Three Anisotropies in Vitreous Silica

Plasma-torch fused-silica boules of low OH content showed an optical anisotropy in the as-grown state which disappeared after annealing in O₂; this behavior was related to an EPR anisotropy which also depended on annealing, probably involving Fe²⁺ and Fe³⁺, respectively. High-temperature (1200° to 1500°C) deformation experiments performed in a modified DTA machine on slabs cut parallel and perpendicular to the preparation symmetry axis also showed an anisotropy, corresponding approximately to a factor of 2 in the viscosity. This anisotropy was also observed in commercial worked fused silica of high and low OH (water) content, made from purified Si compounds or natural quartz, and was not removed by annealing for 5 days at 1200°C in O₂. The deformation rate of fused silica decreases as water content is reduced from high to medium, but there is little further change when water content is decreased to a very low level.     

Deuteroxyl Groups in Vitreous Silica

Vitreous silica exhibits a characteristic absorption band at 3.81 μm which was attributed previously to a combination frequency of SiO₄ fundamentals. Deuterium treatment of water-containing fused silica also gives rise to an absorption band at 3.81-μm. An analysis of the previously characterized 3.81-μm band and its relation to absorption bands in deuterated specimens is presented.     

Diffusion of Oxygen in Vitreous Silica

The diffusivity of the oxygen ion in vitreous silica has been directly determined by exchange measurements employing the stable isotope ¹⁸O and mass spectrometer analysis. It was found that over the temperature range 925° to 1225°C the results can be represented by the equation D = 1.51 × 10⁻² exp (-71,200/ RT ) cm² sec⁻¹. These results are compared with other measurements of oxygen diffusion in silicate glasses. It is proposed that the controlling diffusion step in silicate glasses and nonstoichiometric silica is the rupture of a single oxygen bond to silicon and that the diffusion mechanism is interstitial motion through voids in the lattice. An analysis of theoretical expressions for the pre-exponential term D _O shows that present theories are unable to predict D _O for oxygen diffusion in glasses. It is also shown that the mechanism for electrical conduction in vitreous silica or in electrolytically purified quartz is not migration of oxygen ions.     

Oxygen Tracer Diffusion in Vitreous Silica

Oxygen diffusion in vitreous silica glass is studied using the gas exchange technique. The tracer concentration profiles are consistent with a model based on two mechanisms, one network and the other interstitial. These processes are coupled through limited network–interstitial exchange. Nuclear reaction analysis and secondary ion mass spectrometry techniques are performed and compared. These results are compared to experiments on transport in thin silica films grown on single-crystal silicon.     

Surface Energy of Crystalline and Vitreous Silica

A variant of evaluation of the theoretical values of the surface energy of quartz glass and various forms of crystalline silica based on analysins of the structures of these materials and experimental determination of the zeta-potential of β-quartz and amorphous silica is considered.     

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.     

Structure of Neutron-Irradiated Quartz and Vitreous Silica

The structure of quartz and vitreous silica disordered by heavy irradiation (10²⁰ neutrons per cm.²) of fast neutrons was studied by means of X-ray diffraction and infrared spectroscopy. The results indicate that the structure of the disordered amorphous form of silica resembles closely that of vitreous silica. The average Si–O distance (1.61 a.u.) remains unchanged, but the average Si–O–Si bond angle decreases from 142° to 138°; the radial distances of the more distant neighbor atoms show a wider distribution in the disordered material. It is suggested that the disordered form of silica results from thermal effects (thermal spikes) accompanying the passage of knocked-on atoms through the solid.     

Diffusion of Alkali Ions in Vitreous Silica

The diffusion of Cs⁺, Rb⁺, and K⁺ ions was measured in three grades of vitreous SiO₂ by the radiotracer-sectioning technique or Rutherford, backscattering spectroscopy. The values of the diffusion coefficient, D , at 1000°C decrease strongly with increasing ionic radius, changing by about two and one-half orders of magnitude per row of the periodic table. The difference between D_Cs and D_Rb is largely in the preexponential factor D_o in the Arrhenius expression rather than in the activation enthalpy Q . The values of D are much smaller than the tracer D values for the alkali metal ion in homogeneous Rb or Cs silicate glasses. Residual metallic impurities in the SiO₂ decrease both Q and D_o for the diffusion of Rb. The results are analyzed in terms of the interstitial structure of vitreous SiO₂.     

