Discontinuous Dissolution and Grain-Boundary Migration in Al2O3-Fe2O3 by Oxygen Partial Pressure Change

When sintered 95Al₂O₃-5Fe₂O₃ (wt%) specimens constituting corundum grains and iron aluminate spinel precipitates were annealed under high oxygen partial pressure (P_o2) where only a corundum phase is stable, fast dissolution of particulate spinel precipitates occurred, together with the migration of corundum grain boundaries. Behind the migrating boundaries, a corundum solid solution enriched with Fe₂O₃ formed. Discontinuous dissolution (DD) of particulate spinel precipitates thus occurred by P_o2 increase. In contrast, when 95Al₂O₃-5Fe₂O₃ specimens constituting only corundum grains were annealed under low P_o2 where both corundum and spinel phases are stable, grain boundaries migrated without spinel precipitation, leaving behind a corundum phase depleted of Fe₂O₃, similar to chemically induced grain-boundary migration (CIGM) observed during solute depletion. The volatilization of Fe₂O₃ appeared to cause the boundary migration without precipitation. The observed CIGM and DD would suggest various possibilities of microstructure control in other oxide systems through oxygen partial pressure change.     

Humidity-Sensitive Electrical Conduction of MgCr2O4-TiO2 Porous Ceramics

The crystalline phase, microstructure, semiconduction, and humidity-sensitive electrical conduction of MgCr₂O₄-TiO₂ ceramics were studied. A solid solution with TiO₂ up to 30 mol% occurs as a single phase with a pure MgCr₂O₄-type spinel structure. The humidity-sensitive electrical conduction of the MgCr₂O₄-TiO₂ porous ceramics is the most promising for humidity-sensing devices.     

Optical Spectra of Synthetic Spinels in the System MgAl2O4–MgCr2O4

Unpolarized optical spectra were measured in the wavelength range 322–1666 nm by the diffuse reflection technique from spinel powders synthesized in the system MgAl₂O₄–MgCr₂O₄. The spectra were interpreted by the crystal-field theory on the basis of trigonally distorted spinel octahedra with D_3d symmetry. For chromium-rich solid solutions, including the MgCr₂O₄ end-member, results after peak fittings showed octahedral D_3d local symmetry around Cr³⁺ ions, identical to the crystallographic site symmetry. For chromium-poor solid solutions, however, octahedral C_3v local symmetry was suggested around Cr³⁺ ions, different from the D_3d crystallographically expected.     

Subsolidus Phase Relationships in the System Fe2O3–Al2O3–TiO2 between 1000° and 1300°C

Subsolidus phase equilibria in the system Fe₂O₃–Al₂O₃–TiO₂ were investigated between 1000° and 1300°C. Quenched samples were examined using powder X-ray diffraction and electron probe microanalytical methods. The main features of the phase relations were: (a) the presence of an M₃O₅ solid solution series between end members Fe₂TiO₅ and Al₂TiO₅, (b) a miscibility gap along the Fe₂O₃–Al₂O₃ binary, (c) an α-M₂O₃( ss ) ternary solid-solution region based on mutual solubility between Fe₂O₃, Al₂O₃, and TiO₂, and (d) an extensive three-phase region characterized by the assemblage M₃O₅+α-M₂O₃( ss ) + Cor( ss ). A comparison of results with previously established phase relations for the Fe₂O₃–Al₂O₃–TiO₂ system shows considerable discrepancy.     

The Series MgCr2O4-MgFe2O4

The characteristics of spinels in the series MgCr₂O₄-MgFe₂O₄ were determined. The plot of cell size vs. molar composition is unusual in series showing complete solid solution because an unusually large deviation from Vegard's law was observed. This deviation is caused by changes in spinel structure with composition and temperature, and an equation was derived which applies a correction in terms of the degree of inversion. The effects of temperature on compositions high in MgFe₂O₄ include changes in density and refractive index. Solid solution of forsterite in MgCr₂O₄ decreases the cell size to 8.329 A but apparently is less than 1%. Changes in composition caused by vapor loss or by dissociation are small enough that this series is essentially binary below 1400° C.     

