PNNL

Abstract

Efficient conversion of low-grade heat ( 10) 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 casting and vacuum hot-pressed powders are processed by friction extrusion into meters-long wires having 2.5 mm and 1.0 mm diameters. Friction extrusion 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. This is attributed to the grain refinement, preferential texture alignment, and homogeneous composition achieved by friction extrusion. Furthermore, the unique deformation conditions during friction extrusion enables a much higher extrusion ratio (161:1) than is possible with conventional extrusion. This high ratio is a key enabler for extruding small wire diameters from cast billets.