Al2O3 SiO2 System Research Articles

Effect investigation of a thinning additive on the physical and mechanical properties of celsian ceramics obtained by slip casting

The article discusses the physical and mechanical regularities of ceramics formation based on the BaO—Al2O3—SiO2 system by slip casting into gypsum molds, and also investigates the rheological properties of a slip based on сelsian and the physicochemical properties of products made from it.
 The samples were made according to the following technology: the first stage is synthesis of celsian in a muffle furnace at a temperature of 1200 °C with a maximum temperature holding for 2 hours. The synthesis products were ground in a planetary mill to a residue on the № 0063 sieve of no more than 0.2 %. Then, slips with different moisture content without additives and with the addition of 0.10, 0.15 and 0.20 wt. % thinning additive Dolapix PC 67 were made. The second stage is to obtain samples by casting slip into gypsum molds. Firing of products took place in a muffle furnace at a temperature of 1350 °C with a two-hour holding.
 The optimal amount of thinning additive in the celsian slip was determined, which corresponds to 0.10 wt. %. It was found that, in the studied area of technological parameters, the material acquires the best properties at a sample firing temperature of 1350 °C and is characterized by the following indicators: water absorption — 10.20 %, apparent density — 2.15 g/cm3, open porosity — 21.04 %.

STUDY OF THE INFLUENCE OF INTENSIFYING ADDITIVES ON LOW-TEMPERATURE SYNTHESIS OF SLAVSONITE AND CELZIAN IN THE CREATION OF RADIOTRANSPARENT CERAMIC MATERIALS

The studies carried out concern the development of structural radiotransparent ceramic materials based on the four-component system BaO–SrO–Al2O3–SiO2. Due to the widespread use of radar equipment at modern aviation facilities, the development of radiotransparent fairings for them is a rather important and urgent task.The purpose of the fairings is to protect the antenna devices of radar stations from environmental influences during flight. Based on this, the fairings must meet a complex set of requirements for aerodynamic, thermal, radio engineering and mechanical properties. These requirements at supersonic flight speeds of modern objects increase significantly, since the improvement of the aerodynamic shape, the increase in the mechanical strength and thermal stability of the fairings contradicts the interests of radio engineering, leading to a significant deterioration in their radiotransparency and to distortions of antenna directional patterns. The consequence is a decrease in the range of radar stations and serious deterioration in their accuracy characteristics.The aim of the research was to obtain, on the basis of the BaO–SrO–Al2O3–SiO2 system, crystalline phases of slavsonit and celsian at low temperatures and times of synthesis, by introducing sintering intensifiers with fluxing and modifying action.The influence of a number of additives on the intensification of the low-temperature synthesis of slavsonit and celsian is investigated. The positive effect of the eutectic additive of the SnO2–Li2O system on the preparation of a densely sintered ceramic material based on solid solutions of slavsonit and monoclinic celsian is shown. It has been established that, according to its dielectric properties, the obtained ceramic material can be classified as structural radio-transparent materials.

Effect of glass content on sintering kinetics, microstructure and mechanical properties of glass-ceramics from coal fly ash and waste glass

In this paper, coal fly ash and waste glass were used as raw materials to prepare CaO–Al2O3–SiO2 system glass-ceramics by direct sintering. The effects of glass content on the kinetics, phase, microstructure and mechanical properties of glass-ceramics were studied by means of liquid phase sintering formula, X-ray diffraction, scanning electron microscope and the corresponding property tests. The results showed that as glass content increased, the activation energy of glass-ceramics first decreased and then increased due to the precipitation of tridymite and augite, and the surface of glass-ceramics gradually became smooth and dense. The water absorption and total porosity of the glass-ceramics were decreased with increase of glass content，while the flexural strength and bulk density were increased. When the amount of waste glass added is 40% and the sintering temperature is 1100 °C, the glass-ceramics have dense surface and high bending strength, which is suitable for high-strength building glass-ceramics. The experimental results of this paper provide a basis for direct sintering industrial production of glass-ceramics.

