Study On Preparation And Microwave Absorption Performance Of Double Helical C/Co@HCNT Based On ZIF-67

The rise and rapid progress of the internet of things,wireless electronic devices are flourishing and are widely used in military,communications,medical treatment,smart home and other fields.Considering the wide application of electronic equipment,the electromagnetic wave pollution has become a problem that can not be ignored.Therefore,the development and research of microwave absorbing materials with excellent performance are of great significance of human health and the stable operation of high-precision electronic equipment.Metal-organic framework is a kind of organic-inorganic hybrid material.If the inorganic part is magnetic metal ions,then the pyrolytic derivatives become a kind of carbon-based composites with both dielectric loss and magnetic loss,which provides a new research strategy for the synthesis of lightweight and high performance microwave absorbers.Appropriate materials are the basis of obtaining microwave absorbers with excellent performance,and optimized material structure is a way to further to improve impedance matching and enhance microwave absorption.Most of the existing MOF-derived microwave absorbing materials are obtained by direct pyrolysis of MOF particles,but few attention were paid to form helical structure based on the self-assembly and pyrolysis of MOF.The helical conductive structure endows the material with the cross-coupling of electric field and magnetic field in the electromagnetic field,which produces cross-polarization based on physical induction and enhances the attenuation characteristics of electromagnetic waves.Helical carbon nanotubes（HCNT）are ideal templates for synthetic helical nanocomposites because of the characteristics of light weight and stability.In this paper,using HCNT as template,double helical nanocomposites with hierarchical structure was prepared by controlling the size matching and pyrolysis process between Co-MOF（ZIF-67）and HCNT.In addition,the composite of materials to achieve the purpose of complementary characteristics of materials,in order to achieve the improvement of microwave absorption properties.The main contents and results are as follows:（1）Study on effects of precursor concentration and pyrolysis temperature on the composition and structure of C/Co@HCNT.Benefitting from precursor concentration regulates,composites（ZIF-67@HCNT）with the best size matching degree were prepared by optimizing the size of ZIF-67.Combined with a controllable high-temperature pyrolysis process,helical carbon layer based on ZIF-67 pyrolysis process and HCNT form the double helical nanocomposite material（C/Co@HCNT）with a hierarchical structure.The carbon layer exhibits two characteristics:one is that the carbon layer is mainly deposited on the edge area of the HCNT;the other is that the carbon layer has a multi-level structure,which is represented by catalytically grown CNTs and common graphitized carbon.The helical morphology of HCNT,the size of ZIF-67 and the pyrolysis temperature are essential for the construction of double helical nanocomposites with multi-level structure.The results proved that:ZIF-67 prepared at a concentration of 30 m M Co²⁺has the best size matching with HCNT,which is a good concentration for realizing ZIF-67 helical assembly;on the basis of size matching,the pyrolysis temperature of 700℃is the best Optimum temperature for double helical carbon layer with hierarchical structure.（2）Study on effects of composition and double helical structure of C/Co@HCNT on microwave absorption properties.The main conclusions are:The difference in precursor concentration and pyrolysis temperature leads to the difference in the microwave absorption performance of samples.the double helical composites C/Co@HCNT-3-700 obtained by pyrolysis at 700℃at a concentration of 30 m M Co²⁺exhibited the best microwave absorption performance.The minimum reflection loss at5.68 GHz is-56.8 d B with sample thickness of 3.6 mm.When the sample thickness is adjusted to 1.6 mm,the maximum effective absorption bandwidth was 4.8 GHz.Combining the components,structure and microwave absorption performance,the microwave absorption mechanism was analyzed,it was found that in addition to the conduction loss,interface polarization,defect polarization and magnetic loss,the improvement of the absorption performance is also included in the double cross polarization caused by the double helix structure.