Preparation of ZnFe2O4/SiO2 and CdFe2O4/SiO2 nanocomposites by sol-gel method

The purpose of this paper is to characterise the phase relations in the system ZnFe2O4/SiO2 and CdFe2O4/SiO2 by dependence on the thermal treatment. Both ferrites have the structure of normal spinels. The samples were prepared by the sol--gel method, heated at a temperature of 800--1100 deg C and characterised by high-resolution transmission electron microscopy (HRTEM), X-ray diffraction, and Mossbauer spectroscopy. HRTEM revealed nanocrystals of the size of about 5--10 nm, depending on the heat treatment temperature. The spinel structure of ZnFe2O4 in the amorphous silica matrix proved to be very stable up to 1100 deg C without decomposition to the zinc silicate and iron (III) oxide. On the other hand, CdFe2O4 already starts to decompose at 900 deg C and complete decomposition to the alpha -Fe2O3 and cadmium silicate can be observed at 1100 deg C. This decomposition is also accompanied with the recrystallisation of the silica matrix to quartz and cristobalite.