1250°C liquidus for the CaO–MgO–SiO2–Al2O3–TiO2 system in air

Ti-bearing blast furnace slags have been regarded as an important secondary material in modern society, and the efficient recycling of Ti oxides from it is of key interest. For this reason, more thermodynamic data is needed regarding the phase relations in different composition ranges and sections. Therefore, the equilibrium phase relations of CaO–MgO–SiO₂–Al₂O₃–TiO₂ system in a low w(CaO)/w(SiO₂) ratio of 0.6–0.8 at 1250 °C in air and fixed concentrations of MgO and Al₂O₃, were investigated experimentally using a high temperature equilibration and quenching method followed by SEM-EDS (Scanning Electron Microscope and Energy Dispersive X-ray Spectrometer) analyses. The equilibrium solid phases of perovskite (CaO·TiO₂), a pseudo-brookite solid solution (MgO·2TiO₂, Al₂O₃·TiO₂)_ss, and anorthite (CaO·Al₂O₃·2SiO₂) were found to coexist with the liquid phase at 1250 °C. The calculated results of Factsage and MTDATA were used for comparisons, and significant discrepancies were found between predictions and the experimental results. The 1250 °C isotherm has been constructed and projected on the CaO–SiO₂–TiO₂-8 wt.% MgO-14 wt% Al₂O₃ quasi-ternary plane of the phase diagram. The obtained results provide new fundamental data for Ti-bearing slag recycling processes, and they add new experimental features for thermodynamic modeling of the high-order titanium oxide-containing systems.     

Effect of BaO on the phase composition and properties of aluminates for Ba-W cathodes

In this study, we prepared (4.8+x)BaO·CaO·2Al₂O₃ (0?≤?x?≤?1.6) aluminates by calcining the precursors under static air at 1500?°C for 120?min. The precursor powders were prepared using a liquid phase co-precipitation method. The effects of the molar content of BaO on the phase composition (before and after melting), melting properties, environmental stability, evaporation, and emission properties of the aluminates was investigated systematically The results showed that the phase of the aluminates completely transformed from Ba₅CaAl₄O₁₂ to Ba₃CaAl₂O₇ with an increase in the BaO content. After melting, the phase changed from Ba₅CaAl₄O₁₂ to a mixed phase of Ba₅CaAl₄O₁₂ and Ba₃CaAl₂O₇. In the high-temperature molten state, the aluminates were in the ionic state, which generated a relatively low-energy Ba₅CaAl₄O₁₂ phase during cooling crystallization. With every 0.4?mol increase in the BaO content, the initial melting temperature of the aluminates decreased by 10–20?°C, while the environmental stability deteriorated gradually. When the aluminates reacted with H₂O and CO₂ in the air, the original phase still existed and the characteristic peaks gradually broadened, but with the formation of Ca(OH)₂, CaCO₃, and BaCO₃. At 1050?°C, with an increase in the BaO content, the evaporation rate of the Ba-W cathodes increased and the emission current density first increased and then decreased. The main components of the Ba-W cathode evaporation were Ba and BaO. At n(BaO):n(CaO):n(Al₂O₃)?=?6:1:2, the Ba-W cathode showed the best emission performance, and its pulse emission current density at 1050?°C was as high as 35.31?A/cm².     

Effect of High‐Energy Ball Milling and Silica Fume Addition in BaCO3–Al2O3. Part I: Formation of Cementing Phases

The effects of high‐energy ball milling and subsequent calcination on the formation of barium aluminate cementing phases using mixtures of Al₂O₃ and BaCO₃ were investigated. Silica fume was further added in the raw mixtures to observe its role on the cementing phase formation. Results indicated that the decomposition temperature of BaCO₃ lowered remarkably with the increase in milling time. Barium aluminate cements with grain size in nanometer range were obtained from high‐energy ball‐milled raw mixtures. X‐ray diffraction (XRD) results confirmed several crystalline barium‐silicate and barium aluminate phases present. Formation of crystalline BaO·Al₂O₃ phase was observed between 1000°C and 1100°C in the raw mixtures, which were obtained in amorphous state after milling for 5 h. This temperature is at least 300°C lower than that used in the traditional solid‐state method. Fume SiO₂ additions resulted in BaO·Al₂O₃·2SiO₂ (celsian) formation which acted as a retarder, provides more workability and mechanical strength.     

