SiO2–Al2O3–CaO glass-ceramics: Effects of CaF2 on crystallization,microstructure and properties

The effects of fluorine content on the nucleation and crystallization behavior of SiO₂–Al₂O₃–CaO glass ceramics system have been investigated. The crystalline phases were determined by X-ray diffraction (XRD). The crystallization kinetics was determined by differential thermal analysis (DTA). The microstructures were examined by using scanning electron microscope (SEM). Fourier transformed infrared spectra (FTIR) analysis was used to study the glass structure. The results showed that by increasing the fluorine content, both the crystallization peak temperature (T_p) and activation energy (E) decreased. Wollastonite, anorthite and gehlenite are the main crystalline phases that exist in the glass ceramics system. The study shows that fluorine promoted initial crystallization of glass and can be used as an effective nucleating agent in the SiO₂–Al₂O₃–CaO system.     

Fluoride promoted crystallization and mechanical properties of Sr-fluorphlogopite glass

Crystallization, microstructure and mechanical behavior of strontium fluorphlogopite glass-ceramics, SrO·4MgO·Al₂O₃·6SiO₂·2MgF₂, was studied by varying the fluorine content. A number of glass-ceramics of each glass batch with excess MgF₂ [SR0 (0% MgF₂), SR5 (5% MgF₂) and SR10 (10% MgF₂)] were prepared by heating at its respective nucleation temperature followed by at different crystallization temperatures (780–1150 °C). Differential Thermal Analysis (DTA), X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM) and Micro Hardness Indenter were used to study the crystallization, microstructure and mechanical behavior of resulting three glass batches. DTA analysis revealed that the peak crystallization (T_p) and glass transition (T_g) temperatures decreased with increasing fluorine content that also lowers down the activation energy (E) as evident from crystallization kinetics. Hardness and fracture toughness values are higher for less fluorine containing glass-ceramics when they are treated isothermally. However, more fluorine based glass-ceramics is found to be more machinable than the less fluorine one.     

Thermal and crystallization behavior of SiO2-PbF2 glass system in the presence of ErF3 and Al2O3

Oxyfluoride glass-ceramics have been introduced as one of the best bulk hosts for upconversion process of rare-earth ions. However, less attention to the technological topics in the process of glass crystallization has hindered the development of them in industrial scale applications. In this research, SiO₂–PbF₂ glasses were investigated to clarify the ambiguous role of rare-earth ions and alumina content in the microstructure and crystallization behavior. Results indicated that Er³⁺ addition caused a liquid-liquid phase separation via nucleation and growth mechanism, which led to single phase crystallization of β-PbF₂:Er³⁺ solid solution. Moreover, Er³⁺ had a significant effect on the crystallite size, size distribution, and PbF₂ crystallization temperature. On the other hand, increasing Al₂O₃ content enhanced the transparency and thermal stability of glass samples, whereas it reduced the amount of fluorine loss and increased the dissolution of Er³⁺ in fluoride crystalline structure. These results address some of the most controversial issues about crystallization behavior of rare-earth-doped oxyfluoride glasses.     

Effects of nitrogen and fluorine on crystallisation of Ca-Si-Al-O-N-F glasses

The effect of nitrogen and fluorine substitution on the crystallisation of a new generation of oxyfluoronitride glasses in the Ca-Si-Al-O-N-F system has been studied. Glasses were nucleated for 5 h at the nucleation temperature of Tg + 50 °C and crystallised for 10 h at the maximum crystallisation temperature (900-1050 °C depending on the glass composition) determined from differential thermal analysis. For the oxide glass, crystallisation results in formation of wollastonite (CaSiO₃), gehlenite (Ca₂Al₂SiO₇) and anorthite (CaAl₂Si₂O₈) along with a small amount of residual glass. For crystallisation of oxyfluoride glasses (0 equiv.% N), when fluorine content increases, cuspidine (Ca₄Si₂O₇F₂) is the major crystalline phase at the expense of gehlenite while in oxyfluoronitride glasses containing 20 equiv.% N, gehlenite is always the dominant crystalline phase at different fluorine contents. At constant fluorine content (5 equiv.%), an increase in nitrogen content favours the formation of gehlenite rather than anorthite or wollastonite suggesting that this phase may be able to accommodate N into its crystal structure. While a small amount of nitrogen substitution for oxygen can be assumed in the gehlenite structure, the residual glass in the glass-ceramic is expected to be very N-rich. In terms of properties, hardness is shown to be more sensitive to changes in microstructure, phase morphology and crystal size compared with elastic modulus which is related to the amounts of constituent phases present.     

