The Crystallization Characteristic and Application Study on the Low Temperature Sintering BaO·Al_2O_3·SiO_2 Glass-ceramics

Abstract: The superior BaO·Al_2O_3·SiO_2(BAS) glass-ceramics were an important direction of research. According to abundant investigation, the effects of some nucleating agents on the crystallization, phase transition and properties of the BAS glass-ceramics were studied systematically in this thesis. Finally, the feasibility of BAS glass-ceramics as oxidation resistance coatings for C_f/SiC composites was explored.The BAS glass-ceramics were prepared through sintering method. The melting temperature, sintering temperature and crystallizing temperature were decreased by adding B_2O_3 which can facilitate the melting of glass.The crystallization characteristic, phase transition, density, thermal expansion coefficient(TEC) and flexural strength of BAS glass-ceramics with different nucleating agents were analysed by differential scanning calorimetry and X-ray diffraction methods. In the stoichiometric BAS glass-ceramics, the nucleating agents ZrO_2 can not only promote the phase transfition form hexcelsian to monoclinic celsian but also increase the density and the flexural strength of the glass-ceramics. In the off-stoichiometric BAS glass-ceramics, the transformation of hexcelsian to monoclinic celsian wree restrained by adding ZrO_2 due to which the high TEC of BAS glass-ceramics were obtained. ZrO_2 therefore is an ideal nucleating agents for the BAS glass-ceramics.For the purpose of high density, a set of sintering technics were determined through the research on the effect of every sintering technics parameter (calefactive speed: 2℃·min~(-1), sintering time: 60 min), through which the series of coefficient of thermal expension( 4 to 11 ppm between 40~400℃) can be obtained with the changing of the crystallization time.The crystallization kinetics of hexcelsian and monoclinic celsian were analysed by using the non-isothermal transformation kinetics and the isothermal transformation kinetics respectively. The crystallization of hexcelsian was surface nucleated and the monoclinic celsian was volume nucleated without the nucleating agents, however,once the ZrO_2 was added, it showed surface nucleated .The BAS glass-ceramics oxidation resistance coatings of C_f/SiC composites showed a better thermal shock properties at 1000℃than 800℃because of the softening of the residual glass. The oxidation resistance coatings of BAS glass-ceramics which were made from the green tape had obvious oxidation resistance. The C_f/SiC composites with coatings which had underwent an oxidation test at 1000℃in atmosphere for 10 h sufferd only 5 % mass loss…
Key words: glass-ceramics ; celsian ; nucleating agents ; crystallization kinetics; C_f/SiC composites ; oxidation resistance coatings

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