共查询到20条相似文献,搜索用时 31 毫秒
1.
Anita Fossdal Mari-Ann Einarsrud Tor Grande 《Journal of the American Ceramic Society》2005,88(7):1988-1991
The phase relations in the pseudo-ternary system La2 O3 –SrO–Fe2 O3 have been investigated in air. Isothermal sections at 1100° and 1300°C are presented based on X-ray diffraction and thermal analysis of annealed samples. Extended solid solubility was observed for the compounds Sr n +1− v La v Fe n O3 n +1−δ ( n =1, 2, 3, and ∞) and Sr1− x La x Fe12 O19 , while only limited solubility of La in Sr4− z La z Fe6 O13±δ was observed. At high Fe2 O3 content, a liquid with low La2 O3 content was stable at 1300°C. 相似文献
2.
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
3.
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
4.
Rejini Rajamma Mailadil Thomas Sebastian 《Journal of the American Ceramic Society》2007,90(8):2472-2475
The microwave dielectric properties of two A-site-deficient perovskite-type ceramics in the La6 Mg4 A2 W2 O24 [A=Ta and Nb] system were investigated. The compounds were synthesized by the solid-state ceramic route. The structure and microstructure were analyzed using X-ray diffraction and scanning electron microscopy techniques. The dielectric properties were measured in the microwave frequency range [4–6 GHz] by the resonance method. La6 Mg4 Ta2 W2 O24 had Q u × f =13 600 GHz, ɛr =25.2, and τf =−45 ppm/°C and La6 Mg4 Nb2 W2 O24 had Q u × f =16 400 GHz, ɛr =25.8, and τf =−56 ppm/°C. 相似文献
5.
Riham Michelle Morcos Jean Tangeman Sergey Ushakov Alexandra Navrotsky 《Journal of the American Ceramic Society》2008,91(4):1088-1094
A series of La2 O3 –HfO2 –SiO2 glasses, approximately along the join 0.73SiO2 –0.27( x HfO2 –(1− x )La2 O3 ), 0< x <0.3), was prepared using containerless processing techniques (aerodynamic levitation combined with laser heating in oxygen). The enthalpy of formation and enthalpy of vitrification at 25°C were obtained from drop solution calorimetry of these glasses and appropriate crystalline compounds in a molten lead borate (2PbO–B2 O3 ) solvent at 702°C. The enthalpy of formation from crystalline oxides was exothermic and became less exothermic with increasing HfO2 content. Heat contents were measured by transposed temperature drop calorimetry and depended linearly on the HfO2 content. Differential scanning calorimetry showed that both the onset glass transition and the onset crystallization temperature of these glasses increased with increasing HfO2 content. Upon slow cooling in air, the glasses crystallized to a mixture of baddeleyite, cristobalite, lanthanum disilicate, and hafnon. 相似文献
6.
Zhong Zhuang Xian Ping Wang Dan Li Tao Zhang Qian Feng Fang 《Journal of the American Ceramic Society》2009,92(4):839-844
Dense, crack-free, and uniform La2 Mo2− x W x O9 ( x =0, 0.1, and 0.2) nanocrystalline films were successfully synthesized on poly-alumina substrates via a modified sol–gel method, with inorganic salt of La(NO3 )3 ·6H2 O, (NH4 )6 Mo7 O24 ·4H2 O, and (NH4 )6 H2 W12 O24 as precursors. Pure La2 Mo2 O9 phase was confirmed by X-ray diffractometer when the annealing temperature was >500°C. The average grain size of the La2 Mo2− x W x O9 films is in the range of 90–400 nm, depending upon the conditions of thermal treatment, and the thickness of films can reach 1 μm by repetitive spin-coating. The electrical conductivity increases with decreasing grain size and reaches 0.074 S/cm at 600°C in the film with a grain size of 90 nm, which is one order of magnitude higher than that in the corresponding bulk materials. W-doping can suppress the phase transition that occurs at 580°C in pure La2 Mo2 O9 and enhance the low-temperature ionic conductivity. Furthermore, the activation energy of conductivity in the nanocrystalline La2 Mo2 O9 films decreases to about 0.6 eV in comparison with 1.0 eV in the bulk ones, which implies that the grain resistance prevails in the total resistance, when grain size reduces to nanometer domain. 相似文献
7.
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
8.
