Preparation And Research Of Infrared-Transparent Mullite Ceramic

Mullite can be applied as window material in the rigorous environment because of its excellent dielectric properties and good middle infrared-Transmittance. The infrared window of pure mullite is from 1μm to 5μm. However, it cannot cover with mid-infrared portion (3～5μm) because the cutoff wavelength of Si-O is 5μm. Therefore, It's meaningful to widen infrared transparent range by doping modification. In order to prepare the mullite ceramic with excellent optical property, mullite powder with high-purity, uniform morphology and superfine grain is very important. Mullite powder fabricated by Sol-Gel process can meet the demand of high-purity and uniform morphology, and it is easy to be doped.The precursor of pure mullite was prepared by Sol-Gel method, using tetraethoxysilane(TEOS) and aluminium nitrate as raw materials. In order to prepare the mullite powder with high-purity, uniform morphology and superfine properties, the PH value , mixing method of silicon sol and aluminium sol, dispersant and the concentration of mixing sol during aging were studied. The experimental results reveal that PH value is very important to the mullization temperature. The lower PH value is , the lower the mullization temperature is. The mullite can form at 1150℃when the PH value was 0.5. The formation processing was that the amorphous SiO₂ existed below 1000℃and transition Al-Si spinel existed above 1100℃, finally, the mullite formed above 1150℃. The other influence factors, such as dropping TEOS to ANN, no dispersant and keeping the concentration of mixing sol invariable during aging were all important to the mullization at low temperature.The Ge-doped mullite was prepared based on the optimized technics of pure mullite preparation, using tetraethoxysilane(TEOS), aluminium nitrate and 3-trichloro germanium propionic acid as raw materials. The uniform sol can not be obtained when the incorporation mass of germanium higher than 5mol %. The incorporation mass of germanium was advanced after ameliorating preparation technics, aluminium nitrate being dissolved by ethanol. The mulliation temperature was 900℃when the incorporation mass of germanium was less than or equal to 20mol% , and when the incorporation mass of germanium was more than or equal to 30mol% , the mullization temperature was 1000℃. The Alfa mullite powder was sintered by Spark Plasma Sintering equipment. We obtained the optimized sintering condition, as followed: sintering temperature is 1500℃; heating rate is 100℃/ min; holding time is 5 min, axial pressure is 30MPa, vacuum sintering (vacuum less than 6 Pa).The pure mullite powder and Ge-doped mullite were sintered based on the optimized sintering condition. The results of sintering the pure mullite reveal that the optimized sintering condition was suited for the sintering of pure mullite ceramic. The middle-IR transmittance of pure mullite ceramic was advanced remarkably compared with the middle-IR transmittance of Alfa mullite ceramic. the maximal Transmittance was 69%, and the 4.3μm absorption was weakened compared with that was reported. The Transmittance was more than 50% at 4.3μm absorption band. The results of sintering the Ge-doped mullite reveal that the optimized sintering condition was suited for the sintering of Ge-doped mullite with doping content of 20mol% , for the maximal transmittance was 68%; the infrared window was broadened when the doping content of germanium was more than or equal to 30 mol %.