Sequential Precipitation of MgAl2O4 on Mg1-x CaxAl2O4 in Hot-Pressed MgO Single Crystals

In a hot-pressed and deformed MgO single crystal, precipitates of Mg_1-xCa_xAl₂O₄ spinel upon which MgAl₂O₄ spinel subsequently precipitated were observed and analyzed using transmission electron microscopy and scanning electron microscopy. This behavior is related to the respective solubility limits of CaO and Al₂O₃ in MgO at the hot-pressing temperature and may be aided by impurity segregation to the dislocations. The spinel selectively precipitated at the nodes of a dislocation network which was formed during [001] hot-pressing deformation, as a result of the reaction b₃= b₁+ b₂= (1/2) [011] + (1/2)     = [001]. The dislocation is sessile, and the precipitates have a <100>_matrix≨ <100>_spinel coherent relationship.     

Sintering Kinetics of a MgAl2O4 Spinel Doped with LiF

The sintering kinetics of a pure magnesium aluminate spinel, MgAl₂O₄, and that doped with LiF were determined through the use of the master sintering curve technique developed by Su and Johnson. ²⁰ Powders with 0%, 0.5%, and 1.0% by mass LiF were densified in a vacuum hot press under a range of unaxial pressures. After the sintering mechanisms in each temperature and pressure regime were determined, an optimized vacuum hot-pressing schedule was formulated for spinel powders doped with 1.0% by mass of LiF. In addition to forming a transient liquid phase, the presence of LiF leads to the formation of oxygen vacancies that promote late-stage sintering in MgAl₂O₄.     

Final Sintering of Cr2O3

The effect of oxygen activity on the sintering of high-purity Cr₂O₃ is shown. Theoretical density was approached at the equilibrium O₂ partial pressure needed to maintain the Cr₂O₃ phase ( P _o2=2×10⁻¹² atm). The presence of N₂ in the atmosphere during sintering did not prevent final sintering. The addition of 0.1 wt% MgO at this equilibrium pressure effectively controlled the grain growth and further increased the sintered density to very near the theoretical value. The solute segregation of MgO at the grain boundaries, followed by nucleation of spherulites of magnesium chromite spinel on the boundaries, accounted for the grain-growth control. It is speculated that these isolated spherulites locked the grain boundaries together, changing the fracture mode of the sintered oxide from inter-to intragranular and also that larger MgO additions produced a more continuous spinel formation at the boundaries, resulting in decreased sintered density. Weight loss, which was also monitored as a function of O₂ activity, correlated with the changing predominant volatile species in the Cr-O system.     

Grain Growth of ZnO in ZnO-Bl2O3 Ceramics with Ai2O3 Additions

Grain growth of ZnO during liquid-phase sintering of a ZnO-6 wt% Bi₂O₃ ceramic was investigated for A1₂O₃ additions from 0.10 to 0.80 wt%. Sintering in air for 0.5 to 4 h at 900° to 1400°C was studied. The AI₂O₃ reacted with the ZnO to form ZnAl₂O₄ spinel, which reduced the rate of ZnO grain growth. The ZnO grain-growth exponent was determined to be 4 and the activation energy for ZnO grain growth was estimated to be 400 kJ/mol. These values were compared with the activation parameters for ZnO grain growth in other ceramic systems. It was confirmed that the reduced ZnO grain growth was a result of ZnAl₂O₄ spinel particles pinning the ZnO grain boundaries and reducing their mobility, which explained the grain-growth exponent of 4. It was concluded that the 400 kJ/mol activation energy was related to the transport of the ZnAl₂O₄ spinel particles, most probably controlled by the diffusion of O^2- in the ZnAl₂O₄ spinel structure.     

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.     

