Showing papers on "Devitrification published in 1992"

Design and properties of glass-ceramics

Abstract: Glass-ceramics are microcrystalline solids produced by the controlled devitrification of glass. Glasses are melted, fabricated to shape, and then converted by heat treatment to a predominantly crystalline ceramic. The basis of controlled crystallization lies in efficient internal nucleation ( 1), which allows development of fine, randomly oriented grains without voids, microcracks, or other porosity. A unique manufacturing advantage of glass-ceramics over conventional ceramics is the ability to use high-speed plastic forming processes developed in the glass industry (e.g. pressing, blowing, rolling, etc.) to create complex shapes essentially free of internal inhomogeneities. Because glass-ceramic compositions are designed to crystallize, however, they can­ not be held for long periods at temperatures below the liquidus during the forming process. Therefore, the viscosity at the liquidus temperature is critical both in the choice of a forming process and in the choice of a glass composition. The properties of glass-ceramics depend upon both composition and microstructure. The bulk chemical composition controls the ability to form a glass and its degree of workability. In order to achieve internal nucleation, suitable nucleating agents are melted into the glass. Bulk com­ position also directly determines the potential crystalline phase assem­ blage, and this in turn governs the general physical and chemical charac­ teristics, e.g. hardness, density, acid resistance, etc. Secondly, but equally important, is the importance of microstructure. Microstructure is the key

Fabrication and Secondary‐Phase Crystallization of Rare‐Earth Disilicate–Silicon Nitride Ceramics

Abstract: The fabrication and intergranular-phase devitrification of silicon nitride densified with rare-earth (RE) oxide additives has been investigated. The additions of the oxides of Sm, Gd, Dy, Er, and Yb, having high melting points and behaving similarly to Y2O3, were compositionally controlled to tailor a microstructure with a crystalline secondary phase of RE2Si2O7. The lanthanide oxides were found to be as effective as Y2O3 in densifying Si3N4, resulting in identical microstructures and densities of 98–99% of theoretical density. The crystallization behavior of all six disilicates was similar, characterized by a limited nucleation and rapid growth mechanism resulting in large single crystals. Complete crystallization of the intergranular phase was obtained with the exception of a thin residual amorphous film which was observed at interfaces and believed to be rich in impurities, the cause of incomplete devitrification.

Crystallization of Yttria–Alumina–Silica Glasses

Abstract: Bulk nucleation has been studied for glasses within the Y2O3–Al2O3–SiO2 system, using a two-stage nucleation and growth heat treatment. The crystalline phases formed have been identified. Annealing in an inert atmosphere is required in order to prevent surface nucleation from dominating the results. Despite this, the phases observed are in general agreement with those observed in previous studies done in air. The nucleation kinetics of the y-Y2Si2O7 phase have been measured and the optimum nucleation temperatures have been identified. The kinetic data have been analyzed using two existing models based on classical nucleation theory. The results of this study have been compared with crystallization results of a previous study of glass of similar composition within sintered silicon nitride samples. A two-stage heat treatment is suggested as a process that may lead to improved devitrification of grain boundary glass in such materials.

Effect of Mgo content changes (8 to 15%) on the devitrification of glasses obtained from porphiric sands, Mgo and Tio(in2) (4%)

Abstract: Glasses have been prepared from porphiric sands, TiO2 and MgCO3 and the effect of changing the MgO content on the isothermal devitrification has been studied.

Estimation of the nucleation rate by differential scanning calorimetry

Abstract: A realistic computer model is presented for calculating the time-dependent volume fraction transformed during the devitrification of glasses, assuming the classical theory of nucleation and continuous growth. Time- and cluster-dependent nucleation rates are calculated by modeling directly the evolving cluster distribution. Statistical overlap in the volume fraction transformed is taken into account using the standard Johnson-Mehl-Avrami formalism. Devitrification behavior under isothermal and nonisothermal conditions is described. The model is used to demonstrate that the recent suggestion by Ray and Day (1990) that nonisothermal DSC studies can be used to determine the temperature for the peak nucleation rate, is qualitatively correct for lithium disilicate, the glass investigated.

Medical or dental hardening compositions

Abstract: Medical or dental hardening compositions of the present invention comprise calcium phosphate type devitrification glass-ceramics powder containing apatite and/or calcium phosphate ceramics and a liquid component. The second medical or dental hardening compositions of the present invention comprise strontium phosphate type devitrification glass-ceramics powder containing strontium-apatite crystals and/or calcium strontium phosphate ceramics, or calcium strontium phosphate type devitrification glass-ceramics powder and a liqiud component.

