnæs, Anette Eleonora (2018). Energy Conversion using Phase Transforming Materials - the Cu-Pd-Sn system. Full text in Research Archive
Løberg, Monika; Løvvik, Ole Martin & Gunnæs, Anette Eleonora (2018). Martensitic like phase transformations in the Cu-Pd-Sn system. Full text in Research Archive
Løvvik, Ole Martin (2018). CLEAN ENERGY FET Flagship. Full text in Research Archive
Løvvik, Ole Martin; Flage-Larsen, Espen & Skomedal, Gunstein (2018). Screening silicide thermoelectric materials using ab initio transport calculations. Full text in Research Archive
Løvvik, Ole Martin (2018). Machine learning of energy materials. Full text in Research Archive
Løvvik, Ole Martin (2018). Thermoelectric materials for energy harvesting – new modelling tools with predictive power. Full text in Research Archive
Løvvik, Ole Martin (2018). Følger av Trumps forhold til forskning. [Radio]. Intervju i Ekko, NRK P2. Full text in Research Archive
Løvvik, Ole Martin (2018). Trump ut av storsatsing på fornybar energi. [Radio]. Intervju i Ekko, NRK P2. Full text in Research Archive
Løvvik, Ole Martin (2018). Karakterisering av stålskjeer, varm suppe og sykkelritt. [Radio]. Intervju i Ekko, NRK P2. Full text in Research Archive
Løvvik, Ole Martin (2018). Svenske forskere har designet et molekyl som gjør det mulig å lagre solvarme i 18 år. [Journal]. Kommentar i Teknisk Ukeblad. Full text in Research Archive
Løvvik, Ole Martin (2018). Moderne metoder innen modellering og maskinlæring kan gi nye løsninger for energieffektivisering og utnyttelse av spillvarme. Full text in Research Archive
Løvvik, Ole Martin (2018). High-throughput search for heat harvesting materials. Full text in Research Archive
Løvvik, Ole Martin (2018). Thermoelectric materials for energy harvesting – high-throughput screening using new tools. Full text in Research Archive
Berland, Kristian; Eliassen, Simen Nut Hansen; Løvvik, Ole Martin; Persson, Clas & Schrade, Matthias (2018). Computational Engineering of Thermoelectric Materials. Full text in Research Archive
Tabib, Mandar & Løvvik, Ole Martin (2018). Algoritme på grønn materialjakt. Full text in Research Archive
Tabib, Mandar; Løvvik, Ole Martin; Johannessen, Kjetil Andre; Rasheed, Adil; Sagvolden, Espen & Rustad, Anne Marthine (2018). Discovering Thermoelectric Materials Using Machine Learning: Insights and Challenges. Full text in Research Archive
Løvvik, Ole Martin; Wenner, Sigurd & Ringdalen, Inga Gudem (2018). Fundamental Studies of Precipitates as Strengthening Agents in Aluminium Alloys. Full text in Research Archive
Bondevik, Tarjei & Løvvik, Ole Martin (2018). Applying Machine Learning-based Selective Sampling to determine Grain Boundary Structures. Full text in Research Archive
Løvvik, Ole Martin (2018). Stålskjeer, suppe og sykkelritt. [TV]. Abels tårn, NRK2. Full text in Research Archive
Løvvik, Ole Martin (2018). Handlevogner, teip og heliumtrompet. [Radio]. Intervju i Ekko, NRK P2. Full text in Research Archive
Løvvik, Ole Martin (2018). Handlevogner og heliumtrompet. [TV]. Abels tårn, NRK2. Full text in Research Archive
Pike, Nicholas & Løvvik, Ole Martin (2018). Determination of the pyroelectric coefficient in spontaneously polarized materials. Full text in Research Archive
Pike, Nicholas & Løvvik, Ole Martin (2018). High-throughput calculations of the pyroelectric coefficient in spontaneously polarized materials. Full text in Research Archive
Løvvik, Ole Martin (2018). Dette flyet flyr uten noen bevegelige deler. [Internet]. Kommentar i forskning.no. Full text in Research Archive
Berland, Kristian; Løvvik, Ole Martin & Persson, Clas (2017). How hybrid exchange affects thermoelectric transport properties of GaAs, PbTe, Cu2Se, and half Heuslers: Accurate grid sampling enabled with a corrected k.p scheme. Full text in Research Archive
Løvvik, Ole Martin (2017). Hva skjer hvis du setter et kjøleskap i fryseren? [Radio]. Intervju i Ekko, NRK P2. Full text in Research Archive
Grandcolas, Mathieu; Jelle, Bjørn Petter; Gao, Tao; Løvvik, Ole Martin; Bohne, Rolf André & Mofid, Sohrab Alex [Show all 8 contributors for this article] (2017). Nano Insulation Materials (NIMs) Applying Hollow Silica Nanospheres and Nanofibers. Full text in Research Archive
Schrade, Matthias; Berland, Kristian; Guzik, Matylda Natalia; Løvvik, Ole Martin & Finstad, Terje (2017). Why does the thermal conductivity of XNiSn vary for nominally identical samples? Full text in Research Archive
Berland, Kristian; Eliassen, Simen Nut Hansen; Schrade, Matthias; Tofan, Raluca; Guzik, Matylda Natalia & Gunnæs, Anette Eleonora [Show all 11 contributors for this article] (2017). Using theory to understand thermoelectric materials. Full text in Research Archive
