Crystallization of MgO-Al2O3-SiO2-ZrO2 Glasses

The crystallization of MgO-Al₂O₃-SiO₂-ZrO₂ glasses at 1000°C was studied. Isothermal heat treatments of a cordierite-based glass (2MgO.2Al₂O₃.5SiO₂= Mg₂Al₄Si₅O₁₈) with 7 wt% ZrO₂ produced surface crystallization of α-cordierite and tetragonal ZrO₂ ( t -ZrO₂). These phases advanced into the glass by cocrystallization of t -ZrO₂ rods in an α-cordierite matrix with a well-defined orientation relation. The t -ZrO₂ rods were unstable with respect to diffusional breakup (a Rayleigh instability) and decomposed into rows of aligned ellipsoidal and spheroidal particles. The t -ZrO₂ was very resistant to transformation to monoclinic symmetry. With a similar glass containing 15 wt% ZrO₂, surface crystallization of α-cordierite and t -ZrO₂ was accompanied by internal crystallization of t -ZrO₂ dendrites. Transformation of the dendrites to mono-clinic symmetry was observed under some conditions.     

Direct Observation of the Crystallization of Li2O-Al2O3-SiO2 Glasses Containing TiO2

The nucleation and crystallization of Li₂O-Al₂O₃-SiO₂ glasses containing TiO₂ were investigated using transmission electron microscopy of thin sections produced from bulk samples. Phase separation occurs during cooling from the melt, and on heating, a large number of titanium-aluminum crystals approximately 50 A in diameter are formed. These crystals are the heterogeneous nuclei for the crystallization of the remaining glass. Photomicrographs of various stages of crystallization show the development of the fine-grained glass-ceramic.     

Nucleation and Crystallization of NaNbO3 from Glasses in the Na2O-Nb2O5-SiO2 System

Transparent glass-ceramics, in which the major phase was NaNbO₃, were obtained by heat treatment of glasses in the Na₂O-Nb₂O₅-SiO₂ system. The structure of the glass and the changes occurring during crystallization as a function of temperature and heating rate were examined by X-ray diffraction, transmission and replication electron microscopy, density, and other measurements. On heating, a rather abrupt formation of uniformly dispersed particles was observed. In the early stages of crystallization, these particles contained NaNbO₃ as loose, radially grown dendrites of identical crystal orientation which became dense during later stages of crystallization. The particle sizes ranged from 200 to 10,000 A, depending on the SiO₂ content of the glass. Transparency of the crystallized material was dependent on the particle size rather than on the amount of NaNbO₃ formed. The temperature at which crystallization occurred increased with the heating rate whereas the viscosity at crystallization decreased. For a given value of the rate of crystal formation per °C of temperature increase, the product (viscosity)ⁿ× (heating rate) was constant. The nucleation and growth phenomena which occurred in these glasses was attributed to microheterogeneities of higher Nb₂O₅ content which formed part of the glass structure.     

Electrical Properties of Crystallized Glasses in the System MgO-Al2O3-SiO2-TiO2

The glass composition MgO 15.5, Al₂O₃ 19.5, SiO₂ 58.5, and TiO₂ 6.5 (wt%) was crystallized under controlled conditions in the range 900° to 1140°C and the electrical properties were studied. The activation energy for dc and ac conduction in the uncrystallized specimen was 32.0 and 27.4 kcal/mole, respectively; the corresponding values for the crystallized specimens were about 18.5 and 11.6 kcal/mole. X-ray studies and color of the crystallized specimens suggested the presence of magnesium aluminum titanate crystals in which part of the titanium was reduced to the Ti³⁺ state. These semiconducting phases were suggested as being responsible for the sharp change in the electrical properties and for the appearance of an apparent dielectric relaxation peak. Changes in the electrical properties with increasing crystallization temperature may be related to the nature, size, and shape of the crystals.     

