UCLA

Since the 20th century, deep space missions flown by NASA have been critical for exploration of the solar system and have provided a wealth of fundamental scientific knowledge. Electrical power for these missions is supplied by radioisotope thermoelectric generators (RTG’s) which produce electricity from a radioactive heat source. This technology has a history of reliable power generation and has flown on missions such as Pioneer, Cassini, New Horizons, and, most notably, Voyager which has been in continuous operation for over 45 years. However, device efficiency is limited due to the poor heat-to-electricity conversion of thermoelectric materials, which is quantified by the dimensionless figure of merit, ZT. Therefore, a longstanding goal in the field is that of developing higher ZT materials to improve RTG specific power.Lanthanum telluride (La3-xTe4) has been identified as a prime candidate for next generation RTG’s owing to its ZT of 1.1 at 1275K. However, poor machinability and susceptibility to oxidation are current bottlenecks for device fabrication. The overarching goal of the work presented in this dissertation is to investigate novel processing strategies for La3-xTe4 to facilitate device fabrication and expand pathways for fundamental study of its thermoelectric performance. The first study (Chapter 2) investigates the La3-xTe4 oxidation mechanism to establish the process by which thermoelectric properties degrade and inform oxide mitigation strategies for synthesis and device fabrication. As-synthesized La3-xTe4 powder contains an amorphous surface oxide at room temperature. At elevated temperature, oxidation progresses with the formation of mobile elemental Te, Te-rich lanthanum telluride phases, and crystalline La2O2Te and glassy La2O3 as the bulk material reacts with oxygen. The oxide phases are non-passivating and thermodynamically stable, requiring multifaceted oxide mitigation strategies. The second study (Chapter 3) demonstrates synthesis of La3-xTe4 films by electrophoretic deposition (EPD) using tetrahydrofuran (THF) as solvent. Uniform La3-xTe4 films 10-15 �m thick with a green density of ~65% are deposited on both planar and non-planar substrates. The versatility and scalability of EPD expands thermoelectric device fabrication capabilities and enables study of La3-xTe4-metal composite geometries that are not attainable with current bulk processing methods. The third study (Chapter 4) utilizes ultrafast high-temperature sintering (UHS) to densify La3-xTe4 films and bulk pellets in as little as 10 seconds. Sample densities >90% can be achieved with well-defined grains on the order of 1-10 �m in size. Rapid densification is proposed to result from the extreme heating rate of UHS which maintains a high driving force for densification and quickly activates diffusion mechanisms that attend fast pore elimination and grain growth. Pressure-less sintering of La3-xTe4 via UHS is a significant achievement and demonstrates that complex pressure sintering techniques are not required for densification. The fourth study (Chapter 5) investigates the thermoelectric properties of UHS La3-xTe4. Bulk UHS La3-xTe4 pellets achieve a peak ZT of 1.06 at 1273K with comparable performance to optimized La3-xTe4 prepared by established methods. Initial power factor measurements up to 573K suggest that the performance of UHS La3-xTe4 EPD films is similar to bulk UHS La3-xTe4, but more accurate thin film measurement techniques are necessary to fully characterize their ZT. Achievement of high performance La3-xTe4 via UHS greatly expands and simplifies processing of the material without sacrificing thermoelectric performance. The results of these four studies significantly enhance processing and future device fabrication of La3-xTe4 for high-temperature thermoelectric applications. The detailed understanding of the La3-xTe4 oxidation mechanism informs oxide mitigation strategies for device design. The novel combination of EPD and UHS forms a simple, high-throughput processing scheme that expands possible device architectures while maintaining established thermoelectric performance. Finally, EPD and UHS can be applied to other thermoelectric materials, thus providing unique and adaptable processing enhancements for the broader thermoelectric community.