Preparation and characterizations of SiO2-coated nanoparticles of Mn0.4Zn0.6Fe2O4

Core/shell nanoparticles consisting of a magnetic core of zinc-substituted manganese ferrite (Mn_0.4Zn_0.6Fe₂O₄) and a shell of silica (SiO₂) are prepared by a sol-gel method using tetraethyl orthosilicate (TEOS) as a precursor material for silica and salts of iron, manganese and zinc as the precursor of the ferrite. Three weight percentages of the shell materials of SiO₂ are used to prepare the coated nanoparticles. The X-ray diffractograms (XRD) of the coated and uncoated magnetic nanoparticles confirmed that the magnetic nanoparticles are in their mixed spinel phase in an amorphous matrix of silica. Particles sizes of the samples annealed at different temperatures are estimated from the width of the (3 1 1) line of the XRD pattern using the Debye-Sherrer equation. The information regarding the crystallographic structure together with the particles sizes extracted from the high-resolution transmission electron microscopy (HRTEM) of a few selected samples are in agreement with those obtained from the XRD. HRTEM observations revealed that particles are coated with silica. The calculated thickness is in agreement with that obtained from the HRTEM pictures. Hysteresis loops observed in the temperature range 300 down to 5 K and Mössbauer spectra at room temperature indicate superparamagnetic relaxation of the nanoparticles.     

The superconducting transition temperature of Pb1?xMnx films

Results of the superconducting transition temperatureT _c of amorphous and microcrystalline films of lead doped with manganese as magnetic impurity are reported in this work. The amorphous films show an Abrikosov-Gor'kov behaviour, whereas for the crystalline films there is a much smaller depression and a peak for higher Mn concentrations, which indicates a region of coexistence of superconductivity and magnetic ordering as a spinglass.     

Structural properties of amorphous Fe2O3 nanoparticles

We have investigated the microstructure of amorphous Fe₂O₃ nanoparticles by using molecular dynamics (MD) simulations. Non-periodic boundary conditions with Born-Mayer type pair potentials were used to simulate a spherical model of different diameters of 2, 3, 4 and 5 nm. Structural properties of an amorphous model obtained at 350 K have been analyzed in detail through the partial radial distribution functions (PRPFs), coordination number distributions, bond-angle distributions and interatomic distances. Calculations showed that structural characteristics of the model are in qualitative agreement with the experimental data. The observation of a large amount of structural defects as the particle size is decreased suggested that surface structure strongly depends on the size of nanoparticles. In addition, surface structure of amorphous Fe₂O₃ nanoparticles have been studied and compared with that observed in the core and in the bulk counterpart. Radial density profiles and stoichiometry in morphous Fe₂O₃ nanoparticles were also found and discussed.     

EPR/FMR Investigation of Mn-Doped SiCN Ceramics

SiCN magnetic ceramics doped with Mn²⁺ ions were synthesized at the pyrolysis temperature of 1,100° C, using CERASET™ as liquid polymer precursor and polymer manganese(II) acetylacetonate as dopant, and investigated by electron paramagnetic resonance (EPR)/ferromagnetic resonance (FMR) technique. The predominant source of ferromagnetism in SiCN samples doped with Mn ions, as synthesized here, is the ensemble of ferromagnetic nanoparticles of Mn₅Si₃C _x incorporated into the amorphous SiC/Mn structure. The fluctuation of magnetization due to ferromagnetic Mn₅Si₃C_x particles significantly broadens the EPR lines at the phase-transition temperature (363 K). This is the first fabrication of a SiCN/Mn ceramic, which exhibits room-temperature ferromagnetism.     

Fast degradation of dyes in water using manganese-oxide-coated diatomite for environmental remediation

By a simple wet-chemical procedure using a permanganate in the acidic medium, diatomite coated with amorphous manganese oxide nanoparticles was synthesized. The structural, microstructural and morphological characterizations of the as-synthesized catalysts confirmed the nanostructure of MnO₂ and its stabilization on the support - diatomite. The highly efficient and rapid degradation of methylene blue and methyl orange over synthesized MnO₂ coated Diatomite has been carried out. The results revealed considerably faster degradation of the dyes against the previously reported data. The proposed mechanism of the dye-degradation is considered to be a combinatorial effect of chemical, physicochemical and physical processes. Therefore, the fabricated catalysts have potential application in waste water treatment, and pollution degradation for environmental remediation.     

