Microcompression tests of single-crystalline and ultrafine grain Bi2Te3 thermoelectric material
Highly textured, ultrafine grain pure Bi2Te3 has been obtained by applying large-strain high-pressure torsion (HPT) to hot-pressed (HP) coarse grain material. Its thermal conductivity is significantly smaller than the conductivity of HP Bi2Te3, and its crystallographic texture and mechanical properties significantly improved. The mechanical properties of both, coarse grain and ultrafine grain, samples have been assessed by compression tests of 2 mu m diameter micropillars machined by focused ion beam. The micropillars built from coarse grain samples are single crystalline, those built from ultrafine grain materials are an order of magnitude larger than their grain size. The test results put in evidence the elastic and plastic anisotropy of Bi2Te3 and the significant strengthening and toughening effect of ultrafine grain refining. For instance, after an equivalent strain of about 100, the Vickers hardness (in kg mm(-2)) increases from 60 to 120. Simultaneously, about a 40% reduction of the thermal conductivity has been measured, and a very strong basal texture is developed normal to the torsion axis. Such combination of properties looks very promising for simultaneously enhancing the thermoelectric figure of merit and the mechanical reliability of Bi2Te3-based alloys through HPT processing.