Effect of Cristobalite on the Strength of Sintered Fused Silica Above and Below the Cristobalite Transformation

Porous fused silica ceramics which had been partially devitrified to cristobalite were stronger at 350°C in the beta‐cristobalite stability range, but weaker at 25°C after the transformation to alpha‐cristobalite. The tensile strength distribution for different cristobalite fractions are compared for three types of specimens. These were beta‐cristobalite samples which had never been transformed to alpha‐cristobalite; room‐temperature samples with alpha‐cristobalite; and samples at 350°C which had retransformed to beta‐cristobalite after prior transformation to alpha‐cristobalite. The alpha‐cristobalite samples displayed 50%–75% the strength of samples tested on beta and the difference in strength was dependent on cristobalite content. Specimens retransformed to beta‐cristobalite had strength similar to virgin specimens. Microcracking associated with the transformation to alpha‐cristobalite contributes to the strength changes but cannot fully explain the impact of the beta–alpha transformation on strength.     

Kinetics of Cristobalite Formation from Silicic Acid

Rate experiments made on the transformation of silicic acid into cristobalite, in the temperature range 945° to 1085° C, have demonstrated the existence of a measurable nucleation period. From its variation with temperature, a value of 102.8 kcal. per mole has been calculated for the activation energy of the nucleation process. A second step of the transformation, involving the growth and perfecting of the new phase, has been shown to occur with an activation energy of 50.9 kcal. per mole. From the shape of the rate curves it has been calculated that a superstructure having fairly well-defined sequence rhythms of alternating three- and two-layer structural units might be the end product of heating silicic acid in the temperature range covered by the present experiments. It also has been demonstrated that the degree of disorder increases from the core to the surface of the transforming cristobalite crystals.     

Crystallization and Melting Kinetics of Cristobalite

By measuring internal crystallization in vitreous silica, the kinetics of crystallization and melting of cristobalite were determined near the melting point. Both processes followed linear growth kinetics and could be completely represented by the equation μ=AΔT/η. Analyses of the crystallization data and observations of the crystal morphology indicate that crystallization occurs by a continuous growth mechanism. Melting was heterogeneous, occurring only at the glass-crystal interface and along grain boundaries.     

球形硅微粉的微观结构研究

文章采用XRD、SEM研究了球型硅微粉的晶体结构和微观结构。XRD测试晶体结构,发现高温熔融喷射法制备的球型硅微粉里面有晶态石英存在;SEM分析微观结构发现高温熔融喷射法制备的部分球型硅微粉颗粒内部有孔洞存在。通过对晶体结构和微观结构进行分析,找到了高温熔融喷射法生产的球型硅微粉内结晶态存在的原因。     

Silica Structure Studies: V, The Variable Inversion in Cristobalite

It is demonstrated experimentally that even in the purest laboratory silicas available the temperature of the α-β inversion in cristobalite is variable and depends on the structure of the starting material and on the temperature and length of heat-treatment. It is shown that this variability is an index of the order achieved in the cristobalite structure quite independent of impurities. There is no specific order characteristic of a particular temperature; the completely ordered 3 C stacking of cristobalite is the most stable cristobalite throughout the temperature range and has an α-β inversion temperature of 267°± 2°C. All disordered cristobalites will tend toward the 3C 267° cristobalite with time. The conversion of cristobalite to tridymite involving the 3 C → 2 H stacking change does not alter the cristobalite inversion temperature, nor have any regular polytypes other than those recognized as tridymite been encountered.     

原料对方石英生成的影响

本研究通过采用不同的原料和配方,探讨了高硅质陶瓷中生成方石英的主要途径;不同条件下形成的方石英其结构和组成的差别;以及方石英的形态、含量、分布状况对瓷质性能的影响,制定出了性能良好的高硅质陶瓷料方及宜采用的原料。     

Cristobalite Phase Formation in Glass/Ceramic Composites

The effect of several different aluminum-containing ceramic additions to borosilicate glass on suppressing cris-tobalite precipitation has been examined. The results showed that mullite or aluminum nitride suppresses cristo-balite formation more effectively than alumina or spinel. Although both follow a simple rule of mixtures, glass/mullite composites can be fabricated with lower dielectric constants than glass/alumina composites, while maintaining a thermal expansion coefficient close to Si. Electron micro-analysis using X-ray energy dispersive spectroscope showed that the measured interdiffusion coefficient between alumina and glass is in good agreement with the data which have already been published.     

