Showing papers on "Devitrification published in 1995"

Bioactive material template for in vitro synthesis of bone.

Abstract: We describe the synthesis of a new, porous, modified bioactive glass for use as a template for bone formation in vitro. The porosity of the glass was 36.4%; the pore size ranged from 10-160 mm, and there was no incipient devitrification. Prior to seeding the glass with cells, it was necessary to condition the disks. Optimum conditioning was achieved by immersing the templates in a tris buffer at pH 6.8 for 48 h and then treating the glass with tissue culture medium for 1 h at 37 degrees C. The conditioned glass disks were seeded with 10(6) neonatal rat calvaria osteoblast-like cells; cells on the substrate were maintained in culture for 3-7 days. To prevent pH shifts due to corrosion of the conditioned glass, the medium:glass ratio was maintained at 90 ml/g. We found that the templates were rapidly invaded by cells which maintained the osteoblast phenotype; thus, they exhibited high alkaline phosphatase activity and synthesized type I collagen and osteocalcin. SEM-EDAX showed that the cells elaborated substantial amounts of extracellular matrix and a bonelike tissue was present throughout the entire template thickness. FTIR analysis of material formed in the glass indicated that the mineral phase was a biologic hydroxyapatite. Controls (cells without substrate and substrate without cells) exhibited none of these features. Results of the study suggest that this porous glass can function as a template for generating bone in vitro.

Suevite breccia from the Ries crater, Germany: Origin, cooling history and devitrification of impact glasses

Abstract: — The distribution and petrography of surficial suevite breccias of the Ries impact crater in Southern Germany are reviewed, and the morphology, petrography and chemical composition of impact glasses in suevite breccias and their postdepositional devitrification is synthesized. Origin and thermal history of suevite breccia and suevite glasses are inferred from these data and from recent results of cooling and crystallization experiments with suevite glass melts under controlled conditions. In a montmorillonitic groundmass, the suevite breccia contains pieces of glass, up to some decimeters in size, and crystalline rock clasts of all stages of shock metamorphism. The glass particles originated in impact melt of basement gneisses and cooled by adiabatic pressure release from ∼80 GPa to atmospheric pressure during ejection from the crater. They were deposited on the ground together with the other suevite components at a temperature of ∼750 °C. Fractured glass pieces in the breccia show that during deposition of the suevite the temperature was below the temperature at which undercooled melt transforms to rigid glass. The suevite cooled after deposition mainly by convection of heat by emanating gases and vapors. In chilled layers at the base and at the top of suevite deposits, the glasses are preserved in vitreous state. Between these zones, the glasses were devitrified, yet crystallization of pyroxene, plagioclase and magnetite took place below the glass-transformation temperature. Annealing experiments show that this unusual devitrification below the transformation temperature can be explained by the impact origin of suevite glasses. Due to rapid adiabatic cooling on decompression, the glasses were oversaturated with water and internally strained. Under these conditions, devitrification, especially the formation of plagioclase, was possible at temperatures below the transformation range. The origin from adiabatically cooled impact melt of deep-seated rocks distinguishes water-bearing suevite glasses from the Ries-derived, water-free moldavite tektites, which are interpreted as condensates of vaporized, surficial sediments (Engelhardt et al., 1987).

Effects of nucleating agents on diopside crystallization in new glass-ceramics for tile-glaze application

Abstract: The effect of crystallization produced by addition of TiO2, ZrO2 and P2O5 oxides to glass-ceramic of the system CaO-MgO-SiO2 was studied using structural and thermal techniques. The devitrification process was independent of thermal treatment. X-ray diffraction studies performed on the glass-ceramic system indicated that diopside crystalline phase was more thermodynamically favourable than other phases. The effect of the nucleating agent depends on its nature: TiO2 decreased the activation energy while P2O5 and ZrO2 did not. SEM analysis of the doped glass-ceramics showed randomly distributed crystals with significant dimensional variations from those of the undoped system. All these formulations, showing a high crystallization rate, and a fast heating rate, can be used as tile glazes and/or tile-glaze components.

