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.     

The System Iron–Titanium–Oxygen at 1200°C. and Oxygen Partial Pressures Between 1 Atm. and 2 × 10−14 Atm.

The 1200°C. isotherm in the system Fe-Ti-O has been studied by equilibrating mixtures of iron oxide (Fe₂O₃) and titanium oxide (TiO₂) with atmospheres of controlled oxygen partial pressure. These atmospheresmnsisted of CO₂, air, oxygen, mixtures of CO₂ and H₂, and mixtures of CO₂ and CO. The resulting oxide mixtures were examined at room temperature by chemical analysis and by X-ray diffraction. The stability regions of the α-oxides (Fe_mTi_2-mO₃), of the spinels, (Fe_nTi_3-nO₄), and of the orthorhombic oxides (Fe_pTi_3-p O₅)were determined. Some nonstoichiometry occurs in the spinels and in the α-oxides. The oxygen partial pressures at which spinel is reduced to (iron +α-oxides) and at which α-oxide (ilmenite) is reduced to (iron + orthorhombic oxide) were determined as 2.1 × 10⁻¹³ and 9.3 × 10⁻¹⁴atm., respectively. The orthorhombic solid solution series extends over the whole range of oxygen partial pressures studied.     

Phase Equilibria in the TiO2-Rich Region of the System BaO-TiO2

Phase relations in the system BaO-TiO₂ from 67 to 100 mol% TiO₂ were investigated at 1200° to 1450°C in O₂. Data were obtained by microstructural, X-ray, and thermal analyses. The existence of the stable compounds Ba₆Ti₁₇O₄₀, Ba₄Ti₁₃O₃₀, BaTi₄O₉, and Ba₂Ti₉O₂₀ was confirmed. The compound BaTi₂O₅ is unstable and either forms as a reaction intermediate below the solidus or crystallizes from the melt. The compounds Ba₆Ti₁₇O₄₀ and Ba₄Ti₁₃O₃₀ decompose in peritectic reactions, and BaTiO₃ and Ba₆Ti₁₇O₄₀ react to form a eutectic. Special conditions are required for the formation of Ba₂Ti₉O₂₀, which decomposes in a peritectoid reaction at 1420°C. The new phase diagram is presented.     

Phase Equilibrium Studies in the System Iron Oxide-Al2O3-Cr2O3

Subsolidus phase relations in the system iron oride-Al₂O₂-Cr₂O₃ in air and at 1 atm. O₂ pressure have been studied in the. temperature interval 1250° to 1500°C. At temperatures below 1318° C. only sesquioxides with hexagonal corundum structure are present as equilibrium phases. In the temperature interval 1318° to 1410°C. in air and 1318° to 1495° C. at 1 atm. O₂, pressure the monoclinic phase Fe₂O₃. Al₂O₃ with some Cr₂O₃ in solid solution is present in the phase assemblage of certain mixtures. At temperatures above 1380°C. in air and above 1445°C. at 1 atm. O₂ pressure a complex spinel solid solution is one of the phases present in appropriate composition areas of the system. X-ray data relating d- spacing to composition of solid solution phases are given.     

Properties of Yttrium Oxide Ceramics

The cubic structure of yttrium oxide is stable to 1800°C. in air as indicated by petrographic, X-ray, and differential thermal analyses. A change in lattice parameter of less than ±0.007 a.u. was observed on heating the oxide to 1800°C. The mean specific heat of Y₂O₃ to 1600°C. was 0.13 cal. per gm. per °C. The coefficient of linear expansion to 1400°C. was 9.3 × 10⁻⁶ in. per in. per °C. Compacts of Y₂O₃ required a temperature of 1800°C. for vitrification. In equimolecular binary mixtures heated in the powdered state at 1500°C., Y₂O₃ formed compounds with Al₂O₃ and Fe₂O₃ and solid solutions with ZrO₂ and HfO₂. Y₂O₃ did not react with CaO, MgO, or ThO₂. Crystal types and unit-cell sizes of the reaction products are included.     

Electrical Conduction in Single-Crystal and Polycrystalline Al2O3 at High Temperatures

The electrical conductivity and ion/electron transference numbers in Al₃O₃ were determined in a sample configuration designed to eliminate influences of surface and gas-phase conduction on the bulk behavior. With decreasing O₂ partial pressure over single-crystal Al₂O₃ at 1000° to 1650°C, the conductivity decreased, then remained constant, and finally increased when strongly reducing atmospheres were attained. The intermediate flat region became dominant at the lower temperatures. The emf measurements showed predominantly ionic conduction in the flat region; the electronic conduction state is exhibited in the branches of both ends. In pure O₂ (1 atm) the conductivity above 1400°C was σ≃3×10³ exp (–80 kcal/ RT ) Ω⁻¹ cm⁻¹, which corresponds to electronic conductivity. Below 1400°C, the activation energy was <57 kcal, corresponding to an extrinsic ionic condition. Polycrystalline samples of both undoped hot-pressed Al₂O₃ and MgO-doped Al₂O₃ showed significantly higher conductivity because of additional electronic conduction in the grain boundaries. The gas-phase conduction above 1200°C increased drastically with decreasing O₂ partial pressure (below 10⁻¹⁰ atm).     

