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

Magnetic spinel type oxides such as magnetite, Fe(3)O(4), cobalt, nickel, and zinc ferrites, MFe(2)O(4) (M = Co, Ni, Zn) and the mixed nickel and cobalt ferrite, Ni(x)Co(1 - x)Fe(2)O(4) have been synthesised continuously by the hydrolysis and 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, in selected cases, by transmission electron microscopy (TEM). The bulk composition of the samples was determined by Atomic Absorption analysis (AA). Additionally, Energy-dispersive Detection X-ray analysis (EDX) was carried out on some of the samples. TEM pictures showed a "bimodal" particle size distribution: small particles of ca. 10 nm and larger particles of up to 100 nm, both of which are highly crystalline. Possible reaction mechanisms are discussed, which may be responsible for the observed morphology. The effects of temperature and residence time on the reaction have been studied.