Influence of Al2O3/SiO2 ratio in multicomponent LNAS glasses and glass-ceramics on the crystallization behavior,microstructure and mechanical performance

Transparent glass-ceramics containing eucryptite and nepheline crystalline phases were prepared from alkali (Li, Na) aluminosilicate glasses with various mole substitutions of Al₂O₃ for SiO₂. The relationships between glass network structure and crystallization behavior of Li₂O–Na₂O–Al₂O₃–SiO₂ (LNAS) glasses were investigated. It was found that the crystallization of the eucryptite and nepheline in LNAS glasses significantly depended on the concentration of Al₂O₃. LNAS glasses with the addition of Al₂O₃ from 16 to 18 mol% exhibited increasing Q⁴ (mAl) structural units confirmed by NMR and Raman spectroscopy, which promoted the formation of eucryptite and nepheline crystalline phases. With the Al₂O₃ content increasing to 19–20 mol%, the formation of highly disordered (Li, Na)₃PO₄ phase which can serve as nucleation sites was inhibited and the crystallization mechanism of glass became surface crystallization. Glass-ceramics containing 18 mol% Al₂O₃ showed high transparency ～84% at 550 nm. Moreover, the microhardness, elastic modulus and fracture toughness are 8.56 GPa, 95.7 GPa and 0.78 MPa m^1/2 respectively. The transparent glass-ceramics with good mechanical properties show high potential in the applications of protective cover of displays.     

Effect of metastable phase on the crystallization and mechanical properties of MgO-Al2O3-SiO2 glass-ceramics without nucleating agents

Glass-ceramics without nucleating agents usually undergo surface crystallization, which deteriorates the overall performance of the products. In this paper, we evaluated the effects of the metastable MgAl₂Si₃O₁₀ crystalline phase on the crystallization behavior of a MgO–Al₂O₃–SiO₂ (MAS) glass without nucleating agents and mechanical properties of the glass-ceramics obtained. The results demonstrated that the precipitation of metastable MgAl₂Si₃O₁₀ crystallites promotes the crystallization mechanism transformed from surface crystallization into volume crystallization with two-dimensional crystal growth. Furthermore, the grain size of MgAl₂Si₃O₁₀ near the surface of the prepared glass-ceramics was larger than that of MgAl₂Si₃O₁₀ inside, which helps to generate compressive stress and improves its mechanical properties. The glass-ceramics containing metastable MgAl₂Si₃O₁₀ phase exhibited an enhanced hardness in the range of 7.6 GPa–9.5 GPa for indentation loads ranging from 2.94 N to 98 N, and indentation size effect behavior was observed in Vickers hardness tests of both MAS glass and glass-ceramics. The load-independent hardness values for MAS glass and glass-ceramics were reliably evaluated by the modified proportional specimen resistance (MPSR) model of 7.1 GPa and 7.6 GPa, respectively, with a high correlation coefficient of more than 0.9999. This work reveals the unexploited potential of the metastable phase in improving the crystallization ability and mechanical properties of glass-ceramics.     

Preparation and characterization of alkali-free glass substrates with enhanced properties for TFT–LCDs applications

To obtain an alkali-free glass substrate with enhanced properties for thin-film transistor–liquid crystal displays (TFT–LCDs) applications, we chose a base glass composed of 3B₂O₃-15Al₂O₃-58SiO₂-22MgO-0.5SrO-1.5MgF₂ (mol%) for nucleation–crystallization. The results show that when the nucleation–crystallization processes of the base glass are 810 °C/6 h + 880 °C/6–9 h, the prepared GC/6–GC/9 glass-ceramics exhibit enhanced properties because of the precipitation of nano-sized cordierite. The transmittances in the visible range of the GC/6–GC/9 glass-ceramics exceed 85%, the densities are 2.564–2.567 g/cm³, thermal expansion coefficients are 2.934–3.059 × 10^-6/°C (25–300 °C), compressive strengths are 417–589 MPa, bending strengths are 141–259 MPa, Vickers hardnesses are 6.8–7.8 GPa, and strain points are approximately 735 °C. Considering these properties, the prepared GC/6–GC/9 glass-ceramics have good potential as candidate materials for alkali-free glass substrates. Additionally, these results demonstrate that it is feasible to improve the properties of alkali-free glass substrates by nucleation–crystallization.     

