La0.7-xNdxBa0.3MnO3纳米颗粒的居里温度与磁热效应

用溶胶-凝胶法制备了La0.7-xNdxBa0.3MnO3(x=0,0.05,0.10,0.15,0.20,0.25)多晶纳米颗粒,用XRD分析其相结构并计算晶格常数,用VSM测量样品的磁性能并计算磁熵变和居里温度.结果表明,La-Ba-Mn-O系列中适当的Nd掺杂可调整材料的居里温度至室温附近并有效提高其磁熵变.文中对于Nd掺杂对居里温度和磁熵变影响的机理进行了定性的分析.     

La0.67-xCa0.33MnO3系列样品的磁热效应

用溶胶-凝胶法制备了不同空位浓度掺杂的La0.67-xCa0.33MnO3系列样品(x=0.00,0.02,0.06,0.10),用扫描电子显微镜(SEM)观察了样品的形态和颗粒大小,X射线衍射实验结果显示样品为单相钙钛矿结构,用振动样品磁强计(VSM)测量了样品的磁化强度随温度变化的M-T曲线和起始磁化M-H曲线.研究了空位浓度对样品的居里温度Tc和磁熵变的影响.结果表明,空位浓度的掺杂将样品的居里温度Tc提高至室温附近,磁熵变得到增强.对于经历了一级相变的样品La0.67-xCa0 33MnO3(x=0.02),在居里温度Tc(277 K)附近和1T外磁场下,最大磁熵变达到了2.78 J/(kg·K).     

La0.60Sr0.40-xNaxMnO3钙钛矿材料在室温附近磁电阻的温度稳定性

采用溶胶-凝胶法(sol-gel)制备了名义组分为La0.60Sr0.40-xNaxMnO3(x=0.00,0.10,0.15,0.20,0.30,0.33,0.35)的类钙钛矿型稀土锰氧化物多晶样品。发现用Na+替代部分Sr+2后,可使样品的居里温度降至室温附近,并且使样品的室温磁电阻比替代前明显增大。在1.8T的磁场作用下,样品La0.60Sr0.07Na0.33MnO3在292K时磁电阻为24.4%,比不含Na的La0.60Sr0.40MnO3增大了2.8倍;样品La0.60Sr0.25Na0.15MnO3在285K～345K温区内磁电阻保持在3.9%(±0.2%)左右,受温度影响不大,因此显著提高了样品的室温磁电阻和其温度稳定性。迄今为止还未见这类材料在室温附近具有如此宽范围和高温度上限的MR温度稳定性报道。这对于该类磁电阻材料的应用有很大意义。     

基于Landau二级相变理论的磁热效应分析

将Landau二级相变理论应用于二级磁相变材料的磁热效应,建立了磁熵变与磁场的直接关系表达的理论模型。以La0.7Sr0.3MnO3为例,在居里温度附近利用该理论模型和麦克斯韦关系式计算了磁熵变ΔSM,并进行了对比。结果表明基于Landau理论的计算结果与利用传统方法的计算结果相符合。而根据Landau平均场理论,二级磁相变材料中居里温度TC和磁熵变ΔSM最大的温度Tpeak不一致,但在居里温度附近ΔSM与磁场的相关性ΔSM=kH^n表达的指数为n=2/3。     

La0.8-xNdxNa0.2MnO3的磁卡效应研究

用溶胶-凝胶法制备系列样品La0.8-xNdxNa0.2MnO3(x=0.00,0.05,0.10,0.15和0.20)钙钛矿锰氧化物.研究温度范围在240～340 K、外磁场0～1T下该系列样品的居里温度和磁熵变.发现样品的居里温度TC随x增加而减小,而且x=0.20、温度为295K时,最大磁熵变△SM为1.68 J/kg·K.实验结果表明钙钛矿锰氧化物La0.8-xNdxNa0.2MnO3有可能作为室温下的磁致冷材料的候选者.     

