Dependence of the Thermoelectric Figure of Merit on Orientation in Uniaxial Crystals

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Abstract
The thermoelectric figure of merit has been determined as a function of the angle between the principal symmetry axis and either the field vectors E and −∇T or the current densities j and q for uniaxial crystals assuming isotropy of the Seebeck coefficient. For any orientation of these vectors other than parallel or perpendicular to the principal symmetry axis, the figure of merit in the direction of the current densities is always less than that in the direction of the field vectors. The latter situation is realized experimentally in thin disk samples while the former is characteristic of long rods. When these expressions are averaged over the angle between the cleavage planes and the appropriate vectors, the figure of merit in the direction of the field vectors is 86% while that parallel to the current densities is 72% of the maximum value for Bi2Te3 crystals. These values should be characteristic of sintered powder samples.

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