The System MgO-Cr2O3−Fe2O3 at 1300°c in Air

Phase relations in air at 1300°C were determined for the system MgO-Cr₂O₃−Fe₂O₃ by conventional quenching techniques. Details of the phase equilibria were established for: (1) the sesquioxide solid solution between Cr₂O₃ and Fe₂O₃, (2) the spinel solid solution field between MgCr₂O₄ and MgFe₂O₄, and (3) the periclase solid solution field for MgO. Selected tie lines connecting coexisting compositions were established with X-ray diffractometer data. Diffuse reflectance spectra, diffractometer intensity ratios, and lattice parameter measurements were obtained for quenched samples to study the structural inversion in the spinel series MgCr₂O4-MgFe₂O₄.     

Influence of Cr and Fe on Formation of α-Al2O3 from γ-Al2O3

The effect of Cr and Fe in solid solution in γ-Al₂O₃ on its rate of conversion to α-Al₂O₃ at 1100°C was studied by X-ray diffraction. The δ form of Al₂O₃ was the principal intermediate phase produced from both pure γ-Al₂O₃ and that containing Fe³⁺ in solid solution, although addition of Fe greatly reduced crystallinity. Reflectance spectra and magnetic susceptibilities showed that Cr exists as Cr⁶⁺ in γ-Al₂O₃ and as Cr³⁺ in α-Al₂O₃, with θ-Al₂O₃ as the intermediate phase. The intermediates formed rapidly, and the rates of their conversion to α-Al₂O₃ were increased by 2 and 5 wt% additions of Fe and decreased by 2 and 4 wt% additions of Cr. An approximately linear relation observed between α-Al₂O₃ formation and decrease in specific surface area was only slightly affected by the added ions. This relation can be explained by a mechanism in which the sintering of δ- or θ-Al₂O₃, within the aggregates of their crystallites, is closely coupled with conversion of cubic to hexagonal close packing of O^2- ions by synchro-shear.     

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.     

Kinetics of NiCr2O4 Formation and Diffusion of Cr3+ Ions in NiO

The reaction kinetics for NiCr₂O₄ formation and the diffusion of Cr³⁺ ions into single-crystal NiO were studied between 1300° and 1600°C in air. The experimental activation energy for NiCr₂O₄ formation was about 83 kcal/mol. After incubation, NiCr₂O₄ formed by a diffusion-controlled process. The origin of pores at the NiO/NiCr₂O₄ interface is discussed. The concentration profiles of Cr³⁺ in NiO were linear because the interdiffusion coefficient was directly proportional to the mol fraction Cr³⁺. Theoretical considerations indicate that the interdiffusion coefficient equals 3/2 the self-diffusion coefficient of Cr³⁺, which is rate-determining. The interdiffusion coefficient at 1 mol% Cr₂O₃ can be expressed as =4×10⁻³ exp (−55,000/RT) cm² s⁻¹.     

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.     

Interdiffusion in the System MgO-MgAl2O4

Interdiffusion coefficients in single-crystal MgO were determined using an MgO-MgAl₂O₄ diffusion couple. For a concentration of 1 mol% Al₂O₃ in MgO, the interdiffusion coefficient can be expressed as D =2.0±0.2 exp (−76,000±3,000/ RT ) for the MgO-MgAl₂O₄ couple. This relation compares well with previous measurements in the MgO-Al₂O₃ system. The interdiffusion coefficients, which increased with the mol fraction of cation vacancies, were in the range of 10⁻⁸ to 10⁻¹⁰ cm²s⁻¹ for the concentrations and temperatures studied. Diffusion was enhanced below 1640°C if powdered MgAl₂O₄ was used. Self-diffusion coefficients for Al³⁺ ions in MgO were calculated; Al³⁺ diffuses faster than Cr³⁺ in MgO.     

Activity–Composition Relations in NiAl2O4─MnAl2O4 Solid Solutions and Stabilities of NiAl2O4 and MnAl2O4 at 1300° and 1400°C

Activities of NiO were measured in the oxide and spinel solutions of the system MnO–NiO–Al₂O₃ at 1300° and 1400° C with the aim of deriving information on the thermodynamic properties of the spinel phases. Synthetic samples in selected phase assemblages of the system were equilibrated with metallic nickel and a gas phase of known oxygen partial pressures at a total pressure of 1 atm. The data on NiO activities and directions of conjugation lines between coexisting oxide and spinel phases were used to establish the activity–composition relations in spinel solid solutions at 1300° and 1400°C. The MnAl₂O₄–NiAl₂O₄ solid solutions exhibit considerable negative deviations from ideality at these temperatures. The free energy of formation of MnAl₂O₄ from its oxide components (MnO + Al₂O₃) at 1300° and 1400°C is calculated to be −24.97 and −26.56 kJ. mol⁻¹, respectively. The activities determined in the stoichiometric spinel solid solutions are more negative as compared with those predicted from cation distribution models.     

