Search results for "Corundum"
showing 10 items of 39 documents
Batch-to-Melt Conversion Kinetics in Sodium Aluminosilicate Batches Using Different Alumina Raw Materials
2016
The batch-to-melt conversion in batches of sand, soda ash and corundum (C), alumina spinel (A), boehmite (B), or gibbsite (G) as Al2O3 carrier are studied using thermal analysis, X-ray diffraction, and 27Al nuclear magnetic resonance spectroscopy. Laboratory-scaled batches are either heated continuously or quenched from 1600°C in a series of increasing dwell times. The results show that the conversion from the raw materials to the fresh melt proceeds in two kinetic stages. During the first stage (3–5 min), fast conversion of nearly 95% by mass occurs and the conversion coefficient increases in the order G < C ≈ A < B. The second stage is controlled by the slow dissolution of intermediate cr…
Ab initio simulations on Frenkel pairs of radiation defects in corundum
2015
Large scale first principles periodic calculations based on the density functional theory within the localized atomic orbital approach (DFT-LCAO) using the hybrid exchange- correlation potential B3PW have been performed in order to study the structural and electronic properties of radiation-induced Frenkel pairs Oi+VO in corundum crystal. As an initial approach, we have used conventional 2x2x1 supercell for defective α-Al2O3 lattice containing 120 atoms. After relaxation of the ideal supercell structure, the optimized doi-vo distance has been found to be ~4.5 A while the formation energy of Frenkel pair has achieved 11.7 eV. The interstitial Oi atom, both single and a component of Oi+VO pai…
Influence of preoxidation on the corrosion of steels in liquid lead-bismuth eutectic
2004
Abstract Oxidation studies of martensitic/ferritic and austenitic steels have been conducted in static lead bismuth eutectic (LBE). Samples were preoxidized in an air/H2O gas mixture prior to immersion in LBE. Preoxidation films grown on HT-9 at 800°C for 48 h had a bilayer structure, an outer Fe-rich layer, and an inner Cr-rich layer. Glancing angle x-ray diffraction data found that two distinct structures were present in this oxide: Fe1+xCr2−xO4 spinel (cubic, face-centered cubic [fcc]) and (Cr,Fe)2O3 (rhombohedral, corundum). Magnetite formation (Fe3O4) was ruled out. Immersion in LBE resulted in the growth of an Fe-rich film on top of the preoxidation layer. It was concluded that the gr…
Charged oxygen interstitials in corundum: first principles simulations
2016
Combining supercell models and hybrid B3PW exchange-correlation functionals, ab initio simulations on quasi-stable configurations of interstitial ions in α-Al2O3 (corundum) crystals and possible migration trajectories have been modelled. We have studied crystalline distortion around migrating including interatomic distances and the effective atomic charges, as well as redistributions of the electronic density. Unlike neutral interstitial atom Oi studied by us previously, migrating ion does not form dumbbells with the nearest regular oxygen ions, due to the strong Coulomb interaction with the nearest cations as well as stronger repulsion between and adjacent regular ions. We have also estima…
The adhesion properties of the Ag/α-Al2O3() interface: an ab initio study
2002
Ab initio computer simulations of the atomic and electronic structure of the Ag/a-Al2O3(0 0 0 1) (corundum) interface have been performed for a periodic two-dimensional slab model using the Hartree–Fock method and a posteriori electron correlation corrections.We have considered both Al- and O-terminated corundum substrate surfaces.The dependence of the adhesion energy on the interfacial distance has been analyzed for the two most favorable Ag adsorption positions over corundum and for two different metal coverages (a 1/3 monolayer (ML) of the Ag(1 1 1) crystallographic plane and a full Ag(1 1 1) monolayer).The two different terminations (Al- and O-) give rise to qualitatively different resu…
Formation, diffusion, and aggregation of radiation-induced defects in MgO and α-Al 2 O 3
1997
MgO and M2O3 (corundum) are important ceramic materials with several technological applications including theirpossible use in fusion reactors' . Weare therefore studying the properties ofpoint defects (vacancies, interstitials, andtheir aggregates) and electronic defects (trapped holes and electrons) in these materials. Pair-potential (PP) calculationswere carried out using the Mott-Littleton (ML) two-region strategy, as implemented in the computer code CASCADEdue to Leslie. Details ofthe method, including the construction ofPPs, are in the literature3. For semi-empirical INDO(intermediate neglect ofdifferential overlap) calculations computer codes developed at the University of Riga were …
An ESCA and Mössbauer study of the oxide layer formed on steel in water containing chromate and chloride ions
1983
Abstract The formation of oxidic layers on steel in chromate solutions was studied by photo-electron and Mossbauer spectroscopy. To simulate more aggressive realistic systems, some chloride ions were added to the solution. The layers formed under these conditions were found to be thicker by an order of magnitude than those formed in the absence of chloride ions. They probably consist of ferric and chromic oxides of the corundum type (Fe 1−2 Cr x ) 2 O 3 , with the Cr : Fe ratio being depth dependent. The oxide layer is subdivided into small regions behaving superparamagnetically. This heterogeneity of the layer is ascribed to the attack by the chloride ions. The structure of the oxides foun…
Mechanism of self-trapped hole motion in corundum crystals
1993
Abstract Atomistic simulations of the self-trapped hole eauilibrium geometry and migration in a pure corundum crystal have been carried out using the semiempirical method of intermedia te neglect of differential overlap and atom-atom potentials, as implemented in the CASCADE code. The activation energies for three different hole-hopping mechanisms are calculated. It is shown that the 60° reorientations of a self-trapped hole and hopping to the nearest O-atom triangle reauire almost the same activation energy, approximately 0.9 eV, which agrees auite well with the experi-mental value for hole migration of 0.7 eV. A new mechanism of small-polaron motion is suggested.
Computer modelling of radiation damage in cation sublattice of corundum
1998
Results of quantum chemical computer simulations of close Frenkel defects in corundum crystals are presented and discussed. The conclusion is drawn that the energy barrier for a back recombination up to fourth nearest neighbours is less than 0.3 eV, i.e. such pairs should be unstable at temperatures above 40 K.
Quantum chemical simulations of bound hold polarons (V Mg centers) in corundum crystals
1997
The semi-empirical INDO method has been applied to the calculations of the bound hole small-radius polarons in corundum. Results for optimized atomic and electronic structure using two different approaches (molecular cluster and periodic, supercell model) are critically compared. Both models find that two-site configurations of bound hole polarons have the lowest energy (which does not exclude existence of one-site polarons also characterized by essential relaxation energies). Experimental ENDOR data on V Mg defects are discussed in the light of the calculations.