Development of magneto-dielectric materials based on Li-ferrite ceramics I. Densification behavior and microstructure development

The densification, grain growth and microstructure development of Li-ferrite (Li0.50Fe2.50O4) ceramics, doped with Bi2O3 as sintering aid and Co as magnetic property modifier, were studied. The primary objective of the present study was to develop magneto-dielectric materials, with matching permeability and permittivity, as well as low magnetic and dielectric loss tangents, for miniaturization of HF (3-30 MHz) antennas. Due to its poor densification, fully sintering of pure Li0.50Fe2.50O4 requires high temperature (1200 deg C). High temperature sintering resulted in undesirable high dielectric loss tangent, due to the formation of Fe2+ ions. To reduce the sintering temperature of Li0.50Fe2.50O4, Bi2O3 was selected as sintering aid because of its relatively low melting point (820 deg C). The densification behavior and slow grain growth rate of Li0.50Fe2.50O4 were greatly improved by the addition of Bi2O3, because a liquid phase sintering occurred during the sintering process at high temperature due to the low melting point of Bi2O3. It is found that 1% Bi2O3 is able to promote full densification of Li0.50Fe2.50O4 at a temperature of as low as 850 deg C. Comparatively, the effect of Co on the densification and grain growth behavior of Li0.50Fe2.50O4 ceramics is much less significant.