Abstract
Typical thermoelectric modules are made of a number of p- and n-type thermoelements which are jointed to copper conductors by soldering reaction and arranged in a square array between two ceramic plates. Contact resistance of conductors/thermoelements joints may become a serious limitation to the performance of thermoelectric modules with relatively short or thin-film type thermoelements. In this study we have investigated the impact of interfacial compounds on the contact resistivity of bismuth telluride/Cu soldered junctions. Two different solder alloys, Sn-4Ag-0.5Cu and Sn-37Pb, were used to joint both commercial p-type and n-type bismuth telluride onto thin Cu foils. SnTe and Pb1-xSnxTe were identified to be the major interfacial compounds for the Sn-4Ag-0.5Cu and Sn-37Pb soldered junctions, respectively. The contact resistivity measured is around 10-4 ~ 10-5 □cm2, which depends on both the thickness and composition of interfacial compounds. Sb and Cu element are observed to have a strong influence but in an opposite manner on growth kinetics of the interfacial compounds during soldering reaction. The mechanisms for Sb and Cu on SnTe growth were explored using a simple solder/Te reaction couple. Addition of Sb element in solder considerably enhances the growth rate of SnTe compound, e.g. 24µm/min for 1wt% Sb addition, that is rarely seen in typical soldering reactions. With increasing Sb content in Sn-Sb solder, the growth rate of SnTe may increase accompanied with the reduction in size of SnTe grains. It indicates that the effective channel area for Sn transport in compound may decrease with the consecutive Sb ejection into solder, and hence the rate-limiting step of SnTe formation will change from Sn/Te reaction to the transport of Sn through the compound layer. A dosage of 0.1 wt% Cu in Sn is capable of suppressing the vigorous Sn/Te reaction effectively by forming a thin CuTe at the solder/Te interface. The CuTe morphology changes from irregular clusters into a layered structure with increasing Cu content. With the same reaction time, the CuTe thickness increases proportionally to x1/2 in the Sn-xCu alloys, suggesting a diffusion-controlled growth for CuTe. The contact resistivity of bismuth telluride/Cu soldered junctions using Sn-0.1Cu alloy were measured to ~ 10-5 □cm2. After thermal treatment at 150 ºC for 168 hours, the growth of intermetallic compounds in Sn-0.1Cu/Te couples were still obviously suppressed, indicative of the superior barrier capability for CuTe compound. It is believed that Sn-0.1Cu alloy shall be a good candidate solder for assembly of telluride based thermoelectric modules under the consideration of electrical property and mechanical integrity. This study may be of importance in understanding the interfacial reaction between bismuth telluride and solder, as well as in providing the effective strategy to choose appropriate solder alloys for thermoelectric assembly.