Warm ECAE:aNovelDeformation Process forOptimising Mechanical andThermoelectric Properties ofChalcogenides

Bi05Sb15Te3polycrystalline alloy hasbeenprocessed by Equal Channel Angular Extrusion (ECAE)at573K. Sub-micrometric grain sizehasbeenobtained, witha consequent decrease ofthelattice thermal conductivity andan impressive increase inhardness ofthematerial. A well defined texture isobserved, wherethebasal planes ofthe hexagonal cell ofthecrystals arrange themselves parallel to theshear deformation plane(the plane ofintersection ofthe entry andexit extrusion channels). Thistexture causesanisotropy inthethermoelectric properties; in particular, thetransport properties are maximised intheplane at450totheextrusion direction. WarmECAEapplied toanover-doped p-type material, as inthepresent case,causes anincrease oftheSeebeck coefficient, asa result oftheprevailing concentration of donor-like defects introduced bydeformation. The factorof meritZ reachesthevalueof 2.4x10-3 K-1at300K,say80%higher thanthevalue ofthe starting material.