Effect of Nitriding on Electrolysis and Devitrification of High-Silica Glasses

Devitrification occurs near the cathode when reconstructed high-silica glasses and fused quartz undergo electrolysis at 900°C. The crystalline phase is cristobalite and results from a concentration of alkali ions. Subsequent and more superficial crystallization around the alkali-rich spot is caused by interaction of the glass with alkali vapors coming from the central spot. The degree of devitrification increases with the alkali content of the glass. Devitrification at constant alkali level can be reduced in reconstructed glasses by nitriding the porous glass with ammonia at a high temperature before consolidation. Devitrification decreases and viscosity and electrical resistivity increase as the nitriding temperature increases 500° to 1000°C. Nitrided glasses are more resistant to electrolytically induced devitrification than is fused quartz.     

A Method for Measuring the Flow Point of Glass

The need for a new reference point for glasses, at a temperature substantially above the softening point, is discussed from the standpoint of development of. new glasses, control of properties, and behavior in applications where reworking is required. An apparatus and a procedure are described by which a fiber elongation test indicates the "flow point," or temperature at which a glass reaches a viscosity of 10⁵ poises. A short section of a glass fiber, nominally 0.65 mm. in diameter, is heated under specified conditions of load and heat application. The heating time required for the fiber to draw down to a thread indicates the temperature attained. When properly standardized with one or more known glasses, the precision is equivalent to from 1°C. for soft glasses to 5°C. for hard glasses. The effect of infrared absorption by the sample is examined, and a correction curve is given for use when such absorption is abnormally great.     

Viscosity, Density, and Electrical Resistivity of Molten Alkaline-Earth Borate Glasses with 3 Mole % of Potassium Oxide

Viscosity, density, and electrical resistivity were determined on three series of molten alkaline-earth borate glasses, each containing 3 mole % of potassium oxide. A previous investigation of these properties of the binary alkaline-earth borates was limited by the formation of two immiscible liquids at low alkaline-earth oxide concentrations. To overcome this difficulty 3 mole % of K₂O was added to each composition. The properties of this new series of glasses were compared with the previous work. The densities at room and elevated temperatures were in the order BaO > SrO > CaO on a mole per cent basis. The reverse order was noted for the viscosity data. The electrical resistivity decreased with increasing concentration of alkaline-earth oxide at 1000°C. and at 1100°C. but increased at 800°C. and at 900°C.     

Measurement of Stress-Optical Coefficient and Rate of Stress Release in Commercial Soda-Lime Glasses

A method is described for measuring the stress-birefringence properties of soda-lime glasses in the range 23° to 650°C. Data on three glasses show that the stress-optical coefficient increases slowly with temperature, reaching a definite maximum in the range 570° to 590°C. The same apparatus was used to measure the rate of stress release in two of these glasses. Data obtained under thermal equilibrium conditions can be represented by equations that suggest that two or more concurrent mechanisms are involved in stress release in these glasses within the viscosity range log 12 to log 14.5 (a rapid one of 0.4 to 10 minutes in duration and a slow one of 2 to 60 minutes).     

Effect of Fictive Temperature on Dynamic Fatigue Behavior of Silica and Soda-Lime Glasses

The dynamic fatigue characteristics of silica glasses with fictive temperatures of 1000°, 1100°, and 1300°C and soda-lime glasses with fictive temperatures of 470° and 530°C were measured in air. For both glasses, samples with higher fictive temperatures had a greater fatigue resistance. Inert strength of silica glasses with flctive temperatures of 1000° and 1300°C was also measured at liquid nitrogen temperature. Glass with higher flctive temperature had a greater inert strength.     