Effect of soluble Cr2O3 on the silicate network,crystallization kinetics,mineral phase,microstructure of CaO-MgO-SiO2-(Na2O) glass ceramics with different CaO/MgO ratio

Cr₂O₃ is often used as a glass additive to prepare glass ceramics. Chromium element exists mainly in two parts in the glass ceramics: chromium-containing spinel and soluble chromium in glass matrix. Herein, effect of soluble chromium on the CaO-MgO-SiO₂-(Na₂O) glass system is researched. Glass and glass ceramics were characterized by Raman spectrum, X-ray diffraction, scanning electron microscopy, energy dispersive spectrometry. It is found that the addition of Cr₂O₃ increased the Q²si structure unit in glass networks, especially in glass systems with high MgO content. The crystallization temperatures of the systems were increased with the addition of Cr₂O₃. Soluble chromium reduced the crystallization activation energy of the glass system slightly, but did not alter its crystallization behavior (surface crystallization). With the increase of MgO content, the mineral phases of the glass ceramics gradually changed from wollastonite to diopside. Cr₂O₃ reduced the lattice parameters of the mineral phases. The addition of Cr₂O₃ has a significant effect on grain refinement and structural compactness of the glass ceramics system with high MgO content.     

The influence of TiO2 content on the properties of glass ceramics: Crystallization,microstructure and hardness

The effects of compositional variation, crystallization behavior, crystalline phases and microstructure formed in the SiO₂3Al₂O₃3CaO (SAC) glass system using various amounts of TiO₂ as nucleating agent were investigated by Differential Thermal Analysis (DTA), X-ray powder diffraction (XRD), Scanning Electron Microscope (SEM), Energy-dispersive X-ray spectroscopy (EDAX) and Fourier transform infrared spectroscopy (FTIR) techniques. The crystallization kinetics and mechanical properties of SAC glass ceramics were studied using crystallization peak temperature (T_p) of three different glasses as obtained from DTA, the activation energy (E) and Avrami exponent (n) were also determined. The crystallization peak temperature (T_p) and activation energy (E) were found to increase with the increase in TiO₂ content. The major crystalline phases were anorthite and wollastonite along with gehlenite and titanite as the minor crystalline phases present in the glass ceramic system. The studies showed that the three dimensional crystalline structure and the microhardness increased with the increase of TiO₂ content in the glass ceramics system.     

Kinetics of growth of spinel crystals in a borosilicate glass

Three aspects of the kinetics of spinel crystallization in a high-level waste (HLW) glass were studied: (1) the effect of nucleation agents on the number density (n_s) of spinel crystals, (2) crystallization kinetics in a crushed glass, and (3) crystallization kinetics in a glass preheated at T>T_L (liquidus temperature). In glass lacking in nucleation agents, n_s was a strong function of temperature. In glasses with noble metals (Rh, Ru, Pd, and Pt), n_s increased by up to four orders of magnitude and was nearly independent of temperature. The kinetics of spinel crystallization in crushed glass lacking nucleation agents was dominated by surface crystallization and was described by the Kolmogorov-Johnson-Mehl-Avrami (KJMA) equation with the Avrami exponent n≅0.5. For application to HLW glass melter processing, it was necessary to preheat glass at T>T_L to eliminate the impact of temperature history and surface crystallization on crystal nucleation and growth. In the temperature range of glass processing, crystals descend under gravity when they reach a critical size. Below this critical size, crystallization kinetics is described by the KJMA equation and above the critical size by the Hixson-Crowell equation. At low temperatures, at which glass viscosity is high and diffusion is slow, the KJMA equation represents crystal growth from nucleation to equilibrium. As n_s increases, the temperature interval of the transition from the KJMA to Hixson-Crowell regime shifts to a higher temperature.     

