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.     

Spontaneous Infiltration of Alumina by Copper-Oxygen Alloys

Infiltration of alumina powder compacts by molten copper-oxygen alloys under controlled oxygen partial pressures has been studied. Spontaneous infiltration occurred for copper alloys containing more than approximately 5 at.% oxygen. Despite supersaturation of the copper with oxygen, no Cu₂O or interfacial reaction layers of CuAlO₂ were resolved by TEM, suggesting the critical wettability for infiltration is not determined by interfacial chemical reaction. CuAlO₂ did form, but as discrete inclusions up to 0.5 mm in diameter. Infiltration and microstructure development are discussed in terms of the observed wetting behavior and available data on interfacial oxygen adsorption.     

Modeling and Fabrication of Fine-Grain Alumina-Zirconia Composites Produced from Nanocrystalline Precursors

Fully dense fine-grained 32.6-vol%-zirconia-toughened alumina composites have been fabricated from nanocrystalline rapidly solidified material. A model considering the thermodynamics of the constrained t -ZrO₂ m -ZrO₂ phase transformation was developed for this percolated two-phase material. This analysis indicated that the grain size at which this phase transformation is thermodynamically favorable was 1.26 µm in a composite that contained 32.6 vol% ZrO₂ and was stabilized with 1.50 mol% Y₂O₃. These results of the model compared favorably with experimental results, showing that grains of this size could be retained after heating to temperatures of as high as 1600°C. The rapidly solidified precursor was ball-milled into submicrometer powder and centrifugally cast into green specimens that were pressureless sintered to full density at temperatures as low as 1500°C. A composite containing nearly 100% t -ZrO₂ was produced by pressureless sintering at 1500°C and a composite containing 45 vol% t -ZrO₂/55 vol% m -ZrO₂ was obtained by sintering at 1600°C. The resulting two-phase microstructures contained uniformly distributed, micrometer-size grains whose sizes are consistent with the facilitation of transformation and microcrack toughening.     

Influence of Oxygen Partial Pressure on the Quasi-Ternary System Cr─Mn─Ti Oxide

The quasi-ternary system Cr─Mn─Ti oxide was investigated at 1000°C under oxygen partial pressures ranging from 0.21 bar to 10^-21 bar (1 bar = 10⁵ Pa). X-ray diffraction analysis was used to identify phases and determine lattice parameters. The positions of phase boundaries as a function of oxygen partial pressure were measured using the emf method. The spinel MnCr₂O₄ may be regarded as the most interesting compound in this system. Part of the chromium can be replaced by trivalent titanium at low oxygen partial pressures and by trivalent manganese at high pressures, and the formation of a limited solid solution with the spinel Mn₂TiO₄ is possible in all cases. As a result, a coherent single-phase spinel region exists over the entire oxygen partial pressure range at 1000°C.     

Grain-Growth Behavior during Stepwise Sintering of Barium Titanate in Hydrogen Gas and Air

To study the effect of oxygen partial pressure on grain growth in BaTiO₃, TiO₂-excess samples have been sintered in air with and without a prior H₂ heat treatment. Without prior H₂ treatment, abnormal grain growth occurs below and above the eutectic temperature ( T _e). An introduction of H₂ treatment before air sintering, however, increases the average grain size and suppresses the formation of abnormal grains during subsequent air sintering below and above T _e. This H₂ treatment effect has been explained in terms of a decrease of the driving force for the growth of faceted grains below a critical value for formation of abnormal grains. The observed grain-growth behavior under various atmospheres demonstrates the possibility of having various microstructures via control of oxygen partial pressure and initial grain size.     