Mica (KMg3AlSi3O10F2) based glass-ceramic composite sealant with thermal stability for SOFC application

In this work, pre-synthesized fluorophlogopite mica (KMg3AlSi3O10F2) crystalline powder was added into potassium-magnesium boro-alumino-silicate (K2O–MgO–B2O3–Al2O3–SiO2) glass powder to synthesize glass-ceramic composite (GCC) sealant for solid oxide fuel cell (SOFC) application. The basic purpose was to report the influence of KMg3AlSi3O10F2 on microstructure, thermal expansion and volume shrinkage behavior of GCCs in terms of their SOFC sealing ability. The predominant crystalline phase was also identified as fluorophlogopite mica (KMg3AlSi3O10F2) by X-ray diffraction in the GCCs heat-treated at 900 °C (for 2 h) followed by 800 °C (for 10 h). Higher thermal expansion (10.65–10.81 × 10−6/K at 50–800 °C) compatible with SOFC components (metallic electrode/interconnect, ceramic electrolyte) was realized for 5–10 wt % mica containing GCC. However, it was found decreased linearly with adding further KMg3AlSi3O10F2 content (20–40 wt%) in the composites. GCC with 5 wt % of mica was stable under thermal cycling testing and no significant degradation was observed in thermal expansion till 10 cycles. Field emission scanning electron microscopy revealed the spherical shaped nanocrystallites morphology of those GCCs and that was ascribed for their compact bonding with SOFC components. Heating stage microscopy (HSM) ensured the lower softening temperature (752 ± 7 °C) for 5 wt % mica containing GCC which furthermore showed 11–13% volume shrinkage (to ensure the self-healing) in SOFC operating temperature. In a summary, the above factors confirmed the suitability of 5 wt % mica (KMg3AlSi3O10F2) containing K2O–MgO–B2O3–Al2O3–SiO2 system (GCC) for SOFC sealant application.

Cyclic thermal shock resistance of h-BN composite ceramics with La2O3–Al2O3–SiO2 addition

The effects of La2O3–Al2O3–SiO2 addition on the thermal conductivity, coefficient of thermal expansion (CTE), Young's modulus and cyclic thermal shock resistance of hot-pressed h-BN composite ceramics were investigated. The samples were heated to 1000 °C and then quenched to room temperature with 1–50 cycles, and the residual flexural strength was used to evaluate cyclic thermal shock resistance. h-BN composite ceramics containing 10 vol% La2O3–Al2O3 and 20 vol% SiO2 addition exhibited the highest flexural strength, thermal conductivity and relatively low CTE, which were beneficial to the excellent thermal shock resistance. In addition, the viscous amorphous phase of ternary La2O3–Al2O3–SiO2 system could accommodate and relax thermal stress contributing to the high thermal shock resistance. Therefore, the residual flexural strength still maintained the value of 234.3 MPa (86.9% of initial strength) after 50 cycles of thermal shock.

Formation of Spessartine and CO2 via Rhodochrosite Decarbonation along a Hot Subduction P-T Path

Experimental simulation of rhodochrosite-involving decarbonation reactions resulting in the formation of spessartine and CO2-fluid was performed in a wide range of pressures (P) and temperatures (T) corresponding to a hot subduction P-T path. Experiments were carried out using a multi-anvil high-pressure apparatus of a “split-sphere” type (BARS) in an MnCO3–SiO2–Al2O3 system (3.0–7.5 GPa, 850–1250 °C and 40–100 h.) with a specially designed high-pressure hematite buffered cell. It was experimentally demonstrated that decarbonation in the MnCO3–SiO2–Al2O3 system occurred at 870 ± 20 °C (3.0 GPa), 1070 ± 20 °C (6.3 GPa), and 1170 ± 20 °C (7.5 GPa). Main Raman spectroscopic modes of the synthesized spessartine were 349–350 (R), 552(υ2), and 906–907 (υ1) cm−1. As evidenced by mass spectrometry (IRMS) analysis, the fluid composition corresponded to pure CO2. It has been experimentally shown that rhodochrosite consumption to form spessartine + CO2 can occur at conditions close to those of a hot subduction P-T path but are 300–350 °C lower than pyrope + CO2 formation parameters at constant pressures. We suppose that the presence of rhodocrosite in the subducting slab, even as solid solution with Mg,Ca-carbonates, would result in a decrease of the decarbonation temperatures. Rhodochrosite decarbonation is an important reaction to explain the relationship between Mn-rich garnets and diamonds with subduction/crustal isotopic signature.

