Effect of transition metal doping and carbon doping on thermoelectric properties of YB66 single crystals

We have been investigating the high-temperature thermoelectric properties of some novel rare earth borides with a structure containing B12 icosahedra. Doping effects on the TE properties in such systems were investigated for the first time. A series of Nb-doped YB66 and C-doped YB66 single crystals were grown by the floating zone method. The Nb-doped compounds have approximate chemical formulas ranging from YNb0.30B66 to YNb0.33B66 while the C-doped compound has a formula of YB66C0.6. The effect of Nb-doping on the thermoelectric properties was not monotonic and appears to be complex. As a result of Nb-doping, the room temperature resistivity and the characteristic temperature T0 were considerably reduced. At room temperature the power factor of the Nb-doped YB66 sample with 89% site occupancy was three times greater than that of non-doped YB66. However, in the important high-temperature region, the non-doped sample actually exhibited the highest power factor for image. Furthermore, owing to a structural feature of YB66, thermal conductivity actually increases with doping of transition metals. Taking into account all the thermoelectric properties, transition metal doping of YB66 is therefore not suitable for our purposes. On the other hand, doping of carbon, which is assumed not to go into the same sites as the transition metals, yielded a lowering of the thermal conductivity. Furthermore, contrary to Nb-doping, carbon doping did not result in a reversal of the relative magnitude of resistivity at extremely high temperatures and therefore, an increase in the figure of merit of factor 2 was realized at 1000 K.