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1.
The solubility of Ti4+ in the lattice of apatite-type La9.83Si6−xTixO26.75 corresponds to approximately 28% of the Si-site density. The conductivity of La9.83Si6−xTixO26.75 (x = 1–2) is predominantly oxygen-ionic and independent of the oxygen partial pressure in the p(O2) range from 10−20 to 0.3 atm. The electron transference numbers determined by the modified faradaic efficiency technique are lower than 0.006 at 900–950 °C in air. The open-circuit voltage of oxygen concentration cells with Ti-doped silicate electrolytes is close to the theoretical Nernst value both under oxygen/air and air/10%H2–90%N2 gradients at 700–950 °C, suggesting the stabilization of Ti4+ in the apatite structure. Titanium addition in La9.83Si6−xTixO26.75 (x = 1–2) leads to decreasing ionic conductivity and increasing activation energies from 93 to 137 kJ/mol, and enhanced degradation in reducing atmospheres due to SiO volatilization. At p(O2) = 10−20 atm and 1223 K, the conductivity decrease after 100 h was about 5% for x = 1 and 17% for x = 2. The solubility of Zr4+ in the La9.83Si6−xZrxO26.75 system was found to be negligible, while the maximum concentration of Ce4+ in La9.4−xCexSi6O27−δ is approximately 5% with respect to the number of lanthanum sites.  相似文献   

2.
The oxygen permeability of mixed-conducting Sr1−xCaxFe1−yAlyO3−δ (x=0–1.0; y=0.3–0.5) ceramics at 850–1000 °C, with an apparent activation energy of 120–206 kJ/mol, is mainly limited by the bulk ionic conduction. When the membrane thickness is 1.0 mm, the oxygen permeation fluxes under pO2 gradient of 0.21/0.021 atm vary from 3.7×10−10 mol s−1 cm−2 to 1.5×10−7 mol s−1 cm−2 at 950 °C. The maximum solubility of Al3+ cations in the perovskite lattice of SrFe1−yAlyO3−δ is approximately 40%, whilst the brownmillerite-type solid solution formation range in Sr1−xCaxFe0.5Al0.5O3−δ system corresponds to x>0.75. The oxygen ionic conductivity of SrFeO3-based perovskites decreases moderately on Al doping, but is 100–300 times higher than that of brownmillerites derived from CaFe0.5Al0.5O2.5+δ. Temperature-activated character and relatively low values of hole mobility in SrFe0.7Al0.3O3−δ, estimated from the total conductivity and Seebeck coefficient data, suggest a small-polaron mechanism of p-type electronic conduction under oxidising conditions. Reducing oxygen partial pressure results in increasing ionic conductivity and in the transition from dominant p- to n-type electronic transport, followed by decomposition. The low-pO2 stability limits of Sr1−xCaxFe1−yAlyO3−δ seem essentially independent of composition, varying between that of LaFeO3−δ and the Fe/Fe1−γO boundary. Thermal expansion coefficients of Sr1−xCaxFe1−yAlyO3−δ ceramics in air are 9×10−6 K−1 to 16×10−6 K−1 at 100–650 °C and 12×10−6 K−1 to 24×10−6 K−1 at 650–950 °C. Doping of SrFe1−yAlyO3−δ with aluminum decreases thermal expansion due to decreasing oxygen nonstoichiometry variations.  相似文献   

3.
Rare earth silicates with apatite structure are being actively studied as an alternative electrolyte material for solid oxide fuel cells (SOFC) operating in the intermediate temperature range. In this paper we report on the synthesis of La9.33+x/3AlxSi6?xO26+δ (with x = 0–1.5) and La9.83Fe1.5Si4.5O26 powders using a modified sol–gel process. The parameters involved in the process have been optimized for preparing phase pure, homogeneous and nanometer sized powders. The obtained powders were characterized using scanning electron microscopy, X-ray diffraction and thermal analysis. Pressureless sintering experiments were performed and pellets having relative densities greater than 96% could be obtained after 5 h dwelling in the temperature range between 1500 and 1550 °C.  相似文献   

