Showing papers on "Devitrification published in 1988"

The infrared and Raman spectra of ZrO2-SiO2 glasses prepared by a sol-gel process

Abstract: Infrared and Raman spectra were measured and interpreted on the basis of structure for various ZrO2-SiO2 glasses that were prepared by a sol-gel process involving zirconium nitrate. Interpretation of the spectra indicates the presence of both Zr-O-Si and Si-O-H linkages depending upon heat treatment conditions, and the evidence of zirconium atoms with eight fold coordination for glasses with high ZrO2 contents. The crystalline products formed during glass devitrification were monitored by vibrational spectra. The formation and detection of tetragonal ZrO2, zircon, monoclinic ZrO2 and α-cristobalite depended upon the conditions of devitrification.

Decoupled bulk and surface crystallization in Pd 85 Si 15 glassy metallic alloys: Description of isothermal crystallization by a local value of the Avrami exponent

Abstract: Isothermal devitrification of Pd85Si15 amorphous alloys has been analyzed using differential scanning calorimetry (DSC) and x-ray diffractometry. Both as-quenched and aged amorphous ribbons were investigated. Crystallization of aged samples starts from the surface and proceeds several micrometers into the bulk. The product of this process is a layer of strongly textured palladium (111) followed by a mixture of Pd2Si, Masumoto MSI phase, and untextured palladium. Next, the crystallization occurs via a different (bulk) mechanism, resulting in a mixture of Masumoto MSII phase and untextured palladium. The bulk mechanism is the only one observed in as-quenched samples. The surface and bulk crystallization mechanisms are spatially decoupled and, therefore, the corresponding DSC data can be analyzed separately. This has been done according to the Kolmogorov–Johnson–Mehl–Avrami model and also using the recently developed concept of local value of Avrami exponent n. For both the surface and bulk crystallization the phase transition process cannot be characterized by a single value of n. Observed variation of n with the crystallized fraction x is explained by a considerable variation of the nucleation rate that takes place during devitrification.

Tellurium halide glasses: new materials for transmission in the 8-12 μm range

Abstract: A new class of IR transmitting glasses has been discovered in the binary systems TeCl and in the ternary systems TeClS, TeClSe, TeBrSe, TeBrS, TeBrSe; TeBrS and TeISe. The ternary glasses have a very high resistance towards devitrification. These so-called “TeX glasses” have a glass temperature Tg ranging from 50°C to 85°C depending on the composition. The S or Cl containing glasses have their IR edge located in the 13 μm region while the compositions not containing these light elements have their multiphonoedge near 18 μm. A typical glass composition such as Te3Br2 has a potential high transparency of about 10 dB/km at 10.6 μm estimated from band-gap and multiphonon absorption. Samples prepared from high purity compounds show no parasitic absorption bands due to OH, SH and other complex anions. Vitreous domains, preparation conditions and some properties such as thermal expansion and resistance to corrosion by water are discussed.

Effect of Devitrification on Creep Deformation in Glass‐Containing Ceramics

Abstract: Many commercial structural ceramics contain glass. This glass is often unstable and devitrifies during creep at elevated temperatures. Two models for this process have been developed, in order to understand how devitrification affects measured creep parameters. They indicate that when creep data are measured after a constant amount of creep strain, the effect of devitrification can be large. Moreover, if devitrification is stress-state dependent, a different creep rate on the tensile and compressive sides of a bend bar is predicted. This can contribute to the redistribution of stress across a bend bar and complicate the analysis of creep data obtained in bending.

Characterization of amorphous alloys by thermal analysis techniques

Abstract: Various thermal analysis studies were conducted to characterize metallic glasses. The methods used, namely differential scanning calorimetry, dilatometry, dynamic mechanical analysis and thermomagnetometry, are shown to be useful for studying thermally induced phenomena. Examples covered demonstrate the usefulness of thermal techniques in evaluating structural relaxation. Curie temperature, glass transition, and devitrification processes. A basis of understanding structural relaxation and the glass transition is discussed in terms of a diagram of enthalpy against temperature.