Viscosity Estimation of Multicomponent Slags of the CaO–SiO2–Al2O3–FexO System Based on Microstructure Analysis

Slag viscosity is key factor of liquid steel refining and the smooth slag tapping of entrained flow coal gasifiers, and the accurate estimation of slag viscosity can provide a guidance for the selection of industrial raw material and the control of industrial process. In this study, a structure-related viscosity model about the multicomponent slags of the CaO–SiO₂–Al₂O₃–FeₓO system is developed based on the flow mechanism of silicate melt to improve slag viscosity behavior. The present estimation model improved the expression of activation energy equation on the basis of the model proposed by Nakamoto and expanded the application from the ternary CaO–SiO₂–Al₂O₃ system to the quaternary CaO–SiO₂–Al₂O₃–FeₓO system. Then, the structural roles of all the components were clarified, and the types of oxygen bonds were defined by Raman spectroscopy. The fractions of various oxygen bonds were obtained, and the database of oxygen bonds in any composition within the range of experimental slags was generated by utilizing the Lagrange interpolation method. The result showed that the present model had an advantage in estimating slag viscosity within the compositional range of this experiment slag, and the mean deviations between the viscosities calculated by the present model and measured or published viscosities were within 20%. The model will offer a reference for the estimation and adjustment of slag behavior in the metallurgical slag or coal slag, especially for the iron-rich system.

The viscosity and conductivity of the molten glass and crystallization behavior of the glass ceramics derived from stainless steel slag

The glass and glass ceramics in the CaO–MgO–Al2O3–SiO2 system were successfully synthesized from the stainless steel (SS) slag. The effect of slag content on the viscosity and conductivity of the glass melt, as well as the crystallization behavior and microstructure of the glass ceramics were investigated. The results proved that the viscosity of the molten glass decreases with increasing temperature and SS slag content; however, an inverse relation is observed in case of conductivity. Furthermore, the logarithm of viscosity varies linearly with that of conductivity. Thus, the viscosity and conductivity values can be calculated with respect to each other under melting conditions. The crystallization activation energies range from 335 to 473 kJ/mol, which are significantly higher than the viscous energies and conductance activation energies. Additionally, the obtained glass ceramics comprise augite, gehlenite, and wollastonite. Furthermore, augite grain coarsening and the number of crystals increase with increasing SS slag content.

Preparation of glass–ceramic materials from coal ash and rice husk ash: Microstructural, physical and mechanical properties

Wastes such as coal and rice husk ashes, which are widely available in Colombia, were successfully used to synthesize glass-ceramics in the (Na2O)–CaO–Al2O3–SiO2 system, which are obtained from thermally treating the parent glasses. The raw materials were mechanically conditioned, and the glasses were designed based on the CaO/SiO2 molar ratio, which was varied between 0.25 and 0.39. The glasses were obtained by melting the powders at 1450°C for 2h, and the melted powder was then poured into water. To obtain the glass–ceramic material, the temperature of the glass thermal treatment, which was generally lower than 1000°C in all cases, was determined by differential thermal analysis. The glass-ceramics obtained were microstructurally, physically and mechanically characterized. In addition, the durability was determined in acidic and alkaline environments (HCl and NaOH solutions). Glass–ceramics with densities of 2607–2739kg/m3, water absorption below 0.1%, Vickers hardness above 600MPa and elastic modulus of ∼96GPa were obtained. The fracture toughness Kic was in the range of 0.39–0.59MPam1/2. The chemical durability was considered excellent (with mass losses of ∼0.5mg/cm2), therefore these glass–ceramics can be good candidates for different applications in the construction sector.

