The microstructure evolution in several polycrystalline yttrium iron garnet samples as a result of a sintering scheme was studied in detail, in parallel with the changes in their magnetic properties. Samples with nanometer sized starting powder were synthesized by employing the High-Energy Ball Milling technique and then sintering toroidal compacts of the milled powder. Nine sintered samples were obtained, each corresponding to a particular sintering from 600 °C to 1400 °C. The samples were characterized for their evolution in crystalline phases, microstructure and magnetic hysteresis-loops parameters. The results showed an increasing tendency of the saturation magnetization and saturation induction with grain size, which is attributed to crystallinity increase and to reduction of demagnetizing fields in the grains. The variation in coercivity could be related to anisotropy field changes within the samples due to grain size changes. In particular, the starting appearance of room temperature ferromagnetic order suggested by the sigmoid-shaped B-H loops seems to be dependent on a sufficient number of large enough magnetic domain-containing grains having been formed in the microstructure. Viewed simultaneously, the hysteresis loops appear to belong to three groups with different magnetism-type dominance, respectively dependent on phase purity and three different groups of grain size distributions.
Substituted barium hexaferrite nanoparticles with nominal composition of BaCo1.0Ti1.0Fe10O19 and BaMn0.8Ti0.8Fe10.4O19 were synthesized by high energy ball milling (HEBM). The effects of Co-Ti and Mn-Ti substitution on their microstructure, electromagnetic properties, and microwave absorptive behavior were analyzed. The samples were structurally characterized by X-ray diffractometry (XRD), field emission scanning electron microscopy (FESEM), and energy-dispersive X-ray analysis (EDX). The M-H loops of the composites were determined with a vibrating sample magnetometer (VSM), and the interaction with the microwave radiation in the range of 8-18 GHz of the nanocomposites dispersed in epoxy resin was measured with a vector network analyzer (VNA). This study suggests that by controlling the grain size and different elements of substitution would give a decrease in coercivity and enhanced values of complex permittivity in order to improve microwave absorption. The dielectric constant and loss were enhanced in comparison to the permeability constant and loss over the entire frequency range. Finally, microwave measurement showed that the substituted barium hexaferrite sample with Co-Ti and Mn-Ti could be used as an efficient microwave absorption material with an appropriate absorption at −31.27 and −26.73 dB, respectively. The predicted absorption and reflection loss demonstrates that Co-Ti and Mn-Ti substitution gives low reflectivity at microwave frequency and is a good candidate for electromagnetic materials for radar wave applications.
Problem statement:This study involved an investigation to ascertain the diffusion of NiO and ZnO into the tetrahedral and octahedral sites using mechanical alloying method. The effect of mechanical alloying towards particle size was also reported. Approach: NiO, ZnO and Fe 2 O 3 precursors were mechanically alloyed to synthesis ultrafine powders of Ni 0.5 Zn 0.5 Fe 2 O 4 . Various milling times were employed to study the effect of milling time on the materials. The ultrafine powder was sampled after each milling time and further characterized using XRD to investigate the phases of the powder and the crystallite size, SEM for the morphology and TEM for particle size investigation. Results: The XRD spectra indicated the precursors reacted during milling with the diffusion of ZnO and followed by NiO into their respective crystallographic sites. SEM micrographs showed the agglomeration of powders due to high energy milling and TEM images proved the particles of the materials were of nanosize. Conclusion: It was concluded that samples prepared using mechanical alloying technique appear to be a potential method for large production due to the possible reduction of cost and also reduction of particle size against milling time.
The causes of electromagnetic energy loss in very low loss yttrium iron garnet (YIG) are not fully understood. Thus, we have studied and now report on new findings concerning this problem at MHz and low GHz frequencies. We chose to observe if there would be new revealing data if a polycrystalline YIG sample was subjected to an isochronal recovery behaviour process, a procedure normally employed to study electrical resistivity and shear stress in metals by metallurgist. Thus, isochronal recovery behaviour of YIG samples' complex permeability components, l 0 and l 00 as well as their electrical resistivity (q), was studied in this work. The YIG samples were prepared via mechanical alloying of pure Y 2 O 3 and Fe 2 O 3 powders followed by appropriate sintering, measurement of properties, quenching, re-measurement of properties, isochronal annealing and a final repeat of property measurement. Results obtained from the experiments showed that isochronal recovery behaviour could be clearly exhibited by both the complex permeability components and the electrical resistivity. The measured property values gave their isochronal recovery behaviour in which the values, though suppressed after the quenching were recovered even higher after undergoing the annealing. We believe the mechanisms which produced the changes should involve atomic scale and submicron defects in the form of vacancies, interstitials, microcracks, dislocations etc. created in the quenched samples. It seems plausible that changes in the atomic and submicron defects concentration gave rise to changes in the values of the complex permeability components and electrical resistivity. Generally, the connection between such defects and isochronal recovery behaviour of material properties has been indicated in the literature.
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