Glass fiber forming composition, glass fibers obtained from the composition and substrate for circuit board including the glass fibers as reinforcing material

Abstract: A glass fiber forming composition exhibits a remarkably high dielectric constant .epsilon.r as well as superior chemical resistance, yet it is readily spun into glass fibers. The composition is characterized to show a devitrification temperature which is lower than a spinning temperature at which the glass composition exhibits a viscosity of 102.5 poise, so as to be readily spun into corresponding glass fibers. The composition consists essentially of 40 to 65 mol% of SiO2; 20 to 45 mol% of at least one component selected from the group consisting of MgO, CaO, SrO and BaO; 5 to 25 mol% of at least one component selected from the group consisting of TiO2 and ZrO2; and 0.5 to 15 mol% of NbO5/2 as calculated from an incorporated amount of Nb2O5. Alternately, the composition consist essentially of 40 to 65 mol% of SiO2; 20 to 45 mol% of at least one component selected from the group consisting of CaO, SrO and BaO; 5 to 25 mol% of at least one component selected from the group consisting of TiO2 and ZrO2; 0.5 to 15 mol% of NbO5/2 as calculated from an incorporated amount of Nb2O5; and 0.5 to 15 mol% of AlO3/2 as calculated from an incorporated amount of Al2O3. The composition is also characterized to incorporate at least 85 mol% of a total amount of the oxides and have a dielectric constant [.epsilon.r] of 9 or more at 1 MHz and 25°C.

Crystallization kinetics of binary borosilicate glass composite

Abstract: Kinetics of cristobalite precipitation in a binary glass composite, containing a low-softening borosilicate (BSG) and a high-softening high silica (HSG) glass, have been investigated. XRD results show that the pure glasses do not crystallize under the sintering conditions used, but when mixed in appropriate proportions the cristobalite gradually precipitates out of the initial amorphous binary glass mixture as the sintering continues at temperatures ranging from 800 to 1000 °C. Average linear thermal expansion coefficient (TCE) results show that the TCE increases significantly with increasing precipitation of cristobalite as a function of sintering time. Comparing the experimental TCE results with those theoretically calculated, it is concluded that the precipitation originates most likely in the HSG rather than in the BSG. The precipitation kinetics follow the Avrami equation, and the results show an apparent activation energy of 82 kJ/mol which is close to those for the diffusion of alkali ions in silicate glasses, suggesting mass-transport controlled kinetics. The values of the Avrami exponent are 1.7–1.8, which could be interpreted as a 3-dimension diffusional growth at zero nucleation rate. The linear growth rates of cristobalite, calculated from the precipitation curve, are in the range of 4–8 × 10−5 μm/min, and show slight temperature dependence from 800 to 1000 °C. The linear growth rates of cristobalite are also calculated theoretically using the equation derived by Turnbull et al.,2 and the data are 0–3 orders of magnitude smaller than those observed experimentally. This disparity is attributed to the catalytic effect of the OH and O in air and in the glass network, as well as the diffusion of alkali ions from BSG to HSG.

Thermomechanical Properties of Frozen Sucrose Solutions

Abstract: A thermomechanical (TMA) method was used to study the glass transition of dry amorphous sucrose and the physical changes occurring when frozen sucrose solutions were heated. A modified TMA device allowed the detection of changes in sample dimension and compliance. Results of the TMA study of sucrose solutions suggested that the maximum solute concentration of the amorphous solution was close to 80% (w/w). In nonmaximally freeze concentrated solutions, devitrification generally started at the glass transition temperature of the amorphous phase and continued up to -32°C, where melting of ice crystals began, inducing softening and flow. With dilute solutions, ice recrystallization occurred in the temperature range from–32 to - 10°C. Devitrification and recrystallization had a stabilizing effect on mechanical properties.

The effects of heating and devitrification on the structure and biological activity of aluminosilicate refractory ceramic fibres.

