Field-induced silver injection into lithium disilicate glass

The objective of this work was to study field-induced silver injection into Li₂O · 2SiO₂ glass by the techniques of secondary ion mass spectrometry and thermally stimulated polarization/depolarization current (TSPC/TSDC), with an emphasis on the comparison experiments between gold and silver electrodes. For both anode materials, accumulation of lithium was found near the cathode. The blocking nature of the gold anode was demonstrated by the closeness of the observed and estimated width of the alkali-depleted region near the anode. For silver-anode samples, field-induced silver injection as well as lithium conduction occured and increased the TSPC slope from that of the gold-anode samples.     

Containerless processing of a lithium disilicate glass

Glasses of Li₂O · 2SiO₂ (LS₂), and LS₂ doped with 0.001 wt% platinum (LS₂ + 0.001 wt% Pt) compositions were melted, cooled and reheated at controlled rates while levitated (containerless) inside an electrostatic levitator (ESL) furnace at the NASA Marshall Space Flight Center. The experiments were conducted in vacuum using spherical, 2.5–3 mm diameter, glass samples. The measured critical cooling rate for glass formation, R _c, for the LS₂ and LS₂ + 0.001 wt% Pt glasses processed at ESL were 14 ± 2 °C/min and 130 ± 5 °C/min, respectively. The values of R _c for the same LS₂ and LS₂ + 0.001 wt% Pt glasses processed in a container were 62 ± 3 °C/min and 162 ± 5 °C/min, respectively. The effective activation energy for crystallization, E, for this LS₂ glass processed without a container at ESL, was higher than that for an identical glass processed in a container. These results suggest that the glass formation tendency for a containerless LS₂ melt is significantly increased compared to an identical melt in contact with a container. The absence of heterogeneous nucleation sites that are inherently present in all melts held in containers is believed to be the reason for the increased glass forming tendency of this containerless melt.     

Nucleation in glass and differential thermal analysis

The crystal nucleation of Li₂O-2SiO₂ glass is studied by differential thermal analysis. A simple method, based on the shift of the differential thermal analysis (DTA) crystallization-peak temperature, for evaluating the influence of temperature and time of heat-treatments on bulk nucleation, is described. The experimental results are in good agreement with results obtained by other workers.     

Nucleation and growth of a multicomponent metallic glass

The metallic glass samples of Fe ₆₇Co₁₈B₁₄Si₁ (2605CO), prepared by the melt spinning technique were procured from the Allied Corporation. The kinetics of crystallization of this multicomponent glassy alloy is studied using differential scanning calorimetry (DSC). The crystallization data have been examined in terms of modified Kissinger and Matusita equations for the nonisothermal crystallization. The results show enhanced bulk nucleation in general. At high heating rates added to it is surface induced abnormal grain growth resulting in fractal dimensionality. 5th IUMRS ICA98, October 1998, Bangalore.     

Nucleation and crystal growth in a fly ash derived glass

The devitrification behaviour of a fly ash derived glass, examined by differential thermal analysis (DTA), X-ray diffraction and scanning electron microscopy (SEM), is reported and discussed. The crystallized phases were identified as mullite (3Al₂O₃·2SiO₂) and anorthite (CaO·Al₂O₃·2SiO₂). Kinetic parameters for nucleation and crystal growth were estimated from the DTA curves. The temperature of maximum nucleation rate was 790°C and the activation energy for crystal growth E=370 kJ mol^–1. The crystal morphology was investigated by SEM and the crystal shape found to be consistent with the morphological index n calculated by DTA. The glass-ceramic obtained from a previously nucleated glass showed a fine-grained texture.     

