matthisc

• Ali, Ayaz; Bisht, Prashant; Schrade, Matthias; Xing, Wen; Vullum, Per Erik & Taniguchi, Takashi [Show all 9 contributors for this article] (2025). Optically Programmable Smart WSe2/hBN Heterostructure Gas Sensors. ACS Applied Materials & Interfaces. ISSN 1944-8244. 17(36), p. 50977–50985. doi: 10.1021/acsami.5c09390. Full text in Research Archive Show summary

  Highly sensitive and energy-efficient gas sensors are essential for real-time environmental monitoring and air quality assessment. In this work, we present an optically programmable gas sensor based on WSe2/hBN heterostructure transistors for NOx detection. The hBN interfacial layer enhances device performance by reducing charge trapping and improving transport, enabling the WSe2/hBN configuration to achieve a higher sensing response and faster recovery than WSe2/SiO2 devices. To understand the sensing mechanism, in situ Kelvin probe force microscopy (KPFM) was used, revealing that NOx adsorption at the metal/semiconductor interface modulates the Schottky barrier height (SBH), which governs charge transport and gas sensitivity. Furthermore, we demonstrate that UV-induced charge modulation allows dynamic control of the sensor response, offering a tunable and reversible method for optimizing gas detection. This study highlights the potential of heterostructure engineering and optoelectronic modulation in developing next-generation, low-power, smart gas sensors for environmental monitoring applications.

• 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.

• Ali, Ayaz; Schrade, Matthias; Xing, Wen; Vullum, Per Erik; Koybasi, Ozhan & Taniguchi, Takashi [Show all 8 contributors for this article] (2024). Two-Dimensional Heterostructure Complementary Logic Enabled by Optical Writing. Small Science. 4(5). doi: 10.1002/smsc.202300319. Full text in Research Archive
• 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.

• Rahman, Jamil Ur; Carvalho, Patricia Almeida; Soltani, Nayereh; Schrade, Matthias; Gunnæs, Anette Eleonora & Finstad, Terje Gunnar (2022). Synthesis, microstructure, and thermoelectric properties of Sb-Based high entropy alloys. Intermetallics (Barking). ISSN 0966-9795. 143. doi: 10.1016/j.intermet.2022.107495. Full text in Research Archive Show summary

  In this paper, we explore the synthesis of new antimonide compounds based upon concepts of high entropy alloys (HEA). HEAs are usually multi-element (⪆5) alloys with near the equiatomic composition. They have been given very large attention due to their vast potential within many fields. We investigated the thermoelectric transport properties of antimony-based high entropy alloys. The observed electrical conductivity, Seebeck coefficient, and thermal conductivity were comparable to those of cutting-edge half-Heusler thermoelectric alloys. Alloys with different valence electron counts (VEC) were synthesized and correlated with measured electrical properties.

• 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.

• Schrade, Matthias; Xing, Wen; Thorshaug, Knut & Belle, Branson (2022). Centimeter-Sized Monolayer CVD Graphene with High Power Factor for Scalable Thermoelectric Applications. ACS Applied Electronic Materials. 4(4), p. 1506–1510. doi: 10.1021/acsaelm.1c00995. Full text in Research Archive Show summary

  Among the many extraordinary qualities of graphene, its thermoelectric properties have attracted significant interest, for example, in active cooling applications. Here, we report on the thermoelectric transport properties of centimeter-sized monolayer CVD graphene, electrostatically controlled by a high-capacity ionic gel. The power factor reaches 7 and 5.4 mW m–1 K–2 for hole and electron conduction, respectively, similar to earlier reports obtained for microdevices despite our devices being over 2 orders of magnitude larger. On the basis of these results, we propose nonvolatile ferroelectric polarization as a scalable technology for graphene-based thermoelectric applications.

• Mayandi, Jeyanthinath; Dias, M.; Stange, Marit Synnøve Sæverud; Lind, Anna Maria; Sunding, M.F. & Cerdeira, A.C. [Show all 12 contributors for this article] (2021). Partial oxidation of high entropy alloys: A route toward nanostructured ferromagnets. Materialia. 20. doi: 10.1016/j.mtla.2021.101250. Full text in Research Archive Show summary

  The present work proposes a new approach to the development of nanostructured ferromagnetic composites based on progressive oxidation of high entropy alloys. Sequential ‘subtraction’ of the metals with the lowest oxidation energy destabilizes the remaining solid solution and induces phase separation at the nanoscale. By exploring immiscibility routes in the multidimensional phase diagrams, the strategy can be used to promote intimate contact between mixed oxides and metallic solid solutions with desired magnetic properties. To demonstrate the concept, the relation between oxidation energy, phase separation, and magnetic behavior has been investigated across the (CoCrCuFeNi)Ox composition space, for 0.1≤ x ≤1.3, in materials produced by reactive sputtering. The results point out directions for tuning the composition when aiming for phases with specific magnetic properties arranged in complex microstructures.

