SIZE-DEPENDENT ERRORS IN Z DETERMINATION USING THE HARMAN METHOD

The effect of sample size on the measured thermoelectric figure of merit (Zm) determined by the Peltier–Seebeck or Harman method has been studied by progressively halving the area (As) and length (ls) of bismuth telluride alloy samples. Measurements were carried out with As values from 1 to 0.01 cm2, la values from 2 to 0.5 cm, and three different gauges of current leads (cross-sectional area Aw). The relative decrease of Zm was found to vary approximately as sech [Formula: see text] with Aw and ls constant, as In Aw with As and ls constant, and as T4/s2 with Aw and As constant, where T is the absolute temperature. The variation with As and Aw appears to be due to conduction along the sample leads, while the variation with ls is well accounted for by radiation to and from the sample. With 40 A.W.G. wires, the error in the measured figure of merit is estimated to be about 5% for a sample of dimensions 0.5 × 0.5 × 1 cm and around 50% for a sample of size 0.1 × 0.1 × 2 cm.