ABSTRACT
We investigate the nonlinear behavior of the electric impedance of a kerosene-based ferrofluid (FF) sample subjected to an ac electric voltage of amplitude ranging from 10 mV to 3 V in the frequency range 6.3 mHz, 100 kHz. The FF sample was inserted between two parallel gold electrodes separated by 127 µm distance. The results show that even a sinusoidal voltage of amplitude low as 80 mV can give origin to nonlinear effects for frequency of the applied voltage smaller than 100 mHz. Our experimental data confirm the results obtained by solving numerically the equations of the Poisson-Nernst-Planck model. From this agreement it follows that the model based on the equation of continuity for the mobile ions, and the equation of Poisson for the actual potential across the sample, works well also in its non-linear version.
ABSTRACT
The influence of the free ions on the electric response of cells filled with kerosene-based ferrofluids in the low-frequency region is explored. The experimental investigations have been performed on cells limited by different types of electrodes, with the same kind of ferrofluid, by means of the impedance spectroscopy technique. The electrodes considered in our study are made of titanium, platinum, gold, brass and surgical steel. The analysis of the spectra of the real and imaginary parts of the electric impedance of the cell data has been done by means of a simplified version of the Poisson-Nernst-Planck model, in which only the carriers of a given sign are mobile. The agreement between the theoretical predictions and the experimental data is rather good on the whole frequency range. From the analysis of the data in the low-frequency range, dominated by the properties of the electrodes, we discovered that only gold electrodes behave in a manner different from the other electrodes. From the best fit of the experimental data the free-ions density is determined as well as their diffusion coefficient in kerosene. The estimated dielectric constant of the kerosene is in good agreement with the values reported in the literature. In the framework of our model, the surface conductivity of the electrodes has been also determined.
ABSTRACT
In this paper we review the magnetic properties of spinel ferrite nanoparticles pointing out the primary role of the crystalline structure besides finite size and surface/interface effects. The details of the spinel crystal structure of bulk spinel ferrite materials and their influence on both the magnetization and magnetocrystalline anisotropy are recalled. Moreover, we review some results published in the literature over the last years about how the structure of magnetic nanoparticles influences their magnetic features. Perspectives about the challenges to improve the applications in several fields are finally reported.
ABSTRACT
We compare both magnetic blocking properties and remanence curves for dilute ferrofluid and powder samples of ferrite magnetic nanoparticles. Low field DC magnetization, AC susceptibility, isothermal remanent magnetization and DC demagnetization techniques are employed to investigate the role of interparticle magnetic interactions on the superparamagnetic relaxation, the magnetic anisotropy and on the super-spin-glass state in closely packed particles. The samples used herein are 3 nm sized spinel-type nanocrystals made of a cobalt ferrite core covered by a layer of maghemite on its outermost surface and can be obtained as aqueous colloidal dispersions thanks to this core-shell strategy. They show large anisotropy attributed to an enhanced surface contribution and the blocking temperature is shifted towards higher values as interparticle distance decreases. For all investigated diluted liquids and powder samples the frequency dependency of the peak temperature is well accounted by a Vogel-Fulcher law, with the insertion of a phenomenological temperature associated to the magnitude of interparticle dipolar interactions. The fractional change of the peak temperature per decade of frequency enlights the presence of interactions between particles in dilute liquids and of a spin-glass-like state in powder samples. The remanence curves always show global demagnetizing behavior, attributed to the combination of both spin surface disorder and interparticle dipolar interactions, the former being predominant in isolated nanoparticles and the latter in powder samples. However, in the most compacted powder, exchange interaction between surface ions of different particles becomes more pronounced and promotes an additive magnetizing effect.
ABSTRACT
The temperature dependence of the Soret coefficient S(T)(T) in electrostatically charged magnetic colloids is investigated. Two different ferrofluids, with different particles' mean dimensions, are studied. In both cases we obtain a thermophilic behavior of the Soret effect. The temperature dependence of the Soret coefficient is described assuming that the nanoparticles migrate along the ionic thermoelectric field created by the thermal gradient. A model based on the contributions from the thermoelectrophoresis and variation of the double-layer energy, without fitting parameters, is used to describe the experimental results of the colloid with the bigger particles. To do so, independent measurements of the ζ potential, mass diffusion coefficient, and Seebeck coefficient are performed. The agreement of the theory and the experimental results is rather good. In the case of the ferrofluid with smaller particles, it is not possible to get experimentally reliable values of the ζ potential and the model described is used to evaluate this parameter and its temperature dependence.
ABSTRACT
The Soret coefficient (ST) of positively charged magnetic colloids was measured as a function of the nanoparticles' diameter. The Z-scan technique and the generalization of the thermal lens model proved to be a reliable technique to measure ST. We show that ST is negative and increases with the particle's diameter, being best described by a functional dependence of the type STâd0. Potentiometric and conductometric experiments show that the particle's surface charge decreases as the temperature increases, changing the electrostatic interaction between the nanoparticles. The temperature gradient imposed in the ferrofluid by the Gaussian laser beam leads to the formation of the particle's concentration gradient. The origin of this phenomenon is discussed in terms of the decrease of the particle's surface charge in the hottest region of the sample and the thermoelectric field due to the inhomogeneous distribution of hydrogenous ions present in the colloidal suspension.
ABSTRACT
In this work we focus on the surface charging properties of core shell ferrite nanoparticles dispersed in water, namely magnetic nanocolloids. This structural charge results from the Brönsted acid-base behavior of the particles surface sites and is achieved through hydrolysis reactions. It can be modeled by considering identical charged sites behaving as weak diprotic acids. Then, electrochemical techniques could be implemented to study the acid-base equilibrium between the particle surface and the colloid bulk solution. Simultaneous potentio-conductimetric titrations are therefore performed to determine the thermodynamical constants of the p H-dependent reactions and to obtain the p H variations of the surface charge density. The results reveal that the saturation value of the structural charge strongly depends on the nanoparticle mean size. For large particles, the surface tends to be fully ionized whereas for smaller particles the saturated structural charge decreases drastically. This surface charge reduction is attributed to the existence in smaller particles of metallic surface sites, which cannot be accessible to the proton charge. The existence of such dead sites would be related to hydroxo-bonded sites with very low acidity combined with a quantum size effect, which would affect the charging/discharging process at the surface of the semiconductor ferrite quantum dot.
