Rapid synthesis of La0.7Sr0.3MnO3+λ catalysts by microwave irradiation process

Microwave irradiation processing (MIP) was considered as a potential method to synthesize perovskite-type oxides rapidly, cleanly and energy-efficiently. In this paper, La_0.7Sr_0.3MnO_3+λ, a kind of promising catalyst for automobile exhaust purification, was successfully prepared by MIP in not more than 5 min. The sol–gel method was also used for comparison. All the samples were evaluated by catalytic activity tests in the simulated exhaust and characterized by XRD, BET, TEM and XRF analyses. The results showed that the integrated perovskite-type phase and uniform particle size were obtained in the microwave-treated samples. And those ones exhibited a better oxidation activity under slightly oxygen-rich condition than that by sol–gel method, which may be related to more A-site cation vacancies and larger bulk oxygen content generated in MIP. Possible formation mechanism of perovskites in MIP was also discussed.     

Study on impedance spectra of La0.7Sr0.3MnO3 and Sm0.2Ce0.8O1.9-impregnated La0.7Sr0.3MnO3 cathode in single chamber fuel cell condition

Impedance spectroscopy was used to study the electrochemical performance of pure and ion-impregnated La_0.7Sr_0.3MnO₃ (LSM) cathodes on YSZ (Y₂O₃-stabilized ZrO₂) electrolytes in single chamber fuel cell conditions, i.e. a mixture gas with oxygen as oxidant, methane as fuel and nitrogen as dilute gas. Measurements were taken at the furnace temperature range of 550-750 °C and the CH₄/O₂ ratios from 1 to 2. Polarization resistances (R_p) for pure and impregnated LSM cathodes increased obviously as the CH₄/O₂ ratio increased at 650-750 °C. Polarization resistances of Sm_0.2Ce_0.8O_1.9 (SDC) impregnated LSM cathode were much smaller than the ones of pure LSM cathode under the same conditions. Overtemperatures were occurred at both cathodes due to the partial oxidation of methane.     

Preparation and characterisation of supported La0.8Sr0.2MnO3+x

Three supported La_0.8Sr_0.2MnO_3+x catalysts were prepared, one supported on lanthanum-stabilised alumina and two supported on a NiAl₂O₄ spinel. The catalysts were characterised using X-ray diffraction, transmission electron microscopy and surface area measurements following heat-treatments at temperatures up to 1200°C in air. In the alumina-supported catalyst, a reaction occurred between the active phase and the support at high temperatures, indicating that these materials would be unsuitable for high temperature catalytic combustion. Only in the NiAl₂O₄-supported catalysts were the supported perovskite phases found to be stable at high temperature. These catalysts showed good methane combustion activity.     

Improved electrochemical performance of La0.7Sr0.3MnO3 and carbon co-coated LiFePO4 synthesized by freeze-drying process

Carbon coated LiFePO₄ particles were first synthesized by sol-gel and freeze-drying method. These particles were then coated with La_0.7Sr_0.3MnO₃ nanolayer by a suspension mixing process. The La_0.7Sr_0.3MnO₃ and carbon co-coated LiFePO₄ particles were calcined at 400 °C for 2 h in a reducing atmosphere (5% of hydrogen in nitrogen). Nanolayer structured La_0.7Sr_0.3MnO₃ together with the amorphous carbon layer forms an integrate network arranged on the bare surface of LiFePO₄ as corroborated by high-resolution transmission electron microscopy. X-ray diffraction results proved that the co-coated composite still retained the structure of the LiFePO₄ substrate. The twin coatings can remarkably improve the electrochemical performance at high charge/discharge rates. This improvement may be attributed to the lower charge transfer resistance and higher electronic conductivity resulted from the twin nanolayer coatings compared with the carbon coated LiFePO₄.     

