Reverse micellar synthesis and properties of nanocrystalline GMR materials (LaMnO3, La0.67Sr0.33MnO3 and La0.67Ca0.33MnO3): Ramifications of size considerations

Nanoparticles of complex manganites (viz. LaMnO₃, La_0.67Sr_0.33MnO₃ and La_0.67Ca{0.33}MnO₃) have been synthesized using the reverse micellar route. These manganites are prepared at 800‡C and the monophasic nature of all the oxides has been established by powder X-ray diffraction studies. TEM studies show an average grain size of 68, 80 and 50 nm for LaMnO₃, La_0.67Sr_0.33MnO₃ and La_0.67Ca{0.33}MnO₃respectively. Ferromagnetic ordering is observed at around 250 K for LaMnO₃, 350 K for La_0.67Sr_0.33MnO₃ and 200 K for La_0.67Ca{0.33}MnO₃. These Curie temperatures correspond well with those reported for bulk materials with similar composition.     

Nano-sized La0.8Sr0.2MnO3 as oxygen reduction catalyst in nonaqueous Li/O2 batteries

Nano-sized La_0.8Sr_0.2MnO₃ prepared by the polyethylene glycol assisting sol–gel method was applied as oxygen reduction catalyst in nonaqueous Li/O₂ batteries. The as-synthesized La_0.8Sr_0.2MnO₃ was characterized by X-ray diffraction (XRD), scanning electron microscopy, and Brunauer–Emmet–Teller measurements. The XRD results indicate that the sample possesses a pure perovskite-type crystal structure, even sintered at a temperature as low as 600 °C, whereas for solid-state reaction method it can only be synthesized above 1,200 °C. The as-prepared nano-sized La_0.8Sr_0.2MnO₃ has a specific surface area of 32 m² g⁻¹, which is much larger than the solid-state one (1 m² g⁻¹), and smaller particle size of about 100 nm. Electrochemical results show that the nano-sized La_0.8Sr_0.2MnO₃ has better catalytic activity for oxygen reduction, higher discharge plateau and specific capacity.     

The synthesis and complex anion-vacancy ordered structure of La0.33Sr0.67MnO2.42

The low-temperature topotactic reduction of La_0.33Sr_0.67MnO₃ with NaH results in the formation of La_0.33Sr_0.67MnO_2.42. A combination of neutron powder and electron diffraction data show that La_0.33Sr_0.67MnO_2.42 adopts a novel anion-vacancy ordered structure with a 6-layer OOTOOT' stacking sequence of the ‘octahedral’ and tetrahedral layers (Pcmb, a=5.5804(1) Å, b=23.4104(7) Å, c=11.2441(3) Å). A significant concentration of anion vacancies at the anion site, which links neighbouring ‘octahedral’ layers means that only 25% of the ‘octahedral’ manganese coordination sites actually have 6-fold MnO₆ coordination, the remainder being MnO₅ square-based pyramidal sites. The chains of cooperatively twisted apex-linked MnO₄ tetrahedra adopt an ordered -L-R-L-R- arrangement within each tetrahedral layer. This is the first published example of a fully refined structure of this type which exhibits such intralayer ordering of the twisted tetrahedral chains. The rationale behind the contrasting structures of La_0.33Sr_0.67MnO_2.42 and other previously reported reduced La_1−xSr_xMnO_3−y phases is discussed.     

Synthesis of rod-shaped La0.7Sr0.3MnO3 by a simple solid state ceramic route

The manganese oxides La_0.7Sr_0.3MnO₃ with rod shape have been synthesized by a simple solid state ceramic method. The morphology and magnetic properties of La_0.7Sr_0.3MnO₃ have been studied by XRD, SEM, TEM and VSM. The diameters of rod-shaped oxide were ranging from 180 nm to1.5 μm and the length was averaging 20 μm. The ratio of length to diameter could reach more than 35:1. The magnetic results showed that the Curie temperature of rod-shaped La_0.7Sr_0.3MnO₃ is far lower than single crystal.     

