Crystallization kinetics of gehlenite glass

Isothermal and non-isothermal kinetics of gehlenite glass devitrification have been studied. A Johnson—Mehl—Avrami rate equation was assumed. The values of kinetic parameters obtained from XRD patterns and DTA curves are in a good agreement. The crystallization process is thought to proceed by a rod-like growth controlled by diffusion from a fixed number of nuclei.     

Crystallization of some anorthite-diopside glass precursors

Anorthite and diopside have been obtained from complete devitrification of glasses belonging to the quaternary system MgO-CaO-Al₂O₃-SiO₂. Microstructure, the natural trend of the nucleation mechanism and kinetic studies on the crystallization phenomenon have been investigated by means of optical and electron microscopies, thermal and thermomechanical techniques and X-ray powder diffractometry. All the glasses investigated show a complete crystallization starting from a simple surface nucleation process. The activation energy for the crystallization process proved to be higher than that for viscous flow, leading to an important aspect modification in the sample during ceramization. Thermal stability and physical properties of both glass and glass-ceramic materials have been tested, suggesting the possible use of these materials in industrial application.     

Crystallization behaviour of calcium aluminate glass fibres

Crystallization behaviour of as-spun and fully-nucleated calcium aluminate (CA) glass fibres produced via inviscid melt spinning (IMS), was studied. Differential thermal analysis (DTA) scans on the as-spun and fully-nucleated CA fibres were performed at different heating rates. By applying the Kissinger method to the DTA scan data the activation energy values for crystallization were determined to be 569 and 546 kJ mol–1, respectively for the as-spun and fully-nucleated CA fibres. The Ozawa analysis on the DTA scan data gave the Avrami parameters at 2.7 and 2.4, respectively, for the as-spun and fully-nucleated CA fibres, which indicates high tendency of bulk crystallization mode. The formation of equilibrium phases of Ca12Al14O33 and CaAl2O4 in the crystallized CA fibres was identified by using X-ray diffraction (XRD).     

Crystallization of boro-silicate glass containing lead oxide

Glass of molecular percent composition 7B₂O₃-92.5 SiO₂-0.5 PbO was heat-treated at 600°C and 650°C for different lengths of time. Transmission electron microscopic studies revealed that phase separation is a precursor to crystallization in this glass.     

Crystallization behavior of calcium phosphate glass powder

The crystallization behavior of calcium phosphate glass powder with the molar ratio [CaO]/[P₂O₅] = 0.88, to which 6.38 mol% TiO₂ and 10 mol% Al₂O₃ were added as nucleation agents, was investigated. The results indicate complex crystallization behavior which depends on the powder particle size and the crystallization temperature. In the crystallization temperature range T _c < 900°C the surface mechanism of crystallization dominates in the case of all particle sizes ranging from 0–1 mm and the -Ca₂P₂O₇ phase is formed. At very long annealing times volume crystallization occurs and the TiP₂O₇ and AlPO₄ phases are formed. In the temperature interval T _c > 900°C the dominant crystallization mechanism depends on the particle size. In the size range 0.15–0.5 mm the surface mechanism of crystallization is replaced by the volume one. In the range >0.5 mm the volume mechanism of crystallization is dominant. The phases -Ca₂P₂O₇, TiP₂O₇ and AlPO₄ are formed in that temperature interval for all particle sizes. The additives TiO₂ and Al₂O₃ influence the nucleation and formation of TiP₂O₇ and AlPO₄ but do not influence the formation of -Ca₂P₂O₇.     

Crystallization kinetics of an Fe-Co based metallic glass

Crystallization kinetics of the Fe₆₇Co₁₈Si₁B₁₄ metallic glass has been studied from electrical conductivity measurements. Results are interpreted on the basis of a modification of the Germainet al. model (J. Non-Cryst. Solids 23 (1977) 93). The characteristics of kinetics for different annealing temperatures are analysed and the activation energy is evaluated     

Crystallization in Ni-Si-B glass: the influence of dispersoid additions

Crystallization processes in an Ni-Si-B metallic glass have been examined both for the glass alone and for the glass containing a distribution of fine crystalline particles. In the absence of dispersoids, nucleation of crystals occurs uniformly throughout the glass volume on heating following an initial slow, transient period. Nucleation occurs preferentially at the glass-dispersoid interface for the particle-containing glasses. The rate of nucleation at the dispersoid-particle interface depends on the nature of the particle and can best be correlated with the degree of misfit between the substrate and nucleating crystal for the planes and orientation relationships considered. The rate of nucleation at the dispersoid surface is shown to be completely explained on the basis of the classical heterogeneous nucleation model. Nucleation in the bulk occurs many orders of magnitude faster than predicted by classical homogeneous nucleation: the implication of this on the interpretation of nucleation is discussed.     

