Nucleation and Crystallization of a Lead Halide Phosphate Glass by Differential Thermal Analysis

The nucleation and crystallization mechanisms of a lead halide phosphate glass [40P₂O₅·30PbBr₂·30PbF₂ (mol%)] were investigated by differential thermal analysis (DTA) and X-ray diffraction analysis. There were two crystalline phases in the crystallized samples: the major phase was PbP₂O₄, and the minor phase was PbP₂O₆. The average activation energy for crystallization, E , for two different particle sizes of this glass was determined to be 119 ± 4 kJ/mol by the Kissinger method and 124 ± 4 kJ/mol by the Augis–Bennett method. The Avrami constants were determined to be 1.6 and 2.5 for particle sizes of 203 and 1040 μm, respectively, by the Ozawa equation, and 1.7 and 2.4 for particle sizes of 203 and 1040 μm, respectively, by the Augis–Bennett equation. The decrease in the crystallization peak height in the DTA curve with increasing particle size suggested that the particles crystallize primarily by surface crystallization. A nucleation-rate type curve was determined by plotting either the reciprocal of the temperature corresponding to the crystallization peak maximum, 1/ T _p, or the height of the crystallization peak, (δ T )_p, as a function of nucleation temperature, T _n. The temperature where nucleation can occur for this glass ranges from 360°–450°C and the maximum nucleation rate is at 420°± 10°C.     

Crystallization of Li2O · Al2O3· 6SiO2 Glasses Containing Niobium Pentoxide as Nucleating Dopant

Niobium pentoxide (T form, orthorhombic system) was utilized to promote devitrification in Li₂O · Al₂O₃· 6SiO₂ glasses. Two or more mole percentage of this nucleating dopant enhanced crystallization in these glasses. Glasses containing 4.0 and 8.0 mol% T-Nb₂O₅ exhibited a high tendency to form dispersed TT-Nb₂O₅ (monoclinic system) precipitates during the glass quenching process. The crystallization process in glasses containing 2.0 or 4.0 mol% T-Nb₂O₅ occurred as microphase separation, followed by the formation of dispersed TT-Nb₂O₅ crystalline precipitates (760°C), followed by β-quartz solid-solution ( ss ) formation (850° to 900°C) heterogeneously nucleated from the precipitates. β-quartz( ss ) transformed to β-spodumene( ss ), along with a polymorphic transition from the TT-Nb₂O₅ to M-Nb₂O₅ (tetragonal system) crystalline phase.     

Thermal Analysis of a SiO2─Al2O3─CaO─CaF2 Glass

A SiO₂─Al₂O₃─CaO─CaF₂ ionomer glass was investigated using thermal analysis, X-ray diffraction, and scanning electron microscopy. The purpose of this investigation was to control the susceptibility of the glass to acid attack. The differential thermal analysis trace exhibited a sharp glass transition at about 645°C and two exotherms. The first exotherm corresponded to liquid–liquid phase separation followed by crystallization of fluorite. The second, much larger, exotherm was the result of crystallization of the remaining glass phase to form anorthite. Prolonged heat treatment below the glass-transition temperature demonstrated that crystallization of fluorite can occur without prior liquid–liquid phase separation.     

Effect of Neodymium:Yttrium Aluminum Garnet Laser Irradiation on Crystallization in Li2O–Al2O3–SiO2 Glass

A 355-nm neodymium:yttrium aluminum garnet laser, produced by a harmonic generator, was used for the nucleation process in photosensitive glass containing Ag⁺ and Ce³⁺ ions. The pulse width and frequency of the laser were 8 ns and 10 Hz, respectively. Heat treatment was conducted at 570°C for 1 h, following laser irradiation, to produce crystalline growth, after which a LiAlSi₃O₈ crystal phase appeared in the laser-irradiated Li₂O–Al₂O₃–SiO₂ glass. The present study compares the effect of laser-induced nucleation on glass crystallization with that of spontaneous nucleation by heat treatment.     

Indentation Creep of Na2O · 3SiO2 Glasses with Various Water Contents

Time-dependent deformation behavior induced by an indentation, called indentation creep, was investigated for Na₂O · 3SiO₂ glasses with various water contents by measuring the indentation depth as a function of time using a conical indenter. It was found that the indentation creep behavior of the glasses could be best explained as steady-state inhomogeneous (non-Newtonian viscous) flow. Water in the glasses appeared to reduce the stress (or yield stress) needed to cause this flow. Crack initiation of the glasses was promoted by water through this inhomogeneous flow.     

Determining the Nucleation Rate Curve for Lithium Disilicate Glass by Differential Thermal Analysis

The crystallization of lithium disilicate (Li₂O·2SiO₂) glass nucleated at various temperatures was studied by differential thermal analysis (DTA). A plot of the DTA crystallization peak height versus nucleation temperature closely resembles the classical nucleation rate curve for lithium disilicate glass whose maximum is at 453°C. The glass becomes saturated with internal nuclei when heated at 453°C for 10 h. The DTA technique is a rapid, alternative method for determining the temperature for maximum nucleation. The activation energy for crystallization, E , and the heat of crystallization, H , are independent of the concentration of nuclei and are 249±10 and 67±3 kJ/mol, respectively. The Avrami exponent, n , depends strongly on the concentration of nuclei in the glass.     

