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

Abstract

DC resistivity and dielectric properties of the Li-ferrite ceramics (Li 0.50Fe 2.50O 4) doped with Bi 2O 3 (0–5 wt%) and modified with Co (Li 0.50−0.50 x Co x Fe 2.50−0.50 x O 4, 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 Li 0.50Fe 2.50O 4 ceramics with low dielectric loss tangent because it cannot be sintered at temperature of ≤1100 °C and high temperature sintering will lead the formation Fe 2+ ions. Although 1% Bi 2O 3 is sufficient to obtain fully dense Li 0.50Fe 2.50O 4 ceramics over 850–1100 °C, it is insufficient to produce materials with a dielectric loss tangent of ≤10 −2. A high concentration (3%) Bi 2O 3, combined with sintering temperature of ≤950 °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 Li 0.50Fe 2.50O 4 ceramics as a function of the concentration of Bi 2O 3 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 Li 0.50Fe 2.50O 4 ceramics is much less than that of Bi 2O 3.