A Fluctuational Electrodynamic Analysis Of Micro/Nano Scale Radiative Transfer Of Surfaces

With the development of Micro Electromechanical System (MEMS), nanotechnology and bioscience in recent years, heat transfer is facing a new challenge. At microscale or nanoscale, the radiative properties and radiative transfer performance are extremely different from that at macroscale. The classical research methods can not explain many phenomena at microscale or at nanoscale, such as photon tunneling, coherence, and etc. As the thermal radiation is a kind of electromagnetic wave, the wave characteristics can be interpreted by the electromagnetic theory, which can also give the reasonable illustration of the macroscale radiative transfer.In this paper, the radiative properties and the radiative transfer performance was analyzed, which based on the electromagnetism. We used the Fluctuating-Dissipating Theorem to describe the electromagnetic field induced by the thermal movement of the molecules or the electrons in the medium. Then we used the Green Function Method to solve the Maxwell Equations, which can depict the wave effect of radiation. That is what we used to investigate the laws of evanescent wave.The expression of energy density above a semi-infinite plate and that of the radiative heat flux in the direction vertical to the surface were derived, as well as the radiative heat flux between two parallel semi-infinite plates. The energy of the evanescent wave dissipated in dissipative materials was deduced.The following computation and analysis about the microscale/nanoscale radiative heat transfer were carried out.1. As for the radiative heat transfer between two plates, the propagating wave is coherent when the characteristic wave length is more or less the same to the distance between the two plates. This property leads to the fluctuation of the radiative heat flux as the distance increases.2. As for the radiative spectral properties at microscale/nanoscale, the radiation of SiC, at some angular frequency, shows sharp peak, which increases as the distance goes down and enhances the heat flux and the energy density by several orders. This phenomenon, also named monochromatic effect, was not found when the material is aluminum. The reason is that the real part of the electric permittivity of SiC at this frequency is about 1, while the imaginary part is not very large, and then the surface phonon polarization leads to the resonance of the surface wave.3. The contributions of the propagating/evanescent wave at p/s polarizations to the total energy density and to the total radiative heat flux were compared. For SiC at nanoscale or microscale, the proportion of the s polarization wave can be ignored and that of the evanescent wave at p polarization is much larger than the other parts. This is because of the material properties and the boundary condition of the evanescent wave at the interface.