Thermal Emission beyond the Conventional Kirchhoff's Law

热辐射在我们周围的物体之间不断交换。表征物体发射和接收热辐射能力的两个基本性质分别是发射率和吸收率。基尔霍夫热辐射定律，由基尔霍夫于1860年提出，被认为是热辐射定律中的第一定律。基尔霍夫定律建立了吸收率和发射率之间的基本等效性，150多年来，它已被广泛用于测量和计算热发射器的发射率。然而，基尔霍夫定律的有效性并不是热力学的要求，它在非互易热发射体中失效。尽管非互易热辐射体在太阳能、热整流和循环等方面具有巨大的优势，但人们对非互易热辐射的认识还很缺乏。这个项目的目的是证明一个新的理论适用于所有的热辐射源，包括非互易的。我们将该理论称为广义基尔霍夫定律。通过该项目，将在休斯顿大学建立一个综合研究/教育方案，为代表性不足的群体创造新的职业机会，并让少数民族学生和女学生担任本科生和研究生研究助理。PI将与休斯顿儿童博物馆合作，制作与热科学相关的教育视频。通过对称性分析，热发射器中的热发射过程可以直接连接到发射器伴随系统的吸收过程。因此，这个项目的目标是开发一个新的广义基尔霍夫定律的基础上被忽视的连接。为此，该项目将通过数值模拟和实验测量直接证明传统基尔霍夫定律的崩溃。同样的程序将被应用于展示新的广义基尔霍夫定律的有效性。关于基尔霍夫定律的有效性是否从根本上得到热力学的支持，存在着长期的争论。该项目的结果将阐明这一基本问题。该项目的另一个优点是建立了直接模拟非互易热辐射的理论建模方法，为理解远场和近场非互易热辐射传热提供了一个连贯的理论框架。最后，新的广义基尔霍夫定律将为研究非互易热辐射提供缺少的计量技术。该计划将为理解热辐射提供一个新的范例，超越传统基尔霍夫定律的描述，在热辐射领域创造一个新的前沿。该奖项反映了NSF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。

Thermal radiation is constantly exchanged between objects around us. The two fundamental properties to characterize the ability of an object to emit and receive thermal radiation is the emissivity and absorptivity, respectively. Kirchhoff’s law of thermal radiation, proposed by Kirchhoff in 1860, is considered to be the first of the laws of thermal radiation. Kirchhoff’s law establishes the fundamental equivalence between absorptivity and emissivity, and for over 150 years, it has been widely used in measuring and computing emissivity of thermal emitters. However, the validity of Kirchhoff’s law is not a requirement of thermodynamics, and it breaks down for nonreciprocal thermal emitters. Despite the great benefits of nonreciprocal thermal emitters in solar energy, heat rectification and circulation, the understanding of nonreciprocal thermal radiation is lacking. This project aims to demonstrate a new theory valid for all thermal emitters, including nonreciprocal ones. We refer to this theory as the generalized Kirchhoff’s law. Through the project, an integrated research/education program will be established at the University of Houston by creating new career opportunities for underrepresented groups and involving minority and female students as undergraduate and graduate research assistants. The PI will partner with Children's Museum Houston to create educational videos related to thermal science.From symmetry analysis, thermal emission processes in a thermal emitter could be directly connected to the absorption process of the adjoint system of the emitter. The goal of this project is therefore to develop a new generalized Kirchhoff’s law based on this overlooked connection. To do so, the project will directly demonstrate the breakdown of conventional Kirchhoff’s law through numerical modeling and experimental measurement. The same procedure will be applied to showcase the validity of the new generalized Kirchhoff’s law. There is a long-standing debate on whether the validity of Kirchhoff’s law is fundamentally backed up by thermodynamics or not. The result of the project will shed light on this fundamental question. Another merit of the project is establishing a theoretical modeling method for directly simulating nonreciprocal thermal radiation, providing a coherent theoretical framework to understand nonreciprocal thermal radiative heat transfer in far and near fields. Lastly, the new generalized Kirchhoff’s law will provide the missing metrology technique for studying nonreciprocal thermal radiation. The program will provide a new paradigm for understanding thermal radiation that is beyond the description of traditional Kirchhoff’s law, creating a new frontier in the thermal radiation field.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.