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1.
《Journal of the European Ceramic Society》2007,27(6):2445-2454
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.
《Journal of the European Ceramic Society》2005,25(4):489-499
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.
《Journal of the European Ceramic Society》2006,26(14):2999-3004
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
4.
《Journal of the European Ceramic Society》2006,26(8):1371-1378
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
5.
《Ceramics International》2015,41(4):5821-5829
We report improvement in the magnetocaloric properties of Ce-doped lanthanum manganites. Polycrystalline La0.7−xCexSr0.30MnO3 (0≤x≥0.3) samples were prepared using the conventional solid-state reaction method with phase purity and structure confirmed using X-ray diffraction. Temperature dependent magnetization measurements and Arrott analysis reveal second order ferromagnetic transition in parent sample and as well as in doped sample with Curie temperature decreasing progressively with increasing Ce-concentration from ~370 K for x=0.0 to 310 K for x=0.30. Magnetic entropy change (ΔSM) was calculated by applying the thermodynamic Maxwell equation to a series of isothermal field dependent magnetization curves. A large ΔSM associated with the ferromagnetic–paramagnetic transition in La0.7−xCexSr0.30MnO3 samples has been observed. The value of ΔSM was found to increase with Ce-doping up to x=0.15 and the highest value of 2.12 J kg−1 K−1 (at ΔH=2 T) was observed for La0.55Ce0.15Sr0.30MnO3 sample near the Curie temperature of 356 K. Also, improved relative cooling power of ~122 J kg−1 was observed for the same sample. Due to the large magnetic entropy change and high Curie temperature, the La0.55Ce0.15Sr0.30MnO3 sample is suggested to be used as potential magnetic refrigerants for magnetic refrigeration technology above room temperature. 相似文献
6.
《Journal of the European Ceramic Society》2007,27(6):2455-2461
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
7.
《Ceramics International》2016,42(7):8467-8472
Dielectric properties of Ca1−3x/2YbxCu3−yMgyTi4O12 (x=0.05, y=0.05 and 0.30) prepared using a modified sol–gel method and sintered at 1070 °C for 4 h were investigated. The mean grain sizes of the CaCu3Ti4O12 and co-doped Ca0.925Yb0.05Cu3−yMgyTi4O12 (y=0.05 and 0.30) ceramics were ≈15.86, ≈3.37, and ≈2.32 μm, respectively. Interestingly, the dielectric properties can be effectively improved by co-doping with Yb3+ and Mg2+ ions to simultaneously control the microstructure and properties of grain boundaries, respectively. These properties were improved over those of single-doped and un-doped CaCu3Ti4O12 ceramics. A highly frequency−independent colossal dielectric permittivity (≈104) in the range of 102–106 Hz with very low loss tangent values of 0.018–0.028 at 1 kHz were successfully achieved in the co-doped Ca0.925Yb0.05Cu3−yMgyTi4O12 ceramics. Furthermore, the temperature stability of the colossal dielectric response of Ca1−3x/2YbxCu3−yMgyTi4O12 was also improved to values of less than ±15% in the temperature range from −70 to 100 °C. 相似文献
8.
《Ceramics International》2007,33(6):1133-1136
The structure, thermal expansion coefficients, and electric conductivity of Ce1−xGdxO2−x/2 (x = 0–0.6) solid solution, prepared by the gel-combustion method, were investigated. The uniform small particle size of the gel-combustion prepared materials allows sintering into highly dense ceramic pellets at 1300 °C, a significantly lower temperature compared to that of 1600–1650 °C required for ceria solid electrolytes prepared by traditional solid state techniques. XRD showed that single-phase solid solutions formed in all the investigated range. The maximum conductivity, σ600 °C = 5.26 × 10−3 S/cm, was found at x = 0.2. The thermal expansion coefficient, determined from high-temperature X-ray data, was 8.125 × 10−6 K−1 at x = 0.2. 相似文献
9.
