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.     

Variations in a Boehmite Gel and Oleic Acid Emulsion under Calcination

In this paper, we describe variations in a boehmite (AlOOH) and oleic acid emulsion during the process of forming superfine α-Al₂O₃ crystallites from the mixture of oleic acid and a boehmite gel precursor. We also propose that the oleic acid decomposes under calcination, generating carbon, which can effectively prevent agglomeration of Al₂O₃ particles. Calcination for the present study was conducted under a reduced oxygen atmosphere, in the temperature range 25°–1100°C. Phase variations of the mixture under calcination were identified by Fourier transform infrared spectrometry (FTIR), X-ray diffractometry (XRD), and transmission electron microscopy (TEM). The FTIR spectra were used when the mixed emulsion of oleic acid and boehmite gel was heated, to investigate the adsorption reaction of the aluminum oleate; the C—O—C cross-linking structure of oxygenation, which aided in carbon formation; and the ability of the carbon generated with α-Al₂O₃ during phase transformation to prevent agglomeration (vermicularity). The products were analyzed by XRD at different temperatures, and TEM was used to examine the individual diameters of the α-Al₂O₃ crystallites.     

Reinvestigation of Al–Si Spinel Phase in Diphasic Al2O3–SiO2 Gel

Mechanical mixture of γ-Al₂O₃ and amorphous SiO₂, and diphasic Al₂O₃/SiO₂ gels of three different compositions were synthesized. They were subjected to heat treatment to various temperatures in the range 900°–1600°C. Qualitative X-ray diffraction data show that these diphasic gels do not crystallize to a combined mixture of θ-Al₂O₃ and α-Al₂O₃ polymorphs at the intermediate stage, prior to mullite formation. Estimated mullite formation data show that the course of its formation from mixed oxides was different from that of diphasic gels. Results are compared with previous findings and the concept of Al–Si spinel formation in the phase transformation of stoichiometric diphasic gel system is substantiated.     

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.     

Influence of Cr and Fe on Formation of α-Al2O3 from γ-Al2O3

The effect of Cr and Fe in solid solution in γ-Al₂O₃ on its rate of conversion to α-Al₂O₃ at 1100°C was studied by X-ray diffraction. The δ form of Al₂O₃ was the principal intermediate phase produced from both pure γ-Al₂O₃ and that containing Fe³⁺ in solid solution, although addition of Fe greatly reduced crystallinity. Reflectance spectra and magnetic susceptibilities showed that Cr exists as Cr⁶⁺ in γ-Al₂O₃ and as Cr³⁺ in α-Al₂O₃, with θ-Al₂O₃ as the intermediate phase. The intermediates formed rapidly, and the rates of their conversion to α-Al₂O₃ were increased by 2 and 5 wt% additions of Fe and decreased by 2 and 4 wt% additions of Cr. An approximately linear relation observed between α-Al₂O₃ formation and decrease in specific surface area was only slightly affected by the added ions. This relation can be explained by a mechanism in which the sintering of δ- or θ-Al₂O₃, within the aggregates of their crystallites, is closely coupled with conversion of cubic to hexagonal close packing of O^2- ions by synchro-shear.     

Transformation and Densification of Nanocrystalline θ-Alumina during Sinter Forging

The sinter forging behavior of α-Al₂O₃ seeded and unseeded nanocrystalline θ-Al₂O₃ was investigated as a function of temperature, stress, and strain rate. Seeded samples exhibited the highest degree of plastic deformation during the θ- to α-AI₂O₃ phase transformation. As a result, microstructure control, increased densification, and a higher degree of transformation were obtained. A uniform microstructure of 150 nm α-Al₂O₃ grains developed, reaching 57% relative density after sintering 1.5 wt%α-Al₂O₃ seeded samples for 30 min at 1060°C. When sinter forged at 0.25 mm/min to 63 MPa and 1060°C for 30 min large deformations during the phase transformation increased the relative density to 74%. When the stress was increased to 235 MPa (1060°C, 30 min), 99.7% dense α-Al₂O₃ with a grain size of 230 nm was obtained. By increasing the sinter forging temperature to 1150°C, 99.5% relative density was achieved at 190 MPa for 30 min.     

Formation and Sintering of Yttria-Doped Tetragonal Zirconia with 50 mol% Alumina Prepared by the Hydrazine Method

Al₂O₃/Y₂O₃-doped ZrO₂ composite powders with 50 mol% Al₂O₃ are prepared by the hydrazine method. As-prepared powders are mixtures of AlO(OH) gel and amorphous ZrO₂ solid solutions containing Y₂O₃ and Al₂O₃. The formation process leading to α-Al₂O₃- t -ZrO₂ composite powders is examined. Hot isostatic pressing is performed for 2 h at 1400°C under 196 MPa using θ-Al₂O₃- t -ZrO₂ composite powders. The resulting dense, sintered α-Al₂O₃- t -ZrO₂ composites show excellent mechanical strength.     

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.     

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.     

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.     

Formation of Zirconia Solid Solutions Containing Alumina Prepared by New Preparation Method

In the system ZrO₂–Al₂O₃, a new method for preparing ZrO₂ solid solutions from ZrCl₄ and AlCl₃ using hydrazine monohydrate is investigated. c -ZrO₂ solid solutions containing up to ∼40 mol% Al₂O₃ crystallize at low temperatures from amorphous materials. The formation mechanism is discussed from IR spectral data. The values of the lattice parameter α increase linearly from 0.5072 to 0.5105 nm with increasing Al₂O₃ content. At higher temperatures, transformation of the solid solutions proceeds as follows: c ( SS ) → t ( ss ) → t ( ss ) +α-Al₂O₃→ m +α-Al₂O₃. m -ZrO₂–α-Al₂O₃ composite ceramics are fabricated by hot isostatic pressing for 2 h at 1250°C and 196 MPa. Microstructures and mechanical properties are examined, in connection with increasing Al₂O₃ content.     

