Guillermo Jorge

Anisotropic magnetoresistance and piezoresistivity in structured Fe3O4-silver particles in PDMS elastomers at room temperature.

Magnetorheological elastomers, MREs, based on elastic organic matrices displaying anisotropic magnetoresistance and piezoresistivity at room temperature were prepared and characterized. These materials are dispersions of superparamagnetic magnetite forming cores of aggregated nanoparticles inside silver microparticles that are dispersed in an elastomeric polymer (poly(dimethylsiloxane), PDMS), curing the polymer in the presence of a uniform magnetic field. In this way, the elastic material becomes structured as the application of the field induces the formation of filaments of silver-covered inorganic material agglomerates (needles) aligned in the direction of the field (parallel to the field). Because the magnetic particles are covered with silver, the MREs are not only magnetic but also electrical conductors. The structuration induces elastic, magnetic, and electrical anisotropic properties. For example, with a low concentration of particles in the elastic matrix (5% w/w) it is possible to obtain resistances of a few ohms when measured parallel to the needles or several megaohms in the perpendicular direction. Magnetite nanoparticles (Fe(3)O(4) NP) were synthesized by the coprecipitation method, and then agglomerations of these NPs were covered with Ag. The average size of the obtained magnetite NPs was about 13 nm, and the magnetite-silver particles, referred to as Fe(3)O(4)@Ag, form micrometric aggregates (1.3 μm). Nanoparticles, microparticles, and the MREs were characterized by XRD, TEM, SEM, EDS, diffuse reflectance, voltammetry, VSM, and SQUID. At room temperature, the synthesized magnetite and Fe(3)O(4)@Ag particles are in a superparamagnetic state (T(B) = 205 and 179 K at 0.01 T as determined by SQUID). The elastic properties and Youngs modulus of the MREs were measured as a function of the orientation using a texture analysis device. The magnetic anisotropy in the MRE composite was investigated by FMR. The electrical conductivity of the MRE (σ) increases exponentially when a pressure, P, is applied, and the magnitude of the change strongly depends on what direction P is exerted (anisotropic piezoresistivity). In addition, at a fixed pressure, σ increases exponentially in the presence of an external magnetic field (H) only when the field H is applied in the collinear direction with respect to the electrical flux, J. Excellent fits of the experimental data σ versus H and P were achieved using a model that considers the intergrain electron transport where an H-dependent barrier was considered in addition to the intrinsic intergrain resistance in a percolation process. The H-dependent barrier decreases with the applied field, which is attributed to the increasing match of spin-polarization in the silver covers between grains. The effect is anisotropic (i.e., the sensitivity of the magnetoresistive effect is dependent on the relative orientation between H and the current flow J). In the case of Fe(3)O(4)@ Ag, when H and J are parallel to the needles in the PDMS matrix, we obtain changes in σ up to 50% for fields of 400 mT and with resistances on the order of 1-10 Ω. Magnetoresistive and magnetoelastic properties make these materials very interesting for applications in flexible electronics, electronic skins, anisotropic pressure, and magnetic field sensors.

Magnetic and elastic properties of CoFe2O4- polydimethylsiloxane magnetically oriented elastomer nanocomposites

Magnetic elastic structured composites were prepared by using CoFe2O4 ferromagnetic and superparamagnetic nanoparticles as fillers in polydimethylsiloxane (PDMS) matrixes, which were cured in the presence of a uniform magnetic field. Cobalt-iron oxide nanoparticles of three different average sizes (between 2 and 12 nm) were synthesized and characterized. The smallest nanoparticles presented superparamagnetic behavior, with a blocking temperature of approximately 75 K, while larger particles are already blocked at room temperature. Macroscopically structured-anisotropic PDMS-CoFe2O4 composites were obtained when curing the dispersion of the nanoparticles in the presence of a uniform magnetic field (0.3 T). The formation of the particle’s chains (needles) orientated in the direction of the magnetic field was observed only when loading with the larger magnetically blocked nanoparticles. The SEM images show that the needles are formed by groups of nanoparticles which retain their original average size. The Yo...

