Breakthroughs in Thermoelectric Energy Harvesting Devices by Silicon Nanowires

非技术：热电能量收集装置从废热中产生有用的电能。这种装置是可再生能源的诱人来源，因为它们具有可靠的固态运行，可以扩展到更大的区域。然而，基于传统材料的器件的能量转换效率低、成本高一直是商业化的技术瓶颈。在这里，PI将研究一种基于硅纳米线的节能且经济有效的解决方案。热电器件的两个创新领域将成为目标。首先，具有工程粗糙度的硅纳米线将用于热电器件的热侧，以有效的方式将热转换为电。PI将在回收废热所需的高温下测量单个纳米线和纳米线阵列结构。其次，金属涂层硅纳米线阵列将用作冷侧的选择性发射器，以最大限度地提高辐射冷却。基于纳米线的辐射冷却表面将在热电器件中提供可观的温度梯度，而无需依赖于主动冷却器或昂贵的热交换器。这项研究的成果将加强美国可再生能源生产和废热回收领域的发展。这项研究将通过学生辅导和广泛的外联努力与教育活动相结合。PI的综合教育计划将激发高中生的科学兴趣，并激励本科生和研究生在材料科学和器件工程的交叉领域推进他们的兴趣。技术：该项目旨在通过利用PI独特的热测量和通过金属辅助化学蚀刻控制硅纳米线的粗糙度，回答纳米线表面边界如何在宽温度范围（10-700 K）内影响单个纳米线的热电性能。虽然纳米线的热电特性已经得到了广泛的研究，但我们对纳米尺度上的基本决定因素和与热电能量收集相关的温度范围内的特性的了解仍然存在空白，或者在丰富的低、中等热源可用的地方。该项目将研究人工周期性对粗糙度控制纳米线的影响，并通过理论、数值和实验方法确定热导率降低机制。该项目还试图回答人工周期性如何导致天然材料无法实现的选择性发射特性，并通过开发基于纳米线的选择性发射器为辐射冷却提供新的途径。基于严格耦合波分析的光谱发射率计算将指导纳米线阵列的设计，并在近红外保持低发射率，这将最大限度地减少太阳吸收，而在中红外保持高发射率，这将最大化大气冷却。基于反射率和傅里叶变换红外光谱的光学测量将揭示纳米线系统中人工光谱控制的局限性，并导致环境中辐射冷却的预测模型。所提出的研究将建立对单个纳米线中粗糙度的温度依赖输运现象的新理解，以及对纳米线阵列中间距和涂层尺寸的光谱发射率变化的新理解，这将最终阐明人工周期性如何与热、电和光相互作用。在硅纳米线的合成和加工方法已经建立的基础上，本项目将探索其作为热电材料和表面冷却材料的新型声子和光子特性，以推进热电器件的突破。本项目的成果将进一步推动基于纳米材料的能量收集装置的研究进展。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

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.