Kinetics of Cristobalite Formation in Sintered Silica

The kinetics of the cristobalite transformation are reported for sintered silica glass from 1200°C to 1650°C and plotted as a time–temperature–transformation diagram. The 1200°C–1350°C transformation data were fit to the Johnson–Mehl–Avrami–Kolmogorov expression with an time exponent of 3.0 ± 0.6 and an apparent activation energy of 555 ± 24 kJ/mol for the kinetic constant. The temperature of maximum transformation rate was found to fall between 1500°C and 1600°C. Seeding amorphous silica powder with cristobalite resulted in accelerated transformation kinetics. Silica glass powder containing residual quartz had faster transformation kinetics than fully amorphous powder seeded with cristobalite.     

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。     

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。     

Effect of Alkali Hydroxides on the Strength and Fatigue of Fused Silica Optical Fiber

A two-point bend technique is used to determine the strength of "pristine" bare fused silica optical fiber in aqueous solutions of the group I alkali metal hydroxides (Li through Cs) in the concentration range 10⁻⁴ N to 1 N (pH range 10 to 14). In the highest concentration solutions, a strong cation effect is exhibited with a minimum strength for KOH while fibers in LiOH and CsOH are typically 20% and 10% stronger, respectively. The cation effect decreases with decreasing concentration until at 10⁻⁴ N no effect is apparent. Tetramethylammonium hydroxide gives intermediate strengths between CsOH and LiOH. The results correlate closely with published results for the dissolution rate of silica which exhibit a maximum rate for KOH. In contrast, the results do not correlated with published subcritical crack growth data which show no cation effect for Na through Cs. These results support the pit etching model for pristine fiber strength proposed by Kurkjian et al. and argue against the presence of cracks or cracklike objects in these materials.     

Transformation of Quartz to Cristobalite

The transformation of quartz to cristobalite was studied in the temperature range 1400° to 1650° C. The process which was believed to be a direct one was found to be a consecutive reaction involving an intermediate transition phase. Both Madagascar and Brazilian quartz were used to study the transformation reaction. The reaction rate was found to be much faster for the Madagascar quartz than for the Brazilian quartz, although no difference in their impurity content was found in the spectrographic analysis. An approximate consecutive reaction kinetics was applied and the deviation of the experimentally observed values of the transition phase at a particular temperature from that of the mathematical prediction was noted and is discussed.     

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.     

石英陶瓷的应用

介绍了石英陶瓷在玻璃行业以外，冶金、化工、航空航天等领域的应用，包括应用的石英陶瓷制品、应用特点及条件。     

Hot Pressing of Fused Silica

The hot pressing of powdered fused silica at temperatures below the melting and devitrification regions is described. The fabrication of high-density shapes of fused silica is possible by this process. A relation expressing densification as a function of viscosity, time, applied pressure, and initial compaction is essentially substantiated.     

燃烧器芯数对CVD熔石英玻璃性能的影响

本文通过数值模拟研究了燃烧器芯数对CVD熔石英玻璃性能的影响,讨论了采用五芯、七芯和九芯三种结构燃烧器时沉积面上的温度、OH^-浓度及SiO₂浓度分布,并分析了不同氢氧当量比的影响规律。结果表明:相比于五芯结构燃烧器和七芯结构燃烧器,采用九芯结构燃烧器可以在沉积面获得更加均匀的SiO₂浓度分布和温度分布;当使用九芯燃烧器时,氢氧当量比为1.5可以提高沉积面上温度、OH^-浓度和SiO₂浓度的分布均匀性,从而有利于提高熔石英玻璃的折射率均匀性。     

石英陶瓷的研究应用现状及发展前景

本文综述了石英陶瓷的应用现状,对其研究、应用进行了较为详细的探讨,然后介绍了石英陶瓷增强技术的发展情况,并对石英陶瓷的发展前景和趋势作了介绍。     

聚乙烯醇在石英陶瓷天线罩上的应用

采用泥浆浇注工艺方法制备石英陶瓷天线罩,对比了石英料浆中添加与未添加聚乙烯醇(PVA)对石英陶瓷天线罩轴向各部位弯曲强度、体积密度、介电常数的影响情况。结果显示:聚乙烯醇(PVA)的加入对于提高石英陶瓷天线罩各部位力学性能、介电性能的均匀性有着明显的效果。     

Effect of Organic Binder Segregation on Sintered Strength of Dry-Pressed Alumina

The influence of binder systems on the structure and properties of alumina ceramics was examined. Two types of binders were used to prepare alumina granules by spray drying: poly-acrylic acid (PAA) and poly-vinyl alcohol (PVA). PVA segregated to the surface and subsurface of the granules, whereas PAA was homogeneously distributed. Compaction tests on a single granule showed that the PAA binder provides lower yield stress of granules than PVA. In the die compaction process, the relative density of the compact body started to increase at a lower pressure, and a more homogeneous internal structure was observed in the green compact prepared with the PAA binder than PVA. The homogeneous structure resulted in a sintered body with a strength of 580 vs 485 MPa when PVA was used.     

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.     

Aqueous Gelcasting of Fused Silica Using Colloidal Silica Binder

An aqueous‐based gelcasting of fused silica ceramics by using colloidal silica binder was developed. Fused silica slurries having different volume percentage of solid loading from 63 to 74 vol% in colloidal silica were made and the rheological properties were evaluated. It was found that the slurry with 73 vol% of solid loading with viscosity 0.70 Pa.s is suitable for this gelcasting system. The influence of solid loading on physical and mechanical properties of gelcast green and sintered bodies has been studied. The fabricated green body by using colloidal silica binder exhibited a flexural strength of 9 MPa and 88% of theoretical density with 2.2% of drying shrinkage while the sintered sample exhibited flexural strength of 60 MPa and 95% of theoretical density with 4.3% of sintering shrinkage. It was observed that, the nano silica particles from the colloidal silica binder is filling the interstitial positions in the consolidated fused silica green body and enhancing physical and mechanical properties.     

熔融石英陶瓷的性能及其应用

介绍了石英陶瓷的性能特点、制备过程及应用领域     

Interpretation of the Infrared Spectra of Fused Silica

A vibrational analysis was performed for fused silica assuming short-range order in the glassy network. This order is described by an average structural unit based on the model α-quartz. The analysis uses a group theoretical method and the Wilson FG-matrix techniques to predict 16 normal mode vibrational frequencies based on a diagonal force field. The most probable values of the force constants, which were obtained from the best fit between the calculated vibrational frequencies and the measured spectral values, are:

Although the selection rules are not adhered to, 15 predicted frequencies are within the ranges of the spectra of fused silica reported in the literature.