Controllable Preparation of Al2O3‐MgO·Al2O3‐CaO·6Al2O3 (AMC) Composite with Improved Slag Penetration Resistance

Compact Al₂O₃‐MgO·Al₂O₃‐CaO·6Al₂O₃ (AMC) composite was obtained by melting technology using industrial alumina, light‐burned magnesia, and quick lime as raw materials based on the Al₂O₃‐MgO‐CaO ternary phase diagram. The results show that the phases of MgO·Al₂O₃ and Al₂O₃ are formed as the main framework with plate‐like CaO·6Al₂O₃ crystals mainly discontinuously embedded in MgO·Al₂O₃. The bulk density of AMC composite is up to 3.42 g/cm³, equivalent to 90.5% of the theoretical density. The synthesized compact AMC composite in the work also exhibits better slag penetration resistance than the castable based on tabular corundum due to the formation of liquid phase.     

Experimental determination of the 1300 °C and 1400 °C isotherms for CaO–SiO2–TiO2-10 wt% Al2O3 system in air

Phase equilibrium relations for the CaO–SiO₂–TiO₂-10 wt%Al₂O₃ system were studied at 1300 °C and 1400 °C by a high temperature equilibration and quenching method, with the purpose of extending the equilibrium phase information for the TiO₂-containing slag systems. Equilibrium phase assemblies and phase compositions were analyzed by Scanning Electron Microscopy (SEM) equipped with an X-ray Energy Dispersive Spectroscopy (EDS). The equilibrium solid phases of perovskite CaO·TiO₂, wollastonite CaO·SiO₂, rutile TiO₂, silica SiO₂ and sphene CaO·SiO₂·TiO₂ were confirmed to coexist with a liquid phase. Based on the experimental results, the 1300 °C and 1400 °C isotherms were constructed in the CaO–SiO₂–TiO₂-10 wt% Al₂O₃ quasi-ternary phase diagram. Significant discrepancies were found between the present results and the simulated results by FactSage and MTDATA databases, as well as the results from previous literature.     

Effect of Sodium on Microstructures and Thermoelastic Properties of Calcium Aluminate Cement–Bonded Refractories

The effect of sodium on refractory phase formation in a model Calcium Aluminate Cement–bonded refractory was investigated from 700°C to 1500°C. Sodium reacts with α‐alumina to form sodium β‐alumina (β‐Al₂O₃) via the intermediate NaAlO₂. Formation of β‐Al₂O₃ disrupts the reaction path of calcia with alumina, delaying crystallization of calcium hexaluminate, CaO·6Al₂O₃, from 1350°C to 1500°C. β‐Al₂O₃ is also shown to reduce Young's modulus and delay sintering. The presence of NaAlO₂ and β‐Al₂O₃ result in an increase in internal friction. Increased linear expansion of up to 47% is observed when 1 wt% Na is added. The expansion is shown to scale with the amount of dopant with only 0.3 wt% Na leading to an additional 31% linear expansion. On cooling, the presence of β‐Al₂O₃ can be demonstrated by a peak in internal friction between 1200°C and 1000°C which could be caused by Na⁺ ion hopping along the spinel‐like planes.     

Preparation of calcium aluminate cement by combustion synthesis and application for corundum‐based castables

Calcium aluminate cement was prepared by combustion synthesis with CaO₂, Al, and Al₂O₃ as raw materials. The effects of CaO/Al₂O₃ (C/A) molar ratios in raw materials on the phase compositions and morphologies of calcium aluminate were investigated in detail. It was found that when the C/A reduced from 1.1 to 0.74, the content of CaO·2Al₂O₃ (CA₂) in products increased, whereas contents of CaO·Al₂O₃ (CA) and 12CaO·7Al₂O₃ (C₁₂A₇) decreased; when the C/A was 0.8, the phase composition of product (CS71) was equal to that of Secar71. Additionally, the crystallines of CA and CA₂ in the product were reduced when the C/A molar ratio was decreased. And then, the bulk density, apparent porosity, permanent linear change, cold crushing strength (CCS), and cold modulus of rupture (CMOR) of the corundum‐based castables bonded with CS71, Secar7 were compared. The castables bonded with CS71 demonstrate obviously improved CCS, CMOR, and volume stability.     

