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.     

Mineralizing Magnesium Aluminate Spinel Formation With B2O3

B₂O₃ mineralizes spinel formation from stoichiometric (1:1 mole ratio) calcined magnesia and alumina. After 3 h at 1100°C, X-ray diffraction (XRD) shows the mineralization effect of B₂O₃ is limited to 1.5 wt% additions with higher B₂O₃ contents leading to Mg₃B₂O₆ formation and reduced spinel content. Boron nuclear magnetic resonance, electron probe microanalysis (EPMA), scanning electron microscopy, transmission electron microscopy (TEM) and XRD reveal formation of a boron-containing liquid. Energy dispersive spectroscopy in the TEM and EPMA of the glassy phases formed from solidification of the liquid reveal that initially it is Mg borate, later becoming a magnesia-modified boroaluminate, composition suggesting dissolution–precipitation as opposed to templated growth as the mechanism of this liquid phase mediated mineralization.     

Characterization and Sintering Behavior of Alkoxide-Derived Aluminosilicate Powders

Submicrometer SiO₂-Al₂O₃ powders with compositions of 46.5 to 76.6 wt% Al₂O₃ were prepared by hydrolysis of mixed alkoxides. Phase change, mullite composition, and particle size of powders with heating were analyzed by DTA, XRD, IR, BET, and TEM. As-produced amorphous powders partially transformed to mullite and Al-Si spinel at around 980°C. The compositions of mullite produced at 1400° and 1550°C were richer in Al₂O₃ than the compositions of stable mullite solid solutions predicted from the phase diagram of the SiO₂-Al₂O₃ system. Particle size decreased with increasing Al₂O₃ content. The sintered densities depended upon the amount of SiO₂-rich glassy phase formed during sintering and the green density expressed as a function of particle size.     

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.     

Cation Tracer Diffusion in Cr2O3 and Cr2O3-0.09 wt% Y2O3

The cation diffusivities in the lattice and along dislocations and grain boundaries have been measured on sintered polycrysals of Cr₂O₃; and Cr₂Cr₂O₃-0.09 wt% Y₂O₃ at1100°C and at the pO₂ corresponding to that of Cr/Cr₂O₃ equilibrium at that temperature. Results for lattice and dislocation diffusivities in pure Cr₂O₃ are in good agreement with previous work. The present results indicate that yttrium additions have negligible effect on lattice and dislocation diffusion. However, grain-boundary diffusion in pure Cr₂O₃ is significantly slower than grain-boundary diffusion in Cr₂O₃-0.09 wt% Y₂O₃. The results are discussed in terms of their implications for the reactive-element effect in high-temperature oxidation of chromium-containing alloys.     

Densification of Calcia-Stabilized Zirconia with Borates

Densification studies of submicrometer ZrO₂ powders stabilized with 6.5 wt% CaO (CSZ) showed borate additions (1 to 10 wt%) to be effective sintering aids. Estimated densities >99% of theoretical were obtained on sintering at 1200°Cfor 4 h with 2 wt% B₂O₃ or 5 wt% CaO·2B₂O₃ additions to the CSZ powders. Average grain sizes obtained were typically <1 μm. Partial development of a monoclinic ZrO₂ phase was observed in the sintered samples. The amount of this phase varied from ∼7 to 75 wt% and was approximately linearly dependent on the additive concentration. The effect was most marked for the B₂O₃ additions. Development of the monoclinic phase was attributed to progressive leaching of Ca from the CSZ phase by B₂O₃, in effect partially destabilizing the ZrO₂.     

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.     

Thermal Expansion Behavior of Boron-Doped Cordierite Glass-Ceramics

The linear thermal expansion coefficients of cordierite glass-ceramics that have been doped with a fixed amount of P₂O₅ and 1, 2, and 3 wt% of B₂O₃ show negative expansion in the temperature range of 100°-300°C. The expansion of the undoped cordierite sample is positive. A relative decrease in the degree of negative expansion is observed as the B₂O₃ concentration increases. These negative expansion coefficients are similar to those of cordierite glass-ceramics that have been doped with the potassium cation.     

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.     

