The continuous hydrothermal synthesis of nano-particulate ferrites in nearcritical and supercritical water

Magnetic spinel type oxides such as magnetite, Fe3O4, cobalt, nickel, and z inc ferrites, MFe2O4 (M = Co, Ni, Zn) and the mixed nickel and cobalt ferri te, NixCo1 - xFe2O4 have been synthesised continuously by the hydrolysis an d simultaneous oxidation of mixtures of Fe(ii) acetate and different M(ii) acetates in near-critical and supercritical water using a flow reactor. The materials have been characterised by powder X-ray diffraction (PXD) and, i n selected cases, by transmission electron microscopy (TEM). The bulk compo sition of the samples was determined by Atomic Absorption analysis (AA). Ad ditionally, Energy-dispersive Detection X-ray analysis (EDX) was carried ou t on some of the samples. TEM pictures showed a "bimodal" particle size dis tribution: small particles of ca. 10 nm and larger particles of up to 100 n m, both of which are highly crystalline. Possible reaction mechanisms are d iscussed, which may be responsible for the observed morphology. The effects of temperature and residence time on the reaction have been studied.