| Mullite can be utilized as potential optical window materials at elevated temperature and in severe environment due to its excellent high-temperature strength, thermal-shock resistance, low dielectric constant and good infrared-transparency. It is of great significance to synthesize highly-pure, ultra-fine and homogeneous mullite powder and fabricate transparent mullite ceramics using optimum sintering process. In this thesis, highly-pure and homogeneous monophasic mullite precursors were synthesized using the sol-gel process and transparent mullite ceramics with ultra-fine, equiaxial grains were fabricated by Spark Plasma Sintering (SPS).The sol-gel process parameters were chosen to prepare type I mullite precursors. Monophasic mullite precursor with chemical homogeneity at the molecular lever was prepared from aluminum nitrate nonahydrate (ANN) and tettraethoxysilane (TEOS) dissolved in absolute ethanol. The sharply stirring for 4 days at 60℃and the limited amount of water provided by the crystal water included in ANN effectively controled the hydrolysis rate of Al3+ and Si4+. The hydrolysis and condensation of Al3+ and Si4+ carried out simultaneously and Al3+ entered into the gel structure network to form Al-O-Si bond. The chemical composition of mullite variations and the structure changes over a temperature range were characterized by X-ray diffraction (XRD) and infrared absorption spectra (IR). The results indicated mullite formed at low temperature of 1000℃was aluminum-rich and pseudo-tetragonal. pseudo-tetragonal mullite is metastable and transformes gradually to stable orthorhombic mullite with the composition of 3Al2O3-2SiO2 at temperatures above 1000℃.The sintering processes for fabricating transparent ceramic were optimized. The experimental data indicated transparent mullite ceramic was fabricated by spark plasma sintering (SPS) from monophasic mullite precursor which was calcined at 1100℃for 2h. The sintered body obtained a relative bulk density of above 97.5% and an infrared transmittance of 75-82% in wavelength of 2.5-4.3 u m without additive by heating the sample to 1450℃, holding for 10min, at heating rate of 100℃/min under pressure of 30MPa and vacuum atmosphere. The effects of the calcination temperature on the sintering property of mullite were discussed. When the precursor powder was calcined at below 1000℃, it was unfavorable for completely eliminating the residual OH, H2O and organic compound. However, when calcinations temperature was at above 1100℃, it was unfavorable for full densification due to the absence of viscous flow of amorphous phase SiO2. At the same calcinations temperature of 1000℃, the sintering process could not be accomplished if the sintering temperature was lower than 1450℃·or the holding time was less than 10min. Whereas, if the sintering temperature was higher than 1450℃or the holding time was longer than 10min, the transmittance of mullite would decrease with the increase of temperature and time owing to the elongated grains and pores growth. The densified mechanism of type I mullite precursor in the low temperature and the low pressure environment by SPS was studied by comparing the sintering properties of the precursor with different composition and the same compositional samples calcined at different temperature. The microstructures of the sintered bodies were characterized by scanning electron microscope (SEM) and transmission electron microscope (TEM). The relationships between microstructure and transmittance were analyzed by Rayleigh scatting formula. The result showed the pore sizes severely affected the transmittance of mullite ceramic. The sample sintered at 1450℃had the highest transmittance due to its smallest pores size. However, the transmittance of the sample sintered at 1500℃decreased due its holes size increased although its relative density was higher. In addition, the sintered bodies were annealed at different temperature, holding time and atmosphere. The experimental data showed the black appearance could not be removed by annealing at 1350℃for 8h at oxygen atmosphere. When the annealing temperature was increased to 1500℃and holding time was extended to 50h, the sintered body would become white. However, the transmittance of sample was decreased by 20% due to the holes growth.In additional, we also explored the possibility of fabricating transparent mullite ceramic from spray-drying powder. The powder microstructure and its heating variation with temperature were characterized by SEM and DTA. The result indicated the precursor using spray-drying process could not be fabricated to transparent ceramic because the hollow spherical particles were unfavorable to eliminate -OH, H2O and organic compound. The residue enveloped in the sphere decreased the eutectic temperature. The liquid phase which appearred at lower sintered temperature lead to the elongated grains which decreased the transmittance of sample. In order to widen mullite transparent wavelength range, boron was doped to form solid solution. Boron promotes the atomic diffusing rate, and thus decreased mullite formation temperature and suppressed the spinel formation. Meanwhile, it also reduced the sintering temperature of mullite ceramic. Our data indicated that a part of boron could incorporate into mullite structure and substitute for silicon by measuring the lattice parameters of boron-doped mullite using the XRD pattern of powder and analyzing the element composition of crystal surface etched by chemical solution. The substitution of B3+ for Si4+ leads to the decrease of the cell unite parameter and thus reduces the free space for atom motion. This leads to the decrease of ionic polarizability and thus reduces the dielectric constant of mullite which is favorable to be used as microwave-transparent material. However, boron also decreases infrared-transmittance of mullite ceramic and produces additional intensive absorption bond at 3.9μm.
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