Atomistic Simulation of the Surface Energy of Spinel MgAl2O4

Atomistic simulations with atomic potentials including anion polarizibility have been performed for the low-index surfaces of spinel MgAl₂O₄ with various terminations. The calculations show that for the most stable surface the surface energy is 2.27 J/m² for the {100}, about 2.85 J/m² for the {110}, and 3.07 J/m² for the {111} orientation. The ratio between the experimental values to the calculated relaxed surface energies is about 1.5. Strong surface relaxation was found for the {110} and {111} orientation but only moderate surface relaxation for the {100} surface.     

Preparation of MgAl2O4 Spinel Powders via Freeze-Drying of Alkoxide Precursors

High-sinterability MgAl₂O₄ powder has been produced from alkoxide precursors via a freeze-drying method. Clear alumina sol and magnesium methoxide were used as starting materials in the process. The spinel powders were characterized by various techniques, such as thermal analysis, X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. The tap density and sinterability of the spinel power are affected by the ball-milling techniques. Highly dense, transparent, polycrystalline MgAl₂O₄ has been obtained from these powders by sintering and hot isostatic pressing. Bimodal grain-size microstructure is observed in a HIPed sample.     

Low-Temperature Chemical Synthesis of Nanocrystalline MgAl2O4 Spinel Powder

Nanocrystalline MgAl₂O₄ spinel powder was synthesized by pyrolysis of complex compounds of aluminum and magnesium with triethanolamine (TEA). The soluble metal ion–TEA complexes formed the precursor material on complete dehydration of the complexes of aluminum–TEA and magnesium–TEA. Single-phase MgAl₂O₄ spinel powder resulted after heat treatment of the precursor material at 675°C. The precursor and the heat-treated powders were characterized by X-ray diffractometry (XRD), differential thermal and thermogravimetric analysis, and transmission electron microscopy (TEM). The average crystallite size as measured from the X-ray line broadening was around 14 nm and the average particle size from TEM studies was around 20 nm.     

High-Strain-Rate Superplasticity in Y2O3-Stabilized Tetragonal ZrO2 Dispersed with 30 vol% MgAl2O4 Spinel

High-strain-rate superplasticity is attained in a 3-mol%-Y₂O₃-stabilized tetragonal ZrO₂ polycrystal (3Y-TZP) dispersed with 30 vol% MgAl₂O₄ spinel: tensile elongation at 1823 K reached >300% at strain rates of 1.7 × 10⁻²– 3.3 × 10⁻¹ s⁻¹. The flow behavior and the microstructure of this material indicate that the MgAl₂O₄ dispersion should enhance accommodation processes necessary for grain boundary sliding. Such an effect is assumed to arise from an enhancement of the cation diffusion by the dissolution of Al and Mg ions into the ZrO₂ matrix and from stress relaxation due to the dispersed MgAl₂O₄ grains.     

Synthesis of (Mg,Si)Al2O4 Spinel from Aluminum Dross

The spinel (Mg,Si)Al₂O₄ was synthesized from aluminum dross using an induction synthesis method. X-ray diffraction analyses on products formed at different temperatures provided an understanding of the formation mechanism of the spinel. After removal of soluble components, the induction heating of the dross resulted first in the oxidation of some of the AlN component and the subsequent formation of the spinel by the following reaction: x SiO₂+ (1− x )MgO + [1−( x /3)]Al₂O₃+ (2 x /3)AlN = (Mg_{1− x} ,Si _x )Al₂O₄+ ( x /3)N₂( g ).     

Computer Simulation of Dissociative Adsorption of Water on the Surfaces of Spinel MgAl2O4

Atomistic simulation techniques have been used to model the dissociative adsorption of water onto the low-index {100}, {110}, and {111} surfaces of spinel MgAl₂O₄. The Born model of solids and the shell model for oxygen polarization have been used. The resulting structures and chemical bonding on the clean and hydrated surfaces are described. The calculations show that the dissociative adsorption of water on the low-index surfaces is generally energetically favorable. For the {110} and {111} orientations, the surfaces cleaved between oxygen layers show high absorption and stability. The calculations also show that, for the {111} orientation, the surfaces may absorb chemically water molecules up to ∼90% coverage and have the highest stability. It is suggested that, during fracture, only partial hydration occurs, leading to cleavage preferentially along the {100} orientation.     

Activity–Composition Relations in FeCr2O4–FeAl2O4 and MnCr2O4–MnAl2O4 Solid Solutions at 1500° and 1600°C

Activity–composition relations of FeCr₂O₄–FeAl₂O₄ and MnCr₂O₄–MnAl₂O₄ solid solutions were derived from activity–composition relations of Cr₂O₃–Al₂O₃ solid solutions and directions of conjugation lines between coexisting spinel and sesquioxide phases in the systems FeO–Cr₂O₃–Al₂O₃ and MnO–Cr₂O₃–Al₂O₃. Moderate positive deviations from ideality were observed.     

