Rate of homogeneous nucleation in alkali disilicate glasses

Rates of crystal nucleation in alkali disilicate glasses were measured by counting the number of crystals under an optical microscope. The viscosities of these glasses were measured by the method of beam-bending and penetration. Using the data of rate of nucleation and viscosity obtained in the present study and the data of free energy measured by Takahashi and Yoshio, crystal-glass interfacial energies for alkali disilicate systems were estimated on the basis of homogeneous nucleation theory as follows: 196 erg/cm² for Li₂O·2SiO₂, 126–144 erg/cm² for Na₂O·2SiO₂ and 88–104 erg/cm² for K₂O·2SiO₂. The effects of the viscosity of glass, the free energy difference between crystal and glass and crystal-glass interfacial energy on the rate of nucleation were discussed, and the remarkably higher rate of crystal nucleation in the Li₂O·2SiO₂ glass was attributed to the larger free energy difference.     

Chemical durability of Na2OCaOSiO2 glasses in acid solutions

The resistance of Na₂OCaOSiO₂ glasses to acid solutions has been studied. The compositions studied were Na₂O · 4SiO₂, Na₂O · x CaO · (4?x) SiO₂ and a common electrode glass containing 22.63Na₂O and 5.58 CaO, mol%. The reaction was made at 40°C for about 3 h in 1N solutions of HCl, HNO₃ and H₂SO₄. Powdered glass samples were used and the reaction was followed by analyzing the solution for soda, lime and silica.The extraction rates of each constituent were measured. The effect of acid concentration was also studied for each glass using 10^?3–10N solutions of the three acids for a fixed time. The quantity of calcium extracted increased slowly at first with increasing calcium content in the glass, but rapidly when the lime content exceeded ≈10 mol%. Above this concentration, both calcium and sodium appears to pass into solution in the same proportion in which they are present in the glass. The extraction rate was found to depend on the type and concentration of the acid used, being least in H₂SO₄ and much higher and almost equal in both HCl and HNO₃.An attempt was made to correlate the results of decomposition of the soda-lime-silica glasses to their membrane potentials in acidic solutions.     

Synthesis and characterization of ferrierite

Ferrierite, a medium-port molecular sieve zeolite, was synthesized as the only crystalline phase from an aqueous gel of composition 1.6 Na₂O · 1.6 K₂O · Al₂O₃ · 13.5 SiO₂ · 130 H₂O · 0.8 CO₂ · 2.4 (HCO₂)₂ at 250 °C in reaction periods ranging from 40 to 95 hours. The crystallization time was reduced with seeding. Ferrierite was a metastable phase in this system and it transformed into tridymite, high sanidine and orthoclase at longer reaction periods. At 200 °C, chabazite was crystallized from the same batch composition at a very slow rate. At temperatures higher than 250 °C and in the absence of carbonates and bicarbonates in the batch mixture, tridymite, high sanidine and orthoclase crystallized directly without the formation of ferrierite. The synthetic ferrierite sorbs benzene, toluene and cyclohexane and behaves as a medium port molecular sieve. The sorption capacity of ferrierite was improved by acid treatments with no deterioration of the crystal structure.     

High resolution 29Si NMR study of the structure and devitrification of lead-silicate glasses

High speed magic-angle rotation of glass samples in the strong polarizing field of a superconducting magnet yields high-resolution ²⁹Si NMR spectra. Using this technique glasses of various composition, PbO·SiO₂, (PbO)₂·SiO₂, and (PbO)₄·SiO₂ were studied and the influence of thermal treatment followed. Crystallization of PbO·SiO₂ glass has been found to be a complex process leading to a structure identical with that of the mineral alamosite. The ²⁹Si NMR spectrum of crystalline alamosite consists of three lines in agreement with the structure determined by X-ray diffraction.     

Fabrication of a high lithium ion conducting lithium borosilicate glass

A lithium ion conducting glass, Li₂O-B₂O₃-SiO₂, is fabricated by the conventional melt and quenching technique from a mixture of Li₂CO₃, B₂O₃ and SiO₂ powders. It appears that B₂O₃ decreases the crystallization tendency of the Li₂O-SiO₂ binary glass, resulting in an expanded glass forming region in the Li₂O-B₂O₃-SiO₂ ternary glass. The maximum conductivity is 2 × 10^− 6 S cm^− 1 at 25 °C for the 50Li₂O-38B₂O₃-12SiO₂ glass sample. The observed high conductivity is due to the mixed former effect. The conductivity strongly depends on the Li₂O content, but not on K (SiO₂/B₂O₃) in the Li₂O-B₂O₃-SiO₂ ternary glass.     