熔融石英中方石英的定量分析方法研究

本文主要通过X射线衍射分析方法研究了熔融石英中方石英的定量分析。利用本方法建立的标准曲线得到的相关系数(R2)满足检测分析要求,得出的计算公式简单易行。对此方法进行实验验证,发现此方法误差小,完全满足日常的检测分析。     

Stress Development and Fracture of Surface Nucleated Cristobalite on Silica Glass

Stress development and fracture of isolated cristobalite spherulites in amorphous silica matrix were observed. High purity bulk silica was annealed to produce partial surface crystallization consisting of isolated and impinged spherulites in an amorphous matrix. The stress state of the amorphous silica surrounding cristobalite spherulites was qualitatively examined using crossed‐polars microscopy. Fracture was observed to occur with many spherulites encircled by cracks in the matrix and other spherulites observed to self‐fracture in a “mud‐cracking” pattern. The fracture was found to be size dependent with encircling matrix cracks occurring as a minority phenomena in spherulites 20–70 microns in diameter and “mud‐cracking” self‐fracture to occur in all spherulites over 70 microns in diameter. The stresses develop as a result of the strain associated with the 4.9% volume reduction in the cristobalite on transitioning from beta‐to‐alpha phase at ~250°C. Observed fracture behaviors were modeled. Matrix cracks encircling spherulites were found to be consistent with a Weibull failure model of the glass under a stress field derived from the Eshelby inclusion model. Self‐fractured spherulite failure was found to be consistent with a failure model based on thin films under biaxial stress.     

Pseudopotential Periodic Hartree-Fock Study of the Cristobalite Phases of Silica and Germanium Dioxide

Periodic pseudopotential Hartree–Fock calculations have been performed on the cristobalite strucure of silica and germanium dioxide in order to investigate the topology of the ground-state Born–Oppenheimer energy surface. For both oxides the averaged X–O bond lengths, the two unequivalent O–X–O bond angles, and the tilt angle have been optimized. The tilt angle, which measures the rotation of the XO₄ tetrahedra around their local C ₂ axes, is directly related to the O–X–O bond angle and connects the low-temperature phase twin structures; its zero corresponds to the ideal β-phase. For SiO₂ the calculated values of these coordinates agree within 1% with 10 K experimental data whereas for GeO₂ the agreement between calculation and experiment is much less satisfactory. It is shown that the tilt angle is the most important structural parameter. The structure of β-cristobalite is rationalized on the basis of the energy dependence upon tilt angle and the absence of β-structure in GeO₂ is explained. The one-electron properties, i.e., band structure and density of states, have been calculated and are compared with the results of previous calculations carried out by other methods. As expected in Hartree-Fock calculations, the band gap is overestimated. The analysis of the calculated electron density and of density of states projected onto basis orbitals indicates that these oxides are mostly ionic. It is also shown that the GeO bond is more covalent than the SiO one, which explains the different behavior of these two oxides.     

Kinetics of Dendritic Precipitation of Cristobalite from a Potassium Silicate Melt

The growth of cristobalite precipitates in a 15 wt% K₂O-SiO₂ melt at 770° and 810°C was studied using the optical microscope and electron microprobe. A two-phase layer of dendritic crystals and interstitial melt grew from the surface at a constant rate determined by the chemical interdiffusion coefficient. This mechanism is consistent with most previous results concerning the kinetics of precipitate growth in glass-forming systems. It is evident that the tendency to associate constant growth rate with kinetics controlled by a phase-boundary reaction cannot in general be justified.     

Densification and Sintering Kinetics in Sintered Silicon Nitride

The sintering sequence of Y₂O₃-Al₂O₃-doped Si₃N₄ was investigated with respect to the relationship between densification, α→β transformation, and microstructural development. Quenching studies were performed to reveal these interactions during a complete sintering cycle. Isothermal studies were conducted to examine the sintering kinetics and compared to Kingery's liquid-phase sintering model. The bulk density increased to ≥90% of theoretical density with only minor transformation taking place. Major transformation occurred in a late sintering stage and was accompanied by the development of elongated grains. The kinetic order of the densification process, taking into account an appropriate correction, was larger than any of the rate exponents predicted by the Kingery model, indicating that other single or mixed mechanisms were active.     

