Investigation of solid–solid interactions in NiO/Fe2O3 system doped with ZnO

The solid–solid interactions between pure and ZnO-doped nickel and ferric oxides have been investigated using DTA and XRD techniques. A mixture of equimolar proportions of finely powdered basic nickel carbonate and ferric oxide were impregnated with zinc nitrate dissolved in the minimum amount of distilled water making a paste. The paste was dried then calcined at 700–1100 °C. The amounts of dopant were 0.75, 1.5, 3 and 6 mol% ZnO. The results obtained showed that the addition of zinc nitrate to the reacting mixed solids enhanced the thermal decomposition of nickel carbonate to an extent proportional to its amount added. Fe2O3 interacted readily with NiO at temperatures starting from 700 °C producing crystalline NiFe2O4 phase. The degree of reaction propagation increased as a function of temperature. However, heating of pure mixed solids at 1100 °C for 6 h did not affect the complete conversion of the reacting oxides to NiFe2O4. ZnO-doping of NiO/Fe2O3 system at 700–900 °C effected a limited enhancement in the formation of NiFe2O4. However, an effective enhancement of nickel ferrite was observed upon heating the doped solids at 1000–1100 °C. The activation energy of formation (ΔE) of NiFe2O4 phase was determined for pure and doped solids by following up the change in the peak height of the diffraction line characteristic for NiFe2O4 at d-spacing 2.95 Å as a function of calcination temperature. The computed ΔE values were 103, 85, 70, 41 and 20 kJ mol−1 for pure solids and those doped with 0.75, 1.5, 3 and 6 mol% ZnO, respectively. The decrease in ΔE value of NiFe2O4 formation as a function of dopant added was not only attributed to an effective increase in the mobility of the reacting cations but also to the formation of the mixed ferrite Ni1−xZnxFe2O4 that substitute some of the early formed ferrite. The presence of this mixed ferrite led to an increase in the diffusion of the reacting cations through the ferrite film.