Diffusion of proton in alumina-rich nonstoichiometric magnesium aluminate spinel

The phenomenon of the diffusion of proton and deuteron in a single crystal of magnesium aluminate spinel was studied by infrared absorption. The chemical diffusion coefficient of proton was determined by the relaxation time of the absorption intensity upon the substitution of deuteron with proton. The temperature dependence of the chemical diffusion coefficient of proton for was expressed as . The chemical diffusion coefficient of proton was found to be independent of the composition of spinel and of the atmosphere. Paper presented at the 11th Euro Conference on the Science and Technology of Ionics, Batz-sur-Mer, Sept. 9–15 2007.     

Tuning magnetic properties of magnetoelectric BiFeO3-NiFe2O4 nanostructures

Multifunctional thin film nanostructures containing soft magnetic materials such as nickel ferrite are interesting for potential applications in microwave signal processing because of the possibility to shrink the size of device architecture and limit device power consumption. An essential prerequisite to future applications of such a system is a firm understanding of its magnetic properties. We show that nanostructures composed of ferrimagnetic NiFe₂O₄ pillars in a multiferroic BiFeO₃ matrix can be tuned magnetically by altering the aspect ratio of the pillars by depositing films of varying thickness. Magnetic anisotropy is studied using ferromagnetic resonance, which shows that the uniaxial magnetic anisotropy in the growth direction changes sign upon increasing the film thickness. The magnitude of this anisotropy contribution can be explained via a combination of shape and magnetostatic effects, using the object-oriented micromagnetic framework (OOMMF). The key factors determining the magnetic properties of the films are shown to be the aspect ratio of individual pillars and magnetostatic interactions between neighboring pillars.     

The magnetic behaviour of the magnetically diluted spinel compound Fe0.6Mg1.6Ni0.1Ti0.7O4 studied by Mössbauer spectroscopy

From a new magnetically diluted spinel oxide with composition Fe_0.6Mg_1.6Ni_0.1Ti_0.7O₄ an as-prepared sample and one after reheating three times have been investigated with Mössbauer spectroscopy. The spectra of the as-prepared sample clearly show a typical superparamagnetic behaviour of magnetic clusters with diverging sizes. On the other hand, the reheated sample exhibits a sharp magnetic transition at 16 K. External-field Mössbauer measurements of the latter reveal spin canting to be exclusively present on the octahedral sites which disappears at the magnetic transition temperature. These results show that this spinel compound exhibits a transition to a spin-glass for which the random freezing only occurs on the octahedral sites.     

High-pressure transitions in MgAl2O4 and a new high-pressure phase of Mg2Al2O5

Phase transitions in MgAl₂O₄ were examined at 21-27 GPa and 1400-2500 °C using a multianvil apparatus. A mixture of MgO and Al₂O₃ corundum that are high-pressure dissociation products of MgAl₂O₄ spinel combines into calcium-ferrite type MgAl₂O₄ at 26-27 GPa and 1400-2000 °C. At temperature above 2000 °C at pressure below 25.5 GPa, a mixture of Al₂O₃ corundum and a new phase with Mg₂Al₂O₅ composition is stable. The transition boundary between the two fields has a strongly negative pressure-temperature slope. Structure analysis and Rietveld refinement on the basis of the powder X-ray diffraction profile of the Mg₂Al₂O₅ phase indicated that the phase represented a new structure type with orthorhombic symmetry (Pbam), and the lattice parameters were determined as a=9.3710(6) Å, b=12.1952(6) Å, c=2.7916(2) Å, V=319.03(3) Å³, Z=4. The structure consists of edge-sharing and corner-sharing (Mg, Al)O₆ octahedra, and contains chains of edge-sharing octahedra running along the c-axis. A part of Mg atoms are accommodated in six-coordinated trigonal prism sites in tunnels surrounded by the chains of edge-sharing (Mg, Al)O₆ octahedra. The structure is related with that of ludwigite (Mg, Fe²⁺)₂(Fe³⁺, Al)(BO₃)O₂. The molar volume of the Mg₂Al₂O₅ phase is smaller by 0.18% than sum of molar volumes of 2MgO and Al₂O₃ corundum. High-pressure dissociation to the mixture of corundum-type phase and the phase with ludwigite-related structure has been found only in MgAl₂O₄ among various A²⁺B³⁺₂O₄ compounds.     

