Critical Factors Affecting the Wettability of α-Alumina by Molten Aluminum

The wetting behaviors of α-Al₂O₃ single crystals with three different faces—R(01 1 2), A(011 2 0), and C(0001)—and polycrystals (PC) by molten aluminum were studied over a wide temperature range using both a conventional and an improved sessile-drop method. The critical factors affecting the wettability, such as temperature, atmosphere, substrate surface roughness, and crystallographic orientation, and the influence from the experimental technique, were thoroughly investigated. The results show that the aluminum surface oxidation and the thickness of the oxide film have a pronounced effect on the wettability, especially at low temperatures. To eliminate this effect, the experimental temperature must be over a critical value. Vacuum favors lowering this value compared with atmosphere, and the improved sessile-drop method, particularly using an impingement-dropping mode (I-mode), helps to weaken this effect by mechanical disruption and removal of the oxide film. However, the dropping distance and the dropping force must be controlled to prevent an overspreading of the drop. The effects of the substrate surface roughness and temperature are not significant in the case of a clean aluminum surface and a fine-prepared alumina surface. On the other hand, the effect of the alumina surface crystallographic orientation is noticeable and the wettability is in the order of R > A > PC > C. The intrinsic contact angles of the Al/α-Al₂O₃ system in the temperature range of 1000°–1500°C were estimated to be 76°–85° for the R and A faces, 88°–100° for the C face, and 77°–90° for the polycrystal, depending on the temperature.     

Growth of Tabular α-Al2O3 Grains on Porous Alumina Substrate

Gradient, porous alumina ceramics were prepared with the characteristics of microsized tabular α-Al₂O₃ grains grown on a surface with a fine interlocking feature. The samples were formed by spin-coating diphasic aluminosilicate sol on porous alumina substrates. The sol consisted of nano-sized pseudo-boehmite (AlOOH) and hydrolyzed tetraethyl orthosilicate [Si(OC₂H₅)₄]. After drying and sintering at 1150°–1450°C, the crystallographic and chemical properties of the porous structures were investigated by analytical electron microscopy. The results show that the formation of tabular α-Al₂O₃ grains is controlled by the dissolution of fine Al₂O₃ in the diphasic material at the interface. The nucleation and growth of tabular α-Al₂O₃ grains proceeds heterogeneously at the Al₂O₃/glass interface by ripening nano-sized Al₂O₃ particles.     

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.     

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.     

Synthesis of Magnesium Aluminate Spinel Platelets from α-Alumina Platelet and Magnesium Sulfate Precursors

Spinel platelets were formed from a powder mixture of 3–5 μm wide and 0.2–0.5 μm thick α-Al₂O₃ and 1–8 μm (average 3 μm) MgSO₄ heated 2 h at 1200°C. The hexagonal platelet shape of the original α-Al₂O₃ platelet was maintained in the spinel, although their size was slightly increased and their surface roughened. When a mixture of α-Al₂O₃ platelets and MgO powder was heated 3 h at 1400°C, the spinel formed lost the platelet morphology of the alumina.     

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.     

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.     

Critical Factors Affecting the Wettability of α-Alumina by Molten Aluminum

The wetting behaviors of α-Al₂O₃ single crystals with three different faces—R(01[Onemacr]2), A(01[Twomacr]0), and C(0001)—and polycrystals (PC) by molten aluminum were studied over a wide temperature range using both a conventional and an improved sessile-drop method. The critical factors affecting the wettability, such as temperature, atmosphere, substrate surface roughness, and crystallographic orientation, and the influence from the experimental technique, were thoroughly investigated. The results show that the aluminum surface oxidation and the thickness of the oxide film have a pronounced effect on the wettability, especially at low temperatures. To eliminate this effect, the experimental temperature must be over a critical value. Vacuum favors lowering this value compared with atmosphere, and the improved sessile-drop method, particularly using an impingement-dropping mode (I-mode), helps to weaken this effect by mechanical disruption and removal of the oxide film. However, the dropping distance and the dropping force must be controlled to prevent a receding contact angle. The effects of the substrate surface roughness and temperature are not significant in the case of a clean and a fine-prepared aluminum surface. On the other hand, the effect of the aluminum crystallographic orientation is noticeable and the wettability is in the order of R > A > PC > C. The intrinsic contact angles of the Al/α-Al₂O₃ system in the temperature range 1000°–1500°C were estimated to be 76°–85° for the R and A faces, 88°–100° for the C face, and 77°–90° for the polycrystal, depending on the temperature.     