La0.7Ca0.3MnO3纳米颗粒的聚丙烯酰胺凝胶法制备及性能

采用聚丙烯酰胺凝胶法制备La0.7Ca0.3MnO3纳米颗粒。通过X射线衍射和扫描电镜研究不同pH值和不同络合剂对制备La0.7Ca0.3MnO3纳米颗粒形貌及性能的影响。结果表明：合成La0.7Ca0.3MnO3钙钛矿相的最佳pH=2;分别以醋酸、草酸、柠檬酸和乙二胺四乙酸为络合剂,在600℃煅烧温度制备的高纯La0...     

The effect of particle size on structural,magnetic and transport properties of La0.7Sr0.3MnO3 nanoparticles

This paper reports the synthesis of different particle size La_0.7Sr_0.3MnO₃ (LSMO) nanoparticles using non-aqueous sol gel synthesis route by calcination at temperatures 750 °C, 850 °C and 950 °C. In the present work, the effect of particle size of LSMO nanoparticles on its structural, magnetic and transport properties has been studied in detail. The X-ray diffraction analysis confirms the formation of LSMO nanoparticles having rhombohedral ($R\stackrel{?}{3}c$) structure with average particle size of 20 nm, 22.5 nm and 25.6 nm. An increase in magnetization and decrease in coercivity with increase in particle size is attributed to the magnetically disordered surface layer. The bifurcation in ZFC-FC magnetization indicates the possibility of spin glass like behavior of the LSMO nanoparticles. The effect of particle size on the resistivity and magnetoresistance were studied by using different conduction mechanism for different temperature regions. The upturn in the ρ-T curve at lower temperatures was explained by using Kondo-like transport mechanism. The maximum LFMR achieved was 32.3% at a field of 1 T at 10 K for 20 nm LSMO nanoparticle.     

Electrochemical properties of the single crystal La0.7Pb0.3MnO3 electrode

Electrochemical properties of the single crystal La_0.7Pb_0.3MnO₃ were studied. This oxide is stable only at the anodic potential region in alkaline solution. It was found that the catalytic activities for the oxygen reduction and evolution reactions are relatively high due to the effect of the B site Mn cation on the oxide surface. However, the mechanisms of the oxygen reduction and evolution reactions are different from the case of La_1-xSr_xMnO₃ electrodes and it was presumed that these differences are assigned to the influence of the A site Pb cation in La_0.7Pb_0.3MnO₃ and that of the (100) crystal plane.     

Hydrothermal synthesis and magnetocaloric effect of La0.7Ca0.2Sr0.1MnO3

The hydrothermal synthesis and magnetic entropy change for the perovskite manganite La_0.7Ca_0.2Sr_0.1MnO₃ have been studied. The La_0.7Ca_0.2Sr_0.1MnO₃ can be produced as phase-pure, crystalline powder in one step from solutions of metal salts in aqueous potassium hydroxide solution at a temperature of 240 °C in 72 h. Scanning electron microscopy shows that the materials are made up of cuboid-shaped or rod-shaped particles with typical dimension of 4.0 μm. Heat treatment is necessary to improve the magnetocaloric effect for the hydrothermal sample. The maximum magnetic entropy change (∣ΔS_M∣) for the sample annealed at 1200 °C for 6 h is 2.85 J kg^− 1 K^− 1 at the magnetic field change of 2.0 T and 2.23 J kg^− 1 K^− 1 at the magnetic field change of 1.5 T at 315 K. The hydrothermal synthesis method is a feasible route to prepare high quality perovskite material for magnetic refrigeration application.     