Magnetic and elastic anisotropy in magnetorheological elastomers using nickel-based nanoparticles and nanochains

Nickel (Ni) based nanoparticles and nanochains were incorporated as fillers in polydimethylsiloxane (PDMS) elastomers and then these mixtures were thermally cured in the presence of a uniform magnetic field. In this way, macroscopically structured-anisotropic PDMS-Ni based magnetorheological composites were obtained with the formation of pseudo-chains-like structures (referred as needles) oriented in the direction of the applied magnetic field when curing. Nanoparticles were synthesized at room temperature, under air ambient atmosphere (open air, atmospheric pressure) and then calcined at 400 °C (in air atmosphere also). The size distribution was obtained by fitting Small Angle X-ray Scattering (SAXS) experiments with a polydisperse hard spheres model and a Schulz-Zimm distribution, obtaining a size distribution centered at (10.0 ± 0.6) nm with polydispersivity given by σ = (8.0 ± 0.2) nm. The SAXS, X-ray powder diffraction, and Transmission Electron Microscope (TEM) experiments are consistent with single...

A flexible strain gauge exhibiting reversible piezoresistivity based on an anisotropic magnetorheological polymer

A flexible, anisotropic and portable stress sensor (logarithmic reversible response between 40–350 kPa) was fabricated, in which i) the sensing material, ii) the electrical contacts and iii) the encapsulating material, were based on polydimethylsiloxane (PDMS) composites. The sensing material is a slide of an anisotropic magnetorheological elastomer (MRE), formed by dispersing silver-covered magnetite particles (Fe3O4@Ag) in PDMS and by curing in the presence of a uniform magnetic field. Thus, the MRE is a structure of electrically conducting pseudo-chains (needles) aligned in a specific direction, in which electrical conductivity increases when stress is exclusively applied in the direction of the needles. Electrical conductivity appears only between contact points that face each other at both sides of the MRE slide. An array of electrical contacts was implemented based on PDMS-silver paint metallic composites. The array was encapsulated with PDMS. Using Fe3O4 superparamagnetic nanoparticles also opens up possibilities for a magnetic field sensor, due to the magnetoresistance effects.

Formation and Kinetics of Self-Assembled Structures of Magnetic Microparticles in Rotating Fields

We study the formation of self-assembled structures of magnetic microparticles subjected to the action of a biaxial alternating field. The used device comprises a set of two nested orthogonal Helmholtz coils that can produce rotating fields in the horizontal plane by applying proper alternating currents to the coils. In this way, it is possible to induce a planar organization in a set of magnetic micrometer-size Fe and Ni particles dispersed in a liquid. We investigate, by means of video-microscopy image analysis performed in the MATLAB environment, the time evolution of topological parameters, as the number of objects detected. We also investigate the circularity of the dominant object at different stages of the structure formation and the region of power-law behavior of the time evolution. We compare the characteristic times for pattern formation under different field configurations.

Magnetic and Conducting Properties of Composites of Conducting Polymers and Ferrite Nanoparticles

Composites of ferromagnetic CoFe2O4 nanoparticles and two conducting polymers (polyethylenedioxythiophene-PEDOT- and polypyrrole-Ppy-) were prepared and characterized. Both syntheses were performed by monomer polymerization in presence of a dispersion of the magnetic nanoparticles, at different monomer: CoFe2O4 molar ratios. For PPy-composites, both the coercive field and the applied field required to reach the maximum magnetization decrease as the polymer content increases. For PEDOT-composites, the remanence ratio increases as the polymer content increases, indicating the presence of interactions related to the amount of polymer present. Electrical conductivity measurements indicate that, for both types of composites, a high polymer content gives rise to high electrical conductivity. These results indicate that the composite properties can be modulated by varying the polymer identity and the monomer: CoFe2O4 molar ratio.

Synthesis and Properties of Bifunctional Fe

In this work, a facile synthesis of Fe3O4/Ag nanoparticles, with magnetic properties and electrical conductivity, was successfully developed, by reducing Ag(I) ions with D-Glucose, in the presence of a dispersion of superparamagnetic Fe3O4 nanoparticles, under ultrasound treatment. Poly(vinylpyrrolidone) (PVP) was used as protecting agent and Ag(I) were incorporated in different molar ratios with respect to Fe3O4 nanoparticles. The obtained particles were characterized by XRD studies, SEM and TEM observation, Energy Dispersive X-Ray Spectroscopy (EDS), DC magnetization and conductivity measurements. From TEM and SEM observation it was found that the PVP protection has shown to be partial, as the Fe3O4 nanoparticles have a lower diameter after the reduction treatment. Despite this, the particles retain the superparamagnetic behavior and the saturation magnetization decreases as the Ag content increases. From conductivity measurements, a minimum Ag(I)/Fe3O4 molar ratio = 1.75 was needed in order to observe electrical conductivity in the metallic regime.