New Sol–Gel Route for the Preparation of Pure α-Alumina at 950°C

An anhydrous alumina (Al₂O₃) sol was prepared from aluminum isopropoxide and an organic solvent, using an acetic acid stabilizer. The complete conversion of the dried sol to α-Al₂O₃ was accomplished at a temperature of 950°C by a single transition via γ-Al₂O₃. Al₂O₃ that was deposited via dip coating resulted in amorphous films, even after annealing at 1100°C, because of the silicon diffusion from the substrate. This phenomenon was avoided using a rapid thermal treatment in a flame after dip coating, which resulted in uniform thin films that are converted to α-Al₂O₃ via heat treatment.     

Preparation of Nanometer-Sized α-Alumina Powders by Calcining an Emulsion of Boehmite and Oleic Acid

This study proposes a method to form ultrafine α-Al₂O₃ powders. Oleic acid is mixed with Al(OH)₃ gel. The gel is the precursor of the Al₂O₃. After it is mixed and aged, the mixture is calcined in a depleted oxygen atmosphere between 25° and 1100°C. Oleic acid evaporates and decomposes into carbon during the thermal process. Residual carbon prevents the growth of agglomerates during the formation of α-Al₂O₃. The phase transformation in this process is as follows: emulsion →γ-Al₂O₃→δ-Al₂O₃→θ-Al₂O₃→α-Al₂O₃. This process has no clear θ phase. Aging the mixed sample lowers the formation temperature of α-Al₂O₃ from 1100° to 1000°C. The average crystallite diameter is 60 nm, measured using Scherrer's equation, which is consistent with TEM observations.     

Kinetics and Microstructural Evolution of Heterogeneous Transformation of θ-Alumina to α-Alumina

Ultrafine (<0.1 μm) high-purity θ-Al₂O₃ powder containing 3–17.5 mol%α-Al₂O₃ seeds was used to investigate the kinetics and microstructural evolution of the θ-Al₂O₃ to α-Al₂O₃ transformation. The transformation and densification of the powder that occurred in sequence from 960° to 1100°C were characterized by quantitative X-ray diffractometry, dilatometry, mercury intrusion porosimetry, and transmission and scanning electron microscopy. The relative bulk density and the fraction of α phase increased with annealing temperature and holding time, but the crystal size of the α phase remained ∼50 nm in all cases at the transformation stage (≤1020°C). The activation energy and the time exponent of the θ to α transformation were 650 ± 50 kJ/mol and 1.5, respectively. The results implied the transformation occurred at the interface via structure rearrangement caused by the diffusion of oxygen ions in the Al₂O₃ lattice. A completely transformed α matrix of uniform porosity was the result of appropriate annealing processes (1020°C for 10 h) that considerably enhanced densification and reduced grain growth in the sintering stage. The Al₂O₃ sample sintered at 1490°C for 1 h had a density of 99.4% of the theoretical density and average grain size of 1.67 μm.     

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.     

Nano α-Al2O3 Powder Preparation by Calcining an Emulsion Precursor

An experimental study has been conducted to evaluate the formation of nano α-Al₂O₃ under various conditions, such as different calcining temperatures and emulsion ratios of aqueous aluminum nitrate solutions and oleic acid with a high-speed stirring mixer. Four batches of the precursor powders were calcined at three different temperatures of 1000°, 1050°, and 1100°C for 2 h and a terminal product of nano α-Al₂O₃ powders was obtained. The products have been identified by X-ray diffraction (XRD), specific surface area measurement scanning electron microscope, and transmission electron microscope (TEM). The XRD results show that the phase of powders is determined to be α-Al₂O₃, indicating that the overall process has been effective. The optimum calcination temperature of the precursor powder for crystallization of nano α-Al₂O₃ was found to be 1000°C for 2 h. The TEM image indicates that the particle grains have a sub-spherical shape with a mean size of 50–100 nm.     

