Development of magneto-dielectric materials based on Li-ferrite ceramics II. DC resistivity and complex relative permittivity

DC resistivity and dielectric properties of the Li-ferrite ceramics (Li0.50Fe2.50O4) doped with Bi2O3 (0-5 wt%) and modified with Co (Li0.50-0.50xCoxFe2.50-0.50xO4, with x = 0.01-0.07) were investigated, with a primary objective to develop magneto-dielectric materials with matching permeability and permittivity and low magnetic and dielectric loss tangent, for miniaturization of HF antennas. It is impossible to obtain pure Li0.50Fe2.50O4 ceramics with low dielectric loss tangent because it cannot be sintered at temperature of < =1100 deg C and high temperature sintering will lead the formation Fe2+ ions. Although 1% Bi2O3 is sufficient to obtain fully dense Li0.50Fe2.50O4 ceramics over 850-1100 deg C, it is insufficient to produce materials with a dielectric loss tangent of < =10-2. A high concentration (3%) Bi2O3, combined with sintering temperature of < =950 deg C, is required to attain sufficiently low dielectric loss tangent of < =10-2 over the frequency range of interest (3-30 MHz or HF band). The variation of relative permittivity and dielectric loss tangent of the Li0.50Fe2.50O4 ceramics as a function of the concentration of Bi2O3 and sintering temperature can be understood in terms of the microstructure developed. It is also found that the effect of Co on the dielectric properties of Li0.50Fe2.50O4 ceramics is much less than that of Bi2O3.