Surface treatment by crystalline Co-P coating with ultra-thin CoTe2 layer for improved electrical and mechanical properties of the n-type Bi2Te3/Sn TEG electrode

Bismuth telluride-based thermoelectric generators (TEG) were widely used in the clean recovery of waste heat, but frequent failure problems at the hot-end electrode interface limited the reliable service of TEGs. Diffusion at the electrode interface between n-type Bi2Te3 and Sn-based solder seriously damaged the performance of the TEGs. In this work, the crystalline Co-4 at.% P coating was deposited to protect the n-type thermoelectric material Bi2Te3. The microstructural characterization proved that the Co-P coating effectively resisted the diffusion of Sn, Bi, Te and Cu atoms, and prevented the generation of brittle intermetallic compounds (IMC) and cracks at the interface. Ultra-thin CoTe2 layer was formed between Co-P and Bi2Te3, much thinner than the brittle Ni-Te layer reported in the literatures. It not only showed that a good metallurgical bond was formed between Co-P and Bi2Te3, but also implied the Co-P only consumed little thermoelectric elements. After the 150-hour aging test at 423 K, the shear strength of the electrode joints did not decrease, and the power generation sustainability of thermoelectric pillars was significantly improved compared with uncoated pillars. Overall, the crystalline Co-P coating would be an excellent candidate to protect the n-type Bi2Te3 pillars of the TEGs.