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1.
The partial energies and entropies of O2in perovskite-type oxides La0.6Sr0.4Co1−yFeyO3−δ(y=0, 0.1, 0.25, 0.4, 0.6) were determined as a function of nonstoichiometryδby coulometric titration of oxygen in the temperature range 650–950°C. An absolute reference value ofδwas obtained by thermogravimetry in air. The nonstoichiometry at a given oxygen pressure and temperature decreases with iron contenty. At low nonstoichiometries the oxygen chemical potential decreases withδ. The observed behavior can be interpreted by assuming random distribution of oxygen vacancies, an electronic structure with both localized donor states on Fe, and a partially filled itinerant electron band, of which the density of states at the Fermi level scales with the Co content. The energy of the Fe states is close to the energy at the Fermi level in the conduction band. The observed trends of the thermodynamic quantities can be interpreted in terms of the itinerant electron model only when the iron content is small. At high values ofδthe chemical potential of O2becomes constant, indicating partial decomposition of the perovskite phase. The maximum value ofδat which the compositions are single-phase increases with temperature. 相似文献
2.
A phase-inversion/sintering technique has been employed in the production of La0.6Sr0.4Co0.2Fe0.8O3−α (LSCF) hollow fibre membranes, a bundle of which has then been placed in a high-temperature furnace for production of high purity oxygen from air at temperatures between 980 °C and 1060 °C. By applying a vacuum in the hollow fibre lumens, a product stream containing oxygen purity of 97.15% has been obtained. The downstream vacuum degree higher than 99 kPa shows negligible effect on the oxygen production rate. Studies on long-term operation suggest that the LSCF hollow fibre membranes are less stable for the oxygen production due to the segregation of the constituent membrane elements and the formation of new phases on the outer membrane surfaces. The effect of the operating cycle on the retrogression of membrane performance is much larger than that of duration used in a single cycle. 相似文献
3.
The partial energies and entropies of O2in perovskite-type oxides La0.6Sr0.4Co1−yFeyO3−δ(y=0, 0.1, 0.25, 0.4, 0.6) were determined as a function of nonstoichiometryδby coulometric titration of oxygen in the temperature range 650–950°C. An absolute reference value ofδwas obtained by thermogravimetry in air. The nonstoichiometry at a given oxygen pressure and temperature decreases with iron contenty. At low nonstoichiometries the oxygen chemical potential decreases withδ. The observed behavior can be interpreted by assuming random distribution of oxygen vacancies, an electronic structure with both localized donor states on Fe, and a partially filled itinerant electron band, of which the density of states at the Fermi level scales with the Co content. The energy of the Fe states is close to the energy at the Fermi level in the conduction band. The observed trends of the thermodynamic quantities can be interpreted in terms of the itinerant electron model only when the iron content is small. At high values ofδthe chemical potential of O2becomes constant, indicating partial decomposition of the perovskite phase. The maximum value ofδat which the compositions are single-phase increases with temperature. 相似文献
4.
Jung-Min Kim Gab-Jin Hwang Sang-Ho Lee Chu-Sik Park Jong-Won Kim Young-Ho Kim 《Journal of membrane science》2005,250(1-2):11-16
The oxygen separation membrane having perovskite structure for the partial oxidation of methane to synthesis gas was prepared. La0.7Sr0.3Ga0.6Fe0.4O3−δ (LSGF) perovskite membrane coated with La0.6Sr0.4CoO3−δ (LSC) (M1), and the one side of M1 membrane coated with NiO (M2) was prepared to examine the partial oxidation of methane. The single oxygen permeations of the LSC + LSGF (M1) membrane and NiO coated membrane (M2) were measured. The oxygen permeation flux in M1 membrane was higher than that of M1 membrane at 850 °C.
The partial oxidation experiment of methane using the prepared membranes was examined at 850 °C. The value of CH4 conversion and CO selectivity of M2 membrane was higher than that of M1 membrane.
