Fundamental Studies of Near-field Enhancement in Thermionic Energy Conversion

2014年，美国消耗了超过97个四次方BTU（英国热量单位）的能源。这相当于35亿吨煤或7760亿加仑（美国）汽油的能量。然而，几乎59%的这种能源消耗作为废热损失。必须找到一种创新的方法，从废热源中回收能源，作为一种无排放和成本更低的能源。本研究的目的是探讨近场增强可再生能源回收利用中的电子辐射。传统的电子能量转换（TEC）通常需要超过1500 K的高阴极温度，以从阴极热激发足够的电子，克服其结合电势或功函数，用于发电。低效率是TEC发电的另一个挑战性问题。研究小组将通过实施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来解决这一挑战。电子发射的近场增强可以通过利用阴极中的电子的光激发来增强电子电流生成来降低所需的热发射器温度。此外，由于阴极吸收的大部分辐射将有利于热离子发射，因此能量转换效率将大幅提高，即，来自稍高于阴极带隙的光子能量的光激发以及来自过量光子能量和亚带隙光子能量的热化。该项目的成功将引发电子能量转换的范式转变，并将引发基于电子发射的新型能量回收技术的发展。该项目将促进培训和学习，让学生参与微/纳米纤维，纳米器件的热和红外表征，纳米级传热测量和纳米级仪器。此外，将开发一门新的课程，重点是纳米计量和实验，并向学生提供，以扩大纳米技术教育。研究小组亦会透过以下方法研究近场增强电子能量转换：（1）实验验证亚微米间隙距离内平面结构之间的近场热辐射增强;（2）建立近场增强电子能量转换的理论框架;（3）研究近场增强电子能量转换的方法。（3）实验研究了近场电子发射的增强。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发电子可以显着增强电子电流的产生。他们还将研究对电子发电至关重要的材料特性，包括空间电荷积累的间隙依赖性，功函数的降低以及低带隙半导体材料的高温稳定性。这项研究将提供，第一次，定量测量近场热辐射的两个宏板之间的子100纳米的间隙距离。该实验设计将使精确的平面-平面间隙控制与纳米分辨率和近场热辐射测量在高温超过1000 K。此外，该项目是首次尝试将联合收割机近场热光伏效应和光子效应结合到单一的能量转换过程中。研究团队将为近场热辐射增强光电子发射的可行性提供理论和实验背景。随着光电子隧穿的局部探测，将有可能更好地理解高温下的低带隙半导体材料的光电热行为，这是迄今为止还没有得到很好的理解。该项目的成果将为实现一种新的基于电子的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.