钙铝石涂层对整体型钙钛矿氧化物催化剂活性和热稳定性的影响

 采用固相反应法合成了钙铝石材料 12SrO•7Al2O3, 并以此作为涂层制备了堇青石蜂窝陶瓷型 La0.8Sr0.2MnO3 整体催化剂, 在不同温度 (850~1 050 oC) 下对该催化剂进行了热处理, 并采用 N2 吸附-脱附、X 射线衍射和扫描电镜等手段对其进行了表征, 考察了其催化甲基丙烯酸甲酯燃烧反应的活性. 结果表明, 12SrO•7Al2O3 作为涂层明显改善了整体催化剂的热稳定性, 在 850 oC 下焙烧 6 h 后, 含有 12SrO•7Al2O3 涂层的整体催化剂在 260 oC 即可将甲基丙烯酸甲酯完全转化. 12SrO•7Al2O3 涂层可避免 La0.8Sr0.2MnO3 活性组分与堇青石的接触, 减轻了活性组分在催化剂表面的烧结, 有利于保持 La0.8Sr0.2MnO3 活性组分的晶体结构和分散度, 提高整体催化剂的活性和热稳定性.     

Chemical and physical properties of complex perovskites in the La0.8Sr0.2MnO3−δ–La0.8Sr0.2CuO2.4+δ–La0.8Sr0.2FeO3−δ system

Perovskites resulting from discrete changes in composition within the quasi-ternary system La_0.8Sr_0.2MnO_3−δ–La_0.8Sr_0.2CuO_2.4+δ–La_0.8Sr_0.2FeO_3−δ were investigated under constant experimental conditions with the objective of obtaining an overview of the variation of the properties relevant for possible future applications. Nineteen nominal perovskite compositions within this system were systematically selected and synthesized under identical conditions by the Pechini method. The experimental data obtained on quantitative chemical analysis, powder X-ray diffraction, electrical conductivity and thermal expansion are presented collectively for the first time to facilitate comparisons. The formation and distribution of the different crystallographic phases at 950 °C within this quasi-ternary system are shown. The DC electrical conductivity is strongly influenced by the Cu content and increases up to 276 S cm⁻¹ for La_0.8Sr_0.2CuO_2.4+δ. The thermal expansion is dominated by the Cu/Mn ratio and is almost independent of the Fe content.     

Magnetic transport behavior of nanocrystalline Nd0.67A0.33MnO3 (A = Ca,Sr, Pb and Ba)

Nanocrystalline A-doped manganites, with compositional formula, Nd_0.67A_0.33MnO₃ (A = Ca, Sr, Pb and Ba) were prepared by Polyvinly Alcohol (PVA) gel route. After characterizing the samples by X-ray diffraction studies and by measuring electrical, magnetic transition temperatures, a systematic investigation of magnetization and magnetoresistance measurements both as a function of magnetic field and temperature was undertaken. The variation of electrical and magnetic transition temperatures is explained on the basis of size variance parameter. It has been concluded from the M–H plots that NCMO, NPMO and NBMO samples are exhibiting narrow hystersis loops indicating soft ferromagnetic nature, while the NSMO is found to exhibit double hystersis loop indicating the presence of metamagnetic transition. It is also observed that among the four samples, Nd_0.67Ca_0.33MnO₃ is found to exhibit the highest magnetoresistance (MR) of 91% over a broad temperature region.     

Preparation of La0.75Sr0.25MnO3 Nano-Phase Powders and Films from Alkoxide Precursors

The first purely alkoxide-based sol-gel route to nano-phase powders and thin films of perovskite La_0.75Sr_0.25MnO₃ is described. The phase and microstructure evolution on heat treatment of free gel films to form the target nano-phase oxide were investigated by TGA, IR spectroscopy, powder XRD, SEM and TEM-EDS. The xerogel consisted of a hydrated oxo-carbonate, without remaining alkoxo groups or solvent. Heating at 5°C·min^–1 decomposed the carbonate groups and yielded the pure perovskite La_0.75Sr_0.25MnO₃ at 760°C. The cell dimensions were virtually unchanged from the first observation of perovskite at 680°C, to 1000°C, 4 h. The monoclinic cell of La_0.75Sr_0.25MnO₃ obtained at 1000°C, 4 h, had the dimensions a = 5.475(1), b = 5.504(2), c = 7.771(1) Å, = 90.50(2), fitting the literature data quite well. Crack-free, homogenous, 150 nm thick La_0.75Sr_0.25MnO₃ films were prepared by spin-coating Si/SiO₂/TiO₂/Pt and polycrystalline -Al₂O₃ substrates with a 0.6 M alkoxide solution, followed by heating at 5°C·min^–1 to 800°C, 30 min.     

