Phase equilibria study and thermodynamic description of the BaO‐CaO‐Al2O3 system

A combined experimental investigation and thermodynamic assessment was performed for the BaO‐CaO‐Al₂O₃ system. By using a high‐temperature equilibration/quenching technique and scanning electron microscopy, electron probe microanalysis, and X‐ray powder diffraction analysis, the phase equilibria at 1500°C and phase stability of BaCa₂Al₈O₁₅ phase were determined. An extensive literature survey was conducted for the experimental and thermodynamic modeling data of the BaO‐CaO‐Al₂O₃ system. According to the literature data and the present measurements, a thermodynamic assessment was made in order to obtain a set of self‐consistent thermodynamic parameters to describe the BaO‐CaO‐Al₂O₃ system. Based on the thermodynamic parameters acquired in this work, isothermal sections at 1100°C, 1250°C, 1400°C, 1475°C, and 1500°C and the BaO·Al₂O₃‐CaO·Al₂O₃ and BaO·6Al₂O₃‐CaO·6Al₂O₃ joints were calculated and compared with the available experimental data.     

Visible Light Transparency for Polycrystalline Ceramics of MgO·2Al2O3 and MgO·2.5Al2O3 Spinel Solid Solutions

Magnesium aluminate spinel solid solutions with the alumina‐rich compositions MgO·2Al₂O₃ and MgO·2.5Al₂O₃ have been prepared as polycrystalline ceramics with average in‐line transmissions at 550 nm of 85.5 ± 0.3% and 80.9 ± 0.4%, respectively. Starting powders are prepared from combinations of high purity Mg(OH)₂ and γ‐Al₂O₃ thoroughly mixed in an aqueous slurry, and the solids are collected, dried, calcined, mixed with LiF sintering aid, and sieved. The optimum amount of LiF added varies with the alumina composition of the spinel solid solution. The powders are sintered into dense ceramics by hot pressing at 1600°C under vacuum and 20 MPa uniaxial load followed by hot isostatic pressing at 1850°C under 200 MPa in Ar. Both compositions exhibit exaggerated grain growth with average sizes well over 500 μm. Knoop hardness measurements are 11.2 ± 0.3 GPa for MgO·2Al₂O₃ and 11.0 ± 0.4 GPa for MgO·2.5Al₂O₃.     

High‐pressure synthesis of a 12CaO·7Al2O3–12SrO·7Al2O3 solid solution

Solid solutions of 12CaO·7Al₂O₃ (C12A7) and 12SrO·7Al₂O₃ (S12A7) crystals were synthesized under high pressure. X‐ray diffraction patterns revealed that the lattice constants of the synthesized samples depend linearly on the compositional ratio of C12A7 and S12A7. Electron‐probe X‐ray microanalyses show that the chemical compositions of the crystals are represented by xC12A7·(1?x)S12A7 (0<x<1). These results indicate that the variation in the lattice constants is originated from a difference in the ionic radii of Ca²⁺ and Sr²⁺ ions. From impedance measurements, it was found that S12A7 has the highest conductivity (~1 × 10^?3 Scm^?1 at 550°C) among the solid solutions in the C12A7–S12A7 system.     

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.     

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.     

Microstructure of mayenite 12CaO·7Al2O3 and electron emission characteristics

Electron emission characteristic, electrical conductivity of polycrystalline mayenite (12CaO·7Al₂O₃) electride, formation of [Ca₂₄Al₂₈O₆₄]⁴⁺(e⁻)₄ framework as a function of phase content, and microstructure have been investigated. The mayenite microstructure was investigated using high-resolution transmission microscopy which revealed the type cage structure of 12CaO·7Al₂O₃ partially filled by extra-framework oxygen ions. Incorporation of electrons by means of carbon ion template 12CaO·7Al₂O₃ produces complex structure, and an incomplete ion template 12CaO·7Al₂O₃ structure consisting of mixture of a [Ca₂₄Al₂₈O₆₄]⁴⁺(e⁻)₄ and [Ca₂₄Al₂₈O₆₄]⁴⁺(O²⁻)₂ framework had a direct effect on the electron emission. Surface chemistry and stability of the 12CaO·7Al₂O₃ electride have been studied using x-ray photoelectron spectroscopy. The work function of phase pure 12CaO·7Al₂O₃ electride was determined from direct thermionic emission data and compared to the measurement from ultraviolet photoelectron spectroscopy (UPS). Depending on the extent of ion template of 12CaO·7Al₂O₃ structure, a work function of 0.9–1.2 eV and 2.1–2.4 eV has been measured and thermionic emission initiating at 600°C.     

