Grain Growth in CaO-Stabilized ZrO2 in the Presence of a Liquid Phase

Calcia-stabilized zirconia, CSZ (7.5 wt% CaO), with impurities of A1₂O₃, SiO₂, MgO, Fe₂O₃, and TiO₂, was sintered in air at 1783 K for times (t) up to 230 h. The microstructure consisted of grains of CSZ with small amounts of pores and a CaO-Al₂O₃-SiO₂ eutectic. Grain diameters grew in proportion to t^1/3. The results are consistent with a mechanism in which grain growth is limited by diffusion of Zr⁴⁺ ions through the liquid eutectic.     

Role of Iron in Calcium Phosphate Glasses

Some physical properties okf phosphate glasses con-taining up to about 26 mol% Fe₂O₂ were studied. Pronounced changes in properties were observed at compositions containingabout 6, 10, and 13 mol% Fe₂O₃. The X-ray diffraction spectra of devitrified (heat-treated) samples showed new compounds near these compositions. Electron spin resonance and optical studies confirmed the presence of Fe³⁺ and Fe²⁺ in both 4- and 6-coordination. An increase in total iron in these samples was associated with a decrease in the ratios Fe²⁺ 4-coordinated/Fe³⁺ 6-coordinated 6-coordinated and Fe³⁺ 4-coordinated/Fe³⁺ 6-coordinated up to about 2.0 mol% Fe₂O₃, as shown by the intensity of the optical absorption bands at about 2.0 and 1.0 μm and by the intensity of the ESR lines at g⋍4.2 and 2.0, respectively. Samples containing up to 4.3 mol% Fe₂O₃ showed an increase in Fe³⁺ concentration and a decrease in Fe²⁺ concentration after gamma irradiation. The electrical conductivity and activation energy decreased sharply with increasing Fe₂O₃ content.     

Equilibrium Studies of Fe in Alkali Phosphate Glasses

The Fe²⁺-Fe³⁺ equilibrium in binary Na₂O-P₂O₅, glasses was studied by equilibrating glass melts at different temperatures in air. The enthalpy change (δH) of the reaction 1/2Fe₂O₃⇌FeO+1/4O₂ was calculated for 4 glasses. The results indicate that (1) the equilibrium shifts toward the oxidized state as the Na₂O content of the glass increases (plots of log ([Fe²⁺]/[Fe³⁺]) vs mol% alkali were linear) and (2) Δ H values for glasses of different composition are nearly equal but differ from the standard (calculated) value for the reaction. The experimental ΔH values were nearly equal to that for the reaction FePO₄→1/3 Fe₃(PO₄)²+ 1/6P₂O₅+1/4O₂, indicating that Fe forms phosphate or polyphosphate configurations in the Na₂O-P₂O₅ glasses. In certain of the glasses studied a faint-pink solid precipitated; its X-ray diffraction pattern indicated that its principal component is crystalline Na₂Fe¹¹¹P₃O₁₀.     

Phase Diagram of the System BaO-Fe2O3

The phase diagram of the system BaO-Fe₂0₃ was determined by X-ray diffraction, melting-point measurement, and microscopic methods. Since the reduction of Fe³⁺ to Fe²⁺ was observed by chemical analysis in the samples heated at high temperature, especially in molten samples, the samples were heated at 1 atmosphere pressure of oxygen in the temperature region in which the liquid was in equilibrium; 1 atmosphere pressure of oxygen was su5cient to restrain the reduction of Fe³⁺. In the temperature region of solid-solid equilibrium, the dissociation was not observed even when the samples were heated in air. BaO - 6Fe₂O₃ formed a solid solution with BaO.Fe₂0₃. The BaO:Fe₂0₃ ratio of the solid solution was BaO.4.5Fe₂0₃ at 1350°C. and BaO. 5.0Fe₂0_a at 800°C. The precipitation micro-structures of each primary solid solution were observed.     

Kinetics of Cation Redisribution in Ferrospinels

In situ electrical resistivity measurements were employed to follow the kinetics of cation redistribution in ferrospinels, x Fe₃O₄–(1 – x )MeFe₂O₄ (Me = Co, Mg, Mn, or Ni) or x Fe₃O₄–FeMe₂O₄ (Me = Al) with x ≥ 0.2. Relaxation temperatures (at 20°C/min heating rate) were established and kinetic parameters—activation energies and time constants—were determined for each cation. These parameters were insensitive to grain size, cation ratio, and oxygen nonstoichiometry. The mechanism was shown to be a local "homogeneous" point defect reaction in contrast to the "heterogeneous" nucleation and growth mechanism reported to take place in ferrispinels lacking Fe²⁺. Electron hopping between Fe²⁺ and Fe³⁺ in ferrospinels apparently screens the excess ionic charge associated with the diffusing species and enables homogeneous point defect reactions with lower attendant activation energies to take place.     

Structural Instability and Microstructural Change of La3+-Doped M-Type Calcium Ferrite

Several compositions were selected to investigate the formation of M phase. It was found that by reducing Fe₂O₃ or increasing La₂O₃, we were able to obtain pure M phase, as deduced from X-ray patterns, but CaO.2Fe₂O₃ was always observed in micrographs of the polished surfaces. The instability of the M phases might be due to the distortion of the lattice structure. The development of humps in the microstructures was related to the distribution of La³⁺. Retardation in the formation of M phase when sintered in O₂ could be interpreted by the defect equation.     