NiO/NiAl2O4 catalyst was used to improve the methane conversion, and the partial oxidation experiment of methane with M1 membrane was examined at 850 °C. The CH4 conversion was 88%, and CO selectivity was 100%. 相似文献
5.
Jianxin Yi Yanbo Zuo Wei Liu Louis Winnubst Chusheng Chen 《Journal of membrane science》2006,280(1-2):849-855
A composite of oxygen ion conducting oxide Ce0.8Sm0.2O2−δ (60 vol.%) and electron conducting oxide La0.8Sr0.2CrO3−δ was prepared by sintering a powder compact at a temperature of 1550 °C. No significant reaction between the two constituent oxides was observed under preparation and oxygen permeation conditions. Appreciable oxygen permeation fluxes through the composite membrane were measured at elevated temperatures with one side of it exposed to the ambient air and the other side to a flowing helium gas stream. The oxygen flux initially increased with time, and took a long time to reach a steady value. A steady oxygen permeation flux as high as 1.4 × 10−7 mol cm−2 s−1 was obtained with a 0.3 mm thick membrane at 950 °C under a relatively small oxygen partial pressure difference of 0.21 bar/0.0092 bar. It was revealed that the overall oxygen permeation process was mainly limited by the transport in the bulk of the membrane in the range of the membrane thickness greater than 1.0 mm, and the limitation by the surface oxygen exchange came into play at reduced thickness of 0.6 mm. 相似文献
6.
Wenhui Yuan Xiaofang Hu Li Li College of Chemical Engineering Energy South China University of Technology Guangdong Key Laboratory of Green Chemicals Guangzhou China 《天然气化学杂志》2006,15(1):58-62
Single-phase perovskite La0.6Sr0.4Co0.8Fe0.2O3-δ has been successfully prepared by using citrate-EDTA complexation method at relatively low calcination temperature. The structure and thermal decomposition process of the complex precursor have been investigated by means of differential scanning calorimetry-thermal gravimetric analysis (DSC/TGA), X-ray diffraction (XRD), and Fourier transform infrared spectroscopic (FT-IR) measurements. The precursor decomposed completely and started to form perovskite-type oxide above 420℃ according to the differential scanning calorimetry (DSC) and thermal gravimetric analysis (TGA) results. Single-phase perovskite La0.6Sr0.4Co0.8Fe0.2O3-δ obtained has been confirmed from the XRD pattern, and no peak of SrCO3 was found by XR.D of the oxides synthesized at a relatively low temperature of 800 ℃. The reducibility of La0.6Sr0.4Co0.8Fe0.2O3-δ was also characterized by the temperature programmed reduction (TPR) technique. Disk shaped dense La0.6Sr0.4Co0.8Fe0.2O3-δ membrane was prepared by the isostatical pressing method. The oxygen flux rate of dense La0.6Sr0.4Co0.8Fe0.2O3-δ membrane was (2.8-18)×10-8 mol/(cm2·s) in the temperature range of 800-1 000℃. 相似文献
7.
The bulk superconducting YCa2Cu3O7−δ compounds are prepared at an ordinary pressure of oxygen by conventional solid-state reaction method. The formation of sample is tested by means of XRD and is studied for their ac susceptibility below room temperature up to 77.5 K. The samples are found single-phase orthorhombic structure and found superconducting at 83.5 K. It is shown that the analysis is consistent with published data on YBa2Cu3O7−δ oxide superconductor. 相似文献
8.
9.
