乙二胺对溶胶-凝胶法合成La0.65Sr0.35MnO3的影响

利用溶胶-凝胶工艺制备了钙钛矿结构的亚锰酸盐化合物(La0.65Sr0.35MnO3),通过XRD和SEM研究了乙二胺和氨水对溶胶-凝胶工艺的影响.结果发现,当体系pH值调梧到8.0附近时,添加乙二胺有利于溶胶-凝胶的形成,而添加氨水的样品中有部分沉淀产生,不能很好地凝胶化.热处理后,添加乙二胺的样品形成的产物颗粒细小均匀,而添加氨水的样品出现了比较严重的团聚现象.分析表明:乙二胺在溶胶.凝胶形成过程中起着桥接、交联和螫合的作用,桥接和交联有利于凝胶化过程的发生和稳定,螯合作用有利于金属离子均匀分散在凝胶的三维网络结构中,从而减少后期热处理过程中的团聚.     

Magnetocaloric properties of La0.6Ca0.4MnO3

A theoretic work on magnetocaloric properties of the polycrystalline La_0.6Ca_0.4MnO₃ system near a second-order phase transition from a ferromagnetic to a paramagnetic state is presented. The value of the magnetocaloric effect has been determined from the calculation of magnetization as a function of temperature under different external magnetic field shifts. The magnetic entropy change ?S _M reaches a peak of about 3 J kg^?1 K^?1 at 266 K upon 1.60 KA/m applied field variation. The ?S _M distribution is much more uniform than that of gadolinium, which is desirable for an Ericson-cycle magnetic refrigerator, which is beneficial for the household application of active magnetic refrigerant materials.     

La0.7Ca0.3MnO3薄膜庞磁电阻(CMR)现象的分析

针对La0.7Ca0．3MnO3薄膜材料中磁团簇的特性及其对材料庞磁电阻(CMR)现象的影响进行了研究。根据实验数据，利用Wagner等提出的数学模型，对La0.7Ca0.3MnO3薄膜中的磁团簇随温度的变化行为特性进行了研究；通过对La0.7Ca0．3MnO3薄膜相关实验数据的拟合，发现La0．7Ca0．3MnO3薄膜的居里温度范围应当在245～250K之间，从而进一步缩小了该材料居里温度的误差范围；还通过计算MR随磁团簇总角量子数J的变化，研究了制约和影响MR值的主要因素，发现磁团簇尺寸在La0．7Ca0.3MnO3薄膜的CMR现象中扮演了重要角色。     

La0.8Ca0.2MnO3纳米线的制备及表征

利用Sol-gel法结合氧化铝模板技术制备了La0.8Ca0.2MnO3纳米线, 并研究了两种热处理方法对 La0.8Ca0.2MnO3纳米线结构和形貌的影响. 快速升温到800 ℃得到的La0.8Ca0.2MnO3纳米线较粗, 其直径大于氧化铝模板的孔径, 而经过缓慢升温到400 ℃预处理再升温到800 ℃得到的La0.8Ca0.2MnO3纳米线, 其直径和氧化铝模板的孔径相当, 都约为35 nm. X射线衍射和透射电镜分析结果表明, 两种方法得到的 La0.8Ca0.2MnO3纳米线都是具有钙钛矿结构的属于单斜晶系的多晶材料.     

Magnetocapacitance and impedance spectroscopy of Ba0.7Sr0.3TiO3/La0.67Sr0.33MnO3 and Ba0.8Sr0.2TiO3/La0.67Sr0.33MnO3 thin film heterostructures

The paper discuses synthesis of Ba_0.7Sr_0.3TiO₃/La_0.67Sr_0.33MnO₃ and Ba_0.8Sr_0.2TiO₃/La_0.67Sr_0.33MnO₃ thin film heterostructures by sol–gel (citrate gel route) method. The XRD spectra are determined for confirmation of crystalline phases of Ba_0.7Sr_0.3TiO₃, Ba_0.8Sr_0.2TiO₃ and La_0.67Sr_0.33MnO₃ thin films. The observed variation of Cp, tan δ, magnetocapacitance (Mc), real part of dielectric constant ε′, imaginary part of dielectric constant ε″as a function of frequency, magnetoresistance R(H) and magnetoimpedance Z(H) are presented in the paper. Further the observed impedance spectra as a function of frequency f is also presented in the paper. The observed variation of Mc is qualitatively correlated with the stress induced effect and magnetoresistasnce effect occurring simultaneously.     

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.     

La2/3Ca1/3MnO3中的输运机制与CMR效应

研究了超大磁阻材料La2/3Ca1/3MnO3的磁电特性，发现由电阻测量得到的绝缘体．金属相变与由磁化曲线得到的顺磁-铁磁相变温区完全一致，均随磁场的增强推向高温区。结果证明样品输运行为的变化起因于磁结构的变化，在Tc附近产生超大巨磁电阻效应。     

Galvanostatic synthesis of MnO2 in carbon cloth: an electrochemical impedance spectroscopy study

Journal of Solid State Electrochemistry - In the present work, MnO2 was electrodeposited (galvanostatic mode) on carbon cloth (CC) from manganese nitrate and sulfate precursors. Its electrochemical...     

