Breakthroughs in Thermoelectric Energy Harvesting Devices by Silicon Nanowires

非技术性：热电能量收集设备从废热中产生有用的电能。这类设备是有吸引力的可再生能源，因为它们具有可靠的固态操作，并且可以大规模扩展。然而，基于传统材料的器件的低能量转换效率和高成本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和成本效益高的解决方案。热电设备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被用于热电设备的热端，以高效地将热能转换为电能。PI将在回收废热所需的高温下测量单独的纳米线和纳米线阵列结构。第二，金属涂层硅纳米线阵列将用于冷端作为选择性发射器，以最大限度地提高辐射冷却能力。基于纳米线的辐射冷却表面将在热电设备中提供相当大的温度梯度，而不依赖主动冷却器或昂贵的热交换器。这项研究的结果将加强美国的可再生能源生产和余热回收领域。这项研究将通过学生指导和广泛的外展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设备工程的交叉点上提高他们的兴趣。技术：该项目旨在通过利用PI独特的热计量和通过金属辅助化学腐蚀控制硅纳米线的粗糙度，回答纳米线表面边界如何在广泛的温度范围(10-700K)影响单个纳米线的热电性能。虽然人们对纳米线的热电性质进行了广泛的研究，但我们对纳米尺度的基本决定因素以及与热电能量收集相关的温度范围内的性质的知识仍然存在空白，或者在有丰富的中低等级热源的情况下。该项目将通过理论、数值和实验方法研究人工周期对粗糙度控制纳米线的影响，并确定热导率降低的机理。该项目还试图回答人工周期性如何导致自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来提供一种辐射冷却的新途径。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，并在近红外保持较低的发射率，这将使太阳吸收最小化，同时在中红外保持较高的发射率，这将使大气降温最大化。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。拟议的研究将建立对单个纳米线中粗糙度与温度相关的输运现象的新理解，以及对光谱发射率随间距和纳米线阵列中涂层尺寸的变化的新理解，这将最终阐明人工周期如何与热、电和光相互作用。在硅纳米线的合成和加工方法已经成熟的同时，该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，以推动热电设备的突破。该项目的成果将进一步推动基于纳米材料的能量收集设备的研究进展。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势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.