Effect of substitution of ZrO2 by SnO2 on crystallization and properties of environment-friendly Li2O–Al2O3–SiO2 system (LAS) glass-ceramics

In this study, a transparent and environmentally friendly Li₂O–Al₂O₃–SiO₂ (LAS) glass-ceramic was prepared by melt-quenching and two-step heat treatment. The influence of the substitution amount of ZrO₂ by SnO₂ on the crystallization, microstructure, transparency, and mechanical properties of LAS glass and glass-ceramics was investigated by means of differential scanning calorimetry (DSC), X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), Ultraviolet–visible Spectrophotometer, three-point bending strength test, and microhardness test. The results indicate that the main crystalline phase of LAS glass ceramics was a β-quartz solid solution when heat treated at 780 °C for 2 h and 870 °C for 1.5 h. When the substitution amount of ZrO₂–SnO₂ increased from 0.4 mol% to 2.5 mol%, the grain size and thermal expansion coefficient of LAS glass-ceramics first decreased and then increased, and the crystallinity first increased and then decreased. When the substitution amount of ZrO₂–SnO₂ was 0.8 mol%, the transparency of the LAS glass-ceramics was maximum, the bending strength was 96 MPa, and the Vickers hardness was 10.9 GPa.     

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.     

Effect of TiO2 on crystallization and mechanical properties of MgO–Al2O3–SiO2 glasses containing P2O5

MgO–Al₂O₃–SiO₂ glass-ceramics have been widely used in military, industrial, and construction applications. The nucleating agent is one of the most important factors in the production of glass-ceramics as it can control the crystallization temperature or the grain size. In this study, we investigated the effect of replacing P₂O₅ with different amounts of TiO₂ on the crystallization, structure, and mechanical properties of an MgO–Al₂O₃–SiO₂ system. The crystallization and microstructure were investigated by differential scanning calorimetry, Raman spectroscopy, X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. The mechanical properties were investigated by measuring the Vickers hardness, Young's modulus, and fracture toughness. The results showed that adding TiO₂ favored the precipitation of fine grains and significantly increased the Vickers hardness, Young's modulus, and fracture toughness of the glasses. Introducing an appropriate amount of TiO₂ can make a glass structure more compact, promote crystallization, and improve the mechanical properties of MgO–Al₂O₃–SiO₂ glass-ceramics.     

The influence of MgO on the crystallization behaviour and properties of SrO-rich phosphosilicate glasses

A series of glass was produced to investigate the effect of MgO/SrO replacement on the crystallization characteristics and properties of phosphosilicate glasses containing high SrO content. The glass samples were synthesized by conventional melting technique based on 5CaO-(40-X)SrO-X MgO– 43SiO₂–7P₂O₅–5CaF₂ (where; X = 10, 20, 30 and 40 mol%). The influence of MgO/SrO replacement on phase assemblages, microcrystalline structures, thermal expansion, and mechanical properties was examined as a function of basic chemical compositions and crystallization parameters. Predominant strontium meta-silicates together with strontium fluoroapatite phases are crystallized from the base glass free of magnesium. The substitution of strontium by magnesium up to 50% led to formation strontium akermanite phase Sr₂MgSi₂O₇ at the expense of SrSiO₃ phase. Whereas the increase of the MgO/SrO of more than 50%, which led to the crystallization of the clino-enstatite MgSiO₃ as a predominant phase. The results show that the α-values of the glass-ceramics are ranged in 94–125 × 10⁻⁶ K⁻¹ over the temperature range (25–500 °C). On the other hand, MgO/SrO replacements led to enhancing the microhardness of the resultant crystalline materials from 4713 Mpa to 6744 Mpa. As a result of the designed glass compositions, promising crystalline phases were obtained as well as good thermal and mechanical properties for the resultant glass-ceramics. Therefore, the designed glass-ceramics can be strongly used as biomaterials especially for bone reconstruction applications.     

Some features of the surface modification of MgO–Al2O3–TiO2–SiO2 glass and glass ceramics by Ag diffusion

Diffusion of Ag species into the surface layers of the MgO–Al₂O₃–TiO₂–SiO₂ glass stimulates the crystallization processes therein during subsequent thermal treatment. It was found that the silver doping significantly increases the microhardness of glass and glass ceramics. The effect of the Ag diffusion and the subsequent high-temperature treatment (T > T_g) on the structure and luminescence properties of the MgO–Al₂O₃–TiO₂–SiO₂ glass was studied. The thermal evolution, structure and properties of the glass and glass ceramics were investigated by the DSC method, XRD analysis and luminescence spectroscopy. It was found that the Ag diffusion into the surface layers leads to the formation of numerous different luminescent molecular Ag_n clusters. The MgO–Al₂O₃–TiO₂–SiO₂ glass and glass ceramics subjected to the Ag diffusion can operate as effective down-converters of the radiation demonstrating the effective transformation of UV and blue light into the radiation of red and NIR spectral range.     

