The Crystallization Characters And Application Study Of The Low Temperature Processed BaO-Al₂O₃-SiO₂ Glass-Ceramics

Two groups of BaO-Al₂O₃-SiO₂ (BAS) glass-ceramics with stoichiometric and nonstoichiometric celsian composition were fabricated by sintering at low temperature. According to the principle of quantitative analysis by X-ray Diffraction (XRD), a new method to determinate the crystallinity of glass-ceramics was proposed. Based on the principle of glass technology, main compositions of the two groups of BAS glass with stoichiometric and nonstoichiometric celsian were designed. The effects of ZrO₂ and celsian seeds on crystallization and phase transformation characteristics of the two groups of BAS glass-ceramics were investigated. By controlling the kinds and quantity of crystal phases in these glass-ceramics, Thermal Expansion Coefficient (TEC) of them can easily be designed over a wide range. By taking advantage of this result,a dielectric coating from the BAS glass-ceramics was prepared on 0Crl8Ni9 stainless steel surface, as well, an anti-oxidation coating also from them was prepared on carbon fiber reinforced silicon carbide (C_f/SiC) composite.The new method to determinate the crystallinity of glass-ceramics was proposed only with the intensity data for glass phase diffraction. Mass fractions of crystal-phases was calculated using "Reference Intensity Values" on Joint Committee on Powder Diffraction Standards (JCPDS) . Comparing the results calculated by the method with those by the Rietveld method, the maximum deviation of the crystallinities from experiments is 3.2 %, the maximum experimental deviation of each crystal-phase mass fractions is 2.6 %. Relative toα-Al₂O₃, Reference Intensity Values of monoclinic celsian in the BAS glass-ceramics is 1.74, which is calculated by the so-called calibrated curve method.The basic compositions of nonstoichiometric BAS glasses were designed, through considering effects of oxides in the glasses on their melting temperature, clarified temperature and TECs. B₂O₃ was selected to ruduce the melting temperature of stoichiometric BAS glasses. To preserve the glass performances at high temperature, an appropriate additive fraction of B₂O₃ may be 6 %.ZrO₂ was chosen as nucleation agent. The agent can boost phase transformation from hexacelsian to monoclinic celsian in the stoichiometric BAS glass-ceramics. Reversely, it will restrain the transformation in the nonstoichiometric BAS ones.The effects of ZrO₂ and celsian seeds on crystallization and phase transformation characteristics of the two groups of BAS glass-ceramics were investigated by Differential Scanning Calorimetry (DSC),XRD and dilatometry, according to the isoconversional method of Ozawa and the Johnson-Mehl-Avrami equation. Crystallization characteristic of hexacelsian in each sample is bulk nucleation. Crystallization of monocelsian in the sample doped with celsian seeds characterizes surface nucleation, while it is bulk nucleation in the sample without nucleation agent. The earlier stage crystallization characteristic of monocelsian in the sample doped with 1 %-2 % ZrO₂ is surface nucleation. The crystallization characteristic of monocelsian became bulk nucleation in the later period with 1 %-2 % ZrO₂ or with higher ZrO₂ content. It is the autocatalytic kinetic model, i.e., Sestak-Berggren function, not the JMA function or the Kissinger function, that can most properly describe the processes studied for the all glasses investigated.It is revealed that a content of 1 %-2 % ZrO₂ is the most effective for crystallization and phase transformation in the BAS glass-ceramics investgated. Crystallized at 850℃for 12 h, stoichiometric BAS glass-ceramics doped with 1 % ZrO₂ is completely transformed into monocelsian. No monocelsian in nonstoichiometric BAS glass-ceramics doped with 2 % ZrO₂ can be detected when the glass-ceramics were crystallized at 850℃for 100 h. Crystallization and phase transformation in each formula for the two groups of BAS glass-ceramics doped with the seeds were able to be accelerated greatly. Crystallized at 850℃for 2 h, stoichiometric BAS glass-ceramics doped with 1 % seeds is completely transformed into monocelsian.For the stoichiometric BAS glass-ceramics, by optimizing the content of ZrO₂ or controlling the crystallization time, proportions between monoclinic celsian and hexacelsian quantity can be adjusted. Consequently, TECs from 3.1×10^-6℃^-1 to 6.67×10^-6℃^-1 (40-200℃) can easily be tailored. For the nonstoichiometric BAS glass-ceramics, by controlling the crystallization time, proportions between monoclinic celsian and hexacelsian quantity can similarly be adjusted. Also, TECs from 5.21×10^-6℃^-1to 9.70×10^-6℃^-1(40-200℃) can easily be tailored.The nonstoichiometric BAS glass-ceramics doped with ZrO₂ were employed to prepare thick film dielectric coatings on 0Crl8Ni9 stainless steel substrate. BaO is partially substituted by MgO and CaO in the BAS glass-ceramics, which can decrease the sintering temperature and increase the crystallization temperature, and thus restrain the crystallization. As a result, the sintering processability are obviously modified. TEC of the thick film for the BAS glass-ceramics can match that of 0Crl8Ni9 stainless steel substrate. The coating can meet the requirement of thick film circuit for multilayer co-firing technology. Some results from the present work have been applied into production of the cooperating company.Two different compositions of BAS glass-ceramics, with different soften points and TECs, were employed to prepare three structures of coatings. The all coatings have protection functions for C_f/SiC composite.