Fundamental Studies of Near-field Enhancement in Thermionic Energy Conversion

2014年，美国消耗了超过97万亿英热单位(BTU)的能源。这相当于35亿吨煤或7760亿加仑(美国)汽油的能源量。然而，近59%的能源消耗被浪费为余热。当务之急是找到一种创新的方式来回收来自废热来源的能源，作为一种零排放和成本更低的能源。拟议研究的目的是探索用于可再生能源回收的热离子发射的近场增强。传统的热离子能量转换(TEC)通常需要高于1500K的高阴极温度，才能从阴极热激发足够的电子，克服其束缚势或功函数，用于发电。低效率是TEC发电中另一个具有挑战性的问题。研究小组将实施一种低带隙半导体材料作为阴极，并将其放置在远离热发射器的亚波长距离内，以应对这一挑战。热电子发射的近场增强可以通过增强阴极中电子的光激发产生热离子电流来降低所需的热发射极温度。此外，由于阴极中吸收的大部分辐射将有利于热电子发射，即略高于阴极带隙的光子能量的光激发和过剩的光子能量和亚带隙光子能量的热化，因此能量转换效率将得到显著提高。该项目的成功将引发热离子能量转换的范式转变，并将推动基于热离子发射的新能源回收技术的发展。该项目将通过让学生参与微/纳米制造、纳米设备的热和红外表征、纳米尺度的热传递测量和纳米尺度的仪器来促进培训和学习。此外，还将开发一门以纳米计量和实验为重点的新课程，并向学生提供这门课程，以努力扩大纳米技术教育。研究小组还将向K-12学生进行外联，以促进年轻一代的科学学习。近场增强热离子能量转换将通过以下方式进行研究：(1)通过实验验证亚微米间隙内平面结构之间的近场热辐射增强；(2)建立近场增强热离子能量转换的理论框架；以及(3)实验研究近场热离子发射的增强。研究人员将彻底测试这一假设，即由于吸收的近场热辐射而产生的光激电子可以显著增强热离子电流的产生。他们还将研究对热离子发电至关重要的材料性质，包括空间电荷积累的间隙依赖性，功函数的减少，以及低带隙半导体材料的高温稳定性。这项研究将首次定量测量小于100 nm间距的两块宏板之间的近场热辐射。该实验设计将实现纳米分辨率的精密平面-平面间隙控制和1000K以上高温下的近场热辐射测量。此外，该项目是首次尝试将近场热光电效应和热离子效应结合到单一的能量转换过程中。该研究小组将为利用近场热辐射增强光热发射的可行性提供理论和实验背景。随着光热电子隧穿的局域探测，将有可能更好地理解低禁带半导体材料在高温下的光热电学行为，这一点到目前为止还没有得到很好的理解。该项目的成果将为实现一种基于热离子的新型能源回收技术奠定基础。

In 2014, the Unites States consumed more than 97 quadrillion BTU (British thermal units) of energy. This is equivalent to the amount of energy in 3.5 billion tons of coal or 776 billion gallons (US) of gasoline. However, almost 59% of such energy consumption is being lost as waste heat. It is imperative to find an innovative way of recycling energy from a waste heat source as an emission-free and less-costly energy resource. The objective of the proposed research is to explore the near-field enhancement of thermionic emission for renewable energy recycling. Conventional thermionic energy conversion (TEC) generally requires a high cathode temperature over 1500K to thermally excite enough electrons from the cathode overcoming its binding potential, or work function, for power generation. Low efficiency is another challenging issue in TEC power generation. The research team will address this challenge by implementing a low bandgap semiconducting material as a cathode and placing it in a subwavelength distance away from a thermal emitter. The near-field enhancement of thermionic emission can reduce the required thermal emitter temperature by enhancing thermionic current generation with photoexcitation of electrons in the cathode. In addition, the energy conversion efficiency will be substantially improved because the most radiation absorbed in the cathode will benefit thermionic emission, i.e., photoexcitation from the photon energy slightly above the cathode bandgap and thermalization from the excess photon energy and sub-bandgap photon energy. The success of this project will induce a paradigm shift in thermionic energy conversion, and will spark the development of novel energy recycling technologies based on thermionic emission. The project will promote training and learning by involving students in micro/ nanofabrication, thermal and infrared characterization of nanodevices, nanoscale heat-transfer measurements, and nanoscale instrumentations. In addition, a new course focuses on Nanoscale Metrology and Experimentation will be developed and offered to students in an effort to broaden nanotechnology education. The research team will also pursue outreach to K-12 students to promote scientific learning in younger generations.Near-field enhanced thermionic energy conversion will be examined by (1) experimentally validating the enhancement of near-field thermal radiation between plane structures within sub-micron gap distances; (2) establishing a theoretical framework for the near-field enhanced thermionic energy conversion; and (3) experimentally investigating the enhancement of thermionic emission in the near field. The researchers will thoroughly test the hypothesis that photoexcited electrons due to the absorbed near-field thermal radiation can significantly enhance thermionic current generation. They will also investigate the material properties critical for thermionic power generation, including the gap-dependence of space charge buildup, the reduction of the work function, and the high-temperature stability of low-bandgap semiconducting materials. This research will provide, for the first time, quantitative measurements of near-field thermal radiation between two macro plates within sub-100 nm gap distances. The experimental design will enable the precision plane-plane gap control with a nanometer resolution and near-field thermal radiation measurement at high temperature over 1000K. In addition, this project is the first attempt to combine near-field thermophotovoltaic and thermionic effects into a single energy conversion process. The research team will provide theoretical and experimental background for the feasibility of enhancing photo-thermionic emission with near-field thermal radiation. With the local probing of photo-thermionic electron tunneling, it will be possible to better understand the photothermoelectric behaviors of low-bandgap semiconductor materials at high temperatures, which have not been well understood to date. The accomplishments of this project will constitute a fundamental stepping-stone for the realization of a novel, thermionic-based energy recycling technology.