Structure Formation in Densely Sintered Household Ceramics

The formation specifics of the structure and phase composition of densely sintered ceramics based on a low-melting argillaceous material from Belarus together with a combined flux and a grog additive are investigated. The correlation between the physicochemical properties, structure, phase composition, and the time-temperature firing schedules of the developed materials are investigated. Electron probe microanalysis makes it possible to monitor variations in the chemical composition at various local sites and compare it with the estimated composition of the mixture. The optimum regimes for the heat treatment of articles are determined. __________ Translated from Steklo i Keramika, No. 6, pp. 32 – 36, June, 2005.     

Effect of TiO2, LiF and Cr2O3 in the Crystallization of Cristobalite and Tridymite in Sintered Glass-Ceramics

Based on the poudretteite (K Na₂ B₃ Si₁₂ O₃₀) glass composition, six transparent glass frits were prepared. TiO₂, LiF and Cr₂O₃ were experimented on the base glass composition as nucleation catalysts. Only cristobalite and tridymite were developed through the sintering process of the glasses between 900 and 1100?°C. LiF enhances mainly tridymite with traces of cristobalite and in the opposite side the parent and both TiO₂ and Cr₂O₃ containing glasses catalyzed mainly cristobalite with traces of tridymite. The microstructure of the glass samples heat-treated at 1000?°C showed backbone-like, small rods, quadrant-, hexagonal crystals referred to the cristobalite and tridymite. The EDS microanalysis shows the presence of both alkalis, i.e. Na^?+? and K^?+?, in the cristobalite crystal structure while the residual glass contains K^?+?. The infrared spectroscopy indicates the band patterns of cristobalite and tridymite.     

低温烧结氧化铝陶瓷的动力学研究

用CaO-MgO-SiO2玻璃(CMS-G)和TiO2为烧结助剂,在1450℃下实现了氧化铝陶瓷的致密烧结.探讨了CMS-G和TiO2质量百分含量对氧化铝陶瓷烧结性能、显微结构以及其烧结动力学的影响.结果表明:CMS-G和TiO2可有效促进氧化铝陶瓷的致密化,当CMS-G含量为3%、TiO2含量为1%时,氧化铝陶瓷的相对密度达到98.25%;液相烧结激活能为113.4kJ/mol,表明扩散控制了烧结过程.     

陶瓷烧成中NOx生成的实测分析

通过对两种不同结构、不同烧成气氛梭式窑的整个烧成过程中NOx生成进行测试分析,得出选择含氮量少的洁净燃料有助于减少陶瓷烧成整个过程中NOx的生成;其次,烧嘴的合理布置及加强高温阶段窑内温度场的均匀性可以减少NOx的生成;第三,还原气氛可抑制NOx的生成.     

Crystallization Kinetics of Internally Nucleated Vitreous Silica

It has recently been observed that cristobalite can hetero-geneously nucleate and grow internally in vitreous silica. When the glass-crystal composite is cooled to room temperature, β-cristobalite is metastably retained as a result of the large tensile stresses developed during cooling. Although these crystals are not obvious, they can be readily observed with a polarizing microscope or polariscope. As long as the cristobalite does not transform when cooled to room temperature, repeated increments of crystal growth can be measured on the same growing crystal. Crystal growth was measured on electrically melted quartz glass at 1350° to 1620°C. The observed growth rates were linear with time and were the lowest that have been measured in vitreous silica. The kinetic data agreed with the general form of the crystal growth equation μ= (AΔT)/_ν. Since the growth is internal and free from surface contamination, the measured rates are considered to be very near the intrinsic rates for the material.     

由微晶粉烧结的多孔陶瓷组织结构的形成

研究了加压成型及烧结时二氧化锆微晶粉的性状。查明,甚至在不太高的成型压力50MPa条件下微晶粉的组织结构也会受到强烈的破坏。在加热阶段微晶粉晶粒之间的活化反应促进形成多孔骨架,后者在烧结并保温时发生的进一步致密化也是稳定的。     

鳞石英和方石英的制备

本文介绍了一种从α-石英制取鳞石英和方石英的疗法和条件。