Study On Lithium Zinc Ferrite Low-Frequency Microwave Absorbers And Its Dispersing Technology In Polymer

In recent years,electromagnetic radiation has become a specific type of environmental pollution,due to the rapid growth in utilization of electrical and electronic devices in industrial,commercial and military applications.And electromagnetic radiation in frequency range of 0.5-3 GHz has been proved to be harmful to human tissues and organs severely.Radiation absorption is quite practical and effective among the present measures of radiation protection. Therefore,it would have great significance to study and prepare some effective low-frequency microwave absorbing materials in civil utilization.As far as thickness and working frequency bandwidth are concerned,spinel ferrites and hexaferrites have obvious advantages.However,nano-size ferrites with high quality,which are prepared by sol-gel method,usually have microwave absorption in high frequency region.Some researches have indicated that micron-size pure lithium zinc ferrite(LiZn ferrite) prepared by solid-state reaction method have good low-frequency microwave absorption.But further application of this material is restricted by some inherent disadvantages.So in present work, sol-gel process was combined with subsequent calcination in order to prepare micron-size LiZn ferrite,which could exhibit good microwave absorbing properties in the frequency range of 0.5-3 GHz.Firstly,micron-size pure LiZn ferrites were successfully prepared by a combination of sol-gel process with subsequent calcination,and the microstructure and microwave absorbing properties of them were characterized by X-ray diffraction(XRD),scanning electron microscopy(SEM) and network vector analyzer.The effects of calcination processing parameters on low-frequency microwave absorption were also studied according to kinetics of reaction and ferromagnetic resonance theory.The particle diameter is determined by final calcination temperature,and the heat-treatment speed rate has effect on vaporization loss of lithium and zinc during the calcination.The differences of actual particle size and phase composition of pure LiZn ferrites would have a further influence on their low-frequency microwave absorption generated from ferromagnetic resonance.Pure LiZn ferrite calcined at 500℃with a heating rate of 200℃/h and the one calcined at 1200℃with a heating rate of 240℃/h both exhibite microwave absorption in the frequency range of 0.5-3 GHz,and the latter has better wave absorbing properties.A peak value of reflection loss of-25.89 dB can be obtained for absorber thickness of 10 mm at 2.8 GHz,and this preparation method for LiZn ferrite microwave absorbers has not been reported at present. However,this pure LiZn ferrite mainly has microwave absorption in the frequency range of 2.5-3 GHz,and this working frequency band should be shifted towards lower frequency region.In order to improve microwave absorption and widen working frequency bandwidth in the frequency range of 0.5-3 GHz,LiZn ferrites doped with different amount of magnesium,copper,lanthanum and cerium were prepared by a combination of sol-gel process with subsequent calcination,respectively.The microstructure and low-frequency microwave absorption of these as-doped LiZn ferrites were characterized and compared with pure LiZn ferrite prepared by the same process.The results indicate that doping with magnesium,copper,lanthanum and cerium can modify the distribution of metallic irons within the crystal lattice of LiZn ferrite.And magnetocrystalline anisotropy of LiZn ferrite can be modified after being doped with lanthanum and cerium.All these modifications lead to changes of corresponding equivalent magnetic field,and then have a further effect on low-frequency microwave absorption generated from ferromagnetic resonance. In addition,there is a maximum doping amount for magnesium-doped, copper-doped,lanthanum-doped and cerium-doped LiZn ferrites.If the actual doping amount exceeds the limited amount,MgFe₂O₄,CuFe₂O₄,LaFeO₃ and CeO₂ will be formed in as-prepared ferrites.On one hand,the formation of MgFe₂O₄, CuFe₂O₄ and LaFeO₃ decrease the amount of as-doped LiZn ferrite.On the other hand,MgFe₂O₄,CuFe₂O₄ and LaFeO₃ have relatively poor microwave absorption in low frequency region.As a result,the low-frequency microwave absorption of actual ferrite becomes much worse.However,the formation of CeO₂ has no effect on the amount of LiZn ferrite in actual ferrite when the actual doping amount of cerium exceeds the limited value. It needs to be mentioned that when the doping amount of cerium reaches a specific value,the enrichment of CeO₂ and suitable component ratio between CeO₂ and LiZn ferrite can strengthen the dielectric attenuation for microwave,and improve the low-frequency microwave absorption of actual ferrite material greatly.A peak value of reflection loss of-28.06 dB can be obtained for absorber thickness of 10 mm at 1.68 GHz,and the distribution of working frequency band is in the frequency range of 1.25-2.25 GHz.Therefore,this as-prepared ferrite composite material is a practical and promising microwave absorber in the frequency range of 0.5-3 GHz.According to panorama analysis method and the formula of effective electromagnetic parameters of composite materials,MATLAB was used to compile a model for characterizing the correlations between the reflection loss, absorber thickness and component ratio of composite microwave absorbers at different frequency points.By utilizing this model,a plenty of data about predictive LiZn ferrite/ acetylene carbon black(ACB) composite microwave absorbers,such as matching conditions and component ratios,had been achieved. According to these data,three LiZn ferrite/ACB composite materials with different component ratios were then prepared by ball milling method,and their microwave absorbing properties were measured.The results indicate that these as-prepared composite materials,which are designed by this predictive analysis model mentioned above,do have better low-frequency microwave absorption than pure LiZn ferrite.And this indicates that the predictive analysis model and pre-design method can be useful for the preparation of low-frequency microwave absorbers.In microwave absorbing coating,rapid and uniform dispersion of fine microwave absorbers in polymer has a significant influence on its actual mechanical properties,microwave absorption and practical applications.In this paper,reciprocating fluid injection process(RFIP) and the dispersing device for RFIP were developed in order to promote further practical applications for microwave absorbing coating.And they were used in dispersion experiment for micro-size LiZn ferrite and carbon nanotubes(CNTs) in unsaturated polyester resin(UPR).Viscosity of dispersed precursor mixture and the fracture surface of final composite cured from precursor mixture were characterized.And it can be discovered from the results that in polymer matrix,LiZn ferrite and CNTs dispersed by RFIP could both exhibit more homogenous distribution within less processing time than the ones dispersed by ultrasonication.This technology is not only applicable for rapid and homogeneous dispersion of micro-size microwave absorbers,but also has an excellent dispersive action on nano-size microwave absorbing agents.Therefore,RFIP will be a novel and effective process for dispersing fine microwave absorbing agents in polymer.