Sc3+–Lu3+-Doped α-SiAlONs

Sc³⁺ and dual Sc³⁺–Lu³⁺-doped α-SiAlON compositions were sintered by hot pressing, and the formation behavior, microstructure, and mechanical properties were assessed. It was found that the small cation Sc³⁺ could not be accommodated into the α-SiAlON structure alone. The addition of Lu₂O₃ in the composition induces the Sc³⁺ cation to enter the α-SiAlON structure, and leads to the production of α-SiAlON with an elongated-grain microstructure. Transmission electron microscopy analysis shows that α-SiAlON grains always contain an α-Si₃N₄ core, implicating heterogeneous nucleation to be in present in a mixed lutetium/scandium-doped α-SiAlON system.     

Calculation of the Electronic Structure of sp Elements in β-Si3N4 with Correlation to Solubility and Solution Effects

Local electronic structures around the sp -element solutes in β-Si₃N₄ have been examined from first principles by the discrete-variational (DV) Xα method for the first time. A solute atom, M, was put substitutionally into the Si sites of a (Si₃N₁₀)^18- cluster, and Hartree–Fock–Slater equations were solved self-consistently. The relationship between the calculated overlap population of electrons between the solute and nitrogen and the experimental solubility and the magnitude of the solution effects was examined, and a satisfactory correlation was found. Based on the present calculations, the Sp -element solutes are classified into the following three categories: (1) Elements such as K, Ca, Na, and Mg are almost insoluble and localize at the intergranular glassy phase or precipitates. (2) Remarkable solution effects to lose the local covalency are expected for Li, Be, Ga, and Al. They may enhance the volume diffusivity and also decrease the elastic constants considerably. (3) For B, C, and Ge, no significant reduction in local covalency is expected when they dissolve. The correlations between the calculated overlap population and empirical ionic radii of solute atoms and Pauling's electronegativity were found to be weak, indicating that these empirical parameters are not sufficient to represent the electronic structure change due to the substitution and hence solubility and solution effects, although they are useful in describing general trends.     

Atomistic Simulation of Defect Structures and Ion Transport in α-Fe2O3 and α-Cr2O3

Computer-modelling techniques are applied to the calculation of defect formation and migration energies in α-Fe₂O₃ and α-Cr₂O₃: both electronic and lattice defects are considered. The results are used to predict Arrhenius energies for cation and anion migration in different composition and temperature regimes and show reasonable agreement with experimental data where these are available.     

Structural and Photoluminescence Properties of Ce3+- and Tb3+-Activated Lu-α-Sialon

The crystal structure and photoluminescence properties of undoped and Ce³⁺- or Tb³⁺-doped Lu-α-Sialon are reported. Lu-α-Sialon with the composition of Lu_0.367Si_9.9Al_2.1ON₁₅ crystallizes in a trigonal system with a =7.8013(1) Å, c =5.6827(1) Å, in the space group P 31 c . The Lu³⁺ ion is accommodated at the interstice site at 2 b and directly connected by seven (N, O) atoms with an average bond length of 2.624 Å. The incorporation of large Ce³⁺ and Tb³⁺ ions on small Lu³⁺ site results in the expansion of the unit cell as expected but keeping the average Lu_Ln-(N, O) (Ln=Ce, Tb) distances nearly constant with slight distortion in the lattices. The optical band gap of Lu-α-Sialon is about 5.25 eV determined by the diffused reflection spectrum. Lu-α-Sialon:Ce³⁺ (2 mol%) exhibits efficiently greenish-blue emission at about 486 nm under near-ultraviolet (UV) excitation originating from the 5 d →4 f ¹5 d ⁰ transition of Ce³⁺. Lu-α-Sialon:Ce³⁺ is a potential candidate phosphor for white UV-LED applications due to its high quantum efficiency and excellent thermal stability. Lu-α-Sialon:Tb³⁺ (2 mol%) emits strong mixed blue and green light in equivalent due to the transitions of ⁵D₃→⁷F_J and ⁵D₄→⁷F_J of Tb³⁺.     

Lattice Parameters of the α-Fe2O3-Cr2O3 Solid Solution

Lattice parameter-composition relations are shown in the system α-Fe₂O₃-Cr₂O₃. This information is of some value in determining the composition of scales developed during the oxidation of alloys based on Fe-Cr, such as stainless steels.     

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.     

Nucleation and Growth of α'-SiAlON on α-Si3N4

Morphology, composition, and growth defects of α'-SiAION have been studied in a fine-grained material with an overall composition Y_0.33Si₁₀Al₂O₁N₁₅ prepared from α-Si₃N₄, AlN, Al₂O₃, and Y₂O₃ powders. TEM analysis has shown that fully grown α'-SiAloN grains always contain an α-Si₃N₄ core, implicating heterogeneous nucleation operating in the present system. The growth mode is epitaxial, despite the composition and lattice parameter difference between α-Si₃N₄ and α'-SiAlON. The inversion boundary that separates two domains in the seed crystal is seen to continue in the grown α'-SiAION. Lacking a special growth habit, the growth typically proceeds from more than one site on the seed crystal, and the different growth fronts impinge on each other to give an equiaxed appearance of α'-SiAlON. Misfit dislocations on the α/α'interface are identified as [0001] type ( b = 5.62 Å) and 1/3 [1 2 10] type ( b = 7.75 Å). These nucleation and growth characteristics dictate that microstructural control of α'-SiAlON must rest on the size distribution of the starting α-Si₃N₄ powder.     