不同形态石英玻璃的析晶动力学研究

应用XRD技术对三种形态（块状,粉末和纤维）石英玻璃在1200－1500℃范围内内的等温析动力学过程进行了实验研究,实验结果表明,不同形态石英玻璃的析晶产物都是低温方石英,但形态不同的石英玻璃的析昌开始温度,同一温度下的析昌量及析晶量随温度的变化关系是不同的,三种形态石英玻璃析晶动力学可较好地用Avrami方程表示,其中块状,粉末和纤维的平均n值分别为1.65,1.68和1.92,活化能分klj o 495,445和430kJ/mol,析晶中的成核为表面成核机制,晶体生长受扩散和重排双重控制,随比表面和杂质含量的增大（块状→粉末→纤维）,控制因素中扩散的比例降代,而重排的比例上升,因而n值上升而活化能下降,析晶倾向增大。     

Crystallization and optical properties of a transparent mullite glass ceramic

Transparent mullite - based glass ceramics in the system SiO₂-Al₂O₃-B₂O₃-ZnO-K₂O were prepared. The activation energy of crystallization, the Avrami constants and the mechanism of crystallization of mullite were determined by the Matusita method through differential thermal analysis. X-ray diffraction and scanning electron microscopy methods were also used to determine the type and morphology of the precipitated crystalline phase. In addition, the degree of crystallinity was determined by the internal standard method. The results showed that bulk crystallization was the main mechanism for crystallization of the glasses. Optical absorption and photoluminescence properties of the obtained transparent glass-ceramics and their initial glasses were also studied. The glass-ceramics exhibited two emissions at 680 and 700 nm.     

Crystallization study of Bi5Ge20Se75 glass

Results of thermal analysis performed at different heating rates on the composition Bi₅Ge₂₀Se₇₅ are reported and discussed. The values of crystallization temperature (T _c) and the peak temperature of the crystallization (T _p) were found to be dependent on the heating rates. The activation energy of crystallization E _c was evaluated by five different methods. The average value of E _c is equal to 119.64+ 6.97 kJ mol^–1.The crystallization data are examined in terms of recent analysis developed for non-isothermal conditions. The results indicate a two-dimensional crystal growth for the investigated composition.     

Crystallization and sinterability of cordierite-based glass powders containing CeO2

Dense glass-ceramics for low firing temperature substrates were prepared by the addition of CeO₂ flux to a glass of the MgO-Al₂O₃-SiO₂ system. The glass powders were fabricated by melting at 1500°C and ball milling. Glass powder compacts prepared by dry pressing were heated at 800–1000°C for 0.5–4 h and sintered at 900–1000°C for 3 h. The crystallization behaviour and sinterability of the glass powder compacts were analysed by thermal and thermomechanical techniques. X-ray diffractometry and scanning electron microscopy. The addition of CeO₂ prevents the formation of -cordierite phase in the glass-ceramics and improves the formation of -cordierite phase. The activation energy of the glass containing CeO₂ for crystallization was lower than that of the CeO₂-free glass. Therefore, crystallization properties were enhanced. Because the crystallization onset temperature increased and the softening temperature decreased on the addition of CeO₂, the sinterability increased and dense glass-ceramics were fabricated below 1000°C. The properties of the glass-ceramics containing CeO₂ appeared to be correct for low firing temperature substrates.     

Crystallization of amorphous antimony films deposited onto glass substrates in ultrahigh vacuum

The crystallization of amorphous antimony (a-Sb) films deposited onto glass substrates in an ultrahigh vacuum of 10^?6?10^?7 Pa is investigated through in situ observation with an optical microscope camera. In comparison with the results for deposition in a conventional vacuum of 10^?4?10^?5 Pa, a marked reduction is observed in the critical thickness d_c for crystallization. For thicknesses less than the previous d_c value of about 250 Å, the dependence of the crystallization rate on the thickness is found to weaken drastically and the activation energy of the atoms for crystallization to increase. The antimony crystallites which nucleate in such thin a-Sb films do not take a simple spherical form.