Studies on binary silicate glasses based on the SiKα and SiKß emission X-rays

The SiKα and Kβ X-ray emission spectra of binary silicate glasses of the Li₂O–SiO₂, Na₂O–SiO₂, K₂O–SiO₂, Cs₂O–SiO₂, B₂O₃–SiO₂, and GeO₂–SiO₂ systems were measured with an X-ray fluorescence spectrometer to examine the chemical effects on the spectra. The SiKα peak wavelengths for all the glasses agreed with that for SiO₂ glass, which corresponded to the concept that the coordination number of Si shouldbe four in all the glasses examined in this study. The SiKβ peak wavelength decreased with increasing alkali content in the alkali silicate glasses, indicating that the Si–O bond weakend in average as the alkali oxide was added to SiO₂ glass. On the other hand, no drastic shift in the SiKβ peak wavelength was observed in the B₂O₃–SiO₂ and GeO₂–SiO₂ systems, and this was interpreted as showing the constancy of Si–O bond strength in these glasses.     

Decrease of the alkali signal during auger analysis of glasses

The decrease of the alkali signal during Auger electron spectroscopy (AES) has been studied on glasses with molar composition 20 M₂O · 80 SiO₂ and 20 M₂O · 10 CaO · 70 SiO₂, where M = Na and/or K. The samples were fractured in situ, and beam conditions in the region 1.5–3 keV and 1–40 μA were chosen. The K signal in 20 K₂O · 80 SiO₂ remained sufficiently constant to allow a reliable measurement. Its decrease showed an initial delay which can be explained by assuming a rapid decrease of the K concentration at some depth below the surface. The presence of CaO accelerates the migration of alkali during AES. This is not predicted by diffusion data.     

Non-isothermal crystallization kinetic studies on MgO–Al2O3–SiO2–TiO2 glass

Thermal stability and crystallization kinetics of the glass 21% MgO, 21.36% Al₂O₃, 53.32% SiO₂ and 4.11% TiO₂ (mol%) has been studied using differential thermal analysis (DTA), dilatometry and X-ray diffraction (XRD). Glass in both bulk and frit forms were produced by melting in platinum crucible at 1600 °C for 1–2 h. From variation of DTA peak maximum temperature with heating rate, the activation energies of crystallization were calculated to be 340 kJ mol⁻¹ and 498 kJ mol⁻¹ for first and second crystallization exotherms, respectively. Crystallization of bulk glass was carried out at various temperatures and for different time durations in the range of 850–1000 °C. The influence of the addition of TiO₂ on the crystallization sequence of the glass was experimentally determined and discussed.     

Non-isothermal crystallization and nanostructuring in potassium niobium silicate glasses

Potassium niobium silicate (KNS) glasses the composition of which is characterized by the K₂O/Nb₂O₅ molar ratio ranging from 0.85 to 1.2 and SiO₂ 50-54 mol% were examined in order to clarify the influence of chemical composition on formation of transparent nanostructured state of glasses. Differential thermal analysis, X-ray diffraction and scanning electron microscopy were used to study the non-isothermal crystallization of the KNS glasses as well as their morphological features. It was found that all glasses devitrify in three steps forming unidentified phases at the first two ones while at higher temperature (1000-1100 °C) the crystallization of K₃Nb₃O₆Si₂O₇ takes place. For prolonged heat treatment time (more than 5 h) at high temperature (1050-1100 °C) the transformation of this phase into the KNbSi₂O₇ ferroelectric one occurs in some extent. Nanostructuring occurs at the first stage of the devitrification process. It results from two partially overlapped processes: amorphous phase separation and subsequent crystallization. It was shown that only for the glass with the K₂O/Nb₂O₅ molar ratio equal to 0.85 and SiO₂ 50 mol% it is possible to separate the above processes by isothermal heat treatments at 680 °C obtaining fully transparent nanostructured samples. These samples contain nanocrystals 10 times smaller than the amorphous inhomogeneities of the phase separated matrix in which are dispersed.     

Ion exchange between glass melts of the system Na2ORb2OSiO2

Ion exchange between glass melts of the quasi-binary system Na₂O · 3SiO₂Rb₂O₂ was investigated at 700—1300°C by means of a special capillary method. Concentration profiles were obtained by electron microprobe analysis and were evaluated for a concentration-dependent quasi-binary interdiffusion coefficient $\text{tilde}\text{D}$ using a modified Boltzmann-Matano method. At 700–1000°C interdiffusion could be obtained in pure form with $\text{tilde}\text{D}$ values ranging from about 10^?7 ?5 × 10^?6 cm² s^?1. Above 1000°C convection processes superimposed interdiffusion, making a further evaluation impossible. The data are compared with those from a 0g rocket experiment and are discussed with respect to a mixed-alkali effect and in terms of the Nernst-Planck diffusion model.     

