Showing papers on "Devitrification published in 1993"

Water II is a "strong" liquid

Abstract: Rationalization of anomalous devitrification relationships between hyperquenched vitreous water (and related ASW forms) and vitrified aqueous solutions requires the assumption that the viscosity-temperature relationship of water near but above its glass transition temperature is that of a «strong» liquid, like SiO 2 . This would support Speedy's claim that water II (ASW above T g ) is a separate and distinct state of liquid water. The conclusion is supported by analysis of low-temperature data on water-swollen polymers (hydrogels)

Numerical model for isothermal and non-isothermal crystallization of liquids and glasses☆

Abstract: A new numerical model of isothermal and non-isothermal first order phase transformations, such as the crystallization of liquids and glasses, is presented. This model computes directly the volume fraction transformed, taking into account time-dependent nucleation rates and cluster-size-dependent growth velocities. The model is applied to the crystallization of lithium disilicate glass, using the appropriate kinetic and thermodynamic parameters. The model is used (1) to determine the validity of common methods for computing the volume fraction transformed as a function of time in isothermal experiments when a time-dependent nucleation rate is expected, (2) to simulate non-isothermal differential scanning calorimetry (DSC) studies of glass devitrification as a function of scan rate, and (3) to compute the effect of preannealing on the DSC peak parameters. A novel behavior of the nucleation rate with scan rate is predicted, arising because the relaxation of the cluster distribution cannot be described by a single relaxation time. Comparisons of the calculations with experimental data on this glass demonstrate the validity of the model.

Surface-induced devitrification of glasses: the influence of elastic strains

Abstract: The influence of elastic strains on devitrification processes in glasses is investigated. It is known that in the vicinity and below the vitrification temperature, Tg, thermodynamic inhibition of crystallization has to be taken into account in addition to kinetic inhibition of crystallization connected with the extremely low values of the mobility (large values of the viscosity). This thermodynamic inhibition results from the evolution of elastic fields due to a difference in the specific volumes of the vitreous and crystalline states of the substance. The inhibiting term is of the same order of magnitude as the thermodynamic driving force of crystallization (and may even exceed it). It is considerably diminished for devitrification at the surface of a glass. In this way, elastic strains can be considered as the origin both for the absence of bulk devitrification and for preferential surface crystallization of glasses in systems with different specific volumes of the vitreous and crystalline states.

Creep behavior of a sintered silicon nitride

Abstract: A commercial sintered silicon nitride has been crept in bending and compression at temperatures of 1100°C to 1400°C. In the as-sintered condition the material contains an amorphous intergranular phase. This phase undergoes partial devitrification as a result of high temperature exposure. Preannealing the material to a stable microstructure has very little effect on the creep properties. Deformation behavior compares well with that predicted from a model for creep due to viscous flow of a non-Newtonian grain boundary phase. In bending, the model predicts an initial constant strain rate at low strains as the intergranular phase is squeezed out from between grains under compression. Samples crept in compression are not expected to have this same initial constant strain rate regime. The model also predicts a strong initial strain rate dependence (in bending) on the initial thickness of the amorphous grain boundary layer. Experimentally this strain rate is not affected by partial grain boundary crystallization, suggesting that partial devitrification does not alter the intergranular film thickness or viscosity. This is supported by transmission electron microscopy, which has shown that crystallization of the intergranular phase occurs largely in the pockets between grains, leaving amorphous films between grains.

Alumina as a Devitrification Inhibitor during Sintering of Borosilicate Glass Powders

Abstract: Cristobalite is formed when borosilicate glass (Corning Code 7740) is sintered at temperatures ranging from 700 to 1,000 C The precipitation kinetics, determined by XRD analysis, exhibit a characteristic incubation period which decreases with increasing sintering temperature, from 60-120 min at 700 C to 3-5 min at 1,000 C Activation analysis of precipitation shows an activation energy of 75 kJ/mol, which is close to that for the diffusion of Na[sup +] in borosilicate glass, suggesting mass-transport-controlled kinetics With added alumina content greater than a critical value, however, the cristobalite formation in the borosilicate glass is completely prevented at the sintering temperatures investigated The critical alumina content is found to decrease with decreasing alumina particle size but with increasing sintering temperature The above result, similar to observations previously made in a binary glass mixture containing a low-softening borosilicate glass (BSG) and a high-softening high silica glass (HSG) is attributed to a strong coupling between Al[sup 3+] from alumina and Na[sup +] from borosilicate glass The coupling reaction causes segregation of Na[sup +] in borosilicate glass to alumina, thus forming a Na[sup +]- and Al[sup 3+]-rich reaction layer around alumina particles far too rapid for cristobalite formation

