Stoichiometric Glass Research Articles

Water diffusion, oxygen vacancy annihilation and structural relaxation in silica glasses

Water diffusion into silica glasses causes structural changes which can be readily detected by ultraviolet (UV) and infrared (IR) spectroscopy. One such change is the annihilation of the oxygen vacancies which exist in some silica glasses. This process can be monitored using the UV absorption at ∼ 240 nm (∼ 5.0 eV). Water can also accelerate local structural relaxation of silica glass. This relaxation can be monitored by measuring the IR absorption peak at 1873 cm −1. The diffusion coefficients for water penetration and local structural relaxation were measured for various silica glasses with low water concentrations in the high temperature range of 750–1200°C under a constant water vapor pressure of 355 mm Hg. The diffusion coefficient for oxygen vacancy annihilation was also measured for oxygen-deficient glasses under the same conditions. The water diffusion coefficient was nearly identical for all glasses investigated. The diffusion coefficients for oxygen vacancy annihilation in oxygen-deficient glasses and for local structural relaxation of stoichiometric silica glass were higher than the water diffusion coefficient even though both structural changes are accelerated by water. Water diffusion into silica at high temperatures has been explained in terms of the local equilibrium of the reaction between mobile molecular water and silica glass to form immobile hydroxyl water. The phenomena observed here can be explained by postulating the presence of a minute amount of molecular water species which are not restricted by the equilibrium condition for the reaction and can diffuse faster than the majority of the molecular water species.

Structural chemistry of synthetic cordierite: Evidence for solid solutions and disordered compositional domains in Bi-flux-grown Mg-cordierites

The structure, crystal chemistry, and microstructure of disordered and ordered Mg-cordierites synthesized in a bismuth oxide flux system have been studied by a combination of x-ray and neutron powder diffraction and quantitative x-ray microanalysis using analytical electron microscopy. Microchemical data obtained on Bi-flux cordierites using energy-dispersive x-ray analysis is interpreted through comparison with data collected on stoichiometric Mg2Al4Si5O18 glass and α- and β-cordierite samples synthesized by subsolidus crystallization of the glass. Bi-flux cordierites crystallize in both the hexagonal and orthorhombic polymorphs and contain 5 to 10 at% occupancy of bismuth on the C1 and C2 channel sites. The microstructure of Bi-flux α-cordierite is characterized by the existence of local domains of disordered cordierite solid solutions with variable composition and significant vacancy concentrations on the octahedral site. The β-cordierites have a more homogeneous microstructure but are still Al-deficient, Si-rich solid solutions.

Effects of nitrogen and carbon ion implantation on devitrification of silica glasses

The effects of nitrogen and carbon ion implantation on devitrification of stoichiometric and oxygen deficient silica glasses were investigated. Linear crystal growth was observed in stoichiometric silica glass and was not affected by ion implantation. Parabolic crystal growth was observed in oxygen-deficient glasses, which was retarded by ion implantation, with the rate of crystal growth retardation being greater for nitrogen implantation than for carbon implantation. Activation energy for parabolic rate constant for crystal growth was about ∼ 105 kcal/mol. When the oxygen-deficient silica glasses were hydrated, their crystal growth became linear and the activation energies were smaller. The reduction in activation energy by hydration was approximately equal to the activation energy for water diffusion in silica. It is suggested that nitrogen ion implantation decreased the crystal growth rate in the oxygen-deficient glasses by forming an oxynitride layer, which is an effective water diffusion barrier.

Crystallization of nickel-bearing stoichiometric cordierite glasses

A complete crystalline solid solution exists between magnesium cordierite (Mg2Al4Si5O18) and nickel cordierite (Ni2Al4Si5O18). Isothermal crystallization experiments were conducted at 1 atm on stoichiometric powdered glasses in this compositional range in which high quartz crystallized and then transformed to hexagonal cordierite. When nickel is present, these three phases can be simply identified by colour changes: the glass is brown, the high quartz is red and the cordierite is yellow. The sequence of appearance of the phases is independent of the nickel content of the glass. At any given temperature, the higher the nickel content, the shorter are both the time required for crystallization of high quartz from the glass and that required for the transformation to hexagonal cordierite. This implies that the processes involved in both steps might be similar. Measurements of lattice constants of cordierite crystallization products show a linear decrease in a and a linear increase in c as a function of increasing nickel content between the magnesium and nickel end members. There is no change of the cell volume with the substitution of nickel into the structure. The increase in reaction rates caused by increasing the Ni/Mg ratio of the glass starting material may be due to the stronger network modifying effect of nickel in comparison to magnesium.

