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Thermoelectric Bonding Study

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Lead telluride thermoelectric elements have been used in most thermoelectric power generation devices built and proposed for construction in recent years because of their superior figure of merit in the 100 to 600 degrees C temperature range. For the same reason lead telluride is potentially attractive for several NASA applications. However, poor long term performance continues to limit the usefulness of this otherwise attractive material. The principal causes of thermoelement failures in the material include deterioration of the element to shoe bond and degradation of thermoelectric output because of composition changes within the element. This program had, as its objective, the study of the bonding process and the determination of the mechanism or mechanisms of bond failure in lead telluride thermoelectric elements. A secondary objective was the development of a satisfactory braze and shoe system for the material. It was preferred, but not required, that the selected materials be nonmagnetic. A systematic approach was applied to the selection and screening of potential braze and shoe materials for use with lead telluride. A literature survey reviewing work in bonding lead telluride at other installations was performed. This, plus analytical evaluation of available metallurgical data, led to the selection of a number of metals and alloys for use in the program. Although all materials of potential interest could not be studied, the group selected for evaluation is considered representative. Preliminary screening was accomplished by carrying out wettability tests and accelerated poison effects tests. The first of these measured the ability of the braze materials to flow on and adhere to the surface of lead telluride and the various shoe materials. The poison effects test qualitatively studied the probable effects of long time diffusion of braze and shoe materials into lead telluride.

Subject Categories:

  • Inorganic Chemistry
  • Electricity and Magnetism

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