Magneto-transport study of nanocrystalline and polycrystalline manganites La0.75Ca0.25MnO3

We compare structure and magnetic parameters of the polycrystalline and nanocrystalline manganites La_0.75Ca_0.25MnO₃ prepared by the carbonate precipitation and the sol–gel methods, respectively. The Curie temperature is surprisingly enhanced for nanocrystalline manganite. An effect of magnetic fields on transport parameters is analyzed for the metallic and semiconducting ranges.     

Accurate Crystal Structure Refinement of La3Ta0.25Zr0.50Ga5.25O14

Crystallography Reports - An accurate X-ray diffraction study of a La3Ta0.25Zr0.50Ga5.25O14 single crystal (a = 8.2574(4) Å, c = 5.1465(4) Å, sp. gr. P321, Z = 1, R/R w = 0.62/0.57% for...     

Transport behavior of yttrium substituted polycrystalline La0.7Pb0.3MnO3

The transport properties in La_0.7?xY_xPb_0.3MnO₃ (0.0 ? x ? 0.2) is investigated. The substitution of La³⁺ ions by smaller nonmagnetic Y³⁺ leads to greater spin disorder and induces variations in the magnetotransport behavior. From resistivity versus temperature curves a metal–insulator transition phenomenon is observed at the transition temperature, T_P, decreases as the Y content increases. The resistivity is well fitted using the equation ρ(T) = ρ_nexp[(T¹/T)ⁿ] with n = 1/4 and n = 1/2 at high and intermediate temperatures, respectively. The characteristic temperature T¹ varies with Y content in a manner consistent with the localization model of variable range hopping. Below T_P, resistivity varies as a function of power law contributions, ρ = ρ₀ + ρ₂T² + ρ_5/2T^5/2, corresponding to the electron scattering processes in the ferromagnetic phase.     

Influence of polymerizing agent on structure and spectroscopic properties of nano‐crystalline La0.8Sr0.2MnO3 powders

Strontium‐doped lanthanum manganite (La_1‐xSr_xMnO₃ or LSM) is the material most widely used on solid oxide fuel cells cathode. In this work, nano‐scale La_0.8Sr_0.2MnO₃ powders have been synthesized by polymeric precursor‐based methods using different polymerizing agents with the aim of evaluating the influence of this change in the final powder. The powders calcined at 700 °C for 2 h have been characterized by X‐ray diffraction (XRD), scanning electron microscopy (FE‐SEM) and Fourier transform infrared spectroscopy (FTIR) in order to investigate the quality of both synthesis routes. It is shown that the crystal structure and morphology of the particles are significantly dependent on the preparation conditions. Single phase and La_0.8Sr_0.2MnO₃ nano‐crystalline particles less than 30 nm were obtained using ethylene glycol as polymerizing agent. FTIR results have been indicated that only the powder obtained using gelatin presented small traces of hydroxyl groups on its surface. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Magnetocaloric effect in nano- and polycrystalline La0.8Sr0.2MnO3 manganites

The sol–gel method is applied to prepare the polycrystalline La_0.8Sr_0.2MnO₃ manganite and to reduce grain sizes of nanocrystalline sample down to 23 nm. The magnetocaloric effect was determined from isothermal magnetization measurements. Transition from the polycrystalline to nanocrystalline structure reduces the size of magnetocaloric effect and markedly broadens the temperature interval over which it occurs. The relative cooling power RCP(S) = 35 J/kg in polycrystalline manganite remains almost the same in the nanocrystalline case.     