• Poulia, Anthoula; Azar, Amin S.; Schrade, M.; Graff, J.S.; Bazioti, Kalliopi & Gunnæs, Anette Eleonora [Show all 8 contributors for this article] (2021). Process–Structure–Property Relationship in FeCoNiAlxMnx Complex Concentrated Alloys Processed by Additive Manufacturing. Journal of materials engineering and performance (Print). ISSN 1059-9495. 30(9), p. 6961–6971. doi: 10.1007/s11665-021-06082-8. Full text in Research Archive Show summary

  Fourteen alloys of the FeCoNiAlxMnx system were processed by laser metal deposition (LMD). The feedstock was a weighted and proportional blend of the containing elemental powders, targeting the nominal alloy compositions. Prior to processing, the composition and particle characteristics of the feedstock were assessed. The microstructural features and crystal structures of all LMD processed materials were characterized with scanning electron microscopy/energy dispersive spectroscopy and x-ray diffraction, in both as-received and heat-treated conditions. Selected samples were investigated via scanning transmission electron microscopy and electron backscattered diffraction for further structural understanding. Hardness tests, under various indentation loads and dwelling times, were performed to assess the mechanical properties of the processed samples. The results showed a rise in hardness as Al and Mn contents increase. The variation of hardness with composition follows a reverse sigma-type curve, reflecting the microstructural evolution and grain size variations in the alloys. Based on the hardness data, we suggest a trained and validated predictive model, which can be used in alloy design for future developments.

• Jensen, Ingvild Julie Thue; Ali, Ayaz; Zeller, Patrick; Amati, Matteo; Schrade, Matthias & Vullum, Per Erik [Show all 13 contributors for this article] (2021). Direct Observation of Charge Transfer between NOx and Monolayer MoS2 by Operando Scanning Photoelectron Microscopy. ACS Applied Nano Materials. 4(4), p. 3319–3324. doi: 10.1021/acsanm.1c00137. Full text in Research Archive Show summary

  Atomically thin transition-metal dichalcogenides (MoS2, WSe2, etc.) have long been touted as promising materials for gas detection because of their tunable band gaps; however, the sensing mechanism, based on a charge-transfer process, has not been fully explored. Here, we directly observe the effect of this charge transfer on the doping levels in MoS2 upon exposure to NOx by performing scanning photoelectron microscopy (SPEM) on a monolayer MoS2 transistor under bias conditions in a gas environment. By a comparison of the operando SPEM maps of the transistor with and without exposure to NOx gas, a downward shift in the Fermi level position could be detected, consistent with NOx gas making the MoS2 channel less n-type.

• 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.

• Schrade, Matthias; Berland, Kristian; Kosinskiy, Andrey; Heremans, Joseph P. & Finstad, Terje (2020). Shallow Impurity Band in ZrNiSn. Journal of Applied Physics. ISSN 0021-8979. 127(4). doi: 10.1063/1.5112820. Full text in Research Archive Show summary

  ABSTRACT ZrNiSn and related half Heusler compounds are candidate materials for efficient thermoelectric energy conversion with a reported thermoelectric figure-of-merit of n-type ZrNiSn exceeding unity. Progress on p-type materials has been more limited, which has been attributed to the presence of an impurity band, possibly related to Ni interstitials in a nominally vacant 4d position. The specific energetic position of this band, however, has not been resolved. Here, we report the results of a concerted theory-experiment investigation for a nominally undoped ZrNiSn, based on the electrical resistivity, the Hall coefficient, the Seebeck coefficient, and the Nernst coefficient, measured in a temperature range from 80 to 420 K. The results are analyzed with a semianalytical model combining a density functional theory (DFT) description for ideal ZrNiSn, with a simple analytical correction for the impurity band. The model provides a good quantitative agreement with experiment, describing all salient features in the full temperature span for the Hall, conductivity, and Seebeck measurements, while also reproducing key trends in the Nernst results. This comparison pinpoints the impurity band edge to 40 meV below the conduction band edge, which agrees well with a separate DFT study of a supercell containing Ni interstitials. Moreover, we corroborate our result with a separate study of the ZrNiSn0.9Pb0.1 sample showing similar agreement with an impurity band edge shifted to 32 meV below the conduction band.

• Gunnæs, Anette Eleonora; Tofan, Raluca; Berland, Kristian; Gorantla, Sandeep Madhukar; Storaas, Thomas Aarflot & Desissa, Temesgen Debelo [Show all 12 contributors for this article] (2020). Chemical stability of Ca3Co4−xO9+δ/CaMnO3−δ p–n junction for oxide-based thermoelectric generators. RSC Advances. 10(9), p. 5026–5031. doi: 10.1039/c9ra07159h. Full text in Research Archive Show summary

  An all-oxide thermoelectric generator for high-temperature operation depends on a low electrical resistance of the direct p–n junction. Ca3Co4−xO9+δ and CaMnO3−δ exhibit p-type and n-type electronic conductivity, respectively, and the interface between these compounds is the material system investigated here. The effect of heat treatment (at 900 °C for 10 h in air) on the phase and element distribution within this p–n junction was characterized using advanced transmission electron microscopy combined with X-ray diffraction. The heat treatment resulted in counter diffusion of Ca, Mn and Co cations across the junction, and subsequent formation of a Ca3Co1+yMn1−yO6 interlayer, in addition to precipitation of Co-oxide, and accompanying diffusion and redistribution of Ca across the junction. The Co/Mn ratio in Ca3Co1+yMn1−yO6 varies and is close to 1 (y = 0) at the Ca3Co1+yMn1−yO6–CaMnO3−δ boundary. The existence of a wide homogeneity range of 0 ≤ y ≤ 1 for Ca3Co1+yMn1−yO6 is corroborated with density functional theory (DFT) calculations showing a small negative mixing energy in the whole range.

