Preliminary Observations on Phase Relations in the "V2O3–FeO" and V2O3–TiO2 Systems from 1400°C to 1600°C in Reducing Atmospheres

Phase relations within the "V₂O₃–FeO" and V₂O₃–TiO₂ oxide systems were determined using the quench technique. Experimental conditions were as follows: partial oxygen pressures of 3.02 × 10⁻¹⁰, 2.99 × 10⁻⁹, and 2.31 × 10⁻⁸ atm at 1400°, 1500°, and 1600°C, respectively. Analysis techniques that were used to determine the phase relations within the reacted samples included X-ray diffractometry, electron probe microanalysis (energy-dispersive spectroscopy and wavelength-dispersive spectroscopy), and optical microscopy. The solid-solution phases M₂O₃, M₃O₅, and higher Magneli phases (M _n O_{2 n −1}, where M = V, Ti) were identified in the V₂O₃–TiO₂ system. In the "V₂O₃–FeO" system, the solid-solution phases M₂O₃ and M₃O₄ (where M = V, Ti), as well as liquid, were identified.     

High-Temperature Phase Relations in Selected MnO-CrOx-Containing Systems

Phase relations in the pseudobinary MnO-CrO _x system were studied by the reaction of the individual oxides in the temperature range 1400°-1750°C under a reducing atmosphere with a CO:CO₂ volume ratio of 4.8, yielding oxygen partial pressures in the range 10^-9.98-10^-6.97 atm. In the pseudoternary MnO-CrO _x -containing systems that constitute the pseudoquaternary MnO-CrO _x -CaO-SiO₂ system, phase relations were determined only at 1500°C under an oxygen partial pressure of 10^−8.99 atm. Characteristic of these MnO-CrO _x -containing systems was the dominance of the (Mn,Cr)₃O₄ spinel phase.     

Structural Behavior of Oxygen Permeable SrFe0.2Co0.8Ox Ceramic Membranes with and Without pO2 Gradients

The oxygen partial pressure ( p O₂)-dependent structural behaviors of two dense tubular ceramic membranes in composition SrFe_0.2Co_0.8O _x with cubic perovskite structure have been investigated by high-temperature neutron powder diffraction: one in "static" mode and one in simulated-operation mode in which one side of the membrane was exposed to air and the other side to reducing gases with variable p O₂ levels. Rietveld analysis on data collected for the membrane without p O₂ gradients showed that the perovskite is stable in p O₂ down to ∼10⁻¹² atm, and at ∼10⁻¹⁴ atm it starts to decompose into a three-phase mixture containing layered intergrowth Ruddlesden–Popper phases Sr _{n +1}(Fe,Co) _n O _x with n =2 and 3, along with CoO with rocksalt structure. Similar phase evolution was observed when insufficient air flowed on the air side of the membrane exposed to a p O₂ gradient. The data support a nonlinear model of oxygen content in perovskite across the membrane thickness, corresponding to a p O₂ profile that is shallow inside and steep near the reducing side surface. Gas compositions measured with mass spectrometry indicated that oxygen is permeated from the air side to the reducing side of the membrane. The oxygen permeation fluxes at 900°C were estimated to be 0.4–0.9 sccm/cm² for the ∼1 mm thick membrane containing perovskite, depending upon p O₂ gradient.     

Electrical Properties and Defect Structure of a (0.13YO1.5·0.87ThO2) Electrolyte

High-density sintered disks of the composition 0.13YO_1.5·0.87ThO₂ are shown to be mixed conductors at high oxygen pressures (>10⁻⁶ atm) by electrical conductivity and electrochemical cell measurements. The ac and dc conductivity measurements were made between 900° and 1600°C over a wide range of oxygen partial pressures. A blocking-electrode polarization technique for determining transference numbers was not applicable at high oxygen pressures but appeared to work at the lower pressures, indicating a transition to n -type behavior. The electrochemical cell measurements show essentially completely ionic behavior at low oxygen pressure but indicate at least 0.1% electronic contribution at 10⁻¹³ atm at 1000°C. The lower oxygen pressure limit for completely ionic behavior has not been determined but extends below the equilibrium pressures of an Mn-MnO₂, Cr-Cr₂O₃ electrochemical cell at 1000°C.     