Synthesis and electrocatalytic performance of La0.3Ce0.1Sr0.5Ba0.1TiO3 anode catalyst for solid oxide fuel cells

Ce-doped La_0.4Sr_0.5Ba_0.1TiO_3–δ (LCSBT) perovskite anode catalysts in solid oxide fuel cells were successfully synthesized by a modified rheological phase reaction for the first time. Pure LCSBT could be obtained under a reducing atmosphere and nano-CeO₂ particles could be exsoluted from LCSBT after being sintered in air. The catalytic activity and electrochemical performance of LCSBT anodes for the H₂ oxidation were obviously improved comparing with the pure La_0.4Sr_0.5Ba_0.1TiO_3–δ (LSBT) and LSBT&CeO₂ admixture anodes. The improved performance could be attributed to the nanostructure of LCSBT and the exsoluted nano-CeO₂ particles.     

Application of RF GDMS for trace element analysis of nonconducting La0.65Sr0.3MnO3 ceramic layers

Radio-frequency glow discharge mass spectrometry (RF GDMS) has been applied to the determination of trace elements in ceramic perovskite layers (La_0.65Sr_0.3MnO₃) using synthetic standards. For the preparation of these standards high-purity powder of the basic material (La_0.65Sr_0.3MnO₃) was doped with trace elements in concentrations from 20 to 500 μg/g and the mixture was pressed to compact samples. The resulting calibration curves and the calculated relative sensitivity factors (RSF) differed only from 0.4 to 2 for different elements. For nearly all elements the standard deviations in the determination of trace elements in La_0.65Sr_0.3MnO₃ were better than 15% and detection limits (using the 3σ-criterion) were below 10 μg/g. Changes of the discharge parameters (RF power, pressure of the discharge gas (Ar) in the RF glow discharge) have no significant influence on the sensitivity to elements.     

Magnetic entropy change in perovskite manganites La0.7A0.3MnO3 La0.7A0.3Mn0.9Cr0.1O3 (A = Sr,Ba, Pb) and Banerjee criteria on phase transition

The La_0.7A_0.3MnO₃, La_0.7A_0.3Mn_0.9Cr_0.1O₃ (A = Sr, Ba, Pb) polycrystalline perovskite manganites sample was prepared by the sol–gel technique. The replacement of partial Mn³⁺/Mn⁴⁺ by Cr³⁺ (Cr³⁺ with the same electronic configuration as Mn⁴⁺) cause the variety of magnetocaloric property and magnetic entropy change. The maximum magnetic entropy change ΔS_H = −1.16 J/kg K and the Relative Cooling Power (about 43.3 J/kg) were obtained from La_0.7Sr_0.3Mn_0.9Cr_0.1O₃ in the Cr³⁺ doped series under 1 T magnetic field variation. On this paper, Banerjee criteria had been remarked to distinguish the first-order or second-order phase transition on phase transition of the doped perovskite manganites.     

Preparation and Properties of Highly Oriented Sr0.3Ba0.7Nb2O6 Thin Films by a Sol-Gel Process

We succeeded in the preparation of epitaxial or highly oriented strontium-barium niobate (Sr_0.3Ba_0.7Nb₂O₆) thin film by a sol-gel process. A homogeneous coating solution was prepared with Sr and Ba acetates and Nb(OEt)₅ as raw materials, and acetic acid and diethylene glycol monomethyl ether as solvents. Sr_0.3Ba_0.7Nb₂O₆ film sintered at 900°C on MgO(1 0 0) was oriented with c-axis perpendicular to the substrate surface. Sr_0.3Ba_0.7Nb₂O₆ film sintered at 700°C on SrTiO₃(1 0 0) was an epitaxial and oriented with c-axis in parallel to the substrate surface. Transmittance of Sr_0.3Ba_0.7Nb₂O₆ film (film thickness: 144 nm) was more than 60% at the range from 400 to 800 nm. Refractive index was 2.33 at 633 nm. Dielectric constant and dielectric loss of the Sr_0.3Ba_0.7Nb₂O₆ thin films prepared on polycrystal Pt substrates were 600 and 0.06 at room temperature and 1 kHz, respectively. The curie temperature (T_c) of polycrystalline Sr_0.3Ba_0.7Nb₂O₆ thin films was about 200°C. At room temperature and 50 kHz, remanent polarization (P_r) and coercive field (E_c) of the polycrystalline thin films were 1.79 C/cm² and 2.69 kV/cm, respectively.     

