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.     

Electrical Resistance of Some Refractory Oxides and Their Mixtures in the Temperature Range 600° to 1500°C.

Measurements of electrical resistance in the composition systems Al₂O₃–SiO₂, SiO₂–TiO₂, Al₂O₃–Cr₂O₃, and MgO–NiO were made using, in general, dry-pressed disks about 3 cm. in diameter and 0.4 cm. thick and fired to 1500°C. In the Al₂O₃–SiO₂ series minimum resistance was shown by the samples containing 50% SiO₂, 50% Al₂O₃. The resistance of Al₂O₃ was increased by the addition of small amounts of Cr₂O₃. The same effect was observed in the higher temperature range with small additions of NiO to MgO. In other instances the addition of the relatively inert SiO₂, Al₂O₃, and MgO to the semiconductors TiO₂, Cr₂O₃, and NiO resulted in a dilution effect. The resistance of Cr₂O₃ was decreased by the addition of a slight amount of MgO.     

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.     

Electronically Conducting Doped Chromium Oxides

Samples of polycrystalline chromium oxide doped with Li₂O, MgO, NiO, and Ta₂O₅ were prepared by hot-pressing to test for suitability as current collector materials of the sulfur electrode in the sodium-sulfur cell. Density, grain size, dopant uniformity, and resistivity (to 350°C) were measured. X-ray photoelectron spectroscopy (XPS) tends to support the established model of Ni²⁺ on Cr³⁺ sites to explain the compositional dependence of electrical resistivity after air anneal. A higher level of Cr₂O₃ powder purity was required to obtain the low resistivities with the Li₂O dopant than with the MgO or NiO dopant. An observed increase in the bulk resistivity during several months of electrochemical cycling in sodium polysulfidelsulfur melts is attributed to loss of electronic carriers caused by equilibration of those carriers with the low oxygen partial pressure of the melt. Li₂O- and (to a lesser extent) MgO-doped Cr₂O₃ appear to be suitable as container coating materials.     

On the Mechanism of Densification of Porous Alumina by Infiltration with an Aqueous Solution of Chromium Oxide

Densification of porous alumina by CrO₃ infiltration was investigated on model composites containing a sapphire interface. Gas–solid deposition of Cr₂O₃ from CrO₃ source on sapphire and glass at 600°C was also investigated. The deposited Cr₂O₃ layers were characterized using X-ray diffraction and transmission electron microscopy. It was found that similar to gas-phase deposition on sapphire, in CrO₃-infiltrated porous alumina Cr₂O₃ grows epitaxially on adjacent alumina surfaces, suggesting a gas–solid condensation mechanism. The deposition rate and morphology of the Cr₂O₃ film deposited on alumina depend strongly on the crystallographic orientation.     

Zirconia-Based Metastable Solid Solutions through Self-Propagating High-Temperature Synthesis: Synthesis, Characterization, and Mechanistic Investigations

Cubic Zr_{1− x} Me _x O _y (Me = Fe, Co, Ni, Cu) metastable solid solutions with metal content significantly higher than equilibrium levels have been synthesized by the self-propagating high-temperature synthesis method based on a thermite reaction between metallic zirconium and the transition-metal oxides CoO, Fe₂O₃, CuO, and NiO. Through in situ XRD analysis, it was determined that when heated to 1100°C, the cubic solid solution transformed to the tetragonal phase with the concomitant formation of iron oxide. When cooled to lower temperatures, the tetragonal phase transformed to the monoclinic phase at or below 500°C. Results of auxiliary experiments strongly suggest that the formation of the solid solution takes place behind the combustion front by a reaction between zirconia and the metal.     

Phase Relations in the System Fe2O3–Fe3O4–YFeO3 in Air

Measurements were made of temperature and ternary composition for coexisting liquid and crystalline phases on the air isobar in the system Fe₂O₃-Fe₃O₄-YFeO₃ with particular regard to the stability range and compositional limits of yttrium iron garnet. Phase equilibrium relations were determined by conventional quenching techniques combined with measurements of loss in weight at the reaction temperature to locate true ternary compositions. The intersection of the air isobar with the ternary univariant boundary curve for coexisting magnetite, garnet, and liquid phases results in a eutectic-type situation at the composition Y_0.27Fe_1.73 O_2.87 and 1469°± 2°C. A similar intersection of the isobar with the boundary curve for coexisting garnet, orthoferrite, and liquid phases gives rise to a peritectic-type reaction at 1555° 3°C. and Y_0.44Fe_1.56 O_2.89 The yttrium iron garnet crystallizing from liquids within these temperature and composition limits contains up to 0.5 mole % iron oxide in excess of the stoichiometric formula in terms of the starting composition 37.5 mole % Y₂O₃, 62.5 mole % Fe₂O₃. At 1470° C. the garnet phase in equilibrium with oxide liquid contains 2 mole % FeO in solid solution. The small solubility of excess of iron oxide and partial reduction of the garnet phase in air are unavoidable during equilibrium growth from the melt.     

