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Manufacturing processes for TiOx-based thermoelectric modules: From suboxide synthesis to module testing

: Conze, S.; Poenicke, A.; Martin, H.-P.; Rost, A.; Kinski, I.; Schilm, J.; Michaelis, A.


Journal of Electronic Materials 43 (2014), Nr.10, S.3765-3771
ISSN: 0361-5235
ISSN: 1543-186X
Fraunhofer IKTS ()

An overview of different TiOx synthesis methods with regard to enhancement of thermoelectric properties and transfer of the synthesis process to cost-efficient methods as well as joining techniques for module manufacture is presented. Different synthesis routes were applied and investigated, namely synthesis of TiOx via reduction with less gas formation by mixing TiO2 and TiC [powder-derived (PD)-TiOx], a bottom-up approach via a precursor route for synthesizing TiOx directly [precursor-derived (PDC)-TiOx], and the combination by mixing TiO2 with precursor (PDC-TiOx/TiO2). All the approaches resulted in adjustable phase composition with different oxygen contents and, therefore, adjustable electrical properties as well as different microstructures to enhance the physical and thermoelectric properties. The electrical resistivity could be adjusted from 1 m cm to 1000 m cm through the oxygen content of TiOx. The research included investigations of cost-efficient production processes for thermoelectric material such as spray-drying, spark plasma sintering, hot pressing or pressureless sintering in terms of shaping, sintering, and machining, as well as joining techniques to build a complete thermoelectric module. To realize thermal and electrical connections, technologies for joining and packaging were developed. For a first demonstration of the feasibility of TiOx-based thermoelectric modules for use at high temperatures, a unileg n-type module with footprint of 30 mm × 30 mm was designed. Low-volume fabrication yielded more than 20 single modules. Finally, the modules were successfully tested under conditions close to those of the desired applications with hot-side temperature up to 600°C.