Reactions between Strontium-Substituted Lanthanum Manganite and Yttria-Stabilized Zirconia: II, Diffusion Couples

Formation of secondary phases and diffusion of cations in diffusion couples of yttria-stabilized zirconia and lanthanum manganite substituted with 0 to 60 mol% strontium have been studied by scanning electron microscopy and energy dispersive X-ray spectroscopy. Only the primary phases were observed after 120 h at 1200°C, while formation of secondary phases was identified already after 1 h heat treatment at 1350°C. The phase composition of the reaction layer altered from La₂Zr₂O₇ to SrZrO₃ at increasing Sr content in La _x Sr_{1- x} MnO₃. The thickness of the reaction layer was increasing with heat treatment time. In diffusion couples of La_0.4Sr_0.6MnO₃ formation of manganese oxide was observed in the perovskite layer after 1 h heat treatment at 1350°C, while isolated grains of SrZrO₃ relatively deep inside the zirconia were observed after longer heat treatment time. Diffusion of Mn into zirconia was observed preferenced along grain boundaries in the early stage of the interface reaction.     

Chemical Compatibility between Strontium-Doped Lanthanum Manganite and Yttria-Stabilized Zirconia

Equimolar powder mixtures and multilayer pellets of single-phase Sr-doped lanthanum manganite perovskite materials La_y-xSr_xMnO₃ with La content y = 1 and 0.95 and Sr content 0 ≤ x ≤ 0.5 were annealed in air with 8 mol% Y₂O₃-ZrO₂ at 1470 K, up to 400 h and at 1670 K. up to 200 h. X-ray diffraction and electron probe microanalysis confirmed the formation of La₂Zr₂O₇ or SrZrO₃ depending on the composition of the perovskites. No reaction products could be detected for La_0.95-xSr _xMnO₃ with 0.2 ≤ x ≤ 0.4 after annealing for 400 h at 1470 K, and for the perovskite La_0.65Sr_0.3MnO₃ even after annealing for 200 h at 1670 K. The results demonstrate the improved chemical compatibility of La-deficient perovskites against reaction with zirconia and can provide a basis for the selection of a sufficiently chemically stable material for the air electrode of solid oxide fuel cells.     

Reactions between Strontium-Substituted Lanthanum Manganite and Yttria-Stabilized Zirconia: I, Powder Samples

Sintered submicrometer powder mixtures of strontium-substituted lanthanum manganite and yttria-stabilized zirconia have been studied in order to investigate the chemical stability of these materials as respectively electrode and electrolyte in solid oxide fuel cells. Formation of secondary phases was observed after 1 h heat treatment at 1350°C and more than 13 h at 1200°C. La₂Zr₂O₇ was formed in mixtures with LaMnO₃ at 1350°C, while SrZrO₃ was formed in mixtures containing La_0.6Sr_0.4MnO₃ or La_0.4Sr_0.6MnO₃. Only minor amounts of secondary phases were observed in mixtures with La_0.7Sr_0.3MnO₃. Chemical analysis revealed considerable interdiffusion between the primary phases as well as A-site deficiency of LaMnO₃ and La_0.7Sr_0.3MnO₃ when exposed to cubic zirconia. The oxidative/reductive nature of the chemical reaction between strontium-substituted lanthanum manganite and yttria-stabilized zirconia is discussed in relation to the Sr content in LSM. The lattice parameter of cubic zirconia was observed to be quite sensitive to the interdiffusion and is an excellent tool for investigating reactions on heterophase interfaces involving stabilized zirconia.     

