Nanostructure formation in bulk thermoelectric compounds in the pseudo binary PbTe-Sb₂Te₃ system

Studies on microstructures in thermoelectric compounds in the pseudobinary PbTe-Sb₂Te₃ system are overviewed and strategies to control the microstructure of thermoelectric compounds are discussed on the basis of the phase diagram and phase transformation theories. The morophology of solidification from the melt results in dendrite or lamellar structure depending on composition. The size-scales of the microstructures obtained by solidification can be controlled from the order of micrometers to tens of micrometers by controlling cooling rates (dendrites) or solidification velocity (lamellae). Lamellar and Widmanstatten structures are obtained by eutectoid (Pb₂Sb₆Te₁₁ → PbTe + Sb₂Te ₃) and precipitation (PbTe (Sb₂Te₃) → PbTe + Sb₂Te₃) reactions, respectively. These solid-state transformations show features with nanometer size-scales. For the eutectoid reaction the size-scale depends on annealing temperature and time. For precipitation, the size-scale depends on composition as well as cooling rate or annealing temperature. Such behavior can be understood in terms of phase transformation theories.