Influence of the Characteristics of Spinels on the Slag Resistance of Al2O3–MgO and Al2O3–Spinel Castables

The XRD patterns at ambient temperature and at 1500°C showed that the spinel in the Al₂O₃–MgO castables fired at 1500°C for 3 h has the higher peak intensity, compared to those in Al₂O₃–spinel castables; the interplanar distance in the set (311) is 2.43 Å for the spinel in Al₂O₃–MgO castables as well as the spinels in Al₂O₃–spinel castables using spinels containing 73, 90, and 94 wt% Al₂O₃, respectively. The corresponding alumina contents of the spinels in these castables were estimated to be around 75 wt%. The smaller grain size of the spinel in Al₂O₃–MgO castables compared to that in Al₂O₃–spinel castables is evidenced by the recrystallization of the in situ spinel only occurring in Al₂O₃–MgO castables as revealed by the XRD patterns at ambient temperature and at 1500°C. The larger amount and smaller grain size of the in situ spinel in the matrix mostly account for the better slag resistance of Al₂O₃–MgO castables, compared to Al₂O₃–spinel castables.     

Hydrothermal Synthesis and Low-Temperature Sintering of Zinc Gallate Spinel Fine Particles

Nanosized powders of single-phase zinc gallate (ZnGa₂O₄) spinel were hydrothermally synthesized from solutions in the presence of NaOH over the pH range of 1.9 to 7.0 and from solutions above pH 7.0, i.e., the very basic medium (pH of 13.85), by removing the residual ZnO phase by washing with aqueous H₂SO₄ from the precipitate mixtures of zinc gallate spinel particles and ZnO. A very wide compositional range (Zn/2Ga = 0.705–1.157) of zinc gallate spinel solid solutions could be hydrothermally synthesized in the form of nanosized particles from acid and very basic mediums (pH of 2.4–13.85) in the presence of NaOH. These hydrothermally synthesized spinel powders showed good sinterability and almost full densification at 1100°C for 1 h. Dense sintered bodies consisting of single-phase zinc gallate spinel were fabricated at 1100°C using zinc gallate spinel powders having almost stoichiometric composition formed from the solution at pH 9.95 in the presence of aqueous ammonia.     

Behavior of MgFe2O4 Films on MgO in an Electric Field

Thin films of MgFe₂O₄ spinel on a (001) substrate of MgO have been heated to elevated temperatures in an applied electric field. The externally applied electric field produces a large driving force that influences the kinetic behavior of the spinel film and results in the formation of an MgO layer at the cathode due to the higher mobility of the Mg²⁺ cations in the spinel. Through the use of both scanning and transmission electron microscopy, the evolution of this layer was followed through a series of heat treatments. Analysis of the decomposition process shows that initially isolated pockets of MgO form at the cathode surface. These pockets grow and eventually coalesce to form a continuous MgO layer. The two MgO/spinel heterojunctions behave differently since one is morphologically stable while the other is morphologically unstable. TEM analysis showed that during the decomposition process, dislocation loops are formed in the vicinity of the MgO pockets. It is proposed that these dislocation loops form to accommodate the lattice misfit at the interface between the precipitating MgO and spinel.     

Low-Temperature Synthesis of NiFe2O4 by a Hydrothermal Method

Nickel ferrite (NiFe₂O₄) nanoparticles were successfully synthesized via a hydrothermal process and characterized by X-ray diffraction and transmission electron microscope techniques. The effects of reaction temperature, holding time, and RH ratio (isopropyl alcohol/water) were discussed. The NiFe₂O₄ nanoparticles could be obtained at 60°C within 3 h. The crystallization of the spinel ferrites was promoted by the increase in reaction temperature, holding time, and RH ratio.     

Formation of SiO2, Al2O3, and 3Al2O3. 2SiO2 Particles in a Counterflow Diffusion Flame

SiO₂, Al₂O₃, and 3Al₂O₃.2SiO₂ powders were synthesized by combustion of SiCl₄ or/and AlCl₃ using a counterflow diffusion flame. The SiO₂ and Al₂O₃ powders produced under various operation conditions were all amorphous and the particles were in the form of agglomerates of small particles (mostly 20 to 30 nm in diameter). The 3Al₂O₃.2SiO₂ powder produced with a low-temperature flame was also amorphous and had a similar morphology. However, those produced with high-temperature flames had poorly crystallized mullite and spinel structure, and the particles, in addition to agglomerates of small particles (20 to 30 nm in diameter), contained larger, spherical particles 150 to 130 nm in diameter). Laser light scattering and extinction measurements of the particle size and number density distributions in the flame suggested that rapid fusion leading to the formation of the larger, spherical particles occurred in a specific region of the flame.     