Jelle, Bjørn Petter; Tilset, Bente Gilbu; Gao, Tao; Grandcolas, Mathieu; Løvvik, Ole Martin & Bohne, Rolf André [Show all 9 contributors for this article] (2017). High-Performance Nano Insulation Materials for Energy-Efficient Buildings. Full text in Research Archive Show summary
High-performance nano insulation materials (Hi-Per NIM) may be developed by exploiting the Knudsen effect for reduced thermal conductivity and thus make thermal insulation materials with a nanoporous air-filled structure. Hence, these NIMs are supposed to maintain their thermally insulating properties with nanopores filled with air at atmospheric pressure. NIMs with very low thermal conductivity values will enable the use of normal or thin wall thicknesses in energy-efficient buildings. Very thick building envelopes are not desirable due to several reasons, e.g. considering space issues with respect to both economy, floor area, transport volumes, architectural restrictions and other limitations, material usage and existing building techniques. This study will present an exploration of attempting to achieve NIMs through experimental laboratory development of hollow silica nanospheres (HSNS), hollow silica nanofibres (HSNF) and hollow silica integrated nanospheres and nanofibres, alongside theoretical modelling and sustainability investigations.
Larsson, Andreas; Tollefsen, Torleif Andre; Løvvik, Ole Martin & Aasmundtveit, Knut Eilif (2017). Thermoelectric module for high temperature application. Full text in Research Archive
Larsson, Andreas; Tollefsen, Torleif Andre; Løvvik, Ole Martin & Aasmundtveit, Knut Eilif (2017). Liquid Solid Diffusion (LSD) bonding. Full text in Research Archive
Jemterud, Torkild; Gravensteen, Ida Kathrine; Løvvik, Ole Martin & Gadmar, Tone Charlotte (2017). Abels Tårn, NRK Radio: "Er popcorn junkfoodens svar på norsk gass? [Radio]. Ekko, NRK P2. Full text in Research Archive
Larsson, Andreas; Tollefsen, Torleif Andre; Løvvik, Ole Martin & Aasmundtveit, Knut Eilif (2017). Liquid solid diffusion bonding - A novel joining technology. Full text in Research Archive
Larsson, Andreas; Tollefsen, Torleif Andre; Løvvik, Ole Martin & Aasmundtveit, Knut Eilif (2017). Liquid Solid Diffusion (LSD) Bonding - A novel joining technology. Full text in Research Archive
Løvvik, Ole Martin & Vucetic, Vasilija (2025). First-principles study of structure and phase stability of Cu2AlMn Heusler alloy using TDEP and DFT. Universitetet i Oslo. Full text in Research Archive
Grimnes, Øven Andreas; Berland, Kristian & Løvvik, Ole Martin (2025). Scattering dependent electronic transport in thermoelectric materials with complex band structures. Norges miljø- og biovitenskapelige universitet (NMBU). ISSN 9788257523039. Full text in Research Archive Show summary
Thermoelectric (TE) materials, which can convert heat to electricity and vice versa, have great potential for energy harvesting from waste heat and have seen renewed interest in recent years. These materials are also used in many niche applications, such as electrical generators in remote locations, sensors, and cooling for medical and scientific equipment. With the advent of computational methods like density functional theory (DFT), modeling is becoming an increasingly more important tool in the TE field. Providing, among other things, electronic band structures and electronic transport properties, DFT combined with Boltzmann transport theory provides both predictive and explanatory power. A limiting factor, however, has long been the modeling of electron scattering, resulting in the use of coarse approximations like the constant relaxation time approximation or textbook results based on heavily simplified models. More accurate modeling and understanding of electronic scattering in complex materials and band structures could lift the field of TE and result in new and improved TE materials. To find suitable candidate materials for more detailed analysis, a highthroughput (HT) screening of the Materials Project database was performed. Based on criteria such as crystal structure complexity, elements, energy above convex hull, magnetic properties, and valence electron count, 6965 materials were selected. For more accurate band gap estimations, the MBJ functional was employed. This revealed band gaps in 189 materials predicted to be metallic with PBE, and an average band gap increase of 0.43 eV in 331 materials with narrow PBE band gaps. The electronic transport properties and thermoelectric figure of merit (zT) were calculated with a constant relaxation time (τ ) of 10 fs and with a lattice thermal conductivity (κℓ) of 5 W/mK for each material. Using the same, constant τ and κℓ for every material results in both false positives and negatives, but still yields a rough ranking on how applicable the electronic structure is for TE purposes. Based