Reaction Kinetics and Mechanisms between La0.65Sr0.3MnO3 and 8 mol% Yttria-Stabilized Zirconia

The reaction kinetics and mechanisms between 8 mol% yttria-stabilized zirconia (YSZ) and 30 mol% Sr-doped lanthanum manganite (La_0.65Sr_0.30MnO₃, LSM) with A-site deficiency for the application of planar solid oxide fuel cells (SOFCs) were investigated. The LSM/YSZ green tapes were cofired from 1200° to 1400°C for 1 to 48 h and then annealed at 1000°C for up to 1000 h. The results showed that the diffusion of manganese cations first caused the amorphization of YSZ, and then the formation of small La₂Zr₂O₇ (LZ) or SrZrO₃ (SZ) crystals if treated for a longer time at 1400°C. The ambipolar diffusion of the Mn–O pair, transported through the migration of oxygen vacancy, plays an important role in the formation of secondary phases. The diffusion of LSM to YSZ and substitution of Mn for Zr both result in the enhanced concentration of oxygen vacancy, leading to the formation of a void-free zone (VFZ). No additional reaction products in annealed LSM/YSZ specimens, treated at 1000°C for 1000 h, were detected. The interfacial reactions, detailed reaction kinetics, and mechanisms are reported.     

Chemical Interactions between La-Sr-Mn-Fe-O-Based Perovskites and Yttria-Stabilized Zirconia

Orthoferrite-based perovskites are of interest as materials for the cathode in solid oxide fuel cells (SOFCs). Therefore, the chemical compatibility between perovskites of the composition (La_1−xSr_x)_zFe_1−yMn_yO_3−δ (0 # x # 0.3; 0.2 # y # 1; z = 0.90, 0.95, 1.00) and the solid electrolyte zirconia (ZrO₂) doped with 8 mol% yttria (Y₂O₃) (8YSZ) has been investigated. Powder mixtures of the two materials have been annealed at different temperatures. The formation of monoclinic ZrO₂ at 1000°C, as well as of La₂Zr₂O₇ and SrZrO₃ at 1400°C, has been determined in some samples. The reactions that are observed are discussed, with respect to the thermodynamic activities, tolerance factor, and oxygen-ion migration energies. Some perovskite compositions seem to be compatible with Y₂O₃-stabilized ZrO₂ (YSZ), thereby offering the possibility to use orthoferrite-based perovskites in SOFCs with a solid electrolyte made of YSZ.     

Novel Design of Microchanneled Tubular Solid Oxide Fuel Cells and Synthesis Using a Multipass Extrusion Process

A novel, microchanneled tubular solid oxide fuel cell was fabricated using a multipass extrusion process, with an outside diameter of 2.7 mm that contained 61 cells. Cell materials used in this work were 8 mol% yttria-stabilized zirconia (8YSZ), La_0.8Sr_0.2MnO₃ (LSM), and NiO–8YSZ (50:50 vol%) as electrolyte, cathode, and anode, respectively. Three stages of heat-treatment processes were applied, at 700°C in N₂ condition, at 1000°C in air, and then sintered at 1300°C for 2 h, respectively. The X-ray diffraction analysis confirmed that no reaction phases appeared after sintering. The microstructures of anode and cathode were fairly porous while the electrolyte had a dense microstructure (relative density >96%). The thickness of electrolyte, anode, and cathode were 20, 30, and 40 μm, respectively, and the diameter of the continuous channels was 150 μm.     

Mechanical Properties of Strontium-Doped Lanthanum Manganite

Solid oxide fuel cells that incorporate stabilized-zirconia electrolytes commonly use the transition-metal oxide perovskite La_0.875Sr_0.125MnO_3+δ as the cathode. While in operation, the cathode can be subjected to significant stresses, because of thermal expansion mismatch between mating components. The mechanical behavior of La_0.875Sr_0.125MnO_3+δ has been studied using three-point bend strength measurements at ambient, 400°C, 800°C and 1000°C. The results show that phase transformations have an important role in the mechanical-strength behavior. The results also indicate some unique strengthening effects that are associated with the evolution of the unit cell with temperature and the increasing symmetry of the crystal lattice.     

