Effects of Viscosity and Stress Level on Rates of Stress Release in Soda-Lime, Potash-Barium, and Borosilicate Glasses

Previous work has suggested that two concurrent mechanisms act to relieve stresses in soda-lime commercial glasses in the viscosity range log 12 to log 14.5. The data were found to be best represented by a relation which indicated that the release of stress is the result of at least two distinct, but not necessarily independent, rate processes. The present paper shows stress release and viscosity data on two other types of glasses, namely, a potash-barium glass and a borosilicate. The data show that all three types of glass can be represented by similar equations and that the release of stress in glasses is related to both equilibrium temperature viscosities and delayed elasticity within the annealing zone.     

Stress Relaxation Modulus of a Commercial Glass

The stress relaxation modulus in compression of a container glass was investigated over a wide range of strain, time, modulus, and temperature. The glass is a linear viscoelastic liquid up to 2% strain, and the modulus is a smooth function of time, with no pseudorubbery plateau apparent down to a modulus of 10° dynes/cm². The data cover 4 decades in time and a range of almost 100°C above the glass transition, T ₀ =536°C. Within experimental error, changes in temperature simply shift the modulus-vs-time curve along the time axis without altering its shape. This behavior implies that the same mechanism controls both the bulk and shear spectra. The shift factors fit the WLF equation well with values for the parameters c₁ and c₂ of 16.7±0.2 and 345±5°C, respectively. Data from the literature for silicate glasses agree with these parameters.     

Creep Viscosity and Stress Relaxation of Gel-Derived Silicon Oxycarbide Glasses

The temperature dependence of the viscosity and stress-relaxation kinetics of sol–gel-derived SiOC glasses that contain up to 14 at.% carbon have been characterized in the temperature range of 1000°–1400°C. The viscosity, as determined from relaxation experiments, is in good agreement with the creep viscosity and is typically two orders of magnitude higher than the viscosity of vitreous silica. However, materials suffer from partial crystallization at >1150°C, and the precipitation of β-SiC nanocrystals induces a flow-hardening behavior and results in a dynamic increase in viscosity, especially at >1200°C and for glasses with a high carbon content.     

Temperature Dependence of Static Fatigue of Optical Fibers Coated with a Uv-Curable Polyurethane Acrylate

The static fatigue of fused silica optical fibers coated with a uv-curable polyurethane acrylate was measured, in water, from 40° to 90°C in 10° C intervals. The usual log (time-to-failure) vs log (applied stress) plotting of the data gives a bilinear fit at all test temperatures examined. By assuming exponential temperature dependence for static fatigue parameters, empirical static fatigue equations are generated for the two straight-line segments. These equations are then used to generate a static fatigue design diagram at a given temperature. In view of the possible coating effects on static fatigue, these results should be cautiously applied to optical fibers coated with other polymers.     

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.     

Optical, Structural, and Chemical Characteristics of Lead-Indium Phosphate and Lead-Scandium Phosphate Glasses

Two new glass systems based on a range of lead-indium phosphate and lead-scandium phosphate compositions have been developed and characterized. These glasses have a relatively high index of refraction ( n = 1.75 to 1.83) in the visible region and exhibit moderate dispersion (Abbe number ∼30). The ultraviolet absorption edge occurs near 300 nm and the glasses strongly absorb in the infrared at wavelengths greater than 2800 nm. Both glass types can be prepared at relatively low temperatures (900° to 1000°C) and are easily poured down to ∼800°C because of their low melt viscosities. The glasses exhibit good chemical durability and resistance to both weathering and intense γ-radiation. These materials have a glass transition temperature of about 430°C, a softening point of about 460°C, and thermal expansion coefficients in the range of 10.8 × 10⁻⁶ to 12.0 × 10⁻⁶/°C. The structure of these phosphate glasses is shown to consist of a distribution of chains of PO₄ tetrahedra held together by bonding between the non-bridging oxygens of the tetrahedra and the metal cations. The polyphosphate chain length distribution was determined by a liquid chromatographic technique. Potential aqueous corrosion mechanisms are discussed and some general guidelines for forming a chemically durable phosphate glass are given.     