Solution of MgO, CaO, and TiO2 in α-AI2O3

Atomistic simulation methods are used to predict the preferred solution mechanisms and their associated energies. Both full- and partial-charge models are discussed. It was found that defect clustering always results in a significant lowering of the solution energies but does not change the preferred solution reaction of any of the oxides. The large size of the Ca²⁺ ion results not only in a higher solution energy (lower solubility) for CaO compared to MgO but also to a different preferred compensation mechanism. The predicted energy for the cosolution of MgO and TiO₂ is dramatically smaller than for either oxide separately. Conversely, whereas solution of TiO₂ aids the solution of CaO, additions of CaO do not support TiO₂, Solution.     

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 Investigation of the Atomic and Electronic Structures of α-Al2O3(0001)/Ni(111) Interfaces

Atomic and electronic structures of α-Al₂O₃(0001)/Ni(111) interfaces have been investigated using the first-principles pseudopotential method. Models with different rigid-body translations parallel to the interface for both the O-terminated and Al-terminated interfaces are examined in order to clarify the overall features. Results indicate that the interface stoichiometry as well as the interface configuration has significant effects on the adhesive and electronic properties. The bonding nature of the O-terminated interfaces is explained by strong ionic and Ni-3d/O-2p orbital hybridization interactions, and that of the Al-terminated interfaces is explained mainly by image-charge like electrostatic and Ni–Al hybridization interactions, although there is some Ni–O hybridization for the O-site model. Orbital hybridization and adhesive energies are larger than those in the corresponding Al₂O₃/Cu interfaces, because Ni has higher activity for making bonds with ceramics than Cu.     

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.     

Solubility of KUO3 in BaUO3

The formation of a solid solution between cubic perovskne-type KUO₃ and pseudocubic BaUO₃ was investigated. The reaction begins at 550°C, and the solubility of KUO₃ reaches more than 30 mol% KUO₃ in BaUO₃ at 750°C. The region in which a single-phase solid solution exists was determined. The variation of the lattice parameter of the reacted samples was caused by solid solution formation and by oxygen absorption. The electrical conductivities of the samples varied with composition and showed a distinct maximum. The activation energy for electric conduction was very low compared to that for UO_z+x, or U₃O₈.     

Initial Sintering of α-Cr2O3

Since the difference between oxygen-ion and cation diffusion coefficients is greater for α-Cr₂O₃ than for α-Fe₂O₃ or α-Al₂O₃, a study of initial-sintering kinetics was undertaken to show unequivocally which species is rate controlling. Fine powders of α-Cr₂O₃, obtained by thermal decomposition of reagent-grade (NH₄)₂Cr₂O₇, were lightly compacted and their isothermal rates of shrinkage were determined between 1050° and 1300°C. Resultant data follow volume-diffusion sintering models, and calculated diffusion coefficients agree with, those measured for oxygen ions in α-Cr₂O₃. There is little evidence that oxygen diffusion along grain boundaries becomes so enhanced that chromium ions are left in control of the process.     

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.     

Modification of αAI2O3 Platelet/ZrO2 Matrix Interface by Epitaxial Growth of β-AI2O3 on the α-AI2O3

An epitaxial β-alumina crystal growth method was used to modify α-AI₂O₃ platelet surfaces before inclusion as a reinforcing phase in partially stabilized zirconia (3Y-TZP). The as-grown surface phase was Na-β"-AI₂O₃. This was converted to Ca-β"-AI₂O₃ by ion exchange, as the latter is more temperature-stable at composite sintering temperatures. The conditions of formation, thermal stability, and chemical compatibility of these interfacial phases were examined. α-AI₂O₃ platelets with Ca-β"-AI₂O₃ film were incorporated into 3Y-TZP. The β"-AI₂O₃/ZrO₂ interface was found to promote platelet debonding and pullout, thus enhancing the α-AI₂O₃ platelet/crack interactions during the fracture process.     

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.     

Mixed Protonic–Electronic Conduction in "α-Al2O3": An Alternative Hypothesis

As an alternative, the voltage data of Kurita et al . recently published on galvanic cells with commercial α-Al₂O₃ as a solid electrolyte and with O₂, H₂O/α-Al₂O₃ as well as H₂, H₂O/α-Al₂O₃ as electrodes can be quantitatively described by assuming that α-Al₂O₃ represents a mixed sodium ionic–electronic conductor rather than a protonic–electronic conductor. From the evaluation of the experimental data, numerical values for the p -type electronic conduction parameter are obtained that agree sufficiently well with the data known to date for the sodium ion conductor Na-beta-Al₂O₃.     

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.     

Anion Diffusion in α-Cr2O3

Oxygen-18 exchange between gaseous oxygen, held at a pressure of 125 mm Hg in a Pt10Rh chamber, and spheres of α-Cr₂O₃ containing three or less grains was determined from 1100° to 1450°C. Isotope equilibrium on crystal surfaces appears to be quickly established, and the rate-determining factor is self-diffusion conforming to the relation D = 15.9 exp(-101,000/ RT) cm²sec⁻¹. Changing sphere diameters caused no detectable variation in diffusion coefficients. Anions are the much slower diffusing species in this oxide.