4.
Perovskites La1−xCaxAlyFe1−yO3−δ (x, y = 0 to 1) were prepared by high-temperature solid-state synthesis based on mixtures of oxides produced by colloidal milling. The XRD analysis showed that perovskites La0.5Ca0.5AlyFe1−yO3−δ with a high Fe content (1  y = 0.8–1.0) were of orthorhombic structure, perovskites with a medium Fe content (1  y = 0.8–0.5) were of rhombohedral structure, and perovskite with the lowest Fe content (1  y = 0.2) were of cubic structure. Thermally programmed desorption (TPD) of oxygen revealed that chemical desorption of oxygen in the temperature range from 200 to 1000 °C had proceeded in the two desorption peaks. The low-temperature α-peak (in the 200–550 °C temperature range) was brought about by oxygen liberated from oxygen vacancies; the high-temperature β-peak (in the 550–1000 °C temperature range) corresponded to the reduction of Fe4+ to Fe3+. The chemidesorption oxygen capacity increased with increasing Ca content and decreased with increasing Al content in the perovskites. The Al3+ ions restricted, probably for kinetic reasons, the reduction of Fe4+ and the high-temperature oxygen desorption associated with it.  相似文献   

5.
The maximum solubility of aluminum cations in the perovskite lattice of Sr0.7Ce0.3Mn1−xAlxO3−δ is approximately 15%. The incorporation of Al3+ increases oxygen ionic transport due to increasing oxygen nonstoichiometry, and decreases the tetragonal unit cell volume and thermal expansion at temperatures above 600 °C. The total conductivity of Sr0.7Ce0.3Mn1−xAlxO3−δ (x = 0–0.2), predominantly electronic, decreases with aluminum additions and has an activation energy of 10.2–10.9 kJ/mol at 350–850 °C. Analysis of the electronic conduction and Seebeck coefficient of Sr0.7Ce0.3Mn0.9Al0.1O3−δ, measured in the oxygen partial pressure range from 10−18 to 0.5 atm at 700–950 °C, revealed trends characteristic of broad-band semiconductors, such as temperature-independent mobility. The temperature dependence of the charge carrier concentration is weak, but exhibits a tendency to thermal excitation, whilst oxygen losses from the lattice have an opposite effect. The role of the latter factor becomes significant at temperatures above 800 °C and on reducing p(O2) below 10−4 to 10−2 atm. The oxygen permeability of dense Sr0.7Ce0.3Mn1−xAlxO3−δ (x = 0–0.2) membranes, limited by both bulk ionic conduction and surface exchange, is substantially higher than that of (La, Sr)MnO3-based materials used for solid oxide fuel cell cathodes. The average thermal expansion coefficients of Sr0.7Ce0.3Mn1−xAlxO3−δ ceramics in air are (10.8–11.8) × 10−6 K−1.  相似文献   

6.
《Ceramics International》2017,43(7):5585-5591
Two series of single-phased LaxSr(2−x)Fe(1+y)Mo(1−y)O6 and LaxSr(2−x)Fe(1+0.5y)Al0.5yMo(1−y)O6 (x=3y, y=0.05, 0.1, 0.15 and 0.2) double perovskites were prepared by solid-state reaction. The effects of Al-substitution on the structures, resistivity and magnetic properties of LaxSr(2−x)Fe(1+y)Mo(1−y)O6 were investigated. Although Al-replacement exhibits a negligible influence of on the B-site ordering degree, it results in the suppression of magnetisation caused by non-magnetic Al3+ ions. Reduction of grain sizes leads to increased resistivity, thus an optimised magnetoresistance (MR) behaviour is observed. The greatest MR extent improvement can be obtained when y is 0.15 and the MR% of the Al-doped ceramics reaches −10.5% (10 K, 1 T), which is 2 times greater than that of the undoped ceramics (−4.6%, 10 K, 1 T). Interestingly, the Curie temperature (Tc) of both Al-doped and undoped samples maintained relatively constant values of approximately 420 K and 405 K, respectively, which were different results from the data obtained for similar electron-doping systems in the literature.  相似文献   

7.
The intermediate temperature electrolytes La1?xSrxGa1?yMgyO3?δ (LSGM, where δ = (x + y)/2) with perovskite structure were prepared using a poly(vinyl alcohol) (PVA) solution polymerization method. Three secondary phases were identified by X-ray diffraction, LaSrGaO4, LaSrGa3O7 and La4Ga2O9. The relative amount of these secondary phases depended on the doping compositions. Sr doping produced more Sr rich secondary phases with increasing content, while enhanced solid solubility was observed with Mg addition. Sintered samples showed dense microstructures with well-developed equiaxed grains, and the secondary phases were mainly in the grain boundaries. LaSrGaO4 could not be detected by SEM for the sintered pellets. The oxygen ionic conductivity was enhanced by doping with Sr and Mg. Mg doping showed the increased conductivity activation energy. La0.8Sr0.2Ga0.9Mg0.1O2.85 had the highest ionic conductivity σ = 0.128 S/cm at 800 °C in this work.  相似文献   