Formation of crystalline phases in electroceramics of the system CaO−MgO−Al2O3−SiO2 (A review)

Characteristics are given for the chemical compounds which are contained in the CaO·Al₂O₃·2SiO₂?CaO·SiO₂?MgO·SiO₂?SiO₂ tetrahedron and constitute the main crystalline phase of a large group of electroceramics. It is shown to be possible to analyze natural magnesia-calcium silicates in a four-component system with allowance for the formation of chemical compounds. The use of diopside raw materials is substantiated and their advantages in the manufacture of electroceramics are assessed.     

Phase Evolution in the Transformation of Atomically Mixed Versus Ball‐Milled Mixtures of Nanopowders in the Formation of Composite MO·3Al2O3 Spinels: Bottom‐Up Processing is Not Always Optimal

Liquid‐feed flame spray pyrolysis (LF‐FSP) provides atomically homogeneous mixed metal powders with 30–40 nm average particle sizes, often producing kinetic phases due to the high quench rate As produced LF‐FSP Al₂O₃‐rich spinels, such as MgO·3Al₂O₃, form an Al₂O₃‐rich metastable single‐phase spinel. On heating, the powders phase separate to form MAl₂O₄ and α‐Al₂O₃. Compacts of MO·3Al₂O₃ (M = Co, Ni, Mg) were produced and sintered to evaluate the final duplex microstructure. The same composition was also approached from stoichiometric LF‐FSP MAl₂O₄ nanopowders ball‐milled with Al₂O₃ nanopowders in an attempt to evaluate how the initial length scale of mixing affected the final microstructure. Contrary to traditional sintering, we observe two distinct mechanisms. At 1000°C–1200°C, cation diffusion appears to control densification as a consequence of high vacancy concentrations and atomic mixing where traditionally expected site inversion plays less of a factor given the high quench rates. The second mechanism follows α‐Al₂O₃ exsolution and densification occurs via oxygen diffusion and α‐Al₂O₃ grain growth. When sintering the duplex MAl₂O₄/α‐Al₂O₃ compacts to at least 95% theoretical density, we find final microstructures that do not reflect the initial degrees of mixing. That is, the atomically mixed MgO·3Al₂O₃ does not does not offer an advantage over the submicron length scale of mixing in the ball‐milled samples.     

In situ crystallization and elastic properties of transparent MgO–Al2O3–SiO2 glass‐ceramic

Glass‐ceramics (GC) generally possess enhanced mechanical properties compared to their parent glasses. The knowledge of how crystallization evolves and affects the mechanical properties with increasing temperature is essential to optimize the design of the crystallization cycle. In this study, we crystallized a glass of the MgO–Al₂O₃–SiO₂ system with nucleating agents TiO₂ and ZrO₂. The crystallization cycle comprised a 48 hour nucleation treatment at the glass‐transition temperature followed by a 10 hour growth step at a higher temperature. During this cycle, the evolution of crystalline phases was followed by high‐temperature X‐ray diffraction (HTXRD), which revealed the presence of karooite (MgO·2TiO₂), spinel (MgO·Al₂O₃), rutile (TiO₂), sillimanite (Al₂O₃·SiO₂), and sapphirine (4MgO·5Al₂O₃·2SiO₂). The same heat treatment was applied for in situ measurement of elastic properties: elastic modulus, E, shear modulus, G, and Poisson's ratio, ν. The evolution of these parameters during the heating path from room temperature to the final crystallization temperature and during the nucleation and the crystallization plateaus is discussed. E and G evolve significantly in the first two hours of the growth step. At the end of the crystallization process, the elastic and shear moduli of the GC were approximately 20% larger than those of the parent glass.     

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.     

Disperse Equiaxed α‐Al2O3 Nanoparticles Without Vermicular Microstructure

Nanocrystalline microstructure is regarded as a strategic approach to overcome the brittleness of alumina ceramics, and the preparation of disperse equiaxed α‐Al₂O₃ nanoparticles is an essential step for the preparation of nanocrystalline alumina ceramics. In this work, disperse equiaxed α‐Al₂O₃ nanoparticles were prepared using α‐Fe₂O₃ as seed and isolation phase. At first, the composite of α‐Al₂O₃ nanoparticles embedded in α‐Fe₂O₃ matrix was obtained by calcining the precursor powder containing γ‐AlOOH and Fe(OH)₃ (Fe³⁺/Al³⁺ mole ratio of 5) at 770°C for 2 h. Then disperse equiaxed α‐Al₂O₃ nanoparticles with a mean size of 12 nm and a size distribution from 2 to 40 nm without vermicular microstructure were obtained by removal of α‐Fe₂O₃ and other impurities in the composite through acid corrosion.     

