Dissertation Research Published in July 2019 Issue of Advanced Functional Materials

Naiming Liu’s paper, Improved Thermoelectric Performance of Eco-Friendly β-FeSi2–SiGe Nanocomposite via Synergistic Hierarchical Structuring, Phase Percolation, and Selective Doping, was published in July 2019 in Advanced Functional Materials, a high-impact journal in materials science and engineering. Liu earned his Ph.D. from the University of Virginia School of Engineering in April 2019, working with professors Jerrold Floro and Mona Zebarjadi in the Department of Materials Science and Engineering. Floro and Zebarjadi are among the paper’s co-authors.

The paper relays findings from Liu’s dissertation research with thermoelectric materials, which directly convert heat to electricity. In order to be commercially viable for harvesting waste heat to produce useful electricity, thermoelectric materials must be composed of materials that are abundant, inexpensive and non-toxic. A generous gift from the II-VI Foundation supported his research.

Liu set out to improve upon the “old” thermoelectric materials system, based on iron disilicide (FeSi2), which met these criteria, but suffered from an overall poor performance. Liu’s novel process enhanced overall electron mobility while maintaining the low thermal conductivity needed to improve the performance. As a result, this material achieved a world-record performance level for FeSi2-based thermoelectrics.

Here’s how he did it: Liu took advantage of a eutectoid transformation, in which a new structure is formed within a material, such as steel, with a precise combination of elements at a specific degree of temperature, to create a two-phase nanocomposite. He then added small amounts of chemical components to trigger an electronic reaction that sped up the eutectoid transformation, allowing metallic powder to form a coherent mass by heating without melting. He also developed a highly efficient powder process using ball milling and spark plasma sintering that reduced overall process times by an order of magnitude. 

While further performance enhancements are needed for commercial adoption, the efficient processing and low-cost materials have now made this a material of interest for thermoelectric technologies.