La0.6Ca0.4CoO3, La0.1Ca0.9MnO3 and LaNiO3 as bifunctional oxygen electrodes

A series of perovskite catalysts was investigated for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) in alkaline electrolyte and at room temperature, supplied by oxygen or air. A meniscus cell was used to screen-test candidate catalysts for their bifunctionality and assess their activity for ORR at 3 mm depth of immersion (DOI) in the electrolyte. Based on the meniscus data LaNiO₃, La_0.1Ca_0.9MnO₃ and La_0.6Ca_0.4CoO₃ were selected for further assessment in microelectrode and half-cell studies. Activity tests for the ORR and OER, Tafel slopes at high current densities and apparent activation energies for the ORR were determined using a microelectrode technique on samples of the selected perovskites, La_0.1Ca_0.9MnO₃, La_0.6Ca_0.4CoO₃ and LaNiO₃ with and without graphite support. Tafel slopes of ca. 120 mV per decade and apparent activation energies of approximately 18 kcal mol⁻¹ were measured at high cathodic current densities. Cycle-life and performance of La_0.1Ca_0.9MnO₃, La_0.6Ca_0.4CoO₃ and LaNiO₃-based gas-diffusion electrodes in half-cell configurations were tested at a constant current density of 25 mA cm⁻² with subsequent and intermittent polarizations. Similar activities resulted in the ORR, while increased numbers of cycles were observed for the La_0.1Ca_0.9MnO₃-based electrode. Furthermore, electrode material compositions, especially PTFE contents were optimized to conform to the establishment of the three phase interactions of the electrode structure. Transmission Electron microscopy (TEM) and BET-surface area analyses were carried out in order to find out the morphological and surface properties of the perovskite materials.     

Effect of impregnation of La0.85Sr0.15MnO3/yttria stabilized zirconia solid oxide fuel cell cathodes with La0.85Sr0.15MnO3 or Al2O3 nano-particles

Strontium substituted lanthanum manganite and yttria stabilized zirconia solid oxide fuel cell composite electrodes were impregnated with nano-particles of strontium substituted lanthanum manganite or alumina. A clear positive effect was observed on low performing electrodes and on good performing electrodes if the temperature was kept low after the impregnation with strontium substituted lanthanum manganite. On good performing electrodes the effect disappeared on heating. Alumina nano-particles had a detrimental effect on the activity of the strontium substituted lanthanum manganite based electrodes.     

A bi-layered composite cathode of La0.8Sr0.2MnO3-YSZ and La0.8Sr0.2MnO3-La0.4Ce0.6O1.8 for IT-SOFCs

A bi-layered composite cathode of La_0.8Sr_0.2MnO₃ (LSM)-YSZ and LSM-La_0.4Ce_0.6O_1.8 (LDC) was fabricated for anode-supported solid oxide fuel cells with a thin YSZ electrolyte film. The cell with the bi-layered composite cathode displayed better performance than the cell with the corresponding single-layered composite cathode of LSM-LDC or LSM-YSZ. At 650 °C, the cell with the bi-layered composite cathode gave a higher maximum power density than the cells with the single-layered LSM-LDC and LSM-YSZ composite cathodes, by 52% and 175%, respectively. The impedance spectra results show that the thin LSM-YSZ interlayer not only improves the cathode/electrolyte interface but also reduces the polarization resistance of the cathode. The activation energy for oxygen reduction on the bi-layered composite cathode is much smaller than that on LSM-YSZ composite cathode, and it is suggested that the special redox property of Ce⁴⁺/Ce³⁺ in LDC facilitates the oxygen reduction process on the bi-layered composite cathode. The cell with the bi-layered composite cathode operated quite stably during a 100 h run.     

Growth and characterization of epitaxial (La2/3Sr1/3)MnO3 films by pulsed laser deposition

High quality epitaxial (La_2/3Sr_1/3)MnO₃ (0 0 1) thin films were grown by pulsed laser deposition on SrTiO₃ (0 0 1) substrate at optimized growth parameters. The films quality was confirmed by both structural and physical properties characterization. Channeling Rutherford Backscattering Spectrometry characterization showed the minimal channeling coefficient as low as 4%. The LSMO thin films growth on SrTiO₃ substrate follows the island growth model. The Curie temperature of LSMO films is around 360 K, which is the one of the highest reported in literature. The resistivity of LSMO films showed the metal-insulate transition temperature coincides with the Curie temperature. This high quality LSMO is suitable for room temperature magnetic devices application.     