High Quality and Yield in Potassium Titanate Whiskers Synthesized by Calcination from Hydrous Titania

Hydrous titanium dioxide (TiO₂· n H₂O) was used to prepare K₂Ti₂O₅ single crystals, K₂Ti₄O₉ whiskers and K₂Ti₆O₁₃ whiskers at 820°, 940°, and 1110°C, respectively, by calcination. At T < 820°C, dehydration of hydrous titania, decomposition of potassium carbonate, and reaction between titanate and potassium oxide occurred simultaneously, ending with a crystallization reaction of K₂Ti₂O₅ single crystals at 820°C. Subsequently, K₂Ti₂O₅ single crystals convert into K₂Ti₄O₉ whiskers at 940°C, and K₂Ti₄O₉ whiskers further convert into K₂Ti₆O₁₃ whiskers at 1110°C. The reaction temperatures for the generations of these types of potassium titanates were all 10°–40°C lower than the corresponding temperatures when anatase was used as the reactant. The whiskers synthesized in the present study exhibited uniform size, good morphology, and a high yield.     

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.     

Influence of Processing Conditions on the Electrical Properties of CaCu3Ti4O12 Ceramics

The electrical properties of a series of CaCu₃Ti₄O₁₂ ceramics prepared by the mixed oxide route and sintered at 1115°C in air for 1–24 h to produce different ceramic microstructures have been studied by Impedance Spectroscopy. As-fired ceramics are electrically heterogeneous, consisting of semiconducting grains and insulating grain boundaries, and can be modelled to a first approximation on an equivalent circuit based on two parallel RC elements connected in series. The grain boundary resistance and capacitance values vary as a function of sintering time and correlate with the ceramic microstructure based on the brickwork layer model for electroceramics. The large range of apparent high permittivity values for CaCu₃Ti₄O₁₂ ceramics is therefore attributed to variations in ceramic microstructure. The grain-boundary resistance decreases by three to four orders of magnitude after heat treatment in N₂ at 800°–1000°C but can be recovered to the original value by heat treatment in O₂ at 1000°C. The bulk resistivity decreases from ∼80 to 30 Ω·cm with increasing sintering time but is independent of heat treatment in N₂ or O₂ at 800°–1000°C. The origin of the bulk semiconductivity is discussed and appears to be related to partial decomposition of CaCu₃Ti₄O₁₂ at the high sintering temperatures required to form dense ceramics, and not to oxygen loss.     

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.     

Solubilities of Magnesia, Titania, and Magnesium Titanate in Aluminum Oxide

On heat treatment in air the solubility of MgO or TiO₂, in Al₂₃ is too small to detect by lattice parameter shifts. The solubility of MgTiO₃ in Al₂O₃ in air increased to the measured values, expressed as atomic fractions Mg:A1or Ti:A1of0.82 × lo-², 1.43 × 10^-2, and 1.75 × 10^-2 at 1250°, 1650°, and 1850°C, respectively. In 1 atm hydrogen the TiO₂ solubility expressed as the atomic fraction Ti:A1 is 0.55 × lo^-2, 0.75 × 10W², 1.15 × 10^-2, and 1.50 × 10^p2 at 1400°, 1500°, 1600°, and 1700°C, respectively. The increased solubility in H₂ was attributed to reduction of the titanium ion. The solubility of MgO in A1₂O₃ in vacuum (0.3μ) expressed as the atomic fraction Mg:A1 was measured as 1.10 × loW⁴, 3.00 × 10"⁴, 6.80 × 10–⁴, and 1.40 × 10^-3 at 1530°, 1630°, 1730°, and 183O°C, respectively. These contents did not cause an observable change in lattice parameter, but a slight change was observed when MgO was dissolved in A1₂O₃ in a hydrogen atmosphere.     

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.     

Metal Organic Resin Derived Barium Titanate: I, Formation of Barium Titanium Oxycarbonate Intermediate

Crystallization of BaTiO₃ from an X-ray amorphous, metal organic precursor was investigated by comparing samples heated in O₂, air, argon, and CO₂. It is evident that an intermediate barium titanium oxycarbonate phase forms between 500° and 620°C and that BaTiO₃ forms directly by the endothermic decomposition of this phase between 635° and 700°C. From thermodynamic calculations, thermal analysis, X-ray diffraction, and Raman spectroscopy, it is concluded that the intermediate oxycarbonate is a highly disordered, metastable, and weakly crystalline phase with a stoichiometry close to Ba₂Ti₂O₅CO₃.     

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.     

