Thermoelectric semiconductor and electrode-fabrication and evaluation of SiGe/electrode

Joints between a silicon-germanium thermoelectric semiconductor and electrodes were prepared by hot-pressing. Tungsten or carbon electrodes were joined to either side of n-type Si_0.8Ge_0.2 (P 0.3 at.%) respectively, with thin sheets of titanium foil inserted between them, and then pressed and joined at 1523 K in vacuum. The size of the resulting electrode/SiGe semiconductor/electrode units measured 5 mm× 5 mm×9 mm. Observations through SEM and EPMA showed the presence of many voids in the junction layers of the SiGe/tungsten electrodes, whereas the SiGe/carbon electrodes exhibited no voids in their junction layers. In the SiGe/carbon electrodes, the titanium atoms were enriched in the junction layer, and other atoms also remained undiffused. In the SiGe/tungsten electrodes, the tungsten and silicon atoms showed counter diffusion. Resistance and Seebeck coefficient of the carbon/SiGe/carbon electrodes were measured in the temperature range from 600 to 1100 K. Resistance ranged from 0.012 to 0.015 Ω, and the Seebeck coefficient from 200×10^-6 to 250×10^-6 V/K. To evaluate the upper limit of heating dependence, the carbon/SiGe/carbon electrodes were exposed in an argon atmosphere at 1273 K and 1373 K for 300 hours. After exposure testing, SEM observation showed the presence of many voids in the junction layer of the SiGe/carbon electrode, which had not previously shown any voids either at the stage of joining by hot-pressing or exposure at 1273 K