Devitrification and vitrification of tellurite glasses

Abstract: The crystallization of pure tellurite glass during various heating rates was studied. The activation energy for crystallization was 115 × 1022 eV mol−1. The glass transformation, T g, starting crystallization, T x, crystallization, T c and melting temperatures, T m, have been reported for binary tellurite glasses of the form (1 − x) TeO2−xAnOm [AnOm = MnO2, Co3O4 and MoO3]. Among many different parameters of the glass forming potential the “two-thirds rule”, T g/T m, the glass stabilization range, δT= T x − T g, and the glass forming tendency, K g= (T c− T g)/(T m − T c), are reported for the first time for tellurite glasses.

Chemical and physical compatibilities of fluoride and fluorophosphate glasses

Abstract: Based on a composite preform concept, a systematic investigation has been carried out in order to qualify a low-refractive-index fluorophosphate glass that would be chemically and physically compatible with a low-phonon-energy barium-indium-gallium-based fluoride glass. Special attention is paid to the stability towrds devitrification of the solid solutions that may form at the interface of the two glasses and to the thermomechanical compatibility of the fluoride and fluorophosphate glasses. The most significant result is a preform that exhibits a theoretical refractive index difference, Δn, of nearly 0.07.

Relationship between microstructure and mechanical properties of silicon nitride ceramics

Abstract: The microstructure of silicon nitride ceramics is related to the mechanical properties. It is demonstrated that the room temperature strength and toughness is determined by size and morphology of the Si3N4 grains and the grain boundary phase. The high temperature properties are mainly controlled by the composition and properties of the grain boundary. The grain boundary crystallization, as one strategy for the development of high-temperature resistant materials, is analysed with respect to the phase relationships between Si3N4 and the additives and the impossibility of a complete devitrification. It is shown that small amounts of impurities in the starting powder can be enriched in thin grain boundary films and degrade the high-temperature properties.

Conventional and microwave sintering of condensed silica fume

Abstract: Condensed silica fume, a by-product from the production of silicon alloys, was sintered by (i) conventional heating in a dilatometric furnace, both at constant heating rate and isothermal heating, and (ii) by the microwave heating. The dense products with relative density up to 95% of theoretical can be obtained only by short runs at high heating rates, preferentially accomplished by the microwave treatment. Prolonged heating leads to the devitrification of the original glassy phase to cristobalite, accompanied by an arrest of densification.

A furnace design for use in combined X-ray absorption and diffraction up to a temperature of 1200°C: Study of cordierite ceramic formation using fluorescence QEXAFS/XRD

Abstract: We present a new furnace design which is based on a tube furnace: primarily for ceramics or catalysis research where the sample is heated in air, but at temperatures of up to 1200°C. The design and specifications of this furnace are presented. High temperature in situ studies of glass-ceramic systems would greatly benefit from combined QEXAFS/XRD studies, where it is possible to study the changes in the near environment of dilute nucleating agents and correlate these changes to changes that occur in long range order. A particular example is a glass-ceramic based on the stoichiometry of the mineral cordierite dope with 0.8 wt% Cr 2 O 3 , which acts as a nucleacting agent for devitrification of the glass upon heat treatment.

Devitrification inhibitors in borosilicate glass and binary borosilicate glass composite

Abstract: Cristobalite is known to precipitate out of borosilicate glass (Corning 7740) and a binary glass mixture of borosilicate glass and high silica glass when these glasses are heated to elevated temperatures. To prevent cristobalite from forming in these glass systems, a devitrification inhibitor needs to be found. Among oxides selected for testing, both Al2O3 and Ga2O3 are found to prevent cristobalite from forming in these glass systems.