Determination of CaO–SiO2–MgO–Al2O3–CrOx Liquidus

In this work, the liquidus of synthetic CaO–SiO₂–MgO–Al₂O₃–CrO _x slags is evaluated in the industrially relevant compositional domain. Equilibrium experiments are carried out at 1500°C and partial oxygen pressure ( p _O2) 10^−11.04 atm, and at 1600°C and p _O2=10^−10.16 and 10^−9.36 atm. The studied basicities (CaO/SiO₂) are 1.2 and 0.5. Al₂O₃ levels range from 0 to 30 wt%. Oversaturated liquid is sampled and phase relations are measured with quantitative electron probe microanalysis–wavelength dispersive spectroscopy (EPMA–WDS). The results are compared with the commercially available FactSage thermodynamic databases. Qualitative agreement is always obtained. Also a good quantitative agreement is found at the higher basicity, especially for the spinel liquidus. A minor but systematic deviation can be observed for the eskolaite liquidus. At the lower basicity, the calculated phase diagram deviates strongly from the experimental results, probably due to missing ternary interactions in the database.     

Electrical Properties and Defect Structure of Y2O3

Guarded measurements of the electrical conductivity of high-purity, polycrystalline Y₂O₃ in thermodynamic equilibrium with the gas phase were made under controlled temperature and oxygen partial pressure conditions. Data are presented as isobars from 1200° to 1600°C, and as isotherms from oxygen partial pressures of 10⁻¹ to 10⁻¹⁷ atm. The ionic contribution to the total conductivity, determined by the blocking electrode polarization technique, was less than 1% over the entire range of temperatures and oxygen partial pressures studied. Yttria is shown to be an amphoteric semiconductor with the region of predominant hole conduction shifting to higher pressures at higher temperatures. In the region of p -type conduction, the conductivity is represented by the expression σ= 1.3 × 10³ p O₂^3/16 exp (-1.94/kT). The observed pressure dependence is attributed to the predominance of fully ionized yttrium vacancies. Yttria is shown to be a mixed conductor below 900°C.     

Control of the Thermal Expansion of Strontium-Doped Lanthanum Chromite Perovskites by B-site Doping for High-Temperature Solid Oxide Fuel Cell Separators

The thermal expansion of La_0.9Sr_0.1Cr_{1- x} M _x O₃(M = Mg, Al, Ti, Mn, Fe, Co, Ni; 0 ≤ x ≤ 0.1) perovskites has been studied in oxidizing and reducing atmospheres in the temperature range from 50° to 1000°C. Cobalt doping of La_0.9Sr_0.1CrO₃was an effective way of increasing the average linear thermal expansion coefficient (TEC), whereas titanium doping showed a negative effect. No effect on the TECs was observed for the B-site dopants in perovskites with the remaining dopants. Linear thermal expansion behavior was observed in the La_0.9Sr_0.1Cr_{1- x} M _x O₃ perovskites with doping of ≥1 mol% aluminum or 10 mol% cobalt. TECs of La_0.9Sr_0.1Cr_0.96Co_0.02Al_0.02O₃ were 10.5 × 10⁻⁶/°C in air, 10.7 × 10⁻⁶/°C under He–H₂ atmosphere (oxygen partial pressure of 4 × 10⁻¹⁵ atm at 1000°C), and 11.8 × 10⁻⁶/°C in H₂ atmosphere.     

Thermodynamic Evaluation of the Bi-Cu-O System

A thermodynamic evaluation of the ternary Bi-Cu-O system has been made and a set of parameters consistently describing the system at 1 bar total pressure has been determined. The ternary system contains a liquid phase which is continuous at high temperature and separates into a metal liquid and an oxide liquid at low temperature and the ternary compound Bi₂CuO₄ in addition to the binary compounds. Concentration/temperature diagrams are given for the Bi₂O₃-CuO _x system at oxygen partial pressures of 1, 0.21 and 10⁻⁵ bar, as well as a potential diagram. Selected isothermal sections at 1100 and 1473 K are also presented. Available experimental data are rather limited and often show a relatively large scatter, but nevertheless make it possible to determine a sufficient set of thermodynamic parameters.     