on the high-throughput results, two promising full-Heusler materials were selected for further investigation. CsK2Sb, a known photocatode material, has a complex band structure with features akin to those of two-dimensional (2D) quantum wells. The other material, Na2TlSb, also has a quasi-low-dimensional band structure, but one-dimensional (1D), resulting in a Fermi surface consisting of sheets. Both band structures have a rapidly increasing density of states (DOS) with high-mobility carriers close to the band gap, a favorable trait for thermoelectric materials. Despite high DOS generally being associated with excessive electron scattering, this was not found to be the case in these materials, counterintuitive to conventional wisdom. Both materials were also found to have very low κℓ, resulting in a predicted zT of 2.6 and 4.4 for CsK2Sb and Na2TlSb, respectively. Compressive strain was found to improve the electronic transport properties of CsK2Sb by enhancing the 2D features and reducing electron scattering, but also increased κℓ, resulting in only a marginally increased zT of 2.7 at 1% strain. The changes to materials properties and the resulting electronic scattering rates in strained CsK2Sb prompted further investigation. The materials properties of ZrNiSn were systematically adjusted, and the following changes to scattering rates were observed. While most materials properties changed scattering rates as expected, some gave less intuitive results. Other parameters relevant to scattering, such as carrier concentration (Nc) and temperature (T), were also investigated. In addition to directly affecting different scattering mechanisms, both Nc and T change the location of the Fermi level and the magnitude of free-carrier screening, creating highly interconnected results. The low scattering rates could also, in part, be explained by the exotic geometries of the Fermi surfaces. For momentum-dependent scattering mechanisms, the large separation in k-space prevented scattering between large parts of the surface. Further, the combination of wavefunction overlaps and the geometrical momentum relaxation time factor strongly reduced elastic scattering to large portions of the total scattering space. The results for both materials indicate that Fermi surfaces delocalized in k-space tend to reduce electronic scattering. These insights, combined with knowledge of how materials properties affect scattering, can help build understanding and intuition that can aid the discovery and design of new TE materials.
Høylo-Rosenberg, Jørn-Marcus; Løvvik, Ole Martin & Persson, Clas (2022). An ab initio study of the band gaps of high-entropy silicides based on FeSi2. Universitetet i Oslo. Full text in Research Archive Show summary
In this work we have set out to examine the band gaps of potential high-entropy stabilized transition metal silicides, based on the FeSi2 semiconductor. Here, we have performed a first principles study utilizing density functional theory in combination with the special quasi-random structures method. The band gaps has been evaluated primarily with the PBE GGA functional, but also the SCAN meta-GGA functional and hybrid functional HSE06 has been applied in this project. Potential alloys has been examined by generating a set of distinct supercells of the FeSi2 structure, where the iron sites are populated quasi-randomly by a combination of 3d transition metals. The alloys we have studied in this project are comprised of combinations of Cr, Fe, Mn, Ni, Co and Ti, most emphasis has been put on the Cr-Fe-Mn-Ni-Si system. In the (CrFeMnNi)Si2 composition, we can report several semiconducting supercells with band gaps ranging between 0 - 0.05 eV using PBE GGA. The band gaps displayed significant spin polarization, in the spin up direction most supercells pointed to a gap around 0.3 eV. Contrary, the band gaps in spin down varied between 0 - 0.05 eV. Accordingly, we found a finite magnetic moment in this alloy equal to 0.083 mu_B, this was attributed mainly to chromium and manganese atoms in the lattice. Successive simulations of alloys based on the Cr-Fe-Mn-Ni-Si system with non-equimolar distribution of 3d elements, resulted most frequently in half-metallic structures with a spin up band gap ranging between 0.1 eV and 0.5 eV. Of the different compositions we tested, the ones either rich in manganese and/or poor in chromium showed the most promise with respect to the band gap. In particular, we report a PBE total band gap equal to 0.1 eV in the Cr3Fe5Mn5Ni3Si32 composition. Lastly, we looked at compositions where either Cr, Mn or Ni were replaced by either cobalt or titanium. This yielded predominately metallic compounds, where the lack of band gaps could be ascribed to defect states at the band edges.