High-pressure discharge lamp, method for manufacturing a discharge tube body for high-pressure discharge lamps and method for manufacturing a hollow tube body

Abstract: In a quartz glass tube body for high-pressure discharge lamp, the devitrification occurs during lighting, a light flux decreases and finally the useful life ends, where the main cause of this devitrification phenomenon is reaction between a sealed substance and the quartz glass tube body. It is one object of the present invention to attain the longer useful life, for example, of a high-pressure discharge lamp by preventing such a phenomenon. According to the present invention, a coating is made up by forming one or more oxynitride layers of an element chosen from among aluminum, tantalum, niobium, vanadium, chromium, titanium, zirconium, hafnium, yttrium, scandium, magnesium, silicon and lanthanum rare earth elements. By incorporating a bilayer coating on the inside wall of said hollow tube body, for example, that is composed of an aluminum oxynitride layer and an aluminum nitride layer obtained from application of a high-frequency wave between the sputter electrodes and generation of a glow discharge, a durable coating can be formed, thereby enabling the useful life of a high-pressure discharge lamp to be lengthened.

Glass composition for substrate and substrate for plasma display using the same

Abstract: PURPOSE: To enable forming by float method and improve glass transition point and coefficient of thermal expansion by including SiO 2 , Al 2 O 3 , MgO, CaO, SrO, BaO, Li 2 O+Na 2 O+K 2 O, ZrO 2 , SO 3 , etc., having a specific weight composition. CONSTITUTION: Component raw materials are blended and the blend is continuously charged into a melting furnace and heated and melted to 1500-1600°C to provide the objective melted glass composition composed of 52-62wt.% SiO 2 , 5-12wt.% Al 2 O 3 , 0-4wt.% MgO, 3-5.5wt.% CaO, 6-9wt.% SrO, 0-13wt.% BaO, 17-27wt.% MbO+CaO+SrO+BaO, 7-14wt.% Li 2 O+Na 2 O+K 2 O, 0.2-6wt.% ZrO 2 , 0-0.6wt.% SO 3 , having ≥600°C glass transition point, a devitrification temperature lower than a temperature capable of providing 10 4 poise viscosity and 75 to 95×10 -7 °C -1 coefficient of thermal expansion. The melted glass composition is formed into a plate glass and annealed and cut to provide the objective transparent plate glass. COPYRIGHT: (C)1996,JPO

SAXS/WAXS studies of the devitrification of Cr-doped cordierite glass using synchrotron radiation

Abstract: In situ simultaneous Small Angle X-ray Scattering (SAXS) and Wide Angle X-ray Scattering (WAXS) experiments have been carried out on Cr-doped cordierite (Mg 2 Al 4 Si 5 O 18 ) glass. Samples were initially pre-treated at 875°C close to the glass transition to promote nucleation and this was followed by further treatments at 920°C and 1040°C to generate dispersed crystal growth. The initial treatment revealed no changes in the SAXS or the WAXS data, but the subsequent treatments at higher temperatures revealed large changes in microcrystallinity developing with time. In particular taking the SAXS data for q −1 the average Guinier radius, R g , was found to increase with heat treatment from initial values of ∼ 10Ato final values of ∼ 30A. I(q) versus q −n plots (for q ≥1/R g ) revealed increasing values of n indicative of changes in mesostructure from rough to smooth fractal architecture. From the WAXS data that was obtained concurrently two separate crystalline phases were identified during devitrification-spinel (MgAl 2 O 4 ) and a stuffed quartz (Mg x Al 2 x Si 1− x O 2(1 + x ) )-each growing at different rates. The growth of spinel correlates well with the SAXS invariant, reflecting the major contrast in electron density between this phase and that of the residual glass. For the heat treatment conditions employed, WAXS reveals that the stuffed quartz phase appears later than the spinel phase and also grows less rapidly. For prolonged annealing, a SAXS feature develops at ∼ 0.022A −1 , corresponding to an average interparticle distance of ∼ 280A. This is considerably longer than the statistical separation of Cr atoms in the glass (14A) and suggests some clustering of Cr takes place as devitrfication progresses.