Acoustic microscopy of ceramic-fibre composites: Part III Metal-matrix composites

Abstract: The microstructure of Nicalon-reinforced borosilicate glass was studied using scanning acoustic microscopy (SAM) and scanning electron microscopy (SEM). Acoustic micrographs exhibited high contrast, and the formation of Rayleigh-wave-interference fringes both within and around fibres allowed the in-situ-fibre and matrix elastic moduli to be estimated. The matrix was found to have undergone partial devitrification resulting in the formation of cristobalite. The presence of a high volume fraction of reinforcing fibres was found to have little effect on cristobalite formation and distribution. The thermal expansion mismatch between β-cristobalite and the devitrified glass matrix and the β/α-cristobalite phase change were found to give rise to extensive microcracking around individual cristobalite grains and partial fibre/matrix debonding

Devitrification inhibitor in binary borosilicate glass composite

Abstract: When an appropriate mixture of a low-softening borosilicate glass (BSG) and a high-softening high silica glass (HSG) is sintered at temperatures ranging from 800 to 1000 °C, a crystalline phase, identified as cristobalite by XRD, is known to precipitate out of the initial amorphous binary mixture of glasses as the sintering continues. The precipitation of cristobalite is found to originate in HSG, and is controlled by the transport of alkali ions (e.g., K+, Na+, and Li+) from BSG to HSG.1 In this paper, we report that when a small amount of alumina is present as a dopant in the above binary mixture of BSG and HSG, the cristobalite formation is completely prevented at the sintering temperatures investigated. The above result is attributed to a strong affinity between Al+3 from alumina and alkali ions from BSG, which diverts the diffusion of alkali ions from HSG to alumina, thus forming a K+ and Al+3-rich reaction layer adjacent to the alumina particles far too rapidly compared to that of cristobalite formation.

Nucleation and crystal growth in a fly ash derived glass

Abstract: The devitrification behaviour of a fly ash derived glass, examined by differential thermal analysis (DTA), X-ray diffraction and scanning electron microscopy (SEM), is reported and discussed. The crystallized phases were identified as mullite (3Al2O3·2SiO2) and anorthite (CaO·Al2O3·2SiO2). Kinetic parameters for nucleation and crystal growth were estimated from the DTA curves. The temperature of maximum nucleation rate was 790°C and the activation energy for crystal growth E=370 kJ mol−1. The crystal morphology was investigated by SEM and the crystal shape found to be consistent with the morphological index n calculated by DTA. The glass-ceramic obtained from a previously nucleated glass showed a fine-grained texture.

The non-isothermal devitrification of lithium germanate glasses

Abstract: The non-isothermal devitrification of lithium germanate glasses, examined by DTA and XRD, is reported and discussed. The glass compositions are expressed by the general formula:xLi2O(1−x)GeO2 withx=0.050, 0.125, 0.167, 0.200 and 0.250. All the glasses studied, unlike GeO2 glass, exhibit internal crystal nucleation without the addition of any nucleating agent. The devitrification processes occur in one or more steps. Phases which crystallized at each step are identified and crystallization mechanisms proposed. These crystallization mechanisms are related to structures of the crystallizing phases. Activation energy values as well as those for glass transition temperatures, do not vary linearly with increase in Li2O content but pass through a maximum atx=0.200.

