Crystallization Of Lithium Disilicate Glass Research Articles

Effects of X-rays on Crystal Nucleation in Lithium Disilicate

The effects of exposure to a monochromatic 10 keV X-ray beam on thermally induced crystallization of lithium disilicate glass have been investigated and rendered two unexpected findings. First, it was found that an extended exposure during the nucleation heat treatment increased the number of nucleation sites. Second, it was observed that the effects extended far beyond the sample region that was directly exposed to the X-ray beam. The effects were confined to the direction perpendicular to the polarization of the synchrotron radiation beam that was used. The effects could be attributed to photo electrons created not only by the direct X-ray beam but also by the scattered radiation. Further evidence of the influence of photo electrons due to scattered X-rays is found in the results of an online crystallization study on a lysozyme solution.

Ex situ XRD, TEM, IR, Raman and NMR spectroscopy of crystallization of lithium disilicate glass at high pressure

The structure of Li 2O · 2SiO 2 (LS 2) glass was investigated as a function of pressure and temperature up to 6 GPa and 750 °C, respectively, using XRD, TEM, IR, Raman and NMR spectroscopy. Glass densified at 6 GPa has an average Si–O–Si bond angle ∼7° lower than that found in glass processed at 4.5 GPa. At 4.5 GPa, lithium disilicate crystallizes from the glass, while at 6 GPa new high pressure form of lithium metasilicate crystallizes. This new phase, while having lithium metasilicate crystal symmetry, contains at least four different Si sites. NMR results for 6 GPa indicate the presence of Q 4 species with (Q 4)Si–O–Si(Q 4) bond angles of ∼157°. This is the first reported occurrence of Q 4 species with such large bond angles in alumina free alkali silicate glass. No five- or six-coordinated Si are found.

In situ crystallization of lithium disilicate glass: Effect of pressure on crystal growth rate

Crystallization of a Li 2O · 2SiO 2 (LS 2) glass subjected to a uniform hydrostatic pressure of 4.5 and 6 GPa was investigated up to a temperature of 750 °C. The density of the compressed glass is ∼2% greater at 4.5 GPa than 1 atm and, depending on the processing temperature, up to 10% greater at 6 GPa. Crystal growth rates investigated as a function of temperature and pressure show that lithium disilicate crystal growth is an order of magnitude slower at 4.5 GPa than 1 atm resulting in a shift of +45 °C (±10 °C) in the growth rate curve at high pressure compared to 1 atm conditions. At 6 GPa lithium disilicate crystallization is suppressed entirely, while a new high pressure lithium metasilicate crystallizes at temperatures 95 °C (±10 °C) higher than those reported for lithium disilicate crystallization at 1 atm. The observed decrease in crystal growth rate with increasing pressure for the lithium disilicate glass up to 750 °C is attributed to an increase in viscosity with pressure associated with fundamental changes in glass structure accommodating densification.

The effect of elastic stress and relaxation on crystal nucleation in lithium disilicate glass

In the framework of a general theoretical approach recently developed, the effect of elastic stresses on the rate of nucleation of crystallites of critical sizes in the process of crystallization of lithium disilicate glasses is studied. Hereby both the evolution of elastic stresses – due to changes in the specific volume of the crystallizing system (i.e., differences in the densities of the melt and the crystal) – and their relaxation – connected with viscous flow – are taken into consideration. If the classical theory of nucleation is employed for the determination of the nucleation rates (modified by taking into account elastic stresses), elastic stresses may lead, for the system under consideration, to a decrease of the nucleation rate by up to two orders of magnitude. As shown, the theory allows an at least qualitatively correct incorporation of the effect of elastic stresses and stress relaxation into the description of nucleation phenomena in glass-forming melts.

TEM and XRD study of early crystallization of lithium disilicate glasses

Numerous researchers have speculated the precipitation of metastable phase(s) in the early stages of crystallization of lithium disilicate glass to explain the large discrepancies between the predictions of the classical nucleation theory (CNT) and experimental data. Therefore, we have investigated the early and intermediate stages of crystallization of three glasses on both sides of the stoichiometric composition through direct observations by transmission electron microscopy (TEM)/selected area electron diffraction (SAED) and X-ray diffraction (XRD). In samples heat-treated at ∼ T g=454 °C, two distinct crystalline phases, stable lithium disilicate (LS 2) and metastable lithium metasilicate (LS) coexist up to 120 h at 454 °C (crystalline fraction <1 vol.%). For longer treatments (240–600 h) only the stable phase (LS 2) was observed. These results suggest that in the early stages, simultaneous homogeneous nucleation of both LS and LS 2 takes place. As treatment time and crystallized fraction increase, the relative number of LS crystals decreases. Therefore, the precipitation of the LS phase does not disturb the nucleation of the stable LS 2 phase and thus cannot explain the failure of CNT in predicting the nucleation rates in this glass.

