Ternary System Al2O3—MgO—CaO: Part II, Phase Relationships in the Subsystem Al2O3—MgAl2O4—CaAl4O7

Solid-state compatibility and melting relationships in the subsystem Al₂O₃—MgAl₂O₄—CaAl₄O₇ were studied by firing and quenching selected samples located in the isopletal section (CaO·MgO)—Al₂O₃. The samples then were examined using X-ray diffractomtery, optical microscopy, and scanning and transmission electron microscopies with wavelength- and energy-dispersive spectroscopies, respectively. The temperature, composition, and character of the ternary invariant points of the subsystem were established. The existence of two new ternary phases (Ca₂Mg₂Al₂₈O₄₆ and CaMg₂Al₁₆O₂₇) was confirmed, and the composition, temperature, and peritectic character of their melting points were determined. The isothermal sections at 1650°, 1750°, and 1840°C of this subsystem were plotted, and the solid-solution ranges of CaAl₄O₇, CaAl₁₂O₁₉, MgAl₂O₄, Ca₂Mg₂Al₂₈O₄₆, and CaMg₂Al₁₆O₂₇ were determined at various temperatures. The experimental data obtained in this investigation, those reported in Part I of this work, and those found in the literature were used to establish the projection of the liquidus surface of the ternary system Al₂O₃—MgO—CaO.     

Quaternary System Al2O3–CaO–MgO–SiO2: II Study of the Crystallization Volume of MgAl2O4

Solid-state compatibility and melting relations of MgAl₂O₄ in the quaternary system Al₂O₃–CaO–MgO–SiO₂ were studied by firing and quenching selected samples located in the 65 wt% MgAl₂O₄, plane followed by microstructural and energy dispersive X-ray analysis. A projection of the liquidus surface of the primary crystallization volume of MgAl₂O₄ was constructed from CaO, SiO₂ and exceeding Al₂O₃, not involved in stoichiometric MgAl₂O₄ formation; those three amounts were recalculated to 100 wt%. The temperature and character of six invariant points, where four solids co-exist with a liquid phase, were defined. One maximum point was localized and the positions of the isotherms were tentatively established. The effect of CaO, SiO₂, and Al₂O₃ impurities on the high temperature behavior of spinel materials was also discussed.     

Quaternary System Al2O3–CaO–MgO–SiO2: I, Study of the Crystallization Volume of Al2O3

Compatibility relations of Al₂O₃ in the quaternary system Al₂O₃–CaO–MgO–SiO₂ were studied by firing and quenching followed by microstructural and energy-dispersive X-ray examination. A projection of the liquidus surface of the primary phase volume of Al₂O₃ was constructed in terms of the CaO, SiO₂, and MgO contents of the mixtures recalculated to 100 wt%. Two invariant points, where four solids coexist with a liquid phase, were defined, and the positions of the isotherms were tentatively established. The effect of SiO₂, MgO, and CaO impurities on Al₂O₃ growth also was studied.     

Low-Temperature Chemical Synthesis of Nanocrystalline MgAl2O4 Spinel Powder

Nanocrystalline MgAl₂O₄ spinel powder was synthesized by pyrolysis of complex compounds of aluminum and magnesium with triethanolamine (TEA). The soluble metal ion–TEA complexes formed the precursor material on complete dehydration of the complexes of aluminum–TEA and magnesium–TEA. Single-phase MgAl₂O₄ spinel powder resulted after heat treatment of the precursor material at 675°C. The precursor and the heat-treated powders were characterized by X-ray diffractometry (XRD), differential thermal and thermogravimetric analysis, and transmission electron microscopy (TEM). The average crystallite size as measured from the X-ray line broadening was around 14 nm and the average particle size from TEM studies was around 20 nm.     

Knoop Microhardness Anisotropy of Single-Crystal Stoichiometric MgAl2O4 Spinel

Microhardness anisotropy profiles for the (100) and (111) planes of single-crystal stoichiometric MgAl₂O., spinel were determined at room temperaturé. The (100) microhardness profile has ahardness maximum in tiie [001] and a minimum in the [O11], which supports the previous suggestion that the primary slip system is the {111}〈11¯0〉. The microhardness of the (111) plane is independent of indenter orientation, also consistent, with a {111}〈11¯0〉 primary slip system. It is concluded that these microhardness profiles are in accord with other experimental observations that the {111}〈11¯0〉 is the primary slip system in stoichiometric MgAl₂O₄ spinel.     

Atomistic Simulation of the Surface Energy of Spinel MgAl2O4

Atomistic simulations with atomic potentials including anion polarizibility have been performed for the low-index surfaces of spinel MgAl₂O₄ with various terminations. The calculations show that for the most stable surface the surface energy is 2.27 J/m² for the {100}, about 2.85 J/m² for the {110}, and 3.07 J/m² for the {111} orientation. The ratio between the experimental values to the calculated relaxed surface energies is about 1.5. Strong surface relaxation was found for the {110} and {111} orientation but only moderate surface relaxation for the {100} surface.     