Influences of Zirconia and Silicon Nucleating Agents on the Devitrification of Li2O · Al2O3· 6SiO2 Glasses

The effects of Si and ZrO₂ dopants on the crystallization and phase transformation process in Li₂O · Al₂O₃· 6SiO₂ glasses were investigated using differential thermal analysis, X-ray powder diffractometry (XRD), and high-resolution transmission electron microscopy (TEM) interactively. Phase separation was observed in the studied glasses prior to substantial crystallization. Elemental Si (1 mol%) significantly aided in glass devitrification. Dropletlike phase-separated regions in the as-quenched or heat-treated glass devitrified at ∼760°C, which in turn provided sites for the heterogeneous nucleation and growth of β-quartz(ss) (solid solution), which transformed to β-spodumene(ss) at higher temperature. Low-temperature surface crystallization in these glasses occurred as low as 760°C. ZrO₂ has limited solubility in this glass system. Small ZrO₂ crystallites (·5 nm) in the as-quenched glass acted as sites for the heterogeneous nucleation and subsequent growth of large (<5 μm) β-quartz(ss) crystals in glasses containing 1.0 mol% or more ZrO₂. The transformation from β-quartz(ss) to β-spodumene(ss) was increasingly inhibited with ZrO₂ additions. The nucleating efficiency of Si was significantly greater than that of ZrO₂ in this glass system.     

Crystallization Behavior of CaO–P2O5 Glass with TiO2, SiO2, and Al2O3 Additions

TiO₂ above 4 mol% is an effective nucleating agent for CaO–P₂O₅ glass which also contains substantial SiO₂ and Al₂O₃ additions. Glass ceramics can be made from this glass using a single slow heating ramp with no need for a nucleating heat treatment step. Powder of this composition crystallizes rapidly to β-Ca₂P₂O₇, whereas bulk glass crystallizes from diphasic nuclei consisting of a central cubic Ca-P-Ti-Si-Al oxide phase surrounded by impure AlPO₄ dendrites. Metastable calcium phosphate grows on the AlPO₄ dendrites and later transforms to β-Ca₂P₂O₇.     

Dielectric Characteristics of Na2O · 3SiO2 Glasses with High Water Contents

Dielectric characteristics of Na₂O·3SiO₂ glasses with water contents up to ∼12 wt% were found to be drastically affected by incorporated water. The high-frequency dielectric constant increased with water content, while both the static dielectric constant and the low-frequency dielectric relaxation strength showed a pronounced minimum at a water content of ∼3 wt%.     

Thermal Expansions and Chemical Durabilities of Yttria–Aluminosilicate Glasses Containing Na2O and ZrO2

Properties of glasses in the system Y₂O₃–Al₂O₃–SiO₂ containing Na₂O and ZrO₂ were investigated. The difference between the thermal expansion coefficients (Δα) at temperatures above T _g and those below T _g, microhardness, density, and chemical durability were measured in relation to the Al₂O₃/Y₂O₃ molar ratio. These glasses were found to have a smaller value of Δα than that of a commercial coating glass.     

Differences in Surface Behavior of Alkali Ions in Li2O ·3SiO2 and Na2O · 3SiO2 Glasses

The molecular dynamics computer simulation technique was used to determine the short-time dynamics behavior and resultant structures of ions at the surface of Li₂0·3SiO₂ and Na₂O-3SiO₂ glasses. Room temperature and elevated temperatures were used. Results are compared with similar studies of the K₂O·3SiO₂ glass surface and with recent experimental ion-scattering-spectroscopy data. The simulations indicate that a localized surface rearrangement occurs within picoseconds after formation of the free surface, creating a surface excess of alkali in the Na (and K) case, but not in the Li case. Elevated temperatures, even for brief times, enhance the observed surface excess of Na and K. The results correspond to those obtained from the ion-scattering-spectroscopy studies.     

Oxidation-Reduction Equilibria in Molten Na2O.2SiO2 Glass

The oxidation-reduction equilibria in molten glasses having a soda/silica ratio of 1/2 and containing small amounts of variable-valence ions of the transition elements titanium, vanadium, iron, cobalt, and nickel, the post-transition elements tin and antimony, and the rare-earth element cerium were obtained by equilibrating the melts with various atmospheres. The simple mass expression
was always applicable. In the expression, n is the number of electrons involved in the valence change of the metal M. The value of n is 1 for all systems except for the ions of antimony and tin where n is 2. The ions found are those generally accepted as existing in glass melts which are Ti³⁺, Ti⁴⁺; Fez+, Fe³⁺; Ce³⁺, Ce⁴⁺; Mn²⁺, Mn³; Co²⁺, Co³⁺; Ni²⁺, Ni³; Sb³⁺, Sb⁵⁺; and Sn²⁺, Sn⁴⁺. Two mass action expressions were needed for vanadium-containing glasses to describe the equilibria between 5+, 4+, and 3 f species.     

