Microcompression tests of single-crystalline and ultrafine grain Bi 2 Te 3 thermoelectric material
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Microcompression tests of single-crystalline and ultrafine grain Bi2Te3 thermoelectric material Jon Ander Santamaría,a) Jon Alkorta, and Javier Gil Sevillano CEIT and TECNUN, Universidad de Navarra M. de Lardizabal, 20018 San Sebastián, Spain (Received 15 January 2015; accepted 1 June 2015)
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 lm 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 mm2) 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.
I. INTRODUCTION
Thermoelectric (TE) materials have well-known potential applications in waste heat recovery, air conditioning, and refrigeration. Cooling and power generation based on TE effects have several advantages over other methods, such as absence of moving parts (vibrations are avoided), low maintenance, long life, and high reliability.1 Their obvious disadvantage is their usually poor thermodynamic efficiency. The efficiency of TE devices is determined by the dimensionless figure of merit (ZT), defined as ZT 5 (a2 /qj)T, where a, q, j, and T are the Seebeck coefficients, the electrical resistivity, the thermal conductivity, and the absolute temperature, respectively. Unfortunately, all three (a, q, j) are functions of the carrier concentration, so optimizing all the parameters together turns out to be a difficult challenge.2 For design and in-use reliability of TE systems in many applications, such as heat recovery in vehicles, enhancement of the mechanical properties of TE materials is of paramount importance too, as they are brittle materials. Nanostructuring a TE material leads to a substantial ZT enhancement 3 because phonons are scattered at the Contributing Editor: Terry M. Tritt a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2015.170 J. Mater. Res., Vol. 30, No. 17, Sep 14, 2015
boundaries of crystals when the phonon mean free path approaches the grain dimensions. Many studies report a significant re
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