Phase Equilibria in the Pseudoternary System ZrO2−Y2O3−Cr2O3 at 1600°C in Air

The pseudoternary system ZrO₂-Y₂O₃-Cr₂O₃ was studied at 1600°C in air by the quenching method. Only one intermediate compound, YCrO₃, was observed on the Y₂O₃−Cr₂O₃ join. ZrO₂ and Y₂O₃ formed solid solutions with solubility limits of 47 and 38 mol%, respectively. The apex of the compatibility triangle for the cubic ZrO₂, Cr₂O₃, and YCrO₃ three-phase region was located at =17 mol% Y₂O₃ (83 mol% ZrO₂). Below 17 mol% Y₂O₃, ZrO₂ solid solution coexisted with Cr₂O₃. Cr₂O₃ appears to be slightly soluble in ZrO₂(ss).     

Effect of Cr2O3 on Slag Resistance of Al2O3–SiO2 Refractories

Refractory bodies of 65 wt% Al₂O₃ were prepared from a mixture of calcined alumina and raw kaolin with the addition of Cr₂O₃ up to 15 wt%. The Cr₂O₃ addition effectively enhances slag resistance and reduces mullite formation. Petrographic analysis of the refractories after the slag test suggests that Cr₂O₃ increases the viscosity of both the glassy phase in the refractory as well as the slag, thereby retarding slag penetration and reaction at elevated temperature.     

Preparation of Porous Cr3C2 Grains with Cr2O3

Porous Cr₃C₂ grains (∼300 to 500 μm) with ∼10 wt% of Cr₂O₃ were prepared by heating a mixture of MgCr₂O₄ grains and graphite powder at 1450° to 1650°C for 2 h in an Al₂O₃ crucible covered by an Al₂O₃ lid with a hole in the center. The porous Cr₃C₂ grains exhibited a three-dimensional network skeleton structure. The mean open pore diameter and the specific surface area of the porous grains formed at 1600°C for 2 h were ∼3.5 (μm and ∼6.7 m²/g, respectively. The present work investigated the morphology and the formation conditions of the porous Cr₃C₂ grains, and this paper will discuss the formation mechanism of those grains in terms of chemical thermodynamics.     

Phase Equilibration in ZrO2-Y2O3 Alloys by Liquid-Film Migration

The tetragonal ( t ) and cubic ( c ) ZrO₂ solid solutions in two-phase ZrO₂-8 wt% Y₂O₃ ceramics have low and high solute content, respectively. Annealing samples sintered at 1600°C between 700° and 1400°C requires a change in the volume fraction of the coexisting phases, as well as their equilibrium Y₂O₃ content. The enrichment in Y₂O₃ content of the c -ZrO₂ grains is accomplished by liquid-film migration involving the ubiquitous silicate grain-boundary phase, while the volume fraction of t -ZrO₂ increases by the nucleation and growth of cap-shaped t -ZrO₂ lenses. The interfaces between the c -ZrO₂ matrix and the growing t -ZrO₂ lenses are semicoherent.     

Densification of Cr2O3-ZrO2 Ceramics by Sintering

Cr₂O₃ and ZrO₂ were mixed in various ratios and pressed to form compacts, which were then sintered in carbon powder. Compacts with >30 wt% Cr₂O₃ were sintered to densities >98% of true density at 1500°C. This method of sintering in carbon powder can be used to prepare very dense Cr₂O₃-ZrO₂ ceramics at a relatively low temperature, (∼1500°C) without additives.     

The System Al2O3-Cr2O3-Sio2

Liquidus phase equilibrium data are presented for the system Al₂O₃-Cr₂O₃-SiO₂. The liquidus diagram is dominated by a large, high-temperature, two-liquid region overlying the primary phase field of corundum solid solution. Other important features are a narrow field for mullite solid solution, a very small cristobalite field, and a ternary eutectic at 1580°C. The eutectic liquid (6Al₂O₃-ICr₂O₃-93SiO₂) coexists with a mullite solid solution (61Al₂O₃-10Cr₂O₃-29SiO₂), a corundum solid solution (19Al₂O₃-81Cr₂O₃), and cristobalite (SO₂). Diagrams are presented to show courses of fractional crystallization, courses of equilibrium crystallization, and phase relations on isothermal planes at 1800°, 1700°, and 1575°C. Tie lines were sketched to indicate the composition of coexisting mullite and corundum solid solution phases.     

