High temperature thermoelectric properties of heavily Nb-doped Yb/sub 66/ single crystals

Investigation is being done on the high temperature thermoelectric properties of some new rare earth B12 icosahedra cluster-containing compounds. Doping effects on the TE properties in such systems were investigated for the first time. A series of heavily Nb-doped YB66 single crystals were grown by the floating zone method. The doping dependence on the thermoelectric properties was not monotonic and appears to be complex. As a result of doping, the room temperature resistivity showed a sizable reduction together with a sizable reduction of T0 (increase in D(EF) or elongation of ξ). At room temperature the Nb-doped YB66 sample with 89% site occupancy yielded a factor 3 increase over the power factor of non-doped YB66. However, in the important high temperature region (for these compounds) the non-doped sample actually exhibited the highest power factor for T>550 K. This result is interesting as it advocates manufacturing a high T0, despite meaning an actual higher initial resistivity, for these compounds when considering extremely high temperature properties (which is obvious considering the meaning of T0). Furthermore, due to a special structural feature of YB66, the thermal conductivity actually increases with doping of transition metals and therefore, from these results YB66 is judged to not be a feasible system to pursue among these rare earth boron cluster compounds.