Holistic quantum design of amorphous thermoelectric NbO2 alloyed with transition metals

即使热电设备中没有可移动的机械部件，但在加热和制冷循环下机械性能不佳往往会导致故障。此外，热电效率的提高有望在非晶态固体中实现。申请人证明，非晶态NbO2薄膜的绝对塞贝克系数比相应的晶态NbO2薄膜大。然而，合金化对非晶态NbO2的输运和力学性能的影响尚不清楚。将整体量子设计应用于非晶态NbO_2，不仅探讨了非晶态NbO_2的主要输运性质，而且还考虑了其力学性质，从而得到了高效的热电器件。采用基于密度泛函理论的分子动力学方法研究了合金化对非晶态NbO_2的塞贝克系数和弹性性质的影响。所有3d和5d过渡金属都将被探测到。根据增加的塞贝克系数和柯西压力(一种衡量脆韧性交叉的指标)，将从这两个过渡金属系列中各选择一种元素。这两个非晶系将用反应组合溅射法合成。基于对这些溅射薄膜进行的X射线衍射、X射线光电子能谱、塞贝克系数和电阻率测量以及纳米压痕实验，量子力学预测将得到验证。由于本项目研究的是非晶态固体，它们的导热系数可能已经很低了。然而，调整这些非晶态热电设备的电导率仍然是一个挑战。因此，将使用多层方法，其中与具有增强的塞贝克系数的过渡金属合金化的非晶态NbO2将与导电RuO2交织。通过调节多层膜周期，可以获得较大的塞贝克系数、较大的电导率和较低的导热系数以及较低的热疲劳，从而提高了这些新型热电材料的性能。该项目所获得的结果对无二氧化碳排放的能源生产具有重要意义。

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.