Synthesis and Characterization of Lanthanum Stannate Nanoparticles

Phase-pure lanthanum stannate (La₂Sn₂O₇) nanoparticles with a pyrochlore structure were produced by a simple coprecipitation method for the first time. The optical properties of La₂Sn₂O₇ synthesized by different conditions were investigated. They had different photoluminescence intensities around 400 nm at the same excitation wavelength. The size of La₂Sn₂O₇ nanoparticles synthesized by the reverse-drop method was smaller than those synthesized by the nature-drop method. The nanoparticles became large rapidly with the dilution of the original reagent solution.     

Vibrational Spectroscopic Study of Structural Evolution in the Coprecipitated Precursors to La2Sn2O7 and La2Ti2O7

Structural evolution in the X-ray amorphous precursors to La₂Sn₂O₇ and La₂Ti₂O₇ is examined using IR and Raman spectroscopy. These precursors are prepared by rapid coprecipitation from mixed aqueous solutions of the corresponding metal chlorides. Rapid coprecipitation from an SnCl²⁻₆ and La³⁺-containing aqueous solution yields microcrystalline particles of SnO₂· n H₂O and La(OH)₃, which instantaneously interconnect to form an ultimate, complex colloid particle. The Ti(OH)²⁺₂ and La³⁺ in the other solution system coprecipitate into a different, complex colloid (an unidentified phase), which is definitely not a mixed dispersion of single-component colloids. A comparative examination of the vibrational spectra of the coprecipitates heated to various temperatures indicates that the SnO₂ and anatase phases develop in the respective precursors before crystallization of the desired double oxides. Crystallization itself can be attributed to a solid-state reaction among the various microcrystallites of each single-metal oxide in a gel particle of the precursor.     

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).     

Pore Structure Evolution of Lanthana–Alumina Systems Prepared through Coprecipitation

Pure Al₂O₃ and different compositions of La₂O₃–Al₂O₃ samples have been prepared through coprecipitation. Even after heating at 1300°C, the compositions La₂O₃·11Al₂O₃ and La₂O₃·13Al₂O₃ had higher surface area compared to the pure Al₂O₃ and the La₂O₃·Al₂O₃ composition. Ethanol washing is an effective way for improving the textural stability of pure Al₂O₃ and La₂O₃–Al₂O₃ samples. The effect of steam on the thermal stability of La₂O₃·11Al₂O₃ has also been studied. La₂O₃·11Al₂O₃ sample is found to be stable in steam.     

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.     

Mesoscale Organization of Flower-Like La2O2CO3and La2O3 Microspheres

Well-dispersed spherical lanthanum oxycarbonate (La₂O₂CO₃) and La₂O₃ particles with a flower-like morphology were synthesized via a simultaneous polymerization–precipitate reaction, metamorphic reconstruction, and mineralization under a hydrothermal condition as well as subsequent calcination. The La₂O₂CO₃ and La₂O₃ microspheres obtained consist of about 20 or 40 nm thick nanosheets as petals, respectively. They have an open three-dimensional mesoporous and hollow structure and possess a high surface area, large pore volume, and marked thermal stability. These novel porous structural lanthanum-based materials could be used as catalyst supports for many reactions.     

High-Temperature Stability of Lanthanum Orthophosphate (Monazite) on Silicon Carbide at Low Oxygen Partial Pressures

The stability of lanthanum orthophosphate (LaPO₄) on SiC was investigated using a LaPO₄-coated SiC fiber at 1200°–1400°C at low oxygen partial pressures. A critical oxygen partial pressure exists below which LaPO₄ is reduced in the presence of SiC and reacts to form La₂O₃ or La₂Si₂O₇ and SiO₂ as the solid reaction products. The critical oxygen partial pressure increases from ∼0.5 Pa at 1200°C to ∼50 Pa at 1400°C. Above the critical oxygen partial pressure, a thin SiO₂ film, which acts as a reaction barrier, exists between the SiC fiber and the LaPO₄ coating. Continuous LaPO₄ coatings and high strengths were obtained for coated fibers that were heated at or below 1300°C and just above the critical oxygen partial pressure for each temperature. At temperatures above 1300°C, the thin LaPO₄ coating becomes morphologically unstable due to free-energy minimization as the grain size reaches the coating thickness, which allows the SiO₂ oxidation product to penetrate the coating.     

Sintering of Lanthanum Zirconate

Lanthanum zirconate (La₂Zr₂O₇) was prepared by coprecipitating lanthanum nitrate and zirconyl oxychloride at pH 10, followed by ethanol washing. The initial high surface area of ∼304 m²·g⁻¹ decreased very rapidly with increased sintering temperature and decreased to an immeasurably small value after heating at 1200°C for 15 h. The major parameters studied were phase evolution, crystallite size, porosity, surface area reduction, and shrinkage during sintering. Three temperature regions were identified based on these studies: below the crystallization temperature, between the crystallization temperature and ∼1100°C, and above 1100°C. The main contribution of surface area reduction in the region 800°–1100°C was due to surface diffusion; the main contribution above 1100°C was due to grain-boundary diffusion coupled with surface diffusion.     

Preparation of La2Zr2O7 by Sol—Gel Route

The La₂Zr₂O₇ phase was prepared from metal acetylacetonates by a sol—gel route without any intermediate phase formation. X-ray peaks appeared at a temperature as low as 500°C at the positions expected for La₂Zr₂O₇, although they were broad. The crystal structure of La₂Zr₂O₇ was found to be of the fluorite type below 900°C and of thepyrochlore type above 1000°C. The substitution of a small amount of Eu for La was carried out to investigate the crystal structure from the viewpoint of fluorescence, and these results confirmed the formation of fluorite type La₂Zr₂O₇ below 900°C.     