• Bittner, Michael; Kanas, Nikola; Hinterding, Richard; Steinbach, Frank; Räthel, Jan & Schrade, Matthias [Show all 9 contributors for this article] (2019). A comprehensive study on improved power materials for high-temperature thermoelectric generators. Journal of Power Sources. ISSN 0378-7753. 410-411, p. 143–151. doi: 10.1016/j.jpowsour.2018.10.076. Full text in Research Archive Show summary

  Dense Ca3Co4O9-NaxCoO2-Bi2Ca2Co2O9 (CCO-NCO-BCCO) nanocomposites were produced from sol-gel derived Ca2.25Na0.3Bi0.35Tb0.1Co4O9 powder by four methods: Hot-pressing (HP), spark plasma sintering (SPS) and pressureless sintering in air or O2 atmosphere. Nanocomposites from HP and SPS revealed nanosized grains and showed a thermoelectric power factor of 4.8 and 6.6 μW cm−1 K−2, respectively, at 1073 K in air. A dense 2D nanocomposite with structures on multiple length scales and enhanced thermoelectric properties was obtained from pressureless sintering in O2 atmosphere. The resulting 2D nanocomposite enabled the simultaneous increase in isothermal electrical conductivity σ and Seebeck coefficient α, and showed a thermoelectric power factor of 8.2 μW cm−1 K−2 at 1073 K in air. The impact of materials with enhanced electrical conductivity and power factor on the electrical power output of thermoelectric generators was verified in prototypes. A high electrical power output and power density of 22.7mW and 113.5mW cm−2, respectively, were obtained, when a hot-side temperature of 1073 K and a temperature difference of 251 K were applied. Different p- and n-type materials were used to verify the effect of the thermoelectric figure-of-merit zT and power factor on the performance of thermoelectric generators.

• Guzik, Matylda N.; Schrade, Matthias; Tofan, Raluca; Carvalho, Patricia Almeida; Berland, Kristian & Sørby, Magnus Helgerud [Show all 9 contributors for this article] (2019). Long- and short-range structures of Ti1-xHfxNi1.0/1.1Sn half-Heusler compounds and their electric transport properties. CrysteEngComm. 21, p. 3330–3342. doi: 10.1039/c9ce00046a. Full text in Research Archive
• Schrade, Matthias & Finstad, Terje (2018). Using the Callaway Model to Deduce Relevant Phonon Scattering Processes: The Importance of Phonon Dispersion. Physica status solidi (b). ISSN 0370-1972. 255(12), p. 1–6. doi: 10.1002/pssb.201800208. Full text in Research Archive
• Desissa, Temesgen D.; Schrade, Matthias & Norby, Truls Eivind (2018). Electrical Properties of a p–n Heterojunction of Li-Doped NiO and Al-Doped ZnO for Thermoelectrics. Journal of Electronic Materials. ISSN 0361-5235. 47(9), p. 5296–5301. doi: 10.1007/s11664-018-6394-3. Full text in Research Archive Show summary

  The electrical properties of a p–n heterojunction of polycrystalline p-type Ni0.98Li0.02O and n-type Zn0.98Al0.02O have been investigated for potential applications in high-temperature oxide-based thermoelectric generators without metallic interconnects. Current–voltage characteristics of the junction were measured in a two-electrode setup in ambient air at 500–1000°C. The resistance and rectification of the junction decreased with increasing temperature. A non-ideal Shockley diode model was used to fit the measured current–voltage data in order to extract characteristic parameters of the junction, such as area specific series resistance Rs and parallel shunt resistance Rp, non-ideality factor, and the saturation current density. Rs and Rp decreased exponentially with temperature, with activation energies of 0.4 ± 0.1 eV and 1.1 ± 0.2 eV, respectively. The interface resistance of the direct p–n junction studied here is as such too high for practical applications in thermoelectrics. However, it is demonstrated that it can be reduced by an order of magnitude by using a composite of the individual materials at the interface, yielding a large effective contact area.

• Vøllestad, Einar; Schrade, Matthias; Segalini, Julie; Strandbakke, Ragnar & Norby, Truls Eivind (2017). Relating defect chemistry and electronic transport in the double perovskite Ba1-xGd0.8La0.2+xCo2O6-d (BGLC). Journal of Materials Chemistry A. ISSN 2050-7488. 5(30), p. 15743–15751. doi: 10.1039/c7ta02659e. Full text in Research Archive Show summary

  Rare earth double perovskites comprise a class of functional oxides with interesting physiochemical properties both for low- and high-temperature applications. However, little can be found relating electrical properties with equilibrium thermodynamics of non-stoichiometry and defects. In the present work, a comprehensive and generally applicable defect chemical model is developed to form the link between the defect chemistry and electronic structure of partially substituted BGLC (Ba1−xGd0.8La0.2+xCo2O6−δ, 0 ≤ x ≤ 0.5). The equilibrium oxygen content of 4 different compositions is determined as a function of pO2 and temperature by thermogravimetric analysis, and combined with defect chemical modelling to obtain defect concentrations and thermodynamic parameters. Oxidation enthalpies determined by TG-DSC become increasingly exothermic (−50 to −120 kJ mol−1) with increased temperature and oxygen non-stoichiometry for all compositions, in excellent agreement with the thermodynamic parameters obtained from the defect chemical model. All compositions display high electrical conductivities (500 to 1000 S cm−1) with shallow pO2-dependencies and small and positive Seebeck coefficients (3 to 15 μV K−1), indicating high degree of degeneracy of the electronic charge carriers. The complex electrical properties of BGLC at elevated temperatures is rationalized by a two-band conduction model where highly mobile p-type charge carriers are transported within the valence band, whereas less mobile “n-type” charge carriers are located in narrow Co 3d band.

