Size control of Sb_2Te_3 Widmanstätten precipitates in thermoelectric PbTe

Abstract

The number density and area per unit volume of Sb_2Te_3 Widmanstätten plates in thermoelectric PbTe were controlled through two types of heat treatments of (PbTe)_(1−x)-(Sb_2Te_3)_x, where x = 0.04 and 0.06: isothermal annealing at various temperatures and cooling from a solid-solution regime to a two-phase region with various rates. The microstructure was quantified by image analysis of scanning electron micrographs and Rietveld refinements of X-ray diffraction profiles. Isothermal annealing of (PbTe)_(0.94)-(Sb_2Te_3)_(0.06), results in increasing number density and area per volume of precipitates with decreasing temperature. In controlled cooling rate experiments, faster cooling rates or smaller x result in higher number density and area per volume. These trends are discussed using phase transformation theories. Overall the number density and area per volume of precipitates were controlled in the ranges from 0.4 to 44 μm^(−3) and from 0.5 to 1.8 μm^(−1), respectively. Isothermal annealing was performed for time periods from 10 to 166 h at 723 K to check the stability of the microstructure at the (PbTe)_(0.94)-(Sb_2Te_3)_(0.06) composition. While the Boyd and Nicholson model of the Greenwood–Lifshitz–Slyozov–Wagner theory for the average diameter of plates gives a reasonable value for peripheral interfacial energy, the time dependence was found to decelerate more than the t^(1/3) rule. It has also been found that the coarsening mechanism involves the elongation of plates.