Gd掺杂对La0.67 Sr0.33 CoO3、La0.67 Sr0.33 MnO3体系磁行为的影响

M~T curves and M~H curves of the systems La0.67Sr0.33CoO3, La0.67Sr0.33MnO3 (x=0.00, 0.05, 0.10, 0.20, 0.30, 0.40, 0.50, 0.60, 0.67) were studied by experiments. Experimental results show that the magnetic structure of La0.67-xGdxSr0.33CoO3 system exhibits the cluster glass state with doping Gd increasing. The M~T curves of samples x>0.10 exhibit a peculiar phenomenon that M value rises sharply at low temperature range. The magnetic structure of La0.67-xGdxSr0.33MnO3 system changes from the long rang ferromagnetic order to the cluster glass state, and anti ferromagnetic state. The M~T curves of samples x≥0.50 decline sharply at low temperature range. The different phenomena shown by the two systems come from different coupling function between Gd and Co, Mn and from spin state transition of Co.     

Effect of strontium deficiency on the critical behavior at paramagnetic to ferromagnetic phase transition in La0.57Nd0.1Sr0.33MnO3 manganite oxide

The samples of manganese perovskite La_0.57Nd_0.1Sr_0.33−x_xMnO₃ (0.00 ≤ x ≤ 0.05) ( is the strontium deficiency) are prepared by solid-state methods, and all of them have a rhombohedral perovskite structure, revealed by X-ray diffraction. The critical properties of the samples around the paramagnetic–ferromagnetic phase transition were investigated through various techniques such as modified Arrott plot (MAP), Kouvel–Fisher (KF) method and critical isotherm (CI) analysis based on the data of static magnetic measurements recorded around the Curie temperature T_C. The experimental results have revealed that the samples exhibited the second-order magnetic phase transition and the critical exponents of β and γ for La_0.57Nd_0.1Sr_0.33MnO₃ are close to those found out by the 3D-Heisenberg model. Furthermore, the estimated critical exponents of La_0.57Nd_0.1Sr_0.33−x_xMnO₃ (x = 0.025 and 0.05) are consistent with the prediction of the 3D-Ising model. We deduced, following the Harris criterion, that the disorder in our case is relevant which can be the cause of the change in the universality class and we noted that the critical exponents β are almost similar to the value of the mean-field theory which can be explained by the existence of a long-range dipole–dipole interaction.     

Microstructure and electrical properties of lead zirconate titanate thin films deposited by sol-gel method on La<Subscript>0.7</Subscript>Sr<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript>/SiO<Subscript>2</Subscript>/Si substrate

(La_0.7Sr_0.3)MnO₃ thin films were deposited on SiO₂/Si substrates by a metal-organic decomposition (MOD) method, and then Pb(Zr_0.52Ti_0.48)O₃ (PZT) thin films were grown on (La_0.7Sr_0.3)MnO₃-coated SiO₂/Si substrates by a sol-gel method. The effects of annealing temperature on the crystalline phases, microstructures and electrical properties of the PZT films were investigated. X-ray diffraction analysis results indicated that the PZT films with a perovskite single phase could be obtained by annealing at 650°C. The dielectric constant and the remnant polarization of the PZT films increased with increasing annealing temperature. The remnant polarization and the coercive field of the films annealed at 650°C were 18.3 μC/cm² and 35.5 kV/cm, respectively, whereas the dielectric constant and loss value measured at 1 kHz were approximately 1100 and 0.81, respectively.     

The layer‐by‐layer stoichiometry of La0.7Sr0.3MnO3/SrTiO3 thin films and interfaces

By using a combination of synchrotron photoemission and a finite element model, the layer‐by‐layer stoichiometry of pulsed laser deposited La_0.7Sr_0.3MnO₃ (LSMO) thin films is extracted. For 3‐ and 5‐unit‐cell films, the LSMO stoichiometry at the LSMO/SrTiO₃ interface has been studied, and it is found that the Sr surface segregation is reversed at the lower interface such that it becomes La rich and Sr depleted. Mn is depleted throughout the entire layer, consistent with reports of a magnetically ‘dead’ layer and the formation of a Ruddlesden–Popper phase. The depth‐averaged Mn concentration is less than 50% of the expected value, suggesting that interface mixing or segregation into the bulk is important, whereas the depth‐averaged La and Sr concentrations are close to the expected values. Copyright © 2013 John Wiley & Sons, Ltd.     