Role of Oxygen in Microstructure Development at Solid-State Diffusion-Bonded Cu/α-Al2O3 Interfaces

Microstructure development at solid-state diffusion-bonded Cu/α-Al₂O₃ interfaces has been studied using optical and electron microscopy. High-purity Cu foil was bonded between basal-oriented α-Al₂O₃ single-crystal plates at 1040°C for 24 h in a vacuum of ∼1.3 × 10⁻⁴ Pa (1 × 10⁻⁶ torr). Optical microscopy of as-bonded specimens revealed a large Cu grain size, fine pores, and long needles of Cu₂O at the interface. Bulk specimens were annealed at 1000°C for various times under controlled oxygen partial pressures in CO/CO₂ mixtures. Consistent with a thermochemical analysis, CuAlO₂ could be formed at the interfaces. The CuAlO₂ was acicular and discontinuous, but occurred in a uniform distribution over the bulk specimen interfaces.     

Ambient-Temperature Mechanical Response of Alumina–Fluoromica Laminates

Flexural delamination experiments were used to evaluate the mechanical performance of thermochemically stable alumina–fluoromica laminates. Hot-pressed, precracked laminate specimens, in which two MgAl₂O₄-spinel-coated alumina substrates were separated by a thin layer of fluorophlogopite (KMg₃(AlSi₃)O₁₀F₂), were tested in fourpoint flexure at room temperature. Two types of mechanical response were observed: steady-state delamination and brittle failure. Microstructural analysis showed that the delamination response was associated with fine (≤5 μm) grains of the mica; the brittle response occurred when the mica interphase consisted of large (>30 μm) grains that bridged the interphase. The steady-state strain-energy release rate ( G _ss) measured on the graceful, delaminating beams was 9.1 ± 0.4 Jm^–2 for randomly oriented ∼ 5–μm grains but only 2.8 ± 0.2 Jm^–2 for ∼1–μm grains that were aligned with easy-cleavage planes parallel to the laminate interfaces. The results suggested that debonding of the specimens occurred via cleavage of the mica grains. Observation of delamination cracks confirmed this point: propagation occurred within the fluoromica interphase rather than along the spinel/alumina or spinel/fluorophlogopite interfaces. The mechanical feasibility of laminate specimens without the protective spinel coating on the substrate containing the notch was also tested to address an issue related to the preparation of alumina fiber/mica interphase/alumina matrix composites. The delamination response again occurred for the case of a fine-grained mica interphase.     

Effect of Ba6Ti17O40/BaTiO3 Interface Structure on {111} Twin Formation and Abnormal Grain Growth in BaTiO3

A structural transition of Ba₆Ti₁₇O₄₀/BaTiO₃ interfaces from faceted to rough was induced by reducing oxygen partial pressure in the atmosphere. As the oxygen partial pressure decreased, the number densities of {111} twins and abnormal grain decreased. TEM observation showed that the twin formation was governed only by the faceting of the interface. Experimental evidence of {111} twin-assisted abnormal growth of faceted BaTiO₃ grains was also obtained.     

Influence of Oxygen Partial Pressure and Oxygen Content on the Wettability in the Copper–Oxygen–Alumina System

The influence of oxygen on the wetting behavior of copper on single-crystal Al₂O₃ has been studied. By controlling the oxygen partial pressure ( p _O2) and oxygen content in the copper simultaneously, contact angle can be varied between 125° and 22°. Evaluation of the Gibbs adsorption equation for the liquid/solid interface at 1300°C suggests that adsorption of a Cu-O complex at that interface plays a key role in promoting wetting. Formation of CuAlO₂ and dissolution of Al₂O₃ in the melt also influence the contact angle, especially in the range of p _O2 > 10⁻⁵ bar (1 Pa).     

Limits of the Thermodynamic Stability of Cobalt—Iron—Manganese Mixed Oxides at 1200°C

The thermodynamic stability of spinel and rock-salt structure phases has been investigated for the quasi-ternary mixed oxide system Co-Fe-Mn-O at 1200°C and at total pressures of the order of 1 atm (∼1.01 × 10⁵ Pa) using thermogravimetric and electrical conductivity measurements. The results reveal the stability limits of the spinel and rock-salt structure phases with varying cationic composition and oxygen partial pressure at 1200°C. The oxygen partial pressure was varied over 12 orders of magnitude, from log₁₀ a _o2= 0 to log₁₀ a _o2= -12, by using CO/CO₂ and N₂/O₂ gas mixtures and monitored by an electrochemical, stabilized zirconia-based EMF cell. The maximum cobalt mole fraction used in the investigation was about 0.33.     

