Thermoelectric properties of /spl beta/-FeSi/sub 2/-TiB/sub 2/ composites

Composite media are under consideration with respect to their capability to possibly increase the thermoelectric power factor or even the figure of merit compared with homogeneous phases they are composed of. Series of /spl beta/-FeSi/sub 2/-TiB/sub 2/, composites with composition near the percolation threshold were prepared on different routes. Co-doped /spl beta/-FeSi/sub 2/ powders (5 at% Co) with various amount of TiB/sub 2/ powder added were ball milled. On the other hand, powders of both materials were simply mixed. Both of these precursors were hot pressed after drying and the compacts were annealed. SEM observations show that for the milled powders the FeSi/sub 2/ particles are smaller than those of TiB/sub 2/, while after simply mixing this is vice versa, according to the initial powder grade. The percolation threshold was found at a volume fraction of TiB/sub 2/ of 0.255 (milled) and 0.145 (mixed), respectively. The electrical and thermal conductivity, and the Seebeck coefficient were measured in dependence on temperature. The results show different transport behavior for these two series of composites. The figure of merit Z for both routes is always lower than that of FeSi/sub 2/ without addition of TiB/sub 2/. Whereas the milling route shows better Z values for given TiB/sub 2/ content and a much higher critical volume fraction (percolation threshold) than simple mixing, the latter reaches the percolation threshold at much lower addition of the highly-conducting phase TiB/sub 2/ and at much higher Z. As a peculiarity when milling hard materials like silicides and borides, the modification of the composites by introduction of iron from the milling set has to be considered.