Crystal structure, redox, and magnetic properties for the Pr1−xSrxFeO3−δ solid-solution phase have been studied. Oxidized samples (prepared in air at 900°C) crystallize in the GdFeO3-type structure for 0≤x≤0.80, and probably in the Sr8Fe8O23-type (unpublished) structure for x=0.90. Reduced samples (containing virtually only Fe3+) crystallize as the perovskite aristotype for x=0.50 and 0.67 with randomly distributed vacancies. The Fe4+ content increases linearly in the oxidized samples up to x≈0.70, whereupon it stabilizes at around 55%. Antiferromagnetic ordering of the G type is observed for oxidized samples (0≤x≤0.90) which show decreasing Néel temperature and ordered magnetic moment with increasing x, while the Néel temperature is nearly constant at 700 K for reduced samples. Electronic transitions for iron from an average-valence state via charge-separated to disproportionated states are proposed from anomalies in magnetic susceptibility curves in the temperature ranges 500–600 K and 150–185 K. 相似文献
10.
Rietveld refinement of combined X-ray and neutron diffraction data has, within errors, confirmed the stoichiometries of two, cubic pyrochlore phases in the ZnOBi2O3Sb2O5 system. Neither phase has the ‘ideal’ stoichiometry, Zn2Bi3Sb3O14. One phase, P1, is a Zn-rich, Bi-deficient solid solution Zn2+xBi2.96−(x−y)Sb3.04−yO14.04+δ. The other, P2, is a Bi-rich line phase, stoichiometry Zn2Bi3.08Sb2.92O14+δ. Both structures have a mixture of Bi, Zn on the A-sites and Zn, Sb on the B-sites. However, Zn is displaced off-centre in the A-sites to achieve lower co-ordination number with realistic ZnO bond lengths. Additional structural complexities arise from: displacement of O(2) atoms; partial occupancies of O(1) and O(2) sites; partial occupancy of a third, interstitial oxygen site, O(3). Since the multiplicities of the off-centre sites are much higher than those of the ideal positions, there is considerable possibility for correlated short range order throughout the structures. 相似文献
11.
Ba0.5Sr0.5Co0.8Fe0.2O3−δ (BSCF) hollow fibers were fabricated using a phase inversion/sintering method. As oxygen permeation of BSCF hollow fibers is controlled by the rate surface exchange kinetics, catalytic Ag particles were coated on both inner and outer surfaces using chemical deposition method, as verified by SEM and EDX. The Ag coated BSCF membranes showed up to 100% increase in oxygen permeation at 700 °C, and improvements lower than 10% were measured at 950 °C as compared with unmodified membranes. It was found that Ag catalyst surface loading was non-homogenous and concentrated on the perovskite grain boundaries. As a result, lighter Ag surface loading delivered improved oxygen flux while oxygen flux reached a maximum even though in the presence of excess catalyst loading. The catalytic activity of Ag was beneficial in enhancing surface reaction kinetics up to 850 °C attributed to the spillover effect. Above this temperature, the increase in oxygen permeation rate was marginally diminished due to the reduction of the spillover effect. 相似文献
12.
Mingya Cai Song Liu Konstantin Efimov Jürgen Caro Armin Feldhoff Haihui Wang 《Journal of membrane science》2009,343(1-2):90-96
A mixed proton–electron conducting perovskite made of BaCe0.95Nd0.05O3−δ (BCN) was prepared by EDTA/citric acid complexing method. The precursor was characterized by differential scanning calorimetry (DSC), thermogravimetry (TG), and X-ray diffraction (XRD). In order to learn the perovskite formation process during the calcination, the intermediate, i.e. the sample calcined at 750 °C for 5 h, was investigated by scanning (STEM), energy-filtered (EFTEM), and high-resolution transmission electron microscopy (HRTEM) as well as electron energy-loss spectroscopy (EELS). The results revealed that the perovskite structure was formed via a solid-state reaction between barium–cerium mixed carbonate and cerium–neodymium mixed oxide particles. Dense mixed proton–electron conducting BCN membranes were made by pressing BCN powder followed by sintering. The microstructure of the sintered membranes was investigated by scanning electron microscopy (SEM). Hydrogen permeation through the BCN membrane was studied using a high-temperature permeator. The hydrogen permeation fluxes under wet conditions are higher than those under dry conditions, which is due to increased proton concentrations in the H+ hopping via OH groups. The hydrogen permeation increased with increasing hydrogen and steam concentrations in the feed. For a steam concentration of 15 vol.%, the hydrogen permeation flux reaches 0.026 ml/min cm2. 相似文献
13.
