This proposal is concerned with recovering exhaust waste heat to improve the efficiency of vehicles. An interdisciplinary team of mechanical engineers, a material scientist, and a chemist is assembled to tackle this problem by synthesizing novel nanostructured silicides with improved thermoelectric figures of merit.
Intellectual Merit: The proposed research on nanocomposites and related interfaces will help establish a scientific and engineering foundation for robust and flexible thermomechanical design of thermoelectric systems subjected to thermal cycling and vibration. The PIs will develop metal-matrix composites with tailorable coefficients of thermal exapansion (CTEs) using a filler material with an isotropic negative thermal expansion (NTE). NTE fillers, such as ZrW2O8 powders, can effectively offset the high CTE of the metal phase without significantly degrading the electrical and thermal conductivity. These nanocomposites may be used as electrodes or as interfacial layers for creating segmented thermoeletric elements. Thermomechanical models of TE modules will be developed to guide the design of the nanocomposites, including those with functionally graded compositions. Nanocomposites based on different metals, such as Ag, Al, and Ni, will be synthesized and their thermal and thermomechanical properties will be experimentally characterized. Methods to produce reliable electrical, thermal, and mechanical bonds with TE materials will also be developed.
Broader Impact: The proposed research is designed to maximize the potential for large-scale implementation in vehicle exhaust heat recovery and other related engineering applications. The new material synthesis and bonding methods that will be developed is considered to facilitate economic fabrication of durable TE devices. Another component included in the plan is the training of students in interdisciplinary research whereby fundamental science is coupled to advanced engineering. The PI and graduate students will participate in outreach programs to bring science and engineering to pupils in grades K-12 through summer research programs hosted at UCLA.
Thermomechanical reliability presents a serious challenge to thermoelectric and other solid-state modules being developed for waste heat harvesting and a variety of essential modern devices that dissipate significant amount of heat, including high-performance semiconductor electronic and optoelectronic devices. To address this challenge, the project aimed to develop nano-composites whose coefficients of thermal expansion (CTE) can be tailored to match those of the relevant thermoelectric materials, in particular silicides and other low-cost TE materials. These nanocomposites will be used as electrical contacts/interconnects in TE modules. Liquid-based flexible thermal interfaces to further improve the thermomechanical reliability were also to be explored. The project team successfully developed metal-matrix nano-composites with tailorable CTEs and characterized their thermo-mechanical/transport properties. Nano-composites were developed with silver or copper as the base matrix and ZrW2O8 particles with isotropic negative thermal expansion coefficients as fillers. The experimentally measured CTE values shown below agreed well with the predictions from the rule of mixtures and demonstrated ability to tune CTEs by tailoring the composition. The composites show excellent stability as measurements of CTE using the same samples after weeks of storage in air did not change. Both the thermal and electrical conductivities decrease steadily with increasing amount of ceramic oxide (ZrW2O8) fillers. However, the conductivities of Cu-based samples containing 40+ vol% ZrW2O8 are still >40% those of bulk Cu and comparable to those of aluminum. The project team has also developed liquid-based flexible thermal interfaces that can provide small thermal contact resistances at low loading pressures and associated theoretical models to help systematically design such interfaces. These interfaces are expected to find usage in various energy conversion applications, including pyroelectric waste heat harvesting and solid-state refrigeration. The project allowed rigorous education and training of our undergraduate and graduate students to help them contribute to solving scientific and engineering challenges facing the nation, in particular in the areas of energy efficiency through development of novel materials. As part of the outreach program, the project also specifically targeted high school students from minority and underrepresented groups through our dedicated summer research programs to expand our STEM workforce. The research outcomes have widely been disseminated to the scientific and engineering community through publication in major archival journals and presentations at technical conferences/workshops.