0.73ZrTi2O6–0.27MgNb2O6 microwave dielectric ceramics modified by Al2O3 addition

0.73ZrTi₂O₆–0.27MgNb₂O₆ ceramics with various Al₂O₃ contents (0‐2.0 wt%) were prepared by conventional ceramic route. The effects of Al₂O₃ on the phase composition, microstructure, conductivity, and microwave dielectric properties were systematically investigated. The coexistence of a disordered α–PbO₂‐type phase and a rutile second phase was found in all compact ceramics with low Al₂O₃ contents (x = 0, 0.5, and 1.0 wt%), while a corundum phase was detected when Al₂O₃ additive increased to 1.5 and 2.0 wt% based on X‐ray diffraction results. With the addition of Al₂O₃, the decreased grain size of the matrix phase was observed using field‐emission scanning electron microscope, accompanied with increased resistivity and band‐gap energy. Additionally, Al₂O₃ additives efficiently improved the quality factor of the ceramics. After sintering at 1360°C for 3 hours, the ceramic with 1.0 wt% Al₂O₃ exhibited excellent microwave dielectric properties: a dielectric constant of 43.8, a quality factor of 33 900 GHz (at 6.6 GHz), and a near‐zero temperature coefficient of resonant frequency (3.1 ppm/^°C).     

Slag resistance mechanism of CaO·6Al2O3 refractory and its effect on inclusions of aluminum deoxidized steel

In order to verify the advantage of CaO·6Al₂O₃ (CA₆)-based refractories on the inclusions of aluminum deoxidized steel, the five refractories, CA₆, alumina, spinel, and CA₆-alumina and CA₆-spinel composition refractories were prepared into crucibles, and then the laboratory smelting experiments were conducted. After experiment, the slag resistance of the crucible and the variation on inclusions in steel were characterized and discussed. A dense CaO·2Al₂O₃ (CA₂) layer, which was produced by CA₆ reacting with the slag, was distributed between the original bricklayer and the slag layer, improving the slag resistance of refractories. Meanwhile, the 12CaO·7Al₂O₃ (C₁₂A₇), generated by the reaction between CA₂ and refining slag, would release much Ca into the molten steel. The Ca would react with inclusions to produce low melting point substance to float up and remove, contributing to the reduction of the proportion of large size inclusions. In addition, typical inclusions in steel smelted with CA₆ crucible were small-sized MgO·Al₂O₃ inclusions, whereas those of other crucibles are MnS–MgO·Al₂O₃ composite inclusions with MgO·Al₂O₃ as the core, implying CA₆ may absorb sulfur during the smelting process.     

Separation of bovine serum albumin (BSA) using γ‐Al2O3–clay composite ultrafiltration membrane

BACKGROUND: Ceramic membranes have received more attention than polymeric membranes for the separation and purification of bio‐products owing to their superior chemical, mechanical and thermal properties. Commercially available ceramic membranes are too expensive. This could be overcome by fabricating membranes using low‐cost raw materials. The aim of this work is to fabricate a low‐cost γ‐Al₂O₃–clay composite membrane and evaluate its potential for the separation of bovine serum albumin (BSA) as a function of pH, feed concentration and applied pressure. To achieve this, the membrane support is prepared using low‐cost clay mixtures instead of very expensive alumina, zirconia and titania materials. The cost of the membrane can be further reduced by preparing a γ‐alumina surface layer on the clay support using boehmite sol synthesized from inexpensive aluminium chloride instead of expensive aluminium alkoxide using a dip‐coating technique. RESULTS: The pore size distribution of the γ‐Al₂O₃‐clay composite membrane varied from 5.4–13.6 nm. The membrane was prepared using stable boehmite sol of narrow particle size distribution and mean particle size 30.9 nm. Scanning electron microscopy confirmed that the surface of the γ‐Al₂O₃–clay composite membrane is defect‐free. The pure water permeability of the support and the composite membrane were found to be 4.838 × 10^?6 and 2.357 × 10^?7 m³ m^?2 s^?1 kPa^?1, respectively. The maximum rejection of BSA protein was found to be 95%. It was observed that the separation performance of the membrane in terms of flux and rejection strongly depends on the electrostatic interaction between the protein and charged membrane. CONCLUSION: The successively prepared γ‐Al₂O₃‐clay composite membrane proved to possess good potential for the separation of BSA with high yield and could be employed as a low cost alternate to expensive ceramic membranes. Copyright © 2009 Society of Chemical Industry     

Tribological behavior of micrometer‐ and nanometer‐Al2O3‐particle‐filled poly(phthalazine ether sulfone ketone) copolymer composites used as frictional materials