Preparation and characterization of fully waste-based glass-ceramics from incineration fly ash,waste glass and coal fly ash

Municipal solid waste incineration (MSWI) fly ash (FA) is a typical hazardous waste due to its high contents of toxic heavy metals, and hence its disposal has attracted global concern. In this work, it was recycled into environmental-friendly CaO–Al₂O₃–SiO₂ system glass-ceramics via adding coal fly ash (CFA) and waste glass (WG). The effects of CaO/SiO₂ ratios and sintering temperatures on the crystalline phases, morphologies, mechanical and chemical properties, heavy metals leaching and potential ecological risks of glass-ceramics were investigated. The results showed that wollastonite (CaSiO₃), anorthite (CaAl₂Si₂O₈) and gehlenite (Ca₂Al₂SiO₇) were the dominant crystals in the glass-ceramics, which were not affected by CaO/SiO₂ ratio and sintering temperature. The compressive strength increased, while the Vickers hardness and microhardness decreased as increasing the sintering temperatures from 850 to 1050 °C, which reached their maximum values of 660.69 MPa, 6.14 GPa, and 7.43 GPa, respectively. However, the increase of CaO/SiO₂ ratio resulted into the reduction of the three mechanical parameters. As varying CaO/SiO₂ ratio from 0.48 to 0.86, the maximum compressive strength, Vickers hardness and microhardness were 611.80 MPa, 5.43 GPa, and 6.56 GPa, respectively. Besides, all the glass-ceramics exhibited high alkali resistance of >97%. The extremely low heavy metals leaching concentrations and low potential ecological risk of glass-ceramics further revealed its environmentally friendly property and potential application feasibility.     

The formation of strontium fluoride nano crystals from a phase separated silicate glass

The crystallization of nano crystalline rare earth or alkaline earth fluorides is of high importance for optically transparent partially crystalline materials. In this study, nano crystalline SrF₂ is precipitated. In contrast to recently studied glass systems which allow the crystallization of alkaline earth fluorides, droplet phase separation is the initial step of the SrF₂ crystallization. The large difference in the atomic number of strontium in comparison with the other glass components enables structural studies using electron microscopy. In a glass with the mol% composition 3.7Al₂O₃/12.3SrO/5.3K₂O/8.0Na₂O/60.3SiO₂/10.4SrF₂ nano crystalline SrF₂ is formed after annealing the glass samples at temperatures in the range from 520 to 600 °C for 2 to 160 h. The size of SrF₂ nano crystals remains constant within the limits of error and does neither depend on time nor on temperature. A highly viscous layer enriched in SiO₂ is formed during crystallization hindering further crystal growth.     

Glass-ceramics produced from thin-film transistor liquid-crystal display waste glass and blast oxygen furnace slag

This study employed waste glass from thin-film transistor liquid-crystal displays (TFT-LCD) and slag from a basic oxygen furnace (BOF) to produce CaO–MgO–Al₂O₃–SiO₂ (CMAS) glass-ceramics through vitrification and further heat treatment of compacts of the obtained glass powders for densification and crystallization. CMAS glass-ceramics are known for their excellent mechanical and dielectric properties. MgO and Al₂O₃ were selected as modifying agents to ensure that the composition of the wastes featured these important characteristics.     

Effects of nucleation agents on the preparation of transparent glass–ceramics

Formation of transparent glass–ceramic in the system MgO–SiO₂–Al₂O₃–K₂O–B₂O₃–F with and without addition of LiF and NaF has been investigated. Crystallization of glass-sample was conducted by controlled thermal heat-treatment, at determined nucleation and crystallization temperatures. In this regard, the effects of addition of LiF and NaF were investigated on the crystallization behavior and transparency of the samples.Low transmission (less than 80% at 600 nm) was observed in the basic composition (K).The addition of NaF and LiF caused more intense phase separation in the system.The results indicated that the glass–ceramic can remain transparent if fine grains with nano size are precipitated but will turn into opaque when large grains appear, because of the difference in the refractive index between glass and precipitated crystals.     

