Magnetodielectric Ni Ferrite Ceramics With Bi_2o_3 Additive For Potential Antenna Miniaturizations

摘要

This paper reports on the preparation and characterization of nickel ferrite (NiFe_(1.98)O_4) ceramics doped with Bi_2O_3 as sintering aid. Focus has been on the effects of concentration of Bi_2O_3 and sintering temperature on the densification, grain growth, dielectric, and magnetic properties of the NiFe_(1.98)O_4 ceramics, with an aim at developing magnetodielectric properties, with almost equal real permeability and permittivity, as well as sufficiently low magnetic and dielectric loss tangents, over 3 to 30 MHz (high frequency or HF band). X-ray diffraction results indicated that there is no obvious reaction between NiFe_(1.98)O_4 and Bi_2O_3, at Bi_2O_3 levels of up to 7 wt% and temperatures up to 1150 ℃. The addition of Bi_2O_3 facilitated a liquid phase sintering mechanism for the densification of NiFe_(1.98)O_4 ceramics. The addition of Bi_2O_3 not only improved the densification but also promoted the grain growth of NiFe_(1.98)O_4 ceramics. To achieve sufficiently low dielectric loss tangent, the concentration of Bi_2O_3 should not be less than 5 wt%. The low dielectric loss tangents of the samples doped with high concentrations of Bi_2O_3 can be attributed to the full densification of the ceramics. Magnetic properties of the NiFe_(1.98)O_4 ceramics, as a function of sintering temperature and Bi_2O_3 concentration, can be qualitatively explained by the Globus model. Promising magnetodielectric properties have been obtained in the sample doped with 5% Bi_2O_3.and sintered at 1050 ℃ for 2 h. The sample has almost equal values of permeability and permittivity of ～12, together with low dielectric and magnetic loss tangents, over 3 to 30 MHz. This material might be useful for the miniaturization of HF (3 to 30 MHz) antennas.