Electrical and magnetotransport properties of La0.7Sr0.3MnO3/TiO2 composites

The composite samples with nominal compositions of (1‐x) La_0.7Sr_0.3MnO₃ + x TiO₂ (x = 0, 0.05, 0.10, 0.15 and 0.20) were synthesized via solid state reaction process. The X‐ray diffraction and scanning electronic microscopy observations reveal no reaction between La_0.7Sr_0.3MnO₃ (LSMO) and TiO₂ phases. Temperature dependence resistivity measurements show that TiO₂ phase shifts the metal‐insulator transition temperature (T_p) towards lower temperature and increases the resistivity. Moreover, the magnetization of the composite samples decreases with TiO₂ content. An enhancement in magnetoresistance is observed in the composite samples with x = 0.05 and x = 0.10 at low magnetic fields, which is encouraging for potential application of magnetoresistive materials at low field. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Interface-induced microstrain in La0.67Ca0.33MnO3/YBa2Cu3O7 superlattices

Epitaxial La_0.67Ca_0.33MnO₃/YBa₂Cu₃O₇ superlattices with varying modulation lengths are produced by pulsed laser deposition and their detailed structural characterization is performed by X-ray diffraction. It is shown here that YBa₂Cu₃O₇ layers in all superlattice specimens have considerably high microstrain values, which are probably interface induced. The large distortions in the vicinity of the interfaces may alter the electronic structure and, hence, may have important consequences on the electrical and magnetic properties of the superlattice systems.     

预烧温度对La0.67Ca0.33MnO3陶瓷结构及电学性能的影响

采用溶胶-凝胶法成功制备了La0.67Ca0.33MnO3粉末,并通过改变预烧温度获取电学性能优异的陶瓷.利用X射线衍射(XRD)、扫描电子显微镜(SEM)、四探针法对不同预烧温度处理后的La0.67Ca0.33MnO3陶瓷的物相、结构、微观形貌和电学性能进行测试,分析预烧温度对材料的晶粒尺寸和电学性能的影响,从而摸索出最佳预烧温度.实验结果表明:样品的结晶性能好物相纯,随着预烧温度的增加,晶粒尺寸、致密度、收缩率和电阻温度系数(TCR)在不断减小,电阻率先减小后增加.在300℃预烧,1450℃烧结得到的样品具有较高的TCR值达到了40.8;·K-1.     

Synthesis and characterization of nanocrystalline niobates

Nanocrystalline potassium niobates (KNbO₃, KNb₃O₈ and K₄Nb₆O₁₇) of different morphologies were synthesized with the low-temperature hydrothermal method by adjusting the alkalinity of the reaction system. A possible formation mechanism of the nanocrystalline niobates was proposed based on the experimental results. All of the products were characterized with X-ray powder diffraction and transition electron microscopy. The Fourier transform (FT) infrared, FT-Raman and UV–VIS properties of the niobates were studied.     

烧结工艺对La0.67Ca0.33MnO3多晶物理性质的影响

利用溶胶-凝胶法成功制备了La0.67Ca0.33MnO3粉末,并通过优化烧结工艺获得了物理性能优异的La0.67Ca0.33MnO3多晶靶材.利用X射线衍射仪和扫描电子显微镜分别测试了样品的物相、结构和微观形貌,并利用四探针法测试了靶材的电阻率-温度曲线.结果表明,影响TCR的主要因素有晶粒的化学均匀性、晶粒尺寸和晶界的数量,样品具有接近室温的金属-绝缘转变温度(TM1 =274 K)和高的电阻温度系数(TCR=13.5;).     

Ferromagnetic insulating and reentrant spin glass behavior in Mg doped La0.75Sr0.25MnO3 manganites

In this study, the effect of Mg substitution on structural, magnetic and electrical properties of La_0.75Sr_0.25Mn_1?xMg_xO₃ and La_0.75Sr_0.25?xMg_xMnO₃ (nominal compositions) samples are investigated by XRD, Ac susceptibility and electrical resistivity measurements. It is found that Mg does not replace La in the perovskite lattice. Also the results show that by increasing Mg doping levels, the paramagnetic–ferromagnetic and metal–insulator transition temperatures decrease. The reason for decreasing transition temperatures with increasing Mg concentration is, that the long-range FM order has been destroyed by the Mg, which is randomly occupying Mn site. This leads to the suppression of double-exchange interaction in the Mn³⁺–O–Mn⁴⁺ networks. Also the reentrant spin glass (RSG) state accompanied by FM transition, exists in high doped samples. The RSG state could be understood on the basis of double exchange ferromagnetic interaction in Mn³⁺–O–Mn⁴⁺ and super-exchange antiferromagnetic interaction in the Mn⁴⁺–O–Mn⁴⁺ networks.     

