Mechanical-Activation-Triggered Gibbsite-to-Boehmite Transition and Activation-Derived Alumina Powders

Mechanical activation of monoclinic gibbsite (Al(OH)₃) in nitrogen led to the formation of nanocrystalline orthorhombic boehmite (AlOOH) at room temperature. The boehmite phase formed after merely 3 h of mechanical activation and developed steadily as the mechanical-activation time increased. Forty hours of mechanical activation resulted in essentially single-phase boehmite, together with α-alumina (α-Al₂O₃) nanocrystallites 2–3 nm in size. The sequence of phase transitions in the activation-derived boehmite was as follows: boehmite to γ-Al₂O₃ and then to α-Al₂O₃ when flash-calcined at a heating rate of 10°C/min in air. γ-Al₂O₃ formed at 520°C, and flash calcination to 1100°C led to the formation of an α-Al₂O₃ phase, which exhibited a refined particle size in the range of 100–200 nm. In contrast, the gibbsite-to-boehmite transition in the unactivated gibbsite occurred over the temperature range of 220°–330°C. A flash-calcination temperature of 1400°C was required to complete the conversion to α-Al₂O₃ phase, with both δ-Al₂O₃ and θ-Al₂O₃ as the transitional phases. The resulting alumina powder consisted of irregularly shaped particles 0.4–0.8 μm in size, together with an extensive degree of particle agglomeration.     

Seeding with γ-Alumina for Transformation and Microstructure Control in Boehmite-Derived α-Alumina

The dehydration, transformation, and densification of boehmite (γ-AlOOH) are enhanced by addition of γ-Al₂O₃ seed particles. α-Al₂O₃ microstructures with uniform 1- to 2-μm grain size and sintered densities 98% of theoretical are achieved at 1300°C Thermal analysis shows that γ-Al₂O₃ seed particles transform to α-Al₂O₃ before the matrix, thus controllably nucleating the transformation of θ-AI₂O₃ to α-Al₂O₃.     

Spark-Plasma Sintering of Silicon Carbide Whiskers (SiCw) Reinforced Nanocrystalline Alumina

The combined effect of rapid sintering by spark-plasma-sintering (SPS) technique and mechanical milling of γ-Al₂O₃ nanopowder via high-energy ball milling (HEBM) on the microstructural development and mechanical properties of nanocrystalline alumina matrix composites toughened by 20 vol% silicon carbide whiskers was investigated. SiC_w/γ-Al₂O₃ nanopowders processed by HEBM can be successfully consolidated to full density by SPS at a temperature as low as 1125°C and still retain a near-nanocrystalline matrix grain size (∼118 nm). However, to densify the same nanopowder mixture to full density without the benefit of HEBM procedure, the required temperature for sintering was higher than 1200°C, where one encountered excessive grain growth. X-ray diffraction (XRD) and scanning electron microscopy (SEM) results indicated that HEBM did not lead to the transformation of γ-Al₂O₃ to α-Al₂O₃ of the starting powder but rather induced possible residual stress that enhances the densification at lower temperatures. The SiC_w/HEBMγ-Al₂O₃ nanocomposite with grain size of 118 nm has attractive mechanical properties, i.e., Vickers hardness of 26.1 GPa and fracture toughness of 6.2 MPa·m^1/2.     

Topotactically Modified Stress in Coatings

The unexpected occurrence of compressive stresses in solgel-derived alumina coatings following a densification transformation was found to be related to the topotactic transformation of boehmite (γ-AlOOH) to γ-alumina (γ-Al₂O₃) on heating. The boehmite coating showed a pronounced fiber texture with the [001] crystallographic axis perpendicular to the substrate-coating interface plane. At about 500°C, the orthorhombic boehmite transformed into tetragonal γ-alumina with very little, if any, reorientation of the crystallites. Because of the topotactic nature of the transformation, the fiber texture persisted in the γ-alumina phase. This resulted in an anisotropic change that generated compressive strains in the coating.     

High-Pressure Sintering of Nanocrystalline γAl2O3

High-pressure sintering of nanocrystalline γ-A1₂O₃ has been studied over a temperature range of 923-1323 K and at a pressure of 1 GPa. The γ-Al₂O₃ to α-Al₂O₃ transformation temperature changed from 1473 K without pressure to ∼1023 K at 1 GPa. Full density was obtained at 1273 and 1323 K in 10 min. The microhardness value of fully dense α-alumina with a grain size of 142 nm was found to be 25.3 ± 0.8 GPa. The Hall-Petch slope for the very fine grain size range is different from that of the coarse-grained alumina.     