Maria J. Santillán Alberto Caneiro Francisco C. Lovey Nancy Quaranta Aldo R. Boccaccini 《International Journal of Applied Ceramic Technology》2010,7(1):30-40
Carbon nanotubes (CNTs)/La0.6 Sr0.4 Co0.8 Fe0.2 O3−δ (LSCF) composite films have been fabricated by electrophoretic codeposition on Ce0.9 Gd0.1 O1.95 (CGO) substrates. CNTs are used as a sacrificial phase to produce ordered porous LSCF cathodes for intermediate temperature solid oxide fuel cells. The synthesis of LSCF powder by a modified sol–gel route is presented. The possible mechanism of formation of CNT/LSCF composite nanoparticles in suspension is discussed. Moreover the optimal suspension composition and the conditions for achieving successful electrophoretic deposition (EPD) of CNTs/LSCF composite nanoparticles were evaluated. Experimental results showed that the CNTs were homogeneously distributed and mixed with LSCF nanoparticles forming a mesh-like structure, which resulted in a highly porous LSCF film when the CNTs were burned out during heat treatment in air at 800°C for 2 h. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray diffraction (XRD) techniques were employed to characterize the microstructure of the precursors and of the composite films. 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.
Guangxiu Cao Yaogang Li Qinghong Zhang Hongzhi Wang 《Journal of the American Ceramic Society》2010,93(1):25-27
La-doped TiO2− x F x (La–TiO2− x F x ) powders were prepared by the sol–gel method. X-ray diffraction results showed that La efficiently inhibited grain growth. X-ray photoelectron spectroscopy spectra revealed that La2 O3 and O–Ti–F bonds have formed, the La2 O3 maintained the high surface area of TiO2− x F x after calcination at a temperature above 500°C, while the O–Ti–F bonds increased the oxidation potential of the photogenerated hole in the valence band. The UV-vis spectroscopy of the La–TiO2− x F x showed that the presence of intraband gap states was likely responsible for its absorption of visible light. When the molar ratios of La and F to Ti were 1.5:100 and 5:100, respectively, and calcined at 500°C, the photocatalytic degradation rate of methylene blue over La–TiO2− x F x was about 1.5 times higher than that of F-doping TiO2 . 相似文献
11.
MADELEINE H. WARZEE MARCEL MAURICE FRANZ HALLA WALTER R. RUSTON 《Journal of the American Ceramic Society》1965,48(1):15-17
In the binary system PbO–Laz O3 only one compound, 4PbO.La2 O3 , exists; it is flanked by two eutectics. The structure of the compound, although of lower symmetry, is intimately related to the C modification of the rare earths. Below 800° to 1000°C, metastable solid solutions are formed from oxide mixtures coprecipitated from mixed solutions of the nitrates, the cubic parameter a = 5.66 A, if extrapolated to pure La2 O3 , corresponding to half the a parameter of the C form of La2 O3 . The solid solutions existing between the compositions La2 O3 –2Pb0 and pure La2 O3 have a cubic face–centered lattice and obey Vegard's rule. The systems of PbO with Sm2 O3 and Gd2 O8 are quite similar to that with La2 O3 . The compound Sm2 O3 .4Pb0 decomposes at 1000°C with evaporation of PbO; Sm2 O3 remains in the B modification. 相似文献
12.
Naitao Yang Xiaoyao Tan Zifeng Ma Alan Thursfield 《Journal of the American Ceramic Society》2009,92(11):2544-2550
Samaria-doped ceria (SDC, Ce0.8 Sm0.2 O1.9 ) ceramic powders of submicrometer size were synthesized by a sol–gel auto-combustion method. From these powders microtubes with a dual structure comprising of a dense layer and a porous substrate layer were fabricated in a single step through a phase inversion/sintering technique. A sintering temperature in excess of 1450°C is required for SDC to achieve gastight microtubes. The mechanical strength of the SDC microtubes increases with increasing sintering temperature and may attain up to 208 MPa when sintered at 1500°C. Electrical impedance spectroscopy studies indicate that the SDC microtubes have electrical conductivities of 4.46 × 10−4 –0.072 S/cm and corresponding activation energy of 81.9 kJ/mol at temperatures between 400° and 800°C. Full fuel cells were fabricated by coating Ba0.5 Sr0.5 Co0.8 Fe0.2 O3−δ (BSCF) on to the inner surface and a Ni-SDC cermet on to the outer surface of the gastight microtubes to act as the cathode and the anode, respectively. The resultant BSCF|SDC|Ni-SDC microcells have a stable output maximum of 106 mW/cm2 at 750°C when hydrogen and air were used as fuel and oxidant gas, respectively. 相似文献
13.
An all-alkoxide route to films and nano-phase powders of the La0.5 Sr0.5 CoO3 perovskite is described. To our knowledge, this is the first purely alkoxide-based route to (La1− 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, αr =59.94°. Ninety to 130 nm thick films of La0.5 Sr0.5 CoO3 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. 相似文献
14.
