Rheotechnological properties of mixed suspensions in the SiO2 — Al2O3 system and some properties of materials base on them. II. Molten quartz — Bauxite system

Mixed suspensions in the molten quartz — bauxite system give castings with a much higher concentration and a lower porosity than HCBS based on alumina. Castings with a porosity of σ 14% have been obtained in mixed systems bearing 60 – 70% bauxite. The dependence of the porosity of the castings on the thermal expansion of heat-treated materials with a mixed composition is not additive. Materials bearing 50 – 80% bauxite have maximum strength properties. The ion potential of the solid phase of the initial suspension has the greatest effect on the rheotechnological properties of the suspensions. Transleted from Ogneupory i Technicheskaya Keramika, No. 8, pp. 25 – 29, August, 2000.     

Effect of Al2O3 and K2O content on structure,properties and devitrification of glasses in the Li2O–SiO2 system

The effect of Al₂O₃ and K₂O content on structure, sintering and devitrification behaviour of glasses in the Li₂O–SiO₂ system along with the properties of the resultant glass–ceramics (GCs) was investigated. Glasses containing Al₂O₃ and K₂O and featuring SiO₂/Li₂O molar ratios (3.13–4.88) far beyond that of lithium disilicate (Li₂Si₂O₅) stoichiometry were produced by conventional melt-quenching technique along with a bicomponent glass with a composition 23Li₂O–77SiO₂ (mol.%) (L₂₃S₇₇). The GCs were produced through two different methods: (a) nucleation and crystallization of monolithic bulk glass, (b) sintering and crystallization of glass powder compacts.Scanning electron microscopy (SEM) examination of as cast non-annealed monolithic glasses revealed precipitation of nanosize droplet phase in glassy matrices suggesting the occurrence of phase separation in all investigated compositions. The extent of segregation, as judged from the mean droplet diameter and the packing density of droplet phase, decreased with increasing Al₂O₃ and K₂O content in the glasses. The crystallization of glasses richer in Al₂O₃ and K₂O was dominated by surface nucleation leading to crystallization of lithium metasilicate (Li₂SiO₃) within the temperature range of 550–900 °C. On the other hand, the glass with lowest amount of Al₂O₃ and K₂O and glass L₂₃S₇₇ were prone to volume nucleation and crystallization, resulting in formation of Li₂Si₂O₅ within the temperature interval of 650–800 °C.Sintering and crystallization behaviour of glass powders was followed by hot stage microscopy (HSM) and differential thermal analysis (DTA), respectively. GCs from composition L₂₃S₇₇ demonstrated high fragility along with low flexural strength and density. The addition of Al₂O₃ and K₂O to Li₂O–SiO₂ system resulted in improved densification and mechanical strength.     

New fields of application of materials and articles from refractory fibers in the Al2O3 — SiO2 system

Basic properties of materials and articles from refractory fibers that save resources, cut heat loss, and reduce the consumption of material for furnaces are presented. A new direction in the development of materials and articles from refractory fibers for converter production is described     

Manufacturing process and properties of spherical granules in the MgO — Al2O3 — SiO2 system

A kaolin-talc mixture is used for developing a manufacturing process of spherical granules to be used as a propping agent in the oil industry. The functional properties of the granules are determined.     

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 (T_p) 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 (Na_0.45 Ca_0.55 Al_1.55 Si_2.45 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 m^1/2, 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.     

Characterization and sintering of gels in the system MgO–Al2O3–SiO2

Cordierite aerogels, made by supercritical drying, and xerogels, formed by ambient pressure drying, have been prepared by combining two different recipes. The chemical composition of the gels varied from stoichiometric cordierite 2MgO·Al₂O₃·5SiO₂ to 0·5MgO·1·4Al₂O₃·5SiO₂ due to different procedures for washing of the gels. The crystallization of nearly stoichiometric cordierite gels was shown to be relatively complex involving the formation of several metastable phases such as μ-cordierite (Mg₂Al₄Si₅O₁₈), spinel (Al₆Si₂O₁₃) and sapphirine (Mg₄Al₈Si₂O₂₀) before the equilibrium phase composition was obtained at around 1350°C. On the other hand, during crystallization of gels with stoichiometry close to 0·5MgO·1·4Al₂O₃·5SiO₂ the equilibrium phases mullite, cristobalite and α-cordierite were the major phases formed during heat treatment. A lower densification rate was observed for aerogels compared to xerogels due to a larger pore size. A lower crystallization temperature in aerogels probably due to heterogeneous nucleation reduced the densification. For gels with a composition near 0·5MgO·1·4Al₂O₃·5SiO₂ nucleation and densification occur simultaneously and large differences in the densification behavior was observed. ©     