The Relation of Viscosity, Nuclei Formation, and Crystal Growth in Titania-Opacified Enamel

This investigation shows that there is a relationship between viscosity, number of nuclei, and crystal growth during the firing of titania-opacified enamels. This agrees with a similar relationship which Tammann found in his experiments with organic glasses. The history of the development of the size, shape, and relative number of particles per unit area of titanium dioxide crystals was traced from 650° to 1300°C. As the temperature increased from 700° to 1100°C., the color of the specimens viewed under reflected light changed from light blue to white and then to cream-white. The methods used in this investigation were the measurement of viscosity, X-ray analysis, differential thermal analysis, and the study by light microscopy and electron microscopy of very thin heat-treated films which had been produced by blowing bubbles from high-temperature melts.     

Mechanical Characterization of Sol–Gel-Derived Silicon Oxycarbide Glasses

Silicon oxycarbide glasses have been fabricated, in the shape of thin rods suitable for flexural test experiments, by pyrolysis in an inert atmosphere at 1000° and 1200°C of solgel precursors containing Si–CH₃ and Si–H bonds. The amount of carbon in the silicon oxycarbide network has been controlled by varying the carbon load in the precursor gel. Density and surface area analysis revealed that all of the samples pyrolyzed at 1200°C were well-densified silicon oxycarbide glasses whereas for the glasses treated at 1000°C, compositions with low carbon loads showed the presence of a residual fine porous phase. The elastic modulus ( E ), flexural strength (MOR), and Vickers hardness ( H_v ) increase markedly with the amount of carbon in the oxycarbide glasses reaching the maximum values ( E ∼ 115 GPa, MOR ∼ 550 MPa, and H_v ∼ 9 GPa) for samples with the highest carbon content. The experimental elastic modulus values of the silicon oxycarbide glasses compare well with the theoretical estimations obtained using the Voigt and Reuss models assuming the disordered network formed by the corresponding thermodynamic compositions.     

Nanostructural Characterization of Silicon Oxycarbide Glasses and Glass-Ceramics

Silicon oxycarbide glasses were synthesized by the sol-gel process using precursors such as methyl-, propyl-, and phenyltrimethoxysilanes. The final products contained 14–38 wt% carbon. A TEM study on the nanometer scale revealed that all of the materials were amorphous and monophasic, and that it was not possible to detect any crystalline or otherwise distinct carbon phases. Hot-pressing the glasses led to the crystallization of graphite and silicon carbide within the amorphous matrix. X-ray and electron diffraction showed increasing crystallinity at the higher hot-pressing temperatures. Hot pressing at 1400°C resulted in the appearance of fine-grained silicon carbide, whereas at the highest temperature (1650°C), graphite and both hexagonal and cubic silicon carbide were produced. Subsequent heat treatment of the hot-pressed glasses under an argon atmosphere at 1400°C resulted in the formation of cristobalite. The glass-ceramics produced at the highest hot-pressing temperatures were more resistant to the crystallization of cristobalite during subsequent heat treatments.     

Chemical Durability of Silicon Oxycarbide Glasses

Silicon oxycarbide (SiOC) glasses with controlled amounts of Si—C bonds and free carbon have been produced via the pyrolysis of suitable preceramic networks. Their chemical durability in alkaline and hydrofluoric solutions has been studied and related to the network structure and microstructure of the glasses. SiOC glasses, because of the character of the Si—C bonds, exhibit greater chemical durability in both environments, compared with silica glass. Microphase separation into silicon carbide (SiC), silica (SiO₂), and carbon, which usually occurs in this system at pyrolysis temperatures of >1000°–1200°C, exerts great influence on the durability of these glasses. The chemical durability decreases as the amount of phase separation increases, because the silica/silicate species (without any carbon substituents) are interconnected and can be easily leached out, in comparison with the SiOC phase, which is resistant to attack by OH⁻ or F⁻ ions.     