Effect of Physical Nature of Powders and Firing Atmosphere on ZnAI2O4 Formation

The rate of ZnA1₂O₄ formation for binary powder mixtures of ZnO and α-Al₂O₃ (dense coarse particles and weak agglomerates of fine powder) fired in air or O₂ atmospheres was measured and the microstructures of those systems were observed by scanning electron microscopy. With dispersed dense particles of α-Al₂O₃, the Al₂O₃ surfaces were covered with ZnO and the spinel grew into the particles maintaining essentially a constant reaction interface area. Calculations based on geometric measurements and use of Jander's equation gave a similar high activation energy, 354 kJ/mol, which corresponds to the activation energy of volume diffusion of Zn²⁺ in ZnAl₂O₄. An oxygen atmosphere had no effect. With a matrix of fine α-Al₂O₃ powder and dispersed granules of ZnO, a higher reaction rate occurred because of an increase in reaction interface area due to penetration of the powder compact matrix by ZnO vapor, which was enhanced by an O₂ atmosphere. The reaction layer grew into the alumina matrix adjoining the ZnO granules with a parabolic rate law. Apparent activation energies below ∼200 kJ/mol were calculated.     

Anomalous X-ray Line Broadening Observed in γ-Fe2O3 Derived from γ-FeOOH

Defect structure of fine-grained γ-Fe₂O₃ powder obtained by dehydroxylation of y-FeOOH powder has been studied by X-ray diffraction and high-resolution electron micrographs. An anomalous X-ray line broadening was observed in the γ-Fe₂O₃ powder. It is found that the X-ray line breadth is dependent on the crystal structure factors of each reflection line: the reflections from net planes consisting of only metal ions on tetrahedral sites and octahedral sites of spinel structure are much broader than those from net planes including oxygen ions. The anomalous X-ray line broadening is explained on the basis of stacking faults in the spinel structure.     

Iron-Excess Manganese Ferrite: Electrical Conductivity and Cation Distributions

The electronic conductivity and thermoelectric power of a magnetic ferrite of composition Mn_0.863Fe_2.137O₄ have been measured as functions of oxygen partial pressure and temperature. Within the single-phase region of the spinel ferrite, both electrical properties are essentially independent of the oxygen partial pressure, implying that the electronic charge carrier concentration is governed by the molecularity rather than by oxygen nonstoichiometry. The electrical conduction behavior is successfully interpreted on the basis of a small polaron model, yielding a hopping energy of 0.363 ± 0.008 eV and a drift mobility of 0.05 to 0.10 cm²/(V·s) in the temperature range of 900 to 1300 K.     

Electrical Conductivity and Defect Models of MgO-Doped Cr2O3

The dc electrical conductivity of MgO-doped Cr₂O₃ and the defect models with the defect structure of a sesquioxide were investigated as a function of oxygen partial pressure, temperature, and dopant content. The electrical conductivity of MgO-doped Cr₂O₃ is increased with oxygen partial pressure and temperature. The electrical conductivity of MgO-doped Cr₂O₃ within the solubility limit is increased with MgO content because of the creation of holes and the annihilation of chromium vacancies. Above the solubility limit, however, it is decreased with increasing MgO content owing to the formation of the spinel phase (MgCr₂O₄).     

Evaluation of the Crack Face Bridging Mechanism in a MgAl2O4 Spinel

The effectiveness of a grain bridging mechanism in the following wake zone of two MgAl₂O₄ spinel microstructures has been demonstrated through renotching experiments with double-cantilevered-beam (DCB) specimens. Measurements of d K_R /dδ a agree with previous crack growth resistance curves of similar materials, obtained from single-edgenotch-beam specimens. However, even the extended test ligaments available from the present DCB specimens did not permit the full development of the following wake zone of these coarse microstructures.     