• Song, Xin; Schrade, Matthias; Carcases, Nahum Maso & Finstad, Terje (2017). Zn vacancy formation, Zn evaporation and decomposition of ZnSb at elevated temperatures: Influence on the microstructure and the electrical properties. Journal of Alloys and Compounds. ISSN 0925-8388. 710, p. 762–770. doi: 10.1016/j.jallcom.2017.03.339. Full text in Research Archive Show summary

  The influence of annealing on the microstructure and electrical properties of undoped polycrystalline ZnSb samples has been investigated by different experimental techniques. In situ XRD in an argon at- mosphere showed that ZnSb powders decompose at 300 C, which is attributed to Zn evaporation. In situ SEM in a moist atmosphere showed fast surface deterioration at 450 C and above, reflecting decom- position of ZnSb and the formation of metallic Sb precipitates. The rate of Zn loss in a reducing atmo- sphere was determined by thermogravimetry and related to the Zn partial vapor pressure. The increase of the hole carrier concentration of ZnSb measured at room temperature after heat treatment was correlated with Zn evaporation at elevated temperature. The carrier concentration after annealing at 400 C is consistent with an activation energy for Zn vacancy formation of 0.4 eV and a maximum Zn deficiency x of Zn1-xSb of 1 10 3.

• Cerretti, G; Schrade, Matthias; Song, Xin; Balke, B; Lu, H & Weidner, T [Show all 10 contributors for this article] (2017). Thermal stability and enhanced thermoelectric properties of the tetragonal tungsten bronzes Nb8-xW9+xO47 (0 < x < 5). Journal of Materials Chemistry A. ISSN 2050-7488. 5(20), p. 9768–9774. doi: 10.1039/c7ta01121k. Full text in Research Archive
• Schrade, Matthias; Carcases, Nahum Maso; Perejón, Antonio; Pérez-Maqueda, Luis A. & West, Anthony R. (2017). Defect chemistry and electrical properties of BiFeO3. Journal of Materials Chemistry C. ISSN 2050-7526. 5(38), p. 10077–10086. doi: 10.1039/c7tc03345a. Full text in Research Archive
• Schrade, Matthias; Berland, Kristian; Eliassen, Simen Nut Hansen; Guzik, Matylda Natalia; Echevarria-Bonet, Cristina & Sørby, Magnus Helgerud [Show all 13 contributors for this article] (2017). The role of grain boundary scattering in reducing the thermal conductivity of polycrystalline XNiSn (X=Hf, Zr, Ti) half-Heusler alloys. Scientific Reports. 7. doi: 10.1038/s41598-017-14013-8. Full text in Research Archive Show summary

  Thermoelectric application of half-Heusler compounds suffers from their fairly high thermal conductivities. Insight into how effective various scattering mechanisms are in reducing the thermal conductivity of fabricated XNiSn compounds (X = Hf, Zr, Ti, and mixtures thereof) is therefore crucial. Here, we show that such insight can be obtained through a concerted theory-experiment comparison of how the lattice thermal conductivity κLat(T) depends on temperature and crystallite size. Comparing theory and experiment for a range of Hf0.5Zr0.5NiSn and ZrNiSn samples reported in the literature and in the present paper revealed that grain boundary scattering plays the most important role in bringing down κLat, in particular so for unmixed compounds. Our concerted analysis approach was corroborated by a good qualitative agreement between the measured and calculated κLat of polycrystalline samples, where the experimental average crystallite size was used as an input parameter for the calculations. The calculations were based on the Boltzmann transport equation and ab initio density functional theory. Our analysis explains the significant variation of reported κLat of nominally identical XNiSn samples, and is expected to provide valuable insights into the dominant scattering mechanisms even for other materials.

• Schrade, Matthias; Magraso, Anna; Galeckas, Augustinas; Finstad, Terje & Norby, Truls (2016). The Band Gap of BaPrO3 Studied by Optical and Electrical Methods. Journal of The American Ceramic Society. ISSN 0002-7820. 99(2), p. 492–498. doi: 10.1111/jace.13961. Full text in Research Archive Show summary

  We report on measurements of the electrical and optical properties of BaPrO3. The temperature dependences of the electrical conductivity σ and the Seebeck coefficient α of polycrystalline samples were studied over a wide temperature range (300°C–1050°C). At lower temperatures, the observed charge transport can be described as thermally activated hopping of electron‐based small polarons with an activation energy of 0.37 eV. An observed change in temperature dependence of both σ and α around 700°C was observed and interpreted as a transition from extrinsic to intrinsic carrier transport. The intrinsic conduction can be modeled with an apparent electrical band gap of ~2 eV. Optical absorption and emission spectroscopy in the UV–VIS–NIR range revealed a series of characteristic absorption thresholds and the type of optical transitions was identified by combining transmittance and diffuse‐reflectance spectroscopy methods. An absorption edge of indirect type with onset at 0.6 eV is attributed to small polaron effects. The higher lying absorption thresholds of direct origin positioned at around 1.8 and 3.8 eV are correlated with thermal activation parameters from electrical measurements and discussed in terms of the band gap of BaPrO3.

