We study pattern formation and the aggregation processes in magnetorheological suspensions in the presence of a static magnetic field, and some of their associated physical properties. In particular, we analyze the elastic modes as a function of the intensity of the applied field and for several particle concentrations. We observe that the clusters formed in these systems have multifractal characteristics, which are the result of three well defined stages of the aggregation process. In these stages three generations of clusters are produced sequentially. The structure of the suspension can be well characterized by its mass fractal dimensions and the mass radial distribution. The size distribution of the second-generation clusters written in terms of their mass fractal dimension allows us to calculate the sound speed of the longitudinal modes in the large wavelength regime. This multifractal analysis applied to several kinds of aggregates reveals that the occurrence of at least three stages of aggregation is a common feature to several physical aggregation processes.
Charge trapping effect in silicon-rich oxide (SRO) films was investigated using capacitance versus voltage (C–V), current versus voltage (I–V) and current versus time (I–t) measurements in an Al/SRO/Si MOS-like structure. SRO layers with different thicknesses were deposited. The trapped charge density was found to depend on the thickness of the SRO layers. It was shown that the total trapped charges can be divided into two parts: one part is correlated with the Si nanoclusters located near the SRO/Si interface (interface-NCs); another one is correlated with the Si nanoclusters distributed in the bulk of the SRO layers (bulk-NC). A current peak was observed in the I–V curves in the surface inversion condition. The peak position voltage varies with the thickness of the SRO layers. The current peak is attributed to the charging/discharging of the interface-NCs. The charging/discharging process of interface-NCs was evidenced by a fast decay of current in I–t measurements at the current peak voltage. I–t curves recorded at a large positive voltage are associated with the charge trapping effect of the bulk-NCs. The Coulomb blockade effect was suggested to be the reason of current decay at constant voltage.
In this work, we report the thermal characterization of platelike composite samples made of polyester resin and magnetite inclusions. By means of photoacoustic spectroscopy and thermal relaxation, the thermal diffusivity, conductivity and volumetric heat capacity of the samples were experimentally measured. The volume fraction of inclusions was systematically varied in order to study the changes in the effective thermal conductivity of the composites. In some samples, a static magnetic field was applied during the polymerization process resulting in anisotropic inclusion distributions. Our results show a decrease in the thermal conductivity of some of the anisotropic samples compared to the isotropic randomly distributed ones. Our analysis indicates that the development of elongated inclusion structures leads to the formation of magnetite and resin domains causing this effect. We correlate the complexity of the inclusion structure with the observed thermal response by a multifractal and lacunarity analysis. All the experimental data are contrasted with the well known Maxwell-Garnett's effective media approximation for composite materials.
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