Synthesis,Characterization And Microwave Absorption Properties Of ZnO Nanomaterials

Along with the increasing use of gigahertz electromagnetic waves in industrial,commercial and military application,electromagnetic interference (EMI)has become aserious problem and a matter of crucial concern recently.However,new ways andchances to solve this problem are provided by the fast development of nanoscience andnanotechnology.The nanostructured materials have attracted great interests asmicrowave radiation absorbing and shielding materials in the high-frequency range dueto their many unique chemical and physical properties.Recently,some researches havebeen focused on ZnO,which can be suitable for efficient excitonic emission,shortwavelength optoelectronic applications,and can be used as gas sensors,transducers,piezoelectric nanogenerators etc due to its unique chemical and physical properties.Considering that lightweight and semi-conductive properties of the ZnO nanomaterialsand large-scale synthesis of the ZnO nanomaterials can be easily realized,they may alsohave potential applications in electromagnetic interference shielding materials.Againstthe background of the preparation and application of nanomaterials and with theemphasis of synthesizing ZnO multifunctional nanomaterials,this paper reports on thelatest progress in research of preparing ZnO nanomaterials with different morphologisand their Microwave absorption properties.The main contents,results and originalities ofthis researh are as follows:1.ZnO nanocombs and nanorods with different morphologies have been successfullysynthesized through a simple metal vapor deposition route at 600-750℃using purezinc powder or zinc and graphite powders as source materials.The structures andmorphologies of the products were characterized in detail by using X-ray diffraction,scanning electron microscopy,transmission electron microscopy and laser Ramanspectrometer.The morphologies of the products can be easily controlled by tuningthe following four factors:reaction temperature;the distance between the source andthe substrates;the kind of substrates;the kind of precursors.Possible growth mechanisms for the formation of ZnO nanostructures with different morphologiesare discussed.Photoluminescence (PL)studies show that there are sharp UV andbroad defect-related green emissions for all products.Relative intensity of the UV todefect-related green emissions decreases from ZnO nanorods to nanocombs.2.Intriguing ZnO dendritic nanostructures have been synthesized by a two-stepchemical vapor deposition process.Regular nanorods grow uniformly to thepresynthesized ZnO nanowires on Silicon substrate,the secondary nanorods aresingle-crystal hexagonal ZnO and each nanorod grows along [0001] direction.Therelationship between the secondary-grown nanorods and the primary ZnO nanowireis not epitaxial due to the high temperature increasing rate during the rapid grownprocess.The size and morphology of branches can be controlled by adjusting thetemperature and duration of growth.Room-temperature photoluminescence (PL)properties of the ZnO dendritic nanostructures have been investigated in detail.3.Microwave absorption properties of these ZnO one-dimensional nanorods,two-dimensional nanocombs,and three-dimensional nanotrees are investigated.Theyall have certain degree of microwave absorption properties,and two-dimensionalnanocombs,three-dimensional nanotrees both have strong absorption of microwave.The value of minimum reflection loss is-12 dB at 11 GHz for the ZnO nanocombscomposite (with 50 vol %)with a thickness of 2.5 mm.And the value of minimumreflection loss for the composites with 60 vol % ZnO nanotrees is-58 dB at 4.2 GHzwith a thickness of 4.0 mm.Such strong absorption is attributed to the uniqueisotropic antenna morphology of the ZnO nanotrees in the composite.