Aqueous Combustion Synthesis of Strontium-Doped Lanthanum Chromite Ceramics

An aqueous combustion synthesis is used to produce powders of La_0.8Sr_0.2CrO₃ perovskite. It is shown that interaction between chromium nitrate and glycine controls the process. In addition, it is suggested that glycine reacts with products of nitrate decomposition to yield an intermediate compound, which decomposes exothermically providing high-temperature conditions for complex oxide formation. It is remarkable that although reaction temperature is high (up to 800°C) and characteristic time is small (∼1 s) for synthesis under the self-propagating high-temperature mode, the produced perovskites have high specific surface area (∼40 m²/g) and well-defined crystalline structure. As a result, ceramics sintered by using these powders are dense (∼96% of theoretical) and possess high electronic and low ionic conductivities, important for interconnect applications in solid oxide fuel cells.     

Sol—Gel Synthesis of Strontium-Doped Lanthanum Manganite

Strontium-doped lanthanum manganite powders were prepared using a peroxide acetate salt based solution. The stable sol was peptized by reacting ammonium hydroxide with the precursor solution. The amorphous dried gel powders exhibit a high energy level, due to their high cations coordination and small particles, to develop the perovskite phase. This crystalline phase development from powders containing monocarboxylate ligands was characterized by thermal analysis (TG, DTG, DTA), X-ray diffraction, and IR spectroscopy. The transformation from amorphous powders into a crystallized homogeneous oxycarbonate phase in a first stage corresponds to an exothermal DTA peak at 270°C. X-ray diffraction patterns and IR spectra showed similar behavior of the powders after complete organic removal, during the conversion into perovskite phase starting at approximately 630°C and achieved about 700°C and achieved about 700°C, as well as during the sintering process.     

Strontium-Doped Lanthanum Manganite Films Prepared by Magnetic Deposition

Deposition of La_0.85Sr_0.15MnO₃ (LSM) films from suspensions using a magnetic field was found to be a cheap and quick technique. Ninety weight percent of the particles present in the suspensions were deposited within the first minute of the deposition, and the thickness of the film varied linearly with the concentration of the suspension. Deposition phenomena were explained by modeling the magnetic flux in the deposition cell. Particles aligned with the flux lines, forming chains of LSM particles that, upon sintering, resulted in the formation of porous films with long chains of LSM grains.     

Mechanochemical Synthesis of Lanthanum Oxyfluoride from Lanthanum Oxide and Lanthanum Fluoride

A powder mixture of lanthanum oxide (La₂O₃) and lanthanum fluoride (LaF₃) was ground by a planetary ball-mill to investigate the mechanochemical reaction forming lanthanum oxyfluoride (LaOF) at room temperature. The grinding enables us to form LaOF monophase, and the reaction proceeds with an increase in grinding time, whereas the crystallite size of LaOF formed is about 15–20 nm irrespective of the grinding time. Other rare-earth oxyfluorides (R–OF, R = Pr, Nd, Sm, Gd) can be synthesized by grinding the constituent components (R₂O₃ and RF₃).     

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.     

Effect of Manganese Ion Diffusion on the Microstructural Evolution of Lead Lanthanum Zirconate Titanate Ceramic

Microstructural evolution of lead lanthanum zirconate titanate (PLZT) ceramics caused by diffusion of the Mn ion was observed. Specimens with layered structures were fabricated by copressing a PLZT powder (9/65/35) doped with Mn and same PLZT powder without the dopant. When the copressed specimen was sintered at 1200°C in air, the Mn ion diffused out of the doped region. The region originally containing the Mn ions was totally free of pores while all other regions remained porous. The formation of lattice vacancies, as a result of Mn diffusion, was attributed to the enhanced material transport and the resultant rapid densification.     

Low-Temperature Synthesis and Properties of LaMnO3±d and La0.67R0.33MnO3±d (R = Ca, Sr, Ba) from Citrate Precursors

LaMn_{1− y} ³⁺Mn _y ⁴⁺O_3±d and La_0.67R_0.33Mn_{1− y} ³⁺Mn _y ⁴⁺O_3±d (R = Ca, Sr, Ba) phases were synthesized at 350°C by using very reactive, amorphous precursors obtained from the stoichiometric citrate solutions. The chemical process was optimized with respect to the solution concentration, pH, and additives. The precursor reactions were investigated as a function of the cation stoichiometry and the additive by simultaneous thermal and thermogravimetric analysis and X-ray diffraction. The reaction pathway was found to be independent of the cation stoichiometry, but related to the acid or base additive. The annealing temperature was systematically increased in the 350–1200°C interval and the La_0.67Sr_0.33MnO_3±d properties (i.e., crystal sizes, Mn average valence, Curie temperature, magnetization, magnetic susceptibility) were measured and found to vary consistently as a function of it.     

Mechanochemical Synthesis of Lanthanum Aluminate by Grinding Lanthanum Oxide with Transition Alumina

Grinding lanthanum oxide (La₂O₃) with Al₂O₃ was conducted to investigate their mechanochemical reactions to form lanthanum aluminate (LaAlO₃) powder using a planetary ball mill. Grinding for 120 min allowed us to obtain single-phase LaAlO₃ with a large surface area when transition alumina was used, whereas no formation of LaAlO₃ was achieved when α-Al₂O₃ was used. The mechanochemical process can be applied to synthesize other rare-earth (RE) aluminates (REAlO₃) from mixtures of a rare-earth oxide and transition alumina.     