Helium Migration in TiO2-SiO2 Glasses

Helium permeability, diffusivity, and solubility were measured for a series of TiO₂-SiO₂ glasses containing up to 10.3 mol% TiO₂. The activation energies for helium permeation and diffusion decrease with increasing TiO₂ concentration. The enthalpy of solution is independent of composition, as are the infinite-temperature values for the permeability, diffusivity, and solubility. It is concluded that the observed changes in activation energies result from changes in the strain energy necessary to distort the R-O-R'bond. The average diffusional jump distance appears to be essentially independent of composition, as would be expected from the small changes observed in the molar volume as TiO₂ replaces SiO₂ in these glasses.     

Elastic Moduli and Refractive Indices of Aluminosilicate Glasses Containing Y2O3, La2O3, and TiO2

Glasses in the system Al₂O₃-Y₂O₃-SiO₂, containing TiO₂ and La₂O₃, were investigated. Glasses of high refractive index and elastic modulus were developed. The observed Young's and shear moduli of these glasses show good agreement with theoretical values. Agreement was also found between the observed and calculated values of refractive index when the Appen's empirical coefficients were used.     

Chemical Durability of Sodium Silicate Glasses Containing AI2O3 and ZrO2

A sodium silicate glass and ternary glasses derived from it by substituting AI₂O₃ and ZrO₂ for SiO₂ were exposed to water and aqueous solutions of pH 1.4 to 12.7; the kinetics of the reactions were studied. Diffusion of alkali ions and leaching of alkali and SiO₂ from the glasses were influenced by the occupancy of surface sites by alkali ions above a critical pH; however, the activation energies of the processes varied linearly with the logarithm of mole fraction of surface sites occupied by H^plus;. Identical slopes were obtained for all glasses for a given process. The results are explained on the basis that transport of alkali ions is retarded as a result of increased boundary concentration and that suitable sites for reaction are lacking.     

Sintering and Crystallization of CaO–Al2O3–ZrO2–SiO2 Glasses Containing Different Amount of Al2O3

In this work several complementary techniques have been employed to carefully characterize the sintering and crystallization behavior of CaO–Al₂O₃–ZrO₂–SiO₂ glass powder compacts after different heat treatments. The research started from a new base glass 33.69 CaO–1.00 Al₂O₃–7.68 ZrO₂–55.43SiO₂ (mol%) to which 5 and 10 mol% Al₂O₃ were added. The glasses with higher amounts of alumina sintered at higher temperatures (953°C [lower amount] vs. 987°C [higher amount]). A combination of the linear shrinkage and viscosity data allowed to easily find the viscosity values corresponding to the beginning and the end of the sintering process. Anorthite and wollastonite crystals formed in the sintered samples, especially at lower temperatures. At higher temperatures, a new crystalline phase containing ZrO₂ (2CaO·4SiO₂·ZrO₂) appeared in all studied specimens.     

Raman Spectroscopy of Heat-Treated B2O3-SiO2 Glasses

Raman spectra were measured on B₂O₃-SiO₂ glasses heat-treated at different temperatures and for different periods of time. It was found that the intensity ratio of the 810- and 475-cm⁻¹ bands depended on the thermal history and composition of glass. The intensity vs heat-treatment temperature curve exhibited a maximum in the glass-transition temperature range. High-temperature Raman spectra were measured on 0.5B₂O₃-0.5SiO₂ glass, both with increasing and decreasing temperature, between room temperature and 600°C. Above 300°C with increasing temperature, the intensity of the 810-cm⁻¹ band decreased, whereas the intensity of the 475-cm⁻¹ band increased. The changes reversed with decreasing temperature. The reversible change of the intensity of the two bands was interpreted as a reversible formation and decomposition of boroxy rings with temperature. The reaction becomes very sluggish below the glass transition so that the intensity ratio stayed constant below 300°C.     

