Nanocrystalline α-Al2O3 Using Sucrose

Nanocrystalline α-Al₂O₃ ceramic powders have been prepared from an aqueous solution of aluminum nitrate and sucrose. Soluble Al ion-sucrose solution forms the precursor material once it is completely dehydrated. Heat treatment of the dehydrated precursors at low temperature (600°C) results in the formation of porous single-phase α-Al₂O₃. The precursor and heat-treated powders have been characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), and BET surface area analysis. The phase-pure nanocrystalline α-Al₂O₃ particles had an average specific surface area of >190 m²/g, with an average pore size between 18 and 25 nm.     

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.     

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.     

Sintering of Nanophase γ-Al2O3 Powder

The sintering of ultrafine γ-Al₂O₃ powder (particle size ∼10–20 nm) prepared by an inert gas condensation technique was investigated in air at a constant heating rate of 10°C/min. Qualitatively, the kinetics followed those of transition aluminas prepared by other methods. Measurable shrinkage commenced at ∼ 1000°C and showed a region of rapid sintering between ∼1125° and 1175°C followed by a transition to a much reduced sintering rate at higher temperatures. Starting from an initial density of ∼0.60 relative to the theoretical value, the powder compact reached a relative density of 0.82 after sintering to 1350°C. Compared to compacts prepared from the as-received powder, dispersion of the powder in water prior to compaction produced a drastic change in the microstructural evolution and a significant reduction in the densification rate during sintering. The incorporation of a step involving the rapid heating of the loose powder to ∼1300°C prior to compaction (which resulted in the transformation to α-Al₂O₃) provided a method for significantly increasing the density during sintering.     

Defect Structure of α-Al2O3 Doped with Titanium

Titanium-doped α-Al₂O₃ exhibits a high-temperature conductivity which is ionic at high oxygen pressures and electronic at low oxygen pressures. Both are isotropic. The temperature dependence of conductivity under conditions where equilibrium with the atmosphere is not maintained indicates both the position of the energy level of titanium (Ti_Al^x) in the forbidden gap and the temperature dependence of the mobility of the native ionic defects (Al vacancies, V _Al^m). Optical absorption responsible for the pink color of the reduced crystals is measured as a function of p o₂ and is used to determine concentrations of Ti³⁺ and Ti⁴⁺. Parameters for the equilibrium constants of the reactions involving electrons by which the composition of Al₂O₃:Ti and undoped Al₂O₃ is varied are determined. The chemical diffusion data by Jones et al. are described quantitatively.     

Effect of Seeding and Water Vapor on the Nucleation and Growth of α-Al2O3 from γ-Al2O3

Isothermal transformation kinetics and coarsening rates were studied in unseeded and alpha-Al₂O₃-seeded γ-Al₂O₃ powders heated in dry air and water vapor. Unseeded samples heated in dry air transformed to alpha-Al₂O₃ with an activation energy of 567 kJ/mol. Seeding with alpha-Al₂O₃ increased the transformation rates and reduced incubation times by providing low-energy sites for nucleation/growth of the alpha-Al₂O₃ transformation. The activation energy for the transformation was reduced to 350 kJ/mol in seeded samples heated in dry air. Seeded samples completely transformed to alpha-Al₂O₃ after 1 h at 1050°C when heated in dry air compared to 1 h at 925°C when heated in saturated water vapor. The combined effects of a lower nucleation barrier due to seeding and the increased diffusion due to water vapor reduced the activation energy for the transformation by 390 kJ/mol and the transformation temperature by ∼225°C compared to the unseeded samples heated in dry air. The accelerated kinetics is believed to be due to increased surface diffusion.     

Fabrication of Single-Crystal α-Al2O3 Nanorods by Displacement Reactions

Nanometer-sized Al₂O₃ rods are fabricated by sintering a powder mixture of Al and SiO₂. The sintered product is leached in HF–HNO₃ solution, followed by rinsing and washing before the nanorods are collected. The yield of the product is about 50 wt%. Transmission electron microscopy reveals that these rods are 1 to 2 μm long and have a diameter of 20 to 100 nm, while electron diffraction confirms that these rods are single crystals of α-Al₂O₃. It is observed that these rods have either round or slightly sharp tips, which is different from those having droplet-like tips that are usually found in products fabricated by catalytic reactions.     

Adsorption of PAA on the α-Al2O3 Surface

The specific system of interest is the polyacrylic acid (PAA) and (0001) α-Al₂O₃ surface, which was modeled and simulated by Cerius² 4.9 software with empirical potentials. The simulation predicted that the adsorbed conformations of PAA with a molecular weight ( M _w) of 5000 were train and tail at pH <4 and >10, respectively. After gradually inserting additional PAA molecular chains, the adsorption reached a saturated amount. Gel permeation chromatography experimental results showed that the adsorption amount at pH 3.6 was three times greater than that at pH 11. Based on the results from simulations and experiments, a successively increasing pH environment was modeled to illustrate the possibility of optimizing electro-steric effects by combining the higher adsorption density at a lower pH and strong steric repulsion of tail-adsorbed configuration at a higher pH.     

Structure of Na- and Ca-β"-Al2O3 Coatings on α-Al2O3 Single-Crystal Platelets

The structure of Na- and Ca-β"-Al₂O₃ coatings on α-Al₂O₃ single-crystal platelets has been studied by optical and electron microscopy and X-ray and electron diffraction. The growth features and potential interface weakening effects of the modified platelets in dispersed-particle reinforced composites are discussed.     

