Fabrication & Thermophysical Studies of Hexa Ferrites

Abstract

Strontium hexaferrite nano material with nominal composition SrFe12O19 is prepared by wet chemical methods. The effect of variation in synthesis parameters such as molar ratio of cations (Fe/Sr), volume rate of addition of precipitating agent and the pH of the solution on the phase purity and particle size is studied to optimize them for the synthesis by co- precipitation method. The effect of molar ratio of cations (Fe/Sr) on phase purity is studied by using X-ray diffraction (XRD) patterns. It is observed from indexed XRD patterns that molar ratio of cations does not affect the phase purity of strontium hexaferrites as there is no impurity peak present in any sample and all patterns are almost similar. The effect of volume rate of addition of precipitating agent on phase purity and surface morphology are analyzed by using XRD diffraction patterns and scanning electron micrographs (SEM). The indexed XRD patterns show that the increase in the volume rate of addition of precipitating agent improves the phase purity and SEM micrographs show that the size of the particles also decrease with the increase in the volume rate of addition of precipitating agent. The effect of pH variation on structural and electrical properties of strontium hexaferrite is analyzed by using X-ray diffractometer, scanning electron microscope, temperature dependent dc resistivity measurement system and precision component analyzer. Indexed XRD patterns show that the secondary phases are decreased with the increase in pH of the solution and single phase strontium hexaferrite is obtained for pH=13. The pH of the solution also imparts a significant effect on structural morphology of prepared hexaferrite samples. The SEM micrographs with varying pH samples clearly indicate that most of the particles are of hexagonal shape. It can also be seen that the particle size and their distribution also decrease with the increase in the pH of the solution. The dc resistivity is also increased by increasing pH and this may be due to increase in the grain boundaries. The composition SrFe12-xCrxO19 (X=0.0, 0.2, 0.4, 0.6, 0.8) is prepared in order to increase the coercivity of strontium hexaferrites. Results obtained indicate that Cr doping causes the formation of secondary phases. It is also observed that for X ≤ 0.6, both dielectric constant and coercivity is increased while saturation magnetization is decreased. The increase in coercivity was due to variation in particle size and impurity phases which acted as pinning centers. The decrease in saturation magnetization is because of the replacement of cation (Fe3+) having high magnetic moment (5μB) on octahedral sites with cation (Cr3+) having smaller magnetic moment (3μB). Another composition SrFe12-2xCrxZnxO19 with (X=0.0, 0.2, 0.4, 0.6, 0.8) is prepared with co-precipitation method in order to reduce the dielectric loss tangent. The results show that Cr-Zn doping causes increase in the particle size and decrease in dielectric loss tangent and make the strontium hexaferrite useful for high frequency applications. The hysteresis loops of the Cr-Zn doped samples reveal that both coercivity and saturation magnetization is decreased with increase in doping concentration. The same composition SrFe12-2xCrxZnxO19 with x=0.0, 0.2, 0.4, 0.6, 0.8 is synthesized with WOWS sol- gel method (WOWS stands for Without Water and Surfactants; a new simplified sol-gel method developed in our lab). The structural and dielectric measurements results obtained from the samples prepared with WOWS sol-gel method are better than the results obtained from the same composition prepared with co-precipitation. In some cases, the materials with high loss as well as high dielectric constant may be desired in applications such as electromagnetic (EM) wave absorbing coatings. To achieve these properties, reduction of oxygen from sintered SrFe12O19 is made. This treatment resulted in the increase in the concentration of Fe2+ ions and free iron atoms and hence in the increase in both dielectric constant and dielectric loss and making the material useful for microwave absorption.