Abstract
Thermoelectric modules are important research topic because they can enhance energy usage efficiency by directly converting waste heat into electricity. Bi2Te3-based p-type (Bi,Sb)2Te3 and n-type Bi2(Te,Se)3 alloys are the most commonly used thermoelectric materials in commercialthermoelectric modules. Typically, there are arrays of P-N thermoelectric devices connected to metallic electrodes in thermoelectric modules. Therefore, there are many joints inthermoelectric modules. Nowadays, Sn-based Pb-free solders, such as Sn-3.0wt.%Ag-0.5wt.%Cu, are widely used as solders. In order to prevent direct contact and significant interfacial reactions between solders and thermoelectric substrates, Ni(P) is frequently used as a diffusion barrier layer. Interfacil reactions affect joints’ properties, and the modules’reliablity is directly related with the properties of these joints. Especially, the thermoelectric generators are usually used at relatively high temperatures, understanding of theirinterfacial reactions are even more critical. There are two kinds of interfacial reaction samples. The samples of the first kind are reaction couples made of the related material sub-systems. Those of the second kind are completed thermoelectric modules. The first kind samples include Ni/Bi, Ni/Se, Ni/Sb, Ni/Bi2Te3 and Ni/Bi2Se3 reaction couples. NiBi3 and NiBi phases are formed in the Ni/Bi couples reacted at 150 oC and 200 oC. NiBi phase is only observed at the corner of the couples. Ni3Se2 and Ni1-xSe phases are formed in the Ni/Se couples reacted at 200oC. In the Ni/Sb couples reacted at 200oC, NiSb and NiSb2 phases are observed. NiTe2 and (Bi2)m(Bi2Te3)n phases are formed in the Ni/Bi2Te3 couples both reacted at 150oC and 200oC. Te is the dominant diffusion species, and the reaction path is Ni/ NiTe2/ (Bi2)m(Bi2Te3)n/Bi2Te3. One reaction phase is formed in the Ni/Bi2Se3 couple reacted at 150oC. This phase could be the Ni1-xSe phase or a Ni-Bi-Se ternary phase. A two-phase mixture reaction zone is observed in the Ni/Bi2Se3 couple reacted at 200oC. The samples of the second kind have two different groups of termoelectric modules. Thermoelctric modules of the first group are made by ITRI (Industrial Technology Research Instittue), and those of the second group are made by a Russian company. The ITRI-made modules use Sn-3.0wt%Ag-0.5wt%Cu solders and Au/Ni(P) wetting and barrier layers. The P-type thermoelectric substrate is (Bi0.2Sb0.8)2(Te0.99Se0.01)3 and the N-type thermoelectric substrate is Bi2(Te0.9Se0.1)3. The Russian-made modules use Sn-0.14wt%Cu-0.26wt%Bi solder and Ni(P) barrier layer. The P-type thermoelectric substrate is (Bi0.25Sb0.75)2Te3 and the N-type thermoelectric substrate is Bi2(Te0.9Se0.1)3. The reaction temperatures are at 150oC and 100oC. The reactions at each interface in the modules are examined. The results are compared with literature results. The reaction time is as long as 8 months, and the thickness of the reaction layers are proportional to the square root of reaction time which indicate the interfacial reactions are diffusion controlled.