The Role of GeO 2 and In 2 O 3 in the Glass-Ceramic Formation and Microstructure of Lithium Alumino-Silicate Glasses

The crystallization characteristics of the glasses based on the Li₂O–Al₂O₃–SiO₂ system have been investigated. The base glass composition was modified by partial replacement of GeO₂ for SiO₂ and In₂O₃ for Al₂O₃. The effect of the compositional variation on the crystallization products of the glasses and the type of the solid solution (ss) phases formed as well as the resulting microstructure were traced by differential thermal analysis (DTA), X-ray diffraction analysis (XRD) and scanning electron microscopy (SEM). A decrease in the endothermic and exothermic temperatures was detected by the replacement processes. β -spodumene ss, β -eucryptite ss, lithium meta- and di-silicate, lithium aluminum germanate and two forms of indium-containing phases (LiInSi₂O₆ & In₂Si₂O₇ phases) were mostly developed in the crystallized glasses. The objective of the present work is to understand the role of the glass oxide constituents in determining the type of the crystalline phases formed, their solid solution formed and the microstructure of the resultant glass-ceramic materials.     

Crystallization behavior and mechanical properties of high strength mica-containing glass-ceramics from granite wastes

The high-strength mica-containing glass-ceramics were prepared from granite wastes by bulk crystallization. The influences of SiO₂/Al₂O₃ molar ratio (S/A = 7.72, 9.62, 12.58, 17.82 and 29.67) on the crystallization behavior, microstructure, mechanical properties and machinability of glass-ceramics were investigated. The results demonstrated that the polymerization degree of the glass network decreased with the S/A ratio increasing, which further caused the decrease in glass transition temperature and crystallization temperatures. The increase in the S/A ratio promoted the precipitation of diopside, hectorite, kalsilite and tainiolite in glass-ceramics when the samples were heated at 750 °C, while inhibiting the precipitation of forsterite. For the glass-ceramics crystallized at 800 and 900 °C, the main crystalline phases transformed from diopside, forsterite, and nepheline to diopside, kalsilite, and tainiolite, with the S/A ratio increasing. As the SiO₂ gradually replaced Al₂O₃, the morphology of crystals changed from lamellar to granular, while the mean size of crystals reduced. The Vickers-Hardness values of glass-ceramics crystallized at 800 and 900 °C ascended with S/A ratio rising, and the values were above 6.30 GPa. The bending strength of most glass-ceramics is stable between 90 and 140 MPa, among which the maximum bending strength is 133.28 ± 14.81 MPa. The fracture toughness of the glass-ceramic crystallized at 800 and 900 °C declined, while that at 700 °C increased with a larger S/A ratio. Glass-ceramics after heat-treated at 900 °C with S/A ratio of 9.62 had the largest fracture toughness of 3.28 ± 0.15 MPa m^1/2. In preliminary tests of machinability, glass-ceramic after heat-treated at 900 °C with S/A ratio of 9.62 showed better results.     

Microstructural design of phlogopite glass-ceramics

Fluorophlogopite glass-ceramics in the MgO–Al₂O₃–SiO₂ glass systems were crystallized by two stages thermal treatments. Obtained glass-ceramics were characterized by DTA, X-ray diffraction and scanning electron microscopy. The heat treated samples at 705°C/3 h + 1000°C/15 min showed 2570.32 ± 98 MPa microhardness. The effect of heat treatment and preparation methods on microhardness and microstructure were studied. The microstructure of samples showed phlogopite and forsterite phases and solved or soft edge in the semi-rod like crystals was recorded which can explain the small value of microhardness. The various microhardness were determined in the range between 1591.61 to 9610 ± 147 MPa.     