Electrical Relaxation in Na2O·3SiO2 Glass

The electrical relaxation associated with alkali diffusion in Na₂O·3SiO₂ glass was studied from 0.2 Hz to 700 kHz at –1° to 163°C. A formalism for analysis of electrical relaxation in conducting dielectrics which associates the nonexponential decay of the electric field to zero and the dispersions in the dielectric constant and the conductivity with a distribution of relaxation times for the electric field was developed and is shown to be in qualitative accord with current molecular theories of electrical relaxation in alkali silicate glasses. A relation between the dc conductivity, the limiting high-frequency dielectric constant, and the average electric field or conductivity relaxation time was derived and is verified experimentally for the Na₂O·3SiO₂ glass. The distribution of electric-field relaxation times for the glass is broad, asymmetric on a logarithmic scale, and weighted in favor of the shorter relaxation times; the distribution narrows with increasing temperature. A reduced electrical relaxation curve which can be used to compare electrical and mechanical relaxations in Na₂O·3SiO₂ glass was generated.     

Crystallization of MgO-CaO-SiO2-P2O5 Glass

The crystallization behavior of a glass with a composition of 40 wt% 3CaO · P₂O₅−60 wt% CaO · MgO · 2SiO₂ was investigated. The primary crystalline phase was apatite with a dendritic form and ellipsoidal shape. β-(3CaO · P₂O₅) and CaO · MgO · 2SiO₂ were crystallized as samples heated to 990°C, and a three-layer structure was obtained. The development and morphology of this construction were explained by both the surface crystallization of the apatite and CaO · MgO · 2SiO₂ and the bulk crystallization of apatite and the CaO · MgO · 2SiO₂-β-(3CaO · P₂O₅) composite.     

The System Na2O-CaO-SiO2-H2O

A portion of the quaternary phase diagram for Na₂O-CaO-SiO₂-H₂O has been constructed. Plotting concentrations as their 10th roots allows compounds having solubilities which differ by several orders of magnitude to be represented on a single diagram. The compositional relationships among sodium-substituted calcium silicate hydrate, calcium-substituted sodium silicate hydrate, calcium bydroxide, a quaternary compound of approximate composition 0.25Na₂O · CaO · SiO₂· 3H₂O, sodium hydroxide monohydrate, and miscellaneous sodium silicate hydrates are presented. The quaternary diagram constructed shows the quaternary compound to exist in equilibrium with sodium-substituted calcium silicate hydrate and calcium hydroxide. Conditions in concrete pore solutions which favor the formation of this quaternary compound may also favor the occurrence of the alkali-silica reaction.     

Electrical Conductivity of HF-H2O- Impregnated Na2O.3SiO2 Glass

Up to 5.75 WHO of concentrated (49 wt%) HF solution was placed in Nazo.3SiO₂ glass under hydrothermal conditions. Electrical conductivity of the resulting glasses was primarily determined by the water content.     

Nucleation and Crystal Growth of a MgO–CaO–Al2O3–SiO2Glass with Added Steel Fly Ash

The nucleation and growth of diopside Ca(Mg,Al)(Si,Al)₂O₆crystals on the free surface of a 24 wt% MgO, 14 wt% CaO, 9 wt% Al₂O₃, and 53 wt% SiO₂glass, with a 2 wt% addition of steel fly ash, were investigated through DTA, XRD, SEM, and optical microscopy. Crystallization was complete at ∼920°C with an activation energy of 589 kJ/mol. Samples with polished free surfaces were nucleated at selected temperatures in the range of 730° to 820°C, and then heat-treated at 870°C for 15 min for crystal growth. Nucleation was predominantly observed at the surface, and the number of diopside crystals per unit of area and the mechanism of crystallization were determined. It was concluded that nucleation reaches a maximum at 750°C, corresponding to an average density of diopside crystals of 8.4 × 10⁶ nuclei/cm², and that between 900° and 1100°C, a uniformly crystallized layer is formed at an exponential rate. The crystallized volume fraction increased significantly in the 880°–890°C growth range, and remained almost constant at higher temperatures. In the 860°–910°C range, the size of the diopside crystals formed in the samples nucleated at the temperature of the maximum nucleation rate, and linearly increased, reaching values between 1.0 and 3.0 μm at 870° and at 910°C, respectively.     

Mechanical and Ionic Relaxation in Na2O·3SiO2 Glass

Mechanical relaxation in Na₂O·3SiO₂ glass was measured. The data are compared with dielectric relaxation results for the same glass. The mechanical and electrical relaxation processes have not only the same activation energy but also (within experimental error) the same average relaxation time and distribution of relaxation times. From this result, it may be concluded that the mechanical loss in single-alkali silicate glass arises from the same ionic process that is associated with long-range ionic diffusion.     

Effect of Water Content on the Chemical Durability of Na2O 3SiO2 Glass

Na₂O 3SiO₂ glasses with high water content were prepared under high-pressure, hydrothermal conditions and their chemical durabilities in water were measured at atmospheric pressure and 30°C. Chemical durability decreases with increasing water content, which is probably due to a positive dependence of H⁺ (or H₃O⁺) diffusivity on water content.