Nitrogen doping of fused silica and silicate glasses: a study of transport and optical properties☆

Abstract: Our work has demonstrated for the first time that relatively large amounts of atomic nitrogen can be incorporated in fused silica, rather than Vycor, by a high temperature NH3 cracking process. The nitrogen analysis process shows that the nitrogen is chemically dissolved in the molecular structure of the fused silica glass and we obtain a product which is a stable and transparent glass. A kinetic study of this nitriding process has shown that nitrogen incorporation in a glass will occur even at temperatures as low as 500°C, although the best efficiencies in the reaction are attained at temperatures near or slightly below the glass transition temperature of the glass. The kinetic study also shows that optimum nitriding conditions for fused silica are a NH3/N2 ratio of 3:1 at a nitriding temperature of ∼1000°C. We discuss the possible structural scheme that allows increased lattice stiffness of a nitrided glass relative to a non-treated glass. This in turn has important consequences for understanding the problem of glass stability (i.e. devitrification) in silicate based glasses.

The structure of thin films sputter deposited from a Ba2 Si2TiO8 ceramic target

Abstract: Thin films were sputter deposited from a Fresnoite (Ba 2 Si 2 TiO 8 ) ceramic target at substrate temperatures lower than 175°C. The as-deposited thin films were near amorphous with a void network morphology. In spite of the fact that the film compositions were shifted from stoichiometry, x-ray diffraction studies showed that the films crystallized to form randomly oriented Fresnoite grains. The crystallization kinetics were quite sluggish and the resultant activation energy for the crystallization process was 370 ± 30 kJ/mol. Even after annealing for 10 h at 750°C an appreciable amount of amorphous material remained in the thin films. The short-range order in this amorphous material was changed from that of the as-deposited thin films. The overall devitrification kinetics of amorphous Fresnoite thin films at a fixed temperature were represented theoretically by an equation of Tool's type.

Effect Of Molybdenum And Silicon On The Electrochemical Corrosion Behavior Of Fenib Metallic Glasses

Abstract: Potentiodynamic polarization studies on Vitrovac 0040 (Fe40Ni40B20), Metglas 2826 MB (Fe40Ni38Mo4B18) and Vitrovac 4040 (Fe39Ni39Mo4Si6 B12) commercial metallic glasses in 1 N Na2SO4 of varying pH and in 0.5 N H2SO4 have been performed. It is found that in alkaline and neutral sulfate solution, Mo and Si have a subtle effect in reducing icorr. Mo imparts passivation in neutral sulfate and fails to passivate the glass in H2SO4 solution. No perceptible change in icorr is found with the addition of Mo to the glass in H2SO4 solution. While Si increases the corrosion rate of the glass, it also promotes passivation in H2SO4. However, in the presence of chloride ions, the Si-containing glass is susceptible to pitting. Devitrification of Vitrovac 4040 leads to active dissolution irrespective of the crystalline products present, while the corrosion rate and electrochemical parameters depend on the nature of crystalline phases formed during the devitrification.

Quartz glass products for semiconductor industry

Abstract: PURPOSE:To obtain the title product of high durability without heat distortion and devitrification and with complete prevention of contamination, by growing crystal layers from the nuclei of a dopant on the outer surface of a clear quartz glass tube of Na, K and OH group contents below specific values respectively. CONSTITUTION:Naturally occurring quartz are pulverized, sieved and cleaned by dipping in HF. The purified powder is fused in an electric furnace for about 10-12hr and the melt of less than 10ppm OH content is formed into a clear quartz tube. The tube is placed in a heating furnace and heat-treated for several hours, as Cl2 or HCl gas is allowed to flow to give a tube of 0.1ppm Na and K and 0.3ppm Li. The tube is coated with a solution containing a trivalent cation dopant such as Al on the outer surface and heated over the softening point to effect doping. Further, the doped quartz tube 1 is placed in an electric furnace at a desired stage and heated at about 1,300 deg.C for about 10-15hr to develop the crystalline layer of cristobalite 10-100mu thick.

On the valence state and coordination of iron cations in alumino-silicate glasses

Abstract: Measurements of the Mossbauer effect in 57Fe were performed in selected alumino-silicate glasses. The influence of the preparation temperature of the studied glasses on the valence state and coordination number of iron cations is discussed. It is found that Fe2+ ions are only octahedrally coordinated, while Fe3+ ions are present both in tetrahedral and octahedral coordination. In some partly crystallized galsses both Fe2+ and Fe3+ cations were found to occupy only sites of tetrahedral coordination in the glass matrix. The Mossbauer spectra clearly reveal the presence of magnetite crystallites and the decrease of their contribution with an increase of the glass preparation temperature. The existence of the magnetite phase was confirmed by the X-ray diffraction. The devitrification process is marked by the magnetite-hematite transformation and the decrease of iron content in the glass matrix. Electrochemical methods were used for independent determination of the Fe2+/Fe3+ ratio.