Coherent Precipitation in the System MgO-Al2O3-Cr2O3

An isothermal section of the ternary system MgO–Al₂O₃-Cr₂O₃ was determined at 1700°± 15°C to delineate the stability field for spinel crystalline solutions (cs). Crystalline solutions were found between the pseudobinary joins MgAl₂O₄–Cr₂O₃ and MgCr₂O₄-Al₂O₃, and the binary join MgAl₂O₄-MgO. The first two crystalline solutions exhibit cation vacancy models while the latter can probably be designated as a cation interstitial model. Precipitation from spinel cs may proceed directly to an equilibrium phase, (Al_1-xCr_x)₂O₃, with the corundum structure or through a metastable phase of the probable composition Mg(Al_1-xC_r)₂₆O₄₀. The composition and temperature limits were defined where the precipitation occurs via metastable monoclinic phases. The coherency of the metastable monoclinic phase with the spinel cs matrix can be understood by considering volume changes with equivalent numbers of oxygens and known crystallographic orientation relations. Electron probe and metallographic microscope investigations showed no preferential grain boundary precipitation.     

Phase Equilibria in the Spinel Region of the System FeO-Fe2O3-Al2O3

Phase relations in the spinel region of the system FeO-Fe₂O₃-Al₂O₃ were determined in CO₂ at 1300°, 1400°, and 15000°C and for partial oxygen pressures of 4 × 10⁻⁷ and 7 × 10⁻¹⁰ atmospheres at 15OO°C. The spinel field extends continuously from Fe₃O_4-x to FeAl₂O_4+z.     

Phase Relations in the System Iron Oxide—Cr2O3 in Air

The quenching method has been used to determine approximate phase relations in the system iron oxide-Cr₂O₃ in air. Only two crystalline phases, a sesquioxide solid solution (Fe₂O₃–Cr₂O₃) with corundum structure and a spinel solid solution (approximately FeO ·Fe₂O₃–FeO – Cr₂O₃), occur in this system at conditions of temperature and O₂ partial pressure (0.21 atm.) used in this investigation. Liquidus temperatures increase rapidly as Cr₂O₃ is added to iron oxide, from 1591°C. for the pure iron oxide end member to a maximum of approximately 2265°C. for Cr₂O₃. Spinel(ss) is the primary crystalline phase in iron oxide-rich mixtures and sesquioxide (ss) in Cr₂O₃–rich mixtures. These two crystalline phases are present together in equilibrium with a liquid and gas (po₂= 0.21 atm.) at approximately 2075°C.     

X-Ray Study of the Solid Solution of TiO, Fe2O3, and G, O3 in Mullite (3A12O3a2Si02)

A study of the solid solution of TiO₂, Fe₂O₃, and Cr₂0₃ in mullite was made by measuring the changes in lattice parameters and unit-cell volume. Synthetic mullite (3O₃-2SiO₂) was reacted with up to 12 weight % of the oxides at temperatures ranging from 1000° to 17000C. The approximate minimum temperature required for the formation of solid solution was 12000C. for Fe₂0₃ and 1400°C. for Cr₂O₃ and TiO₃. The maximum amount of solid solution found was 2 to 4% TiO₂ at 1600°C., 10 to 12% Fe₂O_s at 1300°C., and 8 to 10% Cr_ZO₃ at 1600OC. Lattice parameters and unit-cell volumes for each solid solution series increased with increasing amounts of foreign oxide. There was good agreement between the calculated and observed increase in cell dimensions for the iron oxide series. Except in the case of titania, there was good agreement between X-ray data and petrographic observations.     

Phase Equilibrium Studies in the System Iron Oxide-Al2O3-Cr2O3

Subsolidus phase relations in the system iron oride-Al₂O₂-Cr₂O₃ in air and at 1 atm. O₂ pressure have been studied in the. temperature interval 1250° to 1500°C. At temperatures below 1318° C. only sesquioxides with hexagonal corundum structure are present as equilibrium phases. In the temperature interval 1318° to 1410°C. in air and 1318° to 1495° C. at 1 atm. O₂, pressure the monoclinic phase Fe₂O₃. Al₂O₃ with some Cr₂O₃ in solid solution is present in the phase assemblage of certain mixtures. At temperatures above 1380°C. in air and above 1445°C. at 1 atm. O₂ pressure a complex spinel solid solution is one of the phases present in appropriate composition areas of the system. X-ray data relating d- spacing to composition of solid solution phases are given.     

