ZrO2-Transformation-Toughened Glass-Ceramics Prepared by the Sol-Gel Process from Metal Alkoxides

Glasses of composition 3ZrO₂O · 2SiO₂ were prepared by the sol-gel process from metal alkoxides. Tetragonal ZrO₂ was precipitated by appropriate heat treatment at 1000° to 1200°C. The fracture toughness of these glass-ceramics increased with increasing crystallite size of the tetragonal ZrO₂, reaching ∼5.0 MN/m^3/2 at a size of ∼40 nm. The higher fracture toughness was attributed to tetragonal → monoclinic ZrO₂ transformation toughening.     

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.     

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.     

DTA and X-Ray Analysis Study of Nucleation and Crystallization of MgO-Al2O3-SiO2 Glasses Containing ZrO2, TiO2, and CeO2

Crystallization sequences of glasses with compositions in the tridymite primary phase field of the MgO-Al₂O₃-SiO₂ system were studied by DTA, X-ray diffraction, and other techniques. Crystallization was catalyzed by the addition of 7 wt% of either ZrO₂ or TiO₂. Up to 10 wt% CeO₂ was also added to some glasses. Metastable solid solutions with the high-quartz structure exhibiting varying lattice parameters commonly occurred at low temperatures, transforming into a high cordierite at higher temperatures. Depending on the composition and heat treatment, other phases also appeared, e.g. Ce₂Ti₂O₄ (Si₂O₇). The rate of crystallization was markedly dependent on the catalyst. Colloidal precipitation of the catalyst accompanied by bulk crystallization of the glass was observed with ZrO₂, but no crystalline TiO₂ was detected. In the presence of CeO₂, TiO₂ was a more effective catalyst than ZrO₂. Although CeO₂ lowered the melting temperatures of the glass-ceramics, it increased the stability of the glasses and inhibited volume nucleation, causing coarse structures to form on crystallization.     

Use of Stabilized ZrO2 to Measure O2 Permeation

The rate of permeation of CaO-stabilized ZrO₂ (CSZ) by O₂ gas was measured from 640° to 1200°C with the CSZ tubing used simultaneously as the sample and the O₂ pressure detector. The apparent permeation rate depended significantly on the O₂ pressure at the low-pressure side. The rate measured by this method was orders of magnitude smaller than that measured under steady-state conditions, except when the O₂ partial pressure was high (>10⁻⁴ atm), in which case the agreement was good. The difference between steady-state permeability and non-steady-state permeability is related to the deviation in stoichiometry in a sample or detector. The transient response (measured under variable pressure difference) may be very different from steady-state permeation (measured under constant differential pressures across the membranes). To apply CSZ to typical O₂ gas permeability measurements, the O₂ pressure must be kept above ∼ 10^−3.5 atm. In this range, the permeability of CSZ may be regarded as a temperature-dependent material property which is governed by the electron-hole mobility. At lower O₂ pressures the permeation rate is a more complex function of the pressure difference and level.     

Preparation of SiO2-TiO2-ZrO2 Glasses from Alkoxides

Relatively large compact glass samples (∼5 mm in diameter) of 65SiO₂·20TiO₂·15ZrO₂ and other binary and ternary compositions were prepared by the alkoxide gel method. Prolonged storage of the gels in water before they were dried reduced the residual carbon content of the resulting glasses considerably. Optimal preparation conditions are given, and the microstructure of the gels and glasses obtained is discussed.     

Phase Relationships in the ZrO2-Sc2O3 and ZrO2-In2O3 Systems

Zirconia-rich subsolidus phase relationships in the ZrO₂–Sc₂O₃ and ZrO₂–In₂O₃ systems were investigated. Phase inconsistencies in the ZrO₂–Sc₂O₃ system resulted from a diffusionless cubic-to-tetragonal ( t' ) phase transformation not being recognized in the past. Through three different measuring techniques, along with microstructural observations, the solubility limits of the tetragonal and cubic phases were determined.     

Thermodynamic Activities in B2O3-SiO2 Melts at 1475 K

Eight glass samples in the B₂O₃-SiO₂ system with compositions from 20 to 90 mol% B₂O₃ were prepared. The equilibrium vaporization was studied by Knudsen effusion mass spectrometry at temperatures between 1450 and 1500 K. B₂O₃ ( g ) was the most abundant boron-containing species in the vapor; no silicon-containing gaseous species were detected. Thermodynamic activities of B₂O₃ in the liquid were determined at 1475 K. Thermodynamic activities of SiO₂ and integral excess Gibbs energies were estimated from the thermodynamic activities of B₂O₃. The thermodynamic data support the results obtained by other methods indicating the existance of a miscibility gap in the metastable liquid.     

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.     

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.     

Property-Composition-Substructure Relations in Na2O-TiO2-SiO2 Glasses

Published data on optical properties and specific volumes of Na₂O-TiO₂-SiO₂ glasses located in 7 primary phase fields are represented quantitatively in terms of linear mole fraction equations. These equations are used to calculate a primary phase diagram of this system. The fidelity of the representation of experimental data is compared with that obtained using the Huggins-Sun method.     

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.     

Depolymerization of GeO2 and B2O3-GeO2 Network Glasses by Sb2O3

Binary Sb₂O₃-GeO₂ glasses containing 45 mol% Sb₂O₃ and ternary Sb₂O₃-B₂O₃-GeO₂ glasses containing 50 mol% GeO₂ were prepared. Their densities (volumes), refractive indices, and infrared spectra were determined, and their colors and high-temperature viscosities were estimated visually. Small amounts of Sb₂O₃ (∼10 mol%) appear to perturb neither the Ge-O-Ge network nor those B-O-Ge networks with small B/Ge ratios (∼0.2). The B-O-Ge networks with larger B/Ge ratios (∼1.0) depolymerize in the presence of even less Sb₂O₃. Amounts of Sb₂O₃ >10 mol% appear to depolymerize the Ge-O-Ge and Ge-O-B networks progressively, possibly with the formation of chains. A structurally sensitive ir isofrequency contour technique developed for ternary glass systems was applied successfully to these Sb₂O₃-B₂O₃-GeO₂ glasses. These contours can thus readily detect significant network depolymerization in the absence of the usual network modifiers.     

Properties and Structure of Glasses in the System PbO-GeO2-SiO2

The glass-forming region in the system PbO-GeO₂-SiO₂ extends to compositions with 70 mol% PbO. The variations of refractive index, density, and thermal-expansion coefficient with composition (in mol%) are linear. Immiscibility was observed by light-scattering techniques and electron microscopy.