Effects of interface layer on thermoelectric properties of a pn junction prepared via the BMA-HP method

Prealloyed p and n-type bismuth telluride-based materials produced by bulk mechanical alloying are directly one-step hot-pressed to yield a thermoelectric pn junction. Variation of constitutional element concentrations across the pn interface is characterized by EPMA to determine the interface thickness of the pn junction. The electrical resistivity of the interface layer is greater than that of both p and n semiconductor materials. Analytic expressions for Seebeck coefficient and figure of merit versus interface layer size are deduced. The Seebeck coefficient of the pn junction is inversely proportional to the ratio of the interface layer length to the pn junction height (hi/h) and agrees well with experimental results. In a pn junction with an interface layer certain thickness, there is a maximum figure of merit at the optimal hi/h; with the decrease in interface thickness, the maximum increases, and correspondingly, the optimal hi/h decreases. In other words, the pn composite billet with a thinner interface layer can attain a larger figure of merit at the same hi/h than that with a thicker interface. Adjusting the process parameter, with proper cutting, thermoelectric properties can be improved greatly; this method of producing a pn junction is feasible.