Thermodynamics of Phase Relations in the Subsolidus of the MgO – Al2O3 – SiO2 System

Results of a thermodynamic analysis of the relation ΔG = f(T) for solid-phase reactions between stoichiometric components of the MgO – Al₂O₃ – SiO₂ system are reported and the thermodynamic conjugation of solid-phase reactions is considered. The thermodynamic instability of stoichiometric sapphirine and cordierite is discussed. A general scheme for the interaction between nonstoichiometric compounds, with allowance for the possible formation and phase decomposition of sapphirine and cordierite solid solutions and for the disproportion of stoichiometric sapphirine and cordierite, is proposed. It is shown that the structural changes in the crystal lattice of sapphirine and cordierite should be considered as arising from compositional changes of the corresponding solid solutions rather than polymorphous transformations. A diagram for phase relations in the subsolidus of the MgO – Al₂O₃ – SiO₂ system is proposed.     

The Viscosity of Molten Material in Pores and Capillary Channels of the Matrix Phase of High-Alumina Castables Employed in Thermal Power Units of Ferrous Metallurgy. 1. Viscosity Analysis of Melts in the CaO – Al2O3 –FeO – Fe2O3 – SiO2 System

A new approach to the viscosity analysis of high-alumina, low-calcium melts of the CaO – Al₂O₃ – FeO – Fe₂O₃ – SiO₂ system is used. A mathematical formalism for handling experimental viscosity data on the CaO – FeO – Fe₂O₃ – SiO₂ system is proposed that allows viscosity of the system to be evaluated within _av = 0.004 Pa · sec in the temperature range of 1500 – 1700°C at concentrations (mol.%) of CaO = 15.3 – 50.3, Fe_xO_y = 2.3 – 38.5, and SiO₂ = 17.7 – 57.7. An equation describing viscosity as a function of temperature is derived. Using this equation, the viscosity of model melts compositionally analogous to the products of interaction between molten slag and the glassy matrix phase of castables employed in thermal power units of ferrous metallurgy is considered.     

Effect of CaO doping on the properties and crystallization behavior of Y2O3–Al2O3–SiO2 glass

Nowadays, Y₂O₃–Al₂O₃–SiO₂ (YAS) glass joining is considered to be a promising scheme for nuclear-grade continuous silicon carbide (SiC) fiber reinforced SiC matrix composites (SiC/SiC). CaO has great potential for nuclear applications since it has low reactivity and low decay rate under nuclear irradiation. In this paper, the effect of CaO doping on the structure, thermophysical properties, and crystallization behavior of YAS glass was systematically studied. As the CaO doping content increased, the number of bridge oxygens and the viscosity at high temperatures reduced gradually. After heat treatment at 1400 °C, the main phases in YAS glass were β-Y₂Si₂O₇, mullite, and SiO₂ (coexistence of crystalline and glass phases), while that with 3.0% CaO doping turned into a single glassy phase under the same treatment conditions. Moreover, a structural model and the modification mechanism were proposed, which provided a theoretical basis for the subsequent component design and optimization.     

The Effect of Periodic Reactions in the MgO – Al2O3 – SiO2 System on the Phase Composition and Properties of Cordierite-Based Materials

Changes in phase composition and properties of cordierite-based materials depending on the calcination temperature and the calcined talcum/raw talcum ratio in the starting mixture is studied. Periodic features in the behavior of the MgO – Al₂O₃ – SiO₂ system are analyzed in terms of a conjugation scheme of solid-phase reversible chemical reactions. Routes for the spinodal phase decompositions of sapphirine and cordierite solid solutions are proposed. Conditions for formation of a dissipative structure in refractory materials and a specific organization of the phase composition under spinodal decomposition of solid solutions are considered.     

