Study of magnetic relaxation in partially oxidized nanocrystalline iron

The long term magnetic relaxation is investigated in a nanocrystalline ferromagnet. Samples were prepared by ball milling of microcrystalline iron in helium atmosphere and vacuum. We have found logarithmic time dependence of the magnetic moment with the relaxation rate independent of the starting magnetic field in the 0–20 mT low field region.     

高温度稳定性声表面波信道化滤波器组研制

本文报道了作者研制的频率范围分别为200—350MHZ和350一500MHZ的两种15信道声表面波滤波器组的结果。为了克服现有技术温度稳定性较差的缺点，基片材料采用Yll2°LiTaO3，温度稳定性比现有技术提高3．5—5倍。每个滤波器中的两个叉指换能器一个采用特殊抽样的单指换能器，另一个采用三次谐波工作的分裂指换能器，从而避免了采用基频工作的分裂指换能器所需的0．8μm。工艺。输入匹配网络采用改进的串并联匹配网络，降低了输人匹配网络的插人损耗，补偿了因采用Yll2°LITaO3，单个滤波器插损较大的缺陷，使滤波器组总的插入损耗与现有采用YZ或Y128°LiNbO3的滤波器组相当。     

Study of molecular surface coating on the stability of maghemite nanoparticles

In this study γ-Fe₂O₃ nanoparticle, surface-coated with increasing amount of oleic acid, have been prepared while the stability against particle degradation under laser excitation intensity was investigated. Maghemite nanoparticle was obtained via oxidation of magnetite nanoparticle, the latter synthesized by co-precipitation of Fe (II) and Fe (III) ions in alkaline medium. By varying the experimental conditions of surface-coating maghemite nanoparticles with oleic acid, samples with different grafting coefficient were obtained and investigated using X-ray diffraction and different spectroscopic techniques, namely Raman, Mössbauer, and infrared. The amount of oleic acid adsorbed on the maghemite surface was estimated via the carbon content obtained from elemental analysis.     

Exchange bias effect in partially oxidized amorphous Fe-Ni-B based metallic glass nanostructures

The magnetic properties of amorphous Fe-Ni-B based metallic glass nanostructures were investigated. The nanostructures underwent a spin-glass transition at temperatures below 100 K and revealed an irreversible temperature following the linear de Almeida-Thouless dependence. When the nanostructures were cooled below 25 K in a magnetic field, they exhibited an exchange bias effect with enhanced coercivity. The observed onset of exchange bias is associated with the coexistence of the spin-glass phase along with the appearance of another spin-glass phase formed by oxidation of the structurally disordered surface layer, displaying a distinct training effect and cooling field dependence. The latter showed a maximum in exchange bias field and coercivity, which is probably due to competing multiple equivalent spin configurations at the boundary between the two spin-glass phases.     

Wetting and evaporation behaviors of molten Mg on partially oxidized SiC substrates

The wetting and evaporation behaviors of molten Mg drops on pressureless-sintered SiC surfaces were studied in a flowing Ar atmosphere at 973-1173 K by an improved sessile drop method. The initial contact angles are between 83° and 76°, only mildly depending on temperature. The formation of a ridge at the triple junction as a result of reaction between molten Mg and the SiO₂ film on the SiC surface pins the triple line and leads to a constant contact diameter mode during the entire evaporation process. Moreover, the diffusion coefficients of the Mg vapor at different temperatures were evaluated based on a simple model.     

Change in the critical exponent of magnetization in maghemite in the temperature range of the structural phase transition

The magnetization of maghemite and magnetite thin films is measured as a function of the temperature and orientation of the film. It is established that the temperature dependence of the spontaneous magnetization deviates from the Bloch law at low temperatures and is adequately described by the linear function M/M _s = 2.2(1-T/T _C) below the Curie temperature T _C. The linear temperature dependence of the magnetization below the Curie temperature is explained by the change in the spin of iron ions in tetrahedral positions due to local deformations of the crystal lattice.     

Study of Fe-Al thin films oxidized at room temperature

Very thin films of Fe andFe/Al were oxidized at room temperature in dry air andthen studied by means of surface Mössbauer measurements andX-ray photoelectron spectroscopy. A mechanism is proposed for the oxidation process.     

Dimensional effects in the properties of maghemite

Single-phase γ-Fe₂O₃ powders with a coercivity of 161–263 Oe are obtained from iron(III) hydroxide by hydrothermal synthesis followed by heat treatment. Particle size distributions are plotted, based on the TEM data. Magnetic measurements show that a significant decrease in the squareness ratio and coercivity occurs at a mean particle volume of 60000 nm³.     

Structural stability in partially substituted lithium manganese spinel oxide cathode

LiMn₂O₄ is one of the most promising cathode materials for lithium secondary battery because of natural abundance of manganese in the crust and its low toxicity to environment. Lithium ion can almost reversibly intercalate into or deintercalate from lithium manganese spinel oxide LiMn₂O₄. A part of substitution of manganese with other transition metals brings the improvement of cycle life. We focused on the local structure of the spinels and considered the effect of the local distortion on the cycle life of the spinel cathodes. Paper presented at the 8th EuroConference on Ionics, Carvoeiro, Algarve, Portugal, Sept. 16–22, 2001.     

