Study of bulk crystallization in MgO-CaO-SiO2-Na2O glasses in the presence of CaF2 and MoO3 nucleant

The crystallization process of MgO-CaO-SiO₂-Na₂O glasses in the presence of CaF₂ and MoO₃ nucleating agents was investigated by DTA, XRD and SEM. The effect of compositional changes on bulk crystallization and growth morphology has been studied. After a two-stage heat treatment, the phases such as wollastonite, diopside and cristobalite, were identified. The ability of CaF₂ and MoO₃ pair as nucleating agents in inducing bulk nucleation in these glasses was attributed to the surface tension and viscosity reductions. The observed pore formation phenomenon upon the increase of MgO content in these glass-ceramics was related to the difference in density between the residual glass and diopside as the main crystalline phase.     

Rheology and gelation behavior of gel-cast cordierite-based glass suspensions

A cordierite-based glass suspension was shaped by gel-casting method. Effects of various parameters like pH, volume fraction of glass particles, gelling agents and dispersant on the rheology of the prepared slurries were investigated. The results demonstrated that using sodium tripolyphosphate (STPP) dispersant brings about a suspension of minimum viscosity. The bending strength of the dried gels was increased with the gelling agents. In addition, the utilization of cross linkers at a fixed concentration of monomer led to the gradual enhancement of the bending strength of the dried bodies. A bending strength of ∼11 MPa was obtained for the most promising dried gel cast bodies.     

Effect of Ce, Sb, and Sn on Solarization and Crystallization of an X-Ray-Irradiated Photosensitive Glass

The effect of Ce, Sb, and Sn photosensitive elements, individually and in combination with each other, on solarization and crystallization of an X-ray irradiated and a nonirradiated lithium silicate-based glass were investigated. According to the results, considering the crystallization behavior of the nonirradiated glasses, they were divided into Ce-bearing and Ce-free groups, in which the former group showed a clearer solarization tendency that manifested as an appearance of an absorbance peak at 318 nm in the spectrophotometry experiment. However, the results showed that in the irradiated glasses, the presence of Sb was more important in terms of improvement in crystallization view. Antimony decreased the differential thermal analysis (DTA) crystallization peak temperature from 655°C to 594°C and, in combination with the two other elements, changed the surface crystallization mechanism to a bulk one. The reactions that seemed to be responsible for the above-mentioned observations were discussed by spectrophotometry, DTA, X-ray diffraction, and scanning electron microscopic methods.     

The effect of BaO and Al2O3 addition on the crystallization behaviour of cordierite glass ceramics in the presence of V2O5 nucleant

The effect of V₂O₅ nucleant on crystallization of stoichiometric cordierite glass ceramics in the presence of various amounts of BaO and Al₂O₃ additives were investigated by DTA, XRD and SEM. It was shown that 3 wt.% V₂O₅ and 1.5 wt.% BaO were the optimum amounts of the additives effective in inducing both surface and bulk crystallization in the above glass ceramics.This resulted in ～90 wt.% cordierite after a 3 h heat treatment at 1020 °C. The specimens possessing 4–5 wt.% Al₂O₃ in excess of the stoichiometric cordierite composition, developed mullite along with cordierite in the temperature range of 1045–1055 °C, whereas in the specimen containing 6 wt.% excess Al₂O₃, mullite was detected as the sole crystallization product.     

SiO2–PbO–CaO–ZrO2–TiO2–(B2O3–K2O), A New Zirconolite Glass–Ceramic System: Crystallization Behavior and Microstructure Evaluation

Zirconolite (CaZrTi₂O₇) is a mineral that has a high containment capacity for actinides and lanthanides and is considered to be a good candidate for the immobilization of radioactive wastes. The glass–ceramic technique seems to be a very suitable and convenient method to produce zirconolite crystals by precipitating them in a specific glass matrix. In this study, development of a new zirconolite-based glass–ceramic belonging to SiO₂–PbO–CaO–ZrO₂–TiO₂–(B₂O₃–K₂O) system was investigated. The presence of PbO, together with B₂O₃ and K₂O, allowed the preparation of a X-ray diffraction (XRD) amorphous glass with a relatively high concentration of ZrO₂ and TiO₂, which was successfully converted to a glass–ceramic containing 34 wt% of zirconolite after heating at 770°C for 4 h. Differential thermal analysis, XRD, scanning electron microscope, and energy dispersive X-ray spectroscopy were used to determine the crystallization conditions, identify the crystallized phases, determine their compositions and quantities and observe and analyze the microstructures. The zirconolite crystals showed a platelet morphology with a monoclinic structure characterized by a =1.246 nm, b =0.7193 nm, c =1.128 nm, and β=100.508°.     