Liquidus Prediction in Multicomponent Lithium Alumosilicate Glasses

An empirical nine-parameter linear mixing model was developed to estimate the liquidus temperature of zirconia-bearing technical lithium alumosilicate glasses that contain up to 12 major components. The model covers glasses that are suitable as precursor glasses for β-quartz or β-spodumene-type glass ceramics with low coefficients of thermal expansion. Molar coefficients of liquidus temperatures are provided for each component. A self-consistency test results in a standard deviation between predicted and measured liquidus temperatures of 7.5°C, within a range of approximately 200°C and for a typical experimental error of 10°C.     

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.     

Viscoelastic Effects in a Phosphate Glass-Metal Seal

A family of phosphate glasses has been developed with thermal expansion behavior in the elastic range that nearly matches the response of 304 stainless steel. Attempts to make concentric pin-shell seals consistently have yielded fractures between 400° and 300°C during cooling. Elastic stress analyses which neglect glass transitional behavior and utilize constant glassy ("elastic") thermal expansion coefficients predict a residual stress state that is compressive. However, viscoelastic computations which include the effects of structural relaxation during glass transtion show that tensile stresses sufficient to cause failure arise during cooldown of the seal.     

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.     

Role of Grain-Boundary Glass Phase on the Superplastic Deformation of Tetragonal Zirconia Polycrystal

Superplastic deformation of tetragonal zirconia polycrystal (TZP) was investigated by compression test in the temperature range of 1000° to 1500°C. Special attention was paid to the role of the grain-boundary glass phase on hightemperature deformation behavior. A small addition of glass phase markedly improved the high-temperature deformability of TZP. Lithium silicate glass was much superior to aluminosilicate or lithium aluminum silicate glasses for lowering the high-temperature flow stress. The deformation mechanism was discussed on the basis of mechanical testing data and microstructural examinations.     

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.     

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.     

Infrared-Transmitting Glasses in the System K2O-Sb2O3-Sb2S3

Glasses were discovered in the system K₂O-Sb₂O₃b₃ Raw materials used in the preparation of these glasses were potassium pyroantimonate, potassium hydroxide, potassium nitrate, antimony oxide, and antimony trisulfide. Details of the methods of preparing the glasses and the compositions investigated are given. A glass, prepared by melting a mixture of potassium pyroantimonate and antimony trisulfide in air, was investigated in some detail. It was found to have an average infrared transmission of 42% in the range 2 to 7 μ. The glass annealed at about 150°C. and softened at about 230°C. Its coefficient of linear thermal expansion, in the range 240° to 200°C., was 20 × 10⁻⁶ per °C. The glass had a specific gravity of 3.94, a modulus of elasticity of about 5 × 10⁶lb. per sq. in., a Knoop hardness of about 135, and was highly resistant to attack by atmospheric moisture.     

Thermooptic Coefficients of Some Standard Reference Material Glasses

The thermooptic coefficients, i.e., the change in refractive index with temperature (d n /d T ), of four National Institute of Standards and Technology standard reference material (SRM) glasses have been measured over the range of 25° to 125°C. The thermooptic coefficients of all four glasses, NBS-710 (a soda-lime silicate), SRM-711 (a lead silicate), SRM-717 (a borosilicate), and SRM-739 (silica) are positive and range in value from 2 × 10⁻⁶/K to 9.8 × 10⁻⁶/K. The differences in the d n /d T of these glasses arise from differences in the coefficient of thermal expansion and the temperature coefficient of the electronic polarizability.     