《Ceramics International》2017,43(18):16384-16390
The protonic material La2Ce2O7 exhibits good tolerance to H2O and CO2 compared to BaCeO3-based materials and has become increasingly popular for operation at low-to-intermediate temperatures in protonic ceramic fuel cells. In this work, doping La2Ce2O7 with Na in a series with varying compositions is studied. All of the precursors are prepared by a common citrate-nitrate combustion method. X-ray diffraction images reveal that all of the La2-xNaxCe2O7-δ samples have a cubic structure. The La2-xNaxCe2O7-δ pellets are characterized by scanning electron microscopy and are observed to be dense without holes. The effects of Na-doping on the La2Ce2O7 electrical conductivity are carefully investigated in air at 350–800 °C and 5%H2-95% Ar environments at 350–700 °C. It is found that different levels of Na doping in La2Ce2O7 are conducive to improving the electrical conductivity and sinterability. Among the pellets, La1.85Na0.15Ce2O7-δ exhibited the highest electrical conductivity in air and 5% H2-95% Ar atmospheres. Anode-supported half cells with La1.85Na0.15Ce2O7-δ electrolyte are also fabricated via a dry-pressing process, and the corresponding single cell exhibited a desirable power performance of 501 mW cm−2 at 700 °C. The results demonstrate that La1.85Na0.15Ce2O7-δ is a promising proton electrolyte with high conductivity and sufficient sinterability for use in protonic ceramic fuel cells operating at reduced temperatures. 相似文献
10.
《Journal of the European Ceramic Society》2014,34(12):2967-2976
Mn2+-doped Sn1−xMnxP2O7 (x = 0–0.2) are synthesized by a new co-precipitation method using tin(II)oxalate as tin(IV) precursor, which gives pure tin pyrophosphate at 300 °C, as all the reaction by-products are vaporizable at <150 °C. The dopant Mn2+ acts as a sintering aid and leads to dense Sn1−xMnxP2O7 samples on sintering at 1100 °C. Though conductivity of Sn1−xMnxP2O7 samples in the ambient atmosphere is 10−9–10−6 S cm−1 in 300–550 °C range, it increases significantly in humidified (water vapor pressure, pH2O = 0.12 atm) atmosphere and reaches >10−3 S cm−1 in 100–200 °C range. The maximum conductivity is shown by Sn0.88Mn0.12P2O7 with 9.79 × 10−6 S cm−1 at 550 °C in ambient air and 2.29 × 10−3 S cm−1 at 190 °C in humidified air. It is observed that the humidification of Sn1−xMnxP2O7 samples is a slow process and its rate increases at higher temperature. The stability of Sn1−xMnxP2O7 samples is analyzed. 相似文献
11.
《Ceramics International》2017,43(10):7537-7542
The (La1−xYbx)2AlTaO7 ceramics were synthesized by pressureless sintering process at 1600 °C for 10 h in air. The crystal phase, microstructure and thermophysical properties were investigated. Results show that pure (La1−xYbx)2AlTaO7 cermics with single weberite structure are prepared successfully. Owing to the reduction of crystal-lattice tolerance-factor, the thermal conductivity of (La1−xYbx)2AlTaO7 (x>0) ceramics increases with increasing Yb2O3 fraction at identical temperatures, which is lower than that of La2AlTaO7. Due to the relatively high electro-negativity of Yb element, the addition of Yb2O3 increases the thermal expansion coefficient of (La1−xYbx)2AlTaO7 ceramics. 相似文献
12.
《Ceramics International》2016,42(3):4532-4538
The structural, thermal and electrochemical properties of the perovskite-type compound La1−xNdxFe0.5Cr0.5O3 (x=0.10, 0.15, 0.20) are investigated by X-ray diffraction, thermal expansion, thermal diffusion, thermal conductivity and impedance spectroscopy measurements. Rietveld refinement shows that the compounds crystallize with orthorhombic symmetry in the space group Pbnm. The average thermal expansion coefficient decreases as the content of Nd increases. The average coefficient of thermal expansion in the temperature range of 30–850 °C is 10.12×10−6, 9.48×10−6 and 7.51×10−6 °C−1 for samples with x=0.1, 0.15 and 0.2, respectively. Thermogravimetric analyses show small weight gain at high temperatures which correspond to filling up of oxygen vacancies as well as the valence change of the transition metals. The electrical conductivity measured by four-probe method shows that the conductivity increases with the content of Nd; the electrical conductivity at 520 °C is about 4.71×10−3, 6.59×10−3 and 9.62×10−3 S cm−1 for samples with x=0.10, 0.15 and 0.20, respectively. The thermal diffusivity of the samples decreases monotonically as temperature increases. At 600 °C, the thermal diffusivity is 0.00425, 0.00455 and 0.00485 cm2 s−1 for samples with x=0.10, 0.15 and 0.20, respectively. Impedance measurements in symmetrical cell arrangement in air reveal that the polarization resistance decreases from 55 Ω cm−2 to 22.5 Ω cm−2 for increasing temperature from 800 °C to 900 °C, respectively. 相似文献
13.