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.     

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.     

Vaporization Studies of the La2O3–Ga2O3 System

The vaporization of the samples of the compositions Ga₂O₃+ LaGaO₃, LaGaO₃+ La₄Ga₂O₉, and La₄Ga₂O₉+ La₂O₃ was investigated using Knudsen effusion mass spectrometry in the temperature range 1494–1937 K. The partial pressures of the gaseous species O₂, Ga, GaO, Ga₂O, and LaO were determined over the samples investigated. The equilibrium partial pressures were used for the calculation of the thermodynamic activities of the components at 1700 K. Gibbs energies of formation of LaGaO₃( s ) and La₄Ga₂O₉( s ) at 1700 K from the component oxides were derived from the thermodynamic activities as −46.4 ± 4.7 and −99.2 ± 7.9 kJ·mol⁻¹, respectively. The results were compared with the literature data obtained using other methods.     

Electrical Conduction Behavior of La1+xSr1−xGa3O7–δ Melilite-Type Ceramics

Ceramics of the melilite-type compound La_{1+ x} Sr_{1− x} Ga₃O_7−δ were prepared by conventional ceramic processing. Samples prepared represented the entire homogeneity region of the phase (i.e., x =−0.15 to 0.60). Electrochemical characterization under variable temperature and atmospheric conditions in the vicinity of air entailed four-point direct-current conductivity measurements and electromotive force measurements. La_{1+ x} Sr_{1− x} Ga₃O_7−δ samples exhibited a p -type behavior with generally increased conductivity with increased substitution of lanthanum for strontium, which reached a saturation value of ∼10⁻¹ S·cm⁻¹ at 950°C.     

La2Ti2O7 Ceramics

La₂Ti₂O₇ powders were prepared using three different techniques. Single-phase material was obtained at 1150°C by calcination of mixed oxides, at 1000°C by molten salt synthesis, and at 850°C by evaporative decomposition of solutions. Particle sizes and morphologies of the powders differed substantially, as did the sintered microstructures and dielectric properties. Very dense (99%), translucent, grain-oriented lanthanum titanate was fabricated by hot-forging at 1300°C under a 200-kg load. Anisotropy was demonstrated by X-ray diffraction, scanning electron microscopy, thermal expansion, and dielectric measurements.     

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.     

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.     

Effects of La2O3/ZnO Additives on Microstructure And Microwave Dielectric Properties of Zr0.8Sn0.2TiO4 Ceramics

Dielectric ceramics of Zr_0.8Sn_0.2TiO₄ containing La₂O₃ and ZnO as sintering aids were prepared and investigated for microstructure and microwave dielectric properties. Low-level doping with La₂O₃ and ZnO (up to 0.30 wt%) is good for densification and dielectric properties. These additives do not affect the dielectric constant and the temperature coefficient. Dielectric losses increase significantly at additive levels higher than 0.15 wt%. The combined additives La₂O₃ and ZnO act as grain growth enhancers. With 0.15 wt% additives, a ceramic having a dielectric constant, a quality factor, and a temperature coefficient of frequency at 4.2 GHz of 37.6, 12 800, and –2.9 ppm/°C, respectively, was obtained. The quality factor was considerably improved by prolonged sintering.     

Synthesis and Preparation of Lanthanum Aluminate Target for Radio-Frequency Magnetron Sputtering

A polycrystalline LaAlO₃ target for the radio-frequency (rf) magnetron sputterings of LaAlO₃ thin films has been prepared. These films serve as a buffer layer for high- T _c YBa₂Cu₃O_{7– x} superconducting thin films on Si. Synthesis of lanthanum aluminate powder from a mixture of La₂O₃ and Al₂O₃ powders was performed by calcining from 1000° to 1600°C in air. Characterization of the calcined powders by X-ray diffraction indicates that full development of LaAlO₃ phase was evident in the sample calcined for 3 h at 1600°C in air. A polycrystalline LaAlO₃ target was prepared by heat treatment at 1500°C for 2 h in air after pressureless sintering at 1750°C for 3 h in Ar. Thin films of the LaAlO₃ on Si (100) were obtained by rf magnetron sputtering using the target and oxygen-annealing the as-deposited films.     

Quaternary Compounds Prepared in a CaO–Y2O3–SnO2 System

Compounds in a CaO–Y₂O₃–SnO₂ system were prepared by a solid-state reaction at 1673 K. The phase relation in this system was investigated by powder X-ray diffraction. Besides the previously reported ternary compounds, CaSnO₃, Ca₂SnO₄, Y₂Sn₂O₇, and a quaternary compound Ca_0.4Y_1.2Sn_0.4O₃, solid-solution series of Ca_{2− x} Y_{2 x} Sn_{1− x} O₄ with 0≤ x ≤0.5, and Ca_{1− y} Y_{2 y} Sn_{1− y} O₃ with 0≤ y ≤0.2 and 0.95≤ y ≤1.0 were found. The cell parameters of these solid-solution series were refined. The changes of rhombohedral cell parameters in the samples prepared in the range 0.565< y <0.714 of Ca_{1− y} Y_{2 y} Sn_{1− y} O₃ suggested the existence of solid solutions of Ca_0.4Y_1.2Sn_0.4O₃, although their single phases could not be prepared, except at y =0.6.     

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.