Interfacial reactions at the joints of CoSb3-based thermoelectric devices

Abstract

CoSb3-based alloys are among the most promising thermoelectric materials. The Ag-Cu eutectic alloy, Ag-39.9 at%Cu, has a melting point at 779.1 °C and is a good braze candidate for thermoelectric modules used in mid-temperatures. Since the Ag-39.9 at%Cu alloy reacts significantly with the CoSb3 substrate, the introduction of a barrier layer is necessary, and Co, Ni and Ti are potential barrier layers. This study systematically examines the reactions at the joints, Ag-Cu/Co/CoSb3, Ag-Cu/Ni/CoSb3 and Ag-Cu/Ti/CoSb3. Interfacial reactions are observed between the CoSb3 substrate and barrier layers at 450 °C and 600 °C, although they are significantly reduced with the introduction of barrier layers. CoSb2 and CoSb phases are formed at the Co/CoSb3 contact, Ni5Sb2 and (Co,Ni)Sb phases at Ni/CoSb3, and TiSb, TiSb2 and TiCoSb phases at Ti/CoSb3. After reaction at 450 °C for one day, the reaction layer thick is 2 μm, 24 μm, and 0.4 μm in the Co/CoSb3, Ni/CoSb3 and Ti/CoSb3 couples, respectively. Co has both good adhesion and low interfacial reaction rates with CoSb3 and Ag-39.9 at%Cu alloy, and it is concluded that Co is a good barrier layer at 450 °C for the CoSb3-based modules when Ag-Cu eutectic braze is used.