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.     

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.     

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.     

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.     

A Magnetic Resonance Investigation of the Process of Corundum Formation Starting from Sol–Gel Precursors

²⁷Al–MAS (magic angle spinning)–NMR and Fe³⁺ EPR measurements have been performed to follow the local process of corundum formation starting from xerogel on the Al³⁺ and Fe³⁺ sites yielding complementary information. Different heat treatments have been applied to the samples: an isochronous procedure and thermoanalytical measurements stopped by quenching. Despite the different mechanisms of the phase transitions deduced, both seeding and Fe³⁺ doping under the conditions of isochronous procedure favor the formation of α-Al₂O₃ at remarkably low temperatures. The first observed temperature of corundum formation following the isochronous procedure with iron-doped samples is as low as 750°C. The transition alumina, which could be clearly evidenced, is the γ-Al₂O₃ phase (four- and six-fold Al coordination). In undoped or unseeded samples, intermediate Fe species could be detected by ESR and evidence for θ-Al₂O₃ was obtained from ²⁷Al–NMR spectroscopy.     

Anomalous Temperature Dependence of the Growth Rate of the Reaction Layer between Silica and Molten Aluminum

An aluminum/Al₂O₃ composite body is produced by a displacement reaction between SiO₂ and molten aluminum. The growth rate of the reaction layer possesses negative (anomalous) temperature dependence at 1000–1300 K. This study compared reported reaction-kinetic data and investigated causes for this temperature dependence. The reaction product, Al₂O₃, changed from the γ-/θ-Al₂O₃ phase to the α-Al₂O₃ phase in this temperature range and α-Al₂O₃ became the dominant phase at >1273 K. Isothermal transformation of the γ-/θ-Al₂O₃ product phases to the α-Al₂O₃ phase was also observed. Morphologies and scales of the Al₂O₃ phases change drastically at 1173 K; this transition occurred in a spatially discontinuous manner. Reaction-rate retardation was interpreted in terms of occurrence of the competitive and simultaneous reactions to produce different Al₂O₃ phases in this temperature range. It was also found that the hydrogen release from the raw SiO₂ and the SiO₂ phase transformation were not related to the negative temperature dependence.     

Effect of Tween-80 on the Control of Particle Size and Shrinkage Properties of Nanoscale α-Alumina Synthesized by Sol–Gel Processing

The concept of tailored interfaces has been applied to the synthesis of nanoscale α-Al₂O₃. Tween-80 (poly-oxyethylene(20) sorbitan monooleate, T-80) was used as a surface modifier in the sol–gel process for this purpose. High-resolution transmission electron microscopy study of the powder obtained with T-80 confirmed the particle size of α-Al₂O₃ (∼55 nm) and morphology (spherical). The exothermic peak temperature in the differential thermal analysis was shifted to a lower temperature (∼917°C) when the powder was derived from a T-80 modifier content of 10 wt%. X-ray diffraction showed that the α-Al₂O₃ phase was the major phase that existed in modifier-derived powder that was sintered at 1000°C. The experiments, based on linear shrinkage, indicated that the powder with T-80 (10 wt%) could be densified at a low temperature.     

Mechanical Properties of Hot Isostatically Pressed Zirconia-Toughened Alumina Ceramics Prepared from Coprecipitated Powders

Amorphous Al₂O₃–ZrO₂ composite powders with 5–30 mol% ZrO₂ have been prepared by adding aqueous ammonia to the mixed solution of aqueous aluminum sulfate and zirconium alkoxide containing 2-propanol. Simultaneous crystallization of γ-Al₂O₃ and t -ZrO₂ occurs at 870°–980°C. The γ-Al₂O₃ transforms to α-Al₂O₃ at 1160°–1220°C. Hot isostatic pressing has been performed for 1 h at 1400°C under 196 MPa using α-Al₂O₃– t -ZrO₂ composite powders. Dense ZrO₂-toughened Al₂O₃ (ZTA) ceramics with homogeneous-dispersed ZrO₂ particles show excellent mechanical properties. The toughening mechanism is discussed. The microstructures and t / m ratios of ZTA are examined, with emphasis on the relation between strength and fracture toughness.     

Synthesis of AlN Nanopowder from γ-Al2O3 by Reduction–Nitridation in a Mixture of NH3–C3H8

Aluminum nitride (AlN) powders were synthesized by gas reduction–nitridation of γ-Al₂O₃ using NH₃ and C₃H₈ as the reactant gases. AlN was identified in the products synthesized at 1100°–1400°C for 120 min in the NH₃–C₃H₈ gas flow confirming that AlN can be formed by the gas reduction–nitridation of γ-Al₂O₃. The products synthesized at 1100°C for 120 min contained unreacted γ-Al₂O₃. The ²⁷A1 MAS NMR spectra show that Al–N bonding in the product increases with increasing reaction temperature, the tetrahedral AlO₄ resonance decreasing prior to the disappearance of the octahedral AlO₆ resonance. This suggests that the tetrahedral AlO₄ sites of the γ-Al₂O₃ are preferentially nitrided than the AlO₆ sites. AlN nanoparticles were directly formed from γ-Al₂O₃ at low temperature because of this preferred nitridation of AlO₄ sites in the reactant. AlN nanoparticles are formed by gas reduction–nitridation of γ-Al₂O₃ not only because the reaction temperature is sufficiently low to restrict grain growth, but also because γ-Al₂O₃ contains both AlO₄ and AlO₆ sites, by contrast with α-Al₂O₃ which contains only AlO₆.