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Briceno S.,Venezuelan Institute for Scientific Research | Bramer-Escamilla W.,Venezuelan Institute for Scientific Research | Silva P.,Venezuelan Institute for Scientific Research | Garcia J.,University of Los Andes, Venezuela | And 6 more authors.
Journal of Magnetism and Magnetic Materials | Year: 2014

Nickel ferrite (NiFe2O4) was supported on activated carbon (AC) from petroleum coke (petcoke). Potassium hydroxide (KOH) was employed with petcoke to produce activated carbon. NiFe2O4 were synthesized using PEG-Oleic acid assisted hydrothermal method. The structural and magnetic properties were determined using thermogravimetric and differential thermal analysis (TGA-DTA), X-ray diffraction (XRD), Fourier Transform Infrared (IR-FT), surface area (BET), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and vibrating sample magnetometry (VSM). XRD analysis revealed the cubic spinel structure and ferrite phase with high crystallinity. IR-FT studies showed that chemical modification promoted the formation of surface oxygen functionalities. Morphological investigation by SEM showed conglomerates of spherical nanoparticles with an average particle size of 72 nm and TEM showed the formation of NiFe2O4/carbon nanofibers. Chemical modification and activation temperature of 800 C prior to activation dramatically increased the BET surface area of the resulting activated carbon to 842.4 m2/g while the sulfur content was reduced from 6 to 1%. Magnetic properties of nanoparticles show strong dependence on the particle size. © 2014 Elsevier B.V. Source


Vivas L.,Venezuelan Institute for Scientific Research | Costa-Kramer J.L.,CSIC - National Center of Microelectronics | Larionova J.,Montpellier University | Rodriguez P.,Institute Estudios Cientificos y Tecnologicos IDECYT | Silva P.,Venezuelan Institute for Scientific Research
Revista de la Facultad de Ingenieria | Year: 2014

In this work we report ferromagnetism at room temperature and electrical transport effects above room temperature in CaCu3Ti4O12+ (ZnO or TiO2) mixtures. From the Electron Paramagnetic Resonance (EPR) we derived the spectrum of the resonance field (HR) and line width peak to peak (HPP). The magnetization of the samples was studied as a function of the temperature. Weak ferromagnetism was found in four blends made by the mechanical milling method. This fact is attributed to a double magnetic exchange enabled by a possible effect of a partial diffusion at the grain interfaces between the CaCu3Ti4O12 and the oxides (ZnO or TiO2). This possible partial diffusion, due to a closed contact between the particles, is observed in the mixtures by using scanning electron microscopy. We correlate the behavior of the peak to peak line width with the electrical conduction around 400K. Source


Briceno S.,Venezuelan Institute for Scientific Research | Sanchez Y.,Venezuelan Institute for Scientific Research | Bramer-Escamilla W.,Venezuelan Institute for Scientific Research | Silva P.,Venezuelan Institute for Scientific Research | And 3 more authors.
Acta Microscopica | Year: 2013

In this paper a comparative study of the effect of preparation methods, sol-gel auto-combustion, co-precipitation and hydrothermal method in structural and magnetic properties of nanoparticles of cobalt ferrite CoFe2O4has been carried out. The samples were characterized using: X-ray diffraction, scanning electron microscopy and vibrating sample magnetometry. Magnetization measurements as a function of the applied field verified the marked influence of the synthesis method on the magnetic properties of the nanoparticles, reaching a maximum magnetization of 92.64 emu/g for the ferrite prepared using the sol-gel auto-combustion method. Source

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