This project involves researchers from Ohio State University, Northwestern University, and Virginia Polytechnic Institute and State University, with input from industries. Working together, the researchers hope to solve major scientific barriers to commercializing thermoelectric waste heat recovery technology. The goal of project is the creation of a viable system to convert automotive waste heat into usable electrical power using thermoelectric (TE) devices.
Intellectual Merit: The research proposed here will advance work in TE by focusing on five key elements. Materials research (led by OSU and NU) will develop advanced TE materials made from earth-abundant, geographically dispersed elements and compounds, specifically PbSe and Mg2Si-Mg2Sn. Thermal management system design (led by BSST) will create new thermal designs to minimize losses by minimizing the number of interfaces, minimizing the amount of TE material used; these designs will maximize the durability of the product. Work on interfaces, led by VPI&SU and ZTPlus, will focus on the metallization of the TE materials and device interconnection and the flexible bonding of the metallized elements to the heat spreaders to increase durability and reduce device level performance losses. The team will expand capabilities in metrology to measure electrical and thermal contact resistances, and develop a comprehensive and redundant measurement loop system with self-consistent error checking. Durability will be the inherent design criterion in every invention.
This project has the potential to transform progress in TE materials. We will improve the fundamental understanding of the effect of resonant levels on the transport properties of solids, and make it applicable to large classes of semiconductors. The development of matrix encapsulation techniques for Mg2X will expand the repertoire of creative solid-state chemistry approaches in creating nanostructured thermoelectrics. New strong and flexible high-temperature bonding techniques will impact the assembly of semiconductor die. The project will advance understanding on the efficiency of TE generators, TE material durability at high temperatures, and cycle life durability of TE materials, all of which are critical to successful commercialization.
Broader Impacts: This project will create potentially transformative research that promises to save up to 800,000 barrels of oil daily and reduce carbon emissions. Results of the research will be incorporated into classes taught by project investigators in the physics of transport phenomena, materials synthesis and electronic component assembly. The academic PI's will also integrate this research into participation in multidisciplinary collaborative groups. The significance of energy efficiency and usage that this research addresses will be integrated into the well established outreach programs at all three universities. Involvement of corporate partners ensures large scale commercialization, as BSST is the world leader in commercial applications of TE's in automotive and other key industries.
The electrical and thermal contact resistances between thermoelectric devices and their heat-sink substrate are major causes of degradation in device performance. An objective of this project is to engineer a device/heat-sink interface with low electrical/thermal contact resistance and high thermomechanical reliability. Joints formed by sintering silver between devices and heat sink have been shown to be promising interface metallurgy for assembling thermoelectric devices. In this project, we focused on developing a material formulation based on nanoparticles of silver to achieve low-temperature, pressure-free joining of thermoelectric components. We built an analytical model to visualize the effects of heating profile on bond-line defect formation and carried out experiments to verify the simulation results. Our modeling results enabled us to optimize a nanosilver paste formulation which only requires a simple heating profile – pressure-free and under 250°C – for bonding small- as well as large-area thermoelectric devices. The sintered nanosilver joints have uniform microstructure without cracks/voids and delamination, which gives rise to highly conductive and reliable interface quality. Figure 1 shows a comparison of the sintered nanosilver and traditional soldered joints. The electrical and thermal properties of the sintered nanosilver joints were found to be significantly better than those of the lead-free soldered joints. For comparison, Table I lists the properties of both types of joints. The sintered silver joints were also found to be more reliable than the soldered joints. The bonding strength of the sintered silver joint decreases less than 20% after 1000 temperature cycles, while the soldered joint would completely fail after the same number of temperature cycles.