Germanate glass for mid-infrared medical optical fiber

Abstract: This invention relates to a family of rugged, non-toxic, high damage threshold, fiberizable, infrared transmitting germanate glasses that are suitable for medical laser delivery They possess excellent visible transmission to utilize an aim beam in conjunction with the infrared laser The glasses comprise (in mol%) 230-760 % GeO2, 0-390 % Ga2O3, 30-510 % SrO, 80-170 % Ta2O5, 0-230 % Y2O3, 0-350 % ZnO, and optionally one or more of the following components: 0-110 % TiO2, 0-200 % Nb2O5, 0-80 % ZrO2, 0-130 % Cs2O, 0-10 % MgO, 0-27 % La2O3, 0-250 % Gd2O3, 0-230 % Yb2O3 These glasses have intrinsic absorption coefficients of less than 0137, 0048, 0024 and 0021 dB/m at 294, 2796, 271 and 269 νm, respectively The glasses are characterized by excellent chemical durability, low thermal expansion coefficient, high glass transition temperature, and good glass working properties such as low liquidus temperature, flat viscosity-temperature dependence and good resistance to devitrification Specific embodiments are particularly well suited for use in biological applications where high chemical durability, non-toxicity, a high laser damage threshold and good infrared and visible transmission are required

Glass composition for electric lamp

Abstract: PROBLEM TO BE SOLVED: To provide a glass composition for electric lamps that slightly causes devitrification on the boundary face with the sleeve, even when tubular glass is formed by the Danner process, causes no evaporation of B2O3 during the production of the glass and shows good processability and electric insulation. SOLUTION: The objective glass composition comprises 60-75 wt.%; of SiO2, 1-5 wt.% of Al2O3, 3-13 wt.% of Na2O, 2-10 wt.% of K2O, 0-5 wt.% of Li2O, 0-5 wt.% of CaO, 0-5 wt.% of MgO, 1-12 wt.% of SrO, 0-3.8 wt.% of BaO in which the weight ratio of Na2O/K2O is >=1, and (CaO+MgO+SrO+BaO)/(Na2O +K2O+Li2O)>=0.85 and this composition is substantially free from B2O3.

Thermal properties and devitrification behaviour of (1+x)CaO.(1−x)MgO.2SiO2 glasses

Abstract: The thermal properties (glass transformation, Tg, and softening, Ts, temperatures), the crystalline phases formed during heating in a differential thermal analysis (DTA) apparatus, the kinetic parameters and the mechanism of the devitrification process, of glasses of the system diopside-wollastonite were investigated. The substitution of CaO by MgO induces an increase in Tg and the crystal growth activation energy, Ec; this is probably linked to the greater coordination number of Caz+ ions with respect to the Mg2+ ions. The substitution of CaO by MgO lowers the nucleation rates of the diopside phase; wollastonite solid solution nuclei form whose growth appears to leave a glassy matrix in which diopside formation is inhibited. Only surface nucleation was observed, but, in finely powdered samples, which soften and efficiently sinter before devitrifying, surface nuclei behave as bulk nuclei. When bulk crystallization occurs, the Avrami parameter m was found to be 2 for all glasses, except the diopside one, for which m=3.

Chlorine-containing fluorophosphate glass

Abstract: PURPOSE:To obtain fluorophosphate optical glass having a low refractive index and low dispersion, hardly causing crystallization, excellent in stability to devitrification and fit for the production of a large-sized homogeneous glass on an industrial scale. CONSTITUTION:This chlorine-contg. fluorophosphate glass contains, by mol, 0.5-12% P ions, 25-40% Al ions, 3-15% Y ions, 3-12% Mg ions, 15-28% Ca ions, 8-17% Sr ions, 2-14% Ba ions and 0-5% alkali metallic ions as glass forming cations and further contains, by mol, 0.5-15% O ions, 84.5-99% F ions and 0.5-6% Cl ions as glass forming anions.