Thermal evolution and densification of R2O-SiO2 gels

Abstract: R2O-SiO2 materials were prepared with 5 mol% of alkaline oxide by the sol-gel method. The gel-glass transition was achieved by treatment in air or in a controlled atmosphere. The samples present a silica network structure to which the alkaline ions are incorporated in a different way as compared to that of conventional glass. Material densification sets in around 350° C. In the temperature range between 500 and 600° C silica devitrification are produced together with condensation-polymerization of many silanol groups which form part of the material. Densification treatments in a helium atmosphere eliminate these residual groups.

Crystallization of lithium alumino / galliosilicate glasses

Abstract: The effect of glass composition on the crystallization temperature, activation energy for crystallization, and identity of the crystalline phases formed was determined for a large number of lithium gallio and alumino/galliosilicate glasses using differential thermal analysis and X-ray diffraction. A large region exists in the ternary system lithia-gallia-silica in which LiGaSi2O6 is the only crystalline phase formed. The devitrification of glasses containing both gallia and alumina resulted in the formation of solid solutions between the lithium galliosilicate and lithium aluminosilicate phases. These data suggest that gallium and aluminum ions occupy identical sites in the glass structure.

Use of resistivity and magnetic measurements to monitor recrystallization of metallic glasses

Abstract: It is shown that resistivity and magnetization measurements can be used to monitor the controlled devitrification of metallic glasses. These measurements are especially useful in the case of Fe‐based metallic glasses to be used at high frequency. In these materials, transformation of about 1 vol. % of the amorphous matrix into crystalline α‐Fe is known to produce enhanced magnetic properties. Magnetic techniques are especially sensitive when the heat treatment is carried out above the Curie temperature of the amorphous matrix but below the Curie temperature of the α‐Fe precipitates.

Method for bending glass sheet for laminated glass

Abstract: PURPOSE:To produce tough laminated glass with no non-adhered parts by separately heating, bending, and annealing the glass sheets for laminated glass, then laminating the sheets, placing the laminate on a baking die ring, and hot- forming the laminate. CONSTITUTION:At least two glass sheets 5 and 5 for laminated glass are separately placed on a baking die ring 1, and heated in a furnace to the bending temp. Consequently, the sheets 5 and 5 are softened and bent by gravity. The sheets 5 and 5 are annealed for a specified time. The annealed sheets 5 and 5 are laminated and placed on the baking die ring, hot-formed, and matched so that the surfaces of the respective sheets 5 and 5 are conformed. In this case, the ambient temp. is controlled to about 500-650 deg.C, or preferably to about 530-580 deg.C. By this method, the non-adhered parts of the sheets due to the nonconformity in the shape and curvature are eliminated, and the touch laminated glass without any devitrification due to foaming can be obtained.

Investigation of the nucleation kinetics and the linear growth rate during the devitrification of Fe‐B amorphous alloys with the aid of a thermomagnetic balance

Abstract: The kinetics of overall isothermal crystallization of the amorphous Fe75B25 alloy, as well as the kinetics of nucleation and growth in amorphous Fe75B25 and Fe80B20 alloys were investigated with the aid of a thermomagnetic balance. The activation energies of growth for both alloys QG = 167 kJ/mol, which are similar to these, found by other authors, were determined. Curves number of nuclei N vs time t were traced and the activation energy of nucleation in the amorphous alloy Fe80B20 was obtained QN = 211 kJ/mol, which value is also in very good agreement with those determined by means of other independent methods.

THE Nd15Fe77B8 MICROSTRUCTURE : SOME EFFECTS OF OXYGEN FOR DIFFERENT SOLIDIFICATION CONDITIONS

Abstract: Some microstructural effects of oxygen in cast and rapidly quenched Ndl5Fe77B8 alloys have been observed. Starting from as-cast materials, we have studied the effects of an oxidation in air near 300°C, followed by electron beam surface remelting. During the oxidation process, oxygen promotes, along the diffusion paths, the growth of glassy regions containing strongly textured α;-Fe precipitates. After surface remelting of these oxidised samples, various rapid solidification microstructures are obtained. Oxygen contamination effects have also been studied in Ndl5Fe77B8 ribbons directly elaborated by the planar flow casting process. For partially amorphous ribbons, the study of the devitrification mechanisms reveals the formation of very finely crystallized two phase (α-Fe + C-Nd203) regions, in addition to the main phase (Nd2Fel4B) crystallization.

