Development of high-efficiency and high-reliability thermoelectric devices for power generation

Abstract

Thermoelectric (TE) power generation has become popular for application in heat pumps for localized environmental control or waste heat recovery systems to convert heat into electricity. In particular, TE modules are appealing for waste heat recovery because they have no moving parts, require no maintenance, and are scalable in size. Therefore, many researchers have studied TE materials operating at intermediate temperatures (300 – 600$^o$C) to utilize them in steel works, vehicles, and other waste heat sites. The dimensionless figure of merit (ZT) of TE materials has been significantly increased by more than 2, which has resulted in an increasing demand for commercial TE modules with high efficiency. However, these developments in TE materials did not result in the improvement of TE modules or devices because of the lack of fabrication techniques, mechanical reliability, and oxidation or diffusion problems that can occur at intermediate temperatures. For TE generators to be successful, further research at the device level should be conducted and not only for the development of TE materials. This dissertation focuses on the fabrication of TE modules operating at intermediate temperatures as well as device testing. Several techniques such as sintering TE materials, metallization, soldering, and brazing were developed to fabricate high-efficiency and high-reliability TE modules. In addition, for the characterization of TE modules, measurement systems for electrical contact resistance, power output, efficiency, and thermal cycling tests were developed. In this dissertation, we report on a high-efficiency and high-reliability TE module using skutterudite (SKD) and various metallization layers. A series of new technical breakthroughs are reported herein including TE material co-sintering using a spark-plasma sintering system (SPS), a reduction technique of electrical/thermal contact resistance, the fabrication of a metallization layer using indium tin oxide (ITO), and process integration to realize a TE module operating at intermediate temperatures. A TE module with a high power density (2.02 W/cm$^2$) and high efficiency (7.82 %) was fabricated using a metal alloy. The reliability problem was solved by ITO metallization, which can extremely retard the diffusion of elements at the interface. Commercial filled SKD samples ((Mm,Sm)$_y$Co$_4$Sb$_12$ for n-type, DD$_y$Fe$_3$CoSb$_12$ for p-type) were used as TE materials because they exhibit relatively high mechanical strength, thermal stability and low thermal conductivity while maintaining a relatively high power factor. The ZTs of both types of SKD were approximately 0.8 at 500 $^o$C, which was not optimized because we aimed to develop a module manufacturing method using universal materials. Both types of SKD were sintered and metallized simultaneously using the SPS system. Ti, Fe–Ni, and Co–Mo metallization layers were deposited on the SKD as metal-based materials. These metallization layers were effective in minimizing the electrical contact resistance by forming intermetallic compounds (IMCs) at the interfaces. This contributed to the high power density and high efficiency of the module despite the low performance of SKD materials. To improve the reliability of the TE module, we applied ITO to the SKD as a diffusion barrier. The metallization layer was generally developed using metal-based materials with high electrical conductivity. Oxide-based materials are typically not used as metallization layers because they are considered to have high electrical resistivity and form weak bonds with TE materials. However, metal-based layers were limited in preventing elemental diffusion, which generates IMCs at the operating temperature. To address the reliability problems, we proposed a Ti/ITO/SKD structure, and its reliability was demonstrated via thermal aging of the TE leg and thermal cycling of the TE module. The growth of the IMC layer was significantly suppressed in the Ti/ITO/SKD structure before and after thermal aging. In the ITO layer, Ti formed Ti−O bonds which prevented elemental diffusion because the ITO layer was formed using SPS at a high temperature and pressure that favored high reactivity between Ti and oxygen. The electrical contact resistance was also maintained at a low level for the SKD modules. Consequently, the ITO layer functioned as a diffusion barrier and a good electrical contact at the interface. The eight-couple TE module was fabricated using Ti/ITO/SKD-structured TE legs. The performance of the TE module was observed to be lower than that under ideal conditions, which is attributable to additional wiring and thermal contact loss in the module. The maximum power density was recorded to be 1W/cm$^2$ at 500 ℃, indicating the potential for the enhanced performance of the module. The reliability of this module was demonstrated using a thermal cycling test. The power output was maintained even after 1000 thermal cycles. This study demonstrated that a high-reliability SKD module can be achieved using ITO, which can suppress the elemental diffusion while maintaining the electrical contact resistance. These research methods are expected to be effective approaches for further improving the performance of TE modules operating at intermediate temperatures, resulting in practical harvesting devices for waste heat recovery in the industry.