Generation of Nanosized Particles during Mechanical Alloying and Their Evolution through the Hot Extrusion Process in Bismuth-Telluride-Based Alloys

Our extensive studies of extruded alloys have shown that mechanically strong polycrystalline alloys also deliver surprisingly high thermoelectric performance with ZT > 1 for p-type material in the temperature range from 25°C to 90°C, which was traditionally attributed to the strong texture generated by the extrusion process. Optical and low-resolution scanning electron microscopy observations of the powder produced by mechanical alloying show a particle size distribution in the micrometric range. However, high-resolution transmission electron microscopy (HRTEM) observations of the powders obtained after milling clearly show nanosized crystal grains. The larger microparticles appear to be agglomerations of grains whose size goes down to 5 nm to 20 nm. Examination of bulk n-type and p-type materials using x-ray diffraction (XRD) combined with HRTEM shows that nanosized subgrains can also be found in materials after hot extrusion. In this article we present experimental evidence of the generation and evolution of nanocrystalline particles through the mechanical alloying and hot extrusion processes used to produce the thermoelectric alloys. We also discuss the possible influence of nanocrystalline inclusions on the thermoelectric performance of the produced material. The optimization of the hot extrusion process, in order to maximize the influence of the nanostructures, offers new opportunities to increase the thermoelectric performance of bulk materials.