Preparation and Electrical Properties of Highly Conductive Glass-Ceramic Films Containing Tungsten Bronze

Abstract: A series of alkali tungsten bronzes are non-stioichometric compounds having a general formula RxWO3, where R is H, Li, Na, K and Rb and x is less than unity. They show metallic conduction. The present work was done in order to obtain highly conductive glass-ceramic. films containing tungsten bronze and to investigate their electric conductivites. A series of glasses consisted of the compositions containing 30.0-37.5 Na2O⋅57.5-67.5 WO3⋅2.5-10.0 B2O3 (in mol%). About 70 grams batches corresponding to the nominal compositions were melted in alumina crucibles at 1000°C for 1h. The melts were rapidly quenched by the twin roller (3000rpm) to obtain clear glasses without devitrification. Glasses were ground into powders of particle size less 37μm. Films were prepared by two methods. One was the method by dipping of a quartz glass plate into water dissolved Na2O-WO3-B2O3 glasses. The other was the method by melting the glass powders uniformly dispersed on a quartz glass plate in ethanol. Tungsten bronze crystals precipitated from these films when heat-treated in a H2 atomosphere. These glass-ceramic films were shown to be much higher electric conductivities than the sintered tablet. The maximum conductivity of these glass-ceramic films was about 103S/cm at 25°C. The charge carrier of glass-ceramic films was confirmed to be electron by the Seebeck coefficient measurements. The conduction mechanism of glass-ceramics was considered to be electronic conduction due to the large polaron.

Polymorphic mixes of zirconia from a borate glass melt

Abstract: Yttria-stabilized zirconia crystals were produced by devitrification within a sodium borate glass containing zirconia and yttria. Yttria (3, 6, or 10 mol%) in zirconia was added as part of the starting material of the glass batch. Phase separation and crystallization occurred during the cooling and subsequent heat treatment of the glasses. The zirconia powders obtained after leaching away the sodium borate had surface areas between 16 and 48 m2/g. X-ray diffraction traces revealed a mixture of monoclinic, tetragonal, and cubic phases, the phase ratios of which depended on the composition of the glass.

A study of the electrical conduction and devitrification behaviour of the amorphous chalcogenide Te81Ge15As4

Abstract: The thermal spectrum of thedc conductivity of Te81Ge15As4 during a consecutive heatingcooling cycle within the temperature rangeT g > T> T m indicated that the heating and cooling parts of the cycle do not coincide and phase transformation were ohserved The activation energies of conduction (δE) for the initial amorphous, the liquid and the final crystallized phases are 0.35, 0.30 and 0.16 eV, respectively. The growth of conductive regions during isothermal annealing of the amorphous solid phase was studied and correlated to the corresponding structural changes recorded by using microphotography and X-ray diffraction. The structure of the investigated three-component composition can be represented as a solid solution based on tellurium character. A value of 2.4 eV was obtained for the activation energy of crystallization from non-isothermal transition data.

Preparation and reactivity of metal-containing monomers. 9. Low-temperature postradiation polymerization of metalcontaining monomers in devitrification of the matrices

Abstract: 1. The features of phase transitions of metal-containing monomers (MCM) of Ni(II), Co(II), Cu(II), Cr(III), and Mn(II) acrylates and acrylamide complexes and Ti(IV) alkoxy derivatives were investigated in the region of low temperatures in bulk and in 10% ethanol solution. 2. The possibility of postradiation polymerization of MCM in devitrification in the matrix and alcohol solutions was demonstrated. 3. Postpolymerization of MCM in devitrification takes place less efficiently than postpolymerization of their analogs containing no metal atoms.

The effect of Al2O3, CaO, Cr2O3 and MgO on devitrification of silica

Abstract: The effect of doping on devitrification of vitreous silica was studied at 1100, 1200, and 1300 C. Dispersion of dopants on a molecular scale was accomplished via a sol-gel technique. All dopants accelerated the devitrification of silica but to different degrees. The most active was CaO followed by MgO, Al2O3, and Cr2O3. Pure silica and silica containing Cr2O3 and Al2O3 devitrified to alpha-cristobalite only, whereas silica doped with CaO and MgO produced alpha-quartz and alpha-cristobalite. It appears that prolonged heat treatment would cause alpha-quartz to transform to alpha-cristobalite.