Thickness-dependence of the thermoelectric properties of a polycrystalline bismuth telluride alloy

It is shown that the average Seebeck coefficient should depend on thickness in any polycrystalline material in which the local Seebeck coefficient is orientation-dependent. It is known, for example, that the Seebeck coefficient of intrinsic single-crystal bismuth telluride is anisotropic. Thus, measurements of the Seebeck coefficient as a function of thickness have been made on a near-intrinsic polycrystalline bismuth telluride alloy. As expected, the Seebeck coefficient falls when the sample thickness becomes smaller than the grain size. The magnitude of the effect, amounting to about 15 μV/°C, is of the order predicted from the known parameters of single crystal material, though an exact theoretical treatment of the problem has not been possible. On lowering the temperature, so that the material becomes an extrinsic conductor, the thickness-dependence of the average Seebeck coefficient disappears, since the local Seebeck coefficient is then no longer orientation-dependent.

The above results were obtained using careful grinding and polishing techniques. Similar experiments carried out using coarse grinding of the samples led to an increase of the Seebeck coefficient with decreasing thickness. This effect is tentatively explained in terms of mechanical damage, notably the splitting of the grains along their basal planes.