Barium Disilicate Research Articles

An assessment of isothermal crystallization kinetics in glass by mathematical modeling

A mathematical model is presented to assess the devitrification kinetics of glasses under isothermal conditions. The model takes into account all the experimentally observable aspects of nucleation and growth behavior. A particle size distribution function is defined from the nucleation and growth parameters, and used to calculate the transformed volume fraction, the number of crystals and their mean and maximum radii according to the isothermal heat treatment time. The model was validated with a barium disilicate glass that crystallizes uniformly and rapidly into spherical particles; the nucleation and growth curves were therefore determined for BaSi 2O 5 crystals. The simulated and experimental results are compared and discussed. The model closely approximates the actual results, and can describe the devitrification kinetics of any glass matrix for which the nucleation and growth mechanisms have been defined.

Nucleation and growth in cluster dynamics: A quantitative test of the classical kinetic approach

Nucleation and size dependent growth of nanometer sized crystalline particles in glassy media have been studied by numerically solving the Turnbull–Fisher master equations that describe the time evolution of cluster population. Time dependencies of the formation rate and number density are determined for large clusters (built of up to 2×105 formula units, containing 1.8×106 atoms). We demonstrate that the formation rate and number density of such clusters are well approximated by Shneidman’s asymptotically exact analytical solution. A quantitative test of the kinetic Turnbull–Fisher model has been performed: Evaluating the kinetic coefficients and interfacial parameters from the transient time and steady-state nucleation rates measured on six stoichiometric oxide glass compositions (lithium–disilicate, barium–disilicate, lithium–diborate, wollastonite, 1:2:3 and 2:1:3 soda–lime–silica glass compositions), we calculated the macroscopic growth rates and compared with experiments. For wollastonite, lithium–diborate and the 1:2:3 soda–lime–silica glass, differences of 2 to 4 orders of magnitude have been observed between theory and experiment. This inadequacy of the microscopic kinetic parameters in describing macroscopic growth cannot be explained by either the curvature effect on the interfacial free energy or the self-consistency correction for the cluster free energy. The origin of the discrepancy is discussed.

Transient nucleation in oxide glasses: The effect of interface dynamics and subcritical cluster population

To clarify the mechanism of cross-interfacial molecular transport and the role of subcritical cluster population in determining the kinetics of crystal nucleation, cluster dynamics calculations based on viscosity-governed rate coefficients are confronted with experiments on crystal nucleation in six stoichiometric oxide glasses (lithium disilicate, barium disilicate, two soda-lime-silica glasses, wollastonite glass, and lithium diborate). Systematic deviations are observed in the thermal activation of the measured and predicted induction times that lead to a crossover near the glass transition. Below crossover, the viscosity based induction times are higher than the experimental ones, a relation that is reversed at higher temperatures. The differences, that may amount to orders of magnitude far from the crossover temperature, cannot be removed by taking into account the size dependence of the interfacial free energy, the depletion of the monomers, or by enforcing the proper (zero) value of the free energy of monomers. Rather, it appears that while crystal nucleation and viscosity are both diffusion related processes, they are governed by different diffusion modes.

The non-isothermal devitrification of gel-derived lithium, sodium and barium disilicate glasses

The non-isothermal devitrification kinetics of lithium, sodium and barium disilicate gelglasses have been investigated by differential thermal analysis. Using methods proposed by the present authors, the kinetic parameters and the mechanism of crystal growth in the studied glasses were evaluated from DTA curves. The results were compared with those obtained for the glasses of the same composition prepared using a mixture of oxides as starting materials and quenching the melts.

Barium Silicate Glasses and Glass Ceramics Prepared by the Sol-gel Process

An all-alkoxide sol-gel route to barium silicate glass has been developed. A highly reactive barium disilicate (BS2) composition gel was densified into glass with 97% density at 725°C (Tg=695°C). DTA, XRD, TKM and optical studies reveal that the phase separation and crystallization behaviour of the gel glass is similar to that of the corresponding melt glass, although the peak crystallization temperature (Tc) from DTA is lower for the gel glass.

Crystallization of gel-derived barium disilicate glass

Preparation, par le procede sol gel, d'un verre de composition: BaO•2SiO 2 . Etude des avantages a utiliser des gels comme materiaux de depart pour la preparation de vitroceramique

Experimental tests of the classical nucleation theory for glasses

New data for crystal nucleation rates and viscosities were obtained for Li 2O·2SiO 2 and BaO·2SiO 2 glasses. Special efforts were made to minimise impurities in the glasses. Good general agreement with previously published nucleation results was found. The new and previous results were used to test the applicability of classical nucleation theory. For both lithium and barium disilicates the temperature dependence of nucleation rates was satisfactorily described by theory, ln ( Iη/ T) vs. 1/ ΔG 2 T plots yielding straight lines over a wide range of temperatures ( ΔG is the bulk free energy difference per mole between crystal and liquid at temperature T). However, for lithium disilicate, where experimental ΔG data were available, the experimentally determined preexponential factor A in the nucleation equation was much higher than the theoretical value, in agreement with previous studies. This was the case even allowing for reasonable variations in ΔG. A similar result was obtained for barium disilicate using calculated ΔG values. Possible reasons for the discrepancy in A between theory and experiment are discussed including transient nucleation, experimental errors in the nucleation rates and heterogeneous nucleation. None of these effects can account for the observed discrepancy. Other possible explanations are considered.