Nyberg, Fredrik; Løvvik, Ole Martin & Persson, Clas (2020). Ab initio calculation of the coefficient of thermal expansion and the secondary pyroelectric vector. Universitetet i Oslo. Full text in Research Archive
Frøen, Emil Herman; Norby, Truls Eivind; Bjørheim, Tor Svendsen & Løvvik, Ole Martin (2019). Coexistent Oxide p-n Junctions: DFT Analysis of Interfaces Between Nickel Oxide and Zinc Oxide. Universitetet i Oslo. Full text in Research Archive Show summary
NiO and ZnO are a pair of p- and n-type wide band gap metal oxide semiconductors that forms a partially soluble system with each other. Due to these properties, they are considered a potential semiconductor pair for the construction of a p-n heterojunction that remains thermodynamically stable under high-temperature applications. To this end, an in-depth understanding of the properties of the available interface arrangements, as well as of the effects of the formation of the mixed phases on the properties of the semiconductors is necessary. This study seeks to investigate these aspects by a theoretical approach, utilizing DFT to model the properties of the possible interfaces, as well as the behavior of the mixed phase systems at different concentrations of solute ions. The bulk properties of nickel substituted ZnO (Ni:ZnO) and zinc substituted NiO (Zn:NiO) were investigated utilizing a hybrid functional approach. Both systems were found to exhibit an increase in the VBM energy level relative to the pure materials, with maxima of 0.39 eV and 0.26 eV, respectively. The VBM change with solute ion concentrations has been found to reach saturation at 3.70% and 4.69%, again respectively. The VBM shifts of Ni:ZnO and Zn:NiO are found to be caused by the formation of a new valence band due to localized impurity states, and the formation of cubic ZnO bonding states, respectively. For both mixed phase materials, a reduction of the band gap width is observed: The Ni:ZnO band gap is reduced by 0.3 eV, while the Zn:NiO band gap changes linearly with concentration throughout the experimental concentration range, with a band gap reduction of 0.8 eV at 31.25% zinc concentration. The structural, energetic and electronic properties of the NiO-ZnO interfaces formed from pairs of low-Miller index surfaces were investigated utilizing a GGA+U functional approach. The polar-polar interfaces were found to exhibit the most favorable interface formation energies, for either strain distribution considered. The natural band offsets were found to all form a type II heterojunction with valence band offset values ranging from 0.73 to 1.84 eV. The majority of the considered interface arrangements exhibit considerable interface states: Conduction band interface states are the most prevalent, formed from nickel 3d-orbitals. The results indicate that the prevalence of these interface states, and the general NiO-ZnO heterojunction properties, may be manipulated to a considerable extent by choosing a suitable interface, and the growth conditions under which it is formed.
Both, Kevin Gregor; Norby, Truls Eivind; Bjørheim, Tor Svendsen & Løvvik, Ole Martin (2019). Cu2O – ZnO p-n Junctions: Thermodynamics, Structure, and Properties – an Experimental and Theoretical Study. Universitetet i Oslo. Full text in Research Archive
Mjærum, Dagfinn Olaf Øvrevik; Persson, Clas; Berland, Kristian & Løvvik, Ole Martin (2019). Layer Stacking and Electronic Properties of Hexagonal Covalent Organic Frameworks - A Computational Investigation with van der Waals Density Functional Theory. Universitetet i Oslo. Full text in Research Archive Show summary
We have calculated the electronic properties and layer stacking patterns for a group of twelve hexagonal covalent organic frameworks. This has been done in order to explore this group of materials on a fundamental level and aim to lay the ground work for further studies, which will focus on assessing these frameworks as useful materials for electronic and optoelectronic devices. To explore these materials, we have deployed numerical density functional theory calculations. A Van der Waals density functional is used to describe non-local correlation effects that give rise to stabilizing van der Waals forces, essential in layered systems. We found that all the materials prefer to stack in specific configurations close to AA-stacking, where subsequent layers are placed directly on-top of each other. We also found that the materials exhibit dispersion in the out-of-plane direction when we go from two-dimensional single layers to three-dimensional stacks.
