Добірка наукової літератури з теми "Aluminum oxynitride spinel"

There is considerable controversy concerning the point and space group of crystals based on the spinel structure. The debated question is whether the space group of spinel crystals is or . has noncentrosymmetric point group and is symmorphic; has centrosymmetric point group and is nonsymmorphic. One characteristic of the space group that distinguishes it from is that reflections hkl such that h+k, k+l, h+l ≠ 4n, for example {200}, are kinematically forbidden.MgAl2O4 spinel was studied by Steeds and Evans whose observations of {200} reflections in {001} ZAPs lead them to conclude that the space group of this crystal was . de Coomen and Carter also investigated MgAl2O4 spinel and came to the same conclusion. Although these authors do emphasize the importance of a HOLZ contribution to double diffraction, they do not show the effect of changes in accelerating voltage on the HOLZ segment responsible for double diffraction. To clearly show that {200} reflections appear only as a result of double diffraction via HOLZ, it is essential to observe both the {200} reflections and the HOLZ segment as a function of accelerating voltage.

With AlN, Al2O3 and Al powders as starting materials, ALON powders were prepared by high temperature solid state reaction at 1650°C, 1750°C and 1850°C respectively. Nitrogen gas at 1atm was selected as reaction atmosphere. X-ray diffraction analysis and scanning electron microscope analysis were carried out for the as-prepared ALON powders. In this paper, single-component γ-ALON porous materials were obtained at temperatures below 1800 degree with void ratio more than 75%, which is prone to be grinded to powders less than 1μm and applicable for the preparation of transparent ceramics. Aluminum oxynitride spinel γ-ALON transparent ceramics were successfully obtained with the as-prepared ALON powders. The volume density of the transparent ceramics was 3.67g/cm2, which is 99.3 percent of the theoretical density of γ-ALON materials. The bending strength of the transparent ceramics was 296MPa and the transmittance was about 75% in the wavelength range of 1.2 to 5.3μm.

Thesis (Ph.D. (Physics)) -- University of Limpopo, 2019
LiMn2O4 spinel (LMO) is a promising cathode material for secondary lithium-ion batteries which, despite its high average voltage of lithium intercalation, suffers crystal symmetry lowering due to the Jahn-Teller active six-fold Mn3+ cations. Although Ni has been proposed as a suitable substitutional dopant to improve the energy density of LiMn2O4 and enhance the average lithium intercalation voltage, the thermodynamics of Ni incorporation and its effect on the electrochemical properties of this spinel are not fully understood. Firstly, structural, electronic and mechanical properties of spinel LiMn2O4 and LiNixMn2-xO4 have been calculated out using density functional theory employing the pseudo-potential plane-wave approach within the generalised gradient approximation, together with Virtual Cluster Approximation. The structural properties included equilibrium lattice parameters; electronic properties cover both total and partial density of states and mechanical properties investigated elastic properties of all systems. Secondly, the pressure variation of several properties was investigated, from 0 GPa to 50 GPa. Nickel concentration was changed and the systems LiNi0.25Mn1.75O4, LiNi0.5Mn1.5O4 LiNi0.75Mn1.25O4 and LiNi0.875Mn1.125O4 were studied. Calculated lattice parameters for LiMn2O4 and LiNi0.5Mn1.5O4 systems are consistent with the available experimental and literature results. The average Mn(Ni)-O bond length for all systems was found to be 1.9 Å. The bond lengths decreased with an increase in nickel content, except for LiNi0.75Mn1.25O4, which gave the same results as LiNi0.25Mn1.75O4. Generally, analysis of electronic properties predicted the nature of bonding for both pure and doped systems with partial density of states showing the contribution of each metal in our systems. All systems are shown to be metallic as it has been previously observed for pure spinel LiMn2O4, and mechanical properties, as deduced from elastic properties, depicted their stabilities. Furthermore, the cluster expansion formalism was used to investigate the nickel doped LiMn2O4 phase stabilities. The method determines stable multi-component crystal structures and ranks metastable structures by the enthalpy of formation while iv maintaining the predictive power and accuracy of first-principles density functional methods. The ground-state phase diagram with occupancy of Mn 0.81 and Ni 0.31 generated various structures with different concentrations and symmetries. The findings predict that all nickel doped LMO structures on the ground state line are most likely stable. Relevant structures (Li4Ni8O16, Li12MnNi17O48, Li4Mn6Ni2O16, Li4Mn7NiO16 and Li4Mn8O16) were selected on the basis of how well they weighed the cross-validation (CV) score of 1.1 meV, which is a statistical way of describing how good the cluster expansion is at predicting the energy of each stable structure. Although the structures have different symmetries and space groups they were further investigated by calculating the mechanical and vibrational properties, where the elastic constants and phonon vibrations indicated that the structures are stable in accordance with stability conditions of mechanical properties and phonon dispersions. Lastly, a computer program that identifies different site occupancy configurations for any structure with arbitrary supercell size, space group or composition was employed to investigate voltage profiles for LiNixMn2-xO4. The density functional theory calculations, with a Hubbard Hamiltonian (DFT+U), was used to study the thermodynamics of mixing for Li(Mn1-xNix)2O4 solid solution. The results suggested that LiMn1.5Ni0.5O4 is the most stable composition from room temperature up to at least 1000K, which is in excellent agreement with experiments. It was also found that the configurational entropy is much lower than the maximum entropy at 1000K, indicating that higher temperatures are required to reach a fully disordered solid solution. The maximum average lithium intercalation voltage of 4.8 eV was calculated for the LiMn1.5Ni0.5O4 composition which correlates very well with the experimental value. The temperature has a negligible effect on the Li intercalation voltage of the most stable composition. The approach presented here shows that moderate Ni doping of the LiMn2O4 leads to a substantial change in the average voltage of lithium intercalation, suggesting an attractive route for tuning the cathode properties of this spinel.
National Research Foundation (NRF)

Some particulate unit operations (apparatus engineering) are developed to control the particle morphology. The innovative aspect of this operation is the correspondence between the necessary material properties and their apparatus optimization. Spherical powders of aluminum oxynitride and aluminum nitride are directly prepared by flame synthesis (in spite of the fact that oxygen serves as an indispensable reactant). Non-oxide powders are commonly non-spherical with a size below the submicrometer scale. The major limiting factors in their synthesis are free energy, reaction temperature, and reaction rate. The innovative aspect of this flame synthesis technology assisted by DC arc plasma concerns the reducing gas composition beyond 1500 K. A chemical equilibrium calculation indicates that the plasma heating compensates the lack of reaction temperature under a low oxygen condition. This burner realizes a high-speed reaction with the help of reactive species in the arcs. We also report the multilayer-coated cosmetic powders with regard to material application, and the powder bed shear force evaluation equipment with regard to apparatus optimization.