Structure and Magnetic Properties of La0.7Ca0.3MnO3−δ for (3 −δ) < 3.0

Annealing studies were conducted on bulk La_0.7Ca_0.3MnO_3−δ to determine the sensitivity of its structural and magnetic properties to oxygenation conditions. Standard bulk sintering conditions, thin-film annealing conditions for obtaining good magnetoresistive properties, and a reducing anneal, which corresponded to the onset of phase decomposition, were conducted. The main phase formed was a face-centered (fcc) pseudocubic double-perovskite structure, with cell parameters of a ∼ 2 a _p∼ 0.772 nm, where a _p is the single-perovskite cubic cell parameter. A minor superstructure—body-centered pseudotetragonal, with lattice parameters of c = 4 a _p and a =√2 a _p—was observed in samples with (3 −δ) < 3. A maximum of 20% of the superstructure was formed using the most-reducing conditions. The superstructure had a lower critical temperature than the main phase and depressed ferromagnetic order.     

Thermal Stability of Lanthanum Zirconate Plasma-Sprayed Coating

Lanthanum zirconate (La₂Zr₂O₇, LZ) is a newly proposed material for thermal barrier coatings (TBCs). The thermal stability of LZ coating was studied in this work by long-term annealing and thermal cycling. After long-term annealing at 1400°C or thermal cycling, both LZ powder and plasma-sprayed coating still kept the pyrochlore structure, and a preferred crystal growth direction in the coating was observed by X-ray diffraction. A considerable amount of La₂O₃ in the powder was evaporated in the plasma flame, resulting in a nonstoichiometric coating. Additionally, compared with the standard TBC material yttria-stabilized zirconia (YSZ), LZ coating has a lower thermal expansion coefficient, which leads to higher stress levels in a TBC system.     

Hydrothermal Synthesis and Formation Mechanisms of Lanthanum Tin Pyrochlore Oxide

Well-defined La₂Sn₂O₇ with a phase-pure pyrochlore structure was produced by hydrothermal synthesis at temperatures as low as 200°C. Production of phase-pure La₂Sn₂O₇ requires a pH above 10, and higher pH accelerates the crystallization process. The synthesis produced spherical particles of average particle size ∼0.59 μm (±0.12) and surface area ∼14.1 m²/g. SEM and TEM observation for morphologic evolution and kinetic analysis during crystallization indicated that La₂Sn₂O₇ formation probably proceeds via a two-step reaction. First a transient dissolution–precipitation occurs. Then the primary crystallites aggregate because of their colloidal instability, and heterocoagulation with the lanthanum hydrous oxide precursor particles also occurs. The sluggish reaction rate at the later stage of reaction is characterized by an in situ transformation, where the soluble tin species is diffused through the porous La₂Sn₂O₇ aggregates to react with entrapped lanthanum precursors.     

Crystallization of Electrostatically Seeded Lanthanum Hexaluminate Films on Polycrystalline Oxide Fibers

Lanthanum hexaluminate (LaAl₁₁O₁₈), which features easily cleavable atomic planes in the magnetoplumbite (MP) structure, is a good candidate material for the fiber/matrix interface in oxide/oxide-fiber-reinforced composites. Difficulties that are encountered when using sol–gel synthesis to produce this material include the high crystallization temperature and the occurrence of an intermediate phase. This paper presents a method to overcome these difficulties by introducing crystalline seeds into the LaAl₁₁O₁₈ sol during the fiber coating. Seed particles are deposited initially on the fibers via electrostatic forces. Then, the sol is dip-coated on the preseeded fibers, and the final LaAl₁₁O₁₈ coating is achieved via the use of heat treatments. The crystallization temperature can be reduced by at least 50°C, in comparison with that for the unseeded case, according to differential scanning calorimetry measurements, and no intermediate LaAlO₃ phase is formed. The estimated seed numbers are on the order of 10⁹ seeds/cm³ in the preseeded sol-coated fiber surfaces and 10¹¹ seeds/cm³ in the 4-wt%-seeded gels. The crystallization and structure of the La_{1− x} Al_{12− y} O_{19− z} MP phase in the 4-wt%-seeded gels that have been heated at 400° and 1200°C have been studied using powder neutron diffraction.     

Chemical Solution Deposition of Columnar-Grained Metallic Lanthanum Nitrate Thin Films

Columnar and (100)-oriented LaNiO₃ thin films were prepared on silicon substrates by a chemical solution deposition (CSD) process using a 0.05 M solution. By reducing the individual layer thickness to 10 nm, columnar LaNiO₃ films with a lateral grain size of ∼120 nm were obtained. The success of this approach required restricting the individual layer thickness to a value below the grain size observed for equiaxed films. This change in microstructure resulted in an improvement in conductivity. The columnar LaNiO₃ film with a thickness of 300 nm showed a resistivity of 4.5 × 10⁻⁵Ω·cm, which is lower by one order of magnitude than that of fine-grain equiaxed films that typically result from CSD methods.     

