Recovery and thermoelectric performance optimization of p-type bismuth telluride waste by secondary zone-melting

The preparation of bismuth telluride (BST) mainly relies on the zone melting method in the commercial market. However, the zone melted ingot usually exhibits poor performance at both ends, resulting in a low utilization efficiency. Herein, we employed a combination of composition adjustment and secondary zone melting to recycle the scraps at both ends of the p-type zone melted BST ingot. A systematic investigation was conducted to analyze the phase composition, microstructure, and thermoelectric performance uniformity of the recycled ingot. The results reveal that the composition of the recycled ingot is very close to the nominal value after composition adjustment. All the different zones of the recycled ingot display nearly identical composition, microstructure and thermoelectric performance, except for the two ends. Compared with other zones of the ingot, the top end possesses more Te impurity, and the bottom end shows inferior orientation. Meanwhile, both ends exhibit a low electrical conductivity and a high lattice conductivity. Nonetheless, all the zones of the recycled ingot, except the bottom end, show high zT values ranging from 0.93 to 1.26 at room temperature. A maximum zT value of 1.29 at 350 K was obtained for the middle position of the ingot. Our work presents a new environmentally friendly, cost-efficient strategy to recycle the scrap at both ends of the zone melted BST ingot.