Magnetodielectric Ni ferrite ceramics with Bi 2 O 3 additive for potential antenna miniaturizations
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This paper reports on the preparation and characterization of nickel ferrite (NiFe1.98O4) ceramics doped with Bi2O3 as sintering aid. Focus has been on the effects of concentration of Bi2O3 and sintering temperature on the densification, grain growth, dielectric, and magnetic properties of the NiFe1.98O4 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 NiFe1.98O4 and Bi2O3, at Bi2O3 levels of up to 7 wt% and temperatures up to 1150 C. The addition of Bi2O3 facilitated a liquid phase sintering mechanism for the densification of NiFe1.98O4 ceramics. The addition of Bi2O3 not only improved the densification but also promoted the grain growth of NiFe1.98O4 ceramics. To achieve sufficiently low dielectric loss tangent, the concentration of Bi2O3 should not be less than 5 wt%. The low dielectric loss tangents of the samples doped with high concentrations of Bi2O3 can be attributed to the full densification of the ceramics. Magnetic properties of the NiFe1.98O4 ceramics, as a function of sintering temperature and Bi2O3 concentration, can be qualitatively explained by the Globus model. Promising magnetodielectric properties have been obtained in the sample doped with 5% Bi2O3 and sintered at 1050 C 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.
I. INTRODUCTION
Miniaturization of antennas, especially for those working at high frequency (HF, 3–30 MHz) and very high frequency (VHF, 30–300 MHz), has always been a challenge to the research community. Since the physical dimensions of antennas are proportional to their wavelength of working frequencies, the HF and VHF antennas are also used for portable applications. Reduction of antenna physical sizes can be realized by antenna designs based on the understanding of electromagnetic principles.1 An alternative way to shrink antenna is to use material loadings. Theoretically, if an antenna pffiffiffiffiffiffiffiisffi loaded with materials with refraction index (n ¼ m0 e0 , where m0 is relative permeability and e0 is relative permittivity) larger than that of free space,2–5 its physical size can be shrunk by a factor of n. In practical a)
These authors contributed equally to this work. YDSP student from Raffles Junior College, 10 Bishan Street 21, Singapore 574013. c) YDSP student from River Valley High School, 2 Malan Road, Singapore 109433. d) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2009.0057 b)
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J. Mater. Res., Vol. 24, No. 2, Feb 2009
applications, materials for such purposes must meet other requirements2,3; i.e., permeability and permittivity should be as close as possible in order to