Processing and Properties of in Situ-Reinforced α-SiAlONs Stabilized with Y2O3 and Lu2O3

α-SiAlONs with equiaxed and elongated microstructures stabilized with Y₂O₃ and Lu₂O₃ were produced by hot pressing, and the phase structure and room- and high-temperature mechanical properties were assessed. Additional liquid added to the starting composition in the form of 5 wt% rare-earth monosilicate resulted in the formation of elongated microstructures and improvements in room-temperature strength and fracture toughness. The elongated grain growth was promoted by the additional liquid phase, which crystallized to form a secondary grain-boundary phase thought to be J ' (Re₄Si_{2– x} Al _x O_{7+ x} N_{2– x} ). For the equiaxed and the elongated samples, those sintered with Lu₂O₃ showed higher hardness than the comparable Y₂O₃-sintered materials, and, at elevated temperature, the strength retention of the elongated Lu₂O₃ SiAlON was much higher than that of the Y₂O₃ sample, which was attributed to properties of the residual grain-boundary phase associated with the difference in the cationic radius of the stabilizing cation.     

Formation, Characterization, and Hot Isostatic Pressing of Cr2O3-Doped ZrO2 (0,3 mol% Y2O3) Prepared by Hydrazine Method

In the ZrO₂-Cr₂O₃ system, metastable t -ZrO₂ solid solutions containing up to 11 mol% Cr₂O₃ crystallize at low temperatures from amorphous materials prepared by the hydrazine method. The lattice parameter c decreases linearly from 0.5149 to 0.5077 nm with increased Cr₂O₃ content, whereas the lattice parameter a is a constant value ( a = 0.5077 nm) regardless of the starting composition. At higher temperatures, transformation (decomposition) of the solid solutions proceeds in the following way: t (ss)→ t (ss) + m + Cr₂O₃→ m + Cr₂O₃. Above 11 mol% Cr₂O₃ addition, c-ZrO₂ phases are formed in the presence of Cr₂O₃. The t -ZrO₂ solid solution powders have been characterized for particle size, shape, and surface area. They consist of very fine particles (15–30 nm) showing thin platelike morphology. Dense ZrO₂(3Y)-Cr₂O₃ composite ceramics (∼99.7% of theoretical) with an average grain size of 0.3 μm have been fabricated by hot isostatic pressing for 2 h at 1400°C and 196 MPa. Their fracture toughness increases with increased Cr₂O₃ content. The highest K _Ic value of 9.5 MPa·;m^1/2 is achieved in the composite ceramics containing 10 mol% Cr₂O₃.     

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.     

Thermodynamic Modeling of the Isomorphous Phase Diagrams in the Al2O3–Cr2O3 and V2O3–Cr2O3 Systems

The phase diagrams in the Al₂O₃–Cr₂O₃ and V₂O₃–Cr₂O₃ systems have been assessed by thermodynamic modeling with existing data from the literature. While the regular and subregular solution models were used in the Al₂O₃–Cr₂O₃ system to represent the Gibbs free energies of the liquid and solid phases, respectively, the regular solution model was applied to both phases in the V₂O₃–Cr₂O₃ system. By using the liquidus, solidus, and/or miscibility gap data, the interaction parameters of the liquid and solid phases were optimized through a multiple linear regression method. The phase diagrams calculated from these models are in good agreement with experimental data. Also, the solid miscibility gap and chemical spinodal in the V₂O₃–Cr₂O₃ system were estimated.     

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.     

Oxidation/Vaporization Kinetics of Cr2O3

The weight loss of Cr₂O₃ in oxidizing environments (Po₂= 1 to 10⁻³ atm) at 1200°C was measured. Both hot-pressed and sintered Cr₂O₃ pellets were investigated in O₂/Ar gas mixtures, and the dependence of the weight loss on the O₂ partial pressure, the gas flow rate, and the total pressure was determined independently. The experimentally determined O₂ partial pressure dependence (rate ∝ PO₂^3/4) corresponds to that expected for the reaction Cr₂O₃(s)+3/2O₂⇌2CrO₃(g). The flow rate and total pressure dependencies show that mass transport through a gaseous boundary layer is the rate-controlling step in the oxidation/vaporization of Cr₂O₃. Evaporation coefficients for the loss of CrO₃(g) under the experimental conditions were <0.01.     

