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Improving the Mechanical Strength and Power Conversion Efficiency of High Temperature Thermoelectrics


Thermoelectrics are solid state energy conversion materials which are able to generate power through the Seebeck effect or provide cooling through the Peltier effect. Thermoelectric generators have consistently demonstrated their extraordinary reliability and longevity in support of the National Aeronautics Space Administration's (NASA) deep space science and exploration missions. The state-of-practice "heritage" TE materials exhibit only modest thermal-to-electric energy conversion performance, resulting in relatively low system-level conversion efficiencies of 6 to 6.5%.

New thermoelectric devices are sought with improved efficiency to enhance mission capabilities and reduce cost. The refractory material lanthanum telluride (La3-xTe4) is a promising material which is stable up to 1000 oC and has been shown to have an improved thermoelectric efficiency compared to legacy materials at the same temperature. However, a challenge in the translation of La3-xTe4 as a material into a functional thermoelectric device is that it is mechanically weak and brittle. A mechanically robust thermoelectric material is desirable to simplify handling during manufacturing, improve device yield, and to increase tolerance to the thermomechanical stresses encountered during operation.

The initial emphasis this work is the characterization of the mechanical properties of the La3-xTe4. The Vicker's hardness and indentation fracture toughness are employed as a rapid, nondestructive technique to evaluate the material. It provides a measure of hardness, a property strongly interlinked with other mechanical properties and important to the resistance to surface flaw formation. In addition, measurement of the cracks formed during hardness testing provides a measure of brittleness of the material in the form of indentation fracture toughness. The strength of the material is measured through flexural testing. The test is destructive and in additional to flexural strength, provides insight into the dominant failure modes of the material. Identification of the failure modes is important to developing mitigation schemes to improve the mechanical performance of the material.

In addition to mechanical performance, enhancements to the thermoelectric performance of lanthanum telluride are also explored through two means. First, the development of lanthanum telluride composites utilizes the concept of the ideal thermoelectric which would combine the favorable properties of different material types. A 50% improvement in efficiency was achieved for composite of La3-xTe4 with a percolated nickel network. The nickel lowered the electrical resistivity of the material while favorably maintaining other thermoelectric transport properties. Second, alkali earth doping with calcium was explored as a means to modify the band structure of the material to improve the Seebeck coefficient. Although calcium doping did not change performance significantly, it provided important lessons into how to modify the band structure of the material to improve the Seebeck coefficient.

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