Microwave dielectric materials based on the MgO–SiO2–TiO2 system

Ceramics in the system MgO–SiO₂–TiO₂ were prepared by standard mixed oxide route. By adding ZnO–B₂O₃ to the starting mixtures, the firing temperature of the ceramics could be reduced to 1160 °C. Small additions of MnCO₃ and CaTiO₃ improve microwave dielectric properties leading to an increase in insulation resistance and a decrease in temperature coefficient of capacitance. By adding Co₂O₃ grain growth can be inhibited and the dielectric Qf value greatly increased. The resultant ceramic material exhibited low dielectric constant and low dielectric loss: relative permittivity (ε_r): 20±2; temperature coefficient of capacitance (τ_c): 0±30 ppm/°C; Qf: 100,000 (at 10 GHz); insulation resistance: 10¹³ Ω cm:     

Effect of solid loading on properties of Y2O3–Al2O3–Nd2O3 powder mixtures obtained by planetary ball milling and ceramics based on them

The effect of the solid loading (41–50 wt%) of the slurry on granulometric composition and physico-chemical characteristics of Y₂O₃–Al₂O₃–Nd₂O₃ powder mixtures obtained by planetary ball milling has been studied for the first time. It was shown that the particle size distribution of powder, its Zeta potential, and specific surface area depend on the solid loading of the milled slurry and, consequently, on the interparticle distance during milling. The interparticle distance decreases from 200 nm to 142 nm with an increase of solid loading in the range of 41–50 wt%. It was shown that for the solid loading of 47 wt%, the convergence of particles to a distance comparable to their median diameter promotes subsequent clustering of particles. This facilitates the sintering of highly-homogenous ceramics. It was found that solid loadings in the 46–50 wt% range is useful for obtaining high-quality Nd:YAG transparent ceramics. The lowest optical losses optical losses of 1 × 10^?3 cm^?1 and the highest in-line transmittance of 84.1%@1064 nm were obtained for 1 at.% Nd:YAG transparent ceramics (22 × 3 × 4 mm³) prepared from slurries with 47 wt% solid loading (taking all other ball milling parameters fixed). If the interparticle distance in the powder is higher (solid loading of 41 wt%) than the median particle diameter, the ceramics are characterized by significant residual porosity due to the survival of large particles (insufficient milling).     

Optical and mechanical properties of amorphous bulk and eutectic ceramics in the HfO2–Al2O3–Y2O3 system

Bulk glasses containing HfO₂ nano-crystallites of 20–50 nm were prepared by hot-pressing of HfO₂–Al₂O₃–Y₂O₃ glass microspheres at 915 °C for 10 min. By annealing at temperatures below 1200 °C, the bulk glasses were converted into transparent glass-ceramics with HfO₂ nano-crystallites of 100–200 nm, which showed the maximum transmittance of ～70% in the infrared region. An increase of annealing temperature (>1300 °C) resulted in opaque YAG/HfO₂/Al₂O₃ eutectic ceramics. The eutectic ceramics contained fine Al₂O₃ crystallites and showed a high hardness of 19.8 GPa. The fracture toughness of the eutectic ceramics increased with increasing annealing temperature, and reached the maximum of 4.0 MPa m^1/2.     

Influence of Synthesized Super Al2O3 Powder on the Properties of Alumina Castable

The in fluence of three different Al2O3 powder on self-flowing alumina castable was studied.Maximum compaction of sample was computed by Andresen Equation.The result showed that the Al2O3 powder.which has much smaller particle size,could improve the microstructure and the mechanical properties of castable under room temperature and high temperature.With the same powder size,the room tem perature strength of the castable added with Al2O3 powder properties of which were altered by adding mineralizing agent,was higher than that of the castable added with common Al2O3 powder ,but the flowability of these three different castable was almost the same.     

Glass-ceramics in the CaO–MgO–Al2O3–SiO2 system as potential dental restorative materials

This paper reports on development of novel alumina-containing glass-ceramics (GCs) with a high content of Al₂O₃ (12.5 wt.%) in the CaO–MgO–Al₂O₃–SiO₂ system aimed for dental restorations. The thermal properties of the parent glasses, the microstructure and the mechanical properties of the produced sintered and crystallized GCs along with bio-inertia performance were experimentally studied. Dense, white, and bio-inert GCs, comprised of melilite, either as a single-phase or with diopside, were produced. The values of flexural strength ranged between 120 and 171 MPa, the modulus of elasticity varied between 28 and 42 GPa, while the values of the hardness and the fracture toughness (measured by the indentation–Niihara equation) ranged from 6.3 to 7.0 GPa, and from 2.6 to 2.8 MPa m^0.5, respectively. The mechanical properties of the produced GCs, after being meticulously compared with the mechanical properties of GCs of various compositions reported in literature, including commercial ones, are a good match to the properties of dental hard tissues, and satisfy the requirements of the ISO 6872 “Dentistry-Ceramic Materials”.