| Thermal controlling device is one of the important components on a satellite, which is used to keep the satellite temperature at a reasonable range, guaranteeing for normal activities of the inside electronic devices. In order to meet this special task, different thermal controlling systems have been developed. The most popular among them is thermal louver. Although the effective emittance variability of thermal louver is relatively high, its major issues are heavy weight, high power consumption and high production cost. To overcome these obstacles, the development of a lighter, less energy consuming and cheaper device is necessary.In the two recent decades, it has been reported that the perovskite-type manganese oxide La1-x Ax Mn O3(A=Sr, Ca, Ba, etc) material system has a large potential to be a new generation of thermal controlling device due to its particular radiation properties. By proper doping, the material exhibits a low heat emission at low temperature, whereas an abruptly high emission is observed at higher temperature. It means this material may automatically modulate the heat emission by itself according to its temperature without any outer sources. Among foregoing compositions, lanthanum manganite doped with Sr has been the most interesting one due to its remarkably structural, electrical, magnetic and optical properties. The material has been fabricated by different methods and exhibited various results of these properties. Several synthesizing methods are somewhat complex, which demand advanced synthesizing instruments and expensive raw materials. Meanwhile, a combination of gel combustion and hot-pressing route is being an alternative choice for synthesizing LSMO polycrystalline ceramic with low cost and simple procedure. In our scope of knowledge, the influence of this synthesizing combination on structure; electrical, optical, emissive properties of hot-pressed LSMO ceramic has not been reported, and therefore need further studying.In this work, we firstly studied the factors that affect synthesizing process of LSMO powder prepared by gel combustion route. This synthesizing route is an effective combination of chemical sol-gel process, self-combustion and burning process. This is a simple synthesizing method with cheap raw materials. The powder obtained from this synthesizing route usually has good features in homogeneity, purity, particle size, and reactivity. During gelatinizing process, however, there are numbers of factors that can affect the quality and stability of gelatinized solution, as well as subsequent combustion process and quality of powder formed. Two most important factors which need to be optimized in order to prevent precipitation during gelatinization and effectively support the combustion later are p H value and molar ratio of citric acid to metallic ions of the solution(denoted by φ). Raw materials used were citric acid, ethylene glycol, and nitrate salts of metals. The p H value of mixture solution was manipulated by dropping ammonia water. In order able to choose optimal values of p H and φ, visual observations associated with infrared absorptance spectra and XRD analysis on products of gelatinization and combustion were carried out. Analyzed results showed that the gelatinization took place successful without any precipitation and gave the best crystalline structure of after-combusted ash only if p H was 1.8. With this fixed p H value, the crystalline structure of after-combusted powder was like LSMO phase since the φ value was 1.0. It was also found all Sr-doped compositions could reach to single phase only if calcination temperature reached to 1200 °C. The decreases of particle size with increasing dopant content and with reducing calcination temperature were clearly observed as well.With the optimized parameters for the gel combustion route obtained above, we synthesized a large amount of LSMO powder for further experiments. The powder was divided into several parts and these parts were pelletized using hot-pressing method with different parameters, such as compressing pressure, particle size, sintering temperature/time, and doping level. The influences of these parameters on structure, chemical state, and electrical resistivity of the hot-pressed LSMO ceramics were investigated. The results showed that with increasing compressing pressure the electrical resistivity decreased and the transition temperature TP shifted to higher value. The resistivity reduction was also observed from the ceramics hot-pressed with the increase of initial particle size. Particularly, a decrease by one order of magnitude of resistivity, and an obviously change of electrical conduction from insulator to metal-insulator to metal-metal transition with each 100 °C-increasing step of sintering temperature were detected. Further annealing of ceramic for longer duration also resulted in a growth in resistivity magnitude. The increase of Sr dopant content in the parent material La Mn O3 also lowered the ceramic’s resistivity and shifted the TP to higher values.The optical properties of hot-pressed LSMO ceramics were also studied. We measured the spectral reflectance at room temperature in mid and far infrared regions for the samples calcined at different temperatures and for those doped with different levels. All reflectance spectra showed three typical peaks near 170, 360 and 590 cm-1 that correspond to La-site external, Mn-O-Mn bending, and Mn-O stretching phonon modes, respectively. The frequency shifts toward higher frequency side of the bending and stretching modes were observed for the increasing temperature-calcined ceramics. The spectra of optical constants, including refractive index n and extinction coefficient k, calculated by Kramers-Kronig transformation also showed three typical peaks of phonon modes. These peaks were gradually disappeared when the Sr2+ doping content increased. Emissivity and emissive power of different levels-doped ceramics were evaluated via calculation based on temperature-dependence reflectivity. The calculated emissivity in 153-513 K showed the material could self-adjust its emissivity with temperature variation. The sample with doping level 0.2 exhibited the largest change in emissivity. The influence of doping level on band emission, which is characterized by the fraction of emission in a finite wavelength range(2.5-25 μm) and those in an approximately full range(2.5-100 μm), F2.5-25(T), was also discussed. F2.5-25(T) was nearly independent with doping level when T>433 K, while it had maximum value at x~0.33 when T<433 K. |
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