Ion-exchange characteristics of 12CaO·7Al2O3 for halide and hydroxyl ions

Substitution characteristics of the halide ions F⁻ Cl⁻ for the OH⁻ ions in the crystal lattice of 12CaO·7Al₂O₃ solid solution were investigated. Single phases of composition 11CaO·7Al₂O₃·CaF₂ and 11CaO·7Al₂O₃·CaCl₂ were formed at 900 °C or above. The OH⁻ ions in 12CaO·7Al₂O₃ solid solution, i.e. 11CaO·7Al₂O₃·Ca(OH)₂, could be replaced wholly or partially by F⁻ or Cl⁻ ions from the corresponding calcium halide, forming 11CaO·7Al₂O₃·Ca(OH,F)₂ and 11CaO·7Al₂O₃·Ca(OH,Cl)₂ solid solutions above 500 °C and above 700 °C, respectively. Lattice constants of 12CaO·7Al₂O₃ solid solution changed continuously with the proportion of F⁻ ions or Cl⁻ ions. The F⁻ ions in 11CaO·7Al₂O₃·CaF₂ could be wholly or partially substituted by Cl⁻ ions from CaCl₂ at 900 °C or more, forming the solid solution 11CaO·7Al₂O₃·Ca(F,Cl)₂. The Cl⁻ ions in 11CaO·7Al₂O₃·CaCl₂ could be partially replaced F⁻ ions from CaF₂ at 1000 °C or above, apparently due to slow chloride loss by evaporation.     

Synthesis and Characterization of Silicon Carbide Powders Converted from Metakaolin‐Based Geopolymer

Silicon carbide (SiC) ceramic powders were synthesized by carbothermal reduction in specific geopolymers containing carbon nanopowders. Geopolymers containing carbon and having a composition M₂O·Al₂O₃·4.5SiO₂·12H₂O+18C, where M is an alkali metal cation (Na⁺, K⁺, and Cs⁺) were carbothermally reacted at 1400°C, 1500°C, and 1600°C, respectively, for 2 h under flowing argon. X‐ray diffraction and microstructural investigations by SEM/EDS and TEM were made. The geopolymers were gradually crystallized into SiC on heating above 1400°C and underwent significant weight loss. SiC was seen as the major phase resulting from Na‐based geopolymer heated to ≥1400°C, even though a minor amount of Al₂O₃ was also formed. However, phase pure SiC resulted with increasing temperature. While a slight increment of the Al₂O₃ amount was seen in potassium geopolymer, Al₂O₃ essentially replaced cesium geopolymer on heating to 1600°C. SEM revealed that SiC formation and a compositionally variable Al₂O₃ content depended on the alkaline composition. Sodium geopolymer produced high SiC conversion into fibrous and globular shapes ranging from ~5 μm to nanosize, as seen by X‐ray diffraction as well as SEM and TEM, respectively.     

Grain boundary diffusion effect on mineral transition of calcium sulpho-aluminate with sodium dopant

The mineral formation-transition mechanism, microstructure morphology evolution, pulverization property and chemical reactivity of calcium sulpho-aluminate with sodium dopant are systematically studied with the molar ratio of CaO to Al₂O₃ is 0.8, the molar ratio of CaO to SiO₂ is 1.5, the mass ratio of Al₂O₃ to SiO₂ is 2.0, and the SO₃ accounts for 4%. The results show that sodium dopant could promote 3CaO·3Al₂O₃·CaSO₄, CaO·2Al₂O₃ and 2CaO·Al₂O₃·SiO₂ transform into 2CaO·SiO₂, CaO·Al₂O₃, 12CaO·7Al₂O₃ and 2Na₂O·3CaO·5Al₂O₃, and it also can decrease the decomposition temperature of reactive materials and the initial phases formation temperature significantly. Sodium diffusion can destroy the clear grain boundary of calcium aluminate and calcium silicate phases, change the macro-microstructure morphology, improve the crystalline degree of the samples, and then improve the pulverization and alumina leaching property. However, excessive sodium dopant can decrease the melting temperature, inhibit the transformation process of 2CaO·SiO₂ from β to γ, and then deteriorate the pulverization property significantly again.     