Solid-state Equilibrium Relations in the System MgO-Al2O3-SiO2-ZrO2

Using techniques developed by Goldschmidt and lately utilized by Foster, solid-state equilibria were studied in the system MgO-Al₂O₃-SiO₂ ZrO₂. The ternary systems MgO-Al₂OrSiO₂ and MgO-SiO₂-ZrO₂ are the only faces of this four-phase system that have beep studied extensively. Compositions in each of the four faces were heated to temperatures below their fusion point and, after they had been cooled, they were examined by X rays to identify the phases present. After solid-state equilibria had been established or verified in the four faces of the system, quaternary compositions were studied to determine the Alkemade tetrahedra.     

Relative Acidities of Glasses Containing Al2O3 and TiO2 as Determined by the Oxygen Electrode

The oxygen electrode has been utilized to determine the relative acidities of sodium and potassium silicate glasses containing alumina and titania in the temperature range 300° to 500° C. The acidity of a potassium or sodium silicate glass is increased by substituting alumina or titania for the soda or potassia. The first additions of alumina tend to increase the acidity of both glasses by forming A1O₄ tetrahedra which utilize a portion of the nonbridging oxygen ions to fulfill the coordination requirements. The results indicate further that the Ti⁴⁺ ion is in both fourfold and sixfold coordination in the sodium and potassium glasses. Titania increases the acidity when it is in sixfold coordination but tends to decrease it when in fourfold coordination. Both titania and alumina increase the acidity of the potassium glasses more than that of the sodium glasses. This is interpreted on the basis that the oxygen network of the potassium glass, being initially the looser, is tightened more than that of the sodium glass. However, the polymerization of the A1O₄ tetrahedra at the higher alumina concentrations causes a loosening of the oxygen network. Previous assumptions that the oxygen electrode reaction is independent of the metal used for the electrodes have been verified by reproducing the data using platinum foil as the electrode material in place of silver foil.     

Solid Solution and Chromium Oxide Loss in Part of the System MgO-Al2O3-Cr2O3-SiO2

Thermal and X-ray studies show that there is complete solid solution between MgO.Cr₂O₃ and MgO.Al₂O₃ and that the spinel solid solutions are stable with no exsolution down to temperatures as low as 510°C. There is no solid solution of excess Cr₂O₃ in MgO.Cr₂O₃ nor of MgO.Cr₂O₃ in Cr₂O₃. The join MgO.Cr₂O₃–Al₂O₃ is found to be nonbinary; compositions along that join yield mixtures of a chromium oxide-alumina solid solution and a spinel solid solution on firing to temperatures high enough to promote solid-state reaction. Chromium oxide loss by volatilization increases at higher temperature. At a given temperature, chromium oxide loss is found to vary directly with the partial pressure of oxygen in the furnace atmosphere and with the ratio of MgO to SiO₂ in the charges heated.     

Thermochemistry of Glasses in the Y2O3-Al2O3-SiO2 System

Enthalpies of drop solution in molten 2PbO-B₂O₃ at 1078 K were measured for glasses along the 2YAlO₃-3SiO₂ and return ½Y₃Al₅O₁₂-3SiO₂ joins. The onset glass transition temperature increases slightly with increasing silica content and Y/Al. Enthalpies of mixing were calculated on the basis of amorphous end members. Strongly negative heats of mixing support the absence of miscibility gaps except possibly for very high silica content, consistent with experimental phase analyses, which indicate much narrower miscibility gaps compared with the phase diagrams calculated on the basis of previous data and the CALPHAD formalism.     

Eletrical Properties of Glasses in the System PbO-Al2O3-B2O3-SiO2

Glasses in the system Pb0–Al₂O₃-B₂O₃-SiO₂ are chemically stable over a wide composition range and have very desirable electrical characteristics such as high electrical resistivities and activation energies for conduction. Variations in these electrical properties were studied as a function of composition changes within the system, the object being to identify the role of the constituent oxides in achieving the highest activation energy and resistivity values consistent with moderate preparation temperatures. Measurements were made in the temperature range 25° to 400°C on carefully prepared glass disks in which the individual oxide components or different oxide ratios such as PbO/SiO₂, Al₂O₃/SiO₂, and B_sO₃/SiO₁ were systematically varied. The activation energy and resistivity values obtained ranged from 1.2 to 1.6 ev and 10° to 10¹⁴ ohm-cm, with dielectric constants ranging from 9 to 19 and densities from 4.30 to 4.50 g/cmY. Indications were that, for the composition range studied, the behavior manifested was basically that of the binary PbO-SO₂ glass with additions of Al₂O₃ or B₂O₃, even in small concentrations, sharply increasing the activation energy for conduction while lowering the density.     

