Development of High-Durability Substrates for Thermoelectric Modules

Abstract

Thermoelectric modules with a π-structure were fabricated on four types of substrates, and their high-temperature durability was evaluated. A direct bonded aluminum (DBA) substrate was fabricated by using braze material in a vacuum furnace. In addition, an alumina substrate and DBA substrate each coated with pre-sintered Ag layers were fabricated by firing in a furnace in air after screen-printing of an Ag paste containing glass frit on alumina or DBA. Thermoelectric materials were mounted on these substrates by die bonding. In thermal durability tests, the percent increase in internal resistance of modules after 100 thermal cycles (cold side: Tc = 80°C; hot side: $$T_{\rm{h}} = 150{^\circ } {\hbox{C}} \Leftrightarrow 450{^\circ } {\hbox{C}} $$ was measured in air. After the durability testing, cross sections of the modules were prepared and analyzed by electron probe microanalysis and scanning electron microscopy. The percent increase in resistance of the module with pre-sintered Ag layers on an alumina substrate was 33%, and the percent increase in resistance of the module with pre-sintered Ag layers on a DBA substrate was less than 1%. These results demonstrated that, at high temperature in air, the percent increase in internal resistance at the interface between the thermoelectric materials and the metal electrode of a thermoelectric module can be significantly decreased by using Ag as an oxidation-resistant electrode and Al as a buffer layer for stress relaxation.