Corrosion degradation of mullite subject to carbon monoxide atmosphere at 1000oC–1600°C

AbstractThus far, studies on the damage to refractory materials under carbon monoxide atmospheres have mostly concentrated on the effects of carbon deposition, and the testing temperature was always set at approximately 500°C to promote the deposition of carbon. However, this testing temperature is far below the operating temperature of most refractories. In this study, mullite, a widely used high‐temperature structural material, was subjected to a carbon monoxide atmosphere at 1000°C‐1600°C to investigate its phase and microstructural evolutions. Changes to the grain boundaries were initially observed in mullite specimens treated at 1000°C and 1200°C. After treatment at 1400°C, the specimen surface comprised α‐Al2O3, a glass phase, and a small amount of mullite. However, treatment at 1600°C resulted in only α‐Al2O3 and a small amount of glass phase on the surface. Additionally, pores and voids were found in the glass phase on the surface and in the bulk of the specimens treated at 1400°C and 1600°C. This study demonstrated the stability of pure mullite in a carbon monoxide atmosphere and revealed that impurities accelerating generation of the liquid phase in Al2O3–SiO2 system significantly affect the stability of mullite in a carbon monoxide atmosphere.

Experimental Modeling of Decarbonation Reactions Resulting in Mg,Fe-garnets and CO2 fluid at the Mantle P–T parameters

Abstract —Experimental modeling of decarbonation reactions with the formation of Mg,Fe-garnets and CO2 fluid during mantle–crust interactions was carried out in a wide range of the upper-mantle pressures and temperatures. Experimental studies were performed in the MgCO3–Al2O3–SiO2 and (Mg,Fe)CO3–Al2O3–SiO2 systems in the pressure range 3.0–7.5 GPa and temperature range 950–1450 °C (t = 10– 60 h), using a multianvil high-pressure apparatus of the “split-sphere” type (BARS). Experiments were carried out with a specially designed high-pressure buffered cell with a hematite container that prevents the diffusion of hydrogen into a Pt-capsule with a sample. It has been experimentally established that in the MgCO3–Al2O3–SiO2 system decarbonation occurs by the schematic reaction MgCO3 + SiO2 + Al2O3 → Mg3Al2Si3O12 + CO2 at 1100 ± 20 °C (3.0 GPa), 1150 ± 20 °C (6.3 GPa), and 1400 ± 20 °C (7.5 GPa) and in the (Mg,Fe)CO3–Al2O3– SiO2 system, by the reaction (Mg,Fe)CO3 + SiO2 + Al2O3 → (Mg,Fe)3Al2Si3O12 + CO2 at 1000 ± 20 °C (3.0 GPa), 1150 ± 20 °C (6.3 GPa), and 1400 ± 20 °C (7.5 GPa). Based on Raman spectroscopic characterization of the synthesized garnets, the position of the main modes R, υ2, and υ1 in the pyrope has been determined to be 364, 562, and 924–925 cm-1, respectively, and that in pyrope-almandine, 350–351, 556–558, and 918–919 cm-1. The effectiveness of the hematite container was demonstrated by means of mass spectrometry analysis. It has been found that the fluid composition corresponded to pure CO2 in all experiments. The P,T-positions of decarbonation curves leading to the formation of a CO2 fluid in assemblage with pyrope and pyrope-almandine have been experimentally reconstructed and compared with the previous calculation and experimental data. It has been established that the experimentally reproduced reaction lines with the formation of pyrope + CO2 or pyrope-almandine + CO2 assemblages are shifted to lower temperatures by 50–150 °C relative to the calculated ones. When considering the obtained results with regard to the stability of natural carbonates of various compositions in subduction settings, it has been found that at depths of ~90–190 km Mg,Fe-carbonates react with oxides in the temperature range 1000–1250 °C, and at depths of ~225 km, at 1400 °C.

Phase Transformations in High-Temperature Fiber Materials Exposed to Non-Equilibrium Flow of Heat and Light

The changes of the phase composition and structure of modern ceramic materials on exposure to an intense flow of light were investigated. The technological possibilities of a unique facility — Big Solar Furnace — were used to perform the experiment. It was shown that surface treatment of a ceramic material in the system Al2O3–SiO2 by a high-temperature source of energy results in the appearance of two new phase compositions — α-Al2O3 and cristobalite. Treatment of the material in the fusion regime promotes the formation of a eutectic component.

