Suppression of grain growth in nanocrystalline Bi₂Te₃ through oxide particle dispersions

The strategy of suppressing grain growth by dispersing nanoscale particles that pin the grain boundaries is demonstrated in a nanocrystalline thermoelectric compound. Yttria nanoparticles that were incorporated by mechanical alloying enabled nanocrystalline (i.e., d<100nm) Bi₂Te₃ to be retained up to a homologous temperature of 0.94T_m for durations over which the grain size of the unreinforced compound grew to several microns. The nanostructure appeared to saturate at a grain size that depended on volume fraction (f) according to an f ^-1/3 relationship, in accordance with theoretical models in the limit of high volume fractions of particles. Interestingly, at low temperatures, the particles stimulate enhanced grain growth over the unreinforced compound, due to particle-stimulated nucleation of recrystallization. To help prevent this effect, in-situ composites formed by internal oxidation of yttrium are compared with those made ex-situ by incorporation of yttria nanoparticles, with the result that the in-situ dispersion eliminates recrystallization at low temperatures and therefore improves nanostructure stabilization. These developments offer a pathway to thermally stabilized bulk nanocrystalline thermoelectrics processed via a powder route.