The effects of amorphous phase separation on crystal nucleation kinetics in BaO-SiO2 glasses

The nucleation kinetics of the barium disilicate crystal phase were determined in BaO-SiO2 glasses containing 25.3 to 33.1 mol % BaO at temperatures from 673 to 807°C, using quantitative optical microscopy. The highest nucleation rates were found in the 33.1 mol % BaO glass close to the stoichiometric BaO·2SiO2 composition, which was just outside the immiscibility region. Much lower rates were observed in the glasses with lower BaO contents including those exhibiting amorphous phase separation. However phase separation had a marked indirect effect on crystal nucleation. Crystal nucleation was strongly dependent on the composition of the baria-rich matrix phase in the phase separated glasses, so that after amorphous phase separation had occurred at a given temperature the crystal nucleation rates of different glasses tended to converge to similar values. There was no evidence from the present results for a significant enhancement of crystal nucleation rates at the interfaces between the amorphous phases. However, nucleation was significantly influenced by certain impurities, particularly alumina, in the glasses. The early stages of crystallization in the glasses were studied by TEM. Immiscibility temperature measurements and results of DTA and X-ray diffraction are also reported.

Spherulitic growth and recrystallization in barium silicate glasses

The growth and recrystallization of spherulites formed in barium disilicate glasses between 700 and 900° C has been studied by electron microscopy and electron diffraction. Spherulites formed at 700° C consist of fibrillar (∼ 100 A diameter) monoclinic crystals in confocal arrangement with preferred crystallographic growth axes. High temperature (900° C) spherulites are composites of radially oriented plate-shaped orthorhombic crystals with lateral growth of epitaxially nucleated fibrillar monoclinic crystals. At intermediate temperatures “axialites”, consisting of a single orthorhombic “midrib” crystal with monoclinic fibrillar side-growths, grow in competition with the low temperature spherulite morphology. The monoclinic fibrillar phase is believed to be an intermediate metastable structure which is able to grow more rapidly than the orthorhombic phase via cellular transformation in the presence of impurities. Brief comparison is made between the observed morphologies and theories for interface instability and cellular crystallization. Recrystallization, induced mainly by the large interfacial area of spherulite fibrils, produces faulted and twinned monoclinic grains which transform slowly to the orthorhombic stable crystal phase. A glassy intercrystalline residue becomes more prominant with grain growth.

Immiscibility, nucleation and crystal growth in the soda-baria-silica system

The regions of glass formation and liquid-liquid immiscibility have been determined in the soda-baria-silica system. Immiscibility occurs in the high silica corner of the system, and extends over a continuous region from the BaO—SiO2 to the Na2O—SiO2 limiting binaries. In an extensive region close to the glass-forming boundary and including mostly the barium disilicate field, crystallization occurred by homogeneous nucleation to produce a glass ceramic. Some of these compositions also showed metastable immiscibility. Nucleation, growth and coarsening of the crystals were studied; first, a metastable high-temperature polymorph of barium disilicate appears growing as spherulites, but on prolonged heating a lathe-like structure appears.

New data on the barium oxide-silica phase diagram

1. We have studied heterogeneous equilibria at the Ba2Si3O8-BaSi2O5 section of the barium oxide-silica system. In the part of the system near the solidus in the composition range between about 58.5 and 62.5 mole % SiO2 we have found a region of variable composition phases based on dibarium trisilicate Ba2Si3O3. The composition region between 62.5 and 66.7 mole % SiO2 is represented by the equilibrium of two phases: a solid solution of Ba2Si3O3 and barium disilicate BaSi2O5. For barium disilicate we have found two enantiotropic polymorphic varieties: β -(sanbornite) and an α-modification, which is shown on the diagram by corresponding fields of stability of each form. We have detected peritectic equilibrium at 1425°; the liquidus curve has a maximum at 1447°, corresponding to the melting point of Ba2Si3O8. 2. We have determined the crystallooptical characteristics of the end members in the range Ba2Si3O8-BaSi2O5 and have given the change in refractive index of the specimens as a function of the composition. We have given data on intensities and interplanar spacings of the end and intermediate members of the studied composition range.

Field of solid solutions formed by celsian, disarium trisilicate, and barium disilicate (sanbornite)

1. It has been established that there is a field of ternary solid solutions formed by barium disilicate, dibarium trisilicate, and celsian, and its boundaries have bees defined. 2. The refractive index for this region falls with increase in the contents of barium disilicate and alumina. 3. Five polythermal sections for constant silica contents of 45, 42, 33, 36 and 33% have been constructed; these enable the equilibrium relations to be characterized for the field studied.

THE SYSTEM SODIUM DISILICATE-BARIUM DISILICATE*

The sodium disilicate-barium disilicate join of the ternary system, Na2O-BaO-SiO2, has been studied by means of the quenching method, and the phase-equilibrium diagram has been determined. The two compounds form a binary system without intermediate compounds and without appreciable solid solution. The eutectic occurs at approximately 32% of barium disilicate by weight and at a temperature of 797° C. The melting point of barium disilicate was determined as 1418° C. The glasses in the field of stability of barium disilicate show a marked tendency toward devitrification. The refractive indices of the glasses for sodium light were measured by the microscopic immersion method. These varied regularly from 1.505 for sodium disilicate glass to 1.610 for barium disilicate glass. The refractive index-composition curve plotted on a weight percentage basis is definitely convex downward, whereas on a mol percentage basis it is slightly convex upward.