Is the structural relaxation of glasses controlled by equilibrium shear viscosity?

Knowledge of relaxation processes is fundamental in glass science and technology because relaxation is intrinsically related to vitrification, tempering as well as to annealing and several applications of glasses. However, there are conflicting reports—summarized here for different glasses—on whether the structural relaxation time of glass can be calculated using the Maxwell equation, which relates relaxation time with shear viscosity and shear modulus. Hence, this study aimed to verify whether these two relaxation times are comparable. The structural relaxation kinetics of a lead metasilicate glass were studied by measuring the refractive index variation over time at temperatures between 5 and 25 K below the fictive temperature, which was initially set 5 K below the glass-transition temperature. Equilibrium shear viscosity was measured above and below the glass-transition range, expanding the current knowledge by one order of magnitude. The Kohlrausch equation described very well the experimental structural relaxation kinetics throughout the investigated temperature range and the Kohlrausch exponent increased with temperature, in agreement with studies on other glasses. The experimental average structural relaxation times were much longer than the values computed from isostructural viscosity, as expected. Still, they were less than one order of magnitude higher than the average relaxation time computed through the Maxwell equation, which relies on equilibrium shear viscosity. Thus, these results demonstrate that the structural relaxation process is not controlled by isostructural viscosity and that equilibrium shear viscosity only provides a lower boundary for structural relaxation kinetics.     

Temperature dependences of density of sodium borosilicate glasses in equilibrium states at temperatures below a vitrification temperature

The temperature dependences of the density and intensity of SAXRS of a variety of sodium borosilicate glasses in equilibrium states below vitrification temperature are obtained. The existence of a transition of the investigated supercooled liquids in a solid noncrystalline state was discovered that is similar to the one observed in the study of vitreous boron oxide and alkali-borate glasses. The temperature of the transition of supercooled liquids of the investigated compositions to a solid noncrystalline state is about 50°C below the vitrification temperatures determined by the SAXRS data. Thus, the transition occurs at a viscosity of 1014 P in borate and sodium borosilicate glasses.     

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.     

THE ATTACK OF ARSENIC COMPOUNDS ON FIRECLAY REFRACTORY MATERIAL1

McSwiney's statement regarding arsenic compounds being responsible for rapid corrosive action on tank blocks is discussed. It is pointed out that if arsenic compounds exert a marked corrosive action on fire clay, it should be demonstrated by an increased iron oxide or alumina content, or both, in the resulting glass. Data collected in experiments run is presented, which contradicts McSwiney's statement. Only when arsenic in excess of the amount used ordinarily is present in glasses melted at temperatures of 1450 and 1500°C, or above ordinary working temperatures, can corrosion due to arsenic be traced. A full discussion of corrosion of glass house refractories, effects of colors on the heat conductivity of glasses at high and low temperatures, and variations between the types of glasses referred to in Dr. Turner's paper and in McSwiney's, is presented by McSwiney.     

Effect of Temperature on the Relaxation of Radiation-Induced Coloration in Optical Glasses

Optical and radiation-resistant crown and flint glasses exposed to gamma irradiation at room temperature are investigated. The relaxation of radiation-induced absorption in irradiated glasses is examined prior to and after short-term or long-term heat treatment at different temperatures in the range 20–150°C. It is established that an increase in the heat treatment temperature by even several tens of degrees leads to a drastic increase in the relaxation rate of radiation-induced coloration and a decrease in the time of complete bleaching of irradiated glasses by several orders of magnitude.     

THE TEMPERATURE COEFFICIENT OF VISCOSITY AND ITS RELATION TO SOME OTHER PROPERTIES OF LIQUIDS AND GLASSES*

A possible connection between the temperature coefficient of viscosity (not the viscosity itself), the latent heat of fusion or some similar work-function, and the absorption spectrum of liquids and glasses is discussed. The three things seem to be connected in a fairly clear and simple manner. It is shown, for instance, that from viscosity data alone it is possible to predict the position of the ultra-violet absorption band in glasses, and the black color of pitch. Also, that at high temperatures the ultra-violet absorption band invades the visible spectrum and migrates down into the infra-red, and that at room temperature the viscosity of typical soda-lime-silica glasses is of the order 1070 poises. A meaning is attached to the term “viscosity” at temperatures at which it can not be measured mechanically, and a means of tracing it optically down to absolute zero is suggested.     