La_(0.64)Ca_(0.28)Sr_(0.02)MnO_3材料的磁热性能

用溶胶-凝胶法制备了空位掺杂的La0.64Ca0.28Sr0.02MnO3材料样品,从结构,磁化曲线,磁相变等方面研究分析其具有大磁熵变的原因。在外加磁场1 T时,该样品磁熵变|ΔSM|达到3.01 J/(kg.K),居里温度TC为264 K。该样品低磁场下在室温附近有较强的制冷力,可作为良好的室温磁制冷材料。     

La0.67Sr0.33-xAgxMnO3的室温磁电阻效应

采用溶胶-凝胶(sol-gel)法制备了名义组分为La0.67Sr0.33-xAgrMnO3(x=0.15、0．20)的多晶样品,发现用Ag部分替代Sr后样品的室温磁电阻比替代前明显增大。在1．8T下,La0．67Sr0.18Ag0.15MnO3样品的磁电阻在330K出现峰值,其峰值为35％;对于La0．67Sr0.13Ag0.2 MnO3样品,磁电阻峰值为26％．且峰的宽度较大,在290～315K之间的磁电阻随着温度变化不大．因此显著提高了室温时样品的磁电阻和磁电阻的温度稳定性,另外．还提高了样品的磁场灵敏度。这对磁电阻的应用有很大意义。     

La_(0.60)Sr_(0.40-x)K_xMnO_3系列材料的磁电阻效应

利用溶胶-凝胶法(sol-gel)制备了多晶类钙钛矿型稀土锰氧化物La0.60Sr0.40-xKxMnO3(x=0.00,0.15,0.20,0.30)。发现K+取代部分Sr2+后,可使样品的居里温度降至室温附近,并且使样品的室温磁电阻比替代前明显增大。在1.8T的磁场下,x=0.30的样品磁电阻峰值为21%,相应的峰值温度为304K。而母体La0.60Sr0.40MnO3的磁电阻峰值仅为6.4%,峰值温度为373K。可见K+离子替代使室温附近样品的庞磁电阻效应有了明显的改善。     

纳米多晶La0. 7Sr0.3MnO3磁性相变临界行为研究

用溶胶一凝胶方法制备了纳米多晶La0.7Sr0.3MnO3样品.测量了不同温度下烧结的样品的零场冷却交流磁化率与温度和直流磁场的依赖关系.通过对铁磁-顺磁转变点附近临界峰的分析,得到973K烧结的多晶样品居里温度为312.1K±0.2K,临界指数为:δ=3.040,γ=1.007,β=0.493;1173K烧结的多晶样品居里温度为331.7K±0.1K,临界指数分别为:δ=2.950,γ=0.993,β=0.508.两组数据均与平均场理论预言结果一致,表明纳米多晶La0.7Sr0.3MnO3样品在磁性相变点附近存在长程相互作用.     

La空位和Sr离子掺杂对La-Ca-MnO材料磁性能的影响

用溶胶-凝胶法制备了La位空位掺杂La_(0.7-x)Ca_(0.28)Sr_(0.02)MnO_3(x=0.00~0.10)系列样品和Sr离子掺杂La_(0.77-x)Ca_(0.2)Sr_xMnO_3(x=0.00~0.10)系列样品,研究了La位空位掺杂和Sr离子掺杂对样品相结构、居里温度和磁熵变的影响。结果表明,两个系列样品都在近室温有较大磁熵变,对于钙钛矿La-Ca-MnO_3型材料,在La位含少量空位的基础上进行Sr离子掺杂,对居里温度和磁熵都有较积极的调节作用。     

Microwave assisted hydrothermal synthesis and magnetocaloric properties of La0.67Sr0.33MnO3 manganite

We report microwave assisted hydrothermal synthesis and magnetocaloric properties of La0.67Sr0.33MnO3 manganite. The synthesized La0.67Sr0.33MnO3 nanoparticles was characterized using X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), energy dispersive X-ray spectroscopy (EDS) and magnetization measurements. The XRD results indicated that La0.67Sr0.33MnO3 nanoparticles have polycrystalline nature with monoclinic structure. FE-SEM results suggested that La0.67Sr0.33MnO3 nanoparticles are assembled into rod like morphology. Magnetization measurements show that La0.67Sr0.33MnO3 nanoparticles exhibit transition temperature (Tc) above room temperature. The maximum magnetic entropy change (deltaS(M))max was found to be 0.52 J/kg K near Tc approximately 325 K at applied magnetc field of 20 kOe. This compound may considered as potential material for magnetic refrigeration near room temperature.     

