Fundamental Studies of Near-field Enhancement in Thermionic Energy Conversion

2014年，美国消耗了超过97千万亿英热单位（BTU）的能源。这相当于35亿吨煤或7760亿加仑（美制）汽油的能量。然而，近59%的能源消耗作为废热损失掉了。寻找一种创新的方式从废热源中回收能源，使其成为一种无排放和低成本的能源，是势在必行的。本研究的目的是探索可再生能源回收中热离子发射的近场增强。传统的热电子能量转换（TEC）通常需要超过1500K的高阴极温度才能从阴极激发足够的电子，以克服其束缚势或功函数，用于发电。低效率是TEC发电的另一个挑战。研究小组将通过实现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。热离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，由于阴极吸收的大部分辐射将有利于热离子发射，即略高于阴极带隙的光子能量的光激发和多余光子能量和亚带隙光子能量的热化，能量转换效率将大大提高。该项目的成功将引发热离子能量转换的范式转变，并将激发基于热离子发射的新型能量回收技术的发展。该项目将通过让学生参与微/纳米制造、纳米器件的热学和红外表征、纳米尺度传热测量和纳米尺度仪器来促进培训和学习。此外，将开发一门新的课程，重点是纳米计量和实验，并提供给学生，以扩大纳米技术教育。研究小组还将继续向K-12学生推广，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(1)实验验证在亚微米间隙距离内平面结构之间的近场热辐射增强；(2)建立了近场增强热离子能量转换的理论框架；(3)近场热离子发射增强的实验研究。研究人员将彻底验证由于吸收的近场热辐射而产生的光激发电子可以显著增强热离子电流产生的假设。他们还将研究热离子发电的关键材料特性，包括空间电荷积累的间隙依赖性，功函数的减小以及低带隙半导体材料的高温稳定性。该研究将首次提供两个宏观板之间近场热辐射的定量测量，距离小于100 nm。该实验设计将实现纳米分辨率的平面-平面间隙精确控制和1000K以上高温近场热辐射测量。此外，本项目首次尝试将近场热光伏效应和热离子效应结合为单一的能量转换过程。研究团队将为利用近场热辐射增强光热离子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部探测，将有可能更好地理解低带隙半导体材料在高温下的光热电行为，这是迄今为止尚未得到很好理解的。这个项目的成就将为实现一种新颖的、基于热子学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.