Structural and complex electromagnetic properties of cobalt ferrite (CoFe2O4) with an addition of niobium pentoxide

Niobium pentoxide (Nb2O5) was added to cobalt spinel ferrite (CoFe2O4) powders for the first time, at varying amounts of 0, 5, 10 and 15 wt%. The purpose was to evaluate the effect of niobia on the crystalline phases, microstructure and complex electromagnetic behaviour (complex permittivity and permeability between 300 MHz and 10 GHz) of CoFe2O4. The samples were prepared by conventional ceramic methods and sintered at 1475 degrees C, as potential applications are as aerospace materials (radomes) which have to survive at such temperatures upon re-entry. The only crystalline phase observed in all samples was CoFe2O4, but microstructural evaluation showed that a non-crystalline, niobium-rich intergranular region was formed between the grains with niobium addition, and this apparent liquid/glassy phase aided sintering as considerable grain growth was also observed. It was shown by Raman spectroscopy that this niobium-rich amorphous intergranular phase was FeNbO4. The electromagnetic measurements of complex permittivity (epsilon*) and permeability (mu*) measurements indicated a steady decrease in both permittivity and permeability with increasing niobium oxide addition, although the values for each sample were relatively stable between 300 MHz and 10 GHz. The real permittivity, epsilon', decreased from 12 in the pure CoFe2O4 to similar to 5.5 with increasing addition, while its imaginary part assumed values very close to zero. At the same time, the real permeability, mu', decreased from similar to 1.4 to similar to 1.1, and a similar effect can be observed in the permeability curves. The results of the complex measurements also allowed us to obtain reflectivity graphs representing the energy loss of the incident electromagnetic wave when crossing the layer of the evaluated composition. The graphs are presented in the frequency domain, and indicate that the reflection loss increases with the addition of niobia.