Mineralizing Magnesium Aluminate Spinel Formation With B2O3

B₂O₃ mineralizes spinel formation from stoichiometric (1:1 mole ratio) calcined magnesia and alumina. After 3 h at 1100°C, X-ray diffraction (XRD) shows the mineralization effect of B₂O₃ is limited to 1.5 wt% additions with higher B₂O₃ contents leading to Mg₃B₂O₆ formation and reduced spinel content. Boron nuclear magnetic resonance, electron probe microanalysis (EPMA), scanning electron microscopy, transmission electron microscopy (TEM) and XRD reveal formation of a boron-containing liquid. Energy dispersive spectroscopy in the TEM and EPMA of the glassy phases formed from solidification of the liquid reveal that initially it is Mg borate, later becoming a magnesia-modified boroaluminate, composition suggesting dissolution–precipitation as opposed to templated growth as the mechanism of this liquid phase mediated mineralization.     

Crystallization of Na2B4O7 from Its Melt

The temperature dependence of the rate of growth of Na₂B₄O₇ from its melt was determined between 1° and 192°C of undercooling. The maximum rate of growth was 2000 μm/min at 57°C of undercooling. Analysis of the growth data indicated that growth could occur by a screw dislocation mechanism over the entire range of undercooling. When this mechanism was assumed, there was good correlation between the experimental data and the predictions of the Turnbull and Cohen equation.     

Magnesium Aluminate (MgAl2O4) Spinel Powders from Water-Based Sols

Magnesium aluminate (MgAl₂O₄) spinel powders of irregular and spherical morphologies were obtained from the bi-component water-based sols following the sol–gel and sol–emulsion–gel methods, respectively. For the synthesis of the oxide microspheres, the surfactant concentration and viscosity of the sols were found to affect the characteristics of the derived microspheres. The gel and calcined powders were investigated by using thermogravimetry analysis, differential thermal analysis, X-ray diffraction (XRD), optical and scanning electron microscopy (SEM), Fourier transformed infrared spectroscopy, and particle size analysis. XRD results indicated crystallization of the only phase MgAl₂O₄ spinel from 200° to 1000°C. Formation of hollow microspheres with a single cavity was identified by SEM.     

Molten Salt Synthesis of Magnesium Aluminate (MgAl2O4) Spinel Powder

MgAl₂O₄ (MA) spinel powder was synthesized by heating an equimolar composition of MgO and Al₂O₃ in LiCl, KCl, or NaCl. The synthesis temperature can be decreased from >1300°C (required by the conventional solid–solid reaction process) to ∼1100°C in LiCl, or to ∼1150°C in KCl or NaCl. The molten salt synthesized MA powder was pseudomorphic and retained, to a large extent, the size and morphology of the original Al₂O₃ raw material, indicating that a "template formation mechanism" plays an important role in the synthesis process.     

Preparation of Li2B4O7 Films with Preferential Orientation by Sol–Gel Method

Li₂B₄O₇ films, a promising material for surface acoustic wave devices, were prepared by the sol—gel method using metal alkoxide precursors as starting materials. The Li₂B₄O₇ films on silicon (100) and (111) single crystals prepared from a coating solution to which acetic or hydrochloric acid was added were highly oriented along the (122) plane, whereas those without acid additive were randomly oriented. The results were interpreted based on the basic sol-gel chemistry and the lattice matching between the film and substrate.     

The Influence of Water on the Fracture of Magnesium Aluminate (MgAl2O4) Spinel

The influence of water on the fracture of magnesium aluminate (MgAl₂O₄) ceramics and spinel single crystals is investigated in order to determine whether adsorption plays an important role during subcritical crack growth as it does for MnZn ferrites, a compound with the same spinel structure. The fracture toughness of porous and dense polycrystalline ceramic and single crystal specimens are determined using a single edge notched beam setup at different crosshead velocities and humidities. Furthermore, in order to investigate whether a preferred plane of fracture is present, electron backscattered diffraction (EBSD) has been performed. It is found that the fracture toughness decreases with increasing humidity for dense ceramics, indicating that adsorption plays an important role as it does for MnZn ferrites. However, porous ceramics only show a sharp decrease between 0% and 2–10% relative humidity (RH) and hardly decreases onwards. This implies that the pores inhibit in some way the effect of adsorption during fracture. The exact mechanism remains unclear. EBSD measurements indicated that a preferred plane is absent. Therefore, the fracture of spinel single crystal along the (100) and (111) planes was chosen for further detailed investigation. It is revealed that between 2% and 40% RH both planes show a reduction in fracture surface energy of ∼46%. Using reported computer simulations results, it can be concluded that the surfaces are partially hydrated when being fractured.     