Preparation of nanocrystalline MnFe2O4 by doping with Ti ions using solid-state reaction route

Nanostructured manganese ferrites (MnFe₂O₄) with diameters in the range of 45–30 nm were synthesized by Ti⁴⁺ ion doping, using conventional solid-state reaction route. The substitution of Ti⁴⁺ ions created vacancies at Mn²⁺ sites and the coupling of ferrimagnetically active oxygen polyhedra was broken. This created nanoscale regions of ferrites. A reduction of magnetization for decreasing particle size was observed. Coercivity showed an increasing trend. This was explained as arising due to multidomain/monodomain magnetic behaviour of magnetic nanoparticles. DC resistivities of the doped specimens indicated the presence of an interfacial amorphous phase formed by the nanoparticles. Zero-field cooled and field-cooled curves from 30 nm sized particles showed a peak at T_B (∼125 K), typical of superparamagnetic blocking temperature.     

Influence of a dopant source on the structural and optical properties of Mn doped ZnGa<Subscript>2</Subscript>O<Subscript>4</Subscript> thin films

Thin films of ZnGa₂O₄:Mn²⁺ were deposited on quartz substrates using an rf magnetron sputtering technique. The sputtering target, ZnGa₂O₄ doped with 2 at. % manganese, was synthesized by a high temperature solid state reaction. Two different dopant sources were used to incorporate the dopant ions into the target, namely, manganese acetate and manganese oxide. The structural and optical properties of the thin films were studied using XRD, PL and transmission spectra. Polycrystalline ZnGa₂O₄:Mn with a spinel structure could be grown at an optimized substrate–target distance even at room temperature. No luminescence was observed in the as-deposited films grown using (CH₃COO)₂Mn as the dopant source in the target. Substrate heating or post-deposition annealing in the reducing ambient didn’t impart any luminescence to the films, ruling out the possibility of Mn²⁺ incorporation in the films. However, when using MnO as the manganese source in the target, the as-deposited films exhibited green photoluminescent emission (peak maximum at 508 nm) for substrate temperatures at and above 500 °C. This suggests that, in thin films, Mn incorporation and subsequent luminescent outcome is strongly influenced by the dopant source, which is quite different from the bulk phosphor behavior. PACS 81.15.Cd; 78.55.-m; 85.60.-q     

Elucidation of the crystallization kinetics for sodium-alumino-silicate glasses containing different amounts of manganese oxide

The glass samples [40SiO₂?+?5Al₂O₃?+?{55???x}Na₂O?+?xMnO₂] where x?=?0.05, 0.2, 0.4, 0.6, 0.8, and 1?mol% MnO₂ before and after being heat treated were subjected to X-ray diffraction. The diffraction lines provided clear evidence of the nucleation and growth, which are characteristic of sodium silicate phase. Crystallization studies were conducted using differential thermal analysis. Crystallization peak temperatures were identified and the transformed fractions were determined. Both the rate of growth, K ₀, and the activation energy, E, depend on the influence of manganese ions in the glass network as a modifier or as a former and the manganese content. The values of the Avrami parameter, n, were calculated using two methods and were in excellent agreement. The process of nucleation and growth rate depends on the manganese content.     

Cathodic characteristics of amorphous manganese oxides as lithium secondary battery

The amorphous manganese dioxides were prepared by reduction method using various reagents. The particle size and morphology depended on reducing agents, some of which were aggregated round particles of sub micron order, and the others were small fine fibers like of a few nm in width and several tens nm in length. The former showed low Li-insertion capacity of around 200 mAhg⁻¹, while the latter material showed high capacity of 500 mAhg⁻¹ in the OCV measurements until 1 V vs. Li. The as-prepared MnO₂ contains some amount of water in the structure. The H₂O-free MnO₂ obtained by heat-treatment under high oxygen pressure was inferior in electrode property to MnO₂ containing water in the structure. The H₂O content related to the electrode performance. Paper presented at the 8th EuroConference on Ionics, Carvoeiro, Algarve, Portugal, Sept. 16–22, 2001.     

Tetrahedral ↔ octahedral network structure transition in simulated vitreous SiO2

By using molecular dynamics (MD) simulations we found a transition from a tetrahedral to an octahedral network structure in an amorphous SiO₂ model under compression from 2.20 to 5.35 g/cm³. And on heating of a high density amorphous (hda) model of 5.35 g/cm³ at zero pressure, the structure transforms to a low density amorphous (lda) form. Simulations were done in a model containing 3000 particles under periodic boundary conditions with interatomic potentials which have a weak Coulomb interaction and a Morse type short-range interaction.     