A study of tin oxides in silicate based glasses

Abstract: The roles of tin in two silicate based glass systems have been investigated by NMR and Mossbauer spectroscopies and by physical property measurements of the glasses. The first glass system investigated was the stannous silicate (binary SnO-Si02) glass. Glasses with SnO contents ranging from 17 to 72 mol.% have been made by melting pelleted powder in an alumina crucible. It was found that alumina crucibles are unsuitable for making glass with <20 mol.% SnO because of attack on the crucible at the high melting temperature (_>_1600°C). Silica crucibles will not withstand such high temperature and tin will attack a platinum crucible. The ability of this system to form glass past the orthosilicate composition has been discussed in terms of the polarizing power of Sn2+ and the structure of SnO. The 119Sn NMR results did not give much structural information due to the high chemical shift anisotropy of Sn 2+ site but they showed that the glass also contains trace amounts of Sn4+species. The 29Si MAS NMR results showed that SnO does not depolymerise the silicate network to the same extent as Na20 or even Pb0. Computer simulations of the 29 Si MAS NMR spectra showed that, for SnO <-30 mol.%, the disposition of Qn species is consistent with the binary model, which means that SnO is acting the role of modifier. For compositions > 30 mol.% SnO, the Qn distribution follows the statistical model and this has been interpreted as SnO now acting as an intermediate. The 119Sn MOssbauer results confirmed this interpretation. The Sn2+ isomer shift decreases with increase of SnO which is indicative of increasing covalent character of the Sn—O bonds while the larger quadrupole splitting suggests distortion of the SnO polyhedral structure in the glass. The relation of the Sn 2+ isomer shift to the quadrupole splitting and the temperature dependence of the isomer shift of Sn2+ indicate the formation of Si—O—Sn linkages at high SnO contents. The decrease of the viscosity of the glass with increasing SnO is small when compared to the decrease of the viscosity in alkali metal and alkaline-earth oxides silicates when the respective modifier oxide is increased in those glasses. The variation of the density, thermal expansion and refractive index with SnO content showed discontinuities in the region of 30-45 mol.% SnO. This has been interpreted as being the point where SnO changes its role from that of modifier to intermediate. The results of differential thermal analysis and devitrification of SnO-Si02 glasses showed that glass with 40 mol.% SnO can be heat treated in the temperature range of 570° to 680 °C to produce metastable SnSiO3 crystals. SnSiO 3 decomposed to SnO + Si02 at temperatures above —700°C and, at temperatures greater than 720°C, oxidation of SnO to Sn02 and Si02(glass) to Si02 (cristobalite) took place. The second glass system is tin-doped float glass. This is glass of the float composition remelted with tin(II) oxalate in silica crucibles under normal atmosphere conditions. In this way it has been demonstrated that we can mimic the tin oxide distribution found within the tin diffusion region in float glass. Synthesis of the glass has shown that both Sn2+ and SO+ can be assimilated simultaneously in the glass but there is a solubility limit for SO+. The 1195n Mbssbauer results showed that Sn2+ and SO+ played different structural roles in the glass. The environment of Sn2+ in glass is similar to that in amorphous SnO while the SO + structure in glass does not change significantly compared to crystalline Sn02. The Debye temperatures and recoil free fractions showed that Sn2+ is less rigidly bound to the network modifier site while SO + is rigidly bound at network former sites in the glass. The different structural roles of 5n 2+ and SO+ in the glass were reflected in the some of the physical properties of the glasses.