Phase formation and growth mechanisms in Bi2Sr2CaCu2O8 glass ceramics

Abstract: Devitrification of Bi 2 Sr 2 CaCu 2 O 8 (Bi-2212) glasses, including phase formation sequences and mechanisms, has been investigated using both thermal analysis and high-temperature X-ray diffraction (HTXRD). Differential scanning calorimetry (DSC) was used to determine the activation energy for crystallization to be 395 kJ/mol, and in addition showed that the crystallization of these glasses occurs via a bulk mechanism. Differential thermal analysis (DTA) and simultaneous differential thermal and thermogravimetric analysis (STA), in conjunction with HTXRD, were used to establish the phase evolution of both bulk and powdered glass samples heated under O 2 and N 2 atmospheres. The results clearly indicate that Bi 2 Sr 2 Cu 1 O 6 (Bi-2201) crystallizes directly from the glass first, and that Bi-2212 forms at temperatures above ~800°C by the diffusion of Ca, Cu and O into the Bi-2201 lattices.

Gibbs free-energy difference between the glass and crystalline phases of a Ni-Zr alloy

Abstract: The heats of eutectic melting and devitrification, and the specific heats of the crystalline, glass, and liquid phases have been measured for a Ni24Zr76 alloy. The data are used to calculate the Gibbs free-energy difference, Delta G(AC), between the real glass and the crystal on an assumption that the liquid-glass transition is second order. The result shows that Delta G(AC) continuously increases as the temperature decreases in contrast to the ideal glass case where Delta G(AC) is assumed to be independent of temperature.

Growth of crystals from amorphous alloys

Abstract: The paper gives a discussion on the kinetics of crystal growth during devitrification of amorphous alloys. Two examples of kinetic analysis of crystallization are reported: (a) a complete Johnson-Mehl-Avrami treatment of isothermal differential scanning calorimetry (DSC) peaks combined with microscopy observations provide a full set of kinetic parameters for AI91Nd9; (b) a mechanism for the formation of nanocrystals in Fe74Nb3Cu1Si13, B9 is advanced on the basis of X-ray diffraction measurements of the lattice parameter and isothermal DSC traces. A collection of the Arrhenius parameters for the rate of growth of crystals from amorphous alloys is presented and a correlation between them is shown The correlation is of the same type of that earlier found for tracer diffusion and suggests a cooperative mechanism for atomic movements leading to the growth of crystals.

Sialon ceramic compositions and methods of fabrication

Abstract: A method of fabricating a SiAlON ceramic body includes: a) combining quantities of Si3 N4, Al2 O3 and CeO2 to produce a mixture; b) forming the mixture into a desired body shape; c) heating the body to a densification temperature of from about 1550° C. to about 1850° C.; c) maintaining the body at the densification temperature for a period of time effective to densify the body; d) cooling the densified body to a devitrification temperature of from about 1200° C. to about 1400° C.; and e) maintaining the densified body at the devitrification temperature for a period of time effective to produce a β'-SiAlON crystalline phase in the body having elemental or compound form Ce incorporated in the β'-SiAlON crystalline phase. Further, a SiAlON ceramic body comprises: a) an amorphous phase; and b) a crystalline phase, the crystalline phase comprising β'-SiAlON having lattice substituted elemental or compound form Ce.

Outflow apparatus for glass preform

Abstract: PURPOSE: To provide a outflow apparatus for glass preform equalizing the temperature of the whole tube, free from declining of the temperature of the fused glass at the vicinity of the outlet of the pipe for fused glass and capable of stable control of a flow rate even for a fused glass having high devitrification temperature. CONSTITUTION: The objective outflow apparatus for glass preform has a glass fusing vessel 1 connected to outlet for fused glass at the bottom and is provided with a electrically heatable pipe 2 made of platinum or a platinum alloy, an electric device 4 to apply a direct current voltage or an alternate current voltage and generate heat from the pipe and a thermo couple 7 to detect the temperature of the fused glass flowing inside the pipe. The pipe is provided with multiple heat-equalizing plates 6 made of platinum or a platinum allay extending in the axial direction and aligned with a pitch in the peripheral direction on the outer circumference of the pipe. COPYRIGHT: (C)1994,JPO&Japio