The System MgO–MgAl2O4

In the determination of the liquidus, solidus, and subsolidus of the system MgO-MgAl₂O₄ the limits of the solid solution of A1 ions in periclase and Mg ions in spinel were measured. By using both X-ray diffraction and optical techniques, the maximum periclase solid solution was found at 82 wt% MgO, 18 wt% A12O3 (9.5% A1³⁺) and maximum spinel solid solution at 39% MgO, 61 % A1203 (6% Mg⁺⁺). Periclase and spinel solid solutions existed stably in easily detectable amounts at temperatures above approximately 1500°C.     

Thermochemistry of MgAl2O4-Al8/3O4 Defect Spinels

Solution calorimetry of MgAl₂O₄-Al_8/3O₄ solid solutions was performed in a molten 2PbO · B₂O₃ solvent at 975 K. The results indicate small negative heats of mixing, relative to spinel standard states for both end-members. These data were combined with information on the energetics of the α-γ transition in Al₂O₃ and on the MgAl₂O₄-Al_8/3O₄ (MgO-Al₂O₃) subsolidus phase relations to estimate the partial molar entropy of mixing of γ-Al_8/3O₄ in the solid solution. This entropy is much less positive than that calculated from several models for the configurational entropy of mixing of magnesium, aluminum, and vacancies on octahedral and/or tetrahedral sites. The data suggest a good deal of local order to be present in the solid solutions, consistent with negative enthalpies of mixing and entropies of mixing far less than ideal configurational values.     

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.     

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.     

Optical Absorption Spectra of Cr3+ in MgO·Al2O3-MgO·3.5Al2O3 Spinels

Spinel crystals in the system MgO· n Al₂O³:Cr³⁺ (1.04≤ n ≤3.51) were synthesized by flame fusion; their absorption spectra were measured in transmission from 400 to 50,000 cm⁻¹. The O-H band spectra suggest the presence of protons in octahedral site vacancies which appear for n >1. The spectra for Cr³⁺ indicate clearly that these ions occupy only octahedral sites. However, the effective trigonal field, which varies with n , causes the major absorption bands to split in the form of a lower-energy shoulder on a higher-energy component. The red-green transition in these spinels was rationalized and observed to arise from a somewhat different mechanism than that in other oxides in which the site radii and Cr³⁺ concentration vary considerably.     

Exsolution of β-Ga2O3 Crystalline Solutions in the System MgAl2O3-Ga2O3

The subsolidus phase equilibrium diagram for the pseudobinary join MgAl₂O₄-Ga₂O₃ was determined. The shape of the exsolution boundary was obtained by heat-treating samples pre- equilibrated at 1600°C. Crystalline solubility of Ga₂O₃ in MgAl₂O₄ decreased from 73 mole % at 1600°C to 55 mole % at 1200°C. The crystalline solution was formed by the replacement of Mg²⁺ions by Ga³⁺ ions to produce a cation defect spinel. The phase precipitated was the mono-clinic δ-Ga₂O₃ (=δ-Al₂O₃ structure). Changes in the ratios of relative X-ray diffraction intensities indicated that the crystalline solutions also disorder with temperature.     

Kinetics of Precipitation and Measurements of Strain in the System MgO-Al2O3-Cr2O3

A quantitative X-ray technique for measuring precipitation strains has not been previously applied in metallic or oxide systems. The Warren-Averbach analysis of strain was used to determine the buildup of elastic strain energy in the spinel crystalline solution matrix (gross composition = 60 mol% MgAl₂O₂+ 40 mol% Cr₂O₃) during the isothermal (1135°C) precipitation of a metastable (coherent) monoclinic phase. The elastic strain energy of the spinel crystalline solution matrix increased to a maximum of about 3.1 × 10⁷ ergs/cm³ for a reaction time of 8 h. There was a marked decrease in the elastic strain energy during the initial precipitation of the equilibrium corundum crystalline solution with the composition (Al³⁺_0.72 Cr³⁺_0.25)O₃. An overall diffusion activation energy for precipitation of the mono-clinic phase was approximately 86 kcal/mol.     

