A Novel Synthesis of Polycrystalline Bismuth Telluride

Abstract

With the planned phase-out of chlorofluorocarbon (CFC) refrigerants within this decade, considerable interest has developed in altemative refrigeration and cooling technologies. It is possible that alternatives to CFC liquid-gas expansion systems such as solid-state thermoelectric devices could easily develop into a substantial future technology. Thermoelectric cooling devices based upon the Peltier effect have been used for many years in specialized applications. At present, these units do not achieve the performance currently available with CFCs. Typically, CFC systems operate near 40% of Carnot efficiency while the best thermoelectric systems reach only about 10% of Carnot efficiency.' Nonetheless, a significant thermoelectric cooling industry based on the Peltier effect has already developed. Certain semiconductor materials, particularly bismuth telluride-based alloys, are the materials of choice in modem thermoelectric coolers. These alloys are commonly made through metallurgical melt processing,2 Le., by comelting appropriate amounts of the pure elements in sealed vessels at temperatures above 600 C, mixing, and then subjecting the melts to controlled cooling. This batch-processing approach is both equipment and labor intensive, while thermoelectric elements cut from the solidified alloys tend to be somewhat fragile. An altemative approach, amenable to automated production, is the fabrication of thermoelectric elements from polycrystalline powder^.^^^ These powders are commonly obtained by crushing the solidified melts and sieving the resultant particulate material. Polycrystalline thermoelectric elements offer improved structural integrity, although they generally exhibit some degradation in thermoelectric performance due to the anisotropic nature of the matieral. As a step toward the development of an altemative route for the preparation of polycrystalline, Bi/Te-based thermoelectric materials, this publication describes a novel synthesis of the parent compound, bismuth telluride, BiaTe3. This method provides a simple, two-step process for the preparation of fineparticle bismuth telluride. The process features the roomtemperature coprecipitation of a bismuth telluride precursor in aqueous media, followed by its conversion to Bi2Te3 through hydrogen reduction. For scale-up to production levels, the coprecipitation reaction lends itself to continuous powder synthesis through the use of a chemical flow r e a ~ t o r . ~ A very recent publication by Groshens et al. describes another approach to the synthesis of Bi2Te3 and related materials by means of elimination reactions conducted in hexane at -30 OC.6