Ian D. Fawcett Gabriel M. Veith Martha Greenblatt Mark Croft Israel Nowik 《Solid State Sciences》2000,2(8)
The SrMn1−xFexO3−δ (x=1/3, 1/2, 2/3) phases have been prepared and are shown by powder X-ray and neutron (for x=1/2) diffraction to adopt an ideal cubic perovskite structure with a disordered distribution of transition-metal cations over the six-coordinate B-site. Due to synthesis in air, the phases are oxygen deficient and formally contain both Fe3+ and Fe4+. Magnetic susceptibility data show an antiferromagnetic transition at 180 and 140 K for x=1/3 and 1/2, respectively and a spin-glass transition at 5, 25, 45 K for x=1/3, 1/2 and 2/3, respectively. The magnetic properties are explained in terms of super-exchange interactions between Mn4+, Fe(4+δ)+ and Fe(3+)+. The XAS results for the Mn-sites in these compounds indicate small Mn-valence changes, however, the Mn-pre-edge spectra indicate increased localization of the Mn-eg orbitals with Fe substitution. The Mössbauer results show the distinct two-site Fe(3+)+/Fe(4+δ)+ disproportionation in the Mn- substituted materials with strong covalency effects at both sites. This disproportionation is a very concrete reflection of a localization of the Fe-d states due to the Mn-substitution. 相似文献
14.
Y. Brard C. Michel M. Hervieu N. Nguyen F. Studer A. Maignan B. Raveau F. Boure 《Journal of solid state chemistry》2003,170(2):424-434
The possibility to synthesize layered oxycarbonates, with nominal composition Sr4Fe2−xMnxO6CO3 involving trivalent manganese, with 0≤x≤1.5, is reported for the first time. The structural study of Sr4FeMnO6CO3 using NPD, HREM, Mössbauer and XANES, shows that this phase is closely related to n=3 member of the Ruddlesden–Popper family. It derives from the latter by replacing the middle layer of transition metal octahedra by triangular CO3 groups, with two different “flag” and “coat hanger” configurations. The magnetic order is antiferromagnetic and fundamentally different from the magnetic behavior of Sr4Fe2O6CO3. 相似文献
15.
The growth of REBa2Cu3O7−δ (REBCO = rare earth elements) high-temperature superconducting thick films by liquid phase epitaxy is reviewed, which are most promising for electronic device and coated conductor applications. The paper focused on thermodynamic relations, chemical reactions and physical phenomena in the liquid phase epitaxy process, which are closely related to the control of the microstructures and properties of materials. Recent progresses achieved and the problems to be solved have been reviewed in above sections. 相似文献
16.
Shijing Zhan Xuefeng Zhu Baofeng Ji Weiping Wang Xiaoliang Zhang Jibo Wang Weishen Yang Liwu Lin 《Journal of membrane science》2009,340(1-2):241-248
Asymmetrical thin membranes of SrCe0.95Y0.05O3−δ (SCY) were prepared by a conventional and cost-effective dry pressing method. The substrate consisted of SCY, NiO and soluble starch (SS), and the top layer was the SCY. NiO was used as a pore former and soluble starch was used to control the shrinkage of the substrate to match that of the top layer. Crack-free asymmetrical thin membranes with thicknesses of about 50 μm and grain sizes of 5–10 μm were successfully pressed on to the substrates. Hydrogen permeation fluxes (JH2) of these thin membranes were measured under different operating conditions. At 950 °C, JH2 of the 50 μm SCY asymmetrical membrane towards a mixture of 80% H2/He was as high as 7.6 × 10−8 mol/cm2 s, which was about 7 times higher than that of the symmetrical membranes with a thickness of about 620 μm. The hydrogen permeation properties of SCY asymmetrical membranes were investigated and activation energies for hydrogen permeation fluxes were calculated. The slope of the relationship between the hydrogen permeation fluxes and the thickness of the membranes was −0.72, indicating that permeation in SCY asymmetric membranes was controlled by both bulk diffusion and surface reaction in the range investigated. 相似文献
17.