• Schrade, Matthias; Norby, Truls & Finstad, Terje (2015). Hall effect measurements on thermoelectric Ca3Co4O9: On how to determine the charge carrier concentration in strongly correlated misfit cobaltites. Journal of Applied Physics. ISSN 0021-8979. 117(20). doi: 10.1063/1.4921861. Full text in Research Archive Show summary

  The Hall coefficient RH and electrical conductivity of misfit calcium cobalt oxide (Ca2CoO3−δ)q(CoO2) (CCO) were measured at room temperature for different oxygen vacancy concentrations δ. Based on these and numerous previous results, it is shown that the charge carrier concentrations n obtained by the classical formula RH = 1/ne are between 3 and 6 × 1020 cm−3 and thereby much lower than those derived by other experimental techniques and fail to explain the observed electric properties of CCO. We show that the experimental results are well described using an earlier proposed t–J-model for strongly correlated electrons on a triangular lattice. The hopping parameter t for CCO was found to be ≈ −20 K and the charge carrier concentration of fully oxidized CCO to be 5.7 × 1021 cm−3 (0.41 hole type carriers per formula unit), in agreement with other experimental techniques. This research was originally published in the Journal of Applied Physics. © AIP Publishing

• Heinrich, Christophe P.; Schrade, Matthias; Cerretti, Giacomo; Lieberwirth, Ingo; Leidich, Patrick & Schmitz, Andreas [Show all 11 contributors for this article] (2015). Tetragonal tungsten bronzes Nb8-xW9+xO47-d: optimization strategies and transport properties of a new n-type thermoelectric oxide. Materials Horizons. ISSN 2051-6347. 2(5), p. 519–427. doi: 10.1039/c5mh00033e. Full text in Research Archive Show summary

  Engineering of nanoscaled structures may help controlling the electrical and thermal transport in solids, in particular for thermoelectric applications that require the combination of low thermal conductivity and low electrical resistivity. The tetragonal tungsten bronzes Nb8−xW9+xO47 (TTB) allow a continuous variation of the charge carrier concentration while fulfilling at the same time the concept of a “phonon-glass electron-crystal” through a layered nanostructure defined by intrinsic crystallographic shear planes. The thermoelectric properties of the tetragonal tungsten bronzes Nb8−xW9+xO47−δ (0 < x < 2) were studied in the temperature range from 373 to 973 K. Structural defects and the thermal stability under various oxygen partial pressure pO2 were investigated by means of thermogravimetry, HR-TEM, and XRD. Nb8W9O47−δ was found stable at 973 K and a pO2 of ≈10−15 atm. The oxygen nonstoichiometry δ can reach up to 0.3, depending on the applied atmosphere. By increasing the substitution level x, the electrical resistivity ρ and the Seebeck coefficient S decreased. For x = 2, ρ reached 20 mΩ cm at 973 K, combined with a Seebeck coefficient of approximately −120 μV K−1. The thermal conductivity was low for all samples, ranging from 1.6 to 2.0 W K−1 m−1, attributed to the complex crystal structure. The best thermoelectric figure of merit zT of the investigated samples was 0.043, obtained for x = 2 at 973 K, but it is expected to increase significantly upon a further increase of x. The control of the oxygen non-stoichiometry δ opens a second independent optimization strategy for tetragonal tungsten bronzes.

• Schrade, Matthias; Kabir, Rezaul; Li, Sean; Norby, Truls & Finstad, Terje (2014). High temperature transport properties of thermoelectric CaMnO3-d - Indication of strongly interacting small polarons. Journal of Applied Physics. ISSN 0021-8979. 115. doi: 10.1063/1.4868321. Full text in Research Archive Show summary

  The conductivity and Seebeck coefficient of CaMnO3−δ have been studied at temperatures up to 1000 °C and in atmospheres with controlled oxygen partial pressure. Both transport coefficients were varied in situ by the reversible formation of oxygen vacancies up to δ = 0.15. The charge carrier concentration was calculated using a defect chemical model. The Seebeck coefficient could be approximated by Heikes' formula, while the conductivity shows a maximum at a molar charge carrier concentration of 0.25. These results were interpreted as a signature of strong electronic correlation effects, and it was concluded that charge transport in CaMnO3−δ occurs via strongly interacting small polarons. General prospects for strongly correlated materials as potential candidates for high temperature thermoelectric power generation were discussed. This research was originally published in the Journal of Applied Physics. © AIP Publishing

• Schrade, Matthias; Casolo, Simone; Graham, Paul J.; Ulrich, Clemens; Li, Sean & Løvvik, Ole Martin [Show all 8 contributors for this article] (2014). Oxygen nonstoichiometry in (Ca2CoO3)0.62(CoO2): a combined experimental and computational study. Journal of Physical Chemistry C. ISSN 1932-7447. 118(33), p. 18899–18907. doi: 10.1021/jp5048437. Full text in Research Archive Show summary