Perovskite-Type Strontium Zirconate as a New Material for Thermal Barrier Coatings

Perovskite-type SrZrO₃ was investigated as an alternative to yttria-stabilized zirconia (YSZ) material for thermal barrier coating (TBC) applications. Three phase transformations (orthorhombic↔pseudo-tetragonal↔tetragonal↔cubic) were found only by heat capacity measurement, whereas the phase transformation from orthorhombic to pseudo-tetragonal was found in thermal expansion measurements. The thermal expansion coefficients (TECs) of SrZrO₃ coatings were at least 4.5% larger than YSZ coatings up to 1200°C. Mechanical properties (Young's modulus, hardness, and fracture toughness) of dense SrZrO₃ showed lower Young's modulus, hardness, and comparable fracture toughness with respect to YSZ. The "steady-state" sintering rate of a SrZrO₃ coating at 1200°C was 1.04 × 10⁻⁹ s⁻¹, which was less than half that of YSZ coating at 1200°C. Plasma-sprayed coatings were produced and characterized. Thermal cycling with a gas burner showed that at operating temperatures ∼1250°C the cycling lifetime of SrZrO₃/YSZ double-layer coating (DLC) was more than twice as long as SrZrO₃ coating and comparable to YSZ coating. However, at operating temperatures >1300°C, the cycling lifetime of SrZrO₃/YSZ DLC was comparable to the optimized YSZ coating, indicating SrZrO₃ might be a promising material for TBC applications at higher temperatures compared with YSZ.     

Electrochemical Behavior of Gel-Derived Lanthanum Calcium Cobalt Ferrite Cathode in Contact with LAMOX Electrolyte

The electrochemical performance and structural features of (La_{1− y} Ca _y )(Co _x Fe_{1− x} )O₃ cathode prepared via a citrate acid gel route are studied when it is interfaced with the (La_1.8Dy_0.2)(Mo_{2− z} W _z )O₉ electrolyte. The resistance and chemical capacitance of a low-frequency arc are extracted from the impedance results to evaluate its catalytic activity in oxygen reduction reaction (ORR). (La_0.75Ca_0.25)(Co_0.8Fe_0.2)O₃ cathode exhibits the minimum area-specific resistance of 0.9 Ω cm² and maximum capacitance of 5.7 mF/cm² at 800°C among the compositions of x =0.1–0.9 and y =0.25. As the Co content increases, the decrease in resistance outweighs the increase in capacitance so that the product of resistance and capacitance ( RC time constant) decreases. In contrast, when varying the Ca content of the A-site, the changes in resistance and the capacitance compensate each other; hence the RC time constant is virtually unchanged with respect to the calcium content. Thus, Co is a more influential element than Ca on the ORR catalytic activity. The pore structure study reveals a small amount of Mo diffuses from the electrolyte into the cathode, and its quantity is reduced when interfaced to an electrolyte of high W content.     

La(Sr)FeO3 SOFC Cathodes with Marginal Copper Doping

(La_0.8Sr_0.2)_0.98Fe_0.98Cu_0.02O_3−δ can be sintered directly onto YSZ (without the need for a protective ceria interlayer). Though subject to an extended "burn-in" period (∼200 h), anode-supported YSZ cells using the Cu-doped LSF achieve power densities ranging from 1.3 to 1.7 W/cm² at 750°C and 0.7 V. These cells have also demonstrated 500 h of stable performance. The results are somewhat surprising given that XRD indicates an interaction between (La_0.8Sr_0.2)_0.98Fe_0.98-Cu_0.02O_3−δ and YSZ resulting in the formation of strontium zirconate and/or monoclinic zirconia. The amount and type of reaction product was found to be dependent on cathode and electrolyte powder precalcination temperatures.     

Superior Perovskite Oxide-Ion Conductor; Strontium- and Magnesium-Doped LaGaO3: III, Performance Tests of Single Ceramic Fuel Cells

The performance of La_0.8Sr_0.2Ga_0.83Mg_0.17O_2.815 (LSGM) as an optimized electrolyte of a solid oxide fuel cell was tested on single cells having a 500-µm-thick electrolyte membrane. The reactivity of NiO and LSGM suggested use of an interlayer to prevent formation of LaNiO₃. The interlayer Sm-CeO₂ was selected and sandwiched between the electrolyte and anode. Comparison of Sm-CeO₂/Sm-CeO₂+ Ni and Sm-CeO₂+ Ni as anodes showed that Sm-CeO₂/Sm-CeO₂+ Ni gave an exchange current density 4 times higher than that of Sm-CeO₂+ Ni. The peak power density of the interlayered cell is 100 mW higher than that of the standard cell without the interlayer. This improvement is due to a significant reduction of the anode overpotential; the overpotential of the cathode La_0.6Sr_0.4CoO_3-delta (LSCo) remained unchanged. Comparison of the peak power density in this study and with that of a previous study, also with a 500-µm-thick electrolyte, indicates a factor of 2 improvement, i.e., from 270 mW/cm² to 550 mW/cm² at 800°C. The excellent cell performance showed that an LSGM-based thick membrane SOFC operating at temperatures 600° < T _op < 800°C is a realistic goal.     