Formation and crystal growth of needle-like rutile in glass- ceramics

Glass-ceramic, which has negligible dielectric loss, high mechanical strength, excellent drop resistance, low CTE, and low density for lightweight design, is the best option for the back cover of mobile devices in the 5 G era. Herein, the effect of P₂O₅ on the phase separation and crystallization of MgO–Al₂O₃–SiO₂-TiO₂ glass-ceramics is studied. The incorporation of P₂O₅ in the glass structure leads to phase separation, in which the P and Mg-enriched phase was formed in the glass matrix, and promotes the increase of T_g. With the increase of P₂O₅ content, the precipitated crystals change significantly. First, the silicate crystals (Mg₂SiO₄) disappear, whereas the phosphate crystals (LiMgPO₄) emerge when 2 mol% P₂O₅ is introduced. Second, titanate crystal (MgTi₂O₅) can not be observed when 4 mol% P₂O₅ is introduced. The Ti₅O₉ crystals appear simultaneously with LiMgPO₄ crystals and transform to rutile TiO₂ crystals at high temperature. Interestingly, the needle-like rutile TiO₂ crystals, which is 300 nm long and 20 nm wide, have been found in a glass with 4 mol% P₂O₅. The large L/D ratio of needle-like crystals increases the hardness significantly from 6.08 GPa to 7.14 GPa. Similar to other fiber reinforced composites, this needle-like crystals provide a new strategy to improve the mechanical properties of glass ceramics.     

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.     

Effect of Ta2O5 addition on the structure,crystallization mechanism,and properties of CaO–B2O3–SiO2 glasses for LTCC applications

CaO–B₂O₃–SiO₂–Ta₂O₅ (CBST) glass-ceramics, with different Ta₂O₅ content, (up to 6 mol%), have been prepared by using glass melt quenching followed by heat treatment between 800 and 880 °C. The Fourier Transform Infrared (FTIR) results showed that the stronger the attraction of Ta⁵⁺ to the oxygens in the BO₃^3? and SiO₃^2? structures, the more easily the B–O and Si–O bonds will be destroyed. The underlying reason is most probably the high field strength of Ta⁵⁺, which results in a weakening of the vibration intensities of the [BO₃] and [SiO₄] units. Moreover, the Differential Scanning Calorimetry (DSC) results showed that the softening point (T_g), crystallization starting temperature (T_c1), and exothermic crystallization peak temperature (T_p1), of the CaSiO₃ phase, shifted to higher values with the addition of Ta₂O₅. Also, the crystallization activation energy (E_a) and the glass stability factor (ΔT) of the CaSiO₃ phase increased, which indicated that the CaSiO₃ phase of the glass became inhibited by the addition of Ta₂O_5. It was, thus, obvious that there was a need of glass characterization. The results of the crystallization kinetics showed that the critical cooling rate decreased with the addition of Ta₂O₅, which indicated that the viscosity of the system had increased. The CBST glass-ceramics, containing 1 mol% Ta₂O_5, that were sintered at 875 °C for 15 min showed excellent dielectric properties: ε_r = 6.22 and tanδ = 1.19 × 10^?3 (1 MHz). To sum up, CaO–B₂O₃–SiO₂–Ta₂O₅ glass-ceramics are potential low temperature co-fired ceramic substrate materials.     

Highly translucent nanostructured glass-ceramic

Transparent and translucent glass-ceramics (GCs) are found in an increasing number of domestic and high-technology applications. In this paper, we evaluated and optimized the effects of two-stage heat treatments on the resulting crystalline phases and microstructure of a glass of the SiO₂–Li₂O–P₂O₅–TiO₂–CaO–ZnO–Al₂O₃ system. The objective was to develop a transparent nanostructured glass-ceramic (GC). After numerous heat treatment trials, we found that a long nucleation period of 72 h at 455 °C followed by a crystal growth treatment at 660 °C for 2 h resulted in a highly translucent GC having homogenously distributed nanocrystals. The relatively high amount of P₂O₅ (2.5 mol%) induced the formation of lithium disilicate as the main crystal phase. We thus developed a GC having crystals under 50 nm, with a high crystallized fraction (52%vol. Li₂Si₂O₅ and 26% vol Li₂SiO₃), transmittance of approximately 80% in the visible spectrum for 1.2 mm thick specimens, nano hardness of 8.7 ± 0.1 GPa (load of 400 mN), a high elastic modulus of 138 ± 3 GPa as measured by nanoindentation, and good flexural strength (350 ± 40 MPa) as measured by ball-on-3 balls tests. Due to its high content of Li⁺, this GC has the potential to be chemically strengthened and can be further developed to be used in a number of applications, such as on displays of electronic devices.     

