Holistic quantum design of amorphous thermoelectric NbO2 alloyed with transition metals

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Even though there are no movable mechanical parts in thermoelectric devices, poor mechanical performance under heating and cooling cycles often leads to failure. In addition, increased thermoelectric efficiency is expected to be realized in amorphous solids. It was demonstrated by the applicant that amorphous NbO2 thin films exhibit larger absolute Seebeck coefficients than the corresponding crystalline counterpart. However, alloying effects on the transport and mechanical properties of amorphous NbO2 are not known. By applying the holistic quantum design to amorphous NbO2, where not only the key transport properties are probed but also the mechanical properties are considered, efficient thermoelectric devices will be obtained. Density functional theory based molecular dynamics will be used to study the effect of alloying on the Seebeck coefficient and elastic properties of amorphous NbO2. All 3d and 5d transition metals will be probed. Based on the increased Seebeck coefficient and Cauchy pressure, a measure of brittle-ductile crossover, one element from each of these two transition metal series will be selected. These two amorphous systems will be synthesized using reactive combinatorial sputtering. Based on x-ray diffraction, x-ray photoelectron spectroscopy, Seebeck coefficient and resistivity measurements as well as nanoindentation experiments performed on these sputtered thin films, the quantum mechanical predictions will be validated. As amorphous solids are studied in this project, their thermal conductivity may already be low. However, tuning the electrical conductivity of these amorphous thermoelectric devices is still a challenge. Hence, a multilayer approach will be used where amorphous NbO2 alloyed with transition metals exhibiting the enhanced Seebeck coefficient will be interleaved with conductive RuO2. By adjusting the multilayer period, large Seebeck coefficient together with large electrical and low thermal conductivity as well as low thermal fatigue will be achieved leading to enhanced performance of these novel thermoelectrics. The results obtained in this project are relevant for energy generation without CO2 emission.