高能振动球磨法制备纳米钙钛矿粉体La_(0.8)Ca_(0.2)MnO_3

利用高能振动球磨法,以氧化镧、氧化钙和二氧化锰为原料,制备出纳米氧化物粉体La0.8Ca0.2MnO3。采用XRD分析了球磨过程中粉体的晶化过程;以TEM观察了制备出的粉体的颗粒尺寸和分散状况。结果表明,采用高能振动球磨法20 h就能制备出颗粒尺寸在50~100 nm,属正交晶系的纳米钙钛矿粉体La0.8Ca0.2MnO3;在该反应体系的球磨过程中,产物晶粒经历了先长大后细化的过程。将球磨法制备出的粉体压成电极,进行放电分析。结果表明采用该方法制备出的纳米La0.8Ca0.2MnO3粉体具有很好的电催化活性。     

Surface spin glass behavior in sol-gel derived La0.7Ca0.3MnO3 nanotubes

We report the fabrication of La(0.7)Ca(0.3)MnO(3) nanotubes (LCMONTs) with a diameter of about 200 nm, by a modified sol-gel method utilizing nanochannel alumina templates. High resolution transmission electron microscopy confirmed that the obtained LCMONTs are made up of nanoparticles (8-12 nm), which are randomly aligned in the wall of the nanotubes. The strong irreversibility between zero field cooling (ZFC) and field cooling (FC) magnetization curves as well as a cusplike peak in the ZFC curve gives strong support for surface spin glass behavior.     

In situ neutron diffraction study on Pd-doped Mg0.65Sc0.35 electrode material

The behavior of a negative composite electrode made of Pd-doped Mg_0.65Sc_0.35 active material has been studied dynamically by in situ neutron diffraction during a complete charge-discharge electrochemical cycle starting from a virgin alloy. From the analysis of the collected diffraction patterns, phase identification, phase amounts, structural changes and cell volume evolutions have been determined as a function of the electrochemical state of (dis)charge. For the first charge, the active material shows a structural transformation from body-centered cubic (bcc) to face-centered cubic (fcc). During the following discharge, a two-phase behavior with equilibrium between a hydrogen-poor fcc phase and a hydrogen-rich one is observed. Complete discharge beyond the hydrogen-poor phase was not reached and a good electrochemical reversibility is expected between the hydrogen-poor and the hydrogen-rich fcc phases for this electrode material.     

(La1-xDyx)0.67Ca0.33MnO3的磁性和磁电阻特性

在La0.67Ca0.33MnO3中用Dy对La进行了部分替代.结果表明, 随替代量增加, 材料的居里温度和相变温度单调下降, 磁电阻比急剧增大.这些变化可以用稀土离子平均直径的减小和自旋团簇模型来解释.掺Dy使材料中形成大量无序自旋团簇, 外加磁场使自旋团簇取向一致, 从而增加了电子在团簇之间跳跃时的迁移率, 产生了磁场所致的庞磁电阻效应.     

Potassium polytitanate gas-sensor study by impedance spectroscopy

Nanocrystalline potassium polytitanates K₂O·nTiO₂·mH₂O represent a new type of semiconducting compounds which are characterized by a high specific surface that makes them promising for use in gas sensors. In this work, we have studied potassium polytitanate mesoporous nanoparticle agglomerates placed over a SiO₂/Si substrate equipped with multiple coplanar electrodes to measure the electrical response to various organic vapors, 1000 ppm of concentration, mixed with air by impedance spectrometry in range of the 10⁻²–10⁶ Hz. The recorded impedance data for each sensor segment are associated with RC components of an equivalent circuit which are applied to selectively recognize the test vapors exploiting a “multisensor array” approach.     

相分离La0．33Pr0．34Ca0．33MnO3薄膜体系中的交换偏置效应

在相分离La0．33Pr0.34Ca0．33MnO3薄膜体系中发现了大的交换偏置效应．在4K时,交换偏置场的大小达到了约1kOe．交换偏置效应可能源自薄膜内禀的电子相分离特性或薄膜的表面效应．交换偏置效应表现出强的温度、冷却磁场以及厚度依赖的关系．     

六方类钙钛矿体系La2Ca2MnO7-Sm2Ca2MnO7的结构变化和相关系

利用柠檬酸盐前驱物法制备了La2-xSmxCa2MnO7系列样品,并利用Rietveld方法对结构进行了精修.结果表明,体系中存在两个单相区(x≤0.8和x≥1.0)和一个两相区(O.8相似文献   

Structural and Physical Properties of La2/3Ca1/3MnO3 Prepared via a Modified Sol-Gel Method