Massive Transformation in the Y2O3-Bi2O3 System

Cubic solid solutions in the Y₂O₃-Bi₂O₃ system with ∼25% Y₂O₃ undergo a transformation to a rhombohedral phase when annealed at temperatures ≤ 700°C. This transformation is composition-invariant and is thermally activated, and the product phase can propagate across matrix grain boundaries, indicating that there is no special crystallo-graphic orientation relationship between the product and the parent phases. Based on these observations, it is proposed that cubic → rhombohedral phase transformation in the Y₂O₃-Bi₂O₃ system is a massive transformation. Samples of composition 25% Y₂O₃-75% Bi₂O₃ with and without aliovalent dopants were annealed at temperatures ≤ 700°C for up to 10000 h. ZrO₂ as a dopant suppressed while CaO and SrO as dopants enhanced the kinetics of phase transformation. The rate of cubic/rhombohedra1 interface migration (growth rate or interface velocity) was also similarly affected by the additions of dopants; ZrO₂ suppressed while CaO enhanced the growth rate. Diffusion studies further showed that ZrO₂ suppressed while CaO enhanced cation interdiffusion coefficient. These observations are rationalized on the premise that cation interstitials are more mobile compared to cation vacancies in cubic bismuth oxide. The maximum growth rate measured was ∼10⁻¹⁰ m/s, which is orders of magnitude smaller than typical growth rates measured in metallic alloys. This difference is explained in terms of substantially lower diffusion coefficients in these oxide systems compared to metallic alloys.     

Microstructure and Properties of Cr2O3-Doped Ternary Lead Zirconate Titanate Ceramics

The effects of Cr₂O₃ doping (0 to 0.6 wt%) on the microstructure and electrical properties of ternary Pb_0.97Sr_0.03- [Zr_0.458Ti_0.452 (Mn_1/3Nb_2/3) _0.09]O₃ piezoelectric ceramics have been studied. Abnormal grain growth (grain sizes of 2.8 to 28.5 μm) and densification (porosities of 2% to 12%) are found, and the tetragonality c/a nearly keeps constant wherease c and a slightly decrease in the doping range. The results for the temperature dependence of electric conductivity and the dielectric constant show that electric conductivity increase continuously over 0.1 wt% content, activation energies are all close to 0.92 eV in the paraelectric state, and the Curie temperature T_c decreases with increasing doping content. Minor additives of ≤0.1 wt% improve the dielectric and piezoelectric properties, but with more additives of ≥0.4 wt% electrical properties appear to deteriorate. Thus, these phenomena can be ascribed mainly to anomalous developed microstructure.     

Effect of Y2O3 Additions on the Densification of an Al2O3–TiC Composite

The densification of Al₂O₃–30TiC (in weight percent) composite is investigated as a function of Y₂O₃ additions. It is observed that very small amounts of Y₂O₃ are effective in aiding the densification. The density was observed to pass through a maximum at 0.35 wt% of Y₂O₃. The gas-generating reaction of Al₂O₃ with TiC is likely to be suppressed by the addition of Y₂O₃.     

High-Temperature Strength and Fracture Toughness of Y2O3-Partially-Stabilized ZrO2/Al2O3 Composites

The temperature dependence of bending strength, fracture toughness, and Young's modulus of composite materials fabricated in the ZrO₂ (Y₂O₃)-Al₂O₃ system were examined. The addition of A1203 enhanced the high-temperature strength. Isostatically hot-pressed, 60 wt% ZrO₂ (2 mol% Y₂O₃)/40 wt% Al₂O₃ exhibited an extremely high strength, 1000 MPa, at 1000°C.     

Creep Deformation in MgO-Saturated Large-Grain-Size Al2O3

Samples of 65 μm-grain-size AL₂O₃ containing 0.05 wt% Mg and 40 μm-grain-size AL₂O₃ containing 0.13 wt% Mg, both MgO-saturated, undergo compressive deformation in the range 1580° to 1800°C with results interpreted as diffusional creep rate-limited by grain-boundary diffusion with the Coble boundary-diffusion model giving
A lower deformation rate for an unsaturated composition indicates that MgO additions enhance grain-boundary diffusion.     

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.     

Effect of Crystallizing the Grain-Boundary Glass Phase on the High-Temperature Strength of Hot-Pressed Si3N4 Containing Y2O3

The effects of fabrication variables on the high-temperature strength of hot-pressed Si₃N₄ containing 5 wt% Y₂O₃ were studied. Materials containing a crystalline grain-boundary phase, formed as a consequence of a high-temperature presintering heat treatment and identified as Si₃N₄·Y₂O₃, had high-temperature strengths significantly superior to those observed for materials containing a glass phase.     

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).