The joining of low- and high-temperature thermoelectric materials (with ZT optimized to specific temperature ranges) to each other in a segmented configuration can lead to enhanced device efficiency. The resulting joints between these materials must be both chemically and thermally stable over the operating lifetime of the device (~17 years). In addition, using joining methods and/or materials that can reduce the temperature and time at temperature for the bonding cycle can be beneficial in minimizing thermomechanical stresses associated with device fabrication.
One or more of the following alloys for joining thermoelectric segments terminated with Ti, Mo, and Ni metals was used: (a) copper-silver-Ti, (b) gold-germanium, (c) aluminum-silicon, (d) nickel-copper-tin, and (e) copper-silver-indium. Foils of any of the compositions (a)-(d) were chosen depending upon the chemical composition of the segments to be bonded. The bonding temperature and pressure were chosen in accordance with the nature of the segments to be bonded.
Attaching different segments to form a viable thermoelectric couple and/or device requires a very careful consideration of the process conditions because of the differences in properties and stabilities of the components involved. The appropriate choice of the brazing alloy and the brazing conditions is not obvious, even to one skilled in the art. Therefore, the novelty in this work covers the chemical composition of the braze used and the relevant brazing conditions.
This work is critical to further the development of high-performance thermoelectric converter technology for the NASA Advanced Thermoelectric Converter Project (ATEC).