Melt differentiation and crystallization of clinker minerals in a CaO-SiO2-Al2O3-Fe2O3 pseudoquaternary system

In the CaO-SiO₂-Al₂O₃-Fe₂O₃ pseudoquaternary system, the solid solutions of Ca₃SiO₅ [C₃S(ss)], Ca₂SiO₄ [C₂S(ss)], Ca₂(Al_xFe_1 − x)₂O₅ with 0.40 ≤ x ≤ 0.57 (ferrite) and Ca₃Al₂O₆ [C₃A(ss)] were crystallized out of a complete melt with 52.9 mass% CaO and Al₂O₃/Fe₂O₃ = 0.70. When the melt was cooled from 1673 K at 80 K/h, the crystals of ferrite with x = 0.40, C₃S(ss) and C₂S(ss) would start to nucleate from the melt at 1630 K. During further cooling, the x value of the precipitating ferrite would progressively increase and eventually approach 0.57 at 1613 K. The resulting ferrite crystals showed a zonal structure, the x value of which successively increased from the cores toward the rims. Actually, the x values of 0.43 and 0.52 were confirmed for, respectively, the cores and rims by EPMA. As the simultaneous crystallization of zoned ferrite, C₃S(ss) and C₂S(ss) proceeded, the coexisting melt would become progressively enriched in the Al₂O₃ component. After the termination of the ferrite crystallization, the C₃A(ss), C₃S(ss) and C₂S(ss) crystallized out of the differentiated melt. The end result was the four phase mixture of ferrite, C₃A(ss), C₃S(ss) and C₂S(ss), being free from the nucleation of Ca₁₂Al₁₄O₃₃ solid solution.     

Studies of Bone-Bonding Ability and Antibacterial Properties of Ag<Superscript>+</Superscript>, Cu<Superscript>2+</Superscript> or Zn<Superscript>2+</Superscript> ions Doping within Hench’s Bioglass and Glass-Ceramic Derivatives

Derived Hench’s bioglasses with specific ionic dopants Ag⁺, Cu²⁺, or Zn²⁺ have been prepared. The bone-boding ability or bioactivity behavior for the prepared glasses and their glass-ceramic derivatives has been investigated after immersion in phosphate solution for two weeks. Collective Fourier transform infrared absorption spectra (FTIR) and scanning electron microscopic (SEM) studies were conducted in order to study the in-vitro bioactivity behavior. X-ray diffraction (XRD) analysis was carried out to identify the crystallized phases upon thermal heat treatment through a two-step regime. The glasses and their glass-ceramic derivatives were tested to study their antibacterial or antifungal efficiency responding to the doped metal ions. FTIR spectra revealed the generation of two split peaks at about 560 and 605 cm^?1, after immersion in (0.2 M) sodium phosphate solution (Na₃PO₄), signifying the formation of a crystalline calcium phosphate phase, leading to hydroxyapatite formation. SEM examinations show characteristic rounded or nodular microcrystals for hydroxyapatite which support the FTIR data. X-ray diffraction analysis indicated crystallization of the main soda-lime silicate phase (1Na₂O.2CaO.3SiO₂) besides a secondary silicon phosphate phase (SiO₂.P₂O₅) in the studied glass ceramics. The route of crystallization is discussed on the basis of the presence of 6% P₂O_5; which facilitates the formation of phase separation and voluminous bulk crystallization of the main soda-lime silicate phase. The introduction of dopants is identified to cause no changes in the precipitated phases, with only minor changes in the percent of the crystalline phases. Experimental data indicate that the glass-ceramic samples are effective in bioactivity and antimicrobial efficiency.     

Synthesis of glass-ceramic glazes in the ZnO–Al2O3–SiO2–ZrO2 system

Glazes in the ZnO–Al₂O₃–SiO₂–ZrO₂ system with crystallization ability of gahnite (ZnO·Al₂O₃) and β-quartz solid solution (βqss) were synthesized. The compositions were designed based on calcium and magnesium oxide replacement (from a CaO–MgO–Al₂O₃–SiO₂ glass-ceramic glaze system) with zinc oxide and simultaneous increasing aluminum oxide. By this replacement, diopside eliminated and co-precipitation of gahnite and zirconium silicate observed. However, a little addition of Li₂O changes the crystallization path by precipitation of βq_ss and willemite (2ZnO·SiO₂) at low temperatures (800–900 °C) which dissolved into glaze by development of firing temperature. The experiments showed that while the micro-hardness of gahnite-based glass-ceramic glazes is almost equal with the diopside based one, it is more than the traditional floor tile glazes.     

