Electrical properties of Fe-Fe3O4-SiO2 gel nanocomposites.

Nanocomposites containing Fe3O4 and alpha-Fe, respectively, in a SiO2 gel were prepared by subjecting a suitably chosen gel with iron ions to a reduction treatment at 923 K, followed by wet oxidation at the same temperature for 1 hour. The particle sizes of the two phases were estimated to have values in the range of 18 to 25 nm. The dc conductivity of the composites was found to arise due to a variable range hopping mechanism with a density of states calculated as approximately 10(18) eV(-1) cc(-1). The nanoparticles of alpha-Fe are believed to contribute to the latter. The ac conductivity variation as a function of frequency and temperature could be explained because of an overlapping small polaron tunneling mechanism in the Fe3O4 nanoparticles. The density of states estimated in the latter case was approximately 10(18) eV(-1) cc(-1). From the dielectric modulus spectra of the nanocomposites, a Kohlrausch-Williams-Watts (KWW) exponent of approximately 0.30 was extracted. This indicated the presence of a wide distribution of relaxation times in the system.