The Phase Formation,Solid Solution Range and Substitution of La for Ba in La1+zBa2‐zCu3Oy

The phase formation, solid solution range and the substitution of La for Ba in La_1+zBa_2‐zCu₃O_y (La123) system are investigated systematically for obtaining and understanding the optimum condition to prepare La123 superconductors with higher Tc. The solid solution ranges of La_1+zBa_2‐zCu₃O_y are z=0.20‐0.70 in air and z=0 in N₂. Atmosphere of air or O₂ is favourable for the substitution of La for Ba. The substitution is suppressed when La_1+zBa_2‐zCu₃O_y are sintered and cooled in N₂ . It is found that sintering and cooling procedure in N₂ atmosphere is one key procedure to prepare single phase high‐quality LaBa₂Cu₃O_y superconductor with higher Tc.     

高分子辅助化学溶液沉积法制备La0.7Sr0.3MnO3/Si薄膜及电磁输运研究

利用高分子辅助化学溶液沉积法在Si(100)衬底上外延生长La0.7Sr0.3MnO3薄膜.并利用X射线衍射仪结果、扫描电子显微镜结果和电阻率-温度曲线(ρ-T曲线)结果、磁电阻(MR)曲线结果对其晶体结构、表面形貌和电磁输运机制进行了研究.结果显示实验制备的La0.7Sr0.3MnO3薄膜为赝立方钙钛矿多晶结构,薄膜表面均匀平滑,结晶性好,晶粒尺寸约为50~70 nm.随着温度降低,薄膜电输运机制从绝缘体行为向转变金属导电行为.金属绝缘转变转变点温度(TMI)随磁场的增加而升高,在0 T和1 T分别为TMI=238K、246 K.电输运测试结果说明,低温(TTMI)时薄膜按照绝热近似的小极化子输运.     

水热法制备铝掺杂二氧化锰及其储锌性能的研究

可充电水系锌锰电池成本低、环保无毒、安全性好,在大规模储能领域具有广阔应用前景.然而,该电池中不仅存在MnO2正极导电率低、结构稳定性差等问题,而且存在负极锌枝晶生长与析氢腐蚀问题,这严重制约了电池循环稳定性的提升.本文采用水热法制备了Al掺杂二氧化锰作为锌锰电池的高稳定性正极材料,并通过X射线衍射(XRD)、能量色散X射线光谱仪(EDS)、傅里叶变换红外光谱仪(FT-IR)和X射线光电子能谱(XPS)详细讨论了Al掺杂对MnO2物相、形貌、含水量与电化学性能的影响.研究表明,Al掺杂不仅使样品由微米级β-MnO2转变为纳米级α-MnO2,还使产物中结晶水含量增加.作为锌锰电池正极材料,所制备的Al掺杂MnO2在1 A·g-1高电流密度下500次循环后剩余容量高达150.1 mAh·g-1,循环稳定性远优于未掺杂的MnO2样品(500次循环后容量为97.8 mAh·g-1).本研究对高性能锌锰电池的开发具有一定启示意义.     