Substrate for circuit board including the glass fibers as reinforcing material

Abstract: A glass fiber forming composition exhibits a remarkably high dielectric constant e 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 10 2 .5 poise, so as to be readily spun into corresponding glass fibers. The composition consists essentially of 40 to 65 mol % of SiO 2 ; 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 TiO 2 and ZrO 2 ; and 0.5 to 15 mol % of NbO 5/2 as calculated from an incorporated amount of Nb 2 O 5 . Alternately, the composition consist essentially of 40 to 65 mol % of SiO 2 ; 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 TiO 2 and ZrO 2 ; 0.5 to 15 mol % of NbO 5/2 as calculated from an incorporated amount of Nb 2 O 5 ; and 0.5 to 15 mol % of AlO 3/2 as calculated from an incorporated amount of Al 2 O 3 . The composition is also characterized to incorporate at least 85 mol % of a total amount of the oxides and have a dielectric constant [e r ] of 9 or more at 1 MHz and 25° C.

Removal of lead based coating by vitrification

Abstract: Lead in coatings on structures can be removed, without creating hazardous waste, by a flame spray process which utilizes a glass composition having high lead solubility, resistance to devitrification, and an expansion coefficient which is substantially different from that of the metal structure. The process comprises flame spraying particles of a glass powder composition onto the lead based coating to form a layer of glass material, permitting the glass layer to cool sufficiently so that it spalls, thereby forming fragments of the glass layer which readily separate from the structure, collecting the glass fragments, and repeating these steps until any organic material in the coating has been pyrolized and removed as part of the fragments, substantially all red lead present in the coating has been reduced to yellow lead, and all of the lead which was in the lead based coating has been removed from the structure by being incorporated into the glass layers. The preferred glass composition comprises at least one glass forming oxide selected from the group consisting of SiO 2 and B 2 O 3 ; at least one intermediate oxide selected from the group consisting of Al 2 O 3 , TiO 2 , ZnO, PbO, and ZrO 2 ; and at least one glass modifier oxide selected from the group consisting of Na 2 O, K 2 O, CaO, BaO, and NiO.

Optical glass - with high negative part dispersion in blue spectrum range as well as high refraction index and abbe number

Abstract: Optical glass comprises in wt.%:- 0-12 SiO2; 0-2 GeO2; with 0-12 SiO2+GeO2; 9-26 B2O3; 0-2 Al2O3; 0-1 LiO2; 0-1 Na2O; 0-1 K2O; with 0-1 total alkali oxides; 0-4 MgO; 0-4 CaO; 0-4 SrO; 9-27 BaO; 0-4 ZnO; with less than 32 MgO+CaO+SrO+BaO+ZnO; 29-36 La2O3; 0-5 Y2O3; 3-9 ZrO2; 0-2 HfO2; 0-5 TiO2; 0-2 Ta2O5; 6-20 Nb2O5; 0-2 WO3; plus upto 1% purifying agents. ADVANTAGE - Has high negative part dispersion in blue range of the spectrum whilst having high refraction index of above 1.82 and Abbe number of 36-40. This helps to correct for secondary spectrum effect. Glass has high resistance to devitrification and has high transmission capability and is easy to produce.

Silica glass for devitrification resistant discharge lamp

Abstract: PURPOSE: To improve devitrification resistance by specifying non-movable OH group content and alkali (earth) metal content in a silica glass, viscosity and oxygen deficient type defect quantity. CONSTITUTION: A natural quartz powder is treated to make high purity in an atmosphere containing gaseous chlorine or gaseous hydrogen chloride at 900-1100°C for 10-50hr and, after that, is mixed with an aluminum compound, dried and fused by heating to make transparent. A silica glass for devitrification resistant discharge lamp ≤50wt.ppm in non-movable OH group content, ≤2wt. ppm in alkali metal content of Li, Na, K, 5-50wt.ppm in alkaline earth metal element content, ≥10 4 poise in viscosity (1100°C) and ≥5×10 -2 cm -1 in oxygen deficient type defect quantity (absorption coefficient in 250mm absorptive band) is obtained by molding the transparent glass lump into tube like and degassing treating in an atmosphere or a reduced pressure at 900-1100°C for 10-100hr. COPYRIGHT: (C)1994,JPO