Effects of composition and phase relations on mechanical properties and crystallization of silicate glasses

Crystallization, mechanical properties, and workability are all important for the commercialization and optimization of silicate glass compositions. However, the inter-relations of these properties as a function of glass composition have received little investigation. Soda-lime-silica glasses with Na₂O-MgO-CaO-Al₂O₃-SiO₂ compositions relevant to commercial glass manufacture were experimentally studied and multiple liquidus temperature and viscosity models were used to complement the experimental results. Liquidus temperatures of the fabricated glasses were measured by the temperature gradient technique, and Rietveld refinements were applied to X-Ray powder diffraction (XRD) data for devitrified glasses, enabling quantitative determination of the crystalline and amorphous fractions and the nature of the crystals. Structural properties were investigated by Raman spectroscopy. Acoustic echography, micro-Vicker's indentation, and single-edge-notched bend testing methods were used to measure Young's moduli, hardness, and fracture toughness, respectively. It is shown that it is possible to design lower-melting soda-lime-silica glass compositions without compromising their mechanical and crystallization properties. Unlike Young's modulus, brittleness is highly responsive to the composition in soda-lime-silica glasses, and notably low brittleness values can be obtained in glasses with compositions in the wollastonite primary phase field: an effect that is more pronounced in the silica primary phase field. The measured bulk crystal fractions of the glasses subjected to devitrification at the lowest possible industrial conditioning temperatures indicate that soda-lime-silica glass melts can be conditioned close to their liquidus temperatures within the compositional ranges of the primary phase fields of cristobalite, wollastonite, or their combinations.     

Influence of Residual Water in a Glass on Its Rheological and Relaxation Properties (by the Example of a 5Na2O · 95B2O3Glass)

Glasses of the 5Na₂O · 95B₂O₃(mol %) composition synthesized at a temperature of 1100°C for 180 and 20 min are studied. The temperature dependences of the viscosity and the thermal expansion of glasses are obtained. The thermal expansion coefficients and glass transition temperatures of the studied glasses are determined, and the parameters of structural relaxation (the constant characterizing the width of the spectrum of relaxation times, the relaxation modulus equal to the ratio of the viscosity to the relaxation time, and the relaxation time at zero reciprocal temperature) are calculated from the dilatometric curves measured at temperatures close to the glass transition range. The water content in the studied glasses is estimated by comparing the obtained dependence of the viscosity on the water content with the data available in the literature for glasses of a similar composition. The assumption is made that the structural relaxation time in sodium borate glass decreases with an increase in the water content.     

TiO2对粉煤灰玻璃晶化影响的研究

The compositions of basic glasses is decided according to the compositions of the point in CaSiO3-CaAl2Si2O8-SiO2 triangle field of CaO-Al2O3-SiO2 system, on the basis of which akind of glass is made through using fly-ash as the main component, and TiO2 is added to boththe basic glasses and the fly-ash glasses. DTA, XRD and SEM results show that, when theyare subjected to heat treatment, pure CaO-Al2O3-SiO2 glasses, either containing TiO2 orcontaining no TiO2, crystallize only in surface and the crystals which grow from surface tointerior are white. In eontrast, fly-ash glasses, either containing TiO2 or containing noTiO2,crystallize in whole and the crystals which come from the glasses containing TiO2 areyellow and consequently the crystals which come from the glasses containing no TiO2 areblack.TiO2 is used as nucleation agent in either pure CaO-Al2O3-SiO2 glasses or fly-ashglasses, its effect is quite limited. TiO2 can increase the viscosity at low temperatures anddecrease the viscosity at high temperatures, and especially TiO2 can be used as changingcolour agent in fly-ash glasses.     

Specific features of the behavior of mixed CuBr-CuCl nanocrystalline phase in a heterogeneous potassium aluminoborosilicate glass

The structure of copper halide potassium aluminoborosilicate glasses containing chlorine and bromine is investigated by the small-angle X-ray scattering (SAXS) technique. In the samples heat-treated at temperatures of 520, 550, and 600°C for 24 h, the effective radii of liquid phase regions (at the primary heat treatment temperature) are equal to 64, 96, and 150 ?, respectively; and the effective radii of microcrystals of the solid solutions based on copper halides (at room temperature) are 39, 59, and 83 ?, respectively. Upon joint introduction of chlorine and bromine, the crystallization temperature decreases down to 100°C. The heat treatment at temperatures above the liquidus point for the copper halide phase brings about an increase in the size of microcrystals of the solid solutions based on copper halides in cooled samples and an increase in the temperatures of melting and crystallization.     