Preparation of glass–ceramic glazes for fast firing applications by CaF2 substitution with B2O3 in the CaO–CaF2–Al2O3–SiO2 system

The present work aims to obtain glass–ceramic glazes for floor tile applications. In this regard, CaF₂ was gradually replaced by B₂O₃ in the glass compositions belonging to the CaO–CaF₂–Al₂O₃–SiO₂ system. This substitution led to a noticeable decrease of crystallization peak temperatures and to an alteration of the crystallization trend. In the B₂O₃ bearing glazes, anorthite and gehlenite were identified as the major and minor crystalline phases, respectively. During concurrent crystallization and sintering based on the fast firing program, glass–ceramic glazes containing 9 weight parts of fluorine and 12 weight parts of boron oxide showed the most desirable sinterability. The optimized glass–ceramic glazes offered acceptable micro-hardness, whiteness and thermal expansion behavior after fast firing heat treatment.     

Role of MgO in lowering glass transition temperature and increasing hardness of lithium silicate glass and glass-ceramics

The effect of variation of MgO (1.5, 4.5 and 7.5 mol%) content on glass structure, crystallization behavior, microstructure and mechanical properties in a Li₂O–K₂O–Na₂O–CaO–MgO–ZrO₂–Al₂O₃–P₂O₅–SiO₂ glass system has been reported here. Increased amount of MgO enhanced the participation of Al₂O₃ as a glass network former along with [SiO₄] tetrahedra, reducing the amount of non-bridging oxygen (NBO) and increasing bridging oxygen (BO) amount in glass. The increased BO in glass resulted in a polymerized glass structure which suppressed the crystallization and subsequently increased the crystallization temperature, bulk density, nano hardness, elastic modulus in the glasses as well as the corresponding glass-ceramics. MgO addition caused phase separation in higher MgO (7.5 mol%) containing glass system which resulted in larger crystals. The nano hardness (～10 GPa) and elastic modulus (～127 GPa) values were found to be on a much higher side in 7.5 mol% MgO containing glass-ceramics as compared to lower MgO containing glass-ceramics.     

Characterisation of fluorine containing glasses and glass-ceramics by 19F magic angle spinning nuclear magnetic resonance spectroscopy

¹⁹F magic angle spinning nuclear magnetic resonance (MAS-NMR) spectroscopy was used to characterise the local environment of fluorine in three types of fluorine containing glass-ceramics. The glass-ceramic compositions studied included four that crystallised to fluorcanasite, one which crystallised to barium fluorphlogopite and one which crystallised to fluorrichterite. In the fluorcanasite glasses, prior to crystallisation, the fluorine was present solely as an F–Ca(n) species whilst following crystallisation it was also present as an F–Ca(n) species in the fluorcanasite phase and in those glasses containing AlPO₄ it was also present as an F–Ca(n) species in fluorapatite.In the fluorrichterite and fluorphlogopite glasses the fluorine was present predominantly as F–Mg(3) and following crystallisation it was also present as F–Mg(3) in the fluorrichterite and fluorphlogopite phases. In all these glass-ceramics fluorine appears to be preferentially associated with the cations of highest charge to size ratio and the local environment of fluorine in the glass and the crystal phase is almost identical.     

