Iron and Iron-oxide on Silica Nanocomposites Prepared by the Sol-gel Method
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M.P. Morales and C.J. Serna Instituto de Ciencia de Materiales de Madrid, CSIC, 28049, Cantoblanco, Madrid, Spain (Received 21 June 2001; accepted 21 December 2001)
␥–Fe2O3/SiO2 and Fe/SiO2 nanocomposites, with a Fe/Si molar ratio of 0.25, were prepared by the sol-gel method starting from ethanolic solutions of tetraethoxysilane and iron (III) nitrate. After gelation the xerogels were oxidated or reduced. Samples were investigated by transmission electron microscopy, x-ray diffraction, differential scanning calorimetry, and thermogravimetry. Magnetic properties of the samples were investigated at room temperature (RT) and at 77 K. Nanometric particles supported in the silica matrix were obtained in all cases. Bigger particles (10 nm) were obtained in the case of Fe/SiO2 nanocomposites with respect to the ␥–Fe2O3/SiO2 samples (5–8 nm). A slight effect of sol dilution on particle size was observed only in the case of ␥–Fe2O3/SiO2 nanocomposites. A superparamagnetic behavior was shown at RT only by ␥–Fe2O3/SiO2 nanocomposites. Iron-based composites exhibited coercivity values higher than 700 Oe at RT.
I. INTRODUCTION
Nanostructured materials have attracted considerable attention in recent years because they exhibit useful and unusual properties compared to conventional polycrystalline materials.1– 4 In particular, iron- and iron-oxidebased nanomaterials have potential applications in reading and writing technologies on magnetic media,5 in catalysis,6 color imaging,7 magneto-optical devices,8 ferrofluids,9 and particularly as superparamagnets.10 In fact, their superparamagnetic clusters provide magnetic refrigerants with large magnetocaloric effects that can be used at a higher temperature and/or with a smaller applied magnetic field than that of classic paramagnetic refrigerants.11 Moreover they exhibit coercivity and saturation magnetization (Ms), values which strongly depend on the size and magnetic anisotropy of the particles. These materials are often prepared as nanocomposites to avoid the tendency of nanoparticles to aggregate. Further difficulties arise in obtaining samples with controlled particle size and narrow size distribution. The sol-gel-derived amorphous silica matrix is an excellent host for supporting different types of guest nanoparticles,2,12–14 like ␥–Fe2O3 or Fe. The porous nature of the amorphous silica matrix provides nucleation sites for ␥–Fe2O3 or Fe guest particles and minimizes the a)
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J. Mater. Res., Vol. 17, No. 3, Mar 2002 Downloaded: 17 Mar 2015
aggregation phenomena imposing an upper limit to the size of the particles. On the other hand, it has been shown that important features of the final composite, such as the nature of the nanocrystalline guest phase, the average particle size, and the size distribution, are mainly determined by the sol-gel parameters governing the early stages of the preparation, i.e., the metal and silica precursors, the concentration of their solutions, and
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