《Ceramics International》2017,43(9):7153-7158
In this work, Yb3+ was selected to replace the Y3+ in yttrium aluminum garnet (YAG) in order to reduce its thermal conductivity under high temperature. A series of (Y1-xYbx)3Al5O12 (x=0, 0.1, 0.2, 0.3, 0.4) ceramics were prepared by solid-state reaction at 1600 °C for 10 h. The microstructure, thermophysical properties and phase stability under high temperature were investigated. The results showed that all the Yb doped (Y1-xYbx)3Al5O12 ceramics were comprised of a single garnet-type Y3Al5O12 phase. The thermal conductivities of (Y1-xYbx)3Al5O12 ceramics firstly decreased and subsequently increased with Yb ions concentration rising from room temperature to 1200 °C. (Y0.7Yb0.3)3Al5O12 had the lowest thermal conductivity among investigated specimens, which was about 1.62 W m−1 K−1 at 1000 °C, around 30% lower than that of pure YAG (2.3 W m−1 K−1, 1000 °C). Yb had almost no effect on the coefficients of thermal expansion (CTEs) of (Y1-xYbx)3Al5O12 ceramics and the CTE was approximate 10.7×10−6 K−1 at 1200 °C. In addition, (Y0.7Yb0.3)3Al5O12 ceramic remained good phase stability when heating from room temperature to 1450 °C. 相似文献
14.
A new series of rare earth solid solutions Yb2?xLaxW3O12 were successfully synthesized by the solid-state method. Effects of substituted ion lanthanum on the microstructures and thermal expansion properties in the resulting Yb2?xLaxW3O12 ceramics were investigated by X-ray diffraction (XRD), thermogravimetric analyzer (TGA), field emission scanning electron microscope (FESEM) and thermal mechanical analyzer (TMA). Results indicate that the structural phase transition of the Yb2?xLaxW3O12 changes from orthorhombic to monoclinic with increasing substituted content of lanthanum. The pure phases can form in the composition range of 0 ≤ x < 0.5 with orthorhombic structure and 1.5 < x ≤ 2 with monoclinic one. High lanthanum content leads to a low hygroscopicity of Yb2?xLaxW3O12. Negative thermal coefficients of the Yb2?xLaxW3O12 (0 ≤ x ≤ 2) also vary from ?7.78 × 10?6 K?1 to 2.06 × 10?6 K?1 with increasing substituted content of lanthanum. 相似文献
15.
《Journal of the European Ceramic Society》2014,34(10):2553-2561
As one of promising thermal barrier coating (TBC) candidates, La2Ce2O7 (LC) has attracted increasing attention because of its low thermal conductivity and potential capability to be operated above 1250 °C. In this paper, the microstructure evolution and mechanical properties of the plasma-sprayed LC TBC with calcium–magnesium–alumina–silicate (CMAS) glassy deposits at 1250 °C were investigated. Due to chemical reaction between the CMAS deposits and LC coating, a dense sealing layer, mainly composed of Ca2(LaxCe1−x)8(SiO4)6O6−4x and CeO2, was formed on the coating after heat-treatment at 1250 °C and effectively prevented CMAS from further penetration. The interaction layer had the micro-hardness of ∼10–12 GPa, relatively harder than the LC coating. 相似文献
16.
《Journal of the European Ceramic Society》2017,37(1):107-114
Microstructure, temperature and frequency dependent dielectric and energy storage properties of Ba0.3Sr0.475La0.12Ce0.03Ti1−xMnxO3(x = 0 − 0.005) have been investigated with X-ray diffractometer, and scanning electron microscope, broad band dielectric spectrometer and ferroelectric analyzer. The doping of Mn substantially decreased the dielectric loss, but made some of Ce3+ be oxidized to Ce4+ entering into B-site, which resulted in the formation secondary phases. For the Mn-doped composition, extremely low dielectric loss (10−5 order of magnitude at 10 kHz) could be obtained at room temperature. The relaxation mechanism at low temperature is of the dipole type for the undoped composition and that at high temperature (>500 K) is governed by the trap controlled ac conduction, respectively. The energy storage properties were improved by the doping with Mn due to the increase of insulation. Maximum energy density of 0.953 J/cm3 could be obtained for x = 0.003 composition with the BDS of 247 kV/cm and efficiency of 93%. 相似文献
17.