8.
A comparison between theoretically calculated unit cell volume and interatomic distances in the system La0.7Sr0.3Mn1−xMexO3+δ (where Me = Cu, Fe, Cr, Ti) and the experimental data obtained by the full-profile Rietveld X-ray analysis as well as an analysis of magnetic properties allowed us to suggest possible mechanisms of charge compensation occurring when d metals substitute for manganese. It has been shown that in the case when copper, iron, chromium and titanium ions substitute for manganese ions in the system La0.7Sr0.3Mn1−xMexO3 charge compensation is described by the model 2Mn3+  Mn4+ + Cu2+, Mn3+  Fe3+, Mn3+  Cr3+ and Mn4+  Ti4+, respectively. In the latter case, a decrease in oxygen nonstoichiometry occurs with increasing x.  相似文献   

9.
Complex perovskites with large-sized B′-site cations, (Sr1−xBax)(Sr0·33+yTa0·67-y)O3−δ (0  x  1, 0  y≦ 0·17), were fabricated to examine the transformation behavior between the (1:1) and (1:2) order types and dielectric property change with A-site cation substitution. The (1:2) type order was found to appear in a limited Ba-rich composition range. Structural strain induced by size difference between the A-site and large-sized B′-site cations might be responsible for the occurrence of the (1:1) type order in perovskite compounds substituted with more than 50% Sr2+ on the A-site sublattice. Low εr and positive temperature coefficient of εr observed for the (1:2) ordered samples were explained by the restricted ion movement in the (1:2) type order array.  相似文献   

10.
《Ceramics International》2017,43(15):12145-12153
LaxSr1−xTiO3 (LST) nanofibers with pure perovskite structure, smooth surface, uniform diameter and length are prepared by electrospinning technique, and applied as scaffolds of LaxSr1−xTiO3-GdyCe1−yO2−δ (LST-GDC) composite anodes for SOFCs. The optimal La doping ratio of LST scaffold has been found to be 0.4, and 0.2 the optimal Gd doping ratio of GDC impregnation phase. The LST:GDC optimal mass ratio of nanofiber-based composite anode has been found to be 1:1.5481, and the composite anode (electrolyte is yttria-stabilized zirconia) to show low interfacial polarization resistances of 0.7309, 0.4688 and 0.2966 Ω cm2 at 800, 850 and 900 °C, respectively. In addition, the microstructure of LST materials has been found to plays an important role on the electrochemical performance of the anodes, and the LST nanofiber scaffolds to show the higher porosity leading to a larger triple phase (ionic conduction phase, electronic conduction phase and gas phase) boundary (TPB) area for the composite anodes.  相似文献   

11.
Apatite-type silicates are considered as promising electrolytes for solid oxide fuel cells (SOFC). However more studies on the chemical compatibility of these materials with common SOFC electrodes are required. Here, we report the synthesis of single phase La9Sr1Si6O26.5 composition by reactive sintering at 1650 °C for 10 h. Fully dense pellets showed very high oxide-anion conductivity, 25 mS cm?1 at 700 °C. Furthermore, the chemical compatibility of La9Sr1Si6O26.5 with some selected cathode materials has also been investigated. The lowest reaction temperatures were determined to be 1100 °C, 1000 °C and 900 °C for La0.8Sr0.2MnO3?δ, La2Ni0.8Cu0.2O4 and La0.6Sr0.4Co0.8Fe0.2O3, respectively. The segregation of minor amounts of SiO2 seems to be a key limiting factor that must be overcome. Finally, these cathode materials were deposited over dense oxy-apatite pellets and the area specific resistances in symmetrical cells were determined. These values, at 700 °C, were 14.4 and 2.6 Ω cm2 for La0.8Sr0.2MnO3?δ and La0.6Sr0.4Co0.8Fe0.2O3?δ, respectively. Furthermore, the area specific resistances are notably improved 0.6 Ω cm2 when a 50 wt.% composite of La0.6Sr0.4Co0.8Fe0.2O3?δ and Ce0.8Gd0.2O1.9 is used.  相似文献   