Abstract: Three grades of ceramic fibre have been examined for their composition, structures and biological effect in several in vitro assay systems. The fibres were examined in the 'as-manufactured' state and after heating at 1200 and 1400 degrees C. Devitrification of the fibres at 1200 degrees C probably gave mullite crystals on the surface and caused the formation of the high-temperature form of cristobalite and, in zirconia grade fibres, the high-temperature, tetragonal form of zirconia as well. Further heating changed surface structure and led to zircon production in the zirconia fibres. Heating reduced the affinity of the fibres for the surface of V79-4 cells and lowered fibre toxicity toward these cells and towards macrophage-like cells. These changes in toxicity were not due to a reduction in the fibrous nature of the materials although they did become more brittle and powders prepared from them contained more isometric particles than those from as-manufactured materials. This suggests that the devitrification occurring during the use of these materials in high-temperature environments will not necessarily enhance their adverse biological activities despite the production of one phase of crystalline silica.

Alteration of tektite to form weathering products

Abstract: RECENT use of tektites as evidence for a bolide impact at the Cretaceous/Tertiary (K/T) boundary has focused attention on their long-term stability1–5. It was proposed in these studies that residual clay features with the spherical tektite morphology result from in situ alteration of the original glassy material. By contrast, examination of tektite alteration as an analogue for the long-term degradation of nuclear waste glass has revealed no evidence of alteration, hydration or devitrification either for samples found in nature or for those reacted in the laboratory6–9: no residual clay minerals were observed, and therefore the glass was interpreted as having reacted by a complete dissolution or etching process10–12. Here we show that these apparently incongruent observations can be reconciled through understanding the relationship between the environment in which the glass reacts and the chemical processes that control the reaction rate. We have examined both natural and experimental alteration of tektites and have found that, under conditions of restricted water contact, tektite reaction is dominated by water diffusion and in situ hydrolysis of the glass structure, followed by restructuring of the silicate network to form clays. Over time, the effective rate for these processes is lower than that for etching. Thus alteration of tektites to clays, as observed at the K/T boundary, can proceed only under conditions of limited water contact.

Optical glass of fluorophosphoric acid salt

Abstract: PURPOSE: To improve devitrification resistance of glass by forming Al (PO 3 ) 2 -(SrF 2 +BaF 2 )-(La 2 O 3 +Y 2 O 3 )-(SnO+SnO 2 +SnF 2 )-based glass having a specific composition range CONSTITUTION: Optical glass raw materials are mixed in prescribed weight ratio and melted at 1000-1300°C and then gradually cooled to provide the objective optical glass of fluorophosphoric acid salt composed of 15-50wt% Al (PO 3 ) 3 , 0-20wt% Mg(PO 3 ) 2 +Ca(PO 3 ) 2 , 0-20wt% Ba(PO 3 ) 2 , 0-20wt% MgF 2 , 0-10wt% CaF 2 , 0-7wt% SrF 2 , 0-70wt% BaF 2 , with the proviso that SrF 2 +BaF 2 is 20-70wt%, ZnF 2 is 0-15wt%, PbF 2 is 0-15wt%, LiF+NaF+KF is 0-15wt%, AlF 3 is 0-4wt%, LaF 3 is 0-10wt% and YF 3 is 0-10wt%, with the proviso that La 2 O 3 +Y 2 O 3 is 2-30wt%, SnO is 0-2wt% and SnO 2 is 0-2wt%, with the proviso that SnO+SnO 2 +SnF 2 is 001-2wt% This glass has about 150-165 refractive index (Nd) and about 65-77 Abbe's number and is excellent in devitrification resistance and melting property and suitable for mass production COPYRIGHT: (C)1994,JPO&Japio

Cordierite-ZrO2 and cordierite-Al2O3 composites obtained by sonocatalytic methods

Abstract: Cordierite-matrix-based composites were prepared by the sol-gel process involving sonocatalysis during preliminary chemical reactions. ZrO2 and Al2O3 ceramic fibers were employed as reinforcing phases. Sintering was accomplished by hot-pressing techniques. Devitrification of sintered cordierite sonogels showed that, when μ- and α-cordierite forms appear, a substantial increase in the mechanical strength is observed. Addition of the nucleant agent TiO2 further improves mechanical properties.

Crystallization of lithium borate glasses

Abstract: The glass-forming ability and crystallization behaviour of lithium borate compositions, in the diborate to metaborate range, were studied. In particular, the nature and sequence of formation of crystalline phases and the tendency towards devitrification were investigated as functions of temperature, thermal history and batch composition. It was found that the sequence of crystalline phase formation was sensitive to all of the three latter factors, and it was observed that under certain conditions metastable defect structures of the metaborate can appear.