Phase Equilibria in the Ferrite Region of the System FeO-MgO-Fe2O3

Phase equilibria in the ferrite region of the system FeO–MgO–Fe₂O₃ have been investigated up to 1300°C. over an oxygen-pressure range of 1.0 to 0.01 atmospheres. The five most important features of the equilibria in this system are as follows: (1) The spinel phase can exist with a deficiency of oxygen in the high-magnesium compositions, (2) Mg++ replaces Fe⁺⁺ in Fe₃O₄ beyond the "stoichiometric MgFe₂O₄," corresponding to the formula (MgO)_xMgFe₂O₄, where x = 0.092 ± 0.004 and is independent of temperature, (3) the spinel field does not include single-phase composition corresponding to Mg-Fe₂O₄, (4) the lattice constant of spinels in this system depends on cation distribution, and the extent of distribution changes as a function of temperature in turn depends on the magnesium concentration in the spinel, and (5) the spinel field near its terminus has little or no width.     

Dissociation Pressures of Solid Solutions from Fe3O4 to 0.4Fe3O4·0.6CoFe2O4

The dissociation pressures of solid solutions from Fe₃O₄ to 0.4Fe₃O₄·0.6CoFe₂O₄ have been determined as a function of temperature. Within experimental error, solid solutions within this range are thermodynamically ideal.     

Spray Pyrolysis of Fe3O4–BaTiO3 Composite Particles

Fe₃O₄–BaTiO₃ composite particles were successfully prepared by ultrasonic spray pyrolysis. A mixture of iron(III) nitrate, barium acetate and titanium tetrachloride aqueous solution were atomized into the mist, and the mist was dried and pyrolyzed in N₂ (90%) and H₂ (10%) atmosphere. Fe₃O₄–BaTiO₃ composite particle was obtained between 900° and 950°C while the coexistence of FeO was detected at 1000°C. Transmission electron microscope observation revealed that the composite particle is consisted of nanocrystalline having primary particle size of 35 nm. Lattice parameter of the Fe₃O₄–BaTiO₃ nanocomposite particle was 0.8404 nm that is larger than that of pure Fe₃O₄. Coercivity of the nanocomposite particle (390 Oe) was much larger than that of pure Fe₃O₄ (140 Oe). These results suggest that slight diffusion of Ba into Fe₃O₄ occurred.     

Grain Growth in Fe3O4

Sintering and microstructural evolution were studied in Fe₃O₄ as a model system for spinel ferrites. Fe₃O₄ powder, purified by the salt-crystallization method, was sintered to ∼99.5% density in a CO-CO₂ atmosphere. The p _O2 Of the sintering atmosphere drastically affects the microstructure (grain size) of sintered Fe₃O₄ without significantly affecting density. The measured grain-boundary mobilities, M , of Fe₃O₄ fit the equation M=M ₀( T ) p _O2^−1/2 with M ₀( T ) = 2.5×10⁵ exp[-(609kJ·mol-¹/ RT ](m/s)(N/m²)^−l. The grain-boundary migration process appeared to be pore-drag controlled, with lattice diffusion of oxygen as the most likely rate-limiting step.     

Melt Differentiation Induced by Zonal Structure Formation of Calcium Aluminoferrite in a CaO–SiO2–Al2O3–Fe2O3 Pseudoquaternary System

The phase stability in part of the P₂O₅-bearing pseudoquaternary system CaO–SiO₂–Al₂O₃–Fe₂O₃ has been studied by electron probe microanalysis, optical microscopy, and powder X-ray diffractometry. At 1973–1653 K, the α-Ca₂SiO₄ solid solution [α-C₂S(ss)] and melt coexisted in equilibrium, both chemical variations of which were determined as a function of temperature. The three phases of melt, calcium aluminoferrite solid solution (ferrite), and C₂S(ss) coexisted at 1673–1598 K. On the basis of the chemical compositions of these phases, a melt-differentiation mechanism has been, for the first time, suggested to account for the crystallization behavior of Ca₃Al₂O₆ solid solution [C₃A(ss)]. When the α-C₂S(ss) and melt were cooled from high temperatures, the melt would be induced to differentiate by the crystallization of ferrite. Because the local equilibrium would be continually attained between the rims of the precipitating ferrite and coexisting melt during further cooling, the melt would progressively become enriched in Al₂O₃ with respect to Fe₂O₃. The resulting ferrite crystals would show the zonal structure, with the Al/(Al+Fe) value steadily increasing up to 0.7 from the cores toward the rims. The C₃A(ss) would eventually crystallize out of the differentiated melt between the zoned ferrite crystals in contact with their rims.     