Method and apparatus for thermoacoustimetry: applications to polymers and glasses

A method and a convenient inexpensive apparatus are described for carrying out thermoacoustic analysis (TAA) of materials from room temperature to about 350 degrees C by monitoring the changes in the ultrasonic velocity in the sample as a function of temperature with the heating rate as a user-defined variable parameter up to 10 degrees C min-1. The thermoacoustimetric apparatus described is readily constructed from commercially available components with minimum workshop machining. The sample is loaded by sandwiching it between two identical glass buffer rods, one of which is stationary and the other vertically movable, without the need to attach transducers and thermocouples directly to the sample. The ultrasonic velocity is obtained from the time difference between the through signal at the receiving end and the echo at the transmitter end. The apparatus was used to measure the glass transition temperatures of polycarbonate, polyphenylenesulphide in matrix form as part of an aramid fibre-PPS matrix composite, poly(methyl methacrylate) and near stoichiometric As2Se3 glass from the break point of the semilogarithmic plot of the ultrasonic velocity against the temperature as in the case of thermomicrohardness measurements. The measurements, where possible, have been compared with those obtained from conventional differential scanning calorimetry and thermomicrohardness analysis.

X-ray Ge K bands of Bi-Ge-S and Sb-Ge-S glasses

The X-ray Ge K emission and absorption bands were measured in the stoichiometric glass systems (GeS2)x(Bi2S3)1-x and (GeS2)x(Sb2S3)1-x; x=0 to x=0.5. The results do not confirm the existence of the Ge-Bi or Ge-Sb bonds and they imply that the GeS2 clusters are formed in both systems studied.

X-ray scattering studies of the short and medium range ordering in As xTe 1− x glasses

The structures of As x Te 1− x ( x = 0.2, 0.4, and 0.5) glasses are studied using the differential X-ray anomalous scattering technique. The partial distribution functions have also been obtained for the stoichiometric As 2Te 3 glass, giving information on the medium range ordering. All the glasses show chemical disorder to differing extents, the As 2Te 3 glass being the most disordered. The Te coordination number undergoes a change at x=0.4 where it is ∼ 2.5 compared with 2 for AsTe. This change indicates the existence of about 50% of threefold Te sites in the stoichiometric glass, as well as in the Te-rich glasses. Some of the physical properties of the glasses may be explained based on these results.

Crystallization of Anorthite‐Seeded Albite Glass by Solid‐State Epitaxy

Stoichiometric albite glass (NaAlSi3O8) was seeded with 5 wt% crystalline anorthite (CaAl2Si2O8) to make albite glass‐ceramics. The epitaxial crystallization of the albite glass to the glass‐ceramics was investigated by X‐ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive spectrometry (EDS). High albite was observed as the major crystallization product over the temperature range of 800–1200°C. No crystalline albite could be crystallized from pure albite glass without seeds. Small amounts of nepheline (NaAlSiO4), however, crystallized along with albite after heat treatments at temperatures lower than 1000°C. The platelike microstructure of albite crystals was revealed in the seeded glasses. The albite blades grew epitaxially from the anorthite seeds, and the Ca content decreased in the direction away from the seeds. The degree of crystallization and the grain size were dependent upon the heat treatment conditions. By increasing the particle size of the seed, the crystallization process was retarded and the resultant microstructure was degraded. The seeding efficiency was also lowered by adding nonisostructural hexagonal anorthite seeds which produced less albite but more nepheline crystals. Crystallization of albite glass by seeding with 5 wt% anorthite is much greater than with the surface nucleation which takes place in a homogeneous 95 wt% albite + 5 wt% anorthite glass.

Synthesis of monoclinic celsian glass-ceramic from alkoxides

Monoclinic celsian can be made from stoichiometric alkoxide-derived gel glass by prolonged heating at high temperature, cold-pressing glass frit prior to sintering, doping with Li 2O, and seeding gel glass with monoclinic crystals. Completely monoclinic celsian samples were obtained most easily in Li 2O-doped samples. Almost full density was achieved on sintering the gel glasses prior to extensive crystallisation.

Preparation and characterisation of alkoxide-derived celsian glass-ceramic

An all-alkoxide route to stoichiometric celsian glass has been developed. Sintering to 97% of full glass density, prior to the onset of appreciable crystallisation, was achieved after 30 min at 900 °C. However, crystallisation of metastable hexacelsian, rather than the preferred monoclinic celsian, from stoichiometric glass was difficult to avoid.

