The Effect of Surface Coating of Silicic Acid on the Thermal Decomposition of [beta]-FeO(OH)

The thermal decomposition of synthetic acicular microcrystals of β-FeO(OH) coated with silicic acid has been investigated from 200 to 700°C by means of X-ray diffractometry, thermal analyses, electron microscopy, nitrogen adsorption technique and Mössbauer spectroscopy. At lower decomposition temperatures, an intermediate, amorphous-like iro n oxide which was transformed into α-Fe2O3 at higher temperatures was formed. The temperature range of the formation of the intermediate spread over both high and low temperatures by the increase of silicic acid coating. The electron micrograph showed that the intermediate had needle-like shape and porous structure. The peak of pore radius distribution curves of the intermediates obtained by the calcination at 250°C were ca.1. nm independent of silicic acid content. The pore radius became large at higher calcination temperatures. The maximum surface areas were obtained for the intermediate formed by the calcination at 250°C irrespective of silicic acid contents. The pore volume and the specific surface area of the intermediates increased with the increase of silicic acid content. It was thought that the change in the specific surface area depended on the change in the pore structure. These morphological characteristics were compared with those produced from β-FeO(OH) without silicic acid. Chemical analysis showed that the intermediates contained 2∼8%of chlorine in terms of Cl/Fe. The Mössbauer spectra showed that the intermediate, prepared from β-FeO(OH) coated with 6.1% of SiO2 by the calcination at 300°C, had an internal magnetic field of 40.4 T at 20°C and was antiferromagnetic with the transition temperature of 200°C. The internal magnetic field decreased by the increase of the silicic acid contents, and increased by the elevation of the calcination temperature. It was considered that these changes depend on the change in size of unit particle which was contained in a porous structure of the intermediate.