  The oxygen nonstoichiometry in the misfit calcium cobaltite (Ca2CoO3)0.62(CoO2) has been studied experimentally and by density functional theory (DFT) calculations. The standard oxidation enthalpy ΔH0Ox of oxygen deficient (Ca2CoO3)0.62(CoO2) was measured directly using simultaneous thermogravimetry and differential scanning calorimetry. ΔH0Ox was found to be in agreement with the prediction from a previously published defect chemical model based on purely thermogravimetrical analysis. A series of samples with different oxygen vacancy concentration was prepared by annealing in air, followed by rapid quenching. Room-temperature Raman spectroscopy showed a sharp mode at 700 cm–1 decreasing in intensity with increasing vacancy concentration. We discuss this observation as evidence for oxygen vacancies being preferably formed within the central layer of the Ca2CoO3 subsystem. DFT calculations demonstrated that the calculated electronic structure is sensitive to the chosen model of the crystal structure. Still, for all investigated models, the standard formation enthalpy of oxygen vacancies within the Ca2CoO3 moiety was much lower than that for a site within the CoO2 layer, in agreement with the presented experimental data.

• Sagarna, Leyre; Populoh, Sascha; Shkabko, Andrey; Eilertsen, James; Maegli, Alexandra E. & Hauert, Roland [Show all 9 contributors for this article] (2014). Influence of the Oxygen Content on the Electronic Transport Properties of SrxEu1–xTiO3-δ. Journal of Physical Chemistry C. ISSN 1932-7447. 118(15), p. 7821–7831. doi: 10.1021/jp500480c. Full text in Research Archive
• Schrade, Matthias; Fjeld, Harald; Finstad, Terje & Norby, Truls (2014). Electronic Transport Properties of [Ca2CoO3-δ]q[CoO2]. Journal of Physical Chemistry C. ISSN 1932-7447. 118(6), p. 2908–2918. doi: 10.1021/jp409581n. Full text in Research Archive
• Schrade, Matthias; Fjeld, Harald; Norby, Truls & Finstad, Terje (2014). Versatile apparatus for thermoelectric characterization of oxides at high temperatures. Review of Scientific Instruments. ISSN 0034-6748. 85(10). doi: 10.1063/1.4897489. Full text in Research Archive

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• Yang, Juan; Lædre, Sigrid; Knudsen, Ole Øystein; Schrade, Matthias & Simon, Christian Rone (2022). Environmentally friendly POSS based sol gel corrosion protective coatings on steel substrates. 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
• Carvalho, Patricia Almeida; Rahman, Jamil Ur; Soltani, Nayereh; Schrade, Matthias; Stange, Marit Synnøve Sæverud & Gunnæs, Anette Eleonora [Show all 8 contributors for this article] (2021). Functional high entropy antimonides and silicides. Full text in Research Archive
• Carvalho, Patricia Almeida; Mayandi, Jeyanthinath; Rahman, Jamil Ur; Soltani, Nayereh; Schrade, Matthias & Gunnæs, Anette Eleonora [Show all 9 contributors for this article] (2021). Exploring the structure of high entropy oxides, silicides and antimonides for functional applications. 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
• Rahman, Jamil Ur; Carvalho, Patricia Almeida; Soltani, Nayereh; Schrade, Matthias; Karlson, Ole Bjørn & Gunnæs, Anette Eleonora [Show all 7 contributors for this article] (2021). A study of new Sb-based high entropy alloys synthesized by non-conventional methods. Full text in Research Archive
• 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; 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
• 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; 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
• 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.

• Berland, Kristian; Eliassen, Simen Nut Hansen; Løvvik, Ole Martin; Persson, Clas & Schrade, Matthias (2018). Computational Engineering of Thermoelectric Materials. Full text in Research Archive
• Guzik, Matylda N.; Schrade, Matthias; Tofan, Raluca; Berland, Kristian; Gunnæs, Anette Eleonora & Persson, Clas [Show all 8 contributors for this article] (2017). Structural Investigation of Ti1-xHfxNiySn Half-Heusler Compounds. Full text in Research Archive
• Schrade, Matthias (2017). Nye materialer lager strøm av temperaturforskjeller. Apollon : Forskningsmagasin for Universitetet i Oslo. ISSN 0803-6926. 27(1), p. 26–29. 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
• Tofan, Raluca; Echevarria-Bonet, Cristina; Berland, Kristian; Schrade, Matthias; Sørby, Magnus Helgerud & Hauback, Bjørn Christian [Show all 10 contributors for this article] (2016). The effect of spark plasma sintering on structure and phase stability in half-Heusler thermoelectric alloys. Full text in Research Archive
• Tofan, Raluca; Echevarria-Bonet, Cristina; Berland, Kristian; Schrade, Matthias; Sørby, Magnus Helgerud & Hauback, Bjørn Christian [Show all 10 contributors for this article] (2016). Microstructural characterization of spark plasma sintered (X,X’)NiSn half‐Heusler alloys. Full text in Research Archive
• Schrade, Matthias (2016). Å hente strøm fra varme overflater kan bli like vanlig som å hente strøm fra sollys. [Internet]. Teknisk Ukeblad. Full text in Research Archive
• Schrade, Matthias (2016). 900 deltok på festival. [TV]. NRK. Full text in Research Archive
• Schrade, Matthias; Echevarria-Bonet, Cristina; Eliassen, Simen Nut Hansen; Berland, Kristian; Persson, Clas & Tofan, Raluca [Show all 9 contributors for this article] (2016). Thermal Properties of XNiSn(X = Hf, Zr, Ti) half Heusler Alloys. Full text in Research Archive
• Schrade, Matthias; Echevarria-Bonet, Cristina; Guzik, Matylda Natalia; Tofan, Raluca; Gunnæs, Anette Eleonora & Eliassen, Simen Nut Hansen [Show all 10 contributors for this article] (2016). Thermal properties of XNiSN (X = Ti, Zr, Hf) half Heusler alloys. Full text in Research Archive
• Riis, Henrik; Schrade, Matthias & Song, Xin (2016). Termoelektrisk strøm av varme overflater. Full text in Research Archive
• Løvvik, Ole Martin; Eliassen, Simen Nut Hans; Berland, Kristian; Song, Xin; Schrade, Matthias & Shulumba, N. [Show all 9 contributors for this article] (2016). Predicting the thermoelectric figure of merit from first principles. Full text in Research Archive
• Desissa, Temesgen D.; Schrade, Matthias & Norby, Truls (2015). Interface properties of p-Ca3Co4O9 and n-Sr0.9 La0.1 TiO3 junction of oxide thermoelectric materials for oxide thermoelectric modules. Full text in Research Archive Show summary