Defect Structure of α-Al2O3 Doph with Magnesium

The conductivity of single crystals of Al₂O₃+ Mg and the ionic and electronic transference numbers were measured at high temperatures as a function of orientation, oxygen pressure, and temperature. Optical absorption in the visible range was measured on cooled annealed crystals. The results are interpreted on the basis of a model with either Al_i^3- or V _O as the dominant native defects and lead to expressions for the ionic mobility as a function of T and orientation, and for the position of the Mg_Al'level in the forbidden gap.     

Effect of Divalent Cation Additives on the γ-Al2O3-to-α-Al2O3 Phase Transition

The effect on the γ-Al₂O₃-to-α-Al₂O₃ phase transition of adding divalent cations was investigated by differential thermal analysis, X-ray diffractometry, and surface-area measurements. The cations, Cu²⁺, Mn²⁺, Co²⁺, Ni²⁺, Mg²⁺, Ca²⁺, Sr²⁺, and Ba²⁺, were added by impregnation, using the appropriate nitrate solution. These additives were classified into three groups, according to their effect: (1) those with an accelerating effect (Cu²⁺ and Mn²⁺), (2) those with little or no effect (Co²⁺, Ni²⁺, and Mg²⁺), and (3) those with a retarding effect (Ca²⁺, Sr²⁺, and Ba²⁺). The crystalline phase formed by reaction of the additive with γ-Al₂O₃ at high temperature was a spinel-type structure in groups (1) and (2) and a magnetoplumbite-type structure in group (3). In groups (2) and (3), a clear relationship was found between the transition temperature and the difference in ionic radius of Al³⁺ and the additive (Δ r ): The transition temperature increased as Δ r increased. This result indicates that additives with larger ionic radii are more effective in suppressing the diffusion of Al³⁺ and O²⁻ in γ-Al₂O₃, suppressing the grain growth of γ-Al₂O₃, and retarding the transformation into α-Al₂O₃.     

The Dominant Type of Atomic Disorder in α-Al2O3

Comparison of the energy of formation per defect, as deduced from experimental results on α-Al₂O₃ doped with donors and acceptors on the basis of various models, with theoretical values calculated by Dienes et al . for various disorder models shows closest correspondence of the exFrimentd values with the smallest theoretical values (obtained for Schottky disorder). This indicates that the theoretical results are reliable and that Schottky disorder is the major type of atomic disorder in α-Al₂O₃. Creep data on Al₂O₃:Fe by Hollenberg and Gordon make it possible to determine the enthalpy of Frenkel disorder of Al.     

Growth of α-Al2O3 Crystals by Na2O Evaporation

A technique for growing α-Al₂O₃ crystals is described in which Na₂O·11Al₂O₃ is dissolved in a liquid of composition Na₂O·4TiO₂·3Al₂O₃. Alpha Al₂O₃ is precipitated as Na₂O evaporates from the system; Na₂O·11Al₂O₃ serves as a source of Al₂O₃, and Na₂O in the liquid. The content of solids in the mixture is always such that it does not melt completely. The size of the α-Al₂O₃ crystals grown is related to the Na₂O content of the composition. Crystals as large as 4000 by 3000 μm in the α-axis direction and 500 μm in the c -axis direction have been grown.     

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.     

The Defect Structure of Unintentionally Doped α-Al2O3 Crystals

Electrical conductivity and the ionic and electronic transference numbers were determined for two types of unintentionally doped single crystalline sapphire for current directions perpendicular to the r plane (1102). One was acceptor dominated and the other was initially donor (H_i^x) dominated, changing to acceptor domination after a prolonged anneal at 1600°C. The positions of the electronic energy levels of dominant impurities and the constants regulating the oxidation-reduction of these impurities and of pure Al₂O₃ are determined. The latter shows a discrepancy with an expression reported previously.     

Identification of α-Al2O3 Precipitates in Rutile Single Crystals

After high-temperature reaction between Al₂O₃ and TiO₂ crystals, precipitates found in rutile were characterized by electron microprobe and X-ray diffraction methods and by optical and electron microscopy. The precipitates were identified as α-Al₂O₄. Geometric and crystallographic orientation relations with the TiO₂ matrix constitute a reverse case of rutile precipitation in star sapphire .     

Effect of Monovalent Cation Additives on the γ-Al2O3-to-α-Al2O3 Phase Transition

The effect of monovalent cation addition on the γ-Al₂O₃-to-α-Al₂O₃ phase transition was investigated by differential thermal analysis, powder X-ray diffractometry, and specific-surface-area measurements. The cations Li⁺, Na⁺, Ag⁺, K⁺, Rb⁺, and Cs⁺ were added by an impregnation method, using the appropriate nitrate solution. β-Al₂O₃ was the crystalline aluminate phase that formed by reaction between these additives and Al₂O₃ in the vicinity of the γ-to-α-Al₂O₃ transition temperature, with the exception of Li⁺. The transition temperature increased as the ionic radii of the additive increased. The change in specific surface area of these samples after heat treatment showed a trend similar to that of the phase-transition temperature. Thus, Cs⁺ was concluded to be the most effective of the present monovalent additives for enhancing the thermal stability of γ-Al₂O₃. Because the order of the phase-transition temperature coincided with that of the formation temperature of β-Al₂O₃ in these samples, suppression of ionic diffusion in γ-Al₂O₃ by the amorphous phase containing the added cations must have played an important role in retarding the transition to α-Al₂O₃. Larger cations suppressed the diffusion reaction more effectively.     

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.     

Extrusion of α-Al2O3–Boehmite Mixtures

A water-based binder system consisting of colloidal boehmite (AlO(OH)) and 0.3 wt% poly(vinyl alcohol) was developed for low-pressure piston-type extrusion of alumina. Batches using this binder system and two different types of commerical alumina powder were characterized with regard to extrusion pressure, densification, and physical properties in the green and fired states.