Reaction Between Vitreous Silica and Molten Aluminum

Fused silica rods were exposed to molten aluminum and the rate of penetration of the reaction into the silica was measured over a range of temperatures. The reaction obeys the equation:

Three phases of alumina are formed during the reaction. The Group V elements, bismuth and antimony, markedly affect the rate of reaction. It is proposed that they form a lamellar surface compound with oxygen and water already absorbed on the silica surface. This compound then controls the reaction kinetics.     

Temperature Dependence of Elastic Constants of Vitreous Silica

The relation between the elastic moduli and temperature from room temperature to about 1300°C was determined for a group of vitreous silica specimens by a dynamic resonance method. All the curves were approximately parabolic in shape, reaching a maximum near 1050° to 1200°C. At the maximum value, Young's modulus was more than 11% higher and the shear modulus was about 9% higher than their room-temperature values. Poisson's ratio was then computed to rise from about 1/6 at room temperature to about 1/5 at the temperature of maximum elastic modulus. Small but significant differences were observed in the temperature-modulus curves for specimens from different sources. These differences were found to be related to differences in the infrared transmission curves.     

Adherence Between Molybdenum Sealing Foil and Vitreous Silica

The performance of vitreous silica seals is governed by the interaction between the molybdenum sealing foil and vitreous silica with respect to stress level and adherence. Mechanisms of sealing failures, i.e., oxidation from the outside and interface corrosion by the filling substance, are discussed with respect to affected lamps and requirements for the seal. With respect to wetting and chemical bonding, particular attention has to be directed toward the physical and chemical nature of second-phase species in the interface, which is discussed for Y₂O₃ and TiO₂. Using MY (Mo-0.47wt% Y₂O₃–0.08wt% Ce₂O₃) foils, yttrium-rich clusters with a size of a few nm up to 30 nm can be detected within the vitreous silica at a distance of up to 0.6 μm from the molybdenum/vitreous silica interface. In the MY sealing foil/vitreous silica interface, yttrium can be detected only locally, but always in combination with silicon. In the case of Mo-0.55 wt% TiO₂ foils, titanium can be found at each point measured within a distance of 0.58 μm from the interface for a vacuum-collapse seal and 0.30 μm for a pinch seal. Titanium clusters with a size of 3–15 nm can be detected in the case of vacuum-collapse-sealed lamps, whereas in pinch-sealed lamps, only a few clusters up to 5 μm occur very close to the interface.     

Helium Migration in Natural and Synthetic Vitreous Silica

Permeation, diffusion, and solubility of helium in several types of vitreous silica were measured. The activation energy for diffusion increases slightly with increasing hydroxyl concentration; no other significant trends were noted. These results are interpreted in terms of a random-network structural model.     

Alumina Substructure and Electrical Conductivity in Vitreous Silica

A network of alumina cells that remains after a vitreous silica sphere is heated in hydrogen supports the suggestion that alumina and associated sodium and lithium ions in vitreous silica are nonuniformly distributed. This distribution explains the anomalously large diffusion coefficient and electrical mobility of these alkali ions in vitreous silica.     

Quantitative Determination of the Deuteroxyl Content of Vitreous Silica

The deuteroxyl absorptivity for vitreous silica was determined by measurement of the reaction of hydrogen and deuterium with a form of vitreous silica known to form a specific amount of hydroxyl under all experimental conditions. The deuteroxyl absorptivity was found to be 90.4% of that of hydroxyl, or 164 L /(mol·cm).     

The Energy of the Vitreous Lattice of Amorphous Silica

The concept of energy of a vitreous lattice is considered, its meaning is discussed, and a quantitative evaluation of this parameter with respect to quarts glass is carried out.     

Quantitative Determination of the Hydroxyl Content of Vitreous Silica

The molar extinction coefficient for bound water in the form ofhydroxyl in vitreous silica has been determined by a thermogravimetric technique. The value obtained, 181 L/mol cm, is believed to be considerably more accurate than the values now used.     

Effect of Trace Impurities on Devitrification of Vitreous Silica

Devitrification rates were measured for a series of vitreous silica glasses doped with trace levels of alkali metal oxide (1.6 to 57.8 molar ppm) and aluminum oxide (22 to 73.3 molar ppm). The devitrification rate varies inversely with the molar ratio of aluminum oxide to alkali metal oxide, a direct consequence of the effect of these trace impurities on viscosity .