Structure and Mechanism of Conduction of Semiconductor Glasses

The area of glass formation in the system GeO₂-P₄O₁₀-V₂O₅ and the properties of the glasses in this area were determined. The glasses displayed electronic conduction at room temperature (25°C). Resistivity ranged from 500 ohm-cm to 10⁹ ohm-cm at 25°C. Some of the glasses had unusual negative temperature coefficients of resistance of the order of -760,000 ppm °C⁻¹ in the range 25° to -55°C. Volt-ampere characteristics indicated nonlinearity suitable for thermistor application. Other unusual properties included high refractive indices from 1.6 to >2.0 and dielectric constants from 6 to 33 at 1 Mc and 25°C. Values of loss-tangents, however, were high. Infrared spectra indicated that the V⁵⁺ ion existed in sixfold coordination in the glassy state as well as in the devitrified crystalline state. The normal vibrational frequency of the V–O bond at 1015 cm⁻¹ was observed for all glasses in the system. Property versus composition curves indicated that density, refractive index, and dielectric constant of ternary glasses in the system do not obey the additivity rule. The density versus mole % V₂O₅ curve goes through a minimum. Derived quantities from experimental data indicate pronounced influence of V₂O₅ on oxygen packing in the system. Addition of SiO₂, even in small quantities, destroys glass formation. The structure of these glasses, which differs from that of silicate glasses, is discussed. A mechanism of conduction is suggested, based on evidence from magnetic susceptibility, chemical analysis, activation energy, and infrared spectra.     

Porous Silicon Oxycarbide Glasses

High-surface-area silicon oxycarbide gels and glasses were synthesized from mixtures of methyldimethoxysilane (MDMS) and tetraethoxysilane (TEOS) through acidic hydrolysis and condensation. A surface area of ∼275 m²/g and an average pore size of ∼30 Å was obtained for a 50% MDMS-50% TEOS glass at 800°C under a flowing argon atmosphere. The average pore size was increased by aging the precursor gels in ammonium hydroxide. The increased average pore size and the higher strength of the mesoporous gel network enhanced the surface-area stability of the glasses; in this case, surface areas >200 m²/g were retained at 1200°C under an argon atmosphere. ²⁹Si MAS NMR spectra revealed that an oxycarbide structure was established in the mesoporous glasses obtained after pyrolysis of the aged gels. The role of carbon was demonstrated by comparing the surface-area stability of the oxycarbide glasses with that of pure silica and that of oxycarbide glasses where all the carbon groups were removed through low-temperature plasma-oxidation treatments. In the absence of carbon, the thermal stability of the surface area decreased dramatically.     

Nucleation and Crystallization of New Glasses from Fly Ash Originating from Thermal Power Plants

The nucleation and crystallization kinetics of new glasses obtained by melting mixtures of a Spanish carbon fly ash with glass cullet and dolomite slag at 1500°C has been evaluated by a calculation method. These glasses, whose microstructure was examined by TEM carbon replica, were susceptible to controlled crystallization in the 800°–1100°C range. The resulting glass-ceramics developed acicular and branched wollastonite crystals or a network of dendritic pyroxene mixed with anorthite feldspar (SEM and EDX analysis). The time–temperature–transformation curves (processing of the XRD data) showed the crystallization kinetics and the critical cooling rate to be in the 12°–42°C/min range.     

Phase Relations and Glass Formation in the System PbO-GeO,

Phase relations in the system PbO-GeO₂ were determined using the quenching technique. The five compounds detected were: 4PbO-GeO₂, 3PbO-2GeO₂, PbO-GeO₂, and PbO-4GeO₂. The 3:2 and 1:1 compounds melt congruently at 744° and 799°, respectively. The 4:1 compound melts incongruently at 726°C to PbO plus liquid, whereas the 1:4 compound melts incongruently to GeO₂ plus liquid at 790°C. The 1:2 compound has a temperature range of stability between 707° and 730°. The data indicate that no liquid immiscibility gap exists in the system. Indices of refraction for glasses in the system were compared with lead silicate glasses. An addition of ∼65%PbO to GeO₂ is required to prepare a glass with an index near 2.0 whereas with SiO₂, ∼85% PbO is required. It appears that the lead germanate glasses have higher indices than all other two-component oxide glasses. The addition of PbO to GeO₂ decreases the rutile-to-quartz transformation temperature from 1000°C for pure GeO₂ to 990°C. Infrared spectra of lead germanate glasses (∼60w% PbO) show that transmission is good up to 5.5μ but decreases drastically between 5.5 and 6.5μ.     