Preparation and Application of Ca0.8 Sr0.2 TiO3 for Plasma Activation of CO2

Despite a large interest in plasma-assisted catalytic technology (PACT), very little has been reported about the catalytic effects of different dielectric barriers on a dielectric barrier discharge (DBD) reaction. In the present study, Ca_0.8Sr_0.2TiO₃, that possesses a high permittivity, was prepared by liquid phase sintering and used as a dielectric barrier in a DBD reactor to break CO₂. The mechanical and dielectric properties of Ca_0.8Sr_0.2TiO₃ were greatly enhanced by adding 0.5 wt.% Li₂Si₂O₅. A DBD plasma was successfully generated by using this Ca_0.8Sr_0.2TiO₃ as a dielectric barrier and 18.8% CO₂ conversion was achieved with the residence time of 0.17 s at the frequency of 8 kHz, which was much higher than with those using an alumina or a silica glass barrier. It was found that the plasma power increased with the increasing of the permittivity, and finally very dense and strong microdischarges were initiated to decompose CO₂.     

Comparison of LaFeO3, La0.8Sr0.2FeO3, and La0.8Sr0.2Fe0.9Co0.1O3 perovskite oxides as oxygen carrier for partial oxidation of methane

Comparison of LaFeO3, La0.8Sr0.2FeO3, and La0.8Sr0.2Fe0.9CO0.1O3 perovskite oxides as oxygen carrier for partial oxidation of methane in the absence of gaseous oxygen was investigated by continuous flow reaction and sequential redox reaction, Methane was oxidized to syngas with high selectivity by oxygen species of perovskite oxides in the absence of gaseous oxygen. The sequential redox reaction revealed that the structural stability and continuous oxygen supply in redox reaction decreased over La0.8Sr0.2Fe0.9Co0. 1O3 oxide, while LaFeO3 and La0.8Sr0.2FeO3 exhibited excellent structural stability and continuous oxygen supply.     

Enhanced magnetoresistance in La07Sr03MnO3 nanoscaledgranular composites

By soft chemical processing, two kinds of nanoscaled lanthanum manganite granular composites, La₀₇Sr₀₃Mn₃ / CoFe_2O4 (LSMO/CF) and La₀₇Sr_0.3Mn₃ / Nd₀₇Sr₀₃Mn_O3(LSMO/NSMO), were fabricated. Enhancement in magnetoresistance is observed in both of the composites. Different magnetic compounds, CF and NSMO, embedded in LSMO matrix, result in different temperature ranges where the enhancement occurs. It is considered that the phenomenon is associate with electron tunneling between sorts of grains as well as spin-dependent scattering.     

Microstructure,magnetoelectric properties and leakage mechanisms of La0.7Ca0.3MnO3/Bi3.15Nd0.85TiO3 composite thin films

A series of high quality Bi_3.15Nd_0.85TiO₃ (BNT) ferroelectric thin films and La_0.7Ca_0.3MnO₃/Bi_3.15Nd_0.85TiO₃ (LCMO/BNT) multiferroic composite thin films were deposited on Pt(111)/Ti/SiO₂/Si(100) substrates by chemical solution deposition (CSD) method. The microstructure, surface morphology and leakage mechanisms of BNT and LCMO/BNT composite films were revealed by X-ray diffraction, scanning electron microscopy and semiconductor device analyzer, respectively. Ferroelectric behavior along with a remnant polarization (2P_r) of 20 μC/cm², saturated magnetization around 56 emu/cm³ and magnetoelectric effect (ME) voltage coefficient α_ME of 33 mV/cm Oe at 1 kHz for LCMO/BNT composite films were obtained at room temperature, indicating that the coupling effects of electric and magnetic field exist in the fabricated LCMO/BNT multiferroic composite thin films. And our observations provide an effective way to manipulate the conduction behavior and push forward understanding the leakage mechanism in LCMO/BNT composite films.