The stability of partially rigid two-dimensional surfaces in uniform incompressible flow

A theoretical analysis is made of the stability of a partially rigid two-dimensional surface embedded in the uniform flow of an incompressible inviscid fluid. Membranes, simply supported panels and clamped panels, attached at their leading and trailing edges to rigid flat extensions aligned with the undisturbed flow direction, are considered and numerical results are obtained, by using the Galerkin method, showing how the stability varies with the change in length of the upstream and downstream rigid elements. Similar results are obtained for a cantilever panel attached to a leading edge rigid surface modelling the aerofoil or splitter plate used in experiments. The effects of structural damping are included where appropriate and comparisons made with other relevant theoretical and experimental results.     

Multispectral pyrometry for surface temperature measurement of oxidized Zircaloy claddings

Non-contact temperature measurement in a nuclear reactor is still a huge challenge because of the numerous constraints to consider, such as the high temperature, the steam atmosphere, and irradiation. A device is currently developed at CEA to study the nuclear fuel claddings behavior during a Loss-of-Coolant Accident. As a first step of development, we designed and tested an optical pyrometry procedure to measure the surface temperature of nuclear fuel claddings without any contact, under air, in the temperature range 700–850 °C. The temperature of Zircaloy-4 cladding samples was retrieved at various temperature levels. We used Multispectral Radiation Thermometry with the hypothesis of a constant emissivity profile in the spectral ranges 1–1.3 µm and 1.45–1.6 µm. To allow for comparisons, a reference temperature was provided by a thermocouple welded on the cladding surface. Because of thermal losses induced by the presence of the thermocouple, a heat transfer simulation was also performed to estimate the bias. We found a good agreement between the pyrometry measurement and the temperature reference, validating the constant emissivity profile hypothesis used in the MRT estimation. The expanded measurement uncertainty (k = 2) of the temperature obtained by the pyrometry method was ±4 °C, for temperatures between 700 and 850 °C. Emissivity values, between 0.86 and 0.91 were obtained.     

Mössbauer study of synthetic oxidized vivianite at room temperature

Mössbauer spectroscopy has been used to study synthetic vivianites which are oxidized at room temperature in air. Six doublets, three ferrous and three ferric, have been used to fit the spectra recorded at 295 K. They have been attributed to the cations occupying the two different crystallographic sites. These sites were either isolated (I) or in pairs (II). In the case of the paired sites, two types of ferric cations and two types of ferrous cations can be distinguished, depending upon the degree of oxidation of the cation occupying the closest isolated site. Our experimental data showed that the ferrous cations occupying sites I were preferentially oxidized at the beginning of the oxidation process and that the rates of oxidation of the cations occupying two sites were comparable at a high oxidation level. We have also observed that the concentration of Fe³⁺ tends to a stabilized value of approximately 50% after 375 days, which also corresponds to the limit of stability of the vivianite structure.     

Surface composition modification of high-carbon low-alloy steels oxidized at high temperature in air

Segregating alloys across a metal/oxide interface, which occurs when an iron alloy is heated at high temperature in air, was studied on five high-carbon low-alloy steels. The metallographic analyses showed differences between the oxidation kinetics, the decarburisation rates of the surfaces and the number of surface defects. The results of the AES analyses enabled the characterization of the concentration profiles of the alloy elements and to explain the differences observed between the oxidation kinetics, the decarburisation rates and the number of surface defects. All the concentration profiles are discussed and possible mechanisms are proposed in order to understand the profiles obtained.     

High temperature stability of nanocrystalline anatase powders prepared by chemical vapour synthesis under varying process parameters

Systematic variation in the high temperature stability of nanocrystalline anatase powders prepared by chemical vapour synthesis (CVS) using titanium (IV) isopropoxide under varying flow rates of oxygen and helium was obtained by progressively shifting the decomposition product from C₃H₆ to CO₂. The as-synthesised powders were characterised by high temperature X-ray diffraction (HTXRD), simultaneous thermo-gravimetric analyses (STA), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR) and transmission electron microscopy (TEM). It was observed that the anatase to rutile transformation temperature progressively increased for samples synthesised at higher O₂/He flow rate ratios. The improved anatase stability was attributed to the presence of incorporated carbon within the titania structure and confirmed by a high temperature carbon desorption peak.     

Magnetic characterization of maghemite nanoparticles dispersed in surface-treated polymeric template

Magnetic characterization of maghemite nanoparticles dispersed in a polymeric template and treated under different chemical processes is reported in this work. Particle size estimated from magnetic measurements, D _M?≈?10 nm, for the free-surfactant sample, is consistent with values determined from XRD analysis and TEM images. The magnetic collapse of sextets towards a quadrupole doublet as the temperature is increased reveals the thermal relaxation of smaller $\upgamma $ -Fe₂O₃ nanoparticles. Magnetic measurements show a strong irreversibility between ZFC and FC curves suggesting the occurrence of particle–particle interaction.     

The temperature dependence of Eu atom yield resonances in electron-stimulated desorption from oxidized tungsten

This paper reports on the first measurement of the intensity of the resonances in the yield of europium neutrals as a function of temperature observed in electron-stimulated desorption from tungsten surfaces oxidized to different degrees and having different europium coverages. The measurements were carried out by the time-of-flight method with a surface ionization detector. The temperature dependences obtained for resonances due to europium and tungsten core level ionization differ qualitatively. The relation is reversible for temperatures below the onset of europium thermal desorption.