Molten Salt Synthesis of Magnesium Aluminate (MgAl2O4) Spinel on Ti3AlC2 Substrate

A MgAl₂O₄ (MA) spinel layer was synthesized on Ti₃AlC₂ substrate through the molten salt synthesis (MSS) method. The Ti₃AlC₂ substrate was immersed in MgCl₂·6H₂O powders and treated at 800°, 850°, and 900°C for 4 h in air. A continuous and 10-μm-thick MgAl₂O₄ layer was obtained at 900°C, by which the surface hardness of Ti₃AlC₂ can be effectively improved. The combined scanning electron microscopy observations and crystal morphology simulation further revealed that the as-formed MgAl₂O₄ presents tetragonal bipyramids morphology with (400)-orientation.     

Nonstoichiometry in A2B2O7 Pyrochlores

Energies associated with deviations from stoichiometry for an extensive series of A₂B₂O₇ pyrochlores have been predicted. A³⁺ cations range in size from Lu to La and B⁴⁺ cations from Ti to Pb. Results are presented in the form of contour maps as a means of conveying large quantities of data as well as predicting characteristics for pyrochlore compounds not explicitly modeled. These contour maps indicate that the BO₂ excess nonstoichiometry in the pyrochlore structure is distinct from solid-solution fluorite. Within the limitations of this methodology, the contour maps provide a means to understand and predict distinct compositional variations. Defect cluster formation is discussed.     

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.     

Pseudobinary Phase Relations of Li2Ti3O7

The Li₂O-TiO₂ pseudobinary phase diagram was determined from 50 to 100 mol% TiO₂ by DTA, microscopy, and X-ray analysis; Li₂Ti₃O₇ effectively melts congruently at 1300° and decomposes eutectoidally at 940°C. A solid solution based on Li₂TlO₃ from 50 to ∼65 mol% TiO₃ was observed to exist at >930°C. A new metastable phase was discovered with a composition of ∼75 mol% TiO₂ and with a hexagonal unit cell (8.78 by 69.86 × 10⁻¹nm). Discrepancies in the literature regarding some of these phase equilibria are reconciled.     

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.     

Discontinuous Dissolution of Iron Aluminate Spinel in the Al2O3–Fe2O3 System

When sintered 85Al₂O₃–15Fe₂O₃ (in wt%) specimens consisting of corundum grains and spinel particles were annealed at temperature where only a corundum phase was stable, phase transformation of spinel into metastable FeAIO₃ and subsequently complete dissolution of the metastable phase occurred together with the migration of grain boundaries at the surface of the specimens. Since the grain boundary migration was induced by grain boundary diffusion of Fe₂O₃ from the transforming and dissolving particles, the boundary migration by temperature decrease corresponds to a discontinuous dissolution of the spinel particles and a chemically induced grain boundary migration by temperature change. Inside the specimens, however, the transformation—dissolution and the grain boundary migration were suppressed because of unavailable accommodation of the volume expansion due to the transformation.     

Effect of Small Additions of Al2O3 and Ga2O3 on the Immiscibility Temperature of Na2O-SiO2 Glasses

Alumina and gallia were substituted separately for Na₂O in amounts of 0.2, 0.5, 1.0, 1.5, 2.0, and 3.0 wt% in three Na₂O-SiO₂ glass compositions (82, 84, and 86 wt% SiO₂) within the immiscibility region. The immiscibility regions for each system extend to ∼1.5 mol% of the added oxide. In general, the addition reduced the immiscibility temperature ( T _m), but at the edge of the immiscibility region (82% SiO₂) the Na₂O loss effect initially increased T _m. A structural model of the miscibility of Al₂O₃ added to silicate glasses is presented.