Characterization of a low k silica glass composite

Abstract: The effect of alumina on densification, devitrification, thermal expansion and dielectric behaviours of a binary glass mixture of borosilicate glass and fused silica was studied

SiC Fiber/Borosilicate Glass Composite (Part 3)

Abstract: The influence of thermal stresses on the mechanical properties of thermal-shocked Nicalon SiC fibre/Pyrex composites has been analysed by considering the effect of thermal-mismatch stress on the mechanical properties of composite constituents. The thermal-mismatch stresses developed between devitrified cristobalite, and fibre and glass matrix were calculated using the values of material parameters obtained from experiment. This shows that the high thermal mismatch stresses originating from larger coefficient of thermal expansion of devitrified cristobalite extended the preexisting matrix crack, and caused fibre debonding. Thus, the reductions in the modulus and the interfacial stress of thermal-shocked composites in comparison with those of as-fabricated composites resulted from the matrix devitrification. The lower proportional limit of thermal-shocked composite was attributed to the lower interfacial stress and larger pre-existing cracks compared with those of as-fabricated composite. The reduction in ultimate strength of thermalshocked composite was correlated with the decreases of fiber hardness and interfacial stress.

Ultraviolet ray absorbing silica glass and production thereof

Abstract: PURPOSE:To obtain silica glass for a discharge tube having excellent UV absorbing property and devitrification resistance. CONSTITUTION:This silica glass has molecules/cm concn. of hydrogen molecules and contains at least one kind of transition metal having controlled valency.

ZBLAN Microgravity Study

Abstract: One of the greatest obstacles with the fluorozirconate ZBLAN (ZrF4-BaF2-LaF3-AIF3-NaF) is the problem of devitrification. Fluoride glasses have a narrow working range and the viscosity is a strong function of temperature. Rates of nucleation and growth of crystals in the glass depend on the viscosity, making these glasses unstable and prone to crystallization. The viscosity of ZBLAN at the drawing temperature is low, usually between two to five poise, so it is difficult to obtain fibers from their preform melts without crystallization. The preforms usually contain heterogeneous nuclei which grow into microcrystallites above the glass transition temperature, T(g). Since microcrystallites in an optical fiber cause extrinsic light scattering losses of the optical signal, fiber drawing must be completed in a short time to minimize the generation of light scattering centers. To keep these losses to a minimum and to fabricate low scattering loss fibers and other optical components, this research deals with the possibility of minimizing crystallite formation by removing the gravitational influence of solutal segregation of the ZBLAN elements. This report reviews the early work on the KC-135 aircraft, the development of the ZBLAN Rocket Experiment, preparations at the White Sands Missile Range, analysis of the flight and ground test results, lessons learned and future experimentation.

Highly Refined Microstructures in Devitrified Alloys

Abstract: The formation of nanosized microstructures from controlled devitrification of amorphous alloys is reviewed. The driving force for copious nucleation of crystals is discussed in terms of thermodynamic models for the free energy of crystallization and for the interfacial free energy. After fast nucleation and growth of crystals to nanometric size, there are evidences of a change in diffusion mechanism to slow growth. Examples are provided of primary processes in Fe-Zr-B and Al-Sm with data of XRD, TEM, DSC. Cases of eutectic and polymorphic transformations are also at hand (Ni-P, Cu-Ti-Al, Cu-Ti-Ni, Ti-Zr-Co-Cu-Al) showing that the thermodynamic and kinetic conditions leading to nanostructured materials are not unique. Finally chemical effects are discussed such as those of impurities in crystallization (e. g. Zr-based alloys), in leaching of Ni-Zr-Al and in deposition reaction of electroless Ni-P.

Devitrification of Nd3+-doped glasses in the akermanite-gehlenite system

Abstract: The devitrification of glasses in the akermanite-gehlenite system [(C2Al2−2yMgySi1+yO7 (0≤y≤1)] doped with Nd3+ (2% mol) has been studied. DTA was used to determine the kinetics of the process. The Avrami law α=1-exp(−ktn) withn=1 was found to hold for doped gehlenite. For pure gehlenite, parametern was determined to be 1.5. The activation energy of the devitrification process is independent of the presence of neodymium, and it is of the same order of magnitude as that for the viscous flow in molten silicates (ca. 650–750 kJ·mol−1).