Holistic quantum design of amorphous thermoelectric NbO2 alloyed with transition metals
Final Report Abstract
Research on cutting-edge devices is congregated around key physical and chemical properties, but often correlative approaches are lacking. Design of thermoelectrics, devices converting heat directly into electricity, commonly includes the Seebeck coefficient, electrical conductivity, and thermal conductivity. Even though there are no movable mechanical parts in these devices, poor mechanical performance under heating and cooling cycles often leads to failure, but these aspects have less been considered. In this project, amorphous NbO2 was investigated aiming at enhancing its thermoelectric efficiency and enabling durable performance under substantial thermal loads. Out of more than 20 possible alloying elements, Ta was identified, based on correlative experimental and theoretical approaches, to simultaneously increase the thermoelectric efficiency and reduce the thermal fatigue. A hundredfold efficiency increment compared to pure NbO2 was achieved, exceeding many oxide thermoelectrics. Further improvements, especially in terms of lowering the electrical resistivity, were obtained for amorphous Nb-O/Ni-Ta-O multilayers. In terms of thermomechanical properties, NbO2 was put into perspective to a range of other thermoelectrics (telluride, half-Heusler, rock salt, rutile, wurtzite, skutterudite, perovskite, and type-I-clathrate phases). Crystalline oxides seem more prone to thermal stress, thermal shock, and thermal fatigue than the Te-containing thermoelectrics, but modulations are possible by electronic structure tuning, which was achieved in the case of amorphous NbO2 alloyed with Ta. Another important aspect of thermoelectric devices, often omitted in design efforts, is the mechanical stability of metallic contacts. Pt was shown to be the most promising candidate for amorphous NbO2 based on minimizing thermal stresses and enhancing the interfacial strength. Nowadays, flexible and stretchable devices become increasingly important and therefore the holistic design strategy was also employed for these purposes. A common polymeric substrate used for flexible and stretchable devices is Kapton, which is amorphous so that it was important to derive its equation of states. A new theoretical method was developed in this project, termed the rotationannealing algorithm. Alloying NbO2 with Ru enabled a full mechanical flexibility. The holistic methodology employed in this project on amorphous NbO2 was critically appraised for other stateof-the-art thermoelectrics. All goals were reached in this project. A short (popular science) video has been made to promote our scientific efforts on NbO2 based thermoelectric devices (https://youtu.be/e3OWAlnDfMo).
Publications
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High-throughput exploration of thermoelectric and mechanical properties of amorphous NbO2 with transition metal additions, J. Appl. Phys. 120 (2016) 045104
D. Music, R. W. Geyer, and M. Hans
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Thermomechanical response of thermoelectrics, Appl. Phys. Lett. 109 (2016) 223903
D. Music, R. W. Geyer, and P. Keuter
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Adsorption of film-forming species on NbO and NbO2 surfaces, J. Vac. Sci. Technol. A 35 (2017) 061512
D. Music, P. Schmidt, and S. Mráz
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Experimental and theoretical exploration of mechanical stability of Pt/NbO2 interfaces for thermoelectric applications
D. Music, P. Schmidt, and A. Saksena
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Temperature independent Seebeck coefficient through quantum confinement modulation in amorphous Nb-O/Ni-Ta-O multilayers, Solid State Commun. 258 (2107) 33
D. Music, O. Hunold, S. Coultas, and A. Roberts
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Topology and electronic structure of flexible (Nb,Ru)O2 thermoelectrics, J. Phys.: Condens. Matter 29 (2017) 085701
D. Music, V. Schnabel, and J. Bednarcik
