Thermal Emission beyond the Conventional Kirchhoff's Law

University Of Houston, Houston TX

Investigators

Abstract

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.