The System CaO–"CaCr2O4"–CaAl2O4 in Air and under Mildly Reducing Conditions

The system CaO–chromium oxide in air is reinvestigated and the existence of intermediate phases with chromium in oxidation states >3+ (Ca₅Cr₃O₁₂, Ca₃(CrO₄)₂, and Ca₅(CrO₄)₃) confirmed. Under reducing conditions these phases are unstable. A metastable, polymorphic form of calcium chromite, δ -CaCr₂O₄, is observed. In the CaO-rich section of the CaO–Al₂O₃–Cr₂O₃ system a ternary intermediate phase, chrome-haüyne, Ca₄[(Al,Cr³⁺)₆O₁₂](Cr⁶⁺O₄), coexists with calcium chromate and calcium aluminate phases. In air, low melting temperatures are preserved in all assemblages containing calcium chromate phases. Under reducing conditions a new ternary phase, Ca₆Al₄Cr₂O₁₅, coexists with CaO, CaCr₂O₄, chrome-haüyne, and calcium aluminate phases. The influence of chromium oxide additions on the solidus temperatures of the CaO–Al₂O₃ system is insignificant.     

Influence of Oxygen Activity on the Sintering of MgCr2O4

The sintering behavior of MgCr₂O₄ powder compacts was investigated as a function of temperature, time, and oxygen activity. The results show that MgCr₂O₄ cannot be densified to >70% of theoretical density at temperatures up to 1700°C if the oxygen activity exceeds 10⁻⁶ atm. The oxygen activity must be decreased to <10⁻¹⁰ atm before densities exceeding 90% of theoretical can be achieved. Weight loss and X-ray data indicated that maximum density occurred at an oxygen activity just above that where MgCr₂O₄ becomes unstable.     

Incorporation of Aliovalent Dopants into the Bismuth-Layered Perovskite-Like Structure of BaBi4Ti4O15

The solid solubility of the aliovalent dopants Fe³⁺ and Nb⁵⁺ in the BaBi₄Ti₄O₁₅ compound, a member of the family of Aurivillius bismuth-based layer-structure perovskites, has been studied using quantitative wavelength-dispersive spectroscopic microanalysis (SEM/EPMA) in combination with X-ray powder diffractometry (XRPD). The samples with nominal (starting) compositions corresponding to the chemical formulas BaBi₄Ti_{4–4 X} Fe_{4 X} O₁₅ and BaBi₄Ti_{4–4 X} Nb_{4 X} O₁₅ were prepared by hot forging a mixture of BaTiO₃ and Bi₄Ti₃O₁₂ with additions of Fe₂O₃ or Nb₂O₅ followed by a long annealing at 1100°C. The study showed that an excess charge introduced into the structure by the substitution of Ti⁴⁺ ions with aliovalent dopants was preferentially compensated by a change in the ratio of Ba²⁺ to Bi³⁺ ions in the host structure according to the general formulas of the solid solutions Ba_{1–4 X} Bi_{4+4 X} Ti_{4–4 X} Fe^'_{4 X} O₁₅ and Ba_{1+4 X} Bi_{4–4 X} Ti_{4–4 X} Nb^·_{4 X} O₁₅.     

Oxidation State of Chromium in CaO–Al2O3–CrOx–SiO2 Melts under Strongly Reducing Conditions at 1500°C

Equilibrium ratios Cr²⁺/Cr³⁺ of chromium oxide dissolved in CaO–chromium oxide–Al₂O₃–SiO₂ melts have been determined by analysis of samples equilibrated at 1500°C under strongly reducing conditions ( p _o2= 10^−9.56 to 10^−12.50 atm). The majority of the chromium is divalent (Cr²⁺) under these conditions and Cr²⁺/Cr³⁺ ratios at given constant oxygen pressures decrease with increasing basicity of the melts, expressed as CaO/SiO₂ ratios. In addition, Cr²⁺/Cr³⁺ ratios, at a given CaO/SiO₂ ratio, are relatively unaffected by the amount of Al₂O₃ present.     

Optical Spectra of 3d Transition Metal Ions in MgO 3·52O3 Spinel

Large gem-quality boules of MgO·3.5A1₂, O₃ spinel, both pure and incorporating controlled amounts of Ti³, V₃₊, Cr³⁺, Mn₂, Fe₂₊, Co²⁺, or Ni₂₊ were synthesized by a modified verneuil technique. Each crystal was characterized by measuring the lattice parameter, chemical composition, and density. Using thin polished slices as specimens, the absorption spectrum of each crystal was measured from 400 to 50, 000 cm⁻¹, and band centers and order-of-magnitude oscillator strengths were calculated from the spectrum for each specimen. For these well characterized crystals, the absorption spectra were interpreted satisfactorily in terms of crystal field theory when the complete individual spectrum as well as the spectrum of the host matrix was considered. On this basis, the oxidation state and coordination of the 3d ion could be determined. Covalent bonding effects were of definite significance in these spinels and caused an inexact correspondence between observed band centers and those predicted by the Orgel diagrams. The ratio Dq (tetrahedral)/ Dg (octahedral) was 0.67 in the 1:3.5 spinel and verified the theoretical relation expressing this ratio in terms of a constant, 4/9, times the fifth power of the ratio of the site radii. Coordination preferences of the incorporated transition metal ions are also discussed.     

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.