Grain size effect on thermoelectric properties of PbTe prepared by spark plasma sintering

Sintered PbTe materials with average grain sizes of 28-309 μm were prepared by the spark plasma sintering technique. The apparent densities of the sintered PbTe were 8.17-8.23 Mg/m³, which were higher than 99% of the theoretical one. Thermoelectric properties of the sintered materials and those of the as-grown boule by Bridgman method were measured in the temperature range from 77 to 350 K. Resistivity, ρ of the sintered materials increased with decreasing grain size. Temperature dependence of ρ of the sintered PbTe was remarkably different from that of the as-grown boule below 250 K because of potential barriers at grain boundaries. Hall coefficient, R_H of the sintered materials at room temperature increased from 1.4 x 10^-6 to 3.4 x 10^-6 m³/C, as the average grain size increased from 28 to 309 pm, which suggested that oxidation in the crystal grains was caused in the sintering process. Temperature dependence of the thermoelectric power, α of the sintered PbTe below 250 K reflected the effect of carrier scattering at grain boundaries. Lattice thermal conductivity of the sintered PbTe decreased with decreasing grain size below 250 K, while it was independent of the grain size above that temperature. Above 250 K, thermoelectric properties of the sintered PbTe, except those in the 28 μm grain case, were consistent with those of the as-grown boule. The values of α and R_H at 295 K were calculated by using a two-valence-band model involving a non-parabolic and a parabolic valence band. The calculation results were in good agreement with the experimental data.