12.
《Ceramics International》2017,43(12):8715-8720
Electrospun LaxSr1−xCo1−yFeyO3 (LSCF) fibers with y=0.2 – 1.0 have been investigated as the cathode of intermediate solid oxide fuel cells (IT-SOFC). The electrochemical performances of LSCF (y=0.2–1.0) fibers were studied by impedance spectroscopy in symmetrical cells containing gadolinium doped ceria (CGO) electrolyte and LSCF electrode infiltrated with CGO. Impedance measurements showed that the impedance spectra have two or three semicircles, depending on the measurement temperature. The LSCF electrodes with higher cobalt content exhibit lower polarization resistance (Rp) and the La0.6Sr0.4Co0.8Fe0.2O3 electrode displayed the lowest polarization resistance between 500 and 900 °C, classifying this composite cathode as a promising material for intermediate temperature SOFC based on CGO electrolyte.  相似文献   

13.
Synthesis of X-ray-phase-pure (La1−xSrxGa1−yMgyO3−δ, LSGM, where x = 0.1, y = 0.15 and 0.17) powders were achieved at temperatures as low as 1350 °C via organic precursor method using tartaric acid as the carrier material. LSGM materials were characterized for their phase purity, crystallization and electrical properties. Pellets sintered at 1350 °C for 6 h were single phase and dense (>99%). Electron microscopy analysis of X-ray single-phase pellets revealed MgO precipitates with sizes ranging from 50–300 nm. Phase formation and distribution in this complicated multi-cation-oxide system as a function of temperature were reported and discussed. Amorphous LSGM first crystallizes at 625 °C. However, elimination of undesired phases require higher temperatures. Impedance measurements as a function of temperature up to 545 °C revealed that the X-ray phase pure pellets may have extrapolated ionic conductivity values as high as 0.14–0.16 S/cm at 800 °C. Possible implications of limited MgO solubility on the ionic conductivity are presented.  相似文献   

14.
《Ceramics International》2016,42(5):5995-5999
In this paper, a series of novel luminescent Sr1−xAl12O19:xEu2+ phosphors were synthesized by a high temperature solid-state reaction. The phase structure, photoluminescence (PL) properties, as well as the decay curves were investigated. The quenching concentration of Eu2+ in SrAl12O19 was about 0.15 (mol). Upon excitation at 378 nm, the composition-optimized Sr0.85Al12O19:0.15Eu2+ exhibited strong broad-band green emission at 530 nm with the CIE chromaticity (0.2917, 0.5736). The results indicate that Sr1−xAl12O19:xEu2+ phosphors have potential applications as green-emitting phosphors for UV-pumped white-light LEDs.  相似文献   

15.
《Ceramics International》2015,41(7):8931-8935
The densification, microstructural evolution and microwave dielectric properties of (Ba1−xSrx)(Mg0.5W0.5)O3 ceramics with x=0, 0.25, 0.5 and 0.75 are investigated in this study. The sintering temperature of the (Ba1−xSrx)(Mg0.5W0.5)O3 is significantly reduced from 1575 °C to 1400 °C as the x value increases from 0 to 0.25 and 0.50; this result is accompanied by the formation of the (Ba1−ySry)WO4 phase and a small quantity of second phase surrounding the grains. The grain size of the (Ba1−xSrx)(Mg0.5W0.5)O3 ceramics is increased by raising the Sr2+ content, which significantly lowers the sintering temperature. The microstructure of the (Ba0.75Sr0.25)(Mg0.5W0.5)O3 ceramic displays the smallest average grain size of approximately 0.8 μm, with a narrow grain size distribution. Without long annealing time, very high Q×f values are obtained for the (Ba1−xSrx)(Mg0.5W0.5)O3 ceramics sintered at 1400–1575 °C for a duration of only 2 h. The (Ba0.75Sr0.25)(Mg0.5W0.5)O3 ceramic sintered at 1400 °C results in the best microwave dielectric properties, including εr of 20.6, Q×f of 152,600 GHz and τf of +24.0 ppm/°C.  相似文献   

