Contacts and Interface Evolution in Thermoelectric Modules

Abstract

As human civilization faces global warming as the biggest existential threat it has ever faced, harnessing waste heat is a sustainable, renewable source of energy to reduce our dependence on fossil fuels. By directly converting heat into electricity, thermoelectricity offers solutions for waste heat recovery. In the past, very stable and high-performance telluride based thermoelectric generators (TEG) have been used for extraterrestrial missions to provide the energy needed for probing the dark deep space. In the recent years, the interest in application of this technology to automobiles, factories, power plants, military and households has increased. There has been great advancement in discovery of new and smart materials, exhibiting high thermoelectric (TE) performance, but the transition towards their commercialization is not smooth. While many new materials, such as Zn4Sb3, Mg2Si, SnSe, skutterudites, Cu2Se, etc., have been reported to show very high thermoelectric performance, the application of these materials in commercial TEGs is still hindered. One of the key challenges is to form stable and low resistant contacts for intensive working conditions. In this thesis, different joining methods were developed to make contacts between various TE materials and metallic electrodes. Interfacial microstructure evolution was investigated to study the contact stability and degradation mechanism. ZnSb was first chosen because it is non-toxic, low-cost, abundant and light weight thermoelectric material. The successful method developed for ZnSb is then extended to CoSb3 based Skutterudites, which are among the highest performing TE materials in the temperature range (300 ᴼC-600 ᴼC). In the first part, conventional joining method using soldering alloys was investigated. Low-cost high performance ZnSb material was chosen to bond with different metal electrodes such as Ag, Ni and Crofer 22 APU using some commercially available Zn-based solders. The joints were tested for long time at high temperature and the interfacial microstructure and chemical composition were observed using scanning electron microscope equipped with energy dispersive x-ray spectrometer (EDS). It was found that both Ag and Ni electrodes reacted with Zn-based solders and formed a thick diffusion layer comprising of different intermetallic phases. Furthermore, Zn was found to deeply diffuse into ZnSb TE leg, resulting in change of material composition. Introduction of thick films of Ti and Cr as buffer layers could not stop this diffusion. In case of Crofer 22 APU, the reaction layer was found to be minor; suggesting that Crofer 22 APU was a good electrode to be used with Zn-based solders. A novel solder-free joining method using microlayers of Ti and Cr as interconnecting agents was then developed and demonstrated on the ZnSb TE system. It was found that, using microlayers of Ti and Cr as interconnecting agents, a very good interfacial contact was obtained without any gaps or cracks. Interestingly, the starting composition of ZnSb legs was also preserved. The interfacial contact of ZnSb/Cr/Ni was found to be stable after heat treatment at 400 ᴼC for 30 hours, suggesting solder free joining as an effective method for reliable contacts in TE devices in the medium temperature range (200 ᴼC-400 ᴼC). The solder free joining method was further developed and applied to different high-performance materials in higher temperature range. A very stable n-type skutterudite material was chosen to join with Crofer 22 APU electrode. The joint was tested at 550 ᴼC for 300 hours and the interfacial microstructure was studied. Cr and Co based interconnecting layers were systematically investigated and it was found that the Cr/Co multilayer made the best contact.