Effect of MgO/Al2O3 ratio on the crystallization behaviour of Li2O–MgO–Al2O3–SiO2 glass-ceramic and its wettability on Si3N4 ceramic

The composition governs the crystallization ability, the type and content of crystal phases of glass-ceramics. Glass-ceramic joining materials have generated more research interest in recent years. Here, we prepared a novel Li₂O–MgO–Al₂O₃–SiO₂ glass-ceramic for the application of joining Si₃N₄ ceramics. We investigated the influence of the MgO/Al₂O₃ composition ratio on microstructure and crystallization behaviour. The crystallization kinetics demonstrated that the glasses had excellent crystallization ability and high crystallinity. β-LiAlSi₂O₆ and Mg₂SiO₄ were precipitated from the glass-ceramics, and the increase of MgO concentration was conducive to the precipitation of Mg₂SiO₄. Among the glass-ceramic samples, the thermal expansion coefficient of LMAS2 glass-ceramic was 3.1 × 10^?6/°C, which was very close to that of Si₃N₄ ceramics. The wetting test showed that the final contact angle of the glass droplet on the Si₃N₄ ceramic surface was 32° and the interface was well bonded.     

Crystallization characteristics and physico-chemical properties of indium-containing lithium iron aluminosilicate glasses

The crystallization characteristics, microhardness and chemical durability of glasses based on the Li₂O–Al₂O₃–Fe₂O₃ (In₂O₃)–SiO₂ system were investigated. The present work aims to study the effect of In₂O₃/Fe₂O₃ replacements on the crystallization and the resulting microstructure of the glass-ceramics. Very fine grained microstructure was mostly developed by In₂O₃/Fe₂O₃ replacements in the glass-ceramics. The crystalline phases formed after controlled heat-treatment of the glasses were β-eucryptite ss, lithium meta- and di-silicate as well as two forms of pyroxene phases (LiFeSi₂O₆ and LiInSi₂O₆). The microhardness values of the resulting glass-ceramics were decreased with In₂O₃/Fe₂O₃ replacements while the chemical durability was improved. The obtained data were correlated to the type of the crystalline phases formed and the resulting microstructure. Promising glass-ceramic materials of fine microstructure and good chemical durability could be obtained.     

Crystallisation and microstructure of nepheline–forsterite glass-ceramics

This work presents the results of a study focused on the development of forsterite–nepheline glass-ceramic with the use of rice husk ash (RHA) as a silica source. The glass-ceramics were produced by a sintering process of a glassy frit formulated in the MgO–Al₂O₃–SiO₂ base system with the addition of B₂O₃ and Na₂O to facilitate the melting and pouring processes. The crystallisation study was carried out by depicting the TTT curve (Time–Temperature–Transformation). The mineralogical characterisation of the glass-ceramic materials was carried out using the X-ray diffraction (XRD). The crystallisation activation energies were calculated by the Kissinger method. The results obtained show that devitrification of the RHA glass leads to a glass-ceramic material composed of nepheline (Na₂O·Al₂O₃·2SiO₂) and forsterite (2MgO·SiO₂). A study of the microstructure by scanning electron microscopy (SEM) allowed to establish the morphological evolution in both the shape and spatial arrangement of the nepheline and forsterite crystals on heating.     

