High Performance N-Type BiSb Alloys

Bismuth rich BiSb alloys exhibit the best n-type thermoelectric properties known. However, these properties are realized only in single crystals which are difficult to machine because they tend to cleave. Also, the anisotropic nature of BiSb requires that that the device current be passed across the cleavage planes. Therefore, single crystal BiSb has not been utilized in devices because the risk of cleavage in service is too high. Polycrystalline BiSb, made by powder metallurgy techniques, is mechanically robust. However, the thermoelectric properties of polycrystalline BiSb are inferior to single crystals due to misorientation of the grains. The degrading effect of grain misorientation may be more than offset by reducing the grain size. A very fine particle size should cause boundary scattering of phonons thereby reducing the lattice component of the thermal conductivity significantly. The net effect will be to increase the figure of merit of the BiSb making it a feasible n-type material for cooling applications below 200K. The goal of the proposed work is to demonstrate that a mean grain size of less than 10 microns can substantially decrease the lattice thermal conductivity of BiSb.