Physical and Mechanical Properties of Barium Alkali Silicate Glasses for SOFC Sealing Applications

The physical and mechanical properties of two barium alkali silicate glasses were determined as a function of temperature. Their Young's modulus and Poisson's ratio were determined by resonant ultrasound spectroscopy; their viscosity, thermal expansion, and glass transition temperature were determined using a thermomechanical analyzer. The wetting behavior of the two glasses on alumina and 8 mol% yttria stabilized zirconia (8YSZ) substrates was determined by measuring contact angles in air as a function of temperature and time. Values of Young's modulus for both glasses were in good agreement with those predicted by the Makishima and MacKenzie model. The physical and mechanical properties of these glasses are discussed in the context of their potential use for sealing applications in solid-oxide fuel cells.     

Shrinkage behavior after the heat setting of biaxially stretched poly(ethylene 2,6‐naphthalate) films and bottles

Amorphous preforms of poly(ethylene 2,6‐naphthalate) (PEN) were biaxially drawn into bottles up to the desired volume under industrial conditions. These bottles were used to characterize the shrinkage behavior of the drawn bottles with or without heat treatment and to study structural variations during heat setting. During drawing, a rigid phase structure was induced, and the amount of the induced rigid phase structure was linearly related to the square root of the extra first strain invariant under equilibrium conditions. During the production of these bottles, this equilibrium was not attained because of high stretching conditions and rapid cooling after stretching. The structure after orientation contained a rigid amorphous phase and an oriented amorphous phase. The shrinkage behavior was a function of the temperature and time of heat setting. Long heat‐setting times, around 30 min, were used to characterize the possible structural variations of the oriented PEN after heat setting at equilibrium. Under the equilibrium conditions of heat setting, the start temperature of the shrinkage was directly related to the heat‐setting temperature and moved from 60°C without heat treatment up to a temperature of 255°C by a heat‐setting temperature of 255°C; this contrasted with poly(ethylene terephthalate) (PET), for which the start temperature of shrinkage was always around 80°C. For heat‐setting temperatures higher than 220°C, the structural variations changed rapidly as a function of the heat‐setting time, and the corresponding shrinkage of the heat‐set samples sank below 1% in a timescale of 30–60 s for a film thickness of 500 μm. The heat treatment of the oriented films taken out of the bottle walls with fixed ends stabilized the induced structures, and the shrinkage of these heat‐set films was zero for temperatures up to the heat‐setting temperature, between 220 and 265°C, if the heat‐setting time was sufficient. According to the results obtained, a heat‐setting time of 30 s, for a film thickness of 500 μm, was sufficient at a heat‐setting temperature of 255°C to stabilize the produced biaxially oriented PEN bottles and to take them out the mold without further shrinkage. During the drawing of PEN, two different types of rigid amorphous phases seemed to be induced, one with a mean shrinkage temperature of 151°C and another rigid amorphous phase, more temperature‐stable than the first one, that shrank in the temperature range of 200–310°C. During heat setting at high temperatures, a continuous transformation of the less stable phase into the very stable phase took place. The heat‐set method after blow molding is industrially possible with PEN, without the complicated process of subsequent cooling before the molds are opened, in contrast to PET. This constitutes a big advantage for the blow molding of PEN bottles and the production of oriented PEN films. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 87: 1462–1473, 2003     

Mechanical properties of oxynitride glasses

Oxynitride glasses combine a high refractoriness, with Tg typically >850°C, and remarkable mechanical properties in comparison with their parent oxide glasses. Their Young's modulus and fracture toughness reach 170 GPa and 1.4 MPa m^.5, respectively. Most reports show good linear relationships between glass property values and nitrogen content. There is a clear linear dependence of Young's modulus and microhardness on fractional glass compactness (atomic packing density). They also have a better resistance to surface damage induced by indentation or scratch loading. The improvements stem from the increase of the atomic network cross-linking—because of three-fold coordinated nitrogen—and of the atomic packing density, despite nitrogen being lighter than oxygen and the Si–N bond being weaker than the Si–O bond. For constant cation composition, viscosity increases by ∼3 orders of magnitude as ∼17 eq.% oxygen is replaced by nitrogen. For rare earth oxynitride glasses with constant N content, viscosity, Young's modulus, Tg, and other properties increase with increasing cation field strength (decreasing ionic radius). Research continues to find lighter, stiffer materials, including glasses, with superior mechanical properties. With higher elastic moduli, hardness, fracture toughness, strength, surface damage resistance, increased high temperature properties, oxynitride glasses offer advantages over their oxide counterparts.     

