α-Al2O3 (0001) Surfaces: Atomic and Electronic Structure

New results on the     R ± 9° reconstructed α-Al₂O₃ (0001) surface, which can be obtained after heating at high temperature (1400°C) under vacuum, are presented. The atomic structure has been studied by combining low-energy electron diffractometry and grazing incidence X-ray scattering. The surface structure is found to be perfectly commensurable with the underlying bulk lattice. The surface consists of hexagonal zones of two, nearly perfect, close-packed Al (111) planes separated by a defect of hexagonal periodicity with a parameter of 26.44 Å. This model is consistent with previous surface studies of this reconstruction. The electronic structure has been investigated using valence band photoemission spectroscopy, X-ray absorption spectroscopy at the O K edge, electron energy loss spectroscopy, and X-ray-induced Auger electron spectroscopy. Interpretation of these experimental data in the frame of a self-consistent, tight-binding calculation leads to the conclusion that the     R ± 9° reconstructed surface is more covalent than the (1 × 1) surface. Significant changes in the; Al-O hybridizations are observed; these are likely due to a difference in the interatomic distances along the [0001] axis (relaxations). The increase of covalent character is mainly due to a strong decrease of the Madelung field on the reconstructed surface.     

First-Principles Calculation of Electronic, Optical, and Structural Properties of α-Al2O3

The electronic structure, the linear optical properties, and the structural properties of α-Al₂O₃ in the corundum structure are studied by means of first-principles local density calculations. An indirect band gap of 6.29 eV is obtained, which is almost the same as the direct band gap of 6.31 eV at Γ. The calculated density of states are compared with X-ray photoemission and photoabsorption measurements. Real space charge density analysis shows Al,₂O₃ to be highly ionic with an effective charge formula of Al₂^+2.75O₃^−1.83. The calculated dielectric function is in general agreement with the experimental vacuum ultraviolet data. It is shown that the component with c-direction polarization is in better agreement with the experimental data because it is less affected by the exciton formation near the absorption edge. The intrinsic difference between the calculated local density approximation gap and the measured optical gap is pointed out. Various careful test calculations indicate that the remaining discrepancy in the optical spectra may be in the LDA approximation of the electronic structure theory. The total energy calculation for the ground-state structural properties of α-Al₂O₃ shows excellent agreement with experimental data: the calculated equilibrium volume, c/a ratio, bulk modulus, and internal parameters for Al and O atoms differ from measured values by 0.0, −4.3, −4.0,0.85, and 1.96 percent, respectively. The calculations for the electronic structure and the optical properties are repeated with crystal parameters obtained at 2000°C. The result shows only a slight reduction in the band gap (to 5.76 eV at Γ), with the optical spectra essentially unchanged.     

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.     

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.     

Calculated Structures and Energies of Grain Boundaries in α-Al2O3

Static-lattice calculations of the energies and structures of {10 1 n} (n = 0, 1, and 4) and {0001} grain boundaries in α-AI₂O₃ using three different potentials are reported. It was found that the energies of boundaries perpendicular to the (0001) direction vary between 0.3 and 0.9 J·m⁻². These compare favorably to experimental values, and the results are not sensitive to the choice of potential. The energies of the other boundaries do depend on the potential, but their relaxed structures do not. Using the most reliable potential, the energies of boundaries which have been studied by high-resolution electron microscopy range from 1.0 to 1.7 J.m⁻². Simulated images of the calculated structures compare favorably with high-resolution electron microscope micrographs of these boundaries.     

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.     

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.     

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.     

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.     

Role of Oxygen in Microstructure Development at Solid-State Diffusion-Bonded Cu/α-Al2O3 Interfaces

Microstructure development at solid-state diffusion-bonded Cu/α-Al₂O₃ interfaces has been studied using optical and electron microscopy. High-purity Cu foil was bonded between basal-oriented α-Al₂O₃ single-crystal plates at 1040°C for 24 h in a vacuum of ∼1.3 × 10⁻⁴ Pa (1 × 10⁻⁶ torr). Optical microscopy of as-bonded specimens revealed a large Cu grain size, fine pores, and long needles of Cu₂O at the interface. Bulk specimens were annealed at 1000°C for various times under controlled oxygen partial pressures in CO/CO₂ mixtures. Consistent with a thermochemical analysis, CuAlO₂ could be formed at the interfaces. The CuAlO₂ was acicular and discontinuous, but occurred in a uniform distribution over the bulk specimen interfaces.     

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.     

Determination of the Parameters of Native Disorder in α-Al2O3

Compensation of single-crystal, acceptor-dominated samples of α-Al₂O₃ by hydrogen leads to samples showing conductivity by protons, electrons, holes, Al‴_i, and V ‴_Al Values of the partial conductivities as a function of temperature are used to determine the parameters of native ionic and electronic disorder.     

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 .     

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.