Phase relations at liquidus temperatures in the system La2 O3 -TiO2 were studied in air. The existence of two previously unreported compounds, La2 O3 . TiO2 and 2La2 O3 -TiO2 , is postulated on the basis of X-ray and microscopic examination of crystalline samples, and in the case of La2 O3 .-TiO2 , on a maximum in the liquidus curve at that composition. Quenched liquids of the primary-phase field of rutile were found to be semiconducting. This property was related to oxygen loss from both liquid and crystalline phases and is discussed in the light of weight loss experiments, microscopic examination of quenched samples, and information obtained from the literature. Dielectric constant and loss factor of the compounds La2 O3 -TiO2 , La2 O3 -2TiO2 , and 2La2 O3 -9TiO2 are reported at 1 Mc over the temperature range 25° to 500°C. 相似文献
15.
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
16.
Rajamma Rejini Mailadil Thomas Sebastian 《International Journal of Applied Ceramic Technology》2006,3(3):230-235
The La5 CrTi3 O15 and La4 MCrTi3 O15 (M=Pr, Nd, and Sm) microwave dielectric ceramics were prepared by the conventional solid-state ceramic route. The structure and microstructure of the ceramics were studied by X-ray diffraction and scanning electron microscopy methods. The dielectric properties of the ceramics were measured in the microwave frequency region using a network analyzer by the resonance method. The ceramics show a dielectric constant (ɛr ) in the range of 37 to 39.5, a quality factor ( Q u × f o ) 17,300 to 34,000 GHz, and a temperature coefficient of resonant frequency (τf ) in the range from −22 to −38 ppm/°C. 相似文献
17.
Xiuying Guo Ning Zhu Mi Xiao Xiawan Wu 《Journal of the American Ceramic Society》2007,90(8):2467-2471
Sb2 O5 were selected to substitute (Nb0.8 Ta0.2 )2 O5 and the effects of Sb substitution on the dielectric properties of Ag(Nb0.8 Ta0.2 )O3 ceramics were studied. The perovskite Ag(Nb0.8 Ta0.2 )1− x Sb x O3 ceramics showed no obvious change with x value being no more than 0.08, and the pseudoperovskite unit cell parameters a = c , b and monoclinic angle β decrease with Sb concentration increasing. The dielectric properties of Ag(Nb0.8 Ta0.2 )1− x Sb x O3 ceramics were found to be affected greatly by the substitution of Sb for Nb/Ta. The ɛ value of Ag(Nb0.8 Ta0.2 )1− x Sb x O3 ceramics sintered at their densified temperature increased from 480 to 825 with x from 0 to 0.08, the tan δ value decreased sharply from 0.0065 to 0.0023 (at 1 MHz) with x increasing from 0 to 0.04, and then kept a stable lower tan δ value ∼0.0024 with x to 0.08. The temperature coefficient of capacitance values continuously decreased from a positive value of 1450 ppm/°C for x =0 to a negative value of −38.52 ppm/°C for x =0.08. 相似文献
18.
Hilde Lea Lein Kjell Wiik Mari-ann Einarsrud Tor Grande Edgar Lara-curzio 《Journal of the American Ceramic Society》2006,89(9):2895-2898
Steady-state compressive creep rate of La0.5 Sr0.5 Fe0.5 Co0.5 O3−δ (LSFC) and La0.5 Sr0.5 CoO3−δ (LSC) is reported in the temperature region 900°–1050°C and stress range 5–28 MPa. The stress exponents for the two materials were 1.71±0.18 and 1.24±0.15, respectively. The activation energy for creep was considerably higher for LSC (619±56 kJ/mol) than for LSFC (392±28 kJ/mol). The grain size exponent for LSC was 1.28±0.14. Considerably higher creep rates were observed for both materials in N2 compared with air. Relaxation by creep of chemical-induced stresses in oxygen-permeable membranes is addressed, especially at low partial pressure of oxygen. 相似文献
19.
Danjela Kuer Darko Hanel Janez Holc Marko Hrovat Drago Kolar 《Journal of the American Ceramic Society》2001,84(5):1148-1154
La1− y Sr y Fe1− x Al x O3−δ perovskites were studied as potential materials for solid-oxide fuel cell (SOFC) cathodes. The phase relations in the LaFeO3 –SrFeO3−δ –LaAlO3 system were investigated by X-ray powder diffraction analysis. The defect structure of the La1− y Sr y Fe1− x Al x O3−δ perovskites was investigated by Mössbauer spectroscopy and weight-loss analysis. Relations between the nonstoichiometry and the conductivity of the La1− y Sr y Fe1− x Al x O3−δ perovskites were investigated. The incorporation of aluminum ( x ) into LaFe1− x Alx O3 was found to have no influence on the defect structure but to decrease the conductivity. The incorporation of strontium ( y ) into La1− y Sr y Fe1− x Al x O3−δ promotes the formation of anion vacancies and Fe4+ that lead to higher conductivity. 相似文献
20.
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