Abstract
Thermoelectric devices convert waste heat directly into electricity, enhance energy usage efficiencies, and have attracted a lot of research interests. The PbTe compound has good thermoelectric properties and is promising for thermoelectric devices in the temperature range of 300-600oC. Usually, there are many joints in a thermoelectric module. Because of their different application temperatures, different solders/brazes with different melting points are needed. Ag-Sb eutectic alloy, of which melting point is 485oC, is a promising candidate as the joining materials for the mid-temperature thermoelectric modules. Ni and Co are commonly used as barrier layers to prevent interfacial reactions between joining materials and thermoelectric materials. Therefore, the study in the interfacial reactions among thermoelectric material, barrier layer and joining material is crucial for the efficiencies and reliabilities of a thermoelectric device. Phase diagrams contain phase equilibria information and are fundamentally important for understanding and illustration of interfacial reactions. This study thus includes the following discussions: (1) experimental determination of the Ag-Ni-Sb isothermal sections at 400oC and 550oC, (2) thermodynamic calculation of the Ag-Ni-Sb isothermal section, (3) Ni/Ag-Sb interfacial reactions at 350oC and 550oC, (4) Co/Ag-Sb interfacial reactions at 350oC and 550oC, and (5) Co/PbTe interfacial reactions at 350oC and 550oC. In this study, the metallographic analysis, compositions, and structures are determined by SEM, EPMA, and XRD. Similar results are found in the Ag-Ni-Sb isothermal sections at 400oC and 550oC. No ternary compounds are observed and all the binary compounds have very limited ternary solubilities. There are 7 tie-triangles at 400oC. The Ag3Sb+NiSb2+Sb, Ag+Ni5Sb2+NiSb, Ag+(Ni3Sb)+Ni5Sb2, and Ag+Ni+(Ni3Sb) tie-triangles have been experimentally determined. In addition, there are 9 tie-triangles at 550oC. The Sb+liquid+NiSb2, liquid+NiSb+NiSb2, liquid+Ag3Sb+NiSb, and Ag+Ni+(Ni3Sb) tie-triangles have been experimentally determined. Two intermetallic phases are observed in the Ni/Ag-Sb couples reacted at 350oC, and they are NiSb and NiSb2. Ni3Sb, Ni5Sb2 and NiSb are formed in the Ni/Ag-Sb couples reacted at 550oC. According to the phase formation sequence and morphologies, it is concluded that Sb is the fastest diffusion species. One intermetallic phase, CoSb, is observed in the Co/Ag-Sb couples reacted at 350oC. Two intermetallic phases are found in the Co/Ag-Sb couples reacted at 550oC, and they are CoSb and CoSb3. According to the reaction microstructures, Sb is the fastest diffusion species. The IMCs in the Co/Ag-Sb couples are brittle. The Ag-Sb has good wetting toward the Ni substrate. The reaction rates of Ag-Sb eutectic alloys with Ni are comparable to those of commercial solders with Ni. Thus, Ni is a proper barrier layer for Ag-Sb eutectic alloy, and Ag-Sb eutectic alloy is a suitable braze for the mid-temperature thermoelectric modules. No intermetallic phase is in the Co/PbTe couples at 350oC and 550oC. The wettability between Co and PbTe is good. Co thus can be a suitable barrier layer for PbTe.