Effect of different contents of zirconia and yttrium oxide on microstructure and properties of high alumina ceramics

AbstractThe effects of zirconia and yttrium oxide addition on microstructure, bulk density, microhardness, flexural strength, and wear resistance of high alumina ceramics (>97 wt% Al2O3, MSA ceramics) composed of MgO–SiO2–Al2O3 system have been investigated. The results show that the addition of zirconia makes the mechanical properties and wear properties of ceramics composed of MgO–SiO2–Al2O3–ZrO2 (MSAZ ceramics) system have been greatly improved compared with MSA ceramics. In addition, the ceramics composed of MgO–SiO2–Al2O3–ZrO2–Y2O3 (MSAZY ceramics) system have better mechanical properties and wear properties than MSAZ ceramics. With the contents of zirconia and yttrium oxide increase, the bulk density, microhardness, and flexural strength of MSAZ and MSAZY ceramics increased at first and then decreased. However, the wear rate shows the opposite. When 0.4 wt% ZrO2 and 0.6 wt% Y2O3 were added to the matrix, the wear rate of MSAZY ceramics reached a minimum of 0.042%, and the wear resistance was improved by about 73.8% compared with MSA ceramics with a wear rate of 0.16%. In addition, the optimum additions of zirconia and yttria are 0.4% and 0.6%, respectively.

A simple and clean method to prepare SiC-containing vitreous ceramics for solar thermal storage in the clay-feldspar system

A simple and clean in-situ method was designed to prepare SiC whisker-reinforced vitreous ceramics for solar thermal storage. SiC whiskers were in-situ synthesized in the clay-feldspar system via a carbon-buried sintering process to improve the thermal properties of vitreous ceramics. The synthesis mechanism and the optimized processing procedure of SiC-containing vitreous ceramics from the clay-feldspar system were obtained. Results indicated that the in-situ SiC whiskers were synthesized inside the pores of vitreous ceramic matrix through the vapor-solid mechanism and covered by the liquid of K2O–Al2O3–SiO2 system. SiC whiskers with a diameter of 9–23 nm decreased the thermal expansion coefficients of the vitreous ceramics significantly from 6.59 × 10−6 °C−1 to 4.23 × 10−6 °C−1 (room temperature∼1000 °C). Meanwhile, the thermal conductivity increases to 4.78 W·(m·k)−1, 2.6 times higher than that of the original products. The increased porosity was found to improve the amount of SiC whiskers by providing space for their growth. SiC whiskers also enhanced the mechanical properties of the products and the sample with the largest content of SiC exhibited the highest bending strength of 75.7 MPa. The introduction of SiC whiskers through the in-situ synthesis method makes vitreous ceramics more competent for solar thermal storage applications. The in-situ method can save the energy and costs needed for separately producing SiC whiskers and prevent the inhalation toxicity of SiC whiskers.

Effect of ZnO content on the physical, mechanical and chemical properties of glass-ceramics in the CaO–SiO2–Al2O3 system

The objective of this study was to evaluate the effect of ZnO content on the physical, mechanical and chemical properties of CaO–Al2O3–SiO2 (CAS) glass-ceramics produced from Colombian wastes, such as fly ash, granulated blast furnace slag and glass cullet. The CaO/SiO2 molar ratio of the mixtures was held constant (0.36). ZnO was added to the mixtures in proportions of 4, 7 and 10 wt%. The glass-ceramics were produced by the controlled crystallization of a parent glass. The values of crystallization temperature (Tp) show a fall up to 7 wt% and then shoots up with 10 wt% concentration of ZnO, but in general, ZnO addition lowers the temperature required for the formation of crystalline phases. In general, anorthite (CaAl2Si2O8) is the main phase observed in all heat treated samples, in addition to albite (Na(AlSi3O8)) and labradorite (Na0.45 Ca0.55 Al1.55 Si2.45 O8). The crystalline phases hardystonite (Ca2ZnSi2O7) and willemite (Zn2SiO4) were also identified in the samples with 7 and 10 wt% ZnO. The densities of the glass-ceramics were between 2658 and 2848 kg/m3, and it was found that ZnO helps to increase the density of glass-ceramics. The elastic modulus was in the 100–105 GPa range, the fracture toughness was between 0.45 and 0.64 MPa m1/2, and the Vickers microhardness was between 632 and 653 MPa. With regards to the durability, the weight loss of the glass-ceramics immersed in alkaline solution (NaOH) did not exceed 1.5 wt% after immersion for 6 h at 80 °C. The results of this study confirm that the vitrification process is a favorable option to utilize these industrial wastes.