Crystalline Intermediate Phases in the Sol–Gel-Based Synthesis of La2NiO4+δ

X-ray diffraction and transmission electron microscopy were applied to investigate a sol–gel synthetic process for the mixed oxygen ion and electron conductor La₂NiO_4+δ with a K₂NiF₄ structure type. The development of the La₂NiO_4+δ is elucidated considering the influence of calcination temperatures and dwell times. Following the thermal decomposition of nitrate and organic precursors in an intermediate step, the lanthanum nickel oxide is obtained after a short dwell time above 750°C. This occurs by the transformation of an ultrafinely dispersed powder consisting of lanthanum oxycarbonate, lanthanum oxide, and nickel oxide. The pure La₂NiO_4+δ phase was obtained by similar solid-state reactions between nanocrystalline powder particles at just 950°C.     

Alkoxide Route to La0.5Sr0.5CoO3 Films and Powders

An all-alkoxide route to films and nano-phase powders of the La_0.5Sr_0.5CoO₃ perovskite is described. To our knowledge, this is the first purely alkoxide-based route to (La_{1− x} Sr _x )CoO₃, and it yields phase-pure and elementally homogeneous perovskite at 700°C by heating at 2°C/min. At 700°C, a cubic unit cell was obtained with a _c=3.853Å, and after further heating to 1000°C, a rhombohedral cell could be indexed: a _r=5.417 Å, α_r=59.94°. Ninety to 130 nm thick films of La_0.5Sr_0.5CoO₃ were obtained by spin coating. The gel-to-oxide conversion was studied in some detail, using thermo-gravimetric analysis, differential scanning calorimetry, powder X-ray diffraction, IR spectroscopy, and transmission electron microscope equipped with an energy-dispersive X-ray spectrometer.     

Electrical Conduction in Nano-Structured La0.9Sr0.1Al0.85Co0.05Mg0.1O3 Perovskite Oxide

Nanocrystalline La_0.9Sr_0.1Al_0.85Co_0.05Mg_0.1O₃ oxide powder was synthesized by a citrate–nitrate auto-ignition process and characterized by thermal analysis, X-ray diffraction, and impedance spectroscopy measurements. Nanocrystalline (50–100 nm) powder with perovskite structure could be produced at 900°C by this process. The powder could be sintered to a density more than 96% of the theoretical density at 1550°C. Impedance measurements on the sintered samples unequivocally established the potential of this process in developing nanostructured lanthanum aluminate-based oxides. The sintered La_0.9Sr_0.1Al_0.85Co_0.05Mg_0.1O₃ sample exhibited a conductivity of 2.40 × 10⁻² S/cm in air at 1000°C compared with 4.9 × 10⁻³ S/cm exhibited by La_0.9Sr_0.1Al_0.85Mg_0.15O₃.     

Thin Yttrium-Stabilized Zirconia Electrolyte Solid Oxide Fuel Cells by Centrifugal Casting

A centrifugal casting technique was developed for depositing thin 8-mol%-yttrium-stabilized zirconia (YSZ) electrolyte layers on porous NiO-YSZ anode substrates. After the bilayers were cosintered at 1400°C, dense pinhole-free YSZ coatings with thicknesses of ∼25 μm were obtained, while the Ni-YSZ retained porosity. After La_0.6Sr_0.4Co_0.2Fe_0.8O₃ (LSCF)-Ce_0.9Gd_0.1O_1.95 (GDC) or La_0.8Sr_0.2MnO₃ (LSM)-YSZ cathodes were deposited, single SOFCs produced near-theoretical open-circuit voltages and power densities of ∼1 W/cm² at 800°C. Impedance spectra measured during cell tests showed that polarization resistances accounted for ∼70%–80% of the total cell resistance.     