Phase equilibria and liquidus surface of CaO-SiO2–5 wt%MgO-Al2O3-TiO2 slag system

The lack of phase equilibria relations and liquidus surface thermodynamic information for CaO-SiO₂–5?wt%MgO-Al₂O₃-TiO₂ system has seriously restricted the comprehensive utilization of the titanium resources. In present study, the phase equilibrium relationships were investigated for CaO-SiO₂–5?wt%MgO-Al₂O₃-TiO₂ phase diagram system at 1300?°C and 1400?°C using the high temperature equilibrium technique followed by X-Ray Fluoroscopy (XRF), X-Ray Diffraction (XRD), Scanning Electron Microscope (SEM) and Energy Dispersive X-ray spectroscope (EDX) analysis. In the composition range studied, the liquid phase, melilite solid solution phase and CaO·TiO₂ phase were found. The two-phase equilibrium of liquid coexisting with CaO·TiO₂ phase was intensively dicussed, and the spatial liquidus surfaces at 1300?°C, 1400?°C and 1450?°C (data from literatures only) for liquid coexisting with CaO·TiO₂ phase were first constructed in CaO-SiO₂–5?wt%MgO-Al₂O₃-TiO₂ tetrahedral phase diagram, which can realize a visual understanding of the phase relation change trendency in 3-dimensional space.     

Crystallization characteristics of CaO–Al2O3–SiO2–Li2O–B2O3 mold flux with different values of w(CaO)/w(Al2O3)

The effect of C/A ratio (abbreviation of w(CaO)/w(Al₂O₃)) on the crystallization characteristics was investigated. With an increase in C/A ratio from 1.1 to 1.8, the crystallization ability first decreased and then increased; the crystallization ability is weakest and strongest with C/A ratios of 1.5 and 1.8, respectively. Increasing C/A ratio, the crystalline phase changed from LiAlO₂ and CaO·Al₂O₃ to LiAlO₂ and 3CaO·Al₂O₃. The Li⁺ ions in the slag took precedence over Ca²⁺ ions to participate in charge compensation because the mold flux contains Al3+ which is more advantageous for a monovalent cation, and LiAlO₂ formed preferentially over CaO·Al₂O₃. With a further increase in C/A ratio, 3CaO·Al₂O₃ formed from the combination of Ca²⁺ ions and Q_Al² units, and the precipitated amount of 3CaO·Al₂O₃ increased.     

Phase equilibria in the CaO·SiO2Na2O·SiO2Na2O·Al2O3·6SiO2 system

Equilibrium phase relations in the system CaO·SiO₂Na₂O·SiO₂Na₂O·Al₂O₃·6SiO₂ at 40–80 wt% Na₂O·Al₂O₃·6SiO₂ composition range have been experimentally studied at temperatures between 800 °C and 1200 °C. The liquidus temperature was determined with differential scanning calorimetry. The equilibrated samples were quenched with pressurized nitrogen, and examined with electron probe X-ray microanalysis and X-ray diffraction for identification of microstructure and phase relations. Five primary phase fields, CaO·SiO₂, Na₂O·SiO₂, Na₂O·2CaO·3SiO₂, 2Na₂O·CaO·3SiO₂ and Na₂O·Al₂O₃·6SiO₂ were established. The ternary eutectic point of CaO·SiO₂, Na₂O·2CaO·3SiO₂ and Na₂O·Al₂O₃·6SiO₂ was determined to be at 1030 °C with the composition of 29.0 wt% CaO·SiO₂, 12.0 wt% Na₂O·SiO₂ and 59.0 wt% Na₂O·Al₂O₃·6SiO₂. Peritectic reaction of Na₂O·2CaO·3SiO₂, 2Na₂O·CaO·3SiO₂ and Na₂O·Al₂O₃·6SiO₂ occurred at 930 °C with the composition of 13.0 wt% CaO·SiO₂, 29.0 wt% Na₂O·SiO₂ and 58.0 wt% Na₂O·Al₂O₃·6SiO₂. The liquidus surface projection of the ternary system has been constructed in the composition region important for the bottom ash application.     