Effect of Li2O on melt crystallization of CaO–SiO2–CaF2 based glasses

This paper presents the effects of Li₂O on the kinetics and structural aspects of the Cuspidine (Ca₄Si₂F₂O₇) crystallization behavior of CaO–SiO₂–CaF₂ glass (basicity 1.7). In order to elucidate the crystallization characteristics during differential scanning calorimetry (DSC) measurements, the kinetic parameters have been determined using the Johnson-Mehl-Avrami-Kolmogorov (JMAK) equation. The crystallization rate constant and negative activation energy thus calculated indicated that the limiting step of crystallization was nucleation. Also, Raman spectroscopy and Solid-state MAS NMR spectroscopy analyses indicated that lithium could interact with fluorine, thereby disturbing the interaction between calcium and fluorine. This retards Cuspidine nucleation at the initial stage of melt crystallization. These findings on CaO–SiO₂–CaF₂-based glass lubricants can be used to optimize essential properties such as viscosity and crystallization temperature during continuous casting of steels.     

Impact of tellurium on glass transition and crystallization in the Ge-Se-Te-Bi system

Samples with compositions of Ge₂₀Se_67−xTe_xBi₁₃ (x=0, 5, 10, 15 and 20 mol%) were prepared and characterized by XRD, Raman spectroscopy and DSC. XRD patterns and Raman spectra present consistent evidences of glass transition and crystallization in association with the variation of the structure network due to the substitution of Te for Se. Precipitation of thermoelectric (TE) crystals experiences an evolution from Bi₂Se₂Te (BST) to Bi₂SeTe₂ (BTS) phases with the increasing Te content. Based on DSC data performed at five heating rates, crystallization kinetics is analyzed by using Kissinger's relation, Ozawa model and Augis-Bennett approximation. Different methods show the identical evaluations of characteristic activation energies (E_c), while additional parameters of frequency factor (K₀) and Avrami exponent (n) are available with AB method. In particular, calculated E_c and K₀ demonstrate that with the substitution of Se by Te the crystallization process of the target TE BST crystal phase is much enhanced while that of the undesired GeSe₂ phase is slightly hindered.     

Kinetics and in situ observation of nonisothermal crystallization in Bayan Obo tailing-based nanocrystalline glass-ceramic

In recent years, the preparation of CMAS nanocrystalline glass-ceramics has shown potential as an application of secondary resourcing technology in utilizing Bayan Obo iron ore tailings containing rare earth elements. The crystallization mechanism for nanodiopside-type glass-ceramics was studied via an investigation of the nonisothermal crystallization kinetics of the glass system, combined with the in situ observation of softening and crystallization of the basic glass using a high-temperature laser confocal microscope. The results show that the activation energy of nucleation in the glass system is higher than that of crystal growth by using the Ozawa model. The crystallization mechanism changes as the crystallization fraction increases, that is, from the three-dimensional growth in which the nucleation rate increases with time in an interface-controlled manner (a > 1, b = 1, m = 3) at the initial stage of crystallization to a decreased nucleation rate in a diffusion-controlled growth (a = 0.5, b = 0.5, m = 3) at the middle and later stages. This process involves both surface crystallization and volume crystallization. The crystallization was observed in situ, and it was further confirmed that there exists a critical nucleation temperature between T_g and T_x, which is related to the interface free energy and critical Gibbs free energy difference. When the temperature exceeds the critical value of T_g + 55 K, the system begins to exhibit visible crystallization. With an increase in temperature, the basic glass softened considerably, while the crystal grew significantly. In addition, the surface roughness can be used to characterize the crystallization process, providing a new research method for crystal growth.     

Growth behavior,morphology and properties of lithium aluminosilicate glass ceramics with different amount of CaO,MgO and TiO2 additive

Li₂O–Al₂O₃–SiO₂ glass with CaO, MgO and TiO₂ additive were investigated. With more CaO + MgO addition, the crystallization temperature (T_p) and the value of Avrami constant (n) decreased, the activation energy (E) increased. The mechanism of crystallization of the glass ceramics changed from bulk crystallization to surface crystallization. With more TiO₂ addition, the crystallization temperature decreased, E and n had a little change. The crystallization of the glass ceramics changed from surface crystallization to two-dimensional crystallization. Plate-like, high mechanical properties spodumene-diopside glass ceramics were obtained. The mechanical properties related with crystallization and morphology of glass ceramics.     