Computer modeling of non-isothermal crystallization

A realistic computer model for simulating isothermal and non-isothermal phase transformations proceeding by homogeneous and heterogeneous nucleation and interface-limited growth is presented. A new treatment for particle size effects on the crystallization kinetics is developed and is incorporated into the numerical model. Time-dependent nucleation rates, size-dependent growth rates and surface crystallization are also included. Model predictions are compared with experimental measurements of DSC/DTA peak parameters for the crystallization of lithium disilicate glass as a function of particle size, Pt doping levels, and water content. The quantitative agreement that is demonstrated indicates that the numerical model can be used to extract key kinetic data from easily obtained calorimetric data. The model can also be used to probe nucleation and growth behavior in regimes that are otherwise inaccessible. Based on a fit to data, an earlier prediction that the time-dependent nucleation rate in a DSC/DTA scan can rise above the steady-state value at a temperature higher than the peak in the steady-state rate is demonstrated.

Non-isothermal calorimetric studies of the crystallization of lithium disilicate glass

The influence of preannealing treatments on the polymorphic crystallization of lithium disilicate glasses is examined. As expected, glasses heated at different rates through the temperature range where there is significant nucleation develop widely different numbers of nuclei. This can dramatically influence the stability and transformation characteristics of the annealed glass. Non-isothermal differential scanning calorimetry (DSC) and differential thermal analysis (DTA) measurements are demonstrated to be useful to probe the nucleation behavior. The first systematic investigations of particle size effects on the non-isothermal transformation behavior are presented and discussed. Based on DTA and microscopy experiments, we show that small particles of lithium disilicate glasses crystallize primarily by surface crystallization. The relative importance of surface versus volume crystallization is examined by varying particle size, by introducing nucleating agents and by exposing glasses to atmospheres of different water content. These data are analyzed quantitatively using a numerical model developed in a second paper following in this volume.

Effect of Pt doping on nucleation and crystallization in Li2O · 2SiO2 glass: experimental measurements and computer modeling

Heterogeneous nucleation and its effects on the crystallization of lithium disilicate glass containing small amounts of Pt are investigated. Measurements of the nucleation frequencies and induction times with and without Pt are shown to be consistent with predictions based on the classical nucleation theory. A realistic computer model for the transformation is presented. Computed differential thermal analysis data (such as crystallization rates as a function of time and temperature) are shown to be in good agreement with experimental results. This modeling provides a new, more quantitative method for analyzing calorimetric data.

Kinetic Analysis of Nonisothermal Crystallization

ABSTRACTA realistic computer model for polymorphic crystallization under isothermal and nonisothermal conditions, which takes proper account of time-dependent nucleation behavior and cluster-size-dependent growth, is presented. A new correction to the standard Johnson-Mehl-Avrami-Kolmogorov (JMAK) statistical analysis that takes account of finite sample size is incorporated to simulate data taken from fine particles and nano-structured materials. Model predictions compare well with experimental data obtained from calorimetric studies of the polymorphic crystallization of lithium disilicate glass. The computer model is employed to evaluate commonly used methods of analysis for calorimetric data and to suggest new approaches for extracting kinetic parameters.

Numerical model for isothermal and non-isothermal crystallization of liquids and glasses

A new numerical model of isothermal and non-isothermal first order phase transformations, such as the crystallization of liquids and glasses, is presented. This model computes directly the volume fraction transformed, taking into account time-dependent nucleation rates and cluster-size-dependent growth velocities. The model is applied to the crystallization of lithium disilicate glass, using the appropriate kinetic and thermodynamic parameters. The model is used (1) to determine the validity of common methods for computing the volume fraction transformed as a function of time in isothermal experiments when a time-dependent nucleation rate is expected, (2) to simulate non-isothermal differential scanning calorimetry (DSC) studies of glass devitrification as a function of scan rate, and (3) to compute the effect of preannealing on the DSC peak parameters. A novel behavior of the nucleation rate with scan rate is predicted, arising because the relaxation of the cluster distribution cannot be described by a single relaxation time. Comparisons of the calculations with experimental data on this glass demonstrate the validity of the model.