Crystallization of some spodumene-lithium zinc orthosilicate glasses

By using thermal analysis, X-ray diffraction, polarizing and electron microscopy the effect of compositional variation, thermal treatment and nucleation catalysts TiO2 and ZrO2 on the nature, type and stability fields of the crystallizing phases, as well as the resulting microstructures, is described for some stoichiometric glass compositions within the system LiAlSi2O6-Li2ZnSiO4. Intense uniform volume crystallization was achieved. Transparent glass-ceramics with ultrafine microstructures could also be obtained at temperatures near 700 °C. Crystallization begins with the formation of a proto β- and/or βII′-Li2ZnSiO4 followed by, or concomitant with, β-eucryptite ss. The proto β-phase was formed over a narrow temperature range and rapidly transformed into its βII′-modification which, although showing a wider stability range, ultimately transformed into the stable γo-modification. The metastable β-eucryptite ss starts its transformation into β-spodumene around 800 °C. By prolonged heating at temperatures as high as 1000–1040 °C, β-spodumene and γo-Li2ZnSiO4 were the main stable end products. TiO2 and ZrO2 have contrasting effects on the stability of the LiZn orthosilicates and β-eucryptite ss → β-spodumene transformations. The former exhibits a catalytic effect and the latter showed a retarding effect on these processes.

Paramagnetic defects in germanium sulfur glasses

ESR-studies were conducted at 9.5 GHz on GeS 2±x glasses (x=0,0.1). Glasses were melted at 683°C, 789°C and 895°C in Vycor tubes which were evacuated and sealed before fusion. The ESR-spectra of the virgin glasses are characterized by a narrow symmetric (g=2.0037, ΔH pp=3G, A-component) and a broad asymmetric line (g=2.0113, ΔH pp=17G, B-component). The narrow line was not detected in the GeS 2±x glasses fused at 895°C and in the GeS 2.1 glass fused at 796°C. The narrow line intensity of the sulfur deficit glass is approximately two times the corresponding intensity of the stoichiometric and sulfur excess glass. The intensity of the asymmetric line is negligibly small in the GeS glasses fused at 638°C. It dominates in the spectra of the GeS glasses fused at 789°C, and triples in the GeS glasses fused at 895°C. Structural models for the defect centers which cause the detected lines have not yet been developed by us.

Resonance Raman scattering in AsS glasses

The Raman spectra of glasses with compositions in the range As 35S 65 yo As 43S 57 have been recorded using near band-gap excitation ( λ i = 5145 A ̊ ) and also weakly absorbed excitation ( λ i = 6328 A ̊ ). With 5145 Å excitation there is a significant enhancement at ∼231 cm −1 for the glasses with ⩾ 40 at. % As. This band is attributed to the presence of As-As bonds in the AsS network. Both As-As and S-S bonds are believed to occur in the stoichiometric glass, a-As 40S 60, though in small numbers. The enhancement of the 231 cm −1 band for 5145 Å excitation is shown to be primarily a resonance effect, suggesting that As-As bonds constitute electronic “defect” states within the mobility gap.

Reversible photostructural change in melt-quenched GeS 2 glass

Reversible photostructural change accompanying optical absorption edge shift has been observed in melt-quenched Ges 2 glass for the first time. Experimentally determined values of photo-expansion and photo-induced optical absorption edge shift are 0.47% and 0.075 eV, respectively. It has been demonstrated that the structural origin for the reversible photostructural change should not be attributed to wrong bonds inherently involved even in the stoichiometric glass compounds

Kinetics of Crystallization of Stoichiometric SiO2 Glass in H2O Atmospheres

Crystallization rates were measured in vacuum, dry nitrogen, and water-saturated nitrogen atmospheres from 1300° to 1540°C. In all cases the observed rates were linear. Three reactions appeared to contribute to crystallization: the intrinsic crystallization, the impurity effect of H2O vapor, and furnace contamination. Enhancement of crystallization by both water vapor and furnace contamination is attributed to the breaking of silicon—oxygen bonds of the glass structure. Competitive adsorption mechanisms were proposed to characterize the adsorption of water and impurity species. The activation energy for apparent intrinsic crystallization was 134 kcal/mole; the activation energy for crystallization in H2O vapor was 77 kcal/mole.