《Ceramics International》2016,42(4):5218-5225
The synthesis of a material having ferroelectric and ferromagnetic properties in the same phase is a challenging task. In the present work, lanthanum based multiferroic materials, with composition La1−xCoxFe1−yNiyO3 (where, x=0.0–0.5 and y=0.0–1.0), have been synthesized via sol–gel auto-combustion method. The phase of the synthesized materials was confirmed by the X-ray diffraction (XRD) analysis, while the surface morphology and particle size were determined by the scanning electron microscopy (SEM) analysis. The DC electrical resistivity and activation energy were observed to increase from 2.14×107 to 3.04×1010 Ω cm and 0.64 to 0.77 eV, respectively with the increase in concentration of substituents. The drift mobility decreases from 3.27×10−13 to 2.80×10−16 cm2 V−1 S−1. The dielectric constant, dielectric loss and dielectric loss factor decrease with the increase in frequency (1 MHz to 3 GHz) and Ni–Co content as well. The electrical and dielectric results are in good agreement with each other. The increase in electrical resistivity and decrease in dielectric parameters make these materials suitable for applications in microwave devices. 相似文献
18.
《Journal of the European Ceramic Society》2007,27(2-3):813-817
The as-sintered Zn1−xAlxO (0 ≤ x ≤ 0.05) samples crystallized in the ZnO with a wurtzite structure, along with a small amount of the cubic spinel ZnAl2O4. The addition of Al2O3 to ZnO gave rise to a decrease in grain size, ranging from 7.3 to 2.7 μm and in relative density, ranging from 99.2 to 90.1% of the theoretical density. In the Zn0.97Al0.03−yTiyO samples, as the amount of TiO2 increased, the grain size of ZnO grains and second phases, such as Zn2TiO4 and ZnAl2O4, as well as density increased. The co-doping of Al and Ti led to a significant increase in both the electrical conductivity and the absolute value of the Seebeck coefficient, resulting in an increase in the power factor. The highest value of power factor (3.8 × 10−4 W m−1 K−2) was attained for Zn0.97Al0.02Ti0.01O at 800 °C. It is demonstrated that the Al and Ti co-doping is fairly effective for enhancing thermoelectric properties. 相似文献
19.
《Journal of Industrial and Engineering Chemistry》2014,20(5):3303-3312
Mesoporous CexZr1−xO2 solid solutions were prepared by the surfactant-assisted method and used as support of CuO nanocatalysts for catalytic total oxidation of toluene. The prepared CuO/CexZr1−xO2 catalysts have a wormhole-like mesoporous structure with high surface area and uniform pore size distribution, and the CuO nanoparticles were highly dispersed on the surface of CexZr1−xO2. The doping of ZrO2 in CeO2 promotes the dispersion of active copper species and enhances the reducibility of copper species. The effect of Ce/Zr ratio, calcination temperature and CuO loading amount on the catalytic performance of CuO/CexZr1−xO2 was investigated in detail. The 400 °C-calcined 8%CuO/Ce0.8Zr0.2O2 catalyst exhibits the highest activity with the complete toluene conversion temperature of 275 °C at the condition of GHSH = 33,000 h−1 and the toluene concentration of 4400 ppm. The interfacial interaction between CuO and the CexZr1−xO2 support, highly dispersed CuO nanoparticles and the nature of the support contribute to the high catalytic activity of mesoporous CuO/CexZr1−xO2 nanocatalysts. 相似文献
20.
《Ceramics International》2016,42(3):4393-4399
Dense BaZr0.5Ce0.3Y0.2O3−δ (BZCY532) proton conductors were prepared by a spark plasma sintering method. Their conductivities were determined in different atmospheres: dry air, wet N2 and wet H2. Moreover, the potential electronic conductivity contribution to the total conductivity was also identified by testing their total conductivities at different oxygen partial pressures (1–10−24 atm) in combination with an XPS analysis. It is found that the prepared dense BZCY532 ceramics are good proton conductors at 600 °C. In addition, the Ce3+ concentration in the dense BZCY532 ceramics is around 3.5 atm% of the total Ce element, and the electronic contribution to the total conductivity can be neglected after a postheat treatment. 相似文献