穆斯堡尔谱学研究非晶TM80M20合金的微观结构

在室温下和4.2K下测定了非晶Fe₈₀B_20-xM_x合金(M=P,C)的穆斯堡尔吸收谱。利用分布参数拟合程序得到了超精细内场H_i和同质异能移IS同类金属成份的变化关系。利用这些结果考察了非晶合金的微观结构,比较了两类结构模型:Bernal-Polk模型和微晶模型。对比非晶合金和它们的相应晶相的行为得知,这类非晶合金中不存在微晶近程序,Bernal-Polk模型对描述TM-M类非晶合金的微观结构优于微晶模型。 关键词：     

Local atomic structure in molten Si3Sb2Te3 phase change material

By means of ab initio molecular dynamics calculations, we have studied the local structures of liquid and amorphous Si₃Sb₂Te₃. The results show that all the constitute elements in liquid Si₃Sb₂Te₃ are octahedrally coordinated. While in amorphous state, Sb and Te atoms are mainly octahedrally coordinated and Si atoms are mainly tetrahedrally coordinated. In both states, Si is mainly homo-bonded by Si. Finally, we proposed a phase separation model for liquid and amorphous Si₃Sb₂Te₃, which is responsible for the good performance of Si₃Sb₂Te₃ alloy as a phase change material.     

二次锂电池电极材料非晶态MoS3的结构研究

用X射线衍射和光电子能谱分析方法研究了二次锂电池电极材料非晶态MoS₃的结构。结果表明,非晶态MoS₃的结构是由MoS₂基本结构单元S—Mo—S夹心层与无定形S_n链的均匀混合无序堆垛而成。 关键词：     

A study of threshold switching of NbO2 using atom probe tomography and transmission electron microscopy

Threshold switching is a phenomenon where the resistivity of an insulating material changes and the insulator exhibits metallic behavior. This could be explained by phase transformation in oxide materials; however, this behavior is also seen in amorphous insulators. In this study, through an ex-situ experiment using transmission electron microscopy (TEM), we proved that threshold switching of amorphous NbO₂ accompanies local crystallization. The change in I–V characteristics after electroforming was examined by evaluating the concentration profile. Atom probe tomography (APT) combined with in-situ TEM probing technique was performed to understand the threshold switching in amorphous NbO₂. The local crystallization in amorphous NbO₂ was validated by the observed difference in time-of-flight (ToF) between amorphous and crystalline NbO₂. We concluded that the slower ToF of amorphous NbO₂ (a-NbO₂) compared with crystalline NbO₂ (c-NbO₂) is due to the resistivity difference and trap-assisted recombination.     

Some aspects of the precipitation of Feα crystallites in the FePC amorphous alloy

The precipitation of Fe_α in some FePC amorphous alloys has been followed in situ by using neutron diffraction with a multidetector covering 80° (2θ). The quantity of Fe_α which precipitates before the final crystallization of the remaining amorphous phase is a function of the composition of the alloy but is independent of the heating rate. The alloy tends toward a mixture of Fe_α + am′ whose composition is about Fe₇₅(PC)₂₅. The crystallization of am′ occurs as soon as the excess of iron in the amorphous matrix is precipitated. We conclude that the excess of iron stabilizes the amorphous phase.     

静高压下有表面化学反应的非晶合金晶化研究

在静高压3—5GPa,510—660℃温度下,研究了在晶化过程中其表面与Al发生反应的非晶(Fe_0.99,Mo_0.01)₇₈Si₉B₁₃合金的晶化过程。发现在4GPa左右,510—660℃的温度范围内,非晶FMSB晶化为纳米α-Fe(Al)相,在其他压力下,晶化为α-Fe(Mo,Si),(Fe,Mo)₃B或Fe₂B相。利用简单固体模型对其晶化的热力学机制 关键词：     

Measurement and interpretation of the plasmon energy in alumina

The electron energy loss spectra from well categorized samples of α-Al₂O₃, γ-Al₂O₃ and amorphous alumina films were measured using combined electron microscopy and energy analysis. The plasmon energies were compared using an elementary dielectric theory whereby a method of determining the density of the amorphous films was established.     

Room-temperature synthesis of apatite-type lanthanum silicates by mechanically milling constituent oxides

Apatite-type lanthanum silicates have been successfully prepared at room temperature by dry milling hexagonal A−La₂O₃ and either amorphous or low cristobalite SiO₂. Milling a stochiometric mixture of these chemicals in a planetary ball mill with a moderate rotating speed (350 rpm), allows preparing the target phase in only 6 h although longer milling times are needed to eliminate all SiO₂ traces. The mechanically activated chemical reaction proceeds faster when using amorphous silica than low cristobalite as silicon source and pure phases are obtained after only 9 and 18 h respectively. As obtained powder phases are not amorphous and show an XRD pattern as well as IR and Raman bands characteristic of the lanthanum silicate.     

铁基块体非晶合金在纳米压痕过程中的蠕变行为研究

利用纳米压痕技术研究了{［（Fe_0.6Co_0.4）_0.75B_0.2Si_0.05］_0.96Nb_0.04}₉₆Cr₄铁基块体非晶合金的室温蠕变行为及不同的加载速率对该块体非晶合金蠕变变形的影响.{［（Fe_0.6Co_0.4）_0.75B_{0.2< 关键词： 块体非晶合金 蠕变 EVEV模型 蠕变速率敏感指数}