High purity powder of manganese perovskite, La_2/3Ca_1/3MnO₃, is prepared using a modified sol-gel synthesis based on the dissolution and homogenisation of metal salts in ethanol-acetic acid mixture without using any complexing aid (e.g., polyol or polyhydroxy acid, etc.), which is essentially used in the polymeric precursor routes. This modification minimises the organic contamination in the resulting ceramic that is formed as single perovskite at 650°C, after short calcination time periods. The formation of a monophasic material and absence of second phases or phase segregation was confirmed by powder X-ray diffraction, energy dispersive X-ray and electron microscopy of the ceramic calcined at higher temperatures (800–1400°C). The calcined samples are nanocrystalline up to 1000°C (average particle size, 44 nm) however, significant particle growth is observed at higher temperatures with micron-sized grains present in the sample sintered at 1400°C. The sample exhibits the characteristic colossal magnetoresistance behaviour. Owing to the high chemical and structural purity of the obtained ceramic, the intrinsic bulk features like metal-insulator transition and ferromagnet-antiferromagnet behaviour of the polycrystalline sample are comparable to those observed in the single crystal La_2/3Ca_1/3MnO₃ specimen, used as a reference. The single-crystal-like properties are also corroborated by the observation of a sharp metallic fermi edge in the UPS measurements. The variable temperature photoemission spectra reveal a temperature dependent redistribution of spectral weight close to the fermi level corroborating the temperature dependent resistance and magnetoresistance of the sample. The material shows a homogeneous grain size and a high sinterability as shown by TEM and SEM studies, respectively. XPS study indicates a charge carrier hopping between Mn³⁺(3d⁴) and Mn⁴⁺(3d³) sites.     

不同沉淀剂对Ce0.65Zr0.35O2复合氧化物性能的影响

本文选用不同沉淀剂,用共沉淀法制备了稀土储氧材料Ce0.65Zr0.35O2复合氧化物,对用不同沉淀剂制备出的氧化物样品进行了XRD,BET,OSC和H2-TPR的测试和分析。结果表明,用不同沉淀剂制备出的氧化物,均为立方相的铈锆固溶体,经1000℃5小时老化后也无相的分离;以(NH4)2CO3和NH3.H2O的混合物为沉淀剂制备出的氧化物有最大的比表面积,经600℃焙烧后高达120m2/g以上;并且样品具有良好的储氧性能及低温还原能力。     

IrO_2掺杂对La_(0.7)Ca_(0.2)Sr_(0.1)MnO_3室温磁电阻效应的增强

将IrO2掺杂到固相反应法制备的La0.7Ca0.2Sr0.1MnO3(LCSMO)微粉中,制备了(1-x)La0.7Ca0.2Sr0.1MnO3+xIrO2(LCSMO/IrO2)复合体系。通过X射线衍射(XRD)、振动样品磁强计(VSM)以及直流四探针法测试,对复合材料的结构及性能进行了研究。结果发现,在复合体系中,一部分Ir4+离子取代了B位的Mn4+离子,另一部分以IrO2氧化物的形式存在于颗粒的边界处。随着IrO2掺杂量的增加,样品的比饱和磁化强度(σs)快速下降,而居里温度(TC)先下降后上升,电阻率也发生显著变化。与此同时,IrO2的掺杂使样品的低场磁电阻效应(LFMR)和高场磁电阻效应(HFMR)都有所增强,在H=3 kOe,T=307 K,x=5%的样品磁电阻达到11.65%;同样对于x=5%的样品,在H=2 T,T=295 K,磁电阻达到28%,室温磁电阻得到显著增强,这可能与本征磁电阻效应的特点及材料的TC和相变温度接近室温有关,另外掺杂物本身的特性对磁电阻的增强也有一定的影响。     

Influence of silver doping on the electrical and magnetic behavior of La0.7Ca0.3MnO3 manganites

A systematic investigation of structural, magnetic and magnetotransport behavior of La_0.7Ca_0.3?xAg_xMnO₃ manganites has been undertaken. The X-ray diffraction shows a structural transformation from orthorhombic to rhombohedral with increasing Ag concentration. The undoped and 10% Ag substituted samples exhibit double transition in M–T curves. The electrical resistivity in the entire temperature range is fitted to effective medium approximation and phase separation models. The sign of S changes from negative to positive with increase in Ag doping. The low temperature thermopower data has been fitted to an equation containing diffusion, magnon drag and phonon drag terms. The paramagnetic insulting part of the TEP data has been analyzed using small polaron hopping mechanism.     

Ferromagnetic to spin glass cross over in (La,Tb)2/3Ca1/3MnO3

In the series La_2/3?xTb_xCa_1/3MnO₃, it is known that the compositions are ferromagnetic for smaller values of x and show spin glass characteristics at larger values of x. Our studies on the magnetic properties of various compositions in the La_2/3?xTb_xCa_1/3MnO₃ series show that the cross over from ferromagnetic to spin glass region takes place above x ≈ 1/8. Also, a low temperature anomaly at 30 K, observed in the ac susceptibility curves, disappears for compositions above this critical value of x. A mixed phase region coexists in the narrow compositional range 0.1 ≤ x ≤ 0.125, indicating that the ferromagnetic to spin glass cross over is not abrupt.