  Heat to electricity conversion is the technique that is used for waste heat recovery. Thermoelectric generators are potential candidates for this conversion of waste heat in to useful electrical power. Among different classes of thermoelectric materials, oxide thermoelectric materials are gaining much attention and emerging as a promising materials to be utilized for construction of oxide thermoelectric generators (OTEGs). The reason behind is that these materials are stable at high temperature, non-toxic and additionally these oxides are environmentally friendly. OTEGs can be constructed from n and p-type oxide thermoelectric materials and assembling them into a module. The two oxide legs can be brought to direct contact to each other to prevent the usage of metallic interconnects. This will reduce the cost associated to some of the precious metals in the metallic interconnect and some of these metals are toxic and volatile at high temperature. Additionally, these metallic interconnects contribute to the Ohmic loss at their contact between the two n and p-oxide legs. In present work, we are investigating the properties of inter-diffusion of species between the couples and the co-existence of these species at different atmosphere and temperature. Ca3Co4O9 and Sr0.9 La0.1 TiO3 powders were used as p-type and n-type oxide thermoelectric materials, respectively. The powders were mixed using ball milling for several hours in a proper stoichiometric amounts. Then, the powders were filtered and dried in oven. The dried powder was pressed in to a pellet form and subjected to sintering (heating at high temperature) for several hours. Then the sintered sample was characterized. The characterization techniques that could be employed are SEM/EDS, TEM/EDS, EPMA, and XRD analysis. Then, electrical and thermoelectric properties will be measured on the sample.

• Sahoo, Prangya Parimita; Schrade, Matthias; Grandcolas, Mathieu; Polfus, Jonathan M.; Foss, Carl Erik Lie & Sunde, Svein [Show all 7 contributors for this article] (2015). Photoelectrocatalytic conversion of water, carbon dioxide, and nitrogen to fuels and chemicals. Full text in Research Archive
• Echevarria-Bonet, Cristina; Tofan, Raluca; Schrade, Matthias; Gunnæs, Anette Eleonora; Persson, Clas & Sørby, Magnus Helgerud [Show all 7 contributors for this article] (2015). Enhancement of the thermoelectric properties in nanostructured half-Heusler alloys. Full text in Research Archive
• Heinrich, Christophe P.; Schrade, Matthias; Cerretti, Giacomo; Malerød-Fjeld, Harald; Finstad, Terje & Norby, Truls [Show all 7 contributors for this article] (2015). Tetragonal Tungsten Bronzes Nb8−xW9+xO47−δ: Optimization Strategies and Transport Properties of a New Group of n-Type Thermoelectric Oxides. Full text in Research Archive
• Løvvik, Ole Martin; Song, Xin; Valset, Kjetil; Flage-Larsen, Espen; Fjeld, Harald & Graff, Joachim Seland [Show all 15 contributors for this article] (2014). Thermoelectric materials from first principles to final applications: Basic and applied thermoelectrics in Oslo. Full text in Research Archive
• Song, Xin; Persson, Clas; Norby, Truls; Flage-Larsen, Espen; Karlsen, Ole Bjørn & Hansen, Vidar [Show all 16 contributors for this article] (2014). Conversion of Heat and Energy: Thermoelectrics. Full text in Research Archive
• Norby, Truls; Fjeld, Harald; Schrade, Matthias; Finstad, Terje; Løvvik, Ole Martin & Casolo, Simone (2013). On synthesis, phase purity, and oxygen stoichiometry of sodium and calcium cobaltate thermoelectrics. Full text in Research Archive Show summary

  Relatively high concentrations of oxygen vacancies and effects on thermoelectric properties have been reported in the literature on state-of-the-art thermoelectric sodium cobaltates. However, first principle calculations suggest high energies and low concentrations of such vacancies. Experiments in our laboratory indicate that apparent oxygen non-stoichiometry may have stemmed from secondary phases present after decomposition in two-phase regions during synthesis and measurements. In a similar manner, experimental evaluation and defect-chemical interpretation of non-stoichiometry in calcium cobaltate thermoelectrics are troubled by non-trivial synthesis and sample fabrication. Recent results on these matters are presented and discussed.

