High efficiency thermoelectric devices fabricated using quantum well confinement techniques.

摘要

Experimental results are presented of thermoelectric materials, specifically two-dimensional quantum well confinement structures, formed by ion beam sputter deposition methods. Applications of these thermoelectric devices include nearly any system that generates heat including waste heat. The targeted applications of this research include harvesting of waste heat from stand-alone generator systems and automobiles. Thermoelectric generator modules based on an in-plane orientation of nano-scale, thin-film, superlattices have demonstrated very high performance and are appropriate for a wide range of waste heat recovery applications. In this project, the first, fast, ion-beam-based deposition process was developed for producing Si/SiC (n-type) and B4C/B 9C (p-type) superlattices. The deposition process uses low-cost powder targets, a simplified substrate holder with embedded heater, a QCM deposition rate monitor, and stepper-motor-controlled masks. Deposition times for individual layers are shown to be significantly shorter than those achieved in magnetron-based systems. As an example of the speed of the process, a 10-nm thick Si layer can be deposited in as little as 20 sec while a SiC layer can be deposited in less than 100 sec. Electrical resistivities, thermal conductivities and Seebeck coefficients are reported for the deposited films as well as their respective non-dimensional figures of merit (zT). Figures of merit (zT) approaching 20 at modest temperatures of ~600 K were observed. These measurements are made in-plane where enhanced Seebeck values and reduced electrical resistivities have also been reported in the literature. A method for directly measuring thermal conductivity in the plane of the superlattice is described that uses MEMs-based SiN cantilevers. Results are presented for various deposition variables, including film thickness, temperature, deposition energy, and material. Scanning white light interferometry (SWLI) and scanning electron microscopy (SEM) were used to characterize film thickness. In addition to the experimental effort, an analysis was performed to predict the performance of a thermoelectric module fabricated with the superlattice films deposited on ceramic substrates. Thermal efficiencies approaching 15% are predicted for modest cold and hot side temperatures. Thermal conduction through the substrate was found to be the largest factor limiting the performance of the modeled thermoelectric modules.