Røe, Ingeborg Treu; Selbach, Sverre Magnus & Løvvik, Ole Martin (2017). Finite temperature ab initio simulations of the lattice thermal conductivity of XNiBi, X=(Sc, Y, La), based half-Heuslers. Norges teknisk-naturvitenskapelige universitet. Full text in Research Archive Show summary
Over the past few years, the half-Heusler materials have attracted attention for their potential within thermoelectric applications. This is mainly due to the flexibility of half-Heusler crystal structure. The half-Heuslers comprise three elements, XYZ, that crystallize in the face-centered cubic structure F$\overline 4$3m. Thus, materials that comprise nontoxic, environmentally friendly, inexpensive and abundant elements are possible. %, as opposed to many of the commercialized thermoelectric materials such as the lead and bismuth tellurides. Moreover, many of these half-Heuslers are low band gap semiconductors with good electrical properties, making them attractive for thermoelectric applications. On the other hand, the half-Heuslers are associated with too high lattice thermal conductivity to be applicable as thermoelectric materials. However, previous studies suggest that the lattice thermal conductivity may be reduced through materials' engineering, where two of the most common methods proposed in literature are alloying on one of the atomic sites, and nanostructuring. Consequently, the present work investigates the effect of alloying on the X-position relative to the Bi-position of XNiBi, X=(Sc, Y or La), based half-Heuslers using density functional theory together with the temperature dependent effective potential method. The different half-Heusler alloys are described within the virtual crystal approximation, and the lattice thermal conductivity is calculated with the Boltzmann transport equation within the relaxation time approximation. This enables independent investigations of different contributions to the lattice thermal conductivity. In the Sc$_x$Y$_y$La$_{1-x-y}$NiBi alloys, the minimum lattice thermal conductivity was calculated to 4.3 W/mK for Sc$_{0.24}$La$_{0.76}$NiBi even though the maximum mass-disorder occurs for higher concentrations of Sc. This behaviour may be explained by the dominance of the anharmonic contributions to the lattice thermal conductivity. Consistent with the results from the specialization project, increasing the concentration of La in Sc$_x$Y$_y$La$_{1-x-y}$NiBi was shown to increase the anharmonic scattering of the acoustic phonon modes. Since the mass-disorder in Sc$_x$Y$_y$La$_{1-x-y}$NiBi primarily targets the optic phonon modes, which carry less heat than the acoustic, the alloying on the X-position does not reduce the lattice thermal conductivity efficiently. Alloying on the Bi-position of YNiBi with As or Sb, on the other hand, targets the acoustic phonon modes. Thus, the lattice thermal conductivity is reduced to 2.4 W/mK in YNiBi$_{0.36}$As$_{0.64}$. The impact of the scattering of acoustic phonon modes on the lattice thermal conductivity is further emphasized in the nanostructured materials. Through a simple model for grain boundary scattering, nanoscaled grains were introduced to the bulk materials such that the acoustic phonon modes were targeted. Consequently, in the Sc$_x$La$_{1-x}$NiBi binary alloy with grains of 50 nm in diameter, the minimum lattice thermal conductivity is reduced with more than 45\% compared to the minimum value of the bulk alloys; from 4.3 W/mK for Sc$_{0.24}$La$_{0.76}$NiBi to 2.4 W/mK for Sc$_{0.5}$La$_{0.5}$NiBi. The present work indicates that the lattice thermal conductivity of XNiBi, X=(Sc, Y or La), based half-Heuslers may be reduced sufficiently through nanostructuring and alloying to be competitive with state-of-the-art thermoelectric materials. Moreover, the understanding of how different mechanisms influence the lattice thermal conductivity achieved in this work may aid the continued progress of the performance of thermoelectric materials.

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Published Nov. 4, 2010 12:24 PM - Last modified Jan. 22, 2025 4:23 PM

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Solid-state Physics and Quantum Technology