Hot Isostatic Pressing and Characterization of Sol-Gel-Derived Chromium(III) Oxide

Chromium (III) oxide (Cr₂O₃) crystallizes at low temperatures from an amorphous material prepared by adding hydrazine monohydrate to an aqueous solution of Cr(NO₃)₃-9H₂O. Individual particles of Cr₂O₃ tend toward a hexagonal morphology above 800°C. Well-densified Cr₂O₃ pellets (98.8% of theoretical density) have been fabricated by hot isostatic pressing for 2 h at 1100°C and 196 MPa. Their fracture toughness is 4.4 MPa.m^1/2. The sample annealed in air for 12 h at 1300°C exhibits a high electrical conductivity of 3.6 Ω^-1.m^-1at 700°C.     

Effect of Impurities on the Strength of Polycrystalline Magnesia and Alumina

The bending strength of pure and alloyed magnesium oxide and aluminum oxide was studied. The magnesium oxide was doped with nickel oxide and cobalt oxide, both of which form a complete series of solid solutions with magnesia. The aluminum oxide was doped with ferric oxide which has limited solid solubility in aluminum oxide, with titanium dioxide which also has limited solid solubility in alumina, and with chromic oxide which forms a complete series of solid solutions with alumina. The modulus of rupture was corrected to a standard grain size and porosity to determine the intrinsic effects of the impurities on the modulus of rupture. The results indicate that the formation of NiO-MgO and Coo-MgO solid solutions increases the strength of MgO. The formation of Cr₂O₃-Al₂O₃, Fe₂O₃-Al₂O₃, and TiO₂-Al₂O₃ solid solutions, however, has no intrinsic effect on the strength of sintered alumina.     

Decomposition of Higher Oxides of Chromium Under Various Pressures of Oxygen

The formation of Cr₃O₈, Cr₂O₅, CrO₂, and Cr₂O₃ by the decomposition of anhydrous CrO₃ under a high oxygen pressure and their physical and chemical properties are presented. The crystal structure of CrO₂ is a rutile type and its lattice constants do not vary with the ratio O/Cr. The specific gravity of CrO₂ decreases with an increase in the ratio O/Cr. This is explained by assuming that the crystal structure of CrO₂ contains an excess of Cr ions at interstitial sites. This crystal model is supported by the low electrical resistivity of CrO₂. An electron microscopic examination showed an abnormal grain growth of CrO₂ during the decomposition of Cr₂O₅ to CrO₂. The activation energies of the decomposition of Cr₂O₅ and CrO₂ to Cr₂O₃ in air are 61.7 and 48.2 kcal. per mole, respectively. The pressure-temperature diagram of various chromium oxides has been determined. From the pressure-temperature curve of CrO₂-Cr₂O₃ the heat of decomposition is found to be 26.3 kcal. per mole.     

Oxidation Resistance Coating of LSM and LSCF on SOFC Metallic Interconnects by the Aerosol Deposition Process

Conductive La_0.8Sr_0.2MnO₃ (LSM) and La_0.6Sr_0.4Co_0.2Fe_0.8O₃ (LSCF) layers with a thickness of ∼10 μm were deposited on ferritic stainless steel (SS) by the aerosol deposition method, for use as an oxidation resistance-coating layer in the metallic interconnector of a solid oxide fuel cell. The coated layers were fairly dense without pores or cracks, and maintained good adhesion even after oxidation at 800°C for 100 h. The surface of the bare SS after annealing at 800°C for 100 h was covered with Cr₂O₃ and Fe₃O₄ oxide scales, and the electrical conductivity was sharply decreased. However, the LSM- and LSCF-coated SSs showed a surface microstructure with almost no oxidation and maintained good electrical conductivity after annealing at 800°C for 100 h. The area-specific resistance (ASR) of LSM- and LSCF-coated alloys after 100 h of oxidation at 800°C was 20.6 and 11.7 mΩ·cm², respectively.     

Grain Growth and Phase Transformation of Titanium Oxide During Calcination

Additions of NiO, CoO, MnO₂, Fe₂O₃, and CuO promote the anatase-rutile transformation and grain growth of TiO₂. Additions of Na₂O and WO₃ retard the transformation and have no effect on the grain growth. The addition of MoO₃ strongly promotes the grain growth but has only a slight effect on the phase transformation. Both the grain growth and transformation are promoted slightly by Cr₂O₃. The transformation is significantly affected by the method of preparing TiO₂. The grain size of TiO₂ heated in H₂ exceeds that of TiO₂ heated in O₂, in air, in argon, and in vacuum. The rate of transformation decreases with an increase in the partial pressure of oxygen of the atmosphere.     

