Preparation And Microwave Absorption Properties Of Hierarchical MOFs Derived Composites

With the advent of the 5G era,the rapid development of communication technology and the gradual popularization of small electronic devices have gradually aggravated the problem of electromagnetic pollution.The electromagnetic radiation will affect the operation of precision electronic devices and cause harm to human health.Electromagnetic microwave absorbing materials can solve a series of electromagnetic pollution problems.New microwave absorbing materials with light weight,thin thickness,large bandwidth and brilliant absorbing performance have become a current research hotspot.Metal organic frameworks(MOFs)exhibit the advantages of porous,light weight,and adjustable morphology and structure.The metal or metal oxide and carbon composites derived from MOFs have good microwave absorption performance.However,the binding ability between MOFs particles is poor,and it is difficult to connect and form a complex electron transmission path,which can effectively dissipate electromagnetic waves.In this paper,MOFs and various carbon materials are combined to prepare composites,and special hierarchical structure is constructed while connecting MOFs to achieve limited control of the material's absorbing performance.We explore the influence of composition and structure.The specific contents are as follows:(1)The sheet-shaped ZIF-67 is prepared by ultrasonic at room temperature,then it is mixed with graphene oxide solutions of different concentrations.The mixture is freezing-dried and sintered at high temperature to prepare RGO/Co/C composites.The effects of the concentration of graphene oxide and sintering temperature on the microwave absorbing properties are investigated.The reduced graphene oxide is wrapped on the surface of MOF derived carbon nanosheets to produce rich interfaces and promote interfacial polarization.The residual oxygen-containing functional groups and the defects in the composite can promote dipole polarization.The reduced graphene oxide and MOF derived carbon nanosheets are connected to form a channel,which promoted the conduction loss.When the concentration of graphene oxide is 6mg/ml and the sintering temperature is 600?,the reflection loss reaches the minimum value of-47.1 d B at 15.28 GHz,and the effective bandwidth is 5.92 GHz.(2)Using flake-shaped ZIF-67 as the precursor material,mixing it with melamine of different quality,drying and sintering at high temperature under N₂ atmosphere to prepare NCNT/Co/C composite and the influence of melamine content on microwave absorbing properties is investigated.At high temperature,the Co²⁺in the MOF is converted into cobalt nanoparticles,melamine is pyrolyzed to serve as carbon source and nitrogen source and nitrogen doped multiwalled carbon nanotubes are derived under the catalysis of cobalt..By adjusting the amount of melamine,the content of carbon nanotubes can be adjusted,thereby the dielectric properties of the material can be effective adjusted.The carbon nanotubes on different MOF-derived carbon nanosheets are entangled with each other and connected to form a pathway,which is conducive to the transfer of electrons between adjacent carbon nanosheets.The metal nanoparticles are wrapped in carbon nanotubes which effectively promote polarization loss.When the content of melamine is 50 wt%,the sintered product has the best absorbing performance,the minimum reflection loss is-46.6 d B and the effective bandwidth is2.4 GHz.When the thickness is 2 mm,the effective bandwidth is 5.2 GHz.(3)The flower-like Ni-BDC,Ni Co-BDC and Co-BDC are prepared by hydrothermal method.They are sintered in tubular furnace after drying and the corresponding NCNT/Ni/C,NCNT/Ni Co/C and NCNT/Co/C composites are synthesized.The effect of the ratio of Ni²⁺and Co²⁺on microwave absorption properties is investigated.Ni-BDC has compact structure and low porosity.The introduction of Co makes MOF more loose and the distance between adjacent MOFs increases.After sintering,carbon nanotubes grow and connect with adjacent nanosheets to construct a special three-dimensional porous network structure,which improved the conductive loss performance of the materials.When Ni²⁺:Co²⁺=1:1,the minimum reflection loss of NCNT/Ni₁Co₁/C composite is-45.9 d B,the effective bandwidth is 1.44 GHz.Co-BDC is also sintered and the NCNT/Co/C composite is prepared.The minimum reflection loss reaches-51.2 d B at 5.36 GHz and the effective bandwidth is 1.84 GHz.When the thickness is2 mm,the effective bandwidth is 3.92 GHz.