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 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 sigma= (8.0 ± 0.2) nm. The SAXS, XRD and TEM experiments are consistent with single crystal nanoparticles of spherical shape (average particle diameter obtained by TEM: (12 ± 1) nm). Nickel-based nanochains (average diameter: 360 nm; average length: 3 mm, obtained by SEM; aspect ratio=length/diameter ~10) were obtained at 85 ºC and ambient atmosphere (open air, atmospheric pressure). The magnetic properties of Ni-based nanoparticles and nanochains at room temperature are compared and discussed in terms of surface and size effects. Both Ni-based nanoparticles and nanochains were used as fillers for obtaining the PDMS structured magnetorheological composites, observing the presence of oriented needles. Magnetization curves, ferromagnetic resonance spectra (FMR) and strain-stress curves of low filler´s loading composites (2% w/w of fillers) were determined as functions of the relative orientation respect to the needles. The results indicate that even at low loadings it is possible to obtain magnetorheological composites with anisotropic properties, with larger anisotropy when using nanochains. For instance, the magnetic remanence, the FMR-resonance field and the elastic response to compression are higher when measured parallel to the needles (about 30% with nanochains as fillers). Analogously, the elastic response is also anisotropic, with larger anisotropy when using nanochains as fillers. Therefore, all experiments performed confirm the high potential of nickel nanochains to induce anisotropic effects in magnetorheological materials.Fil: Landa, Romina Ailín. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinacion Administrativa Ciudad Universitaria. Instituto de Química Física de los Materiales del Medioambiente y Energía; Argentina;Fil: P Soledad Antonel. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinacion Administrativa Ciudad Universitaria. Instituto de Química Física de los Materiales del Medioambiente y Energía; Argentina;Fil: Mariano M. Ruiz. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinacion Administrativa Ciudad Universitaria. Instituto de Química Física de los Materiales del Medioambiente y Energía; Argentina;Fil: Oscar E Pérez. Universidad de Buenos Aires. Facultad de Cs.exactas y Naturales. Departamento de Industrias;Fil: Alejandro Butera. Comisión Nacional de Energía Atómica;Fil: Guillermo Jorge. Universidad Nacional de General Sarmiento;Fil: Cristiano L. P. Oliveira. Instituto de Física, Universidade De São Paulo; Brasil;Fil: Martín Negri. Universidad de Buenos Aires. Facultad de Cs.exactas y Naturales. Departamento de Industrias

Magnetoresistencia y elasticidad anisotrópica en elastómeros formados por cadenas de nanopartículas y nanotubos orientadas magnéticamente [magnetoresistance and anisotropic elasticity in elastomers formed by chains of magnetically oriented nanoparticles and nanotubes]

La meta de éste trabajo es obtener magneto-elastómeros compuestos por dispersión de nanopartículas magnéticas en polidimetilsiloxano (PDMS), curando el polímero en presencia de un campo magnético uniforme. [Le but de ce travail est d’obtenir des composites magnéto-élastomériques par dispersion de nanoparticules magnétiques dans du polydiméthylsiloxane (PDMS), en réticulant le polymère en présence d’un champ magnétique uniforme.] [The goal of this work is to obtain magneto-elastomeric composites through dispersion of magnetic nanoparticles in polydimethylsiloxane (PDMS), by curing the polymer in presence of a uniform magnetic field.

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

Magnetic elastic structured composites were prepared by using CoFe 2O4 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 Young's moduli of the structured composites are four times larger when measured along the oriented needles than in the perpendicular direction. Magnetization (VSM) and ferromagnetic resonance curves of the structured composites were determined as a function of the relative orientation between the needles and the probe field. The remanence magnetization was 30 higher when measured parallel to the needles, while the coercive field remains isotropic. These observations are discussed in terms of the individual nanoparticle's properties and its aggregation in the composites. © 2011 American Institute of Physics.Fil:Soledad Antonel, P. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina.Fil:Jorge, G. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina.Fil:Perez, O.E. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina.Fil:Martín Negri, R. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina

Synthesis and characterization of CoFe2O4 magnetic nanotubes, nanorods and nanowires. Formation of magnetic structured elastomers by magnetic field-induced alignment of CoFe2O4 nanorods

Magnetic CoFe2O4 nanotubes, nanorods and nanowires were synthesized by the template method. The materials are highly crystalline and formed by compactly packed ceramic particles whose equivalent size diameter depends on the nanostructure type. Nanotubes and nanorods present the remarkable characteristic of having very large coercive fields (1000-1100 Oe) in comparison with nanoparticles of the same crystallite size (400 Oe) while keeping similar saturation magnetization (53-55 emu/g). Nanorods were used as filler material in polydimethylsiloxane (PDMS) elastomer composites, which were structured by curing in the presence of uniform magnetic field, Hcuring. In that way the nanorods agglomerate in the cured elastomer, forming needles-like structures (pseudo-chains) oriented in the direction of Hcuring. SEM analysis show that pseudo-chains are formed by bunches of nanorods oriented in that direction. At the considered filler concentration (1 % w/w), the structured elastomers conserve the magnetic properties of the fillers, that is, high coercive fields without observing magnetic anisotropy. The elastomer composites present strong elastic anisotropy, with compression constants about ten times larger in the direction parallel to the pseudo-chains than in the perpendicular direction, as determined by compression stress-strain curves. That anisotropic factor is about three-four times higher than that observed when using spherical CoFe2O4 nanoparticles or elongated Ni nanochains. Hence, the use of morphological anisotropic structures (nanorods) results in composites with enhanced elastic anisotropy. It is also remarkable that the large elastic anisotropy was obtained at lower filler concentration compared with the above mentioned systems (1 % w/w vs. 5-10 % w/w).Fil: Antonel, Paula Soledad. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Química, Física de los Materiales, Medioambiente y Energía. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Química, Física de los Materiales, Medioambiente y Energía; ArgentinaFil: Oliveira, Cristiano L. P.. Universidade de Sao Paulo; BrasilFil: Jorge, Guillermo Antonio. Universidad Nacional de General Sarmiento. Instituto de Ciencias; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Perez, Oscar Edgardo. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Química Biológica de la Facultad de Ciencias Exactas y Naturales. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Química Biológica de la Facultad de Ciencias Exactas y Naturales; ArgentinaFil: Leyva de Guglielmino, Ana Gabriela. Comisión Nacional de Energía Atómica; ArgentinaFil: Negri, Ricardo Martin. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Química, Física de los Materiales, Medioambiente y Energía. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Química, Física de los Materiales, Medioambiente y Energía; Argentin

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 (Fe₃O₄ 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 Fe₃O₄@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 Fe₃O₄@Ag particles are in a superparamagnetic state (<i>T</i>_B = 205 and 179 K at 0.01 T as determined by SQUID). The elastic properties and Young’s 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, <i>P</i>, is applied, and the magnitude of the change strongly depends on what direction <i>P</i> is exerted (anisotropic piezoresistivity). In addition, at a fixed pressure, σ increases exponentially in the presence of an external magnetic field (<b>H</b>) only when the field <b>H</b> is applied in the collinear direction with respect to the electrical flux, <b>J</b>. Excellent fits of the experimental data σ versus <b>H</b> and <i>P</i> were achieved using a model that considers the intergrain electron transport where an <b>H</b>-dependent barrier was considered in addition to the intrinsic intergrain resistance in a percolation process. The <b>H</b>-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 <b>H</b> and the current flow <b>J</b>). In the case of Fe₃O₄@ Ag, when <b>H</b> and <b>J</b> 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