Growth of α-Al2O3 Within a Transition Alumina Matrix

The growth of α-Al₂O₃ from a planar specimen of thermally grown γ-alumina on a molybdenum transmission electron microscope grid was studied. The α-Al₂O₃ grows into the transition alumina matrix and then thickens via a ledge growth mechanism. Faceted Mo crystallites cause pinning of α-Al₂O₃ ledges and are larger on α-Al₂O₃ than on the transition alumina matrix.     

Synthesis of α-Alumina Hexagonal Platelets Using a Mixture of Boehmite and Potassium Sulfate

Single-crystal α-alumina (Al₂O₃) hexagonal platelets with a diameter of about 200 nm and 25 nm in thickness were synthesized by heating a mixture of boehmite and potassium sulfate at 1000°C for 2 h and washing with water. The potassium sulfate addition effects on the Al₂O₃ phase and morphology were investigated using differential thermal analysis (DTA), X-ray diffraction (XRD), and transmission electron microscopy (TEM). It was found that potassium sulfate addition helps in the formation of single-crystal α-Al₂O₃ hexagonal platelets and promotes phase transformation from intermediate γ-Al₂O₃ to α-Al₂O₃.     

Transformation, Microstructure Development, and Densification in α-Fe2O3-Seeded Boehmite-Derived Alumina

Addition of α-Fe₂O₃ seed particles to alkoxide-derived boehmite sols resulted in a 10-fold increase in isothermal rate constants for the transformation of γ- to α-Al₂O₃. Changes in porosity and surface area with sintering temperature showed no effect of seeding on coarsening of the transition alumina gels, but the 200-fold decrease in surface area associated with transformation to α-Al₂O₃ occurred ∼ 100°C lower in seeded gels compared with unseeded materials. As a result of high nucleation frequency and reduced microstructure coarsening, fully transformed seeded alumina retained specific surface areas >22 m²/g and exhibited narrow pore size distributions, permitting development of fully dense, submicrometer α-Al₂O₃ at ∼ 1200°C.     

Low-Temperature Sintering of Seeded Sol—Gel-Derived, ZrO2-Toughened Al2O3 Composites

Seeding a mixture of boehmite (AIOOH) and colloidal ZrO₂ with α-alumina particles and sintering at 1400°C for 100 min results in 98% density. The low sintering temperature, relative to conventional powder processing, is a result of the small alumina particle size (∼0.3 μm) obtained during the θ-to α-alumina transformation, homogeneous mixing, and the uniform structure of the sol-gel system. Complete retention of pure ZrO₂ in the tetragonal phase was obtained to 14 vol% ZTA because of the low-temperature sintering. The critical grain size for tetragonal ZrO₂ was determined to be ∼0.4 μm for the 14 vol% ZrO₂—Al₂O₃ composite. From these results it is proposed that seeded boehmite gels offer significant advantages for process control and alumina matrix composite fabrication.     

Fabrication of Nano-Scaled α-Al2O3 Crystallites Through Heterogeneous Precipitation of Boehmite in a Well-Dispersed θ-Al2O3-Suspension

The possibility of eliminating finger or vermicular growth of α-Al₂O₃ particles obtained by calcination of boehmite was examined. Heterogeneous precipitation of boehmite in a well-dispersed θ-Al₂O₃ suspension was first prepared, in which the mass ratio of boehmite to θ-crystallite was evaluated to form agglomerates of similar sizes that will form α-Al₂O₃ crystallites of <100 nm in diameter. θ- to α-phase transformation of alumina experiences a nucleation and growth mechanism, with the critical size of nucleation being ∼25 nm for θ-Al₂O₃ and the size for accomplishment of transformation followed by finger growth being ∼100 nm. Hence, fabricating agglomerates that would form α-Al₂O₃ crystallites with sizes <100 nm accompanied with appropriate thermal treatments can be a method for obtaining α-Al₂O₃ crystallites free of finger growth. It is found that proper preparation of the agglomerate with appropriate size may initiate a simultaneous and lower temperature θ- to α-Al₂O₃ phase transformation for such powder systems, substantially limiting the mass transfer among the newly formed α-Al₂O₃ particles. Moreover, α-Al₂O₃ crystallites free of finger growth can be obtained.     

