共查询到20条相似文献,搜索用时 31 毫秒
1.
Shumei Wang Guangjun Zhou Mengkkai Lu Yuanyuan Zhou Shufen Wang Zhongsen Yang 《Journal of the American Ceramic Society》2006,89(9):2956-2959
Phase-pure lanthanum stannate (La2 Sn2 O7 ) nanoparticles with a pyrochlore structure were produced by a simple coprecipitation method for the first time. The optical properties of La2 Sn2 O7 synthesized by different conditions were investigated. They had different photoluminescence intensities around 400 nm at the same excitation wavelength. The size of La2 Sn2 O7 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. 相似文献
2.
Structural evolution in the X-ray amorphous precursors to La2 Sn2 O7 and La2 Ti2 O7 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 H2 O 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. 相似文献
3.
Mechanochemical Synthesis of Lanthanum Oxyfluoride from Lanthanum Oxide and Lanthanum Fluoride 总被引:1,自引:0,他引:1
A powder mixture of lanthanum oxide (La2 O3 ) 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 (R2 O3 and RF3 ). 相似文献
4.
Jalajakumari Nair Padmakumar Nair Fujio Mizukami Jan G. Van Ommen Giel B. M. Doesburg Julian R. H. Ross Anthonie J. Burggraaf 《Journal of the American Ceramic Society》2000,83(8):1942-1946
Pure Al2 O3 and different compositions of La2 O3 –Al2 O3 samples have been prepared through coprecipitation. Even after heating at 1300°C, the compositions La2 O3 ·11Al2 O3 and La2 O3 ·13Al2 O3 had higher surface area compared to the pure Al2 O3 and the La2 O3 ·Al2 O3 composition. Ethanol washing is an effective way for improving the textural stability of pure Al2 O3 and La2 O3 –Al2 O3 samples. The effect of steam on the thermal stability of La2 O3 ·11Al2 O3 has also been studied. La2 O3 ·11Al2 O3 sample is found to be stable in steam. 相似文献
5.
Mechanochemical Synthesis of Lanthanum Aluminate by Grinding Lanthanum Oxide with Transition Alumina 总被引:1,自引:0,他引:1
Grinding lanthanum oxide (La2 O3 ) with Al2 O3 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-Al2 O3 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. 相似文献
6.
Chunwen Sun Guoliang Xiao Hong Li Liquan Chen 《Journal of the American Ceramic Society》2007,90(8):2576-2581
Well-dispersed spherical lanthanum oxycarbonate (La2 O2 CO3 ) and La2 O3 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 La2 O2 CO3 and La2 O3 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. 相似文献
7.
Michael K. Cinibulk Geoff E. Fair Ronald J. Kerans 《Journal of the American Ceramic Society》2008,91(7):2290-2297
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 La2 O3 or La2 Si2 O7 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. 相似文献
8.
Jalajakumari Nair Padmakumar Nair Giel B. M. Doesburg Jan G. Van Ommen Julian R. H. Ross Anthonie J. Burggraaf Fujio Mizukami 《Journal of the American Ceramic Society》1999,82(8):2066-2072
Lanthanum zirconate (La2 Zr2 O7 ) 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. 相似文献
9.
Hiroyasu Kido Sridhar Komarneni Rustum Roy 《Journal of the American Ceramic Society》1991,74(2):422-424
The La2 Zr2 O7 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 La2 Zr2 O7 , although they were broad. The crystal structure of La2 Zr2 O7 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 La2 Zr2 O7 below 900°C. 相似文献
10.
The phase diagram for the CuO-rich part of the La2 O3 ─CuO join was redetermined. La2 Cu2 O5 was found to have a lower limit of stability at 1002°± 5°C and an incongruent melting temperature of ∼1035°C. LagCu7 O19 had both a lower (1012°± 5°C) and an upper (1027°± 5°C) limit of stability. Subsolidus phase relations were studied in the La2 O3 ─CuO─CaO system at 1000°, 1020°, and 1050°C in air. Two ternary phases, La1.9 Ca1.1 Cu2 O5.9 and LaCa2 Cu3 O8.6 , were stable at these temperatures, with three binary phases, Ca2 CuO3 , CaCu2 O3 , and La2 CuO4 . La2 Cu2 O5 and La8 Cu7 O19 were stable only at 1020°C, and did not support solid-solution formation. 相似文献
11.