Discoloration of Fired Kaolinitic Clays (Study of Fe3+ Coordination by Mössbauer and UV-ViS-NIR Spectroscopy)

The final color of a ceramic body is generally determined by the contents of Fe³⁺ ions. The Central European market accepts kaolinitic clays for the production of tableware, electro-insulators, and wall tiles only if the Fe₂O₃ in chemical analyses of the kaolin does not exceed 1.0 wt%. The chemical analyses calculate the content of all iron components as if they were in the form of Fe₂O₃ and then their quantity excludes even good-quality clay from the ceramic industry of white bodies. We found that oxidizing firing of the samples fired at over 1180°C changes the color in cases where the initial Fe³⁺ component is in the form of hydro ferric oxide [FeO(OH)] or if the ferric ions were added to the white kaolin samples in the form of ferric nitrate [Fe(NO₃)₃·9H₂O]. The Mössbauer spectroscopy confirms the presence of only diluted Fe³⁺ ions. The UV-ViS-NIR spectroscopy confirms that even if the concentration of Fe₂O₃ is above 3.0 wt%, these ions of Fe³⁺ in the case of their initial hydro ferric oxide formed in clay are incorporated into the mullitic structure in tetrahedral coordination. The iron-coloring effect depends on the coordination of Fe³⁺ ions with the studied discoloring effect of fired bodies—the very small and well-distributed particles enter into the formatted mullitic structure.     

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.     

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.     

Formation Mechanisms of La3+-Doped CaO·6Fe2O3 with the Magnetoplumbite Structue

Several experiments were conducted to investigate the formation mechanisms of the magnetoplumbite phase in La³⁺-doped CaO 6Fe₂O₃. It is shown that the CaO·2Fe₂O₃ phase plays a crucial role in forming the magnetoplumbite phase. The formation mechanisms are proposed and verified.     

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.     

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.     

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.     

Melting Relations of the Common Rock-Forming Oxides

Geological science is concerned with the nature of the materials of the earth and particularly with the processes by which earth materials have been changed and modified. Laboratory studies of the melting behavior of the common rock-forming oxides have been an important adjunct to the observations of the field geologist. For the past fifty years investigators at the Geophysical Laboratory have been obtaining quantitative information on the melting relations of many of the important rock-forming minerals. These studies of the fundamental chemistry of igneous and metamorphic rocks have yielded much information of value to ceramists, metallurgists, and mineral technologists. This paper summarizes the most important phase-equilibrium studies of unary, binary, ternary, quaternary, and portions of quinary systems of the common rock-forming oxides-SiO₂, Al₂O₃, FeO, Fe₂O₃, CaO, MgO, Na₂O, and K₂O.     

Water of Hydration in the System Fe–O

Fe or Fe + Fe₂O₃ was heated in a stream of carefully dried oxygen, and the amount of water carried by the gas was determined as a function of time and temperature in the range 300° to 1100°C. Water was evolved as a result of a reaction with the crucible. This water was absorbed by FeO when FeO was present, and was released when the FeO was converted to Fe₃O₄. The apparent excess of O or deficit of Fe in FeO, reported in the literature, therefore may be due to the presence of water or hydroxide in the FeO. The other oxides, which occur in the phase diagram with exact stoichiometry, absorbed and held much less water, if any.     

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.     

Effect of Molybdenum on the Structure and on the Crystallization of SiO2–Na2O–CaO–B2O3 Glasses

Crystallization of the poorly durable Na₂MoO₄ phase able to incorporate radioactive cesium must be avoided in SiO₂–Al₂O₃–B₂O₃–Na₂O–CaO glasses developed for the immobilization of Mo-rich nuclear wastes. Increasing amounts of B₂O₃ and MoO₃ were added to a SiO₂–Na₂O–CaO glass, and crystallization tendency was studied. Na₂MoO₄ crystallization tendency decreased with the increase of B₂O₃ concentration whereas the tendency of CaMoO₄ to crystallize increased due to preferential charge compensation of BO₄⁻ entities by Na⁺ ions. ²⁹Si MAS NMR showed that molybdenum acts as a reticulating agent in glass structure. Trivalent actinides surrogate (Nd³⁺) were shown to enter into CaMoO₄ crystals formed in glasses.     

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.     

Iron Diffusion in Iron-Aluminate Spinels

The diffusion coefficient of ⁵⁹Fe in iron-aluminate spinels (Fe₃O₄-FeAl₂O₄) was measured by serial sectioning. The results were analyzed with a defect model, assuming that the predominant defects were cation vacancies and interstitials. At 1380°C, diffusion occurs in Fe₃O₄ and Fe(Fe_1.51Al_0.49))O₄ by an interstitial mechanism at low P_O2 and a vacancy mechanism at high P_O2* Diffusion of iron is by the vacancy mechanism for higher hercynite concentrations, except that the results for the end-member, FeAl₂O₄, suggest an interstitial mechanism. At 10⁻⁶ atm O₂, an Arrhenius plot of D _Fe exhibits a minimum for Fe₃O₄ and Fe(Fe_1.51 A1_0.49)O₄. This effect is explained in terms of a mechanism change. The D _Fe is essentially independent of temperature in Fe(Fe_1.03Al_0.97)O₄ and Fe(Fe_0.54Al_1.46)O₄.     

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.