Studies On Preparation And Microwaveabsorption Properties Of Rare Earth Ionssubstitued Strontium Ferrites Their Composite

The electromagnetic absorption material has become a hot research topic and development focus as a consequence of the booming research and widely application of GHz electromagnetic waves in modern society. As a solution of electromagnetic pollution and some special usage in military and aerospace field, electromagnetic absorption material is required to have strong absorption, wide bandwidth and proper mechanical properties. The hexagonal strontium ferrite is a good candidate material due to its high magnetic anisotropy, high saturation magnetization and excellent chemical stability. However, disadvantages like relatively weaker absorption performance and narrower frequency bandwidth constrains its application. The purpose of this work is to fabricate a Sr ferrite material with better absorption performance and wider bandwidth, as well as studying the absorption mechanism and material preparation process.Firstly, absorption performance was remarkably improved by introducing rare-earth ion substitution into Sr ferrite. La/Ce/Nd ion substituted strontium ferrites were prepared by sol-gel technique and self-propagating combustion method. XRD analysis shows that the maximum substitute amount of La/Ce/Nd ion is 0.15, 0.1 and 0 under the process condition of 1000℃, 1.5h. Ce can't enter the lattice due to its stable +4 valence. The substitute amount of rare-earth ions depends on their valence and ion radius. Compare to pure strontium ferrite, the lattice dimension and particle size of the ion substituted ferrite are smaller, that can remarkably improve the electromagnetic absorption performance by increasing the coercivity without causing any significant deterioration in specific saturation magnetization. Especially for La-substituted ferrite, when the substitute amount is 0.15, compared with pure strontium ferrite, the matching thickness can be reduced from 2.6mm to 2.3mm, the minimum reflection loss decreases from -24dB to -37.7dB. and 10db bandwidth can be expanded from 2.7GHz (6.4-9.1GHz) to 4GHz (6-10GHz). The M?ssbauer spectroscopy showed that when the La3+ substituted the Sr2+ in the ferrite, there will be some Fe3+ turns into Fe2+ for the valence balance at 4 f1 and 2a site. That means although its Sr2+ is substituted by the rare-earth ions, but the hyperfine field electron cloud density and electron orbit of the Fe ions are also influenced. The reason why the ion substituted strontium ferrite gets better absorption performance is: smaller lattice dimension brought bigger boundary loss, increasing of coercivity enlarged the hysteretic attenation and lattice distortion induced by ion substitution increased the dielectric loss.Secondly, to achieve a further improvement of absorption bandwidth, composite powder of strontium and zinc ferrite was prepared using coating and one-step method respectively. Experimental result showed that 15% zinc ferrite can improve the absorption performance but too much zinc ferrite (over 30%) will do the other way because zinc ferrite is not an originally electromagnetic absorption material. it can reach -35dB reflecting loss and over 4GHz 10dB bandwidth at 2.3mm matching thickness. Because Sr ferrite material is very liable to congregate as cluster, it's hard to achieve a complete coating on the un-uniform particles even with grinding, ultrasonic dispersing and mechanical stirring. That compromised the final performance of coating prepared ferrites. Different with pure Sr ferrite and ion-substituted ferrite, the composite powder has two reflecting loss peak. As a result of the interaction of two kinds of ferrites, the location of the peak at around 15GHz is independent with the sample thickness.Finally, based on the research above, to prepare the La substituted ferrite and zinc ferrite composite powder using one-step method. Using this composite powder as absorption material and epoxy-resin as matrix, composite sample can be prepared with different powder vs. matrix ratio. Test result showed that when powder/matrix ratio at 1.25, good absorption performance can be obtained with 2.2mm matching thickness, -41dB minimum reflection loss and 5.7GHz 10dB bandwidth. For practical application, this composite material's absorption performance is much better than pure Sr ferrite even at very low thickness (<2mm), well meet the requirements of real application: strong absorption, wide bandwidth, being thinner and lighter. Results showed that this composite possess both of good absorption ability and wide absorption bandwidth. M?ssbauer spectroscopy analysis showed that the composite process of the two kind of ferrite can make positive zinc ferrite turns into mixed spinal ferrite type, and when La ion exists, Zn ion can enter the Sr ferrite lattice and substitute the Fe ion at 4 f1 site. That process can enhance the specific saturation magnetization and improve the electromagnetic absorption performance.