Research On The Control Methods Of Infrared Thermal Radiation Characteristics Of Microstructured Surfaces

In recent decades, with the rapid development of electronic devices and equipments, a series of micro/nano-scale heat scientific problems have been put forward. The basic phenomena of micro/nano-scale heat radiation have been found in various applications, such as electronics cooling, infrared thermal radiator, the efficient use of renewable energy and spacecraft thermal control and so on. It is very important to understand, control and utilize the energy transfer process by micro/nano-scale heat radiation. In the past decades of continual development, the researchers have summarized a series of coupling mechanisms of between micro-structure and electromagnetic wave, which have greatly promoted the development of microscaled thermal control technology. These mechanisms mainly involve surface plasmon polaritons (SPPs), surface phonon polaritons (SPhPs), microcavity resonant effect, Fabry-Perot (FP) resonance, the photonic band-gap effect, photon tunneling effect and so on. This paper focuses on infrared thermal radiation control of the micro-structured surfaces as well as their applications. The main contents of this paper include the following aspects.1. Narrow-band thermal radiative properties of circular micro-cavity arraysNarrow-band thermal emitter is vital in various applications of the thermal imaging, sensing and spectroscopy of chemical and biological agents, environmental monitoring, and energy conversion technology. So far, many researchers have conducted constructive and fruitful studies on narrow-band thermal emitters, which were mainly based on the enhanced spectral absorption due to SPPs excitation and FP resonance. The emitters based on these two mechanisms are radiative-angle-dependent, i.e., the radiative properties of the structured surfaces are different with the observation angle. Therefore, it is difficult to utilize those two mechanisms to design a structured surface which simultaneously possesses the following two features:Firstly, the surface has high emission at the same wavelength in all directions; Secondly, the wavelength position corresponding to high emission can be adjusted by changing the structural parameters. Based on microcavity resonant effect, we put forward an Ag/Si structured surfaces with periodic micro-cavity array. This structure can be used as a narrow-band heat radiator simultaneously possessing the above two features. This paper has carefully analyses the influence of various structural parameters on the performances of narrow-band radiative properties. Finally, a narrow-band radiator is fabricated by using the photolithography process and its spectral properties are tested. A series of samples are fabricated by using the lithography process. The relationships between the structural parameters and the processing technic are investigated. Their surface topographies are observed by scanning electron microscopy. The spectral emittance characteristics of these samples are tested by using spectrometer and compared with the results of numerical simulation. In addition, the direction-dependance of the emittace spectral is also investigated.2. Thermal radiative properties of grating coupled thermochromic materialsPerovskite-type manganese oxide La0.875Sr0.i25MnO3 (LSMO) is a typical thermochromic material. A smart radiation device (SRD) made of doped perovskite-type manganese oxides have potential applications in miniature satellites. LSMO shows metal-insulator transition in the vicinity of the Curie temperature Tc.LSMO exhibits lower emissivity at a temperature less than Tc, and higher emissivity at a temperature larger than Tc.From the perspective of surface morphology, a one-dimensional grating is introduced into LSMO bulk to improve the thermochromic property. The spectral emissivities of structured LSMO with different structural parameters are calculated and analysed. The spectral features are reasonably interpreted by the microcavity resonance effect. The temperature dependence of averaged emittance of LSMO with grating surface and smooth surface is also calculated. The results show that LSMO with grating structured surface has a better thermochromic performance compared with LSMO with smooth surface. The weight control is prerequisite and severe for some specified cases such as thermal management of spacecrafts.Consequently, it is of vital importance to enhance absorption ability of the thermochromic oxides to reduce the material consumption. In this paper, a one-dimensional metal/dielectric grating is introduced into LSMO film. Such a hybrid structure can remarkably reduce the material consumption and improve the thermochromic performance of LSMO film.3. Nagetive refractive index properties of fishnet micro-structurePerforated metal/insulator/metal sandwiched fishnet structure is a typical metamaterial. An effective negative refractive index can be obtained by utilizing this kind of metamaterial. The physical mechanism on the negative index of this metamaterial has been debated for a while. Numerical computation is conducted to obtain the reflection and transmission coefficients of this metamaterial. By inverting the simulated reflection and transmission results, the effective electromagnetic parameters can be acquired utilizing a retrieval procedure. A negative refractive index is observed from the retrieved results. In this paper, the phenomenon of the negative refractive index of the metamaterial is attributed to the excitation of internal SPP. It is of great significance to achieve a tunable negative refractive index material in extending its applications. In this paper, we propose a novel structure of metamaterials with tunable negative refractive index. In the proposed structure, an LSMO thin film is attached to an Ag/MgF2/Ag fishnet structure. By combining thermochromic feature of LSMO and the nagetive index feature of Ag/MgF2/Ag fishnet structure, we realize the automatic adjustment of negative refraction index of metamaterial by simply tuning the temperature.4. Multi-band compatible invisablility properties of periodic micro-structureThe multi-band compatible invisablility technology is vital significant to the survival ability of the military targets. The spectral characteristics can be effectively controlled at multiple wavelength bands by micro-scale structures, which are hardly achieved by macro-scale structures. In the present paper, ZnS/Ag/ZnS asymmetric film stack is designed for the compatible control in visible and infrared regions. This structure exhibits high transmission in the visible region and high reflectance in the infrared region. We attempt to explain the spectral features by impedance matching and the FP resonance. Then the periodic aperture arrays are introduced into the film stack, expected to achieve a compatible spectrum control for three wavelength bands of visible, infrared and laser (10.6μm) regions.5. Thermal rectification based on microscale near-field thermal radiationThe so-called thermal rectification is a phenomenon in which thermal transport is biased toward a specific direction. That is, heat flux along a specific direction is quite large, while heat flux along the opposite direction is relatively small under the same temperature difference. The previous investigations on thermal rectification were mainly limited to heat conduction and convection. So far, there are few reports about the thermal rectification based on microscale near-field thermal radiation. In this paper, we attempt to achieve the thermal rectification by thermochromic materials VO2 and LCSMO based on near-field thermal radiation. Firstly, we investigate a thermal rectification structure consisting of two homogeneous lossy medium. The factors influencing the near-field thermal radiation are carefully analysed to figure out the mechanism of near-field thermal radiation. These factors include the gap between the two plates, the real and imaginary part of the refractive index and surface polaritons. Then, the thermal rectification properties of the structure consisting of VO2 and LCSMO are studied. The ratio of the heat flux for the forward biased case to that for the reverse biased case amounts to 8.7. The SPhPs and photon tunneling effect in near-field are used to interpret the thermal rectification.