ELECTRICAL-PROPERTIES OF YTTRIUM SESQUISULFIDE (Y2S3) MECHANICALLY ALLOYED WITH COPPER, BORON, AND ALUMINUM

The electrical resistivity, thermoelectric power (Seebeck coefficient) and Hall effect have been measured for (Y2S3)(1-y)X(y) compounds with the 6-monoclinic structure (where X = Cu, B or Al and y = 0.05 for Cu and B, and 0.025 less than or equal to y less than or equal to 0.075 for Al) in order to determine their potential as high temperature (300 -1000 degrees C) thermoelectric materials. The Cu- and B-doped Y2S3 ar e insulators. The Al-doped Y2S3 behaves as a metastable degenerate sem iconductor with itinerant conduction. The room temperature electrical resistivity decreases as the Al doping level increases, while the Seeb eck coefficient remains constant at ca. 400 mu V degrees C-1. These ma terials, however, become insulators after a 2 week heat treatment at 1 000 degrees C or higher. Both the electrical resistivity and Seebeck c oefficient increased with increasing temperature, and exhibited lower values during the cool down run to room temperature. These data sugges t that the (Y2S3)(1-y)Al-y alloys are in a metastable state in which t he Al atoms occupy the interstitial sites of the Y2S3 lattice, where i t acts as a current carrier donor. When heated, the Al atoms diffuse o ut of the lattice and no longer contribute to the electrical conductiv ity, thus making the alloy an insulator. The maximum value of the powe r factor was 0.6 mu W cm(-1)degrees C-Z, and decreased with increasing temperature. The results of this study indicate that Cu-, B-, and Al- doped Y2S3 are not useful as high temperature thermoelectric materials .