Phase Relationships in the System CaO—AI2O3—P2O5

Primary fields of crystallization in the system CaO-Al₂O₃-P₂O₅ at temperatures from 900° to 1600°C. were determined by the method of quenching. Three ternary eutectics were established: CaO·P₂O₅-Al₂O₃·3P₂O₅-Al₂O₃·P₂O₅, 2CaO·P₂O₆-CaO·P₂O₅-Al₂O₃·P₂O₅, and 3CaO·P₂O₆-2CaO·P₂O₅-Al₂O₃·P₂O₅. The rate of decomposition of Al₂O₃·3P₂O₅ was determined at several temperatures. The boundary was established between the field A1₂O₃·P₂O₅, which covers about 35% of the ternary diagram, and the fields Al₂O₃·3P₂O₅, 2CaO·P₂O₅, and CaO·P₂O₅. A portion of the Al₂O₃·P₂O₅-3CaO·P₂O₅ boundary also was established. A compound with the composition 2Al₂O₃·3P₂O₅ did not appear in the system. No calcium aluminum phosphates were found.     

Effect of CaO Addition on Compressive Deformation of Silicon Nitride Ceramic with Y‐Mg‐Si‐O‐N Glassy System

Creep behavior of Si₃N₄ polycrystals containing Y₂O₃‐MgO‐SiO₂ glass phase (with and without Calcium oxide [CaO] additive) was studied by compression tests between 1500 and 1700°C. We studied the effect of CaO additive on flow stress, microstructural evolutions, and thermal stability of the intergranular glass phase during deformation. While the addition of CaO did not affect grain size, the flow stress decreased with the amount of CaO. This result suggested that the addition of CaO reduced the viscosity of intergranular glass phase. The addition of CaO further improved the thermal stability of the glass phase by suppressing the evaporation at elevated temperatures.     

Carbon nanotube/graphene oxide‐added CaO‐B2O3‐SiO2 glass/Al2O3 composite as substrate for chip‐type supercapacitor

A CaO‐B₂O₃‐SiO₂ (CBS) glass/40 wt% Al₂O₃ composite sintered at 900°C exhibited a dense microstructure with a low porosity of 0.21%. This composite contained Al₂O₃ and anorthite phases, but pure glass sintered at 900°C has small quantities of wollastonite and diopside phases. This composite was measured to have a high bending strength of 323 MPa and thermal conductivity of 3.75 W/(mK). The thermal conductivity increased when the composite was annealed at 850°C after sintering at 900°C, because of the increase in the amount of the anorthite phase. 0.25 wt% graphene oxide and 0.75 wt% multi‐wall carbon nanotubes were added to the CBS/40 wt% Al₂O₃ composite to further enhance the thermal conductivity and bending strength. The specimen sintered at 900°C and subsequently annealed at 850°C exhibited a large bending strength of 420 MPa and thermal conductivity of 5.51 W/(mK), indicating that it would be a highly effective substrate for a chip‐type supercapacitor.     

Surface Properties of CaO (or BaO)–La2O3–MgO Catalysts and Their Performance in Oxidative Coupling of Methane

CaO–La₂O₃–MgO and BaO–La₂O₃–MgO catalysts with different compositions have been studied for their bulk and surface properties (viz. crystal phases, surface area, acidity/acid strength distribution, basicity/base strength distribution, etc.) and catalytic activity/selectivity in the oxidative coupling of methane (OCM) at different processing conditions (reaction temperature, 700–850°C; CH₄/O₂ ratio in feed, 3·0, 4·0 and 8·0 and GHSV, 102000 and 204000 cm³ g⁻¹ h⁻¹). The surface acidity and strong basicity of La₂O₃–MgO are found to be increased due to the addition of a third component (CaO or BaO), depending upon its concentration in the catalyst. The addition of CaO or BaO to La₂O₃–MgO OCM catalyst causes a significant improvement in its performance. Both the CaO- and BaO-containing catalysts show a high activity and selectivity at 800°C, whereas, the activity and selectivity of BaO-containing catalysts at 700°C is lower than that of CaO-containing catalysts. © 1997 SCI.     