Structural Understanding of MnO–SiO2–Al2O3–Ce2O3 Slag via Raman, 27Al NMR and X-ray Photoelectron Spectroscopies

In order to understand sulfur solubility in the MnO–SiO2–Al2O3–Ce2O3 slag or inclusion system (mol MnO/mol SiO2 = M/S = 2.2 or 0.9, Al2O3 = 14.2[± 1.8] mol%, Ce2O3 = 0–5.6 mol%), the effect of Ce3+ ions on the structure of the Mn–aluminosilicate system with different M/S molar ratios has been studied by micro-Raman spectroscopy, solid-state nuclear magnetic resonance (NMR) spectroscopy and X-ray photoelectron spectroscopy (XPS). Because constant oxygen partial pressure was maintained at p(O2) = 2.8 × 10−7 atm, oxidation state of Mn and Ce would be in Mn2+ and Ce3+ in the MnO–SiO2–Al2O3–Ce2O3 system, which were also confirmed by XPS analysis. Although it was difficult to measure the 27Al solid-state NMR spectra of the quenched MnO–SiO2–Al2O3 system due to the paramagnetic effect, 27Al NMR spectra did show structural changes in aluminate when cerium was added to the Mn–aluminosilicate melts. Addition of Ce2O3 in the high M/S(= 2.2) system caused both an explosive enhancement in the intensity of Raman bands at 600 cm−1 and a peak shift in 27Al NMR spectra, indicating the transition from the [AlO4]–:0.5Mn2+ tetrahedron to a [AlO6]3–:Ce3+ octahedron complex due to a strong attraction between aluminate and the cerium ion. XPS spectra show that the transition of the aluminate structure upon introduction of Ce3+ ions consumes free oxygen in Mn–aluminosilicate melts in high M/S(= 2.2) system. However, the same structural changes were not observed when Ce2O3 was added to lower M/S(= 0.9) system, because Ce3+ ions primarily interact with the pre-existing [AlO6]-octahedron in low M/S system.

Separation of Cenospheres from Lignite Fly Ash Using Acetone–Water Mixture

This work reports on the separation of cenospheres from lignite fly ash through a wet separation process-the sink-float method. A better quality of cenospheres could be achieved through a physical–chemical approach using an acetone–water mixture as a medium. This work aimed to elucidate the correlation between the structure, morphology, and composition and medium fraction variables, with data for the freshly prepared and the reused mixtures presented for comparison. The work covers a study of the macrocomponent composition of an Fe2O3–SiO2–Al2O3 system, highlighting the pair dependences of SiO2–Al2O3, Al2O3–Fe2O3, and SiO2/Al2O3–Al2O3 and revealing an interesting result in terms of geochemical characteristics categorizing the collected cenosphere fractions separated from high-calcium class C fly ash produced from a lignite coal power plant in Thailand (as magnetic cenospheres). The CaO and SO3 contents increased monotonically with increased water content, particularly for the CaO composition profile, which was found to be similar to the increased carbonate concentration measured from the mixtures after use. The physicochemical properties in terms of the self-association ability of the acetone–water mixing phase is believed to have played an important role in determining the intermolecular interactions and reactivity of ions in the liquid phase, consequently affecting the separation efficiency, recovery yield, and quality of cenospheres.

