Publications

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Nanostructuring of thermoelectric materials is expected to enhance thermoelectric properties by reducing the thermal conductivity and improving the power factor from that of homogeneous bulk materials. In multiphase, nanostructured thermoelectric materials, an understanding of precipitation mechanisms and phase stability is crucial for engineering systems with optimal thermoelectric performance. In this presentation we will discuss our investigations of the morphological evolution, orientation relationship, and composition of Ag{sub 2}Te precipitates in PbTe using transmission electron microscopy (TEM) and atom probe tomography (APT). Annealing in the region of two phase equilibrium between Ag{sub 2}Te and PbTe results in the formation of monoclinic {beta}-Ag{sub 2}Te precipitates as determined by x-ray and electron diffraction studies. These precipitates are aligned to the PbTe matrix with an orientation relationship that aligns the Te sub-lattices in the monoclinic and rock salt structures. This relationship is the same as we have reported earlier for {beta}-Ag{sub 2}Te precipitates in rocksalt AgSbTe{sub 2}. Observations using TEM and APT suggest that the Ag{sub 2}Te precipitates initially form as coherent spherical precipitates which upon coarsening evolve into flattened semi-coherent disks along the <100>PbTe directions which is consistent with theoretical predictions for elastically strained precipitates in a matrix. Our HRTEM observations show that sufficiently small precipitates are coherently embedded, while larger precipitates exhibit misfit dislocations and multiple monoclinic variants to relieve the elastic strain. Analysis of the composition of both precipitate groups using APT indicates that the larger precipitates exhibit compositions close to equilibrium while the smaller nanoscale precipitates exhibit enhanced Pb compositions. This detailed analysis of the orientation relationship, morphology, composition, and coarsening behavior of embedded Ag{sub 2}Te precipitates may be helpful in understanding the precipitation mechanisms and microstructure of related thermoelectric materials, such as LAST.

The precipitation of Ag{sub 2}Te in a PbTe matrix is investigated using electron microscopy and atom probe tomography. We observe the formation of oriented nanoscale Ag{sub 2}Te precipitates in PbTe. These precipitates initially form as coherent spherical nanoparticles and evolve into flattened semi-coherent disks during coarsening. This change in morphology is consistent with equilibrium shape theory for coherently strained precipitates. Upon annealing at elevated temperatures these precipitates eventually revert to an equiaxed morphology. We suggest this shape change occurs once the precipitates grow beyond a critical size, making it favorable to relieve the elastic coherency strains by forming interfacial misfit dislocations. These investigations of the shape and coherency of Ag{sub 2}Te precipitates in PbTe should prove useful in the design of nanostructured thermoelectric materials.

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