TiOx based thermoelectric modules - manufacturing, properties and operational behavior

The class of reduced (non-stoichiometric) titanium dioxide is a promising material for thermoelectric modules with regard to its availability, economy, and operation and alteration behavior. The thermoelectric efficiency of TiOx is minor compared to established thermoelectric materials. However TiOx is very attractive for economic reasons and there are still expectations for efficiency rise by modification of the material structure. This paper gives the latest results of manufacturing technologies and test results of TiOx modules close to real conditions of service. A manufacturing process for unileg n-type modules based on hot pressed and shaped n-type TiOxlegs was established including joining of modules with reliable electrical connections and supporting substrates. Modules with copper metallized AlN, Al2O3 and Si3N4 substrates were built to evaluate the influence of ceramic substrate material on module behavior. In contrast to the widely used direct copper bonding process in combination with Al2O3substrates in this study the active metal brazing technology for the realization of planar and structured metal to ceramic compounds was applied. A lab scale testing device was used for acceptance trials of the modules with a maximum hot side temperature of 600 °C while the cold side was kept at 100 °C. This initial high temperature test revealed similar power output and internal resistance of all fabricated modules. To investigate the reliability and durability of the modules thermal cycles were carried out on the basis of a reduced version of a thermoelectric generator unit, which was equipped with 2 x 3 modules arranged along a planar gas heat exchanger. The test rig was designed to simulate the thermal stresses during power generation in the exhaust line of a passenger car. The characterization of the module resistance and microstructural investigations of cross-sections after 1000 thermal cycles showed differences in the behavior and failure modes of the modules which can be correlated to the ceramic substrates used.