Liquation phenomena in Sn/Bi2Te3, In/Bi2Te3 and Cu/Bi2Te3 couples

Abstract Bi2Te3-based materials are the most commonly used materials in commercial thermoelectric devices. There are many joints in thermoelectric devices, thus knowledge of interfacial reactions with Bi2Te3 substrate is important. Interfacial reactions are examined in the following couples, Sn/Bi2Te3 at 160 °C, In/Bi2Te3 at 140 °C and Cu/Bi2Te3 at 350 °C. The reaction temperatures are all lower than the melting points of the respective reaction substrates, so all these couples are solid/solid couples; however, liquation phenomena are observed in these couples. A reaction zone composed of two regions is formed in the Sn/Bi2Te3 couple reacted at 160 °C. The phase region adjacent to Sn is liquid phase and that adjacent to Bi2Te3 is SnTe with Bi solubility. The liquid phase is of near Bi-Sn eutectic composition and is formed due to alloying between Sn and Bi. The reaction path is Sn/liquid/SnTe/Bi2Te3. In the In/Bi2Te3 couple reacted at 140 °C, a liquid phase is formed among various reaction phases. The reaction path is In/In4Te3/liquid/Bi/(Bi2)m(Bi2Te3)n/Bi2Te3. Te atoms diffuse toward In and an In4Te3 phase is formed. Liquid phase is formed due to the alloying between In and remaining Bi. A metastable CuxBi2Te3, an oversaturated Bi2Te3 phase, is formed adjacent to the Bi2Te3 substrate in the Cu/Bi2Te3 couple reacted at 350 °C. The oversaturated phase decomposes, and a stable Cu2Te phase is formed. The remaining Bi adjacent to CuxBi2Te3 phase is molten at 350 °C, and the remaining Bi in contact with Cu, alloying with Cu and is also molten. Once the liquid phase is formed, the reaction rates become very fast. The reaction paths in the couples are illustrated with the proposed related phase equilibria isothermal sections.