Pressureless Sintering of Apatite/Wollastonite–Phlogopite Glass–Ceramics

Apatite/wollastonite (A/W)—phlogopite (Ph) glass-ceramics of various compositions were prepared by means of pressureless sintering of two frit mixtures. Sintering conditions were studied by differential thermal analysis (DTA) and hot-stage microscopy. The sintered samples were characterized by X-ray diffraction. The results showed that by addition of a frit of phlogopite composition to a frit of A/W composition, the crystallization temperature decreases which results in suppressing the sintering process. The 50–50 frits mixture could be sintered to 0.90 relative density after 1 h heating at 1100°C. The type and sequence of formation of crystalline phases were also modified. Whereas the A/W glass–ceramic crystallized to apatite and wollastonite, the crystallization products of 50–50 frits mixture were fluoro-apatite, wollastonite, diopside, and mellilite.     

Sinterability, crystallization and properties of glass–ceramic tiles belonging to CaF2–CaO–MgO–Al2O3–SiO2 system

The effect of CaO substitution by different amounts of MgO on crystallization and various properties of CaF₂–CaO–Al₂O₃–SiO₂ glass system were investigated. It was shown that the properties of obtained glass–ceramics are greatly influenced by MgO contents. The Avrami exponent and activation energy for crystallization of the most promising and the MgO-free specimens were determined. Results suggested that surface crystallization was the main precipitation mechanism of both samples and while the activation energy for crystallization of MgO containing sample was less than MgO-free one, its ability to crystallize was diminished. SEM results confirm occurrence of surface crystallization of samples and depicts the phenomenon of microstructure coarsening by increasing MgO content and also reduced densification of specimen with magnesium oxide more than 9 wt.%.     

Devitrification and Phase Separation of Zinc Borosilicate Glasses in the Presence of MgO,P2O5, and ZrO2

A base glass composition which undergoes phase separation was chosen in the system Na₂O-ZnO-B₂O₃-SiO₂. Spontaneous and reheat-treatment opal glasses were synthesized by addition of different amounts of oxides such as P₂O₅ and ZrO₂. To clarify devitrification, visible spectroscopy was performed for all samples. Reheat-treated samples showed Tyndall effect in low temperatures, while they were completely opacified after soaking in 800°C for 3 h. P₂O₅-or ZrO₂-containing samples opacified spontaneously. The mechanism of opacification in glasses containing the MgO and ZrO₂ was partial crystallization, while in the mechanism of P₂O₅-bearing one was liquid–liquid phase separation.     

Effect of alumina content and heat treatment on microstructure and upconversion emission of Er3+ions in oxyfluoride glass-ceramics

The effect of alumina content and heat treatment temperature and time, on microstructure and Er3+ (0.5 mol.%) emission of oxy-fluoride glass-ceramics were investigated in this research. Two values of 1.8 (SA1.8Er0.5) and 2.18 (SA2.18Er0.5) were selected in this re-search for SiO2/Al2O3 ratio. According to DTA results, precursor glasses were heat treated at 630, 660 and 690 °C for 4 h and some glasses were also heat treated at 630 °C for 48 and 72 h. The results indicated that alumina content had significant effect on phase separation and vis-cosity of the glasses. Therefore the size, size distribution, and volume concentration of nano CaF2 crystals which precipitated during the heat treatment depended on alumina content of the glass. Due to the much smaller size of the precipitated CaF2 crystals in the glasses of low alumina content, these samples maintained excellent transparency and had narrower crystal size distribution than the high alumina glasses. The crystal size was increased markedly with the temperature increasing from 630 to 690 °C. On the other hand a slight increase was observed in the crystal size by raising the heat treatment time in both glasses. Results indicated that in low alumina content glass (SA2.18Er0.5) the size of CaF2 nanocrystals was controlled in one order of magnitude. The increase of heat treatment time and temperature led to the incorporation of Er3+ ions into CaF2 crystalline phase, increasing significantly the upconversion intensity. After heat treatment at 690 ℃ for 4 h, atomic force microscope (AFM) re-vealed the development of small crystals with an average size of 80 and 30 nm in SA1.8Er0.5 and SA2.18Er0.5 samples, respectively.     

Crystallization behavior,microstructure and dielectric properties of lead titanate glass ceramics in the presence of Bi2O3 as a nucleating agent

The crystallization behavior of PbO–TiO₂–B₂O₃–SiO₂ glasses in the presence of Bi₂O₃ as a nucleating agent were studied utilizing XRD, DTA, SEM. The glass samples heat treated in the range of 557–630 °C for different soaking times, all developed PbTiO₃ (PT) with perovskite structure. It was found that the addition of 0.5–1.0 mol% Bi₂O₃ resulted in the formation of homogenous, nano-structured glass ceramics with a mean crystallite size of 20–25 nm and PbTiO₃ as the major crystalline phase. The dielectric constant and dissipation factors for the prepared glass ceramics were in ～140–400 and ～0.04–0.4 ranges respectively.