Optical and Photocatalytic Properties of Spinel ZnCr2O4 Nanoparticles Synthesized by a Hydrothermal Route

In this paper, a simple hydrothermal route has been developed to synthesize ZnCr₂O₄ nanoparticles. Experimental results show that the as-prepared ZnCr₂O₄ nanoparticles have an average particle size of <5 nm. The ZnCr₂O₄ nanoparticles have a direct band gap about 3.46 eV and exhibit blue emission in the range of 300–430 nm, centered at 358 nm when excited at 220 nm. Furthermore, the nanoparticles show apparent photocatalytic activities for the degradation of methylene blue under UV light irradiation.     

Single-Source Sol-Gel Synthesis of Nanocrystalline ZnAl2O4: Structural and Optical Properties

Nanometer-sized zinc aluminate (ZnAl₂O₄) particles were synthesized from heterometal alkoxides, [ZnAl₂(OR)₈], possessing an ideal cation stoichiometry for the ZnAl₂O₄ spinel. ZnAl₂O₄ is formed at 400°C, which is the lowest temperature reported for the formation of monophasic ZnAl₂O₄. ²⁷Al magic-angle spinning nuclear magnetic resonance spectroscopy revealed that ZnAl₂O₄ possesses an inverse structure at <900°C, while the normal spinel phase is observed at higher temperatures. The homogeneity of the in-depth composition and Zn:Al stoichiometry (1:2) was confirmed by electron spectroscopy for chemical analysis. Evaluation of the valence-band spectra of ZnAl₂O₄ and ZnS suggested that the hybridization of O 2 p and Zn 3 d orbitals is responsible for lowering the bandgap in the latter. The average crystallite size showed an exponential relationship to the calcination temperature (X-ray diffractometry and transmission electron microscopy data). The optical spectra of different spinel powders (average particle sizes, 20–250 nm) showed that the absorption edge exhibits a blue shift as particle size decreases.     

Thermodynamic Properties of Fe3O4-FeV2O4 and Fe3O4-FeCr2O4 Spinel Solid Solutions

Solid solutions of Fe304-FeV204 and Fe304-FeCr204 were prepared and equilibrated with Pt under controlled streams of CO/CO, gas mixtures at 1673 K. The concentration of Fe in Pt was used to determine the activity of Fe304 in the solid solutions. The activity of the second component was calculated by Gibbshhem integration. From these data, the Gibbs energy of mixing was derived for both systems. The experimental results and theoretical values which are determined from calculated cation distribution compare favorably in the case of vanadite solid solutions but not in the case of chromite solid solutions. The difference is attributed to a heat term arising from lattice distortion due to cation size difference. The positive heat of mixing will give rise to a miscibility gap in the system Fe304-FeCr204 at lower temperatures.     

Hydrothermal Synthesis of Nanosized CoAl2O4 on ZnAl2O4 Seed Crystallites

ZnAl₂O₄-seeded CoAl₂O₄, with a core-shell structure, has been prepared under hydrothermal conditions when the Co²⁺ salt solution is substituted by 10% Zn²⁺ as a precursor. The ZnAl₂O₄ seed is generated during the synthesis process. The seeding process can decrease the synthesis temperature from 245° to 230°C and the particle size from 67 to 20 nm. The process can economize the consumption of Co²⁺ and control the particle size effectively.     

Effect of Controlling Parameters on the Reaction Sequences of Formation of Nitrogen-Containing Magnesium Aluminate Spinel from MgO, Al2O3, and AlN

The reaction sequences of the formation of nitrogen-containing magnesium aluminate spinel from MgO, Al₂O₃, and AlN were investigated as a function of temperature through dilatometric study and as a function of time through isothermal heat treatments. The natures of reactions are described through the appearance of phases in conjunction with densification behavior and the change in lattice parameter of the spinel phase. Although the dilatometric study provides the detail insights of the formation sequence, the isothermal runs reveal new information about the differential rate of reactivity of the reacting species that suggested a tentative controlling mechanism. Through the initial formation of magnesium aluminate, oxygen-rich solid solution (MgAlON) forms, which ultimately reacts with the rest of AlN to reach its nominal composition. Nitrogen diffusion through MgAlON lattice seems to be rate controlling.     

Quaternary System Al2O3–CaO–MgO–SiO2: I, Study of the Crystallization Volume of Al2O3

Compatibility relations of Al₂O₃ in the quaternary system Al₂O₃–CaO–MgO–SiO₂ were studied by firing and quenching followed by microstructural and energy-dispersive X-ray examination. A projection of the liquidus surface of the primary phase volume of Al₂O₃ was constructed in terms of the CaO, SiO₂, and MgO contents of the mixtures recalculated to 100 wt%. Two invariant points, where four solids coexist with a liquid phase, were defined, and the positions of the isotherms were tentatively established. The effect of SiO₂, MgO, and CaO impurities on Al₂O₃ growth also was studied.