Fabrication of Lanthanum Manganese Oxide Thin Films on Yttria-Stabilized Zirconia Substrates by a Chemically Modified Alkoxide Method

Perovskite-type thin films of lanthanum manganese oxide (LaMnO₃) were prepared on yttria (8%) stabilized zirconia substrate by the sol–gel process from an alkoxide solution of lanthanum isopropoxide (La(O- i -C₃H₇)₃) and manganese isopropoxide (Mn(O- i -C₃H₇)₂). The alkoxide solution was chelated with 2-ethyacetoacetate, and further modified with polyethylene glycol (PEG). The obtained LaMnO₃ thin film was transparent and macroscopically crackless. X-ray diffraction, differential thermal analysis–thermogravimetry analysis, and scanning electron microscope observations indicated that single-phase LaMnO₃ thin films with a grain size of 80 to 100 nm are formed when a spin-coated LaMnO₃ gelled film is heated at 600°C for 1 h. The porous and homogeneous grain structure with a grain size of <100 nm can be obtained when the LaMnO₃ gelled film is heated at 600° and 800°C. It was considered that PEG might accelerate the crystallization of the perovskite phase, which indicates that PEG assists the formation of the La-O-Mn frame network during partial hydrolysis and condensation reactions in sol–gel processes.     

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.     

Preparation, Structure, and Selected Catalytic Properties of the System LaMn1-xCuxO3-y

Samples of LaMn_1-xCu_xO_3-y in the range 0≤x≤0.8 were prepared from freeze-dried solutions of the nitrates. Samples with x≤0.6 were single-phase perovskites. At higher values of x , the samples contained La₂CuO₄ and CuO as well as the perovskite phase. Samples of LaMn_1−x,Cu_x,O_3−y supported on ceramic monoliths or when mixed with powdered A1₂O₃ exhibit catalytic activity for the oxidation of CO. Greatest activity is shown for 0.4≤x≤0.7. Although the catalysts are severely poisoned by SO₂, 2% H₂O in the gas stream causes only slight deactivation. Activities of other oxide catalysts were also measured and compared. Rate constants per unit surface area at 200° to 400°C follow the order Co₃O₄>Pt>LaMn_1−xCu_xO_3−y (0.4≤x≤0.7)>copper chromite>La_1−xSr_x,MnO₃≤ other substituted LaMnO₃ materials, CuO, or La₂CuO₄. The perovskite catalyst is more stable than Co₃O₄ or copper chromite when heated in 10% H₂+ 90% N₂.     

Subsolidus Relations in the La2O3─CuO─CaO Phase Diagram and the La2O3─CuO Binary Join

The phase diagram for the CuO-rich part of the La₂O₃─CuO join was redetermined. La₂Cu₂O₅ was found to have a lower limit of stability at 1002°± 5°C and an incongruent melting temperature of ∼1035°C. LagCu₇O₁₉ had both a lower (1012°± 5°C) and an upper (1027°± 5°C) limit of stability. Subsolidus phase relations were studied in the La₂O₃─CuO─CaO system at 1000°, 1020°, and 1050°C in air. Two ternary phases, La_1.9Ca_1.1Cu₂O_5.9 and LaCa₂Cu₃O_8.6, were stable at these temperatures, with three binary phases, Ca₂CuO₃, CaCu₂O₃, and La₂CuO₄. La₂Cu₂O₅ and La₈Cu₇O₁₉ were stable only at 1020°C, and did not support solid-solution formation.     

Preparation of Larnthanum β-Alumina with High Surface Area by Coprecipitation

Mixtures of La₂O₃ and Al₂O₃ with various La contents were prepared by co-precipitation from La(NO₃)₃ and Al(NO₃)₃ solutions and calcined at 800° to 1400°C. The addition of small amounts of La₂O₃ (2 to 10 mol%) to Al₂O₃ gives rise to the formation of lanthanum β-alumina (La 2 O₃·11–14Al₂O₃) upon heating to above 1000°C and retards the transformation of γ-Al₂O₃ to α-Al₂O₃ and associated sintering.