Thermal Radiative Properties Of Surface And Its Control Method Based On The Thermochromic Phenomenon

With the power density of heat dissipation and the heat load in a system increas-ing, there is a growing demand for a better performance of thermal radiative surface because these with constant emissivity cannot meet the demand any more. For ex-ample, thermal radiative surface, such as ZnO coating and OSR reflector, are usually used to dissipate heat from the satellite surface. However, these traditional thermal radiative surface have a constant emissivity, they don’t adjust their thermal radiative properties alone in response to the violent fluctuation of exterior heat flux. Therefore, there is a serious need for developing novel thermal radiative surface with its radia-tive properties automatically adjusting in response to a variation temperature for the next generation thermal control system applications. In this dissertation, perovskite-type manganese oxides La1-xAxMn03(A=Ca, Sr, K) were selected as a basic material to prepare a novel thermal radiative surface, and investigation on thermal radiative properties of surface and its control method based on thermochromic phenomenon was performed. The contents of this thesis mainly include the following parts.(1) Preparation of thermochromic functional materialSolid stated reaction is used to prepare the bulk thermochromic functional ma-terial, and the plasma spray method is used to prepare the thermochromic coating. XRD, SEM, AFM, VSM, and PPMS are used to detect the phase structure, surface to-pography, RMS roughness, magnetic properties and electronic properties, respectively. These effects of doping element, doping level, or sintering temperature on the phase structure, densification, or phase transition temperature of thermochromic functional material are analyzed.(2) Thermal radiative properties of thermochromic functional materialReflectivity spectra of thermochromic device or coating are measured using the FT-IR spectrometer and spectrophotometer. The emissivity and solar absorptivity are deduced from the experimental reflectivity data. The mainly factors to affect the ra-diation properties of surface are presented. Experimental investigations are reported as follow:For the Ca and/or Sr doped samples, their reflectivity spectra increase with temperature decreasing. In an appropriate doping level of Ca and/or Sr, ther- mochromic functional surface shows a transition from metallic state to insulator, as a result, its emissivity is increases with temperature. If only the Ca element was doped in A site, the emissivity of sample changed dramatically in the vicinity of transition temperature, but the transition temperature is low. If only the Sr element was doped in A site, the emissivity of sample shows a sudden change near the transition temper-ature, and the transition temperature is high. So, it is possible that the the transition temperature can be adjust according to application needs and the emissivity variation can be maintained. Thermochromic coating with tunable emissivity properties is ob-tained by adjusting the plasma spraying parameters. The variation range of emissivity of thermochromic coatings are relation to the surface condition, and spray parameter. The largest increment of emissivity is0.33at present. A limitation of application was found in the thermal radiative surface due to its large solar absorptivity, which is difficult to be reduced only changing the doping level or element, such as Ca, Sr, or K.(3) Coupling control method of radiation properties of surface based on ther-mochromic phenomenonThe optical control method based on the multi-layer thin films is introduced to investigate the control of radiation properties of surface. A multi-layer thin films structure is designed onto the surface based on thermochromic phenomenon using sim-ulated annealing genetic algorithm. The effect factors on the thermal radiative prop-erties of complex functional surface, which consists of surface based on thermochromic phenomenon and multi-layer thin films, are shown in this thesis. The calculated re-sults reveal that multi-layer thin films can effectively reduce the solar radiation, which arrives to the surface of thermal radiative surface, and transmit infrared radiation simultaneously.According to the calculated results, the multi-layer thin films are deposited onto the thermochromic functional surface using the electron beam evaporation. The ther-mal radiative properties of complex functional surface are experimental studied. The experimental results show that complex functional surface is not only obtain a low solar absorptivity, but also maintain the temperature-dependent emissivity properties of thermochromic functional surface. Thus, the coupling control of solar absorptivity and infrared emissivity is confirmed.(4) Particles irradiation-induced degradation of radiation properties for the sur-face basing on thermochromic phenomenonInfluence of particles irradiation,60-1000KeV electrons and100KeV protons in energy, on the thermal radiative behaviors of surface basing on thermochromic phe-nomenon (i.e., thermochromic functional surface and complex functional surface) have been investigated. To evaluate thermal radiative degradation, their solar absorptivity, temperature-dependent reflectivity, and thermal emissivity are studied before and after particles irradiation. The thermal radiative degradation behavior in thermochromic functional surface is reported, and the degradation mechanism is discussed. In addi-tion, it is revealed that the thermal radiative properties of complex functional surface shows a excellent stability without degradation in simulation environment.