高电阻温度系数的La0.67(Ca0.33-xSrx)MnO3多晶陶瓷制备与电学性能研究

采用溶-凝胶法在1450 ℃烧结12 h的条件下,制备了x组分为0、0.01、0.03、0.05、0.06、0.07的La0.67(Ca0.33-xSrx)MnO3 (LCSMO)多晶陶瓷,通过R-T、XRD和SEM测试分析,结果表明,溶胶-凝胶方法制备的LCSMO多晶陶瓷其物相较纯,阻温特性良好,晶粒尺寸大,致密度高.在未掺Sr时,该体系电阻温度系数(TCR)达到最高值42.5;.随着Sr掺杂比增加,金属-绝缘体转变温度TP升高,TCR降低,而在TP达到室温时,LCSMO多晶陶瓷具有较大的TCR(9;以上).在LCSMO多晶体系中如此大的TCR较为罕见,使其具有重要的潜在应用价值.     

Bi3.25La0.75Ti3O12纳米片的水热合成及表征

以硝酸铋、硫酸钛和硝酸镧为原料,氢氧化钠为矿化剂,采用水热法制备了镧掺杂的钛酸铋片状纳米材料,研究了水热温度和水热时间对其结构的影响.采用XRD和SEM对产品的物相和结构进行了表征.结果表明:以硫酸钛作为无机钛源,在较低的水热温度(200 ℃)、水热时间为6 h时可以制备出花状的Bi3.25La0.75Ti3O12纳米片.     

钛酸锶钡纳米管阵列薄膜的水热合成及其性能研究

以阳极氧化法制备的二氧化钛纳米管阵列为模板,结合水热法制备了钛酸锶钡纳米管阵列薄膜.讨论了Ba1-xSrxTiO3纳米管阵列薄膜的结构、形貌和电学性能.用X射线衍射仪表征其晶体结构;扫描电子显微镜观察其表面及断口形貌;以及用宽频介电阻抗谱仪测试其介电性能.结果表明:在较为温和的条件下用水热法制备了立方相及四方相的Ba1-xSrxTiO3纳米管阵列薄膜;纳米管孔径在65～ 80 nm之间,薄膜厚度可达10 μm以上;经热处理之后的薄膜样品在1 kHz介电常数可达338,介电损耗为0.46.     

高性能Ag/La0.7Mg0.3MnO3/p+-Si阻变器件的研究

采用溶胶-凝胶工艺在p型单晶硅衬底上制备了La0.7Mg0.3MnO3薄膜,对薄膜的微观结构及Ag/La0.7Mg0.3MnO3/p+-Si器件的电致阻变性能进行了研究.结果表明:La0.7Mg0.3MnO3薄膜在经过700℃退火2h后为单一的钙钛矿结构,沿(112)晶向择优生长,薄膜致密平整;Ag/La0.7Mg0.3MnO3/p+-Si阻变器件具有典型的双极型阻变特性,具有非常高的电阻开关比,其高阻态(HRS)与低阻态(LRS)的比值高于105,以及较佳的耐疲劳性能,器件在1000次循环后高、低阻态比值没有明显变化;器件在高阻态(HRS)时的导电机制为Schotty势垒发射效应,低阻态(LRS)导电机制为导电细丝机制.     

Self-Organized Quantum Dots from a Material with Colossal Magnetoresistance (La0.7Ca0.3MnO3), Grown on MgO Crystals

Crystallography Reports - Films in the form of superlattices with alternating La0.7Ca0.3MnO3 layers, possessing colossal magnetoresistance, and dielectric MgO layers have been grown on MgO(100)...     

Crystal structure of La3Nb0.5Ga5.5O14 at 20 K

The atomic structure of La₃Nb_0.5Ga_5.5O₁₄ single crystals (space group P321, R = 2.21%, R _w = 2.31%) at 20 K was determined using X-ray diffraction. A comparative analysis of the structural characteristics determined at temperatures of 293 and 20 K did not reveal a noticeable redistribution of bond valences that could be considered precursors of a possible phase transition accompanied by the reduction of the symmetry of the crystal. The assumption was made that the anomalous behavior of the permittivity e ₃₃ of the La₃Nb_0.5Ga_5.5O₁₄ single crystals is associated with the decrease in the possible relative displacements of the Ga(1) and Nb atoms occupying the cation position with point symmetry 32 in a 1: 1 ratio.