ICT2014 Abstracts

CoSi is a cheap, non-polluting thermoelectric material for medium temperatures (200-700°C). Its power factor is similar to the state of the art materials; however, its thermal conductivity is too large. Then, increasing the scattering of the phonons by nanostructuring the material can enhance its thermoelectric performances. Nonetheless rapid sintering techniques are recommended because the high temperatures involved in sintering can destroy nanostructuration. In this work we investigated the effect of sintering parameters on microstructural and thermoelectric properties of CoSi.

We obtained nanometric powders of CoSi, pure and doped, of about 10 nm by mechanical milling bulk CoSi. Powders were shaped using Spark Plasma Sintering; we investigated the effect of sintering temperature and duration. Grain size, phonon mean free path and the eventual appearance of a secondary phase on pellets after sintering were investigated by XRD, and Raman spectroscopy. Then we investigated thermoelectric performances and compared the latter with arc-melted bulk samples and with pellets obtained by natural sintering of the same powders.

We showed that after sintering powders remains nanostructured even though grain coarsened and the density are superior to 95%. Doping can lower sintering temperature without reducing density. Thermoelectric performances are encouraging: the low porosity does not harm electrical conductivity and nanostructuration effectively reduces thermal conductivity. Nevertheless the appearance of a secondary phase, Co₂Si, and some difficulties in obtaining pure CoSi with a good stoichiometry by arc melting lowered the Seebeck coefficient.

Finally, mechanical milling combined with Spark Plasma Sintering can effectively increase thermoelectric performances of silicides in general and pure or doped CoSi in particular.