The Tb Doping Effect on the Room Temperature Magnetoresistance in (La1-xTbx)0.67Sr0.33MnO3 (x≤ 0.4)

by Tb in (La1-xTbx)0.67Sr0.33MnO3, the room temperature magnetoresistance △R/R0 drops at first, then undergoes an increase near x≈0.1, and finally drops again. The value of room temperature magnetoresistance at a field H=12 kOe for (La1-xTbx)0.67Sr0.33MnO3 is -3.56%. The enhancement of the room temperature magnetoresistance induced by an appropriate Tb substitution in (La1-xTbx)0.67Sr0.33MnO3 is correlated with the shifts of the Curie temperature and metal-insulator temperature to near room temperature. The drop of the room temperature magnetoresistance at large Tb doping-contents may be due to its lower TC and TMI far from the room temperature.     

Research on a New Process of Preparation for Nano-SiO2 with High Activity and Mesopores

Nano-SiO_2 with high activity and mesopores was prepared through sol-gel synthesis followed by low-temperatureheat treatment and ball milling firstly in our experiments. TEM was performed to measure particle sizes. Nitrogenadsorption experiments were carried out to estimate specific surface area, porous distribution and porous ratio by BETand BJH methods. The content of Si-OH in SiO_2 surface was calculated by analysis of the results of hydrogen-oxygencontent mensuration (HOCM). As a result, appropriate heat treatment system and ball milling time are important topreparation for nano-SiO_2 with high activity and mesopores, which are 5～50 nm particles, 5～6 nm average aperture,85%～93% porous ratio, and 51%～55% Si-OH content in surface. Nano-SiO_2 with that structure has high surfaceenergy and activity. This process, which has simple facilities and operation rules, is a new way of preparation fornano-SiO_2 with high activity and mesopores.     

(La0.6Dy0.1)Sr0.3MnO3的磁热效应研究

对庞磁电阻材料(La0.6Dy0．1)Sr0.3MnO3的磁热效应进行了研究．通过不同温度下的等温磁化(M-H)曲线的测量和计算，发现伴随铁磁-顺磁(PM—FM)相变出现大的磁热效应，额外的磁性交换作用将导致额外的磁熵变化．结果表明，(La0.6Dy0．1)Sr0.3MnO3可以作为室温下使用的磁制冷工质候选材料．     

La0.67Sr0.33MnO3中Ag-Ti的掺杂效应

将La0.67Sr0.33MnO3（LSMO）、Ag2O及TiO2粉混合经高温烧结后制备了钙钛矿相/xAg两相复合体系（x是Ag与钙钛矿材料的物质的量比）,系统地研究了Ag-Ti的共掺杂对LSMO电性和磁电阻效应的影响.0.07摩尔比Ti4＋离子的B位掺杂使LSMO的居里温度降至室温.Ag的掺入对Tc影响不大,Tp逐渐升高.由于钙钛矿颗粒属性的改善和金属导电通道的出现,材料的电阻率明显下降.Ag掺杂使室温磁电阻得到显著增强,室温下从x=0.30样品中得到最大的磁电阻,约为32%,是La0.67Sr0.33MnO3样品的8倍,La0.67Sr0.33Mn0.93Ti0.07O3样品的1.6倍.     