Crystallization and Ability of Hydroxyapatite Formation in Some Trivalent Oxides Containing Na-Ca-Silicate Glass-Ceramics

The formation of glass-ceramics based on Na₂O–CaO-silicates containing P₂O₅ with minor additives of some trivalent oxides (e.g. La, In, Ga, and Al), has been investigated. Different crystalline phases including sodium-orthosilicate containing lanthanum, or aluminum were formed together with Na₂Ca₂Si₃O₉, NaInSi₂O₆, and Na₃Ga₂Si₃O₁₀. The nature and mechanism of HA formed in the glass-ceramics are considered. In general, the presence of trivalent oxides in the glass-ceramics progressively reduced the ability to form a calcium phosphate layer on the surfaces of the materials. The addition of In₂O₃ decreased the crystallization of the hydroxyapatite layer. However, in the presence of either Ga₂O₃ or Al₂O₃ only the amorphous calcium phosphate layer was formed after the immersion of the crystallized specimens in the SBF solution.     

Effect of ZnO/MgO ratio on the crystallization and optical properties of spinel opaque glazes

The effect of the ZnO/MgO ratio on the crystallization and optical properties of glass‐ceramic glazes from the SiO₂–Al₂O₃–ZnO–MgO–CaO–K₂O–Na₂O–B₂O₃ system was studied. The glazes with different ZnO/MgO ratios were characterized by differential scanning calorimetry, X‐ray powder diffraction, Raman spectroscopy, scanning electron microscopy, energy dispersive spectroscopy analysis and a spectrophotometer. The results reveal that the A glaze without ZnO content contains forsterite and sapphirine. The B and C glazes with intermediate ZnO/MgO ratio contain enstatite and spinel solid solution. The D to F glazes with higher ZnO/MgO ratio crystallize spinel solid solution as the only crystalline phase. The amount of spinel solid solution, lightness values (L*), gloss values and the reflectance of the studied glazes increase with the ZnO/MgO ratio.     

Bioactive glass-ceramics prepared by powder sintering and crystallization of polyphosphate glass containing strontium

Melt-quenching method was employed for obtaining a glass-ceramic with the following composition 42P₂O₅·40CaO·5SrO·10Na₂O·3TiO₂ (mol%) glass. The crystallization and sintering behavior of glass have been studied by using DTA, HSM, XRD, FTIR and SEM methods. It was determined that the surface and volume crystallization mechanisms act simultaneously in bulk glass samples. The comparison of DTA and HSM data revealed that the sintering and crystallization processes are independent. The sintered calcium phosphate glass-ceramic which contained bioactive β-Ca₃(PO₄)₂ and β-Ca₂P₂O₇ phases was successfully prepared. It was determined that during crystallization the primary phase in the precipitate was β-Ca(PO₃)_2. Other phases appearing in the resulting glass-ceramic were: α-Ca₂P₂O₇, γ-Ca₂P₂O₇, Ca₄P₆O₁₉ and CaHPO₄(H₂O)₂. Crystalline phases containing Sr and Ti were not detected. SEM analysis of the glass-ceramic microstructure revealed surface crystallization of glass particles and plate-like morphology of crystal growth. The result of the in vitro bioactivity showed that no apatite layer was formed on the surface of the as-prepared glass-ceramic samples after immersion in the simulated body fluid (SBF).     

A Raman Spectroscopic Study of Phase Transformations in Titanium-Containing Magnesium Aluminosilicate Glasses

The evolution of the Raman spectra of the 20MgO · 20Al₂O₃ · 60SiO₂ glass with 10 mol % TiO₂ and glasses doped with 0.5 wt % NiO or 2.5 and 5.0 mol % Ga₂O₃ is investigated in the course of heat treatment resulting in phase separation and crystallization. The composition and structure of the amorphous and crystalline phases (precipitating in the course of phase separation) and the residual glass are analyzed on the basis of Raman scattering, small-angle X-ray scattering (SAXS), and X-ray powder diffraction data. The size of nanoparticles of the precipitated phases is determined by low-frequency Raman spectroscopy, and the results obtained are compared with the nanoparticle size derived from the SAXS data.     

