Properties of Nanocrystalline Nickel Particles in Ni-Si0 2 Composites

Abstract

Nickel-Silica nanocomposites with a nickel content equal to 10, 15, 20 wt% have been prepared by a sol-gel method starting from ethanolic solutions of tetraethoxysilane and nickel nitrate hexahydrate. After selation the samples were reduced in H-> flow at selected temperature (450 °C < T < 600 °C).The morphological, structural and magnetic properties were investigated by transmission electron microscopy (TEM), wide and small angle X-ray scattering (WAXS, SAXS), magnetic susceptibility in zero field cooled and field cooled mode (ZFC and FC), and magnetic hysteresis loop.Nanometric nickel particles are observed in all the investigated samples. TEM, WAXS and SAXS techniques indicate that the average nickel particle size grows slightly but almost regularly with the nickel concentration. TEM results moreover indicate that also the width of the particle size distribution, which can be simulated by log-normal functions, follows this trend.All the sample treated in hydrogen show superparamagnetic behaviour. The magnetisation falls to reach saturation up to highest measuring field of 70 kOe even at 3 K, while the observed coercivity H c is much higher than the theoretical bulk one. Some uncertainty in the complete interpretation of the sequence of magnetic measurements is attributed to a progressive oxidation of the samples when these are air exposed.

Magnetostatic Fields in Multilayer Composite Films with a Bimodal Granule Size Distribution

“…The sizes of such particles depend on the concentration and the method of manufacture. Various situations are possible, for instance, when the granules have practically the same size [6] or the granule size distribution has a certain character (log-normal distribution [9,10], bimodal distribution [6,7]). A bimodal distribution can be observed at metal concentrations approaching the percolation threshold, when closely spaced finer granules coalesce into larger complexes.…”

“…Calculation of magnetostatic fields for particle ensembles has been carried out in many papers with the aim of obtaining analytical expressions for regular systems (for example, [4]). However, in real systems there are certain deviations from the ideal models, such as nonperiodic spatial arrangement of particles, different sizes and shapes of granules, as well as the type of their distributions (mono-, bimodal or log-normal) [5][6][7][8][9][10]. In addition, due to the long-range nature of the dipole interaction the results of calculations can also depend on the size of the simulated system.…”