Structural changes between soda-lime silicate glass and melt

Neutron diffraction has been used to study the structure of a glass and melt of composition 75SiO₂-15Na₂O-10CaO. RMC modeling of the neutron and X-ray diffraction data for the glass allowed the determination of the Na and Ca environment. The structure has been investigated at 300K, just below the glass transition at 823 K and in the melt at 1273 K. The short range order does not present important modifications with temperature while significant reorganization appears at the medium range order. These latter changes can be associated with the Si and O pairs and indicate the relaxation of the silicate network. This indicates that the glass formation involved structural rearrangement during cooling.     

Structural analysis of some sodium and alumina rich high-level nuclear waste glasses

Sodium- and aluminum-rich high-level nuclear waste glasses are prone to nepheline (NaAlSiO₄) crystallization. Since nepheline removes three moles of glass-forming oxides (Al₂O₃ and SiO₂) per mole of Na₂O, the formation of this phase can result in severe deterioration of the chemical durability in a given glass. The present study aims to investigate the relationships between the molecular-level structure and the crystallization behavior of sodium alumino-borosilicate-based simulated high-level nuclear waste glasses with infrared spectroscopy (FTIR) and X-ray diffraction, respectively. The molecular structure of most of the investigated glasses comprise a mixture of Q² and Q³ (Si) units while aluminum and boron are predominantly present in tetrahedral and trigonal coordination, respectively. The increasing boron content has been shown to suppress the nepheline formation in the glasses. The structural influence of various glass components on nepheline crystallization is discussed.     

Continuous compositional changes of crystal and liquid during crystallization of a sodium calcium silicate glass

This paper deals with a systematic study of crystal nucleation and growth kinetics in a 14.6Na₂O–34.0CaO–51.4SiO₂ mol% glass, which is close to the CaO · SiO₂–Na₂O · SiO₂ pseudo-binary section, just left of the stoichiometric Na₂O · 2CaO · 3SiO₂ (N₁C₂S₃) compound. We show that crystallization begins with nucleation of a Na_4+2xCa_4−x[Si₆O₁₈] (0 < x < 1) solid solution that is enriched in sodium as compared with both parent glass and the N₁C₂S₃ compound; while a fully crystallized sample is composed only by a solid solution that is stable at very high temperatures, but is metastable in the temperatures under investigation. We thus confirm a continuous compositional change of the crystals during the course of crystallization.     

Effect of BaO on the crystallization kinetics of glasses along the Diopside-Ca-Tschermak join

We report on the effect of BaO on the crystallization kinetics of glasses in the diopside (CaMgSi₂O₆)-Ca-Tschermak (CaAl₂SiO₆) system. Partial substitution (i.e. 5%, 10% and 20%) of Ba²⁺ for Ca²⁺ was attempted in composition CaMg_0.8Al_0.4Si_1.8O₆, in three different glasses while partial substitution of B³⁺ for Al³⁺ was made in the fourth glass. Structural investigations on the glasses have been made by density measurements, molar volume and Infra-red spectroscopy (FTIR). Non-isothermal crystallization kinetic studies have been employed to study the mechanism of crystallization in all the four glasses. The Avrami parameter for the glass powders is ∼2, indicating the existence of intermediate mechanism of crystallization. Crystallization sequence in the glasses has been followed by X-ray diffraction (XRD) analysis, scanning electron microscopy (SEM) and FTIR. Augite crystallized out being the dominant phase in all the glass-ceramics, while different polymorphs of BaAl₂Si₂O8 were present as secondary or minor phases.     

Chemical durability and weathering resistance of canasite based glass and glass-ceramics

Classic composition 8.4Na₂O·5K₂O·10.8CaO·64SiO₂·10.5CaF₂·1.3Al₂O₃ (G1/GC1) and high silicon composition 7.6Na₂O·4K₂O·8.4CaO·71SiO₂·8CaF₂·1.0Al₂O₃ (G2/GC2) canasite-based glass and glass-ceramics were prepared, and the chemical durability and weathering of samples were studied with XRD, ICP-AES, SEM and optical microscopy. Interestingly, a kind of color fringe pattern caused by the acid leaching was directly observed on the glass ceramic surface under optical microscopy. The 20 day weight losses of glass and glass ceramic in acid (1 M HCl) and alkali (1 M NaOH/1 M Na₂CO₃) solution were measured. Accelerated weathering was used to demonstrate that increasing silicon content contributes to the weathering performance of glass and glass-ceramics. For different micro-structures and compositions, the weight loss of each glass and glass-ceramic is quite different. In general, through increasing the network interconnectivity of residual glass network and suppressing the crystallization of the less durable canasite phase, the addition of SiO₂ (from 60 mol% to 71 mol%) enhanced the chemical durability of canasite-based glass and glass ceramic relatively under acid, alkali and weathering conditions.     