Effects of Addition of Na2O, Sb2O3, CaO, and ZrO2 on the Properties of Lithium Zinc Ferrite

Some effects of CaO, Na₂O, Sb₂O₃, and ZrO₂ additions on Li_0.3Zn_0.4Fe_2.3O₄ ferrite were studied to improve its density and other material properties. The densification behavior of the ferrite depended on the amount and type of additive. A relative density of ∼98.5% was achieved with the addition of CaO. The grain size decreased with the addition of Na₂O, CaO, and Sb₂O₃. The permeability and electrical resistivity increased with additives. CaO remarkably increased resistvity, whereas, ZrO₂ increased permeability. Na₂O and Sb₂O₃ increased the Curie temperature, whereas CaO and ZrO₂ decreased it. These effects were attributed to mainly additive segregation on the grain boundaries, which suppressed grain-size development during the sintering of lithium zinc ferrite.     

Quaternary System Al2O3–CaO–MgO–SiO2: II Study of the Crystallization Volume of MgAl2O4

Solid-state compatibility and melting relations of MgAl₂O₄ in the quaternary system Al₂O₃–CaO–MgO–SiO₂ were studied by firing and quenching selected samples located in the 65 wt% MgAl₂O₄, plane followed by microstructural and energy dispersive X-ray analysis. A projection of the liquidus surface of the primary crystallization volume of MgAl₂O₄ was constructed from CaO, SiO₂ and exceeding Al₂O₃, not involved in stoichiometric MgAl₂O₄ formation; those three amounts were recalculated to 100 wt%. The temperature and character of six invariant points, where four solids co-exist with a liquid phase, were defined. One maximum point was localized and the positions of the isotherms were tentatively established. The effect of CaO, SiO₂, and Al₂O₃ impurities on the high temperature behavior of spinel materials was also discussed.     

Calcium Hexaluminate and Its Stability Relations in the System CaO–Al2O3–SiO2

By a combination of solid-state sintering and quenching experiments the validity of calcium hexaluminate as a stable phase and the extent of its primary field in the system CaO–Al₂O₃–SiO₂ have been established. The size of the primary field is considerably reduced from that suggested by earlier work. The anorthite-corundum-calcium hexaluminate invariant point has been relocated at 28.0% CaO, 39.7% Al₂O₂, and 32.3% SiO₂ and at 1405°± 5°C.     

The System CaO-Ga2O3

CaO and Ga₂O₃ form three compounds: 3CaO-Ga₂O₃, CaO-Ga₂O₃, and CaO-2Ga₂O₃. 3CaO-Ga₂O₃ melts incongruently to CaO plus liquid at 1263°C.; CaO Ga₂O₃ and CaO 2Ga₂O₃ melt congruently at 1369° and 1504°C. respectively. Eutectics are located at the following temperatures and compositions (in mole% Ga₂O₃): between 3CaO Ga₂O₃ and CaO Ga₂O₃, 1245°C. and 37.5%; between CaO Ga₂O₃ and CaO-2Ga₂O₃,1323^oC. and 57.0%; and between CaO -2Ga₂O₃ and β-Ga₂O₃,1457°C. and 68.0%. There is a peritectic at 1263°C. and 36.0%. Three polymorphs of CaO Ga₂O₃ are described. Compositions from approximately 35 to 70 mole% Ga₂O₃ can be quenched to yield homogeneous glasses.     

Reducing the Sintering Temperature for MgO–Al2O3Mixtures by Addition of Cryolite (Na3AlF6)

The preparation of near stoichiometric spinel and alumina-rich spinel composites from Al₂O₃and MgO powders with the addition of Na₃AlF₆up to 4 wt% in the temperature range 700°–1600°C was studied; 98 wt% spinel containing 72 wt% Al₂O₃can be produced from the mixture of 72 wt% (50 at.%) Al₂O₃+ 28 wt% (50 at.%) MgO powders with the addition of 1 wt% Na₃AlF₆fired at 1300°C for 1 h. Spinels containing 81–85 wt% Al₂O₃can be produced from either the mixture of 90 wt% (78 at.%) Al₂O₃+ 10 wt% (22 at.%) MgO or the mixture of 95 wt% (88 at.%) Al₂O₃+ 5 wt% (12 at.%) MgO powders with the addition of 4 wt% Na₃AlF₆in the temperature range 1300°–1600°C by using a torch-flame firing for 3 min, followed by quenching in water, while the same system under slow cooling in a furnace results in spinel containing 74–76 wt% Al₂O₃. Microscopic studies indicate that the alumina-rich spinel composites consist of a continuous majority spinel phase and an isolated minority corundum phase, regardless of slow cooling in a furnace or quenching in water.     

