Homogeneous composites of Fe and silica containing 5–40 wt % Fe have been prepared by infiltration of ferric nitrate solution into a colloidal silica: potassium silicate gel at room temperature. Previous electron microscopy, small angle neutron scattering, and nitrogen sorption (BET) measurements on such gels have shown they are comprised of an interconnected silica matrix, having pores ranging from 2–800 nm in size. Mössbauer effect and magnetization data show the Fe is in a paramagnetic environment with weak antiferromagnetic interactions. Curie–Weiss analysis indicated negative intercepts of the temperature axis for all the samples and that the magnetic moment per Fe atom (μFe) decreased as the Fe content increased at a rate of ∼0.0058 μB/% Fe. For most of the samples μFe≊2.1 μB. By contrast, in silica gel/Fe nanocomposites formed by the polymerization of an aqueous solution of tetraethoxysilane and iron nitrate, the Fe possessed magnetic moments varying from 3.9 to 2.9 μB. It is suggested that the difference is a result of the different pore structures of the gels, and not due to the different methods (e.g., postmatrix infiltration as compared to premixing prior to matrix polymerization) used to introduce the Fe.
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Shull et al. (1994) studied this question.
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