A kinetic treatment of glass formation. VIII: Critical cooling rates for Na2OSiO2 and K2OSiO2 glasses

The critical cooling rates required to form glass have been measured for Na₂OSiO₂ compositions containing 15.4, 20.6, 29.9 and 34.0 wt% Na₂O and for K₂OSiO₂ compositions containing 15.3, 21.7, 34.3, 41.8 and 43.9 wt% K₂O. Pronounced minima in critical cooling rate are observed in the ranges about 25 wt% Na₂O and 33 wt% K₂O. The locations of these minima correlate with regions of low liquidus temperatures (near eutectics) in the phase diagrams.Calculations of critical cooling rates have been carried out using the analysis of crystallization statistics and the simplified model of glass formation. In both cases, the models predict well the measured critical cooling rates and their variation with composition.     

Preparation and properties of porous glass using fly ash as a raw material

The porous glasses were prepared by a conventional phase separation method using coal fly ash as a raw material, and the properties of these porous glasses were investigated. The composition of coal fly ash is basically composed of SiO₂–Al₂O₃–Fe₂O₃–CaO system and the SiO₂–B₂O₃–Al₂O₃–CaO–Na₂O system of glass was chosen as base glass composition. The pore diameter increases proportional to cube root of heating time (t^1/3), however, the early stage of phase separation is not clear. It is estimated that the rate determining step may be the diffusion process of structural units involving oxygen ions and the phase separation may take place by the nucleation and growth mechanism, and the relatively larger pores of above 1 μm can be obtained easily. The chemical composition of porous glasses is SiO₂–B₂O₃–Al₂O₃(–CaO–Na₂O). A relatively large amount of fly ash (>40%) can be used successfully for the preparation of porous glass.     

On the Synthesis and Characterization of Zeolite ZSM-20

ZSM-20 is a silicon-rich analog of the faujasite-structure, the synthesis of which is complicated. Starting from known synthesis recipes its crystallization in the system Na₂O · Al₂O₃ · SiO₂ · H₂O is investigated in the presence of tetraethylammoniumhydroxyde as templating agent. The aim was to find the critical conditions for its synthesis. The synthesis products obtained are investigated by X-ray diffraction, thermal analysis and i.r. spectroscopy.     

Elastic properties and structure of alkaline-earth silicate glasses

The densities and elastic properties of RO · SiO₂ and RO · 2 SiO₂ glasses (R = Mg, Ca, Sr, Ba or their mixtures) have been determined to obtain the compositional dependences of volume and bulk modulus of alkaline-earth silicate glasses. The mean atomic volume was found to vary almost linearly with glass composition for a series of RO · SiO₂ or RO · 2SiO₂ glasses, although there exists a slight negative deviation from the linearity for RO · 2SiO₂ glasses when two kinds of alkaline-earth oxides were mixed. The change in glass volume agreed fairly well with the value calculated on the basis of the difference in ionic size of alkali-earth ions being subtituted each other. The bulk moduli of RO · SiO₂ and RO · 2Si_O2 glasses were found to vary in reciprocal proportion to the four-thirds power of the volume when R was changed from one kind to another, indicating that the difference in internal energy of glass arises mainly from the change in Coulombic attraction due to their size difference. These results seemed to show that the glass structure of a series of RO · 2SiO₂ or RO · 2SiO₂ glasses remains almost unchanged except the local a change around R ions when R is subtituted from one kind to another.     

The mechanism of crystal growth in glass forming systems

A critical survey on experimental results on the mode of growth in simple glass forming melts is given, attention being mainly concentrated to data obtained at small undercoolings. Dissolution rates, change of interfacial conditions at constant undercooling as well as detailed structural determinations are considered as experimental evidences, complementary to a thorough analysis of growth-temperature dependences. For network glass formers (SiO₂, GeO₂, P₂O₅, Na₂B₄O₇) with melt structures, similar to those of the corresponding crystals, the normal mode of growth is typical. For a number of simple glass forming substances in which the crystallization is connected with a process of molecular reconstruction (NaPO₃, LiPO₃), spiral growth could be proved. Dislocation-free crystals of high entropy of melting glass forming substances (Na₂S₂O₃ · 5 H₂O, thymol) are obtained after prolonged annealing and growth in thin bored capillaries. Two-dimensional growth is verified for the resulting perfect crystals.