MAGNETIC AND TEXTURAL STUDIES OF XEROGEL AND AEROGEL NANOCOMPOSITES FORMED BY Ni FERRITES DISPERSED IN SILICA MATRIX

Nickel ferrites have been extensively studied due to their numerous applications in electronic devices, microwave adsorbents, corrosion protectors, magnetic fluids and catalysts [1-5], among others. Nickel ferrite is a soft material with inverted spinel with the tetrahedral site (A) occupied by Fe and the octahedral site occupied by Fe and Ni. It is applied to devices that require easy magnetization and demagnetization to produce high magnetic flux and magnetic induction by an external field [3]. Usually, nickel ferrites is synthesized as dispersed particles or nanoparticles, but these kind of materials have a strong tendency to aggregate, mainly when these particles have nanometric dimension. The dispersion of ferrites in an inert matrix reduces particle agglomeration and controls the particle distribution. This procedure reduces energy loss of the material and provokes coupling effects, with resulting property enhancement. Important materials to be used as inert matrices are silica xerogels and aerogels obtained by sol-gel process. Previous paper on nanocomposites prepared by the sol-gel process indicated that there is no direct interaction between the silica oxide and the ferrites [6], which could influence the properties of the nanocomposites. In the absence of such interactions, the distribution of the nanoparticles depends on the pore structure of the matrix network, which affects the maximum size of particles formed. The drying step also plays an important role in determining the final pore structure of the materials obtained by sol-gel. When the solvent is slowly removed from the wet gel at room pressure and temperature, the xerogel formed presents a large structure shrinking and pore size, but when the solvent is removed by supercritical drying, aerogels with higher pore volumes are obtained. Some parameters are important in the control of magnetic properties of nanocomposites, such as crystallite size, concentration and distribution of the magnetic phase in the matrix. The crystallite size control is justified by existence of an average diameter range of single domain crystallites, between 10 nm < d < 70 nm, depending on the desired optimal magnetic properties. Crystallites with diameter smaller than 10 nm show superparamagnetic behavior, while with diameters larger than 70 nm (critical particle size/Dc) show multi-domain microstructure, with the consequent decrease in coercivity. When the ferrite concentration is low (< 10%), the crystallites are isolated, having single domains and showing superparamagnetism. Concentrations above 50% of ferrite provoke the agglomeration of the crystallites, which results in multi-domains. Other important characteristic of magnetic nanocomposites is the texture of the matrix, which has important influence in their final applications such as magnetic devices and catalysts, due to the transport and interaction of fluids within their connected network formed by meso and macropores.