Prominent Nanocrystallization of 25K2O·25Nb2O5·50GeO2 Glass

Some K₂O-Nb₂O₅-GeO₂ glasses are prepared, and their crystallization behaviors are examined. 25K₂O·25Nb₂O₅·50GeO₂ glass with the glass transition temperature T _g= 622°3C and crystallization onset temperature T _x= 668°3C shows a prominent nanocrystallization. The crystalline phase is K_3,8Nb₅Ge₃O_20,4 with an orthorhombic structure. The sizes of crystals in the crystallized glasses heat-treated at 630° and 720°3C for 1 h are °10 and 20–30 nm, respectively, and the crystallized glasses obtained by heat treatments at 620°-850°3C for 1 h maintain good transparency. The density of crystallized glasses increases gradually with increasing heat-treatment temperature, and the volume fraction of crystals in the sample heat-treated at 630°3C for 1 h is estimated to be ∼35%. The usual Vickers hardness and Martens hardness (estimated by nanoindentation) of 25K₂O·25Nb₂O₅·50GeO₂ glass change steeply by heat treatment at T _g, i.e., at around 35% volume fraction of nanocrystals. The present study demonstrates that the composite of nanocrystals and the glassy phase has a strong resistance against deformation during Vickers indenter loading in crystallized glasses.     

Transmission Electron Microscopy and Electron Energy-Loss Spectroscopy Study of Nonstoichiometric Silicon-Carbon-Oxygen Glasses

Crystallization behavior of Si-C-O glasses in the temperature range of 1000°–1400°C was investigated using transmission electron microscopy (TEM) in conjunction with electron energy-loss spectroscopy (EELS). Si-C-O glasses were prepared by pyrolysis of polysiloxane networks obtained from homogeneous mixtures of triethoxysilane, T^H, and methyldiethoxysilane, D^H. Si-C-O glass composition depended on the molar ratio of the precursors utilized. At a ratio of T^H/D^H= 1, the formation of a carbon-rich glass was observed, whereas a ratio of T^H/D^H= 9 yielded a Si-C-O glass with excess free silicon. Both materials were amorphous at 1000°C, but showed a distinct difference in crystallization behavior on annealing at high temperature. Although T^H/D^H= 1 revealed a small volume fraction of SiC precipitates in addition to a very small amount of residual free carbon at 1400°C, T^H/D^H= 9 showed, in addition to SiC crystallites, numerous larger silicon precipitates (20–50 nm), even at 1200°C. Both materials underwent a phase separation process, SiC _x O_2(1-x)→ x SiC + (1 - x )SiO₂, when annealed at temperatures exceeding 1200°C.     

Fracture of Glass in Vacuum

The fracture of 6 glasses was studied in vacuum, <10⁻⁴ torr (10⁻² N/m²), as a function of temperature from 25° to 775°C. Subcritical crack growth was observed in 4 of the glasses. Activation energies for crack motion ranged from 60 to 176 kcal/mol. The glasses which did not exhibit slow crack growth were "anomalous" glasses with abnormal thermal and elastic properties. Critical stress intensity factors for these 2 glasses increased ∼10% as the temperature increased to ∼600°C. It is felt that subcritical crack growth is not the result of alkali-ion diffusion or viscous flow but rather of a thermally activated growth process which depends on the crack-tip structure in the glass. A narrow cohesive region at the crack tip favors subcritical crack growth, whereas a wide region favors abrupt fracture.     