A synthetic route for advanced perovskites is elucidated that consists of a combined EDTA/citrate complexing of metal cations and the subsequent drying and firing at moderate temperatures. A fine-scale intermixing of cations is maintained during all the processing steps. The perovskite-type oxide is formed already in an intermediate step at 700 °C by the reaction of an ultra-finely dispersed powder consisting of a mixed barium–strontium carbonate, a zinc–iron spinel, and zinc oxide. The process yields powders of good sinterability and finally dense ceramics composed of stoichiometric perovskite grains of the type (Ba,Sr)(Zn,Fe)O3−δ. 相似文献
18.
Meng-Fei Luo Zong-Lan Yan Ling-Yun Jin 《Journal of molecular catalysis. A, Chemical》2006,260(1-2):157-162
A series of CexPr1−xO2−δ mixed oxides were synthesized by a sol–gel method and characterized by Raman, XRD and TPR techniques. The oxidation activity for CO, CH3OH and CH4 on these mixed oxides was investigated. When the value x was changed from 1.0 to 0.8, only a cubic phase CeO2 was observed. The samples were greatly crystallized in the range of the value x from 0.99 to 0.80, which is due to the formation of solid solutions caused by the complete insertion of Pr into the CeO2 crystal lattices. Raman bands at 465 and 1150 cm−1 in CexPr1−xO2−δ samples are attributed to the Raman active F2g mode of CeO2. The broad band at around 570 cm−1 in the region of 0.3 ≤ x ≤ 0.99 can be linked to oxygen vacancies. The new band at 195 cm−1 may be ascribed to the asymmetric vibration caused by the formation of oxygen vacancies. The TPR profile of Pr6O11 shows two reduction peaks and the reduction process is followed: . The reduction temperature of CexPr1−xO2−δ mixed oxides is lower than those of Pr6O11 or CeO2. TPR results indicate that CexPr1−xO2−δ mixed oxides have higher redox properties because of the formation of CexPr1−xO2−δ solid solutions. The presence of the oxygen vacancies favors CO and CH3OH oxidation, while the activity of CH4 oxidation is mostly related to reduction temperatures and redox properties. 相似文献
19.
Among the perovskites, the rare earth manganites find application in several electrochemical devices because of their enhanced thermodynamic stability. In this paper, we present the results obtained on the preparation and characterization of La0.95MnO3+δ and Sm0.95MnO3+δ which were prepared by the solid state and sol–gel methods. XRD characterization of the manganites indicated that the crystal structure depends on the method of preparation and heat treatments. The ratio of Mn3+ to Mn4+ in these samples also depended on the method of preparation and heat treatments, as indicated by thermogravimetric (TG) and temperature programmed reduction (TPR) studies in Ar + 5% H2 atmosphere. The standard molar enthalpy of formation, which is a measure of the thermodynamic stability of these compounds were determined using an isoperibol calorimeter. 相似文献
20.
The space group symmetry and crystal structure of Tl3SbS3−xSex compounds in the composition range 0 < x < 3 have been determined by a combination of powder X-ray diffraction, electron diffraction, and high-resolution electron microscopy. The incongruently melting compound Tl3SbSe3 has been shown to crystallize in cubic space group P213 with a = 9.435Å in a structure related to that of Langbeinite. The convergent beam electron diffraction pattern of Tl3SbS3 is in accord with the space group R3m determined by X-ray diffraction. The cubic Langbeinite-type structure is found for Tl3SbS3−xSex for 0.5 < x < 3 and for Tl3SbyAs1−ySe3 for 0.077 < y < 1.0. A five-component compound Tl3Sb0.5As0.5Se1.5S1.5 was also found to be cubic. 相似文献