Preparation and Electrochemical Properties of xLi2MnO3·(1-x)LiMn2O4 Composites

Li₂MnO₃-doped spinel LiMn₂O₄ composites were synthesized by sol-gel method to improve the electrochemical performance of LiMn₂O₄. The microstructures, morphologies and electrochemical performance of the obtained xLi₂MnO₃·(1-x)LiMn₂O₄ composites were characterized by X-ray diffraction(XRD), scan electron microscopy(SEM) and a galvanostatic charge-discharge process. It was found that both Li₂MnO₃ and LiMn₂O₄ components exist in xLi₂MnO₃·(1-x)LiMn₂O₄(02MnO₃·0.7LiMn₂O₄ composite shows the optimized electrochemical performance, including discharge capacity and cycle stability. It was demonstrated that Li₂MnO₃-doped spinel LiMn₂O₄ cathode material can work at wide potential window with quite good capacity retention and considerably larger reversible capacity compared to single-phase LiMn₂O₄ component.     

紫外激光调控CoFe2O4表面氧空位以增强析氧反应催化活性

本文以尖晶石型材料CoFe₂O₄为模型催化剂,研究证实脉冲紫外激光辐照可以有效调控材料表面的氧空位含量,进而改变其析氧催化活性,得到了催化活性随辐照时间的火山型变化趋势. 这种激光辐照方法可用于定量研究过渡金属化合物的表面阳离子价态、阴离子空位和物化性质间的关联.     

Enhanced microwave magnetic and attenuation properties of composites with free-standing spinel ferrite thick films as fillers

Free-standing thick films of spinel ferrite, Ni_0.89−xCu_0.11Zn_xFe₂O₄ with x=0.55 and 0.60, were prepared as fillers to fabricate electromagnetic composites. Compared to those made with conventional spherical fillers, the composites made with thick film fillers showed enhanced static permeability (μ′₀) and maximum imaginary permeability (μ″_max). At the same time, complex permittivity (ε′ and ε″) were almost unchanged. A relative bandwidth W_R of 7–8 was achieved, which is about 75% of the theoretical maximum relative bandwidth. These composites are potential candidates as electromagnetic attenuation materials with ultrabroad absorption bandwidth in L and S bands.     

Study of magnetic and structural properties of ferrofluids based on cobalt-zinc ferrite nanoparticles

Ferrofluids are colloidal systems composed of a single domain of magnetic nanoparticles with a mean diameter around 30 nm, dispersed in a liquid carrier. Magnetic Co_(1−x)Zn_xFe₂O₄ (x=0.25, 0.50, 0.75) ferrite nanoparticles were prepared via co-precipitation method from aqueous salt solutions in an alkaline medium. The composition and structure of the samples were characterized through Energy Dispersive X-ray Spectroscopy and X-ray diffraction, respectively. Transmission Electron Microscopy (TEM) studies permitted determining nanoparticle size; grain size of nanoparticle conglomerates was established via Atomic Force Microscopy. The magnetic behavior of ferrofluids was characterized by Vibrating Sample Magnetometer (VSM); and finally, a magnetic force microscope was used to visualize the magnetic domains of Co_(1−x)Zn_xFe₂O₄ nanoparticles. X-ray diffraction patterns of Co_(1−x)Zn_xFe₂O₄ show the presence of the most intense peak corresponding to the (311) crystallographic orientation of the spinel phase of CoFe₂O₄. Fourier Transform Infrared Spectroscopy confirmed the presence of the bonds associated to the spinel structures; particularly for ferrites. The mean size of the crystallite of nanoparticles determined from the full-width at half maximum of the strongest reflection of the (311) peak by using the Scherrer approximation diminished from (9.5±0.3) nm to (5.4±0.2) nm when the Zn concentration increases from 0.21 to 0.75. The size of the Co-Zn ferrite nanoparticles obtained by TEM is in good agreement with the crystallite size calculated from X-ray diffraction patterns, using Scherer's formula. The magnetic properties investigated with the aid of a VSM at room temperature presented super-paramagnetic behavior, determined by the shape of the hysteresis loop. In this study, we established that the coercive field of Co_(1−x)Zn_xFe₂O₄ magnetic nanoparticles, the crystal and nanoparticle sizes determined by X-ray Diffraction and TEM, respectively, decrease with the increase of the Zn at%. Finally, our magnetic nanoparticles are not very hard magnetic materials given that the hysteresis loop is small and for this reason Co_(1−x)Zn_xFe₂O₄ nanoparticles are considered as soft magnetic material.     

尖晶石LiMn_2O_4的表面改性研究

采用溶胶_凝胶法合成尖晶石LiMn2 O4 ,并以LiCoO2 对其进行包覆 ,用XRD、SEM、EPMA等方法对修饰的尖晶石结构和性能进行研究 .结果表明 ,经包覆的LiMn2 O4 在 70 0℃焙烧 10h所得的晶粒是表层富含Co的立方尖晶石 ,而且晶粒中Co3+的含量呈现出从表到里递减的梯度分布 .以该材料作锂离子电池正极 ,虽初始容量稍有降低 ,但能有效地降低Mn2 +在电解质中的溶解 ,而且对Jahn_Teller效应有一定的抑制作用 ,包覆的LiMn2 O4 尖晶石正极材料比未包覆的有更好的循环性能