Liquid-Phase-Assisted Transformation of Seeded γ-Alumina

α-Al₂O₃-seeded, boehmite-derived γ-Al₂O₃ was transformed in the presence of V₂O₅, resulting in a 205°C decrease in the α-Al₂O₃ transformation temperature and a 74% reduction in the apparent activation energy for the γ- to α-Al₂O₃ transformation at temperatures greater than 850°C. These changes are attributed to the lowered energy barrier for nucleation by seeding and the lowered activation energy for material transport through the liquid relative to the unseeded, solid-state transformation. Growth of the transforming alumina yielded fine-grained α-Al₂O₃ particles which exhibited a highly faceted morphology. It is proposed that the combined control of both nucleation and growth during liquid-phase-assisted transformation provides a potentially powerful technique for tailoring powder characteristics in many material systems which undergo nucleation and growth processes.     

Hydrothermal Synthesis of Petal-Like Alumina Flakes

Petal-like alumina flakes were prepared by hydrothermal reaction at 170°C for 6 h with aqua-based precursor materials. The gel particles were calcined at 550°–1200°C. The gel and calcined particles were characterized by differential thermal analysis (DTA), thermogravimetry (TG), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), multipoint Brunauer–Emmett–Teller (BET) surface area, and field emission scanning electron microscopy (FESEM). Crystallization of alumina was confirmed by DTA and XRD studies. XRD results showed that boehmite particles were formed in as-prepared sample while γ-alumina (γ-Al₂O₃) persisted up to 800°C followed by the appearance of θ-alumina (θ-Al₂O₃) at 1000°–1200°C. The characteristics Al–O vibrations in alumina polymorphs were examined by FTIR study. FESEM confirmed the formation of petal-like alumina flakes with the assembly of alumina nanorods.     

Electron Beam-Induced "Nanocalcination" of Boehmite Nanostrips to Mesoporous α-Alumina Phase

Phase transformation of boehmite (AlOOH) nanostrips to α-Al₂O₃ nanolaces was investigated under electron beam irradiation of a 200 keV transmission electron microscope (TEM). X-ray diffraction pattern showed that the powder consisted of boehmite phase before the electron exposure, while selected area electron diffraction patterns demonstrated the formation of stable α-alumina phase after bombardment with high-energy electrons. The in situ TEM observations revealed that the initial morphology remained unchanged while a lace-like mesostructure of α-alumina was developed.     

Growth of α-AI2O3 Platelets in the HF-γy-AI2O3 System

In the presence of a fluorine mineralizer, highly aggregated, <5 μm α-Al₂O₃ platelet particles form by vapor transport during the thermal transformation of γ-alumina. Platelet aggregation was determined to occur by platelet inter-growth and by edge nucleation on primary α-Al₂0₃ platelets. The addition of 10¹⁰α-alumina seed particles/cm³γ–Al₂O₃ resulted in the development of discrete particles during the initial stage of transformation. Impingement of the growing platelets during the latter stage of transformation, however, resulted in intergrowth, a process which was not changed by seeding. Particle size distribution broadening was observed to increase with increasing HF and H₂O concentrations because vapor reactant supersaturation increases the degree of edge nucleation. When initially low HF and H₂O concentrations were used in seeded systems, however, essentially aggregate-free α-Al₂O₃ platelets of 10–15 μm were obtained.     

Microstructure and Mechanical Properties of Hot-Pressed α-Al2O3-Seeded γ-Alumina Ceramics

High-quality alumina ceramics were fabricated by a hot pressing with MgO and SiO₂ as additives using α-Al₂O₃-seeded nanocrystalline γ-Al₂O₃ powders as the raw material. Densification behavior, microstructure evolution, and mechanical properties of alumina were investigated from 1250°C to 1450°C. The seeded γ-Al₂O₃ sintered to 98% relative density at 1300°C. Obvious grain growth was observed at 1400°C and plate-like grains formed at 1450°C. For the 1350°C hot-pressed alumina ceramics, the grain boundary regions were generally clean. Spinel and mullite formed in the triple-grain junction regions. The bending strength and fracture toughness were 565 MPa and 4.5 MPa·m^1/2, respectively. For the 1300°C sintered alumina ceramics, the corresponding values were 492 MPa and 4.9 MPa·m^1/2.     