Crystallization process and some properties of novel transparent machinable calcium-mica glass-ceramics

Bulk glass having a calcium-mica composition (Ca_0.5Mg₃AlSi₃O₁₀F₂) is homogeneous glass. The crystallization mechanism of the mica is surface crystallization and transparency is lost completely when crystallization occurs on the surface. In this study, by decreasing SiO₂ and increasing CaO and Al₂O₃ from the chemical composition of Ca_0.5Mg₃AlSi₃O₁₀F₂, and moreover by replacing a small amount of K₂O instead of CaO, the phase separation appears in the glasses. Because of this phase separation, the mica begins to be crystallized not only on the surface but also in the bulk at lower temperatures. Consequently, the novel transparent machinable mica glass-ceramic can be obtained by heating the glasses having the chemical composition of Ca_0.6Mg₃Al_1.2Si_2.8O₁₀F₂ and K_0.01Ca_0.595Mg₃Al_1.2Si_2.8O₁₀F₂. As a larger amount of calcium-mica is separated, the bending strength decreases and the fracture toughness increases. Furthermore, by replacing K⁺ ion instead of Ca²⁺ ion in the interlayer of calcium-mica, the interlayer bonding strength becomes high, resulting in the increase of the bending strength.     

Influence of Ca/Al ratio on physical and dielectric properties of Al2O3/CaO-B2O3-SiO2 glass-ceramics composite coatings prepared by high enthalpy atmospheric plasma spraying

The dielectric layer plays a key role in regulating electromagnetic wave broadband scattering based on meta-surface technology. Herein, the physical properties of composite powder, prepared by spray drying of CaO-B₂O₃-SiO₂ (CBS) glass-ceramic powder and Al₂O₃ in different mass ratios, are systematically investigated. Meanwhile, a high enthalpy atmospheric plasma spraying equipment is utilized to prepare CBS/Al₂O₃ composite coatings, and the morphology, physical properties and dielectric properties of the composite coating are analyzed. The XRD and DSC data of the composite coating reveal that the crystallization behavior of β-CaSiO₃ and CaB₂O₄ gradually disappear with the increase of Al₂O₃ content. Hence, only CaAl₂Si₂O₈ phase is observed during heat treatment. The experimental results confirm that the dielectric properties of CBS/Al₂O₃ composite coating conform to the rule of mixture for composite materials. Also, the dielectric properties are affected by porosity and crystallization rate.     

Annealing temperature controlled crystallization mechanism and properties of gallium oxide film in forming gas atmosphere

Gallium oxide (Ga₂O₃) films had been fabricated on Al₂O₃(0001) substrate by employing pulsed laser deposition (PLD) and annealed at different temperatures under forming gas (FG) atmosphere (95% N₂ + 5% H₂). The influence of annealing temperature on the structural, optical, chemical composition, and surface morphological properties of the Ga₂O₃ thin films was investigated comprehensively. The annealing processes with hydrogen gas play a crucial role in the characteristics of Ga₂O₃ thin films. A crystallization mechanism of Ga₂O₃ films controlled by annealing temperature has been proposed firstly and analyzed systematically, which contains three kinds of competitive mechanism, namely the thermal enhanced crystallization, the enhanced H₂ dissociative adsorption on Ga₂O₃ surfaces, and the high-temperature decomposition of Ga₂O₃. Both Ga⁺ and Ga³⁺ oxidation valence states were presented in all samples, which indicated lattice oxygen deficiency in Ga₂O₃ films. The variation of the non-lattice oxygen proportion of Ga₂O₃ films related to the crystallization mechanism firstly increased and then decreased with the increase of annealing temperature. The detailed crystallization mechanism of PLD-Ga₂O₃ films annealed in FG offers a guideline and references for the further fabrication of high-quality Ga₂O₃ films and their applications in high-performance devices.     

