Abstract
Manufacture of a high quality and low cost ferrite material for power transformer cores in switched mode power supplies is an important aspect for investigation. These cores require high saturation flux densities, low hysteresis and low eddy current losses at high frequencies (upto a few megahertz). Eddy current losses can be minimized by, besides optimizing the microstructure, increasing the resistivity of a ferrite material. Hysteresis loss can be lowered by a decrease in anisotropy constant and magnetostriction which have inverse relations with the permeability [1, 2]. Snoek [3] demonstrated that an increase in permeability would increase the operating frequency of a ferrite only when there is a simultaneous increase in saturation magnetization. Therefore, it necessitates careful planning even at the compositional stage in the development of these materials. The aim of this paper is to examine and discuss the possibilities of producing materials with optimum characteristics required for high frequency power applications using low-cost raw materials and by inexpensive preparation methodology. Since Ni-Zn ferrites normally exhibit high resistivities and can be easily sintered in air atmospheres, a nickel-zinc ferrite composition with high room temperature saturation magnetisation, Ni0.65Zn0.35Fe2O4, was considered to be suitable for the current study. However, this composition has some disadvantages since its anisotropy constant, |K1| and the associated losses are high [4]. Therefore, compositional modifications are necessary and before making such substitutions two aspects have to be considered; the first is to optimize the sintering conditions as well as the microstructure for the substituted samples, and the other is that the substitutions are to be made in such a way that the resultant magnetization of the final product should remain at its peak or close to peak. It should mention here that in this work the selection of the sintering conditions for the substituted samples has to be made mainly with an emphasis to improve upon their magnetic properties. With these considerations, compositional modifications have been aimed at to the basic nickel-zinc ferrite with varying concentrations of Zn and Sc. The concept of simultaneous substitutions [5] has been tried in an attempt to obtain as high as possible magnetization out of a given system. Zn2+ ions are selected for substitution in the expectation that they will improve magnetization and reduce the anisotropy constant [6] as well as magnetostriction [7]; thus an increase in permeability and a decrease in hysteresis loss may be realized. On the other hand, the choice of scandium ions, expected to be in octahedral sites, is made to increase the resistivity and thereby decreasing the eddy current losses. Polycrystalline Ni-Zn-Sc ferrites with the formula Ni0.65−x Zn0.35+x Fe2−x Scx O4, where x values range from 0.000 to 0.125 in steps of 0.025, have been prepared by conventional ceramic technique using the procedure described elsewhere [8]. Sintering of the samples has been carried out at 1300 ◦C for 4 h in air atmosphere followed by natural cooling. X-ray studies confirm single phase spinel structure in all the samples. Regarding characterisation of the samples, lattice constant and Curie temperature of the basic composition were measured and these were found to be in good agreement with the respective parameters of the same composition reported earlier [9, 10]. DC resistivity measurements have been made on the samples by standard two probe method using spring loaded copper electrodes connected to a Keithley model 614 digital electrometer. Permeability and loss factor (tan δ) measurements were made in the range from 10 kHz to 13 MHz by using HP4192A LF Impedance Analyzer. Saturation magnetization studies were made by the Ponderometer method. Bulk density and grain size estimations have been made by using Archimedes principle and Jeol T330A scanning electron microscope respectively. Table I gives the values of saturation magnetization (4IIMs), initial permeability (μi), tan δ/μi, dc resistivity (ρ), grain size and bulk density of various Ni-Zn-Sc ferrites. Density and grain sizes are found to increase for initial concentrations of scandium and for subsequent concentrations they remained either almost unaltered or decreased slightly. These results indicate that the presence of scandium enhances sinterability due to an increase in bulk diffusion [11] thus promoting densification as well as grain growth processes for smaller concentrations. Increased zinc content is known to decrease crystalline anisotropy and exchange energy [6] thus causing a decrease in the grain size as is the case with higher concentrations in the study. The substitutions have resulted in higher DC resistivities with the increase in x . This increase in resistivity is accounted for by a larger ionic radii and specific site preferences of scandium ions in different lattice sites [8] and will contribute to lower eddy current losses. The simultaneous substitution of zinc and scandium ions aiming at dilution of both the sublattices because
Published Version
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