Structural and Phase Transformations in Plasma-Spray ZrB2–SiC–AlN Coatings on a C/C–SiC Substrate After High-Temperature Thermal Cyclic Heating

The composition and structure of a plasma-spray coating produced from a 60ZrB2 + 20SiC + 20AlN (wt.%) composite powder on a C/C–SiC substrate were examined. The coating 320–370 μm thick is characterized by a heterophase structure, has no pores or cracks, and densely adheres to the substrate. The coating phase composition corresponds to that of the starting powder. A coated sample was oxidized in a supersonic oxygen–propane/butane flow at ~2000°C in thermal cyclic mode (2 min heating, 10 min cooling, 15 cycles). The oxidation rate was <7 μm/min. The main coating phases in the near-surface layers were zirconium dioxide m-ZrO2 and a mullite Al2SiO5 solid solution. Cross-sectional microstructural and elemental chemical analyses of the starting and oxidized coatings established the high-temperature oxidation mechanism. An Al2SiO5-based layer with inclusions of spheroidal m-ZrO2 grains 1–2 μm in size was found to form. Thin SiO2-based amorphous films in the SiO2–Al2O3 system developed on the layer surface. The results testify that the coating is resistant to high-temperature oxidation to ≤2000°C.

Effect of B2O3 on the crystallization, structure and properties of MgO–Al2O3–SiO2 glass-ceramics

The formation of α-cordierite glass-ceramics in the MgO–Al2O3–SiO2 system without and with addition of B2O3 was investigated. Moreover, the effects of different contents of B2O3 on the crystallization behavior, structure variation, thermal and physical properties of glass-ceramics were studied, and the action mechanism of B2O3 was systematically discussed by XRD, FT-IR and XPS. The results show that a proper amount of B2O3 shortens the crystallization route and reduces formation temperature of α-cordierite, which further promotes the direct precipitation of α-cordierite in MAS system. The glass-ceramics undergo the phase transitions from intermediate phases of MgO·Al2O3·SiO2 and Mg0·6Al1·2Si1·8O6 to pure α-cordierite without the addition of B2O3. However, the pure α-cordierite directly precipitates in the glass-ceramics adding 4 mol% B2O3 without intermediate phases. Low B2O3 content (4 mol%) promotes the crystallization of α-cordierite, while high B2O3 content (8–12 mol%) inhibits it, which is accompanied by the increase of thermal expansion coefficient from 1.70 × 10−6 °C−1 to 3.72 × 10−6 °C−1 and the decrease of Vickers hardness from 9.4 GPa to 6.9 GPa. B2O3 is confirmed to be very important in controlling the crystallization process of α-cordierite in this glass-ceramics system, and this study is helpful for providing the reference value for others to choose B2O3 and its content.

Phase and microstructural evolution in lithium silicate glass‐ceramics with externally added nucleating agent

AbstractDevelopment of lithium disilicate‐based glass‐ceramics critically depends on use of nucleating agent in the glass matrix. The present study reports the effect of externally added nucleating agent Li3PO4 in Li2O–K2O–MgO–ZnO–ZrO2–Al2O3–SiO2 system which is compared with a reference composition (GC1) (SiO2:Li2O = 2.16:1) prepared with in situ formed Li3PO4. For externally added Li3PO4, two compositions were studied. In one case (GC2) before addition of Li3PO4, SiO2:Li2O ratio in glass was maintained as 2.87:1 and in another case (GC3) SiO2:Li2O ratio in glass was maintained same as reference GC1 that is, 2.16:1. The glasses were characterized by using MAS‐NMR spectroscopy. Sintering and crystallization behavior of the glass‐ceramics was characterized by using XRD, SEM, DTA. Due to in situ formation of Li3PO4, GC1 resulted in a dense sample with finer crystals of lithium disilicate. In GC2 and GC3, externally added lithium phosphate, which was in the form of ultrafine aggregated particles, formed flower‐like colonies of radially outward crystals. Higher SiO2:Li2O ratio in GC2 resulted in lithium disilicate crystals and high viscous glass causing large air entrapment and so less densification. GC3 with higher lithia in glass showed higher densification than GC2 but only lithium metasilicate crystals were formed.