Publications
Metallurgy, thermal stability, and failure mode of the commercial Bi-Te-based thermoelectric modules
Yang, Nancy Y.; Morales, Alfredo M.
Bi-Te-based thermoelectric (TE) alloys are excellent candidates for power generation modules. We are interested in reliable TE modules for long-term use at or below 200 C. It is known that the metallurgical characteristics of TE materials and of interconnect components affect the performance of TE modules. Thus, we have conducted an extensive scientific investigation of several commercial TE modules to determine whether they meet our technical requirements. Our main focus is on the metallurgy and thermal stability of (Bi,Sb){sup 2}(Te,Se){sup 3} TE compounds and of other materials used in TE modules in the temperature range between 25 C and 200 C. Our study confirms the material suite used in the construction of TE modules. The module consists of three major components: AlN cover plates; electrical interconnects; and the TE legs, P-doped (Bi{sub 8}Sb{sub 32})(Te{sub 60}) and N-doped (Bi{sub 37}Sb{sub 3})(Te{sub 56}Se{sub 4}). The interconnect assembly contains Sn (Sb {approx} 1wt%) solder, sandwiched between Cu conductor with Ni diffusion barriers on the outside. Potential failure modes of the TE modules in this temperature range were discovered and analyzed. The results show that the metallurgical characteristics of the alloys used in the P and N legs are stable up to 200 C. However, whole TE modules are thermally unstable at temperatures above 160 C, lower than the nominal melting point of the solder suggested by the manufacture. Two failure modes were observed when they were heated above 160 C: solder melting and flowing out of the interconnect assembly; and solder reacting with the TE leg, causing dimensional swelling of the TE legs. The reaction of the solder with the TE leg occurs as the lack of a nickel diffusion barrier on the side of the TE leg where the displaced solder and/or the preexisting solder beads is directly contact the TE material. This study concludes that the present TE modules are not suitable for long-term use at temperatures above 160 C due to the reactivity between the Sn-solder and the (Bi,Sb){sup 2}(Te,Se){sup 3} TE alloys. In order to deploy a reliable TE power generator for use at or below 200 C, alternate interconnect materials must be used and/or a modified module fabrication technique must be developed.