(251f) Liquation Phenomena in Solid/Solid Thermoelectric Bi2Te3 Reaction Couples

Thermoelectric devices can convert heat into electricity and have attracted a lot of interests of research and development. There are various contacts in thermoelectric devices. Understanding of their interfacial reactions is fundamentally important for the processing routes determinations and products’ reliability assessment. Bi₂Te₃-based alloys are the materials most commonly used in commercial thermoelectric devices. Couples with Bi₂Te₃ substrates are prepared for examinations of interfacial reactions. Interesting liquation phenomena are observed in the solid/solid reaction couples, Sn/Bi₂Te₃, In/Bi₂Te₃ and Cu/Bi₂Te₃ reacted at 160℃, 140℃ and 350℃, respectively. Both Sn and Bi₂Te₃ are solid at 160℃. After reaction at 160℃ for 40 min, a 160μm-thick reaction zone composed of two regions is formed in the Sn/Bi₂Te₃ couple. 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 diffusing from Bi₂Te₃ substrate. The reaction path is Sn/liquid/SnTe/Bi₂Te₃ and is illustrated based on the Bi-Sn-Te 160℃isothermal section. Both In and Bi₂Te₃ are solid at 140℃. After reaction at 140℃ for one day, a liquid phase is formed among various reaction phases in the reaction zone of 158 μm thick in the In/Bi₂Te₃ couple. The reaction path is In/In₄Te₃/liquid/Bi/(Bi₂)_m(Bi₂Te₃)_n/Bi₂Te₃. Te diffuses toward In and a In₄Te₃ phase is formed. Bi and Bi/(Bi₂)_m(Bi₂Te₃)_n phases are formed in the Bi₂Te₃ substrate with depletion of Te. Liquid phase is formed due to the alloying between In and Bi. A Bi-In-Te 140℃isothermal section is proposed and the reaction path is illustrated thereupon. Since the melting points of Cu and Bi₂Te₃ are higher than 350℃, the 350℃ Cu/Bi₂Te₃ couples are solid/solid couples. However, a liquid phase is observed after reaction for 5 minutes. A metastable Cu_xBi₂Te₃, an oversaturated Bi₂Te₃ phase, is formed in the Cu/Bi₂Te₃ couple due to the fast penetration of Cu atoms into the Bi₂Te₃ substrate. The oversaturated phase then decomposes, the stable Cu₂Te phase is formed, and the remaining Bi becomes liquid phase at 350^oC. Once the liquid phase is formed, the reaction rates become very fast. When the couple is removed from the furnace at 350oC, the liquid phase solidifies, and Bi and (Bi₂)_m(Bi₂Te₃)_n phases are formed. The reaction path is illustrated in the Bi-Cu-Te isothermal section with and without the metastable Cu_xBi₂Te₃ phase.