• Fjeld, Harald; Schrade, Matthias; Norby, Truls & Finstad, Terje (2013). Defects and transport in thermoelectric calcium cobalt oxide. Full text in Research Archive
• Løvvik, Ole Martin; Casolo, Simone; Fjeld, Harald; Schrade, Matthias & Norby, Truls (2013). Fundamental properties of oxygen deficient calcium cobalt oxide. Full text in Research Archive
• Schrade, Matthias; Kabir, Rezaul; Li, Sean; Norby, Truls & Finstad, Terje (2013). High-temperature thermoelectric properties of CaMnO3-x. Full text in Research Archive
• Schrade, Matthias; Kabir, Rezaul; Li, Sean; Norby, Truls & Finstad, Terje (2013). Effect of oxygen content on the thermoelectric properties of CaMnO3-x. Full text in Research Archive
• Schrade, Matthias; Fjeld, Harald; Finstad, Terje & Norby, Truls (2013). High temperature electronic transport in (Ca2CoO3-x)q(CoO2). Full text in Research Archive
• Schrade, Matthias; Fjeld, Harald; Norby, Truls & Finstad, Terje (2012). Influence of oxygen non-stoichiometry on thermoelectric properties of Ca3Co4O9+x. Full text in Research Archive
• Fjeld, Harald; Schrade, Matthias & Norby, Truls (2012). The role of phase relations and Ca to Co ratio in thermoelectric properties of Ca3Co4O9+d. Full text in Research Archive
• Schrade, Matthias; Fjeld, Harald & Norby, Truls (2012). Simultaneous measurement of Seebeck coefficient and conductivity at high temperatures and in controlled atmospheres. Full text in Research Archive
• Røsvik, Mari Mathillas; Gunnæs, Anette Eleonora; Carvalho, Patricia Almeida & Schrade, Matthias (2021). Structural Characterization of High Entropy Metal Silicide Alloys An investigation of the Si:Cr:Fe:Co:Ni:Cu system. Universitetet i Oslo. Full text in Research Archive
• Schrade, Matthias (2014). On the Oxygen Nonstoichiometry in Thermoelectric Oxides. Universitetet i Oslo. Full text in Research Archive Show summary

  The interest in thermoelectric oxides as candidate materials for high temperature waste heat recovery has generated vital scientific activity during the last years. However, while it is well established in several other fields of materials science that the electronic structure of oxides at high temperatures can be significantly modified by the formation of oxygen vacancies, the precise control of the sample and its surrounding atmosphere is rather seldomly seen in scientific publications on high temperature thermoelectric oxides. Therefore, during this thesis, I have investigated the influence of the oxygen content on the properties of two of the most prominent thermoelectric oxides. A variation of the oxygen content of a material at high temperatures can be achieved by a variation of the surrounding atmosphere and the subsequent in- and out-diffusion of oxygen ions until the new thermodynamic equilibrium state is reached. Oxygen vacancies can usually be described as effectively charged point defects and the precise control of their concentration provides a means to change the charge carrier concentration of a material in situ. Therefore, one goal of this thesis is to establish that the high temperature thermoelectric characterisation of oxides should preferably be done under controlled atmospheric conditions. In fact, part of the scatter observed in published results on nominally identical samples can be explained by (unintentionally) different oxygen content, due to different measurement atmospheres or sample kinetics. Moreover, the results from this thesis aim to contribute to the fundamental understanding of the charge transport processes in the studied and related materials. The thesis comprises the design and characterisation of an appropriate system to measure the electrical transport properties of the materials under investigation (Manuscript 3). Manuscript 1 and 4 study one of the most prominent thermoelectric oxides: Misfit calcium cobalt oxide (Ca₂CoO_3-δ)_q(CoO₂) (CCO), which shows one of the best reproducible p-type thermoelectric performances among all oxides. In Manuscript 1, we established a defect chemical model of this material. Due to its misfit structure, it is inherently mixed-valent, so that a modified defect notation was chosen. The dependency of both electrical conductivity and Seebeck coefficient on the oxygen content was measured in a wide range of temperature and oxygen partial pressure. It was concluded that – at high temperatures – charge carriers should be described as itinerant in this material. We further showed that the often used Heikes formula cannot be used for a quantitative analysis of the Seebeck coeffcient in CCO. Instead, we suggested a modified Mott formula with a significant contribution from the energy dependent mobility to describe the Seebeck coefficient in CCO. In Manuscript 4, a combined experimental and theoretical study of the oxygen nonstoichiometry in CCO is presented. Based on DFT-calculations and experimental Raman-spectroscopy, it is shown that oxygen is preferentially removed from an atomic position within the central layer of the Ca₂CoO₃-subsystem. The computational results further indicated that the electronic properties are sensitive to small variations in the crystal structure. The thermodynamics of oxidation were investigated by three different techniques (TG, TG-DSC, and DFT) and differences were discussed. In Manuscript 2, the high temperature charge transport in CaMnO_3-δ (CMO) was investigated. CMO is – when doped with small amounts of niobium – among the most promising n-type oxides with a figure of merit reaching 0.3 at high temperatures. When forming oxygen vacancies – thereby increasing the electron concentration of the material – we observed an unusual simultaneous decrease of both conductivity and the absolute value of the Seebeck coefficient. These findings were analysed as an indication of strongly interacting small polarons as the charge carrier in this material. We generalised this result to develop a simple model for the powerfactor and concluded that mutual Coulomb repulsion limits the thermoelectric performance of these materials.

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