Behavior of the refractive index of lithium disilicate glass ceramic processed at high pressure and high temperature

The aim of this work was to investigate the effect of high pressure and high temperature on the refractive index of lithium disilicate glass ceramic with the stoichiometric composition Li₂O·2SiO₂ (LS₂). A first group of monolithic LS₂ glass samples were processed at 2.5 GPa, 4 GPa and 7.7 GPa at room temperature and a second group was submitted to high pressure and, simultaneously, to heat treatments for nucleation and growth of the crystalline phases. For comparison, samples submitted to the same heat treatments at 1 atm were also investigated. The refractive index of the samples was obtained by spectral ellipsometry and the results were clearly dependent on the particular pressure and temperature conditions. The crystallization of the samples was investigated by X-ray diffraction. For the samples processed under high temperature at 1 atm and at 2.5 GPa a fraction of the originally amorphous glass was transformed to a monoclinic phase of lithium disilicate. For the samples processed under high temperature and at 4 GPa, a large fraction of the originally amorphous glass transformed to an orthorhombic phase while, at 7.7 GPa, it was observed the formation of lithium metasilicate.     

Nucleation and crystallization kinetics of CaO-Al2O3-2SiO2 in powdered anorthite glass

The nucleation and crystallization kinetics of CaO-Al₂O₃-2SiO₂ crystals in powdered anorthite glass with particle size <44 m in which CaO-Al₂O₃-2SiO₂ crystals were found to crystallize in the heating process of the glass, were studied by nonisothermal measurements using differential thermal analysis (DTA). The temperature of maximum nucleation rate was determined from the DTA curves of samples heat treated at different temperatures. The activation energy and kinetic parameters were simultaneously calculated from the DTA data using previously reported kinetic models. The crystallization process of a sample heat treated at the temperature of the maximum nucleation rate was fitted to kinetic equations with an Avrami constant, n2 and a dimensionality of crystal growth, m2, indicating that the constant number of nuclei of CaO-Al₂O₃-2SiO₂ precipitated in a glass matrix grew two-dimensionally with an activation energy taken as an average of the values calculated by the Kissinger and also the Augis and Bennett method of 679±4 kJ mol^–1.     

Nucleation kinetics in a glass-ceramic enamel

The kinetics of nucleation in a glass-forming melt having the composition (in mol %) 71.80 SiO₂, 1.63 Al₂O₃, 23.20 Li₂O, 2.20 K₂O and 1.17 MoO₃ is investigated. X-ray analysis, DTA, optical and electron microscopy are used together with precise viscometric data to follow the crystallization process and to characterize the properties of both the initial melt and the resulting crystalline phase — the rhombic Li₂O · 2SiO₂. By virtue of its composition, structure and properties the resulting semi-crystalline material is a model of typical glass-ceramic enamel. The analysis of the kinetic measurements demonstrated the non-steady state character of the investigated process of nucleation. Furthermore it is shown that the presence of small amounts of a surface tension lowering oxide (MoO₃) in the melt considerably decreases the interface energy at the crystal/melt boundary. This explains the role of MoO₃ and similar surface active oxides in inducing the bulk crystallization in glass-ceramic enamels.     

Crystallizing phases and kinetics of crystal growth in potassium tetragermanate glass

Abstracts are not published in this journal This revised version was published online in November 2006 with corrections to the Cover Date.     

Mechanisms of nucleation and crystallization of lithium disilicate in Li-Zn-silicate glass-ceramic fibres

The preferred crystal orientation of Li₂Si₂O₅ crystallites has been evaluated in several partially crystallized glass fibres with the following chemical composition in mol %: 66.75 SiO₂; 23.45 Li₂O; 8.00 ZnO; 1.00 K₂O and 0.80 P₂O₅. The crystallites were shown to be grown radially with thec-axis directed from the surface towards the core of the fibre. When glass fibres have been subjected to a nucleation treatment step, this preferred orientation slightly increases in the temperature range 590 to 620° C, by increasing the crystallinity content. This behaviour was attributed to the presence of active nuclei. A kinetic study on the crystalline volume coarsening, carried out according to the Johnson-Mehl-Avrami equations, showed a mechanism of linear growth for the lithium disilicate phase with the (00/) growing planes moved inward from the surface to the core of the fibre. The small-angle X-ray scattering results were obtained by using a masking liquid with an electron density close to that of the glass fibres investigated, in order to eliminate the total external reflection and surface scattering. Results indicated that two distinct particle fractions are precipitated inside the fibres after glass-in glass phase separation and nucleation at 500° C for various periods of time. The fraction of the largest particles,13 to20 nm in size, is probably constituted with vitreous SiO₂-rich droplets, whereas the smallest particles, 2.5 to 6.5 nm in size, are probably Li₂O-rich clusters containing P₂O₅.     