Mechanical Properties and Microstructure of Ca2SiO4–CaZrO3 Composites

Three types of dicalcium silicate (Ca₂SiO₄–calcium zirconate (CaZrO₃) composites were fabricated and their microstructures correlated with their mechanical properties. In the first type, Ca₂SiO₄ was added as a minor phase. The second type consisted of a 50 vol% Ca₂SiO₄-50 vol% CaZrO₃ mixture, while in the third type, CaZrO₃ constituted the minor phase. Pure CaZrO₃ was also studied as a control and found to have a toughness which depended on its grain size. In composites with Ca₂SiO₄ as the minor phase, a toughness increase was observed and found to be a function of matrix grain size. The composite with the second type of microstructure had the highest toughness of about 4.0 Mpa. m^1/2, which was about double that of the monolithic CaZrO₃. No evidence was found for transformation toughening by the orthorhombic (β) to monoclinic (γ) transformation in Ca₂SiO₄. The main toughening mechanisms identified were crack deflection and crack branching. Microstructural observations indicated the existence of weak grain boundaries in CaZrO₃ agglomerates as well as weak interfaces between the two phases.     

Processing Effects on Microstructure and Superconducting Properties of Sintered YBa2Cu3O6+x

The relationships between the microstructure of sintered YBa₂Cu₃O_{6+ x} superconductors and processing variables (sintering time, sintering temperature, and oxygen partial pressure) were examined. Large-grained microstructures were obtained in 100 kPa oxygen sintering atmospheres, while fine-grained microstructures were obtained in 2 kPa oxygen. The formation of liquid phases below the peritectic decomposition temperature of YBa₂Cu₃O_{6+ x} was found to have an effect on both the microstructure (as observed by optical and transmission electron microscopy) and the transport critical current density ( J_c ). The critical current density was found to be highest for sintering below the lowest invariant point, which is a function of the oxygen partial pressure. However, over the range of conditions examined here, there does not appear to be any correlation between microstructural features, such as average grain size and aspect ratio, and the transport J_c .     

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.     

Oxidation Inhibition Mechanisms in Coated Carbon–Carbon Composites

The mechanism of oxidation resistance in SiC-coated carbon-carbon composites containing boron-based inhibitors has been investigated using thermodynamic calculations, thermogravimetric analysis, and electron microscopy. A model is developed based on the formation of a volatile B₂O₂ sub-oxide in the interior of the composite which condenses to B₂O₃ upon encountering a locally high oxygen partial pressure in coating cracks. The active-to-passive transition for the oxidation of elemental boron has been determined.     

Early Stages of Composite Formation by Oxidation ofLiquid Aluminum Alloys

Microstructure evolution during the early stages in the directed oxidation of molten Al-Mg and Al-Mg-Si alloys was investigated to provide needed insight into the origins of the incubation period and its practical elimination by SiO₂ additions. Oxidation experiments were performed primarily in thermogravimetric balances and microstructures were analyzed by optical and scanning electron microscopy. Continuous heating above the alloy liquidus produces first a thin MgO layer and then a brief rapid growth of a spinel + metal mixture within a temperature range which depends on the alloy Mg content and the heating rate. The initial rapid oxidation terminates abruptly with the formation of a dense spinel layer at the surface, leading to a long incubation period of negligible weight gain. The surface MgO regenerates in this regime, while the metal channels slowly advance upward by dissolution of the dense spinel, eventually reaching the MgO and inducing the formation of composite nodules. These consist initially of spinel + metal upon which the conventional A1₂O₃+ metal growth starts after the Mg in the near-surface alloy is depleted to a critical level. SiO₂ surface additions promote composite nucleation by locally hindering surface passivation, acting as an O source for continued spinel growth, and modifying the local chemistry to facilitate the formation of A1₂O₃.