Hydrothermal synthesis of ultrafine barium hexaferrite nanoparticles and the preparation of their stable suspensions

The hydrothermal treatment of an appropriate suspension of Ba and Fe hydroxides in the presence of a large excess of OH− results in the formation of Ba hexaferrite at temperatures as low as 150 °C. This low formation temperature enables the synthesis of uniform, ultrafine Ba hexaferrite nanoparticles. These nanoparticles have a disc-like shape, approximately 10 nm wide, but only approximately 3 nm thick. When the temperature of the hydrothermal treatment is increased, large platelet Ba hexaferrite crystals appear as a consequence of secondary re-crystallization (Ostwald ripening). In this work, this undesired process of secondary re-crystallization has been evaluated. We show that the secondary re-crystallization can be totally suppressed with the use of an oleic acid surfactant. The addition of oleic acid enabled the synthesis of uniform, ultrafine nanoparticles at temperatures up to 240 °C. The nanoparticles were hydrophobic and could be suspended in nonpolar liquids to form relatively concentrated ferrofluids. Such stable suspensions of hexaferrite nanoparticles will be technologically important, especially as precursors for the preparation of new nanostructured materials, for example nanocomposites or nanostructured ceramic films.