Nucleation kinetics in thin film growth III transient nucleation and nucleation on preferred substrate sites

An analytical method is described by which the initial (transient) stage of thin film nucleation may be generally treated. The method is extended to cover simultaneous nucleation at substrate defect sites and at normal lattice sites. Post-transient nucleation and growth under these latter conditions is also considered in detail. The analysis is applied to defect-controlled nucleation under complete condensation conditions and the results are compared with those previously derived for nucleation on a perfect defect-free substrate. The application of this analysis to computer simulation of thin film nucleation and growth is discussed.     

Crystal nucleation in P2O5-doped lithium disilicate glasses

The bulk (volume) crystallisation of Li₂O·2SiO₂ (LS₂) glasses with additions of 1, 2 and 5 mol % P₂O₅ (G1P, G2P and G5P) was investigated. In the G1P and G2P glasses heated at 454°C metastable - and -LS₂ crystal phases formed initially and only stable LS₂ at later stages. In G1P heated at 463°C transmission electron microscopy (TEM) revealed particles (2 m) of stable LS₂ but also smaller (<0.1 m) crystalline particles which were unstable under the electron beam and could not be identified. Crystalline lithium orthophosphate (Li₃PO₄) was detected by XRD and ³¹P magic angle spinning nuclear magnetic resonance (MAS-NMR) in both as-quenched and heat treated G5P but not in G1P and G2P. In G5P heated at 476°C, metastable -LS₂ phase initially formed and at later times stable LS₂. In as-quenched G5P, a trace of lithium metasilicate (LS) was observed. Li₃PO₄ crystals were not detected in G5P by TEM but a marked increase in nucleation rate due to P₂O₅ addition was observed. ²⁹Si MAS-NMR revealed amorphous Q² (LS) and Q⁴ (SiO₂) species in an average Q³ (LS₂) environment in the as-quenched glasses. Increased P₂O₅ concentration caused a greater degree of amorphous phase separation. ³¹P MAS-NMR showed amorphous lithium phosphate units in the as-quenched and nucleated G1P and G2P. The presence of crystalline Li₃PO₄ phase in glass G5P suggests that Li₃PO₄ crystals may act as sites for heterogeneous nucleation of stable lithium disilicate.     

Nucleation kinetics of a Li2O·2SiO2 glass based on a liquid model

A liquid model was proposed wherein crystal embryos should exist in equilibrated melts at temperatures even above the melting point or liquidus temperature and the number density of embryos should decrease as the melting temperature increases. Crystallization behavior of a Li₂O·2SiO₂ glass was studied. Secondary heat-treatment (600 °C for 20–60 min) of the Li₂O·2SiO₂ glass induced different number of crystals depending upon the melting temperature in the range 1040°–1300 °C and duration of heat-treatment t. The number density of crystals N decreased with increasing temperature of melting. The number density of pre-existing embryos was estimated from (N – t) diagram and was found consistent with what the proposed model claims.     

Nucleation and growth of poly-Si films deposited directly on glass substrate in reactive thermal-chemical vapour deposition

We prepared device-grade polycrystalline silicon thin films on glass substrates at 450 °C, by reactive thermal-chemical vapour deposition employing Si₂H₆ and F₂ as source gases. The nucleation and growth of the poly-Si thin films were investigated, and the process pressures were varied from 533 to 933 Pa. Thin films with thickness of 2 to 200 nm were prepared and their crystallinity and morphological properties were characterized. The incubation time is shorter than 30 s; 533 Pa yields higher crystallinity. However a trade-off exists between growth rates and grain sizes. We proposed a model for comparing our technique with conventional low- pressure chemical vapor deposition.