Reduction of crystallization ability of sodium zinc phosphate glass under X-ray radiation

The effect of X-ray radiation on the crystallization ability of maximally homogenized sodium zinc phosphate glass with a minimal light scattering value of V _v = 4 × 10^?6 cm^?1 has been investigated. The crystallization kinetics of the sodium zinc phosphate glass of the 33.9P₂O₅ · 56.6ZnO · 9.5Na₂O (mol % from analysis) composition under the conditions of homogeneous nucleation and X-ray radiation have been studied. It has been demonstrated that the crystallization properties of phosphate glasses are more sensitive to the synthesis method than those of silicate glasses. It has been established that sodium zinc phosphate glass crystals represent the main crystalline phase precipitated in the glass of the above composition. The main parameters of nucleation have been determined in glass without preliminary radiation, including stationary nucleation rate I _st, nonstationary nucleation time ??, and nucleation activation energy E _??. It have been found that the effect of the CuK _?? X-ray radiation leads to the slowing down or even cessation of the nucleation of crystals in glass (the result depends on the change in the radiation intensity along the sample depth) in the case when radiation takes place immediately during the nucleation thermal treatment of the sample in a high-temperature chamber of the X-ray device. The kinetics of sodium zinc phosphate crystallization in the samples upon their pretreatment by CuK _?? X-ray irradiation has been investigated. It has been shown that the rate of crystal nucleation in glasses exposed to X-ray radiation is lower than that in glasses without preliminary irradiation.     

Properties and Crystallization Behavior of Sodium Iron Phosphate Glasses

Ternary Na₂O–Fe₂O₃–P₂O₅ (NFP) glasses with varying Na₂O/Fe₂O₃, Na₂O/P₂O₅, and Fe₂O₃/P₂O₅ ratios were prepared. The properties and crystallization tendencies were systemically investigated. It is shown that both density and chemical stability of the glass increase with Fe₂O₃. In contrast the Na₂O/P₂O₅ ratio has little effect on the glass properties for a fixed Fe₂O₃ content. The crystallization behavior of the glasses was analyzed by DTA and XRD. Unlike Li₂O–Fe₂O₃–P₂O₅ glasses NFP glasses were found to be stable against crystallization. 15Na₂O–27Fe₂O₃–58P₂O₅ glass was found to have the highest chemical stability among the studied NFP samples; the influence of TiO₂, ZrO₂ on crystallization in this composition was studied. It is found that addition of 3.4 mol% TiO₂ or 2.2 mol% ZrO₂ had little effect on the crystallization behavior of this glass. However, when the amounts of TiO₂ or ZrO₂ were increased to 8.4 or 5.5 mol% respectively the glass readily devitrified. Furthermore the addition of fluorine (introduced by replacing Na₂CO₃ with NaF in the glass batch) leads to amorphous glasses which could be crystallized to form NaFeP₂O₇ upon controlled thermal treatment. With increasing NaF additions the activation emergy for crystallization decreased from 428 to 381 kJ/mol.     

Crystallization characteristics of glass based on phosphorite ore enrichment wastes

It has been found that when multicomponent glass containing phosphorus oxide as an additive is subjected to low-temperature treatment (< t _g), crystallization accompanied by precipitation of calcium silicophosphate, not previously described in the literature, occurs in the glass powder. The compound obtained possesses cubic symmetry with space group P _p ³ and lattice parameter a = 7.98 Å.     

