Investigation of rare earth and transition metals substituted X-type hexagonal ferrites for microwave applications

Abstract

The present work is focused on the fabrication of Sr-Ba based X-type ferrites substituted with transition metals (TM: Ni, Co), and trivalent rare-earth cations (Nd, Dy, Pr, Gd) for microwave applications. This dissertation presents a systematic study on four series namely, SrBaCu2-xNixNdyFe28-yO46, SrBaMg2-xNixCoyFe28-yO46, SrBaZn2-xNixPryFe28-yO46 and SrBaMn2-xCoxGdyFe28-yO46 (x=0 to 1.0 and y=0 to 0.1) of hexagonal ferrites. These X-type materials were fabricated by sol-gel method at 1250 °C temperature. The thermal decomposition of nitrate-citrate gel of as-synthesized samples and estimation of sintering temperature to obtain X-type phase was determined by DSC, TGA, and DTA analysis. X-Ray Diffraction patterns revealed pure X-type hexagonal phase of the investigated X-type hexaferrites, except for the series doped with Dy-Co an extra peak of orthorhombic phasedetected as DyFeO3 appeared for higher concentrations of Dy content. Lattice parameters (lattice constants, porosity, X-ray density, bulk density, crystalline size) showed variations with dopants (RE-TM) and their concentrations. There was increase in lattice constants (a, c) with increase in dopants concentration which was attributed to introduction of higher ionic radius of RE3+ cations in octahedral sites of Fe3+ ions.FTIR spectral study revealed the presence of absorption bands in the range 400-600 cm-1which are characteristics of hexaferrites. It was observed that substitution of RE-TM ions caused shifting of band positions due to change in metal-oxygen bond length at A- and B-sites. From hysteresis loops magnitudes of saturation magnetization, coercivity as well as retentivity were calculated. It was observed that magnetic parameters (saturation magnetization, retentivity, coercivity) were enhanced with RE-TM doping in these X-type ferrites. For all samples, the values of saturation magnetization varied in range from 40 to 65 emu/g, and coercivity was found in range 1500-3500 Oe, the high values of Ms and Hcsuggested application of these RE-TM doped X-type ferrites in high density video recording and permanent magnets. The dielectric characteristics were carried out in frequency range 1 MHz to 3 GHz. The real and imaginary parts of dielectric constant and tangent loss were decreased with the increase of frequency on the basis of Wagner and Koop's theory. The resonance peak appeared in all samples above 2 GHz (i.e. in MW band) which is characteristics of materials used for EMI attenuation. The doping of RE-TM resulted in increase of dielectric constant, AC conductivity and Q-value for all series. In the Present studies it had been found that Q-factor decreased with concentration of RE-TM ions which reflected the fact that with addition of RE-TM substitution the loss of energy has been enhanced. Reduced size, high porosity, high losses, low Q values and high saturation magnetization endorsed these X-type materials for microwave absorption applications such as in mobiles, medicine, satellite communication, stealth and RAM etc.