Crystallization of aluminium-based metallic glasses: processes and kinetics

Abstract: Differential scanning calorimetry was used to study the crystallization processes and kinetics of melt-spun Al 90 Y 10 , Al 90 Fe 5 Y 5 and Al 85 Fe 7.5 Y 7.5 metallic glasses. The characteristics of structural relaxation and glass transition have been observed only in the two ternary alloys. All the glasses have a relatively high thermal stability. The isothermal experiments show that the application of Johnson-Mehl-Avrami theory is limited to the late crystallization processes, particularl for the two low concentration alloys. The crystal nucleation and growth in the early stage devitrification of the glasses are discussed in terms of the local Avrami exponent.

Si3N4-SiC particulate composites: Devitrification of the intergranular phase and its effect on creep

Abstract: Pressureless sintered β′-Si3N4 materials containing either 0, 17 or 34 wt% of SiC and 10·5wt% Al2O3 + 4·5 wt% Y2O3 additives were heat-treated to stabilize the microstructure by devitrification of their glassy intergranular phase. Heat-treatments at 1523 K result in the formation of YAG (3Y2O3 . 5Al2O3) and Y2S (Y2O3 . 2SiO2). Using TEM and EDS characterizations, the nucleation of YAG or Y2S crystals was related to the presence or to the absence of SiC inclusions in the glassy interphase. At 1523 K, a complete devitrification cannot be achieved whatever the annealing time. SiC appears to increase devitrification and YAG formation. At 1573 K or 1623 K less complete devitrification occurs and only the YAG phase is detected. The creep resistance during compression tests at 1523 K is clearly increased by annealing but the residual glassy interphase still influences deformation.

Microstructural Characterization of Cofired Tungsten‐Metallized High‐Alumina Electronic Substrates

Abstract: Microstructural characterization of a high-Al2O3 substrate containing cofired thick-film tungsten metallization, with particular emphasis on the metal/ceramic interface, was conducted. The substrate contained tabular Al2O3 grains surrounded by a continuous calcium magnesium aluminum silicate glass containing particles of monoclinic ZrO2 and reduced rutile (TiO2-x). The metal/ceramic adhesion was caused by mechanical interlocking between the W and Al2O3 grains by the glass phase which penetrated the porous W layers during sintering; there was no interfacial reaction or diffusion zone. The mechanical properties of the W metallization did not limit interfacial strength. Heat treatments of the substrate at 1400 K in air and under vacuum resulted in the devitrification of the intergranular glass. The most abundant devitrification product was anorthite (CaAl2Si2O8), accompanied by magnesium aluminate titanate, magnesium aluminate spinel, α-cristobalite (SiO2), and α-cordierite (Mg2Al4Si5O18). In addition, small rutile particles precipitated within the Al2O3 grains.

Nucleation and crystallization kinetics on float glass surfaces

Abstract: The superficial crystallization kinetics of a commercial float glass is investigated in the temperature range from 665 to 925°C. The following phases were observed: cristobalite, devitrite, sporadically sodium metasilicate and additionally β-wollastonite if a reducing atmosphere is applied. The devitrification process is divided into a primary crystallization of cristobalite and a secondary appearance of devitrite after an induction time (t>7 min). The growth rates of devitrite are up to 4 times larger than those of cristobalite, thus, a fractured surface perpendicular to the float glass plate shows a two-stage devitrification image after longer times of heat treatment. The growth rates depend on the furnace atmosphere as well as on the compositional variations of the two float glass surfaces. The activation enthalpies of crystal growth rates are (in kJ/mol):≃110 for cristobalite, ≃150 for β-wollastonite and ≃220 for devitrite. A superficial phase separation occurs at the tin bath side if oxidizing furnace atmosphere is applied. This can be related to a redox interaction of the tin ions and has consequences on nucleation, induction period, crystal growth mechanism and bloom effect. Heterogeneous surface nucleation arises from a certain number of sites which are specific for the various crystal phases. In the case of cristobalite the time and temperature dependence of the number of crystals are measured and nucleation rates were determined on the atmosphere side under oxidizing conditions

Preliminary study of fiber drawing of AlF3-based glasses

Abstract: Although AlF3-based glasses have shown superior chemical durability and mechanical strength relative to ZrF4-based glasses, devitrification problems near the drawing temperature have limited their fiber applications. We have investigated the effect of atmosphere, particularly moisture, on the surface crystallization during the fiber drawing of these glasses. The crystallization behavior has been monitored by optical microscopy, scanning electron microscopy (SEM) and differential scanning calorimetry (DSC) techniques. Preforms of AlF3-based glasses have also been drawn into long fibers in a tower enclosed by a unique three-stage vertical glove box in which the moisture levels are maintained below 1 ppm.

