Synthesis of nanosized ferrite powders from hydrothermal and supercritical solutions

Nanosized powders (with a grain size of 10-20 nm) of ZnFe2O4, NiFezO4, Ni0.2Zn0.8Fe2O4 and Ni0.5Zn0.5Fe2O4 and of iron(III), nickel, and zinc oxides were synthesized hydrothermally (1-6 h at 423-513 K) after precipitating amorphous metal hydroxides by ammonia from 0.25-0.50 M aqueous solutions of the proper nitrates (pH 7.0-11.0). ZnFe2O4 and NiFe2O4 were directly prepared in 30 and 50% yields by rapid expansion of supercritical solutions (the RESS process). The powders were characterized by X-ray powder diffraction, X-ray crystallography, thermogravimetry, scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The variable-field specific magnetic susceptibility chi was determined for the ferrite samples synthesized between 20 and 298 K. For the NiFe2O4, Ni0.2Zn0.8Fe2O4, and Ni0.2Zn0.5Fe2O4 samples, the chi -T curve showed maxima at T-b (where T-b is the blocking temperature), which are characteristic of superparamagnetic particles. Three nickel ferrite samples serve as an example of a decrease in T-b With increasing powder dispersion.