Effect of processing route on the bipolar contribution to the thermoelectric properties of n-type eutectic Bi_22.5Sb_7.5Te₇₀ alloy

Abstract

Narrow bandgap thermoelectric materials based on bismuth-telluride exhibit bipolar conduction that is detrimental to their overall thermoelectric performance. Through fabrication of composites of (Bi,Sb)₂Te₃ matrix with embedded random colony of (Bi,Sb)₂Te₃/Te lamellar (eutectic) with different length scales through a combination of different processing routes such as normal flame melting, cryomilling followed by spark plasma sintering and melt-spinning techniques followed by spark plasma sintering, interfacial charge defects and selective potential barrier were created leading to the injection of excess electron and activation of energy filtering effect of minority carriers. The selective-filtering of charge carriers lead to the delay in the onset of bipolar conduction to a higher temperature as a result of an increase in the band gap. This was found to be responsible for shifting the Peak ZT and the optimum operating regime of our alloys to a higher temperature. Changes in microstructure length scale from normal flame-melting to melt-spinning plus spark plasma sintering resulted in 20% reduction in the total thermal conductivity.