Morphological Forms of α-Alumina Particles Synthesized in 1,4-Butanediol Solution

Phase-pure, monodispersed, hexagonal plates of single-crystal α-alumina (∼ 2 μm wide and ∼0.5 μm thick) have been prepared via precipitation by treating an aluminum hydrous oxide precursor in 1,4-butanediol at 300°C under autogenous vapor pressure. Present work shows that KOH is the only reagent that precipitates an aluminum hydrous oxide precursor suitable to synthesize α-alumina in 1,4-butanediol solution. In contrast, the use of NaOH or NH₄OH as the precipitating reagent for the precursor material does not yield the alpha phase. The solution pH at which the precursor materials are precipitated is also a critical factor for the formation of α-Al₂O₃. Phase-pure α-alumina powders were also only synthesized from the aluminum hydrous oxide precursors precipitated in the pH range from 10 to 10.5. The results of X-ray diffraction and scanning electron microscopy indicate that longer reaction times promote the phase transformation from the intermediate boehmite phase to α-alumina. The complete transformation from boehmite to α-alumina requires reaction times of about 12 h.     

Seeding of the Reaction-Bonded Aluminum Oxide Process

The effect of the initial α-Al₂O₃ particle size in the reaction-bonded aluminum oxide (RBAO) process on the phase transformation of aluminum-derived γ-Al₂O₃ to α-Al₂O₃, and subsequently densification, was investigated. It has been demonstrated that if the initial α-Al₂O₃ particles are fine (∼0.2 μm, i.e., 2.9 × 10¹⁴γ-Al₂O₃ particles/cm³), then they seed the phase transformation. The fine α-Al₂O₃ decreases the transformation temperature to ∼962°C and results in a finer microstructure. The smaller particle size of the seeded RBAO decreases the sintering temperature to as low as ∼1135°C. The results confirm that seeding can be utilized to improve phase transformations and densification and subsequently to tailor final microstructures in RBAO-derived ceramics.     

Preparation and Properties of Porous α-Al2O3 Membrane Supports

Strong and permeable macro-porous α-Al₂O₃ membrane supports are made by colloidal filtration of 20 vol% dispersions of α-Al₂O₃ with an average particle size of 600 nm. Intact compacts with very good surface quality were obtained at an optimum pH of 9.5 and dosage of 0.2 wt% ammonium aurintricarboxylate (Aluminon), based on dry alumina. The colloidal stability of the aluminon-stabilized slurries is confirmed by ξ potential measurements. Slight sintering of dense-packed α-Al₂O₃ compacts was found to result in >67% packing density and a bimodal pore-size distribution as derived from shrinkage behavior and gas adsorption studies. Non-stationary single gas permeation measurements showed improved gas permeability, compared with α-Al₂O₃ compacts prepared using powder with a smaller particle size (300 nm). The strength of the disk-shaped alumina compacts within the porosity range of 30%–20% increased from 100 to 300 MPa with a standard deviation of 20 and 50 MPa, respectively.     

Preparation of Larnthanum β-Alumina with High Surface Area by Coprecipitation

Mixtures of La₂O₃ and Al₂O₃ with various La contents were prepared by co-precipitation from La(NO₃)₃ and Al(NO₃)₃ solutions and calcined at 800° to 1400°C. The addition of small amounts of La₂O₃ (2 to 10 mol%) to Al₂O₃ gives rise to the formation of lanthanum β-alumina (La 2 O₃·11–14Al₂O₃) upon heating to above 1000°C and retards the transformation of γ-Al₂O₃ to α-Al₂O₃ and associated sintering.     

Analysis of Phase Distribution for Ceramic Coatings Formed by Microarc Oxidation on Aluminum Alloy

The phase distribution for ceramic coatings formed by microarc oxidation (MAO) on 2024 aluminum alloy was investigated using X-ray diffraction. The results showed that the ceramic coatings mainly consisted of α-Al₂O₃ and γ-Al₂O₃ phases. The percentage of α-Al₂O₃ gradually increased from the external surface to the interface between the coating and the substrate of samples. The surface layer of coatings mainly contained the γ-Al₂O₃ phase, and its fraction of the composition remained almost constant with oxidation time. It is believed that the difference in the amounts of α-Al₂O₃ and γ-Al₂O₃ phases in the different layers of coatings was caused by the various cooling rates of molten Al₂O₃, which temporarily existed in the microarc zone.