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Fabrication of composite materials by in situ generation of gamma- and epsilon-Fe(2)O(3) nanoparticles in a SiO(2) matrix through sol-gel process is reported. The process involves the hydrolysis and condensation of 1:3:10:x (x = 0.05, 0.1 and 0.2) molar ratios of tetraethoxysilane, absolute ethanol, nitric acid (0.16 N) and ferric nitrate, respectively, and subsequent thermal-treatment at temperatures ranging from 110 to 1000 degrees c. The in situ generation and growth of gamma- and epsilon-Fe(2)O(3) nanoparticles, and their distribution in SiO(2) matrix strongly depend on the concentration of Fe(3+) ions and thermal-treatment temperatures. The restricted growth of Fe(2)O(3) in SiO(2) matrix seems to stabilize the metastable epsilon-Fe(2)O(3) phase and prevent the formation of alpha-Fe(2)O(3) even at 1000 degrees C. Further, the presence of Fe(2)O(3) nanoparticles in SiO(2) matrix modified the gel morphology on thermal-treatment, leading to strong structural and chemical changes which influence the magnetic properties to a large extent. The concentration of individual magnetic phase (gamma- and epsilon-Fe(2)O(3)) in the samples, the particle size and distribution, and thermal-treatment temperature determine the net magnetic moment, shape of the hysteresis loop (symmetric or concentric), coercivity and magnetic phase transition. (C) 2009

Transparent crack-free and bubble-free Fe3+ doped SiO2 nanostructured gel-glasses were obtained by the sol-gel process. The process involves the hydrolysis and condensation of an appropriate molar ratio of tetraethoxysilane (TEOS), absolute ethanol, nitric acid and ferric nitrate, followed by stepwise annealing at temperatures ranging from 110 degrees C to 1000 degrees C. The structural variation of the gel-glasses and their influence on physical properties during annealing has been studied. It has been observed that monolithicity and chemical environment around Fe3+ in the gel-glasses are strongly dependent on the annealing temperatures. The colour of gel-glass samples is different for different annealing temperatures, mainly due to the different co-ordination state of Fe3+ and the generation of Fe2O3 colloids of size 20-60 nm in the silica matrix. The annealing process facilitates the tuning of the UV-visible transmission cut-off edge in high optical quality Fe3+ doped silica gel-glasses. A marked difference in the magnetic properties of these glasses is, also observed with annealing temperatures. (c) 2005