Synthesis and Characterization of MgAl2O4 Spinel Precursor from a Heterogeneous Alkoxide Solution Containing Fine MgO Powder

A precursor was synthesized from a heterogeneous alkoxide solution that contained fine MgO powder, which allowed the preparation of MgAl₂O₄ spinel powder with high sinterability characteristics. The precursor consisted of a mixture of boehmite (AlO(OH)) and a mixed hydroxide (Mg₄Al₂(OH)₁₄· 3H₂O). The spinel phase formed through two steps: (i) decomposition of the mixed hydroxide at low temperature and (ii) solid-state reaction between MgO and γ-Al₂O₃ at higher temperatures. Dense polycrystalline spinel could be obtained from the calcined powders at sintering temperatures as low as 1400°C.     

Densification and Thermal Conductivity of AIN Doped with Y2O3, CaO, and Li2O

Small amounts of Li₂O result in sintering in the AIN-Y₂O₃-CaO and AIN-CaO systems at firing temperatures <1600°C. The effect is ascribed to reduction of the liquidus temperature. Furthermore, Li₂O is removed by volatization at temperatures from 1300° to 1600°C, and its content decreases several ppm from the initial 0.3 wt%. Li₂O-doped AIN specimens containing Y₂O₃ and CaO additives are well densified by firing at 1600°C for 6 h, and their thermal conductivity is 135 W^.m^−1.K⁻¹.The effect of Li₂O addition on sintering and thermal conductivity also is discussed through thermo-dynamic considerations.     

Crystallization of MgO-CaO-SiO2-P2O5 Glass

The crystallization behavior of a glass with a composition of 40 wt% 3CaO · P₂O₅−60 wt% CaO · MgO · 2SiO₂ was investigated. The primary crystalline phase was apatite with a dendritic form and ellipsoidal shape. β-(3CaO · P₂O₅) and CaO · MgO · 2SiO₂ were crystallized as samples heated to 990°C, and a three-layer structure was obtained. The development and morphology of this construction were explained by both the surface crystallization of the apatite and CaO · MgO · 2SiO₂ and the bulk crystallization of apatite and the CaO · MgO · 2SiO₂-β-(3CaO · P₂O₅) composite.     

Studies in Lithium Oxide Systems: II, Li2O Al2O3–Al2 O 3

Solid-state reactions between Li₂O and Al₂ O₃ were studied in the region between Li₂O.Al₂ O ₃ and Al₂ O ₃. The compound Li₂ O Al₂ O ₃ melts at 1610°± 15°C. and undergoes a rapid reversible inversion between 1200° and 1300°C. Vaporization of Li₂ O from compositions in the system proceeds at an appreciable rate at 1400°C, as shown by fluorescence. Lithium spinel, Li₂ O -5Al₂O₃, was the only other compound observed. The effect of Li₂ O on the sintering of alumina was investigated.     

TEM Characterization of Nanostructured MgAl2O4 Synthesized by a Direct Conversion Process from γ-Al2O3

Nanostructured MgAl₂O₄ spinel was synthesized by a direct conversion process from cubic γ-Al₂O₃. The effect of post-annealing temperature (300°, 500°, and 800°C) on MgAl₂O₄ phase formation was investigated using transmission electron microscopy, selected area electron diffraction (SAED), electron energy loss spectroscopy (EELS), and energy-dispersive spectroscopy (EDS). Relative diffraction intensities as well as lattice parameter measurements from SAED revealed that MgAl₂O₄ spinel structure starts forming at temperatures as low as 300°C. EELS and EDS spectrum images also revealed an increase in elemental homogeneity with increasing annealing temperature. The degree of ordering of Mg and Al between octahedral and tetrahedral sites has been determined from relative diffraction intensities. Results show that annealing to 800°C leads to a spinel phase with an order parameter of 0.78.     

