Interaction of PbTe with and its alloys at diffusion bonded interfaces: Temperature dependent evolution of phases and microstructure

Abstract

The past years have seen an increasing interest in high-efficient thermoelectric materials because of their promising application to harvest the widely distributed waste heat. Realizing high-efficient TE materials into actual devices remains a challenge. The suitability of thermoelectric material is judged by a dimensionless parameter called thermoelectric figure-of-merit, zT. The efficiency of a thermoelectric (TE) device depends on material parameters and, to a large extent, on joints/contacts these semiconducting materials form with metallic conductors for completing an electrical circuit. The maximum power output gets significantly reduced due to parasitic losses occurring at these metal-semiconductor junctions because of many interdependent factors. One of the critical factors is the chemical interaction of TE materials with the conducting connectors like Cu, Ag, or Al. The interaction of TE materials with these materials results in intermetallic phases that deteriorate the interface properties, leading to decreased bond strength and, in severe cases, mechanical detachment of the joints. Additionally, since these devices are intended to work at higher ranges of temperatures, unwarranted growth of phases formed during joining or during operation due to atomistic diffusion of elements significantly affects these joints' reliability. A diffusion barrier is inserted between the semiconductor and metal conduct/solder alloys to prevent this interaction. The diffusion barrier, which is in immediate contact with TE material, is called the contact material for that TE. PbTe is a state-of-the-art thermoelectric alloy for power generation in intermediate-temperature range applications (600-900K) whose maximum zT values have already reached 2.2-2.5. This thesis investigates the interaction between PbTe (TE material) and Ni (and its alloys) as contact material by conducting a structural and microstructural study of temperature-dependent phase evolution at diffusion bonded interfaces. The thesis is divided into seven chapters.