The crystallisation and optical properties of gallium cerium lanthanum sulphide glass

Abstract: A series of gallium lanthanum sulphide glasses (70Ga 2 S 3 -30La 2 S 3 ) were melted with cerium sulphide contents ranging from 0 to 15 mol%, i.e. up to half of the lanthanide sulphide content. The stability of these glasses was investigated using thermal analysis and x-ray diffraction. Replacement of half of the lanthanum sulphide content was found to improve the stability against devitrification on heating by a 19% expansion of the working range. Glasses with and without cerium were doped with 3 mol% neodymium sulphide and the optical properties examined. Cerium sulphide addition appeared to result in energy transfer to the neodymium, enhancing the lower energy fluorescence

Interdiffusion-induced devitrification of porcelain enamel on steel

Abstract: Devitrification which occurred in the ground coat (fired at 830°C for 0–30 min) due to interdiffusion near the steel-enamel interface was characterized by electron microscopy (SEM and TEM) coupled with energy-dispersive X-ray analysis. Spinel oxide, Fe3O4, which was alloyed with Ni if nickel-dip pre-treatment was adopted and was compatible with α-Fe, crystallized in the ground coat near the glass-metal interface. Independent of nickel-dip pre-treatment the spinel crystals were extensively developed at the edges of the specimen slab, but were fewer and smaller in the centre of the flat faces. The quantity and size of crystals increased with the firing time. Within the titania-opacified cover coat (fired subsequently at 800°C for 3 min) equiaxed TiO2 crystals (anatase with a minor amount of rutile) were distributed uniformly. The TiO2 crystals grew inwards to some extent into the ground coat and became needle-like with their long axes aligned diagonally to the interface. The devitrification of enamel was affected significantly by diffusion across the interfaces which resulted in a saturation with oxide components.

Thermal evolution of Co1-x-Px electrodeposited ribbons and its influence on the electrical resistivity

Abstract: Co-P ribbons have been obtained by electrodeposition on aluminium substrates. The effect of changing the current density (in the range 8–19 A dm−2) and the H3PO3 concentration on the nature of the phases present in the “as-prepared” samples and their thermal evolution has been studied by X-ray diffraction analysis and differential scanning calorimetry. Amorphous and crystalline (fully or partly) products have been obtained, by changing the bath composition. Devitrification of the initially amorphous samples leads to cobalt (in the hexagonal and/or cubic form) and cobalt phosphide (Co2P) crystalline phases. A devitrification mechanism has been proposed. The presence of two amorphous phases in the glassy samples and the possibility of obtaining intermediate samples with crystalline and amorphous phases intermixed with each other, is suggested from the experimental results. The electrical resistivities of the products obtained have been measured. They are affected by the phosphorus content and develop after thermal treatments, in agreement with both microstructure changes and the nature of the crystalline cobalt phases formed.

Nucleation-and-growth process in Fe80B20 amorphous alloys

Abstract: A nucleation-and-growth process in Fe80B20 amorphous alloy ribbons has been studied using differential scanning calorimetry using an isothermal analysis mode. Measurements have been performed in the temperature range between the glass transition and the onset of crystallization. Experimental data have been fitted using the theoretical Johnson–Mehl–Avrami equation. Kinetic parameters as well as the activation energy of crystallization have been determinated. It is shown that the mechanism, which governs the nucleation-and-growth process is independent of annealing temperature; while the nucleation frequency and growth rate are dependent on temperature value. Finally it is shown that the whole devitrification of the samples cannot be completely described using only the nucleation-and-growth process.

Identification of crystalline phases found in multicomponent AlF3-based glasses

Abstract: Glasses in the system AlF 3 [MgF 2 CaF 2 SrF 2 BaF 2 ]YF 3 (i.e., AMCSBY) have been optimised to reduce devitrification on quenching the melt. Cast specimens are in large part crystal-free. Nevertheless, microcrystallites tend to be present in a characteristics distribution within the glass bulk. The morphology and identity of these crystals are discussed especially in relation to the crystallisation which occurs in reheating the glass.