Structural Characterization and High-Temperature Behavior of Silicon Oxycarbide Glasses Prepared from Sol-Gel Precursors Containing Si-H Bonds

Silicon oxycarbide glasses have been synthesized by inert atmosphere pyrolysis at 1000°C of gel precursors obtained by cohydrolysis of triethoxysilane, HSi(OEt)₃, and methyl-diethoxysilane, HMeSi(OEt)₂. The oxycarbide structures have been carefully characterized by means of different techniques such as ²⁹Si magic angle spinning nuclear magnetic resonance (MAS-NMR) and Raman spectroscopies, X-ray diffraction (XRD), and chemical analysis. Experimental results clearly indicate that, depending on the composition of the starting gels, the resulting oxycarbide glass either is formed by a pure oxycarbide phase or contains an extra carbon or silicon phase. By increasing the temperature up to 1500°C, the oxycarbide glasses display compositional and weight stability; however, the amorphous network undergoes structural rearrangements that lead to the precipitation of nano-sized β-SiC crystallites into amorphous silica. Crystallization of metallic silicon is also clearly observed at 1500°C for the samples in which the presence of Si-Si bonds was postulated at 1000°C.     

Preparation of High-Silica Glasses from Colloidal Gels: II, Sintering

The sintering of dried colloidal SiO₂ gels, whose preparation and properties are reported in Part I, is described. The effects of various sintering parameters were studied and the conditions for achievement of the best optical quality include the use of: a pretreatment of the SiO₂ at ∼925°C, moderate heating rate (∼400°C/h), He+CI₂ atmosphere, 1500° to 1600°C sintering temperature, and 1 to 4 h sintering time. Dynamic sintering kinetic studies (heating rate=400°C/h) show that this SiO₂ sinters to nearly theoretical density by about 1380°C. However, optical transparency is achieved by removal of minor residual porosity at above 1500°C. Isothermal sintering data fit to a model assuming interconnecting cylinders of SiO₂ predict the proper activation energy for the viscosity if initial stages of sintering are considered. Residual porosity in sintered glasses is related to large interstices in the unsintered gel.     

Oxycarbide Glasses in the Mg-Al-Si-O-C System

Oxycarbide glasses in the Mg-Al-Si-O-C system were produced at initial carbon levels of 0.0, 0.5, 1.0, 1.5, 2.0, and 2.5 wt%. Carbon was incorporated into the glass melts by means of SiC additions. The glasses were melted between 1750° and 1800°C under nitrogen atmospheres. The limit of carbon incorporation was reached at the 2.5% carbon level, as these glasses crystallized (predominantly cordierite, 2MgO˙2Al₂O₃˙5SiO₂) upon gradual cooling from the meit. Glasses containing less than 2.5% carbon were amorphous according to standard X-ray diffraction methods. Further examination of these oxycarbide glasses by transmission electron microscopy indicated the lack of microcrystalline phases and the potential for producing clear inclusion-free glasses. The Mg-Al-Si-O-C glasses showed significant increases in density, Young's elastic modulus, shear modulus, Vickers hardness, and fracture toughness with increasing carbon content.     

Nonequiaxial Grain Growth and Polytype Transformation of Sintered α-Silicon Carbide and β-Silicon Carbide

α(6 H )- and β(3 C )-SiC powders were sintered with the addition of AlB₂ and carbon. α-SiC powder could be densified to ∼98% of the theoretical density over a wide range of temperatures from 1900° to 2150°C and with the additives of 0.67–2.7 mass% of AlB₂ and 2.0 mass% of carbon. Sintering of the β-SiC powder required a temperature of >2000°C for densification with these additives. Grains in the α-SiC specimens grew gradually from spherical-shaped to plate-shaped grains at 2000°C; the 6 H polytype transformed mainly to 4 H . On the other hand, grains in the β-SiC largely grew at >2000°C; the 3 C polytype transformed to 4 H , 6 H , and 15 R . The stacking faults introduced in grains were denser in β-SiC than in α-SiC. The rapid grain growth in the β-SiC specimen was attributed to polytype transformation from the unstable 3 C polytype at the sintering temperature.