Effect of MgO Addition on the Microstructure Development of 3 mol% Y2O3—ZrO2

MgO addition to 3 mol% Y₂O₃–ZrO₂ resulted in enhanced densification at 1350°C by a liquid-phase sintering mechanism. This liquid phase resulted from reaction of MgO with trace impurities of CaO and SiO₂ in the starting powder. The bimodal grain structure thus obtained was characterized by large cubic ZrO₂ grains with tetragonal ZrO₂ precipitates, which were surrounded by either small tetragonal grains or monoclinic grains, depending on the heat-treatment schedule.     

Phase Equilibria in the System CaO-Yb2O3

Phase equilibrium relations in the system CaO-Yb₂O₃ were studied. Results of this work demonstrated the existence of four crystalline phases: Yb₂O₃.3CaO, Yb₂O₃.2CaO, Yb₂O₃°CaO, and 2Yb₂O₃°CaO. The 2Yb₂O₃°CaO phase is metastable at all temperatures and was obtained only by rapid quenching from the melt. The crystalline solubility limit of Yb_aO₃ in CaO at 1850°C is slightly greater than 8 mole %, whereas no solubility of CaO in Yb₂O₃ was detected. All four compounds have subsolidus minimums of stability and dissociate into the component oxides below 1800°C. Data are also presented for the systems CaO-Gd₂O₃ and CaO-La₂O₃.     

Investigation of High-Alumina Spinel: Effect of LiF and CaCO3 Addition

This work examines the effects of LiF or LiF/CaCO₃ additives on the phases, microstructure, sintering temperature, and activation energy of MgO-Al₂O₃ spinel formation. According to these results, the spinel formation temperature decreases and the spinel phase content increases by incorporating sintering additives. Such an effect is due to the formation of liquid phases at sintering temperatures. The Al composition in spinel is substantially higher in samples containing LiF or LiF/CaCO₃ than in samples without sintering additives. Results obtained from the Arrhenius equation and Jander's model indicate that the activation energy for forming spinel decreases by incorporating LiF or LiF/CaCO₃.     

Microstructure of (Fe0.2Co0.8)0.8(Fe2.38Co0.62O4): A Magnetic Oxidation Resistant Composite Formed by Coprecipitation

The metal–ferrite composite (Fe_0.2Co_0.8)_0.8(Fe_2.38Co_0.62O₄) has been studied by X-ray diffractometry measurements and high-resolution transmission electron microscopy. Spinel ferrite occurs in highly crystalline domains 100–150 nm in size, and the iron–cobalt alloy occurs in smaller and less-crystalline domains (10–20 nm). Both phases are heterogeneous in composition. The metal is embedded in the spinel phase, located near the edges, and overlaid by a poorly crystallized layer or misshapen regions containing small spinel crystals and amorphous phases. By annealing under vacuum up to 800°C, the misshapen regions disappear and the size of the metallic regions increases. The concentration of iron in the metallic regions decreases and their structure changes to face-centered cubic, while the spinel becomes enriched in iron.     

Effect of Rapid Solidification on Microstructural Evolution in MgO–MgAl2O4

The response of the MgO–20 and 30 mol% Al₂O₃ compositions to rapid solidification has been studied. The oxides were twin-roller quenched and the resulting flakes were characterized by X-ray diffraction and transmission electron microscopy. The results indicate that metastable extensions of spinel and periclase occurred and the microstructural pathway was determined from the final microstructure. The flakes having MgO–20 mol% Al₂O₃ show a dendritic structure consisting of periclase and spinel. In the MgO–30 mol% Al₂O₃ composition, the liquid transforms to spinel partitionlessly. The spinel is believed to undergo decomposition by a modulation in composition, and the resulting microstructure consists of spinel and periclase. Kinetic and thermodynamic aspects of phase selection have been rationalized based on the metastable extensions of the different phase fields in the phase diagram. It has been proposed that composition fluctuations in spinel are stabilized because of the formation of disordered phases with a continuous range of order parameter on the tetrahedral sublattice.