Oxygen-Enhanced Crystallization of Solution-Derived 12CaO·7Al2O3

12CaO·7Al₂O₃ (C12A7) composed of nanosize cage structure can clathrate oxygen radicals (O⁻) and has a high potential to application of strong oxidizing catalysis. In the present report, we demonstrate a fabrication route to C12A7 fine powders by Chemical Solution Deposition method in order to enhance the catalytic reactivity. Aluminum sec-butoxide, calcium nitrate tetrahydrate, acetylacetone, 2-methoxyethanol, and nitric acid were used as raw materials. Precursor solution was dried and annealed at 800°–900°C in air or O₂ atmosphere. Crystalline C12A7 powders were obtained by annealing at 900°C in O₂ atmosphere. Scanning electron microscope and transmission electron microscope images of the obtained powders revealed C12A7 particles were sintered and formed several micrometer particles with many pores. BET specific surface area of the powders was 4.2 m²/g. Possibility for synthesizing C12A7 powder with higher specific surface area by the solution process was indicated.     

Ionic Transference Numbers and Electrical Conduction in MgAl2O4 Spinel

Ionic transference numbers were obtained for T =1000 to 1700 K and for oxygen pressures 10⁵ to 10⁻¹¹ Pa by measuring potentials produced across samples having different oxygen partial pressures at the two faces. Alternating and direct current conductivities were also measured for the same samples over the same range of temperature and pressure. The results show that conduction in MgAl₂O₄ is almost purely ionic and that the conductivity is not greatly influenced by either ambient oxygen pressure or iron impurity up to 0.1%. It is proposed that the current carriers are cation vacancies that are present as a result of cation nonstoichiometry.     

Discontinuous Dissolution of Iron Aluminate Spinel in the Al2O3–Fe2O3 System

When sintered 85Al₂O₃–15Fe₂O₃ (in wt%) specimens consisting of corundum grains and spinel particles were annealed at temperature where only a corundum phase was stable, phase transformation of spinel into metastable FeAIO₃ and subsequently complete dissolution of the metastable phase occurred together with the migration of grain boundaries at the surface of the specimens. Since the grain boundary migration was induced by grain boundary diffusion of Fe₂O₃ from the transforming and dissolving particles, the boundary migration by temperature decrease corresponds to a discontinuous dissolution of the spinel particles and a chemically induced grain boundary migration by temperature change. Inside the specimens, however, the transformation—dissolution and the grain boundary migration were suppressed because of unavailable accommodation of the volume expansion due to the transformation.     

Sintering Kinetics of a MgAl2O4 Spinel Doped with LiF

The sintering kinetics of a pure magnesium aluminate spinel, MgAl₂O₄, and that doped with LiF were determined through the use of the master sintering curve technique developed by Su and Johnson. ²⁰ Powders with 0%, 0.5%, and 1.0% by mass LiF were densified in a vacuum hot press under a range of unaxial pressures. After the sintering mechanisms in each temperature and pressure regime were determined, an optimized vacuum hot-pressing schedule was formulated for spinel powders doped with 1.0% by mass of LiF. In addition to forming a transient liquid phase, the presence of LiF leads to the formation of oxygen vacancies that promote late-stage sintering in MgAl₂O₄.     

Primary Recrystallization During the Hot-Pressing of MgAl2O4

Magnesium aluminate powders, both pure and doped with 0.5 wt% LiF, were hot-pressed under vacuum at 1050°, 1170°, and 1290°C and 2500 and 5000 psi. Densification data were analyzed according to the classical hot-pressing equation, In P/P ₀= -Kt; deviations are discussed in terms of the Occurrence of primary recrystallization. Examination of fracture surfaces and thermally etched polished surfaces by SEM corroborated this approach, indicating that strain-induced primary recrystallization Occurred during hot-pressing. Increased hydrostatic components of stress resulting from the presence of a liquid phase associated with LiF are proposed as the cause of plastic flow in the LiF-doped spinel at low temperatures. Analogies are suggested between densification during primary recrystallization and reactive hot-pressing.     

