Effect of Twin-roll Casting Process Parameters on the Thermoelectric Transport Properties of Ag-Bi-Te Alloy

Abstract

Typical alloy development of a eutectic, compositional system of Ag18.34Bi29Te52.66 (at.%) was successfully obtained by alloying very pure elemental raw materials, viz., Silver, Bismuth, and Tellurium by flame melting. The obtained master ingot processed by Twin-roll casting with varied rollers speed from slow to fast (600-2400 rpm) by keeping the rest parameters, viz., the gap between rollers, cooling rate, the distance of head melt to rollers, melting temperature, the dimension and loads of rollers constant. Very thin and silvery stripes obtained from the casting process with different thickness sizes depending on the casting rollers speed. The stripes were ground into raw powder, and sintered by a spark plasma-sintering unit. The relative density of the sintered samples measured by Archimedes methods and all samples density measurement result was above 95%. As the examination result of a scanning electron microscope, the structures are multidirectional, unlike to other results that processed by another equilibrium process. Again, the structures are with refined grain size, and due to increased grain boundaries density, the cumulative material’s thermal conductivity became lowered and with moderate power-factor to result in improved thermoelectric performance and evidenced by peak ZT of 1.18 at 303K that corresponds to low revolution per minute (600 rpm) processed sample. The achievement of this work can reported as, due to increasing rate of solidification with the increasing casting rollers speed, the Bi2Te3 phases become amorphous as witnessed by the X-ray diffraction pattern. Therefore, with increasing rollers speed, the transport properties decrease as seen from each measured and calculated value, and leading to decreasing total thermoelectric performance efficiency.