High Temperature Thermoelectrics

With global energy consumption approaching 50 TW/yr, new power generation strategies are needed to meet the demand. Waste heat is an abundant source that is underutilized as an energy resource. Thermoelectric (TE) technology provides direct conversion of heat to electric power by utilizing the Seebeck effect. Thermoelectrics are solid-state convertors and therefore they are extremely reliable. Due to such reliability, NASA has used them to power their deep space probes such as Cassini; operational over 14 years with no maintenance, and most currently, Curiosity. However, commercial applications of thermoelectric technology have been limited due to low conversion efficiency and cost. Over the last decade, progress in higher conversion efficiency has been achieved by implementation of nano-technology. This created a renewed interest in thermoelectrics from industry, especially the automotive industry. Electrical power generation from waste heat will help reduce fossil fuel consumption but also improve system efficiency. However, successful commercialization of thermoelectric technology will be dependent upon conversion efficiency, material cost and environmental toxicity. Currently the thermoelectrics with largest conversion efficiencies contain Tellerium, a rare and relatively toxic material.

TE materials with figure of merit (ZT) ~ 1 are adequate for waste heat recovery if the cost is low ($/W). To maximize the figure of merit a material has to have high Seebeck coefficient and electrical conductivity and low thermal conductivity. Introduction of nano-precipitates with coherent interfaces has been successful in decreasing the thermal conductivity with no adverse effect on electrical conductivity. However, the stability of these nano-precipitates at high temperatures is a current problem for power generation. Quantum well super- lattices have recently been obtained for metal oxide systems by alternating hetero-interface layers. These film structures have demonstrated, at the laboratory scale, the potential to achieve higher conversion efficiency (ZT=2.4). This increase was due to coherent interfaces that scattered phonons but had no effect on electrical charge carriers. However, the mass production of super-lattices is a big challenge to date. Our research focuses on duplication of these thin film hetero-structures for bulk materials by using self-assemble processes, e.g., spinodal decomposition, directional solidification of eutectic structures and nano-checkerboard structures. Self-assembled nano-structures are usually controlled by composition and temperature, and provide a simple process to fabricate nano-structures in bulk solids for commercialization. These thermodynamically stable nano-structures can be obtained in systems that are environmentally friendly and abundant (i.e., W-Si/Ge, SnO2/TiO2).