Superplastic Deformation in Fine-Grained MgO 2Al2O3 Spinel

Fine-grained polycrystals of MgO 2A1₂O₃ spinel were deformed to large strains at strain rates ranging from 1O^-5 to 1O^-3 s^-1 and at temperatures from 1723 to 1885 K. These polycrystals were ductile at low strain rates and high temperatures; the ductility was especially remarkable at temperatures near the solvus. The mechanism of deformation, which was determined by measuring the change of flow stress with grain size, was dislocation creep, with the stress exponent in the power-law creep equation having a value of 2.1 ± 0.4. Despite deformation of several hundred percent, the grain size remained equiaxed, and deformation led to the evolution of a grain size which depended only on the strain rate and temperature. The initial microstructure had an influence on whether the poly-crystal would fracture or flow; a small initial grain size and a supersaturated solid solution were conducive to ductile flow. The ductility is attributed to dynamic recrystallization. It is proposed that the onset of fracture and the onset of dynamic recrystallization are competitive processes. Conditions which promote dynamic recrystallization also promote ductile flow.     

Evaluation of the Crack Face Bridging Mechanism in a MgAl2O4 Spinel

The effectiveness of a grain bridging mechanism in the following wake zone of two MgAl₂O₄ spinel microstructures has been demonstrated through renotching experiments with double-cantilevered-beam (DCB) specimens. Measurements of d K_R /dδ a agree with previous crack growth resistance curves of similar materials, obtained from single-edgenotch-beam specimens. However, even the extended test ligaments available from the present DCB specimens did not permit the full development of the following wake zone of these coarse microstructures.     

Phase Equilibria in the Spinel Region of the System FeO-Fe2O3-Al2O3

Phase relations in the spinel region of the system FeO-Fe₂O₃-Al₂O₃ were determined in CO₂ at 1300°, 1400°, and 15000°C and for partial oxygen pressures of 4 × 10⁻⁷ and 7 × 10⁻¹⁰ atmospheres at 15OO°C. The spinel field extends continuously from Fe₃O_4-x to FeAl₂O_4+z.     

Spark-Plasma-Sintering Condition Optimization for Producing Transparent MgAl2O4 Spinel Polycrystal

By controlling the heating rate at <10°C/min during spark-plasma-sintering (SPS) processing, transparent polycrystalline spinel with an in-line transmission of 50% and 70% in the visible- and infrared-wavelengths, respectively, can be successfully fabricated for only a 20-min soak at 1300°C. The high transmission can be attained by reducing the residual porosity and pore size, which was achieved by the low-heating rate. At high heating rates, many closed pores are formed due to the high densification rate during the heating process and remain as large pores around grain junctions. At temperatures >1300°C, the coalescence of the residual pores and the precipitation of second phases, which are caused by rapid grain growth, degrade the transparency. The present study demonstrates that although the high heating rates have been regarded as a primary advantage for the SPS processing, the low heating rate is highly effective in attaining a high transparency in the spinel even at low temperatures and for short sintering times.     

Chemical Interaction Between LiF and MgAl2O4 Spinel During Sintering

Reactions between LiF and MgAl₂O₄ at temperatures up to 1500°C are examined with a variety of tools, including differential scanning calorimetry, thermo-gravimetric analysis, X-ray diffraction, and scanning electron microscopy. LiAlO₂ and MgF₂ are found to be the active reaction products at these temperatures. A transient liquid phase comprising MgF₂ and LiF forms at intermediate temperatures, but then is consumed at higher temperatures during the reformation of MgAl₂O₄. If processed as an uncompacted powder mixture, all of the initial LiF in the system eventually vaporizes at temperatures exceeding 1300°C. A new reaction sequence relevant to the densification of LiF-doped MgAl₂O₄ spinel is proposed.     

Phase Relations in the System Al2O3–B2O3–Nd2O3

The phase relations at a temperature below "subsolidus" in the system Al₂O₃–B₂O₃–Nd₂O₃ are reported. Specimens were prepared from various compositions of Al₂O₃, B₂O₃, and Nd₂O₃ of purity 99.5%, 99.99%, and 99.9%, respectively, and fired at 1100°C. There are six binary compounds and one ternary compound in this system. The ternary compound, NdAl₃(BO₃)₄ (NAB), has a phase transition at 950°C ± 15°C. The high-temperature form of NAB has a second harmonic generation (SHG) efficiency of KH₂PO₄ (KDP) of the order of magnitude of the form which has been used as a good self-activated laser material, and the low-temperature form of NAB has no SHG efficiency.     

Preparation of MgAl2O4 Spinel Powders via Freeze-Drying of Alkoxide Precursors

High-sinterability MgAl₂O₄ powder has been produced from alkoxide precursors via a freeze-drying method. Clear alumina sol and magnesium methoxide were used as starting materials in the process. The spinel powders were characterized by various techniques, such as thermal analysis, X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. The tap density and sinterability of the spinel power are affected by the ball-milling techniques. Highly dense, transparent, polycrystalline MgAl₂O₄ has been obtained from these powders by sintering and hot isostatic pressing. Bimodal grain-size microstructure is observed in a HIPed sample.