Thermoelectric properties of undoped p-type CoSb3 prepared by vertical Bridgman crystal growth and spark plasma sintering

A crystal of p-type CoSb3 was grown using the vertical Bridgman method. In the as-grown condition, polycrystalline CoSb3 grains surrounded by metallic Sb were obtained. Post-annealing was used to remove the excess Sb because the residual Sb degrades the thermoelectric properties. Subsequent heat treatment at 1073 K for 5 h removed the residual Sb, resulting in the formation of voids in the crystal. For the annealed samples, an increase in the Seebeck coefficient from 40 to 300 μV/K and a decrease in the electrical conductivity were observed. To improve the conductivity, metal was deposited on the surfaces of post-annealed samples. The maximum power factor was 5.1 × 10−5 W/cm K2 obtained at 523 K for a post-annealed sample with 100 nm of metal deposited. To understand the effect of voids in the annealed samples, highly condensed CoSb3 samples were treated with a typical spark plasma sintering (SPS) method using the grown polycrystalline CoSb3 as starting material. The thermoelectric properties of the SPS samples were compared with those of the post-annealed samples. The 100 nm Au-deposited on the post-annealed samples gave rise to an overall decrease in the value of the thermal conductivity reported previously. The maximum value of the figure of merit obtained was 0.84 at 573 K.