Effect of Al2O3 content on BaO–Al2O3–B2O3–SiO2 glass sealant for solid oxide fuel cell

The glass structure, wetting behavior and crystallization of BaO–Al₂O₃–B₂O₃–SiO₂ system glass containing 2–10 mol% Al₂O₃ were investigated. The introduction of Al₂O₃ caused the conversion of [BO₃] units and [BO₄] units to each other and it played as glass network former when the content was up to 10 mol%, accompanied by [BO₄] → [BO₃]. The stability of the glass improved first and then decreased as Al₂O₃ increased from 2 to 10 mol%, the glass with 5 mol% Al₂O₃ being the most stable one. The wetting behavior of the glasses indicates that excess Al₂O₃ leads to high sealing temperature. The glass containing 5 mol% Al₂O₃ characterized by a lower sealing temperature is suitable for SOFC sealing. Al₂O₃ improves the crystallization temperature of the glass. The crystal phases in the reheated glasses are mainly composed of Ba₂Si₃O₈, BaSiO₃, BaB₂O₄ and BaAl₂Si₂O₈. Al₂O₃ helps the crystallization of BaSiO₃ and BaAl₂Si₂O₈.     

Glass forming,crystallization, and physical properties of MgO-Al2O3-SiO2-B2O3 glass-ceramics modified by ZnO replacing MgO

This study focused on the glass forming, crystallization, and physical properties of ZnO doped MgO-Al₂O₃-SiO₂-B₂O₃ glass-ceramics. The results show that the glass forming ability enhances first with ZnO increasing from 0 to 0.5 mol%, and then weakens with further addition of ZnO which acted as network modifier. No nucleating agent was used and the crystallization of studied glasses is controlled by a surface crystallization mechanism. The predominant phase in glass-ceramics changed from α-cordierite to spinel/gahnite as ZnO gradually replaced MgO. The phase type did not change; however, the crystallinity and grain size in glass-ceramics increased when the glasses were treated from 1030 °C to 1100 °C. The introduction of ZnO can improve the thermal, mechanical, and dielectric properties of the glass-ceramics. The results reveal a rational mechanism of glass formation, crystal precipitation, and evolution between structure and performance in the xZnO-(20-x)MgO-20Al₂O₃-57SiO₂-3B₂O₃ (0 ≤ x ≤ 20 mol%) system.     

The role of K2O on sintering and crystallization of glass powder compacts in the Li2O–K2O–Al2O3–SiO2 system

The effects of K₂O content on sintering and crystallization of glass powder compacts in the Li₂O–K₂O–Al₂O₃–SiO₂ system were investigated. Glasses featuring SiO₂/Li₂O molar ratios of 2.69–3.13, far beyond the lithium disilicate (LD-Li₂Si₂O₅) stoichiometry, were produced by conventional melt-quenching technique. The sintering and crystallization behaviour of glass powders was explored using hot stage microscopy (HSM), scanning electron microscopy (SEM), differential thermal (DTA) and X-ray diffraction (XRD) analyses. Increasing K₂O content at the expense of SiO₂ was shown to lower the temperature of maximum shrinkage, eventually resulting in early densification of the glass-powder compacts. Lithium metasilicate was the main crystalline phase formed upon heat treating the glass powders with higher amounts of K₂O. In contrast, lithium disilicate predominantly crystallized from the compositions with lower K₂O contents resulting in strong glass–ceramics with high chemical and electrical resistance. The total content of K₂O should be kept below 4.63 mol% for obtaining LD-based glass–ceramics.     

Rapid preparation of low thermal expansion transparent LAS nanocrystalline glass by one-step thermoelectric treatment

The Li₂O–Al₂O₃–SiO₂ (LAS) glass ceramics are prepared by one-step thermoelectric treatment. The influence of thermoelectric treatments on LAS glass-ceramics were studied. The crystal phase composition and microstructure of the LAS glass-ceramics were investigated by DSC, SEM, XRD and FTIR. Moreover, the thermal expansion performance and light transmittance of LAS nanocrystalline glass were characterized. The results show that low-expansion transparent LAS nanocrystalline glass can be produced in a short time by thermoelectric treatment. The free energy of nucleation and the degree of polymerization of the glass network are reduced by the electric field. The key is that the electric field polarizes Ti and Zr ions at the crystallization temperature, so that the crystal nuclei repel the same poles. This allows uniform crystal distribution, promotes crystallization and reduces one-step crystallization of crystal agglomeration. This proves that the rapid preparation of nanocrystalline glass by the one-step method is feasible, and provides a reference for the future one-step processing of glass-ceramics.