Specific features of the behavior of mixed CuBr-CuCl nanocrystalline phase in a heterogeneous potassium aluminoborosilicate glass

The structure of copper halide potassium aluminoborosilicate glasses containing chlorine and bromine is investigated by the small-angle X-ray scattering (SAXS) technique. In the samples heat-treated at temperatures of 520, 550, and 600°C for 24 h, the effective radii of liquid phase regions (at the primary heat treatment temperature) are equal to 64, 96, and 150 å, respectively; and the effective radii of microcrystals of the solid solutions based on copper halides (at room temperature) are 39, 59, and 83 å, respectively. Upon joint introduction of chlorine and bromine, the crystallization temperature decreases down to 100°C. The heat treatment at temperatures above the liquidus point for the copper halide phase brings about an increase in the size of microcrystals of the solid solutions based on copper halides in cooled samples and an increase in the temperatures of melting and crystallization.     

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).     

DISINTEGRATION OF SODA LIME GLASSES IN WATER1

Soda lime glasses have been treated in water at temperatures helow boiling point, at boiling point, and up to 25 lb. pressure in an autoclave. Effects produced are illustrated with photographs and show that glass disintegrates, i. e., cracks, spalls, and loses weight when treated at any temperature used. The rate of disintegration depends upon time, temperature, composition, and heat treatment of glass surface during forming, the latter factor apparently affecting considerably the tendency to spall or chip. Treatment given glasses conforms in many ways to treatment glass receives in service, either when sterilized when used for food packing or continued washing and sterilizing as when used for delivering or serving food daily. The relative endurance of glasses may be studied by the methods used, and standard methods of testing based upon them can be worked out. An article must stand at least six hours in boiling water without apparent disintegration or chipping to be of good quality. Spalling always appeared on the exterior only of pressed or blown ware. Disintegration of the glass is cumulative with time of treatment whether it be periodic or continuous treatment. Treating glass in hot water with increasing pressures does not seem to give greatly increased disintegration as the temperature and pressure rises. Surfaces of blown or pressed articles are shown to be laminated rather than smooth and impervious.     

Creep Behavior of Soda-Lime Glass in the 100–500 K Temperature Range by Indentation Creep Test

The loading time and temperature dependences viscoelastic behavior of a soda-lime-silicate glass (SLS) were studied by indentation creep experiments. Experiments were conducted in air, water and silicone oil with 15–10 000 s loading times, and temperatures ranging between 100 and 500 K. The indentation size was found to depend much on the loading time and temperature. Hardness was found to decrease significantly with increasing loading time, even at 173 K, and to decrease rapidly with rising temperature, even well below the glass transition temperature ( T _g). Water on the surface of glasses appeared to reduce the hardness and indentation viscosity of the glass.     

Effects of high-temperature heat treatment on the mechanical properties of unidirectional carbon fiber reinforced geopolymer composites

Unidirectional carbon fiber reinforced geopolymer composite (C_uf/geopolymer) is prepared by a simple ultrasonic-assisted slurry infiltration method, and then heat treated at elevated temperatures. Effects of high-temperature heat treatment on the microstructure and mechanical properties of the composites are studied. Mechanical properties and fracture behavior are correlated with their microstructure evolution including fiber/matrix interface change. When the composites are heat treated in a temperature range from 1100 to 1300 °C, it is found that mechanical properties can be greatly improved. For the composite heat treated at 1100 °C, flexural strength, work of fracture and Young's modulus reach their highest values increasing by 76%, 15% and 75%, respectively, relative to their original state before heat treatment. The property improvement can be attributed to the densified and crystallized matrix, and the enhanced fiber/matrix interface bonding based on the fine-integrity of carbon fibers. In contrast, for composite heat treated at 1400 °C, the mechanical properties lower substantially and it tends to fracture in a very brittle manner owing to the seriously degraded carbon fibers together with matrix melting and crystal phases dissolve.     

Transformation Range Behavior of Lithium Galliosilicate Glasses

The transformation range viscosity and thermal expansion behavior of lithium galliosilicate glasses were studied. The glass transformation and dilatometric softening temperatures exhibit maxima at gallium to lithium ratios of somewhat greater than one. Similar behavior was observed for the transformation range viscosity. The effect of phase separation on the dilatometric softening temperature is discussed in detail. These results are compared with those obtained for lithium alumino-silicate glasses in other studies. The results are discussed in terms of the structural role of aluminum and gallium in the glass network.