Fabrication of bismuth-telluride thermoelectric wires by friction extrusion

• Friction extrusion fabricates meters-long bismuth-telluride wires of 2.5 mm and 1.0 mm diameter. • Average grain size <5 µm, strong c-axis texture alignment perpendicular to extrusion direction. • Wires extruded from cast billets achieved a power factor exceeding vacuum hot-pressed powder. • Enables high-aspect ratio elements needed for harvesting thermal energy from low grade heat. Efficient conversion of low-grade heat into electric power is challenging for thermoelectric materials and systems. This is because the high-aspect ratio thermoelectric elements needed for maximum conversion efficiency are difficult and costly to manufacture. In this work, friction extrusion is explored as a new method for fabricating bulk bismuth-telluride (BiTe) wires that can be sectioned into high-aspect ratio thermoelectric elements. Bismuth-telluride feedstock materials in the form of vacuum hot-pressed powder and castings are processed by friction extrusion into meters-long wires having 2.5 mm and 1.0 mm diameters. The novel deformation process gives an average grain size below 5 µm and preferential c-axis texture alignment perpendicular to the extrusion direction. Results for Seebeck coefficient, resistivity, power factor, and weighted mobility show that transport properties exceeding that of high-performance vacuum hot-pressed powders are achieved through friction extrusion of simple castings. The unique deformation conditions at the rotating die/feedstock interface enables a much higher extrusion ratio (161:1) than is possible with conventional extrusion. This is a key enabler for extruding small wire diameters from cast billets.