Structural and electrical properties of La0.7Sr0.3Co0.5Fe0.5O3 powders synthesized by solid state reaction

This work investigates the effect of synthesis parameters (calcination temperature, milling conditions and sintering temperature) on the structural, morphological and electrical properties of La_0.7Sr_0.3Co_0.5Fe_0.5O₃ (LSCF) powders prepared by the solid state reaction. The thermogravimetric profile showed that the minimum temperature needed for the carbonate decomposition and formation of perovskite phase is 800 °C. SEM analysis revealed the loose and porous structure of the powder materials. The XRD patterns demonstrate that milling parameters such as grinding balls:sample ratio, rotational speed, and milling time influence the structural properties. The results revealed that powders synthesized with grinding balls:sample ratio of 8:1, 500 rpm and 4 h of milling present pure LSCF phase. Porosity of the pellets decreased with increasing sintering temperature from 950 to 1100 °C. Electrical conductivity was measured at 400–1000 °C and correlated with sintering temperature.     

Large temperature coefficient of resistance at near room temperature in Sr-doped La0.72Ca0.28MnO3:Ag0.2 polycrystalline composites

A series of La_0.72Ca_0.28?xSr_xMnO₃:Ag_0.2 (x = 0.00, 0.01, 0.03, 0.05, 0.07, and 0.10) composites was prepared by sol–gel method using methanol as solvent. Sample composition was analyzed by X-Ray diffraction, which revealed that all samples are pseudo-perovskite. Grain size and morphology were characterized by scanning electron microscopy. The results showed that all samples are of high density and large grain size in the range of 50–100 µm. All samples exhibited sharp metal–insulator (M-I) transitions, along with a drastic transition from paramagnetism to ferromagnetism. With increased Sr doping, radius of A-site ion increased, both Curie temperature T_C and M-I transition temperature shift to higher temperature increases, and transition width turns widener which makes a decreased temperature coefficient of resistance (TCR). Larger ionic radius of Sr²⁺, which resulted in enhancement of double exchange, enlarges Mn–O–Mn bond angle, hence improves electrical and magnetic properties. It was found that TCR could reach 24.3% K^?1 by Ag addition at near room temperature, which makes this composite promising potential applications in infrared detection and night vision.     

Effect of temperature on the porosity, microstructure, and properties of porous La0.8Sr0.2MnO3 cathode materials

Porous La_0.8Sr_0.2MnO₃ cathode materials were prepared by the gelcasting technology. Carbon was employed to produce pores. It is shown that the open porosity decreases with increasing temperature. The proper sintering temperature is 1100 °C and the median pore size of the sample obtained at this temperature is about 460 nm. The microstructure indicates that the grain grows as the sintering temperature increases, which leads to the decrease of the number of open pores. Generally, the pores are located at multi-grain boundaries. Some closed pores appear in the sample prepared at 1100 °C and more. Both the conductivity and the interface bonding between La_0.8Sr_0.2MnO₃ and YSZ get better as the temperature increases. When the sintering temperature is more than 1250 °C, La and Mn ions begin to diffuse into YSZ, and therefore interface reactions happen. According to ln(σT)  1/T curves, E_a was calculated to be 10.18 kJ/mol.     

Catalytic combustion of CH4 and CO on La1−xMxMnO3 perovskites

The activities of perovskites depend on compositions and preparation methods. Various perovskites, La_1−xM_xMnO₃ (M=Ag, Sr, Ce, La), have been prepared by two different methods (co-precipitation and spray decomposition). The new preparation method, spray decomposition, produced perovskites of a high surface area of over 10 m²/g. The catalytic activities for CH₄ and CO oxidation have been studied on a series of catalysts, La_1−xM_xMnO₃. The perovskite-type oxide, La_0.7Ag_0.3MnO₃, shows the highest catalytic activity: the complete conversion of CO and CH₄ at 370 and 825 K, respectively.     