16.
Tubular membranes of La0.6Ca0.4Fe0.75Co0.25O3−δ and La0.5Sr0.5Fe1−yTiyO3−δ (y = 0, 0.2) for the application of partial oxidation of methane to syngas were produced by thermoplastic extrusion and investigated by oxygen permeation measurements. The optimum ceramic content in the feedstock for extrusion was found to be 51 vol% as a result of rheology measurements. Tubes with an outer diameter of 4.8–5.5 mm and thickness of 0.25–0.47 mm were produced with densities higher than 95% of the theoretical density. The oxygen permeation flux of the tubular membranes was measured with air on one side and Ar or Ar + CH4 mixture on the other side. The oxygen permeation rate decreased with Ti-substitution while it was considerably increased by introduction of 5% methane into the system. The normalized oxygen fluxes in air/Ar gradient at 900 °C were measured to be 0.06, 0.051, and 0.012 μmol cm−2 s−1 for LCFC, LSF, and LSFT2, respectively, and 0.18 μmol cm−2 s−1 for LSFT2 with 5% methane.  相似文献   

17.
The environmental impacts of nitrous oxide (N2O) have received much attention, including contributions to the greenhouse effect and ozone depletion. Currently, the direct catalytic decomposition of N2O is considered to be the simplest and most promising method for N2O abatement. In this study, we focused on the high activity of rhodium and the oxide-ion conducting property of lanthanum silicate and prepared novel Rh/La10Si6  xFexO27  δ catalysts. From the results of catalytic N2O decomposition activities, Rh/La10Si6  xFexO27  δ (x = 1.0) exhibited the highest catalytic activity and N2O was completely decomposed at 600 °C.  相似文献   

18.
La0.85Sr0.15Ga0.8Mg0.2O3?δ pellets obtained by the polymeric organic complex solution method, isostatic pressing and sintering at 1350 °C have been electrical and mechanically studied. Electrical measurements evidenced reasonable ionic conductivities (0.01 S cm?1 at 800 °C), which were comparable to those reported for the La1?xSrxGa1?yMgyO3?δ prepared by other synthesis methods. On the other hand, the mechanical properties (elastic modulus, E and hardness, H) have been determined at micro/nanometric scale using the instrumented indentation technique. While E did not vary significantly with the increasing indentation depth (h), H values strongly decreased with the indentation depth up to 500 nm. For h > 500 nm, both mechanical properties remained almost constant, thus obtaining E = 271 ± 6 GPa and H = 13.2 ± 0.4 GPa. Finally, the residual imprints and fracture mechanisms have been observed by atomic force microscopy (AFM).  相似文献   

19.
BaxSr1−xCo0.8Fe0.2O3−δ (0.3  x  0.7) composite oxides were prepared and characterized. The crystal structure, thermal expansion and electrical conductivity were studied by X-ray diffraction, dilatometer and four-point DC, respectively. For x  0.6 compositions, cubic perovskite structure was obtained and the lattice constant increased with increasing Ba content. Large amount of lattice oxygen was lost below 550 °C, which had significant effects on thermal and electrical properties. All the dilatometric curves had an inflection at about 350–500 °C, and thermal expansion coefficients were very high between 50 and 1000 °C with the value larger than 20 × 10−6 °C−1. The conductivity were larger than 30 S cm−1 above 500 °C except for x > 0.5 compositions. Furthermore, conductivity relaxation behaviors were also investigated at temperature 400–550 °C. Generally, Ba0.4Sr0.6Co0.8Fe0‘2O3−δ and Ba0.5Sr0.5Co0.8Fe0.2O3−δ are potential cathode materials.  相似文献   

20.
Niobium doping effect on phase structure, phase stability and electrical conductivity of SrCoOx oxides and oxygen permeability of the corresponding membranes were systematically investigated. Niobium was successfully incorporated into A-site, B-site or simultaneously A and B double sites of SrCoOx oxide and stabilized the perovskite structure with a cubic symmetry in air down to room temperature at a proper doping amount. However, the A-site doping could not stabilize the cubic structure under a more reducing atmosphere of nitrogen, as to SrCo1?yNbyO3?δ, y  0.1 is necessary to sustain the cubic perovskite structure. Simultaneous doping of Nb at A and B sites is the most effective way to stabilize the perovskite structure under nitrogen atmosphere. Irrespective of doping site, the electrical conductivity decreased monotonously with Nb-doping amount. Both NbxSr1?xCoO3?δ and NbzSr1?zCo1?zNbzO3?δ envisaged a decrease in oxygen permeation flux with Nb-doping amount while SrCo1?yNbyO3?δ reached the maximum flux at y = 0.1. Among all the membranes, SrCo0.9Nb0.1O3?δ and Nb0.05Sr0.95Co0.95Nb0.05O3?δ show the highest oxygen fluxes of 3.5 and 2.7 ml cm?2 min?1 at 900 °C under an air/helium gradient, respectively.  相似文献   

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