Synthesis and investigation of Ag-Cs-<Emphasis Type="Italic">X</Emphasis> (<Emphasis Type="Italic">X</Emphasis> = I,Br, Cl) glasses doped with Er<Superscript>3+</Superscript>

The glass formation and crystallization of ErI₃-doped melts in the Ag-Cs-Pb-X (X = I, Br, Cl) are investigated in the section corresponding to the cation ratio Ag: Cs: Pb = 59: 39: 2 with ErI₃ contents of 0.2–7.5 mol %. The glass transition, crystallization, and melting temperatures are determined for a number of compositions. The primary crystalline phases that precipitate from melts upon crystallization are identified. It is demonstrated that the iodide, bromide, and chloride glasses containing up to 0.5 mol % ErI₃ have the lowest crystallization ability. An increase in the ErI₃ content leads to the precipitation of unidentified erbium-containing phases from melts upon cooling and to an increase in the crystallization ability of glasses. The spectral and luminescence properties of the glass of the composition 52Ag · 17AgCl · 39CsBr · 2PbCl₂ + 0.5 mol % ErI₃ are studied in the range of the ⁴ I _13/2 → ⁴ I _15/2 transition of the Er³⁺ ion. It is found that the lifetime of the ⁴ I _13/2 → ⁴ I _15/2 level of the Er³⁺ ion is equal to 10 ms.     

Properties of Ca–(Y)–Si–Al–O–N–F Glasses: Independent and Additive Effects of Fluorine and Nitrogen

Thirty glasses of composition (in equivalent percent) 20‐xCa:xY:50Si:30Al:(100‐y‐z)O:yN:zF, with x = 0, 10; y = 0, 10, 20, and z = 0, 1, 3, 5, 7 were prepared by melting and casting. All glasses were X‐ray amorphous. Glass molar volumes (MV) decreased with nitrogen substitution for oxygen for all fluorine contents and, correspondingly, glass fractional compactness increased. Fluorine substitution of oxygen had virtually no effect on molar volume or fractional glass compactness for the three nitrogen contents tested. Young's modulus and microhardness were virtually unaffected by fluorine substitution for oxygen while nitrogen substitution for oxygen caused increases in these two properties. Glass‐transition temperature and dilatometric‐softening point values all decreased with increasing fluorine substitution levels, while increasing nitrogen substitution caused values for these thermal properties to increase. Correspondingly, the thermal expansion coefficient increased with fluorine and decreased with nitrogen substitution levels. Using property value differences between glasses containing fluorine and the corresponding glass containing 0 eq.% F enabled 24 data points to be used to determine the effect of fluorine on T_g,dil and T_DS. The trends were linear with a gradient for both properties of the order of ?22°C (eq.% F)^?1. For the nitrogen effect, 20 data points were analyzed for trend effects. As expected from earlier work, all trends had good linearity. Gradients were for T_g,dil and T_DS +2.5°C (eq.% N)^?1, which are fairly similar to previous results in oxynitride systems. All of the data collected and its analysis clearly shows that the substitution effects of fluorine for oxygen and nitrogen for oxygen are independent and additive with the fluorine substitution. The property trends of the glasses are discussed in terms of their implications for glass structure.     

Characterization of Li–Zn–Fe crystalline phases in low temperature ceramic glaze

Ceramic glaze containing Li₂O and ZnO was prepared at a low firing temperature of 1100 °C. Addition of 0–30 wt.% iron oxide content developed brown color with a metallic sparkling effect from crystallization after soaking at 980–1080 °C. Using XRD, SEM/EDS and Raman microscopy the crystalline phases were determined as lithium zinc ferrite (Li_xZn_1?2xFe_2+xO₄ where x = 0.05–0.20), hematite (α-Fe₂O₃) and anorthite (CaAl₂Si₂O₈). The most preferable metallic sparkling effect was caused by the lithium zinc ferrite phase obtained from the glaze containing 10 wt.% of iron oxide. Thermal analysis by STA after heat treatment indicated that crystallization temperature of lithium zinc ferrite and the effective soaking temperature depended on the iron oxide content in the glaze. The influence of excessive iron oxide content on the crystallization behavior of lithium zinc ferrite, anorthite and hematite phases is discussed.