Inhibition of Sintering and Surface Area Loss in Phosphorus-Doped Corundum Derived from Diaspore

The influence of magnesium, phosphorus, and iron additions on the low-temperature (≤1000°C) sintering of nanocrystalline α-Al₂O₃ derived from α-AlOOH has been investigated. α-AlOOH powder with a surface area of 50 m²/g yielded α-Al₂O₃ products with surface areas of 150 and 80 m²/g after dehydration at temperatures of 400° and 500°C, respectively. However, these products were prone to sintering at >600°C, and the surface area was reduced to 15 m²/g within only 1 h at 1000°C. Although magnesium and iron doping had no discernible effect, the presence of phosphorus inhibited sintering and surface-area loss significantly. Samples doped with 1%–2% phosphorus had surface areas of >31 m²/g after 100 h at 1000°C. Atomic force microscopy studies of α-Al₂O₃ pseudomorphs derived from α-AlOOH single crystals also demonstrated the inhibiting effect of phosphorus, as the rate of crack elimination was reduced on phosphorus-modified surfaces. The effects of the dopants are discussed with regard to their potential influence on α-Al₂O₃ surface energy and diffusivity.     

Abnormal Grain Growth During Rapid Annealing of Sol–Gel Alumina Thin Film Deposited on Ni-Based Superalloy

A ∼50 nm thick alumina layer was deposited on an Ni-based superalloy substrate by a sol–gel method. α-AlOOH particles presented in the layer after drying at 140°C transformed mostly to α-Al₂O₃ grains within ∼1 min at 1100°C under a low oxygen partial pressure annealing environment. During the same time period, the α-Al₂O₃ grains grew significantly in the lateral direction, resulting in the aspect ratio of grain diameter to thickness of ∼20. The presence of a preferred orientation in the α-Al₂O₃ layer suggested that the mechanism for the lateral growth was abnormal. The lateral growth mechanism appeared to become very slow when a critical thickness (∼100 nm) was reached.     

Ultrafine Alumina Particles Prepared by Mechanochemical/Thermal Processing

Ultrafine alumina particles have been prepared by the mechanical milling and subsequent heat treatment of a mixture of AICI3 and CaO. Heat treatment of the as-milled powder at temperatures above 350°C and washing with water resulted in γ-Al₂O₃ particles 10–20 nm in size. Single phase α-Al₂O₃ was formed in the sample after heat treatment at 1250°C. This study demonstrates a novel process for synthesizing nanoscale alumina particles.     

Synthesis of Nanocrystalline Aluminum Nitride by Nitridation of δ-Al2O3 Nanoparticles in Flowing Ammonia

Nanocrystalline aluminum nitride (AlN) with surface area more than 30 m²/g was synthesized by nitridation of nanosized δ-Al₂O₃ particles using NH₃ as a reacting gas. The resulting powders were characterized by CHN elemental analysis, X-ray diffraction (XRD), Fourier transform infrared spectra, X-ray photoelectron spectra, field-emission scanning electron microscopy, transmission electron microscopy, and Brunauer–Emmett–Teller surface area techniques. It was found that nanocrystalline δ-Al₂O₃ was converted into AlN completely (by XRD) at 1350°–1400°C within 5.0 h in a single-step synthesis process. The complete nitridation of nanosized alumina at relatively lower temperatures was attributed to the lack of coarsening of the initial δ-Al₂O₃ powder. The effect of precursor powder types on the conversion was also investigated, and it was found that α-Al₂O₃ was hard to convert to AlN under the same conditions.     

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.     

Lowering Crystallization Temperatures by Seeding in Structurally Diphasic Al2O3-MgO Xerogels

Thermal reactions in 93% Al₂O₃-7% MgO and 95.8% Al₂O₃-4.2% MgO gels seeded with α-Al₂O₃, MgAl₂O₄, α-Fe₂O₃, and SiO₂, sols were investigated by differential thermal analysis to determine the extent of nucleation catalysis of solid-state reactions. Seeding with α-Al₂O₃ lowered the α-Al₂O₃ crystallization temperature in these xerogels by 100° to 150°C. Spinel seeds have much less effect on the γ-α transition, and α-Fe₂O₃ and SiO₂ seeds do not affect it significantly. Isostructural seeding of gels may therefore permit lower ceramic processing temperatures.     

Solid-Phase Epitaxy of Boehmite-Derived α-Alumina on Hematite Seed Crystals

The seeded transformation of boehmite-derived alumina was studied by transmission electron microscopy. Crystallographic analysis confirmed that the growth of α-Al₂O₃ on α-Fe₂O₃ seed crystals occurs by solid-phase epitaxy, with the orientation relationship [0001]_Al2O₃||[0001]_Fe2O₃ and [11 2 0]_Al2O₃||[11 2 0_Fe2O₃.