Breakthroughs in Thermoelectric Energy Harvesting Devices by Silicon Nanowires

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Non-technical: Thermoelectric energy harvesting devices create useful electrical energy from waste heat. Such devices are attractive sources of renewable power as they have reliable solid-state operation and can be scaled to large areas. However, low energy conversion efficiency and high cost in devices based on conventional materials have been technological bottlenecks to commercialization. Here, the PI will investigate an energy-efficient and cost-effective solution to these problems that is based on silicon nanowires. Two areas of innovation in thermoelectric devices will be targeted. First, silicon nanowires with engineered roughness will be used on the hot side of thermoelectric devices to convert heat to electricity in an efficient manner. The PI will measure individual nanowires and nanowire array structures at the high temperatures needed to recover waste heat. Second, metal-coated silicon nanowire arrays will be used on the cold side as a selective emitter to maximize radiative cooling. The nanowire-based radiative cooling surface will provide a substantial temperature gradient in the thermoelectric device without relying on active coolers or costly heat exchangers. The outcomes of this research will strengthen the areas of renewable energy production and waste heat recovery in the United States. The research will be integrated with educational activities through student mentoring and broad outreach efforts. The PI's integrated education plan will spark scientific interest of high school students and inspire undergraduate and graduate students to advance their interests at the intersection of materials science and device engineering.Technical: The project seeks to answer how the nanowire surface boundary influences thermoelectric properties of individual nanowires over a wide range of temperature (10-700 K) by utilizing the PI's unique thermal metrology and controlling the roughness in silicon nanowires via metal-assisted chemical etching. While nanowire thermoelectric properties have been extensively studied, there remain gaps in our knowledge of fundamental determinants at the nanoscale and the properties at temperature ranges that are relevant to thermoelectric energy harvesting or where abundant low-and mid-grade heat sources are available. The project will investigate the effects of artificial periodicities in the roughness-controlled nanowires and identify thermal conductivity reduction mechanisms through theoretical, numerical, and experimental approaches. The project also seeks to answer how artificial periodicities lead to selective emission properties that are unachievable by natural materials and present a new pathway for radiative cooling by developing a nanowire-based selective emitter. Spectral emissivity computations based on rigorous coupled wave analysis will guide designs of nanowire arrays and keep the emissivity low in the near-infrared, which will minimize solar absorption, while keeping the emissivity high in the mid-infrared, which will maximize atmospheric cooling. Optical measurements based on reflectance and Fourier transform infrared spectroscopy will reveal the limits of artificial spectral control in nanowire systems and lead to a predictive model for radiative cooling in the ambient environment. The proposed investigations will establish new understanding of temperature-dependent transport phenomena with respect to roughness in individual nanowires and new understanding of spectral emissivity variations with respect to pitch and coating dimensions in nanowire arrays, which will ultimately elucidate how artificial periodicities interact with heat, electricity, and light. While synthesis and processing methods of silicon nanowires are well established, the project will explore their novel phononic and photonic properties as the thermoelectric material and as the surface cooling material to advance breakthroughs in thermoelectric devices. The outcomes of this project will further promote the progress of nanomaterial-based energy harvesting device research.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.