The System Manganese Oxide–Cr2O3 in Air

The quenching technique has been used to determine equilibrium relations in the system manganese oxide-Cr₂O₃ in air in the temperature range 600° to 1980°C. The following isobaric invariant situations have been determined: At 910°± 5°C tetragonal Mn₃O₄ solid solution, cubic Mn₃O₄ solid solution (=spinel), Mn₂O₃ solid solution, and gas coexist in equilibrium. Cubic Mn₃O₄ solid solution, Cr₂O₃ solid solution, liquid, and gas are present together in equilibrium at 1970°± 20°C. The invariant situation at which cubic Mn₃O₄ solid solution, Mn₂O₃ solid solution, Cr₂O₃ solid solution, and gas exist together in equilibrium is below 600°C.     

Phase Relations In The Pseudo-Ternary System La2O3–SrO–Fe2O3

The phase relations in the pseudo-ternary system La₂O₃–SrO–Fe₂O₃ have been investigated in air. Isothermal sections at 1100° and 1300°C are presented based on X-ray diffraction and thermal analysis of annealed samples. Extended solid solubility was observed for the compounds Sr _{n +1− v} La _v Fe _n O_{3 n +1−δ} ( n =1, 2, 3, and ∞) and Sr_{1− x} La _x Fe₁₂O₁₉, while only limited solubility of La in Sr_{4− z} La _z Fe₆O_13±δ was observed. At high Fe₂O₃ content, a liquid with low La₂O₃ content was stable at 1300°C.     

Solid Solution and Chromium Oxide Loss in Part of the System MgO-Al2O3-Cr2O3-SiO2

Thermal and X-ray studies show that there is complete solid solution between MgO.Cr₂O₃ and MgO.Al₂O₃ and that the spinel solid solutions are stable with no exsolution down to temperatures as low as 510°C. There is no solid solution of excess Cr₂O₃ in MgO.Cr₂O₃ nor of MgO.Cr₂O₃ in Cr₂O₃. The join MgO.Cr₂O₃–Al₂O₃ is found to be nonbinary; compositions along that join yield mixtures of a chromium oxide-alumina solid solution and a spinel solid solution on firing to temperatures high enough to promote solid-state reaction. Chromium oxide loss by volatilization increases at higher temperature. At a given temperature, chromium oxide loss is found to vary directly with the partial pressure of oxygen in the furnace atmosphere and with the ratio of MgO to SiO₂ in the charges heated.     

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.     

Indirect Dissolution of (Al, Cr)2O3 in CaO—MgO—Al2O3—SiO2 (CMAS) Melts

The dissolution of (Al, Cr)₂O₃ into CaO—MgO—Al₂O₃—SiO₂ melts, under static and forced-convective conditions was investigated at 1550°C in air. With sufficient MgO in the melt, or sufficient Cr₂O₃ in (Al, Cr)₂O₃, a layer consisting of a spinel solid solution, Mg(Al, Cr)₂O₄, formed at the (Al, Cr)₂O₃/melt interface. The dissolution kinetics of 1.5 and 10 wt% Cr₂O₃ specimens were determined as a function of immersion time, specimen rotation rate, and magnesia content of the melt. Electron microprobe analysis was used to characterize concentration gradients in the (Al, Cr)₂O₃ sample, the Mg(Al, Cr)₂O₄ spinel, or in the melt after immersion of specimens containing 1.5 to 78 mol% Cr₂O₃. The dissolution kinetics and microprobe analyses indicated that a steady-state condition was reached during forced-convective, indirect (Al, Cr)₂O₃ dissolution such that spinel layer formation was rate limited by solid-state diffusion through the spinel layer and/or through the specimen, and spinel layer dissolution was rate limited by liquid-phase diffusion through a boundary layer in the melt. This is consistent with a model previously developed for the indirect dissolution of sapphire in CMAS melts.