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Study on Preparation and Microwave Absorption Properties of Low Density Hollow Microsphere/Barium Ferrite Composite Powders

Author: MouGuoHong
Tutor: GuMingYuan
School: Shanghai Jiaotong University
Course: Materials Science
Keywords: Hollow ceramic microsphere Barium ferrite Titania Coating Composite powders with low density
CLC: TB383.1
Type: PhD thesis
Year: 2008
Downloads: 649
Quote: 5
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Abstract

Due to the increasing use of GHz electromagnetic waves in military, industrial and commercial applications, the electromagnetic wave interference (EMI) has become a new pollution. The electromagnetic radiation disturbes other electronic facilities, and also deteriorates the health of human being. So, the electromagnetic wave absorption materials have been widely studied and applied. An ideal electromagnetic wave absorption material should have the advantages of strong absorption, broad absorption frequency, small thickness and light mass. Barium ferrite has been widely used as a microwave absorber due to its high magnetic anisotropy, high saturation magnetization and excellent chemical stability. However, it is quite heavy. Hollow ceramic microsphere can be used as a carrier of microwave absorption materials because it has low density and special hollow structure, etc. In this paper, a novel concept to prepare a low density microwave absorption material is proposed through combining the advantages of barium ferrite and hollow microsphere together. Then the titania doping was used to improve the absorption properties, whose effect has been investigated. Based on the above results, low density hollow microsphere/titania/barium ferrite composite powders with mulit-layer structure have been designed and prepared. The mechanism of titania intermediate layer and hollow structure on the enhanced absorption properties of composite is also discussed.A light magnetic composite powder including barium ferrite and hollow ceramic microsphere was prepared through combining sol-gel technique and self-propagating combustion method. The influence of hollow microsphere on the phase formation, microstructure and magnetic properties of composite powders had been studied. By using DSC/TG analysis, the phase transition process was also revealed. The XRD results show that the composites are composed of barium ferrite, hematite and minor mullite. The fraction of barium ferrite, the specific saturation magnetization and coercivity of composite powders decrease with increasing amount of hollow microsphere. It is indicated that some of maghemite resulted from the combusted powders react with barium carbonate forming into barium ferrite, while the others convert into hematite. The reason is that due both hematite and corundum in the mullite belong to corundum-type structure with small misfit to form semi-coherent interface easily. So hematite is prone to be nucleated on the surface of microsphere.For uncoated hollow microsphere, the surface is quite smooth. Coating made it be covered with a layer of barium ferrite and hematite, whose thickness is about 170nm. The granular size of most ferrite is below 80nm and belongs to single domain. In a frequency range of 2-18GHz, the minimum reflection loss (R.L.) and bandwidth below -10dB of composite powders decrease with increasing amount of hollow microsphere. For the addition of 30wt% hollow microsphere, its minimum R.L. is -19.5dB, the corresponding matching thickness and frequency are 2.8mm and 8.16GHz, respectively. And the absorption bandwidth for R.L. < -10dB is 3.44GHz.The low density composite powders doped with rutile and anatase titania respectively were prepared by wet chemical method. When doped with rutile titania, the powders include titania, hexagonal barium ferrite, hematite and a little mullite. But for the samples with anatase titania doping, XRD and magnetic property measurement indicated that anatase titania doping is not benefical to the barium ferrite formation. The reason is that there exists the corundum phase in the precursor and the ratio of Fe/Ti is too high during the phase transition process. The specimens doped with rutile titania have an enhanced microwave absorption property. For the specimen with the same thickness of 2mm and addition of 50wt% hollow microsphere, its minimum reflection loss decreases to -13.9dB in comparison with -11.6dB without titania. Its bandwidth increases from 2.48GHz to 4.24GHz.In order to improve the microwave absorption property of composites further, the low density multilayer magnetic powders with hollow microsphere (core)/rutiel titania (intermediate layer)/barium ferrite (magnetic shell) (M/T/B) were prepared using two-step sol-gel technique. They are composed of titania, barium ferrite, hematite and mullite. The SEM, TEM and EDS analyses indicated that rutile titania and barium ferrite have been directly coated on hollow microsphere and the rutile titania-coated hollow microsphere, respectively. The formed ferrite particles mostly belong to single domain, too.The results show that the introduction of rutile titania as an intermediate layer has inhibited the maghemite to hematite phase transition, and then promoted the formation of barium ferrite. Therefore, the specific saturation magnetization and the microwave absorption property of composite powders are enhanced, and the matching thickness is decreased. For the samples with 40wt% barium ferrite, its specific saturation magnetization and coercivity with titania as the intermediate layer are increased to 17.88emu/g and 4850Oe in comparison with 9.60emu/g and 4550Oe without titania, respectively. For the composite powders with 50wt% hollow microsphere, the minimum R.L. with 25wt% titania addition decreases to -30.1dB, compared with -14.6dB without the addition of titania. The matching thickness decreases from 2.9mm to 2.4mm. With increasing addition of titania, the minimum R. L. decreases first and then increases. In contrary, its bandwidth below -20dB increases first and then decreases. For composite powders containing 25wt% titania over an absorber thickness of 2.0-3.0mm, reflection loss less than -20dB is obtained between 7.5-9.4GHz. Its density decreases from 5.29g/cm3 of barium ferrite to 2.90g/cm3 of composite powder. The enhancement microwave absorption property is related to the increased magnetic and dielectric loss, having better impedance match among different layers due to the introduction of titania as the intermediate layer, the special hollow structure, and so forth.

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