Preparation and transport properties of polycrystalline Bi and Bi-SiO2 nanocomposites

Bismuth-silica nanocomposites and polycrystalline bismuth were prepared via powder metallurgy in order to study the influence of silica inclusions on the thermoelectric properties of bismuth. Bi-SiO2 powders containing from 0 .5 to 15 vol. % of silica and pure Bi powders were produced by an arc-plasm a processing. Transmission electron microscopy investigations revealed the presence of a nanometric silica shell around the Bi grains. The powders wer e cold pressed and sintered close to the melting temperature of bismuth. Th e bulk microstructures are very different for the bismuth and the Bi-SiO2 n anocomposites because silica, which is primarily dispersed at grain boundar ies, inhibits the grain growth during sintering. The electrical resistivity was measured from 5 to 300 K, while the thermoelectric power and the therm al conductivity were measured from 65 to 300 K on both polycrystalline bism uth and Bi-SiO2 samples containing 0.5, 4, and 15 vol. % of silica, respect ively. The transport properties are mainly discussed with regard to the mic rostructures. In spite of a strong reduction of the thermal conductivity fo r the nanocomposites, the thermoelectric figure of merit is not improved co mpared to bismuth due to a dominating concurrent increase of the electrical resistivity resulting from a finite-size effect. (C) 2000 American Institu te of Physics. [S0021- 8979(00)02419-1].