| Infrared detective technology has been widely used in military affairs. How to avoid important targets to be detected and reduce the loss is of great urgency. So, the development of infrared stealth materials has been paid much attention by scientists. Since the spectral characteristics of photonic crystal will change with the variation of its structure, it has been used in information process, optical device and thermal photovoltaic system, etc.. However, controlling infrared radiation effectively with photonic crystal is a novel research realm.Firstly, in the preliminary analysis about the suppression effect of photonic crystal on atmospheric window band emission (3～5μm and 8～14μm), the physical model of one dimensional photonic crystal is set up with transfer matrix method. The effects of several combinations of substrates and photonic crystals and their spontaneous emissions on inhibition effect are discussed. It is found that, when the high and low refractive index medias composing the photonic crystals are Silicon (Si) and Potassium Chloride (KCL), respectively, and the substrates are Aluminum (AL), Silicon Carbide (SiC) and an imaginary surface (of emissivity equal to 1) at 100℃orderly, all the surface emissions are inhibited well in both bands (3～5μm and 8～14μm). For the same substrate, the inhibition effect could be increased if the ratio of the high and low refractive indices of the composing materials is higher, such as Germanium (Ge) and KCL.Secondly, without considering the emission between 3 and 5μm, the transfer matrix method is used to analyze the inhibition effect of photonic crystal on atmospheric window emission between 8 and 14μm. According to the optical characteristics of the materials, Ge and Zinc Sulfide (ZnS) are chosen as the composing materials of the photonic crystal. The structure of the photonic crystal is optimized, and the optimal thicknesses of germanium and zinc sulfide are 0.63μm and 1.11μm respectively, while the ratio of optical thickness is 1:1 and the period is 8. The photonic crystal is prepared by vacuum evaporation coating method, and the optical properties of the photonic crystal are measured. The inhibition of the photonic crystal to atmospheric window emission is verified by both experimental and theoretical results.Thirdly, the inhibition of photonic crystal to atmospheric window band emission is investigated by further field experiments. The actual inhibition effect is explored when substrates are at different temperatures. By vacuum evaporation coating method, Ge/ZnS type photonic crystals are prepared on different substrates. The infrared thermographies are photographed by infrared thermal imager at different substrate temperatures, when the substrate is coverd with photonic crystal or not, respectively. And the characteristic of photonic crystal inhibiting the atmospheric window emission can be shown sufficiently by comparing the display temperature of thermography with the actual temperature of the substrate. It is found that, if the smoothness of the substrate can be ensured effectively, the stop band characteristic of photonic crystal can be exhibited sufficiently. For example, when the actual temperature of the substrate is 100℃, the lowest display temperature in infrared thermal imager of the five groups of experiments is 26.5℃.Fourthly, finite difference time domain method is used to design the spectral properties of Ge/Sodium Fluoride (NaF) photonic crystal. With the analysis results of reflection and transmission spectrum of Transverse Magnetic (TM) and Transverse Electronic (TE) waves, it is found that the reflectivity of the photonic crystal of 7 periods can reach 0.99 between 8 and 14μm. For the realization of transmission peak at a given wavelength among the atmospheric window (for example,10.6μm), the methods of introducing a third medium and tuning the original film thickness are explored, respectively. Introducing a third medium which is transparent in the infrared spectrum can not achieve the aim of forming transmission peak at the special wavelength, but tuning the original film thickness does work, for example, the transmittivity at 10.6μm could reach 0.938.Finally, the thermophysical model of Thermo PhotoVoltaic (TPV) system adopting silicon cell is set up while one dimensional Magnesium Fluoride (MgF2)/ZnS type photonic crystal filter is used. In view of the mismatch between the energy distribution of high-temperature radiator and the response band range of silicon cell, one kind of photonic crystal filter is designed and its basic period structure is|L/2HL/2|s. And another improved filter is obtained by tuning the central wavelength of the main stop band. When the blackbody radiator is at 1800K and the central wavelength of main stop band of the filter is 1.69μm, the output power density of cell and the cooling power density in the TPV system are equal to 1.57 and 36.30Wcm-2, respectively. By comparing with the TPV system using the filter whose central wavelength of main stop band is 1.39μm, the output power density of cell and cooling power density are increased by 8.3% and decreased by 10.4%, respectively. In order to solve the problem of large cooling power density, on the basis of the filter structure whose central wavelength of main stop band is 1.69μm, the scheme of combining several filters of different main stop band central wavelengths orderly is proposed (the reformed structure is |L/2HL/2|s|L/2HL/2|sl…|L/2HL/2|sn). And a wide reflection band can be formed by the combination of the main stop band of each filter. It is found that, if the output power density of cell is sacrificed in some extent, the cooling power density will decrease significantly. Then, the performance of TPV system with the ytterbia selective radiator is analyzed.
|
PDF Full Text Request