Phase studies in the systems CaOAl2O3CaCrO4 and SrOAl2O3SrCrO4

Exploratory studies in the two systems have established the joins which exist between the 3:3:1 compounds and the binary compounds in the respective CaOAl₂O₃ and SrOAl₂O₃ systems. More detailed studies showed that the compound 3CaO·3Al₂O₃·CaCrO₄ takes CaO·Al₂O₃, CaO·2Al₂O₃ and Al₂O₃ into solid solution to the extent of 30, 20, and 25 mole percent, respectively. The compound 3SrO·3Al₂O₃·SrCrO₄ takes SrO·Al₂O₃, SrO·2Al₂O₃, SrO·6Al₂O₃ and Al₂O₃ into solid solution to the extent of 35, 30, 10 and 50 mole percent, respectively. The haüynite solid solutions have potential as yellow or yellow-green pigments in applications where hexavalent chromium can be tolerated.     

Characteristics,Thermodynamics, and Preparation of Nanocaged 12CaO·7Al2O3 and Its Derivatives

Crystal s tructures, characteristics, and preparations of 12CaO·7Al₂O₃ family and CaO–Al₂O₃ (C–A) system have been reviewed in detail with relevant thermodynamic parameters being assessed or recalculated. 12CaO·7Al₂O₃ (shortened as C12A7) can form several derivatives of type C12A7:M^n? or C12A7–M^n? through replacing so‐called “free oxygen ion” by many anions including OH^?, H^?, O^?, , F^?, Cl^?, and e^? in their cages, or being adopted by rare earth metals or alkaline earth metal oxides at cation sites (Ca²⁺ or Al³⁺). These doped C12A7 derivatives show unique material properties of transparent conduction, catalysis, and antibacterial with potential applications in fast ion conductors, optoelectronics, oxidants, and catalysts etc.     

High‐temperature calorimetric study of oxide component dissolution in a CaO–MgO–Al2O3–SiO2 slag at 1450°C

Blast‐furnace slags are formed, as iron ore is reduced to metal, as a molten a mixture of refractory and not easily reducible oxides, largely silica, alumina, lime, and magnesia. Their relatively low silica content makes them basic and poor glass formers. Their thermodynamic properties, though important for modeling their formation and reactivity, as well as furnace heat balance, are poorly known. Solution calorimetry of small amounts of solid oxides in a molten oxide solvent at high temperature (up to about 1500°C) permits direct assessment of energetics of dissolution. The enthalpies of solution of slag forming oxides: CaO, SiO₂, Al₂O₃, MgO, and Fe₂O₃ in a simplified model slag of composition: CaO (45.9 mol%), SiO₂ (35.1 mol%), Al₂O₃ (8.3 mol%), MgO (10.7 mol%) were measured by high‐temperature drop solution calorimetry at 1450°C. For this slag composition, enthalpies of solution become more exothermic in the order: Fe₂O₃ (279.3 ± 20.8 kJ/mol), MgO (56.7 ± 9.1 kJ/mol), Al₂O, (41.6 ± 11.3 kJ/mol), CaO (?4.3 ± 2.3 kJ/mol), and SiO₂, (?20.4 ± 4.4 kJ/mol), reflecting the relatively basic character of this low‐silica melt. Within these fairly large experimental errors, characteristic of calorimetry at this high temperature, there is little or no discernible concentration dependence for these heats of solution. The trends seen for these five solutes parallel those seen for heats of solution of the same oxides in other melts at various temperatures, with changes in magnitude reflecting the differences in acid‐base character of the melts. The new data for quartz show systematic behavior which extends the range of basicity studied for the enthalpy of dissolution of silica. The results provide reliable data for future modeling of the thermal balance of steel‐making furnaces and geologic and ceramic systems.