Synthesis and sintering behavior of La0.8Sr0.2CrO3 by a glycine nitrate process

La_0.8Sr_0.2CrO₃ powder was synthesized by a glycine nitrate process from an aqueous solution of lanthanum, strontium, and chromium nitrates, and glycine. The apparent density, size and morphology of the La_0.8Sr_0.2CrO₃ powder depended on the glycine-to-nitrate ratio. However, the pH value of the precursor solution had no significant effect on these properties. It was found that a single-phase perovskite, La_0.8Sr_0.2CrO₃, was synthesized when the dried ash was calcined at 1200 °C for 5 h. A secondary minor phase, SrCrO₄, was observed in the powder calcined at temperatures lower than 1100 °C. The presence of the SrCrO₄ phase has a significant effect on the sinterability and microstructural evolution of the La_0.8Sr_0.2CrO₃. A relative density higher than 90% could be achieved when the 1000 °C-calcined La_0.8Sr_0.2CrO₃ powder was sintered at 1450 °C.     

Monodisperse magnetic composite poly(glycidyl methacrylate)/La0.75Sr0.25MnO3 microspheres by the dispersion polymerization

Monodisperse magnetic poly(glycidyl methacrylate) microspheres were prepared by dispersion polymerization of glycidyl methacrylate in cyclohexane in the presence of La_0.75Sr_0.25MnO₃ nanoparticles, surface of which was modified with penta(propylene glycol) methacrylate phosphate (PPGMAP). However, only agglomerates were formed by the dispersion polymerization in toluene. Sterical stabilizer was poly(styrene-block-hydrogenated butadiene-block-styrene) and initiators investigated were 2,2′-azobisisobutyronitrile (AIBN) and 4,4′-azobis(4-cyanovaleric acid) (ACVA). Effects of initiators and other reaction conditions on properties of the composite microspheres were evaluated. The phase composition of the magnetic polymer composite microspheres and the size of magnetic cores were determined by X-ray powder diffraction. The characterization was completed by magnetization measurements, atomic absorption spectroscopy, TEM, SEM and ATR FTIR spectroscopy.     

The NO selective reduction on the La1−xSrxMnO3 catalysts

La_1−xSr_xMnO₃ (x=0, 0.1, 0.3, 0.5, 0.7) perovskite-type oxides (PTOs) were prepared by coprecipitation under various calcination temperature, and their performances for the NO reduction were evaluated under a simulated exhaust gas mixture. The X-ray diffraction (XRD) and thermogravimetric analysis were carried out to find the formation process of the perovskite. The NO reduction rate under different reaction temperature, the concentration of oxygen and the presence of hydrocarbon were observed by the input/output analysis. In the presence of 10% excess oxygen, the catalyst La_0.7Sr_0.3MnO₃ calcined at 900 °C showed a NO reduction rate of 61% at 380 °C. The study of the reaction curves showed that C₃H₈ could act as the reducer for the NO reduction below 400 °C. The NO reduction is highly affected by increasing the O₂ concentration from 0.5 to 10%, especially at high temperatures when oxygen becomes more competitive than NO on the oxidation of C₃H₈, leading to a decrease of the NO reduction from 100% to zero at 560 °C.     

Characterization of La0.67Sr0.33MnOz thin films synthesized by metal-organic decomposition on different substrates

La_0.67Sr_0.33MnO_z (LSMO) thin films were synthesized by means of metal-organic decomposition on the substrates including amorphous quartz, (1 0 0) Si chip, (1 0 0) MgO single crystal and polycrystalline Al₂O₃ ceramic plate. The structure and magnetotransport properties of the films were characterized. X-ray diffraction spectra show that all samples are polycrystalline with (2 0 2) preferred orientation. All films present metal–insulator transition and enhanced magnetoresistance (MR) effect below metal–insulator transition peak temperature (T_p). At room temperature (RT) low-field magnetoresistance effect (LFMR) and linear change of resistivity under applied field are exhibited by all the films. These magnetotransport properties were first ascribed to the porous structural characteristics in the films observed by atomic force microscope. Furthermore, the LSMO film synthesized on (1 0 0) MgO substrate presents a bit different magnetic properties and magnetotransport from the other samples, including broad ferromagnetic–paramagnetic transition zone, lower T_p and weaker LFMR at RT. However, for the samples synthesized on the other substrates, the LFMR effect is very similar to each other and their MR ratio reaches near 5% under 10 kOe field. Thus the substrate effect of LSMO film on (1 0 0) MgO is more intensive than that of the other samples.