Warm ECAE: a Novel Deformation Process for Optimising Mechanical and Thermoelectric Properties of Chalcogenides

Bi_0.5Sb_1.5Te₃ polycrystalline alloy has been processed by Equal Channel Angular Extrusion (ECAE) at 573 K. Sub-micrometric grain size has been obtained, with a consequent decrease of the lattice thermal conductivity and an impressive increase in hardness of the material. A well defined texture is observed, where the basal planes of the hexagonal cell of the crystals arrange themselves parallel to the shear deformation plane (the plane of intersection of the entry and exit extrusion channels). This texture causes anisotropy in the thermoelectric properties; in particular, the transport properties are maximised in the plane at 45deg to the extrusion direction. Warm ECAE applied to an over-doped p-type material, as in the present case, causes an increase of the Seebeck coefficient, as a result of the prevailing concentration of donor-like defects introduced by deformation. The factor of merit Z reaches the value of 2.4 times 10^-3 K^-1 at 300 K, say 80% higher than the value of the starting material