Thermal and devitrification behavior of CaO–Ga2O3–SiO2 glasses

A study of the influence of the substitution of Ga₂O₃ for CaO, at constant O/Si ratio, on thermal properties and non-isothermal devitrification of 2.5CaO.2SiO₂ is reported. Differential thermal analysis (DTA) and X-ray diffraction analysis were used. The X-ray diffraction pattern of the crystallized Ga₂O₃ base glass shows that the αCaO SiO₂, that should be stable only above 1125°C, forms in the temperature range 900–1000°C. A new ternary crystalline phase, whose reflections are not reported in the JCPDS cards, was found to form during crystallization of the glass. The glass transformation temperature, T_g, and softening, T_s, temperature decrease as Ga₂O₃ is substituted for CaO. This is the result of the substitution of the network modifying cation Ca²⁺, of higher coordination number, by a network forming cation Ga³⁺ in fourfold coordination, in a composition range of relative insensitivity to changes of covalent cross-linking density. The crystal growth activation energy, E_c, decreases with substitution; this is the consequence of the decrease of the structural rigidity and of the shift of crystallization to a higher temperature range. Devitrification involves a mechanism of surface nucleation; surface nuclei behaving as bulk nuclei in samples that soften and sinter before devitryfing.     

A study on the wetting behavior of liquid iron on forsterite,mullite, spinel and quasi-corundum substrates

Wetting characteristics of liquid iron on magnesia, alumina and silica mixture substrates were studied by sessile drop experiments. Chromium-free forsterite, mullite, spinel and quasi-corundum phases were selected as alternative refractories in MgO-Al₂O₃-SiO₂. Morphological changes of molten electrolytic iron on the oxide substrates were investigated via apparent contact angle measurements. The results showed that the wetting behavior was significantly influenced by FeO compounds that were formed via oxidation of the liquid iron. Morphologies of the reacted layer were studied by Scanning Electron Microscope (SEM)/EDX analysis. The ternary phases FeO-MgO-SiO₂ and FeO-Al₂O₃-SiO₂ improved the wetting of liquid iron on the forsterite and mullite substrates by providing liquid phases at solid (refractory)–liquid (iron) interfaces. However, corrosion by liquid iron was significantly inhibited at spinel phase which did not feature FeO based compounds at the interface. Quasi-corundum (10MgO-25SiO₂-65Al₂O₃) showed a much enhanced resistance to liquid iron compared to forsterite or mullite refractories.     

Synthesis of Glass-Ceramic Materials in the BaO–PbO–B2O3–Al2O3–TiO2 System

The influence of PbO, B₂O₃, and Al₂O₃ additives on the glass formation and crystallization of glasses with a high total content of BaO and TiO₂ (65–75 wt % or 76–86 mol %) is investigated. It is shown that glasses of the compositions (wt %) 31–35 BaO, 12–17 PbO, 34–42 TiO₂, 10–13 Al₂O₃, and 2–3 B₂O₃ are promising materials for use in preparing glass-ceramic ferroelectrics based on the melting–molding–crystallization technology. These compounds are characterized by a relatively low melting temperature (1450°C), the absence of spontaneous crystallization during molding, and the possibility of controlling the phase composition of the material through the appropriate choice of the crystallization temperature.     

Nucleating role of P2O5 in nepheline-based transparent glass-ceramics

The nepheline-based transparent glass-ceramics are promising candidates for cover glass applications in electronic displays owing to their superior mechanical properties (than glasses) and ability to be chemically strengthened. However, our poor understanding about the kinetic and thermodynamic drivers controlling their crystallization processes usually results in their opacification and development of large internal stresses. The present work focuses on the development of nepheline-based nanocrystalline transparent glass-ceramic designed in the Na₂O–Al₂O₃–SiO₂ ternary system nucleated with P₂O₅. The temporal evolution of the phosphate and nepheline nanocrystal formation has been followed using X-ray diffraction, scanning/transmission electron microscopy, and energy-dispersive spectroscopy. The incorporation of P₂O₅ in the glass structure leads to the phase separation resulting in the crystallization of nanocrystalline Na₃PO₄ as an intermediate phase; thus, acting as a nucleating site for volume crystallization of nepheline. The optimization of nucleation and growth profile in the designed composition results in the formation of a transparent glass-ceramic with high optical transmittance (91.5 ± 0.1%).