Influence of zinc substitution with cobalt on thermal behaviour, structure and morphology of zinc ferrite embedded in silica matrix

The influence of Zn2+ substitution with Co2+ on the thermal behaviour, morphology and structure of ZnFe2O4 was studied. The ZnFe2O4 and Co-substituted ZnFe2O4 (Co0.3Zn0.7Fe2O4 and Co0.7Zn0.3Fe2O4) nanoparticles embedded in SiO2 matrix were synthesized by a modified sol-gel method. The thermal analysis revealed the formation of Zn-, Fe- and Co-succinate precursors at 130-200 degrees C and their decomposition to ferrites at 220-400 degrees C. Fourier transformed infrared spectroscopy confirmed the formation of succinates and their decomposition into ferrites. Isotherms were used to calculate the rate constants for the decomposition of succinates to ferrites and the activation energy for each ferrite. The nanocrystallites have cubic spinel structure and an average diameter of 12-31 nm. At 1000 degrees C, for ZnFe2O4/SiO2 gel, X-ray diffraction (XRD) revealed a main (ZnFe2O4) and two secondary (Zn2SiO4 and SiO2) phases. By partially substitution (Co0.3Zn0.3Fe2O4/SiO2 gel), the secondary phases disappear, XRD revealing only the presence of mixed Zn-Co ferrite. Due to presence of non-magnetic SiO2 matrix, nearly spherical and non-agglomerated nanoparticles were showed by transmission and scanning electron microscopy. Ferrites are coloured in dark brown to black shades and can be used as ceramic pigments.