Micrometer‐ and nanometer‐Al₂O₃‐particle‐filled poly(phthalazine ether sulfone ketone) (PPESK) composites with filler volume fractions ranging from 1 to 12.5 vol % were prepared by hot compression molding. We evaluated the tribological behaviors of the PPESK composites with the block‐on‐ring test rig by sliding PPESK‐based composite blocks against a mild carbon steel ring under dry‐friction conditions. The effects of different temperatures on the wear rate of the PPESK composites were also investigated with a ball‐on‐disc test rig. The wear debris and the worn surfaces of the PPESK composites were investigated with scanning electron microscopy, and the structures of the PPESK composites were analyzed with IR spectra. The lowest wear rate, 7.31 × 10^?6 mm³ N^?1 m^?1, was obtained for the composite filled with 1 vol %‐nanometer Al₂O₃ particles. The composite with nanometer particles exhibited a higher friction coefficient (0.58–0.64) than unfilled PPESK (0.55). The wear rate of 1 vol %‐nanometer‐Al₂O₃‐particle‐filled PPESK was stable and was lower than that of unfilled PPESK from the ambient temperature to 270°C. We anticipate that 1 vol %‐nanometer‐Al₂O₃‐particle‐filled PPESK can be used as a good frictional material. We also found that micrometer‐Al₂O₃‐particle‐filled PPESK had a lower friction coefficient at a filler volume fraction below 5%. The filling of micrometer Al₂O₃ particles greatly increased the wear resistance of PPESK under filler volume fractions from 1 to 12.5%. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 95: 993–1001, 2005     

Synthesis of Nanofiber‐like Mesoporous γ‐Al2O3 toward Its Adsorption Efficiency for Congo Red

Nanofiber‐like mesoporous γ‐Al₂O₃ was synthesized using freshly prepared boehmite sol in the presence of triblock copolymer, P123 following evaporation‐induced self‐assembly (EISA) process followed by calcinations at 400°C–1000°C. The samples were characterized by thermogravimetry (TG), differential thermal analysis (DTA), X‐ray diffraction (XRD), N₂ adsorption–desorption, and transmission electron microscopy (TEM). The adsorption efficiency of the samples with Congo red (CR) was studied by UV – vis spectroscopy. XRD results showed boehmite phase in the as‐prepared sample while γ‐Al₂O₃ phase obtained at 400°C was stable up to 900°C, a little transformation of θ‐Al₂O₃ resulted at 1000°C. The Brunauer‐Emmett‐Teller surface area of the 400°C‐treated sample was found to be 175.5 m²g ^{? 1}. The TEM micrograph showed nanofiber‐like morphology of γ‐Al₂O_3. The 400°C‐treated sample showed about 100% CR adsorption within 60 min.     

Effect of CeO2 and Al2O3 contents on Ce‐ZrO2/Al2O3 composites

Effect of CeO₂ and Al₂O₃ contents on phase composition, microstructures, and mechanical properties of Ce–ZrO₂/Al₂O₃ composites was studied. The CeO₂ content in CeO₂–ZrO₂ varied from 7 to 16 mol%, and the Al₂O₃ content in Ce‐ZrO₂/Al₂O₃ composites were 7 and 22 wt%. When CeO₂ content was ≤10 mol%, high Al₂O₃ content contributed to hinder the tetragonal‐to‐monoclinic ZrO₂ phase transformation during cooling and decrease the density of microcracks in the composites. Tetragonal ZrO₂ single‐phase was obtained in the composites with ≥12 mol% CeO₂, regardless of the Al₂O₃ content. Hardness, flexural strength, and toughness were dependent on CeO₂ and Al₂O₃ contents which were related to the microcracks, grain size, and phase transformation. The high flexural strength and toughness of the composites with 7wt% Al₂O₃ could be obtained at an optimum CeO₂ content of 12 mol%, whereas those of the composites with 22 wt% Al₂O₃ could be achieved in the wide CeO₂ content range of 8.5‐12 mol%.     

Selective Corrosion Preparation and Sintering of Disperse α‐Al2O3 Nanoparticles

Disperse fine equiaxed α‐Al₂O₃ nanoparticles with a mean particle size of 9 nm and a narrow size distribution of 2–27 nm were synthesized using α‐Fe₂O₃ as seeds and isolation via homogeneous precipitation‐calcination‐selective corrosion processing. The presence of α‐Fe₂O₃ acting as seeds and isolation phase can reduce the formation temperature to 700°C and prevent agglomeration and growth of α‐Al₂O₃ nanoparticles, resulting in disperse fine equiaxed α‐Al₂O₃ nanoparticles. These α‐Al₂O₃ nanoparticles were pressed into green compacts at 500 MPa and sintered first by normal sintering to study their sintering behavior and finally by two‐step sintering (heated to 1175°C without hold and decreased to 1025°C with a 20 h hold in air) to obtain nanocrystalline α‐Al₂O₃ ceramics. The two‐step sintered bodies are nanocrystalline α‐Al₂O₃ with an average grain size of 55 nm and a relative density of 99.6%. The almost fully dense nanocrystalline α‐Al₂O₃ ceramic with finest grains achieved so far by pressureless sintering reveals that these α‐Al₂O₃ nanoparticles have an excellent sintering activity.     

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.