Chemical Preparation of Pure and Strontium- and/or Magnesium-Doped Lanthanum Gallate Powders

Powder compositions of LaGaO₃, La_0.9Sr_0.1GaO_2.95, and La_0.8Sr_0.2Ga_0.83Mg_0.17O_2.815 were prepared via a Pechini-type process that uses citric acid and ethylene glycol. The calcination behavior of the precursor powders of the above-mentioned phases was studied in the temperature range of 200°–1400°C in an air atmosphere. Characterization of the powder samples were performed using several processes, including X-ray diffractometry, thermogravimetry/differential thermal analysis, Fourier transform infrared spectroscopy, scanning electron microscopy, inductively coupled plasma–atomic emission spectroscopy, and carbon and nitrogen analyses.     

Electrochemical Stability of Strontium-Doped Ceria Electrolyte in Solid-Oxide Fuel Cell Applications

SrO-doped CeO₂ electrolyte has been evaluated in single-cell configuration under solid-oxide fuel cell operating conditions. Because of oxygen loss from the crystal lattice, the material experiences a macroscopic expansion of several percent at 1000°C. On extended cell operation, strontium precipitates-out at/near the anode, resulting in irreversible cell degradation in the case of SrO-doped CeO₂. Precipitation and diffusion of SrO causes decreased ionic conductivity and may result in anode delamination. SrO precipitation is attributed to insolubility of the dopant in the reduced CeO₂ phase. The diffusion of strontium seems to be related to the flux of oxygen through the sample, but an exact mechanism is unknown.     

Mechanism of Reaction between Lanthanum Manganite and Yttria-Stabilized Zirconia

The reaction of La_{1- x} Ca _x MnO₃ ( x = 0, 0.1, 0.2) with ZrO₂-8 mol% Y₂O₃ (YSZ) has been investigated at temperatures ranging from 1300° to 1425°C in air. Substitution of Ca for La in LaMnO₃ depresses the reactivity with YSZ. A layer of La₂Zr₂O₇ is formed at the La_{1- x} Ca _x MnO₃/YSZ interface after an induction period, and its formation is accelerated when the La_{1- x} Ca _x MnO₃ phase is porous. The reaction proceeds by unidirectional diffusion of La, Mn, and/or Ca ions, mainly Mn ions, into YSZ. The diffusion coefficients of La and Mn ions in YSZ, which are estimated using a LaMnO₃/single-crystal YSZ couple, are much lower than that of oxygen ion. From the experimental data, a reaction mechanism is proposed.     

Mechanical Activation of Perovskite-Type Oxides for Catalytic Combustion

The effect of grinding on the catalytic properties of La_0.8Sr_0.2MnO_3± powders prepared via a ceramic or sol–gel process was studied with respect to the carbon black combustion temperature (T ^C). For the ceramic process, a milling time of 5 h led to a decrease of 77°C in the T ^C (from 615 to 538°C) in relation with increasing BET surface area. Regarding the sol–gel process, the T ^C decreased from 535 to 505°C after 1 h of grinding. Nevertheless, upon further milling (10 h), the benefit of the grinding effect disappeared due to a continuous decrease in the BET surface area. Finally, grinding (when well controlled) enabled one to obtain ceramic powders showing performances as good as sol–gel materials (T ^C540°C).     

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.     

Synthesis of Perovskite-Type Lanthanum Iron Oxide by Glycothermal Reaction of a Lanthanum–Iron Precursor

Perovskite-type lanthanum iron oxide (LaFeO₃) powders were synthesized by glycothermal reaction at 300°C of a precursor that had been previously prepared by solvothermal decomposition of a mixture of lanthanum(III) isopropoxide and iron(III) butoxide (or methoxide) in toluene. Direct glycothermal reaction of the mixture yielded no double oxide, indicating that preparation of the precursor was important for crystallization of LaFeO₃ by the glycothermal method.     

Characterization of Lanthanum Zirconate Formation at the A-Site-Deficient Strontium-Doped Lanthanum Manganite Cathode/Yttrium-Stabilized Zirconia Electrolyte Interface of Solid Oxide Fuel Cells

Lanthanum zirconate (LZO) formation and growth kinetics between 1273–1673 K at the A-site (La, Sr)-deficient Sr-doped lanthanum manganite (LSM)/yttrium-stabilized zirconia (YSZ) interface were studied using high-resolution transmission electron microscopy–energy-dispersive X-ray spectrometry (HRTEM–EDS). A cross-section TEM sample preparation technique was developed using a dual-beam focused ion beam and an in situ Omniprobe manipulator. The LZO pyrochlore phase was identified at the LSM/YSZ interface by TEM. The LZO is epitaxial with respect to the YSZ phase but not to the LSM phase. EDS results coupled with structural analysis suggest that La diffusion is a critical step in LZO formation. This study shows that LZO formation can occur even for A-site-deficient LSM with a La composition of 78 mol% (<86 mol%). The activation energy for LZO formation was found to be 168±8 KJ/mol.