Electrical and magnetic properties of magnesium ferrite ceramics doped with Bi 2O 3

Abstract

The effects of concentration of Bi 2O 3 and sintering temperature on DC resistivity, complex relative permittivity and permeability of MgFe 1.98O 4 ferrite ceramics were studied. The objective of the study was to develop magneto-dielectric materials, with almost equal values of permeability and permittivity, as well as low magnetic and dielectric loss tangent, for the design of antennas with reduced physical dimensions. It was found that the poor densification and slow grain growth rate of MgFe 1.98O 4 can be greatly improved by the addition of Bi 2O 3, because liquid phase sintering was facilitated by the formation of a liquid phase layer due to the low melting point of Bi 2O 3. It was found that 3% Bi 2O 3 can result in fully sintered MgFe 1.98O 4 ceramics. The DC resistivities of the MgFe 1.98O 4 ceramics were increased as a result of the addition of Bi 2O 3, except for 0.5%. The exceptionally low resistivities of the 0.5% samples were explained by a ‘cleaning’ effect of the small amount of liquid phase at the samples’ grain boundaries. The electrical and magnetic properties of the MgFe 1.98O 4 ceramics exhibited a strong dependence on the concentration of Bi 2O 3. The 0.5% samples were found to have the highest dielectric loss tangents, which can be understood similarly to the DC resistivity results. The 2–3% Bi 2O 3 is required to attain low dielectric loss MgFe 1.98O 4 ceramics for antenna application. Low concentration of Bi 2O 3 increased the static permeability of the MgFe 1.98O 4 ceramics owing to the improved densification and grain growth, while too high a concentration led to decreased permeability owing to the incorporation of the non-magnetic component (Bi 2O 3) and retarded grain growth. However, the addition of Bi 2O 3 alone is not able to produce magneto-dielectric materials based on MgFe 1.98O 4 ceramics, and further work is necessary to modify the permeability using cobalt (Co).