Effect of Processing Route on the Microstructure and Thermoelectric Properties of Bismuth Telluride-Based Alloys

Abstract

Considerable work has been done to engineer materials with high efficiencies of thermoelectric heat-to-electricity conversion and the mechanical strength necessary to withstand the demands of practical applications. In particular, in the bismuth telluride system, extrusion pressing has been found to be effective for improving the mechanical strength of alloys via grain refinement. We review some of the literature relating to processing approaches for the bismuth telluride system. We also present preliminary data for a series of samples obtained by incorporating C 60 via ball milling and spark plasma sintering into a matrix consisting of a (Bi,Sb) 2 Te 3 alloy, with a focus on the texture of the composites and its relation to thermoelectric transport properties, in comparison to the parent material. The viability of improving the thermoelectric performance of bismuth telluride alloys by the insertion of nanoparticles into a composite is also considered.

Spark plasma sintered bismuth telluride-based thermoelectric materials incorporating dispersed boron carbide

“…66 [23] 90 [12] More typically 1.1 (∥) and 0.8 (⊥) [7,9] Best room temperature values of up to 1.1 (⊥) and 0.8 (∥) quoted [7], typical value 0.9 (⊥) [16,19]. Note: max zT occurs ⊥ , the opposite of directionally solidified material…”

“…The highest thermoelectric figure of merit (zT) is achieved in materials produced in directionally solidified form, parallel to the direction of solidification [7]. The strength and toughness is poor perpendicular to solidification due to cleavage along the basal crystallographic plane which is aligned parallel to the growth direction [7][8][9].…”

“…The strength and toughness is poor perpendicular to solidification due to cleavage along the basal crystallographic plane which is aligned parallel to the growth direction [7][8][9]. Strength is also governed by flaws [10] in common with any brittle solid.…”

“…Mechanical property improvement has been the justification for the development of polycrystalline materials produced by a variety of processes including hot-pressing [11,12], extrusion [8,13], plasma activated sintering [14,15] and spark plasma sintering (SPS) [7,[16][17][18][19][20][21][22][23][24]. The addition of nanoscale particulate reinforcements (SiC [17,18,23], C 60 [7], multi-wall carbon nanotubes [12] and Al 2 O 3 [22]) have been pursued. The intention is to maintain thermoelectric performance in polycrystalline materials while improving their mechanical properties.…”

“…In pressed or sintered materials the direction of maximum zT switches from parallel with solidification to perpendicular to the pressing direction. This effect has been attributed to preferential alignment of the basal poles with the pressing direction [7]. The addition of a dispersed nanoscale particle phase may increase the zT relative to a control polycrystalline specimen providing the volume fraction is optimised, largely due to reduced thermal conductivity [18,22,23].…”