Phase Relations in the Pseudobinary System Na2TiSi4O11-Na2Ti2Si2O9

The quenching technique was used to study subliquidus and subsolidus phase relations in the pseudobinary system Na₂ Ti₂Si₂ O₁₁-Na₂ Ti₂ Si₂ O₉. Both narsarukite (Na₂TiSi₄O₁₁) and lorenzenite (Na₂Ti₂Si₂O₉) melt incongruently. Narsarsukite melts at 911°±°C to SiO₂+liquid, with the liquidus at 1016°C. Lorenzenite melts at 910°±5°C to Na₂ Ti₆ O₁₃+liquid; Na₂ Ti₆ O₁₃ reacts with liquid to form TiO₂ and is thus consumed by 985°±5°C. The liquidus occurs at 1252°C.     

High-Temperature Automotive Catalytic Nitrogen Oxide Reduction over Copper–Lanthanum–Alumina

The synthesis and nitrogen oxide (NO) removal activities of composite powders in the copper-lanthanum-alumina(Cu-La-Al₂O₃)system were studied, in regard to their high-temperature application as an automotive lean-burn exhaust catalyst. The solid-state reaction between the copper, the lanthanum, and the Al₂O₃support, at elevated temperature, was examined by X-ray diffractometry and electron-spin resonance. Lanthanum impregnation into the Al₂O₃support successfully helped to form active composite powders that could remove nitrogen oxides and consisted of gamma-Al₂O₃and LaAlO₃, after heat treatment at 900°C, and mainly CuLaAl11O19 at 1000°C. Cu²⁺cations were observed to be isolated in the present powders, even after heat treatment up to 1000°C. The evaluation of NO removal activity over lanthanum-modified copper-Al2O₃ catalysts that were subjected to harsh heat treatment was performed using a model exhaust-gas mixture of NO, carbon monoxide (CO), propene (C₃H₆), carbon dioxide (CO₂), water vapor (H₂O), and an excess of oxygen (O₂). The catalyst that was composed of 20 mol% copper, 10 mol% lanthanum, and Al2O₃ and heat treated at 900° and 1000°C in air showed NO removal conversion efficiencies of 14% and 8%, respectively, under the lean-burn conditions of an air:fuel (A/F) ratio of 18 and a space velocity (SV) of 100000 h-1. Lanthanum-modified copper-Al2O₃ powders are potentially useful as a NO removal catalyst, when applied to an automotive lean-burn exhaust.     

Direct-Current Conductivity and Iron Tracer Diffusion in Hematite at High Temperatures

The dc conductivity of natural single-crystal α-Fe₂O₃ was measured as a function of O partial pressure from 10⁻⁴ to 1 atm at 950° to 1422°C. The conductivity was independent of O₂ partial pressure, indicating that hematite is an intrinsic semiconductor with lattice defect concentrations much lower than the concentration of intrinsic electrons (holes). The activation energy of the dc conductivity was 1.18 eV. The iron tracer (⁵⁵Fe) diffusion coefficients, measured as a function of O₂ partial pressure at 1200° and 1300°C, increased as the O₂ partial pressure decreased, with a pressure dependence of -0.75; the iron therefore migrates interstitially.     

The System PbO-Chromium Oxide in Air

Phase relations in the system PbO-chromium oxide were determined in air at high temperatures and are considered in terms of the PbO-Cr₂O₃−O₂ ternary because of reactions with atmospheric oxygen. Although not necessarily binary at all temperatures, four established subsystems characterize the air equilibria and indicate oxidation-reduction of chromium. The PbO-PbCrO₄ join contains the compounds Pb₅CrO₃ and Pb₅CrO₈ and consists of two portions, PbO-Pb₂CrO₆ and Pb₂CrO₅−PbCrO₄ The former is binary at all temperatures studied, while the latter exists only below 753° C because of the decomposition of PbCrO₄ at 753°C to Pb₂CrO₅ and Cr₂O₃ with oxygen evolution. Similarly, the join PbCrO₄−Cr₂O₃ exists only below 753°C yielding to Pb₂CrO₃−Cr₂O₃ equilibria above this temperature.     

Phase Equilibria in the System La2O3—Iron Oxide in Air

Phase equilibria data are presented for compositions in the system La₂O₃-iron oxide in air. Liquidus and solidus curves were obtained by the quenching method in the iron-rich portion of the system. The remainder of the diagram was determined using a strip-furnace technique. Two compounds have been found, the ortho-rhombic perovskite LaFeO₃ and a compound with the magnetoplumbite structure corresponding to a composition LaFe₁₂O₁₉. LaFeO₃ was determined to melt congruently at about 1890°C. whereas LaFe₁₂O₁₉ has both a stability minimum and maximum at 1380° and 1421°C., respectively. The iron-rich portion of the system is essentially ternary whereas the remainder can be considered to be a simple binary. Liquid compositions have been determined and are plotted in terms of the ternary system La₂O₃-Fe₂O₃-FeO.