Piezoelectric Properties of Pb(Mg1/3Nb2/3)O3 PbTiO3 -PbZrO3 Ceramics Modified with Certain Additives

Effects of additives on the piezoelectric properties of Pb(Mg_1/3Nb_2/3)O₃-PbTiO₃-PbZrO₃ ceramics in a perovskite-type structure are described. The tetragonality of Pb(Mg_1/3Nb_2/3)_0.375-Ti_0.375Zr_0.25O₃ ceramics increased with the addition of NiO, Cr₂O₃, or Fe₂O₃ but decreased with the addition of MnO₂ or CoO. The dielectric and piezoelectric properties of the base composition were improved markedly through selection of additives in proper amounts. Addition of NiO yielded a high dielectric constant and planar coupling coefficient for compositions at the morphotropic transition boundary. High mechanical Q -factors and low electrical dissipation factors were obtained by addition of MnO₂. Addition of both NiO and MnO₂ produced a mechanical Q -factor of 2051 and a planar coupling coefficient of 0.553. The resonant frequency of Pb(Mg_1/2Nb_2/3)_0.4375Ti_0.4375 zr_0.125O₃ containing MnO₂ had very low temperature and time dependence. The microstructure indicated that ceramics with a high mechanical Q -factor had a fine, uniform grain structure. Addition of Cr₂O₃ retarded grain growth and addition of MnO₂, NiO, CoO, or Fe₂O₃ promoted grain growth in the ternary system.     

Synthesis Mechanism of an Iron–Chromium Ceramic Pigment

The synthesis of a black Fe₂O₃–Cr₂O₃ pigment has been studied to better understand the synthesis mechanism. A mechanism is proposed according to which, under the tested operating conditions, one of the starting constituents (Cr₂O₃) is transferred, largely in a gas phase (after subliming), to the particle surface of the other constituent (Fe₂O₃), at which it is deposited or chemisorbed, forming a layer from which it diffuses inward into the Fe₂O₃ particles. The process stops when the composition of the resulting solid solution has become uniform.     

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.     

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.     

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.     

Eletrical Properties of Glasses in the System PbO-Al2O3-B2O3-SiO2

Glasses in the system Pb0–Al₂O₃-B₂O₃-SiO₂ are chemically stable over a wide composition range and have very desirable electrical characteristics such as high electrical resistivities and activation energies for conduction. Variations in these electrical properties were studied as a function of composition changes within the system, the object being to identify the role of the constituent oxides in achieving the highest activation energy and resistivity values consistent with moderate preparation temperatures. Measurements were made in the temperature range 25° to 400°C on carefully prepared glass disks in which the individual oxide components or different oxide ratios such as PbO/SiO₂, Al₂O₃/SiO₂, and B_sO₃/SiO₁ were systematically varied. The activation energy and resistivity values obtained ranged from 1.2 to 1.6 ev and 10° to 10¹⁴ ohm-cm, with dielectric constants ranging from 9 to 19 and densities from 4.30 to 4.50 g/cmY. Indications were that, for the composition range studied, the behavior manifested was basically that of the binary PbO-SO₂ glass with additions of Al₂O₃ or B₂O₃, even in small concentrations, sharply increasing the activation energy for conduction while lowering the density.     

Vaporization and Corrosion of Refractories in the Presence of Pressurized Pulverized Coal Combustion Slag

The corrosion and vaporization behavior of eight commercial refractories containing Cr₂O₃ were investigated under simulated pressurized pulverized coal combustion (PPCC) conditions at 1723 K in the presence of liquid slag. The corrosion resistance of the refractory materials decreased with increased content of free Cr₂O₃, because bursting (reaction of Fe₃O₄ with Cr₂O₃) occurred. Refractories containing MgCr₂O₄ with dissolved Al₂O₃ showed the highest corrosion resistance. Thermodynamic calculations showed that CrO₃ and CrO₂(OH)₂ were the most volatile species in air and in PPCC flue gas, whereas additions of A₂O (A = Na and K, minor slag component) increased the chromium vaporization significantly because of A₂CrO₄ formation. Knudsen effusion mass spectrometry measurements showed that the chromium vaporization of refractories was directly correlated to the Cr₂O₃ content of the material. In contrast, refractories containing MgCr₂O₄ showed a significant decrease in chromium vaporization.     

Lead Zirconate-Lead Titanate Piezoelectric Ceramics with Iron Oxide Additions

An investigation of the effects of small additions of Fe₂O₃ to lead zirconate-lead titanate ceramics with compositions between 45 and 60 mol% lead zirconate has shown that, with 2 moles of PbO to balance each mole of Fe₂O₃, the solubility of Fe₂O₃ was about 0.8 wt% in compositions near the tetragonal-rhombohedra1 boundary. The dielectric constant and dissipation factor for both rhombohedra1 and tetragonal materials were decreased by addition of iron oxide, whereas the mechanical quality factor and frequency constant were increased. The dependence of the electromechanical properties on grain size was qualitatively similar for both undoped and iron oxide-doped materials; the presence of iron oxide inhibited grain growth and lowered the limiting grain size below which the electromechanical properties change rapidly with grain size.