Mineralizing Effect of Li2B4O7 and Na2B4O7 on Magnesium Aluminate Spinel Formation

Lithium borate (Li₂B₄O₇) and sodium borate (Na₂B₄O₇) mineralize spinel formation from stoichiometric MgO and Al₂O₃ between 1000° and 1100°C. Mineralization with both compounds is shown to be mediated by B-containing liquids which form glass on cooling. However, the liquid compositions depend on the type of mineralizer and temperature, suggesting that templated grain growth or dissolution–precipitation mechanisms are operating, one dominating over the other under certain conditions. Na₂B₄O₇-mineralized compositions show predominantly templated grain growth at 1000°C, which changes to dissolution–precipitation at 1100°C, whereas Li₂B₄O₇-mineralized compositions show dissolution–precipitation from 1000°C. Li₂B₄O₇ is a stronger mineralizer as spinel formation is complete with 3 wt% Li₂B₄O₇ at 1000°C and with ≥1.5 wt% addition at 1100°C, whereas Na₂B₄O₇-mineralized compositions are found to retain some unreacted corundum even at 1100°C.     

Structure of B2O3 and Binary Aluminoborate Melts at 1600°C

Viscosity and density data were obtained up to 1700°C for a series of binary aluminoborate melts that contained as much as 15 mole% (∼21 wt%) Al₂O₃ and up to 1620°C for pure molten B₂O₃. Large expansion coefficient decreases and a slight activation energy increase for B₂O₃ above 1400°C suggested a tightening of its structure. The addition of Al₂O₃ reduced viscosity and increased activation energy. The decreased compositional dependence of molar volume (compared to SiO₂ additions) and the increased expansion coefficients accompanying Al₂O₃ additions suggested a loosening of the O—B—O structure at 1600°C. Molar volume deviations from ideality were similar to but smaller than those for SiO₂ and GeO₂ additions at 1300°C. Microclustering of aluminum-bearing polyhedra appeared to occur at slightly higher boron atom contents than with SiO₂ and GeO₂ additions.     

Mullite Transformation Kinetics in P2O5-, TiO2-, and B2O3-Doped Aluminosilicate Gels

Mullite transformation kinetics of sol-gel-derived diphasic mullite gels doped with P₂O₅, TiO₂, and B₂O₃ were studied using quantitative X-ray diffraction and differential thermal analysis (DTA). The mullite transformation temperature initially increased with P₂O₅ doping because of phase separation and formation of α-alumina and cristobalite. In TiO₂-doped samples, the mullite transformation temperature decreased with TiO₂ doping, and the transformation rate increased with decreasing TiO₂ particle size. Kinetic studies showed that titania reduced the activation energy for both nucleation and growth relative to pure diphasic mullite gels by lowering the glass viscosity and/or enhancing the solid-state mass transport through lattice defects. B₂O₃ doping decreased the mullite transformation temperature and lowered the activation energy for both nucleation and growth but especially affected the mullite nucleation process, as indicated by the much smaller grain size.     

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.     

Crystallization Kinetics of Mullite in Alumina–Silica Glass Fibers

The mechanism of mullite crystallization in Al₂O₃-SiO₂ glass fibers with compositions of 49 and 69 mass% Al₂O₃ was investigated using isothermal methods. The activation energies ( E _a) of the nucleation and nucleation-growth of mullite were obtained from the temperature dependence of the incubation times and rate constants of mullite formation, respectively. The formation of mullite in both glasses occurred in two stages. The amount of mullite increased very steeply in the first stage within very short firing time, whereas the increase was more gradual in the second stage. The E _a values for nucleation and nucleation-growth in the first stage were 864 and 1288 kJ/mol in the 49-mass%-Al₂O₃ glass fiber and 980 and 1138 kJ/mol in the 69-mass%-Al₂O₃ glass fiber, respectively. Arrhenius plots of the second-stage data showed that the slope ratios were different at temperatures below and above 1200°C. The E _a values at <1200°C were 1195 and 1099 kJ/mol in the 49- and 69-mass%-Al₂O₃ glass fibers, whereas those at >1200°C were 696 and 645 kJ/mol, respectively. Observation of the microstructures and crystallite-size data indicates that the crystallization of mullite proceeds via three different mechanisms: nucleation, nucleation–growth, and coalescence of mullite grains.     