Bilge Saruhan Isaac Robin Abothu Sridhar Komarneni Chun-Keung Loong 《Journal of the American Ceramic Society》2000,83(12):3172-3178
Lanthanum hexaluminate (LaAl11 O18 ), 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 LaAl11 O18 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 LaAl11 O18 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 La1− x Al12− y O19− 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. 相似文献
12.
Wioletta Kuncewicz-Kupczyk Dietmar Kobertz Miroslaw Miller Christian Chatillon Lorenz Singheiser Klaus Hilpert 《Journal of the American Ceramic Society》2002,85(9):2299-2305
The vaporization of the samples of the compositions Ga2 O3 + LaGaO3 , LaGaO3 + La4 Ga2 O9 , and La4 Ga2 O9 + La2 O3 was investigated using Knudsen effusion mass spectrometry in the temperature range 1494–1937 K. The partial pressures of the gaseous species O2 , Ga, GaO, Ga2 O, 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 La4 Ga2 O9 ( 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. 相似文献
13.
Michael Rozumek Peter Majewski Heike Schluckwerder Fritz Aldinger Klaus Künstler Gerhard Tomandl 《Journal of the American Ceramic Society》2004,87(9):1795-1798
Ceramics of the melilite-type compound La1+ x Sr1− x Ga3 O7−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. La1+ x Sr1− x Ga3 O7−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. 相似文献
14.
La2 Ti2 O7 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. 相似文献
15.
Hitoshi Taimatsu Keiji Wada Hiroyuki Kaneko Hiroshi Yamamura 《Journal of the American Ceramic Society》1992,75(2):401-405
The reaction of La1- x Ca x MnO3 ( x = 0, 0.1, 0.2) with ZrO2 -8 mol% Y2 O3 (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 La2 Zr2 O7 is formed at the La1- x Ca x MnO3 /YSZ interface after an induction period, and its formation is accelerated when the La1- 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. 相似文献
16.
Kishori Deshpande Alexander Mukasyan Arvind Varma 《Journal of the American Ceramic Society》2003,86(7):1149-1154
An aqueous combustion synthesis is used to produce powders of La0.8 Sr0.2 CrO3 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. 相似文献
17.
Rajiv Kudesia Anna E. McHale Robert L. Snyder 《Journal of the American Ceramic Society》1994,77(12):3215-3220
Dielectric ceramics of Zr0.8 Sn0.2 TiO4 containing La2 O3 and ZnO as sintering aids were prepared and investigated for microstructure and microwave dielectric properties. Low-level doping with La2 O3 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 La2 O3 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. 相似文献
18.
Synthesis and Preparation of Lanthanum Aluminate Target for Radio-Frequency Magnetron Sputtering 总被引:1,自引:0,他引:1
Gun Yong Sung Kwang Yong Kang Sin-Chong Park 《Journal of the American Ceramic Society》1991,74(2):437-439
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 YBa2 Cu3 O7– x superconducting thin films on Si. Synthesis of lanthanum aluminate powder from a mixture of La2 O3 and Al2 O3 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. 相似文献
19.
Yusuke Kaminaga Hisanori Yamane Takahiro Yamada 《Journal of the American Ceramic Society》2007,90(6):1917-1920
Compounds in a CaO–Y2 O3 –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 , Ca2 SnO4 , Y2 Sn2 O7 , and a quaternary compound Ca0.4 Y1.2 Sn0.4 O3 , solid-solution series of Ca2− x Y2 x Sn1− x O4 with 0≤ x ≤0.5, and Ca1− y Y2 y Sn1− 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 Ca1− y Y2 y Sn1− y O3 suggested the existence of solid solutions of Ca0.4 Y1.2 Sn0.4 O3 , although their single phases could not be prepared, except at y =0.6. 相似文献
20.
Thin Yttrium-Stabilized Zirconia Electrolyte Solid Oxide Fuel Cells by Centrifugal Casting 总被引:4,自引:0,他引:4
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 La0.6 Sr0.4 Co0.2 Fe0.8 O3 (LSCF)-Ce0.9 Gd0.1 O1.95 (GDC) or La0.8 Sr0.2 MnO3 (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. 相似文献