La0.67Sr0.33-xCuxMnO3的室温磁电阻效应

采用溶胶一凝胶(sol-gel)法制备了名义组分为La0.67Sr0.33-XCuXMnO3(x=0～0.33)的多晶样品,发现用Cu替代少部分Sr后样品的室温磁电阻比替代前的明显增大.在1.8T磁场作用下,当x＝0.15时,磁电阻峰值为27.7%,峰值温度为306K,当温度低于306K时磁电阻值随温度的升高而增大,当温度高于306K时磁电阻值随温度的升高而减小;当x＝0.1时,在295K-310K温度之间磁电阻值达19%左右,受温度影响很小;因此在提高了室温磁电阻值的同时,又提高了磁电阻的温度稳定性.这对于该类磁电阻材料的应用具有很大意义.     

La0.8-xCa0.2MnO3钙钛矿氧化物居里温度研究

采用溶胶-凝胶法制备了系列Lao.8-xCa0.2MnO3钙钛矿多晶样品,用X射线衍射分析确定了样品的钙钛矿结构,用透射电子显微镜观察了样品的形貌并估算了颗粒度及粒径分布情况,用PARl55型振动样品磁强计测量了样品磁性并确定各样品的居里温度.结果表明,随焙烧温度的提高,样品的平均粒径略有增大,但居里温度的变化没有明显的规律性,对于不同温度焙烧的样品居里温度Tc随La3+离子空位浓度x的变化表现出相似的规律性,当x＞0.03时Tc随x变化不大,当x＜0.03时,Tc随x的增大有明显的提高.     

Synthesis and characterization of pure and doped (Na, Ca, Sr) nanograined LaMnO3 magnetoresistive ceramics

Pure and Na, Ca, and Sr doped lanthanum manganites (LaMnO3, La0.85Na0.15MnO3, La0.7Ca0.3 MnO3, La0.7Sr0.3MnO3) were prepared in form of bulk nano-structured materials through a two-step process. Nanometric powders were synthesized by the Pechini method and subsequently densified by Spark Plasma Sintering at 1273 K for 5 minutes under a pressure of 140 MPa. The method allowed the preparation of single phase manganites with a theoretical density above 98% together with a very limited growth of the grain size. Grain sizes below 70 nm were obtained for all materials except La0.85Na0.15MnO3 which showed a much larger grain size. Curie temperatures (Tc) and magnetoresistivity properties of the samples were measured by a Superconducting Quantum Interference Device (SQUID). The decrease in the resistivity below Tc occurred in a much wider range of temperature compared to ceramics having micron-size grains.     

Lattice Effects and Irreversible Magnetoresistance in Gd Doped La—Ca—Mn—O

The Gd substituting effects for La in La0.67Ca0.33MnO3 has been studied .With increasing the substituting amount of Gd,the phase transition temperature of metal-isolator for the samples decreases,the corresponding peak resistivity increases,the Curie temperature decreases monotonically.The substitution of La-Ca-Mn-O with 11% Gd for La improved the magnetoresistance ratio by an order of magnitude.The effects of substituting Gd can be explained in terms of the lattice effects.An irreversible MR behaviour was observed in Gd-substituting compounds.This effect became marked when the substituting amount of Gd was greater than 7%.A maximum irreversible increment of MR ratio as large as 91% was obtained when Gd substituting amount was 11%.     

Piezoresistivity in films of nanocrystalline manganites

Rare earth manganites having perovskite structure are susceptible to lattice strain. So far most investigations have been done with hydrostatic pressure or biaxial strain. We have observed that hole doped rare-earth manganites, which are known to display colossal magnetoresistance (CMR) also show change in its resistance under the influence of uniaxial strain. We report the direct measurement of piezoresistive response of La0.67Ca0.33MnO3 (LCMO) and La0.67Sr0.33MnO3 (LSMO) of this manganite family. The measurements were carried out on nanostructured polycrystalline films of LCMO and LSMO grown on oxidized Si(100) substrates. The piezoresistance was measured by bending the Si cantilevers (on which the film is grown) in flexural mode both with compressive and tensile strain. At room temperature the gauge factor approximately 10-20 and it increases to a large value near metal-insulator transition temperature (Tp) where the resistivity shows a peak.