Effect of Metal Oxide Additions on the High-Temperature Electrical Conductivity of Alumina

Several metal oxide additions were made to typical 99 and 96% alumina compositions to study their effect on the electrical conductivity of alumina from 500° to 1400°C. The metal oxide additions investigated were CO₂O₃, Cr₂O₃, CuO, Fe₂O₃, MnO₂, NiO, and TiO₂. Using a guarded two-probe technique, dc resistivities were measured on nonporous ceramic specimens. Additions of 0.5 to 2 mole % Co₂O₃, 2 mole % CuO, 1 mole % Fe₂O₃, or 2 mole % NiO to either a 96 or a 99% alumina composition increased the electrical resistivity. The addition of 1 mole % Cr₂O₃ to either a 96 or a 99% alumina showed practically no change in the resistivity. All changes in resistivity seemed to be structure dependent.     

Reinvestigation of Al–Si Spinel Phase in Diphasic Al2O3–SiO2 Gel

Mechanical mixture of γ-Al₂O₃ and amorphous SiO₂, and diphasic Al₂O₃/SiO₂ gels of three different compositions were synthesized. They were subjected to heat treatment to various temperatures in the range 900°–1600°C. Qualitative X-ray diffraction data show that these diphasic gels do not crystallize to a combined mixture of θ-Al₂O₃ and α-Al₂O₃ polymorphs at the intermediate stage, prior to mullite formation. Estimated mullite formation data show that the course of its formation from mixed oxides was different from that of diphasic gels. Results are compared with previous findings and the concept of Al–Si spinel formation in the phase transformation of stoichiometric diphasic gel system is substantiated.     

Effect of CaO Content on the Hot Strength of Alumina-Spinel Castables in the Temperature Range of 1000° to 1500°C

The modulus of rupture of Al₂O₃-spinel castables containing 20 wt% Al₂O₃-rich MgO-Al₂O₃ spinel and 1.36-2.04 wt% CaO generally increases with an increase in both CaO content and temperature from 1300° to 1500°C, but it remains virtually constant from 1000° to 1300°C. Microscopic observation of the castable fired at 1500°C for 3 h reveals the growth of some CA₆ crystals out of the Al₂O₃-rich spinel grains in the bonding matrix of the castable. The bond linkage between the CA₆ and spinel grains in the matrix is believed to cause both the CaO content and temperature dependence of the hot strength of the Al₂O₃-spinel castables as well as the hot strength enhancement of high-Al₂O₃ castables with addition of Al₂O₃-rich spinel.     

Low-Temperature Sintering and Microwave Dielectric Properties of Zinc Metatitanate-Rutile Mixtures Using Boron

Mixtures of zinc metatitanate and rutile (ZnTiO₃+ x TiO₂, where x = 0-0.5) have been prepared via the conventional mixed-oxide method. Centrifugal planetary milling with zirconia beads 1 mm in diameter produced very fine powders (mean particle size of 0.2 µm), which allowed the synthesis of ZnTiO₃ and sintering at temperatures <945°C, which is the decomposition temperature of ZnTiO₃. Sintering of the mixtures was enhanced further by the addition of B₂O₃. Densities of >94% of the theoretical density have been attained for the specimens that were sintered at 875°C for 4 h with B₂O₃ additions of <1 wt%. Microwave dielectric properties of the aforementioned compositions were as follows: dielectric constant of 29-31, normalized quality factor of 56000-69000 GHz, and a temperature coefficient of resonance frequency between -10 and +10 ppm/°C. Sintering was enhanced by the formation of a ZnO-B₂O₃ liquid phase, which affected the microwave properties, because of variation in the phase composition.     

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.     

Phase Transformation of Diphasic Aluminosilicate Gels

Aluminosilicate gels with compositions Al₂O₂/SiO₂ and 2 were prepared by gelling a mixture of colloidal pseudo-boehmite and a silica sol prepared from acid-hydrolyzed Si(OC₂H₅)₄. Upon heating the pseudo-boehmite transforms to γ-Al₂O₃ around 400°C, then to δ-Al₂O₃ at 1050°C, and at 1200°C reacts with amorphous SiO₂ to form mullite. Some twinned θ-Al₂O₃ forms before mullite. Nonstoichiometric specimens have a similar transformation sequence, but form mullite grains with inclusions of either Al₂O₃ or cristobalite, often associated with dislocation networks or micropores. Mullite grains are formed by nucleation and growth and have equiaxed shape.