Abstract
Thermoelectric devices can convert heat into electricity directly, and have attracted enormous research interests. There are usually numerous solder (or braze) joints in thermoelectric devices. To prevent direct contact and interfacial reactions between solder (or braze) and thermoelectric materials, barrier layer is often used. Ni is the most commonly used barrier layer material. Examination of the interfacial reactions between Ni and thermoelectric substrates is fundamentally important for reliability assessment of the thermoelectric devices. This study investigates interfacial reactions between Ni and three kinds of thermoelectric materials: Ag2Te, (Bi,Sb)2Te3, and Bi2(Te,Se)3. The thermoelectric substrates are prepared with proper amounts of pure constituent elements, and are then plated with Ni. The thickness of Ni layer is 60μm. Ni3Te2, NiTe0.775 and NiTe2 are formed in the Ni/Te couple reacted at 200oC for 720 hours. Two reaction regions are observed in the Ni/Ag2Te couple reacted at 200oC for 720 hours. A continuous Ag layer is formed adjacent to the Ni substrate. The other reaction region is a two-phase finger-type mixture. The darker phase in this two-phase region is the Ni3Te2 phase and the other brighter phase is Ag2Te phase with Ni solubility. This study also investigates the interfacial reactions between Ni/Te couple and Ag2Te couple at 250 oC. Ni3Te2, NiTe0.775 and NiTe2 are also formed in the Ni/Te couple reacted at 250oC for 720 hours, and the thickness of reaction layer is about 102.6μm. Two reaction regions are observed in the Ni/Ag2Te couple reacted at 250oC for 720 hours. A continuous Ag layer is formed adjacent to the Ni substrate. Comparing with those in the Ni/Ag2Te 200oC reaction couple, a continuous Ni3Te2 reaction layer is formed adjacent to the Ag2Te substrate. The interfacial reactions between the Ni barrier layer and the P-type (Bi1-xSbx)2Te3 and n-type Bi2(Te1-ySey)3 thermoelectric materials at 300oC are examined. Two reaction phase layers are observed in the Ni/(Bi0.25Sb0.75)2Te3 couples reacted at 300oC. The phase layer adjacent to the Ni substrate is likely a Sb-Ni-Te ternary compound or the Ni3Te2 phase with significant Sb solubility.In the In the Ni/ Bi2(Te0.9Se0.1)3 couple reacted at 300oC two reaction phase layers are also found. The BiTe phase is adjacent to the Bi2(Te0.9Se0.1) substrate, while a Bi-Ni-Te ternary compound or the NiTe2 phase with Bi solubility is formed adjacent to the Ni substrate. It can be found that the thickness of the reaction layers in the Ni/Bi2(Te0.9Se0.1)3 couple is thicker than that in the Ni/(Bi0.25Sb0.75)2Te3. Furthermore, the reaction rate at 300oC is faster than that at 250oC.