Devitrification of High-SiO2 Glasses of the System Al2O3-SiO2

Pertinent works of previous investigators are reviewed briefly. A superior technique for the preparation of experimental glasses of high purity and chemical homogeneity is presented. Devitrification growth-rate data and traction-viscosity data are reported for high-silica glasses of the system Al₂O₃-SiO₂. Glasses with added Al₂O₃ were found to devitrify to cristobalite more rapidly than ultrapure silica glasses at a given temperature, and yet their fluidities (i.e., reciprocal viscosities) were found to be less. The devitrification growth rate, g, was found to be of zero order and to follow the temperature dependence In( g/T ) = -(B/T) + In A over the temperature range 1561°K. T 1730°K.     

Phase Separation of Glasses in the System SrO-B2O3-SiO2

The region of stable liquid-phase separation in the system SrO-B₂O₃SiO₂ was found to extend from the SrO-SiO₂ binary join, across the ternary region, to the SrO-B₂O₃ join. The boundary of the region rich in network-forming oxides lies between the SiO₂-B₂O₃ boundary and the 2.5-mol% SrO isopleth, whereas the maximum SrO content is between 30.3 and 35.2 mol% (42 to 48 wt%) SrO at an SiO₂: B₂O₃ ratio of 4:1. The microstructures of glasses quenched from the immiscibility region were characterized by scanning electron microscopy. Evidence is presented of both stable and metastable phase separation and of microseparation and coring in disperse-phase spheres rich in network-forming oxides.     

Rapid Crystallization of SiO2-Al2O3 Glasses

The crystallization of Al₂O₃-rich glasses in the system SiO₂-Al₂O₃ which were prepared by flame-spraying and/or splat-cooling was studied by DTA, electron microscopy, and X-ray diffraction. Over a wide range of compositions, the crystallization temperature ( T_x ) remained near 1000°C, changing smoothly with composition. In all cases crystallization of mullite was detected by X-ray diffraction. In the low-Al₂O₃ region, coarsening of the microstructure during crystallization was observed by electron microscopy. In the high-Al₂O₃ region mullite and γ-Al₂O₃ cocrystallized; this behavior may be interpreted as evidence of a cooperative process of crystallization at the respective T_x 's. The crystallite size of the mullite immediately after rapid crystallization increased continuously with increasing Al₂O₃ content. In light of the T_x data, the adequacy of the evidence for the proposed metastable miscibility gap in the SiO₂-Al₂O₃ system is questioned.     

Crystallization of a PbO-BaO-TiO2-Al2O3-SiO2 Glass

The phase development, microstructure development, and Curie temperature of PbO-BaO-TiO₂-Al₂O₃-SiO₂ glass- ceramics were investigated by X-ray diffractometry, electron microscopy, and dilatometry. The primary crystalline phase was perovskite titanate, which exhibited bulk nucle- ation. When the samples were heated at low temperatures, the Curie inversion was hindered and cubic perovskite was obtained. Increased heating temperature/time resulted in tetragonal perovskite. Growth of a surface layer due to the crystallization of the secondary crystalline phase PbO- Al₂O₃-2SiO₂ resulted in the coexistence of coarse and fine perovskite particles. The increase in the relative number of coarse-to-fine perovskite particles with the heating temperature/time was the major factor responsible for increased spontaneous deformation and higher Curie temperature of the perovskite phase. Substitution of barium for lead in the lead titanate phase probably occurred and is explained by the use of SiO₂ instead of B₂O₃ as the glass former in the present study.