Nanostructuration of CoSi by mechanical milling and mechanical alloying

CoSi is an inexpensive thermoelectric material for medium temperature (200-500 degrees C). Its power factor is as large as the state of the art materials; however, its thermal conductivity is too large. Then, improving its thermoelectric performances implies increasing the scattering of phonons, which can be performed by nanostructuring the material. In this paper we investigate the effect of nanostructuration on the structure, microstructure, lattice dynamics and stability of CoSi. We obtained powders of about 13 nm by mechanical milling bulk CoSi for only four hours or by mechanical alloying pure elements for twelve hours. Nanostructuration induces a 0.1% expansion of the lattice parameter. Raman spectroscopy, associated to ab initio calculations, highlights the effectiveness of nanostructuration on phonon scattering, showing a reduction of the phonon relaxation time by as much as 80%. Powders are stable up to 450 degrees C; then grains coarsen and a partial degradation of the material occurs, probably due to silicon sublimation. Our results indicate that nanostructuration should be considered when interested to reduce CoSi thermal conductivity. (C) 2014 Elsevier Masson SAS. All rights reserved.