Sintering of Lunar Glass and Basalt

Abstract: Experiments were conducted to study the sintering behavior of glass and basalt lunar soil simulants. The degree of sintering was assessed by compressive strength testing and microanalysis. Both crushed glass and basalt sinter significantly at 1000 C, with the basalt attaining its maximum strength at 1100 C. Initial sintering occurs in less than 15 min, and the degree of sintering does not increase significantly with time after about 30 min. Glass sinters more readily than crystalline material. Sintering and devitrification both occur on a time scale of minutes in the heated glass, but sintering is apparently more rapid. The processes of sintering and oxygen release by hydrogen reduction of lunar soil are synergistic, and could be combined to produce two extremely useful products at a lunar base.

Crystallizing phases and kinetics of crystal growth in Li2O 19GeO2 glass

Abstract: This study was part of a research program in which the devitrification behaviour of lithium germanate glasses is investigated. The crystallizing phases and the kinetic parameters of crystal growth in a lithium germanate glass containing 5mol% lithium oxide were studied

Partial Devitrification of Sintered Silicon Nitride During Static Fatigue Testing

Abstract: The static fatigue behavior of sintered Y2O3/Al2O3-ftuxed Si3N4 in air is controlled by slow crack growth or creep fracture. Partial devitrification of the amorphous grain boundary phase at 1000°C and 1100°C improves the static fatigue resistance with specimens surviving up to 1500 hrs. during stress rupture experiments. In this study the early stages of partial devitrification during static fatigue testing at 1000°C are investigated by conventional and analytical transmission electron microscopy with emphasis on nucleation and growth of δ-Y2Si2O7 and X1-Y2SiO5 and possible constraints from different stress states. The results show that the stress state does not affect the nature of the secondary phase assemblage. However, the amount of crystallization is higher within the tensile region of the flexural specimens than in areas which experienced compressive stresses.

Light emitting tube for high pressure discharge lamp

Abstract: PURPOSE: To obtain a light emitting tube for high pressure discharge lamp, in which the devitrification of a quartz glass is restricted and the generation of deformation in the case of long-term lighting is reduced. CONSTITUTION: An outer layer 11 is made of natural quartz glass having its viscosity at 10 14.8 poise or more at 1025°C, and an inner layer 12 is provided with a compound quartz glass tube made of synthetic quartz glass, which includes the mounts of sodium and potassium and lithium all of which are 0.2ppm or less. COPYRIGHT: (C)1994,JPO&Japio

Influence of water vapour on high-temperature oxidation of Al2O3-MgO-doped hot-pressed silicon nitride

Abstract: Silicon nitride is particularly sensitive to high-temperature oxidation. The intensity of oxidation is influenced by the chemical composition of the amorphous phases present at the grain boundaries and consequently by the sintering additives responsible for their formation. The presence of water vapour increases Si 3 N 4 oxidation also in intermediate temperature conditions. In this study the influence of water vapour pressure at high temperature (1200°C) on the corrosion of hot-pressed silicon nitride (HPSN) doped with Al 2 O 3 -MgO was evaluated. The water vapour has a great influence on the devitrification of the amorphous oxide upper layer, due to the formation of crystalline oxides (primarily cristobalite and tridymite). This process increases the oxidation rate, consequently increasing the porosity of the exposed surface. The microstructural evolution of HPSN in the presence of water vapour at 1200°C was analysed by SEM and XRD.

Aluminum borate devitrification inhibitor in low dielectric borosilicate glass

Abstract: A ceramic composition for forming a ceramic dielectric body having a dielectric constant of less than about 5.5 and a TCE of less than about 3.5 ppm/°C. The composition comprises a mixture of finely divided particles of 20-80 vol. % borosilicate glass and 20-80 vol. % of Alx By Oz wherein x is in the range of 4 to 22, y is in the range of 2 to 5, and z is in the range of 9 to 36. The Alx By Oz inhibits the formation of crystalline forms of silica. The composition can be used with a polymeric binder to produce an unfired green tape which is co-fireable with high conductivity metallurgies such as gold, silver and silver/palladium.