Fatigue Crack Propagation in Ceria-Partially-Stabilized Zirconia (Ce-TZP)-Alumina Composites

Fatigue crack propagation rates in tension-tension load cycling were measured in ZrO₂-12 mol% CeO₂-10 wt% Al₂O₃ ceramics using precracked and annealed compact tension specimens. The fatigue crack growth behavior was examined for Ce-TZPs of different transformation yield stresses obtained by sintering for 2 h at temperatures of 1500°C (type A), 1475°C (type B), 1450°C (type C), and 1425°C (type D). The threshold stress-intensity range, ΔK_th, for initiation of fatigue crack propagation increased systematically with decreasing transformation yield stress obtained with increasing sintering temperature. However, the critical stress-intensity range for fast fracture, ΔK_c, as well as the stress-intensity exponent in a power-law correlation (log (da/d N ) vs log ΔK) were relatively insensitive to the transformation yield stress. The fatigue crack growth behavior was also strongly influenced by the history of crack shielding via the development of the crack-tip transformation zones. In particular, the threshold stess-intensity range, Δ K _th, increased with increasing size of the transformation zone formed in prior quasi-static loading. Crack growth rates under sustained peak loads were also measured and found to be significantly lower and occurred at higher peak stress intensities as compared to the fatigue crack growth rates. Calculations of crack shielding from the transformation zones indicated that the enhanced crack growth susceptibility of Ce-TZP ceramics in fatigue is not due to reduced zone shielding. Alternate mechanisms that can lead to reduced crack shielding in tension-tension cyclic loading and result in higher crack-growth rates are explored.     

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.     

Mechanical Properties of Polycrystalline TiC

The mechanical properties of fine-grained polycrystalline TiC were studied using both four-point bending and compression tests. The ductile-brittle transition (D-B) temperature in compression was determined to be =800°C and was found to depend on grain size. Yield-point behavior was observed for the first time in fine-grained TiC deformed in compression and was found to depend on grain size and test temperature. The yield stress as a function of grain size can be described by a Hall-Petch type of relation, i.e. yield stress α (grain size)-1/2. The dislocations resulting from deformation in compression at lower temperatures were predominately screw in character, with edge dipoles and dislocation loops being present. As the temperature of deformation was increased, the dipoles and loops were gradually annihilated by climb and the dislocations were observed in the form of hexagonal networks with a much-reduced dislocation density. A plot of log yield stress vs 1/T showed a change in slope, which suggests that two rate-controlling mechanisms are in operation during deformation at different test temperatures. Thermal activation analysis at T = 1050° to 1500°C suggested that the rate controlling mechanism during deformation in this temperature range is associated with cross slip.     

Zinc Tellurite Glasses

Glass formation was observed in the system ZnO-TeO₂ in mixtures containing 20 to 40 mole % ZnO. Softening occurred at 350°C and the glasses crystallized at 400°C. High dielectric constants (ca. 20) and high electrical resistivities (ca. 10¹¹ ohm-cm⁻¹) were measured. The zinc tellurite glasses have a higher density and refractive index and are considerably softer than silicate glasses. The optical transmission range was between 0.38 and 6.6μ with two absorption bands in the infrared. Spectra of Cu²⁺ and Nd³⁺ doped samples are also given.     

Mechanical Properties of Chemically Strengthened Glasses at High Temperatures

Young's modulus, shear modulus, and modulus of rapture for two chemically strengthened glasses were determined at high temperatures. The Young's modulus and shear modulus decreased with increasing temperature, with a sharp inflection slightly above room temperature. The region of inflection indicated an internal friction peak. For comparison Young's modulus and shear modulus were determined as a function of temperature on a thermally tempered soda-lime-silica glass and on a semitempered borosilicate glass. Curves of these moduli, in contrast to those for the chemically strengthened glasses, did not reveal regions of inflection. The modulus of rupture is not affected by short exposure to heat up to 260°C., but decreases appreciably when exposed to temperatures above 204°C for 200 hr or more. Deflection measurements at room temperature showed that the two chemically strengthened glasses had about five times as much delayed elasticity as did thermally tempered soda-lime-silica glass.