Phase Equilibria in the Spinel Region of the System FeO-Fe2O3-Al2O3

Phase relations in the spinel region of the system FeO-Fe₂O₃-Al₂O₃ were determined in CO₂ at 1300°, 1400°, and 15000°C and for partial oxygen pressures of 4 × 10⁻⁷ and 7 × 10⁻¹⁰ atmospheres at 15OO°C. The spinel field extends continuously from Fe₃O_4-x to FeAl₂O_4+z.     

Thermal Expansion Behavior of Titanium-Doped La(Sr)CrO3 Solid Oxide Fuel Cell Interconnects

La_0.8Sr_0.2Cr_0.9Ti_0.1O₃ perovskite has been designed as an interconnect material in high-temperature solid oxide fuel cells (SOFCs) because of its thermal expansion compatibility in both oxidizing and reducing atmospheres. La_0.8Sr_0.2Cr_0.9Ti_0.1O₃ shows a single phase with a hexagonal unit cell of a = 5.459(1) Å, c = 13.507(2) Å, Z = 6 and a space group of R -3 C . Average linear thermal expansion coefficients of this material in the temperature range from 50° to 1000°C were 10.4 × 10⁻⁶/°C in air, 10.5 × 10⁻⁶/°C under a He–H₂ atmosphere (oxygen partial pressure of 4 × 10⁻¹⁵ atm at 1000°C), and 10.9 × 10⁻⁶/°C in a H₂ atmosphere (oxygen partial pressure of 4 × 10⁻¹⁹ atm at 1000°C). La_0.8Sr_0.2Cr_0.9Ti_0.1O₃ perovskite with a linear thermal expansion in both oxidizing and reducing environments is a promising candidate material for an SOFC interconnect. However, there still remains an air-sintering problem to be solved in using this material as an SOFC interconnect.     

Influence of Oxygen Activity on the Sintering of MgCr2O4

The sintering behavior of MgCr₂O₄ powder compacts was investigated as a function of temperature, time, and oxygen activity. The results show that MgCr₂O₄ cannot be densified to >70% of theoretical density at temperatures up to 1700°C if the oxygen activity exceeds 10⁻⁶ atm. The oxygen activity must be decreased to <10⁻¹⁰ atm before densities exceeding 90% of theoretical can be achieved. Weight loss and X-ray data indicated that maximum density occurred at an oxygen activity just above that where MgCr₂O₄ becomes unstable.     

Preparation of Ca3Co4O9 and Improvement of its Thermoelectric Properties by Spark Plasma Sintering

The precursor powders of Ca₃Co₄O₉ were synthesized by a sol–gel method. The results of X-ray diffraction and thermogravimetric and differential thermal analyses patterns indicate that pure Ca₃Co₄O₉ powders could be obtained by calcining the precursor at 800°C for 2 h. High dense Ca₃Co₄O₉ ceramic samples (∼99% of theoretical density) were prepared by the spark plasma sintering (SPS) method. Compared with the conventional sintering (CS), the SPS samples exhibit much higher electrical conductivity and power factor which are respectively about 118 S/cm and 3.51 × 10⁻⁴ W·(m·K²)⁻¹. The SPS method is greatly effective for improving the thermoelectric properties of Ca₃Co₄O₉ oxide ceramics.     

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.     

Thermal Stability of Barium Ferrite (BaFe12O19)

Compositions in the system Fe₂O3-FeO-BaO in the vicinity of the compound BaFe₁₂O₁₉ were studied at temperatures from 1300° to 1550°C and oxygen pressures from 10⁻² to 10² atm. Equilibrium relations involving several barium ferrous ferrites are described. Barium ferrite can be crystallized congruently from the melt at 40 atm oxygen pressure and 15400°C.     

Fe-Nb Oxides: Phase Relations at 1180°C

Subsolidus phase relations of the oxides in the system Fe–Nb–O were experimentally determined at 1180°C, 1 atm total pressure, and variable partial pressures of oxygen. Niobium pentoxide reacts readily with either ferrous or ferric oxide at subsolidus temperatures and the following ternary compounds were synthesized: Fe₄Nb₂O₉, FeNbO₄, and FeNb₂O₆. It is shown that the rutile structure of NbO₂ will take Fe-Nb₂O₆ into solid solution and that the spinel structure of Fe₃O₄ will incorporate up to 7 at.% Nb.     

