A Novel Approach to Join Large Coefficient of Thermal Expansion (CTE) Mismatched Thermoelectric (TE) Materials for High Temperature Applications

Abstract

Radioisotope thermoelectric generators (RTGs) have been used to enable deep space exploration where solar power is scarce. Various NASA missions such as Voyager 1 and Voyager 2 have used RTGs and have traveled over 11 billion miles from Earth. Heritage RTGs have reliably provided power throughout the lifetime of the various missions but thus far have been relatively inefficient and convert less than 10% of heat to electricity. A method to achieve higher thermal to electrical conversion efficiencies is via the segmentation of advanced thermoelectric (TE) materials. High thermoelectric efficiencies can be achieved by constructing a TE device that includes combining different TE materials optimized at different temperatures and joining them together. Though advantageous to achieve high performance, segmentation can be challenging due to differences in material properties such as the coefficient of thermal expansion (CTE). Thermal stresses induced by large CTE mismatch of TE materials during the joining process can cause cracking at the bonded interface, these stresses need to be minimized in order to reduce the probability of cracking at the joint interface. Additionally, the bonds formed between materials need to be both chemically and mechanically stable and require high thermal conductivity along with low electrical contact resistance. In this study two thermoelectric materials with dissimilar CTE at room temperature were joined by using a graded-CTE bonding approach. The thermal stress states of the two thermoelectric materials were modified to avoid cracking at the bonding interface. Optical and scanning electron microscopy (SEM) was used to characterize the cross-section of the bonded thermoelectric materials. Preliminary results of a stress model shows a reduction of stress at the bonded interface and is in agreement with experimental results.