Thermodynamics and Phase Equilibria in the Sr─Cu─O System

Phase equilibria in the SrO─CuO─Cu system were determined at 1173 K from the results of X-ray diffraction measurements using specimens annealed under low oxygen partial pressure ( p o₂= 0.21–10⁻⁸ atm). Electromotive force (EMF) measurements using ZrO₂ solid electrolyte cells were carried out in the ternary phase equilibria. Gibbs free energies for five reactions in the cell electrodes were summarized by equations with linear temperature dependence. The standard free energies of formation for Sr₂CuO₃, SrCuO₂, Sr₁₄Cu₂₄O₄₁, and SrCu₂O₂ were derived. The stability conditions of all oxides are displayed in the p-T-x diagram using the new data.     

Mn-Ta Oxides: Phase Relations at 1200°C

Subsolidus phase relations among oxides in the system Mn–Ta–O were experimentally determined by the quenching method. The conditions during the heating period were 1200°C, 1 atm total pressure, and various partial pressures of oxygen between 10^–17 atm and 1 atm. At the limits of this range, the stable assemblages at f -= 10^–17 atm are: MnO, Mn₆Ta₂O₁₁, Mn₄-Ta₂O₉, Mn_1.4 TaO_3.9, MnTa₂O₆, and β-Ta₂O₅; at p _o2= 1 atm they are: Mn₃O₄, Mn_1.4TaO_4.2, MnTa₂O₆, and β-Ta₂O₅. There are five univariant assemblages of three solid phases plus vapor in the phase diagram.     

Fe-Ta Oxides: Phase Relations at 1200°C

Subsolidus phase relations of the oxides in the system Fe-Ta-O were experimentally determined at 1200°C, 1 atm total pressure, and variable partial pressures of oxygen. Tantalum pentoxide reacts readily with either ferrous or ferric oxide at subsolidus temperatures and the following ternary compounds have been synthesized: Fe₄Ta₂0₉, Fe₃Ta₂O_8-1, FeTaO₄, FeTa₂O₆, solid solutions between the latter two compounds, and tantalian magnetites. The compositions of solid solutions between FeTaO₄ and FeTa₂O₆ were very sensitive to variation of oxygen pressure. This sensitivity explains the oxidation behavior of tapiolite. The incorporation into magnetite of up to 7 at.% Ta was demonstrated, with a resulting increase in the size of the unit cell and decrease in magnetic permeability.     

Effect of Oxide Defect Structure on the Electrical Properties of ZrO2

The defect structure of monoclinic ZrO₂ was studied by measuring the transfer numbers and electrical conductivity as functions of O₂ pressure and temperature. The data suggest a defect structure of doubly ionized oxygen vacancies at low pressures, i.e. <10⁻¹⁹ atm, and singly ionized oxygen interstitials at pressures >10⁻⁹ atm. Zirconia is primarily an ionic conductor below #700°C and an electronic conductor at 700° to 1000°C for 10⁻²²≤P_o2≤1 atm.     

Measurement of Oxygen Diffusion in Dy2O3 and Gd2O3 by Studying High-Temperature Oxidation of the Metals

Studies of the oxidation of Gd and Dy at P _O2's from 10^−0.3 to 10^−14.5 atm and temperatures from 727° to 1327°C indicate both semiconducting and ionic-conducting domains in the sesquioxides formed. At higher temperatures, where dense coarsegrained oxide layers developed, the rate of oxidation in the high- P ₀₂ semiconducting domain yielded oxygen diffusion coefficients in Dy₂O₃ in excellent agreement with literature values derived from oxidation of partially reduced oxide single crystals. Under the same conditions, the oxidation of Gd yielded oxygen diffusion coefficients in cubic Gd₂O₃ which are considerably below literature values for monoclinic single-crystal Gd₂O₃. At lower temperatures, porous scales were formed, and apparent diffusion coefficients derived from oxidation rates show a smaller temperature dependence than the high-temperature data. At low P _O2, the oxides behave as ionic conductors, and metal oxidation rates result in estimates of the electronic contribution to the electrical conductivity of the order of 10⁻⁶ to 10⁻⁷Ω⁻¹ cm⁻¹.