Preparation and Electromagnetic Wave Absorption Properties of Zeolitic Imidazolate Frameworks Derivatives and Graphene Composites

Author:Liu Zhong Fei

Supervisor:jin shao wei

Database:Doctor

Degree Year:2019

Download:74

Pages:123

Size:10438K

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With the rapid development of science and technology in recent years,the widely applications of electronic equipment in biological systems,communication security,medical care and electronic devices has caused many electromagnetic radiation pollution problems.In order to solve these problems,it is extremely urgent to prepare and study the high-performance electromagnetic wave absorbing(EWA)materials.Zeolitic-imidazole-framework materials(ZIFs)are a class of metal organics with zeolite skeleton structure which are obtained through self-assembling reactions between the metal source(Zn or Co)and organic ligands(imidazole or imidazole derivatives)in a solvent.Metal-organic framework materials(MOFs)exhibit superior performance in electromagnetic absorption due to their high stability,large specific surface area,porous structure and diversified framework.At the same time,the chemical composition and structural adjustability of ZIFs have provide a great probability for design and research of new types of EWA materials.As a kind of efficient conductive material,reduced graphene oxide(RGO)has the characteristics of low density,large surface area and abundant defects.It is also a kind of high-efficiency electromagnetic wave absorbing material.In general,the composite materials based on RGO and magnetic materials can obviously improve the matching degree of dielectric constant and magnetic permeability to achieve in enhancement of electromagnetic wave absorption.In this paper,zeolitic-imidazole-framework materials(ZIFs)and graphene oxide(GO)were used as precursors to prepare composites of zeolitic imidazolate skeleton(ZIFs)material derivatives and reduced graphene oxide(RGO)through self-assembly process and high-temperature pyrolysis.The mechanism analysis,morphological characterization and structural characterization of the composites of ZIFs derivative/RGO materials were carried out by adjusting the different molar ratios of ZIFs and graphene and different annealing temperatures,and then the electromagnetic wave absorption properties of ZIFs derivatives/RGO composites were studied.The specific research content is as follows:(1)The research group used the Hummers method to prepare graphene oxide(GO).The reduced graphene oxide(RGO)obtained by calcination at high temperature was characterized by XRD and SEM,and vector network analyzer(VNA).In order to evaluate the electromagnetic wave absorption performance of the as-prepared samples,the electromagnetic parameters was tested,and the simulated calculation was carried out to obtain reflection loss(RL)value of RGO(-6.5 dB).(2)The graphene oxide(GO)was prepared by Hummers method and the ZIF-8 materials were synthesized through coordination reaction between the 2-methylimidazole solution and Zn(NO3)2·6H2O methanol solution by slowly adding the former into the latter.After that,ZnO@N-C/RGO composite was prepared by using GO and ZIF-8 as precursors through solvothermal method and carbonization process(800℃ calcination).The phase,morphology,structure and electromagnetic wave absorption properties of the ZnO@N-C/RGO composite were characterized and analyzed by XRD,Raman,XPS,SEM,TEM,TG,BET and VNA.The electromagnetic parameters of ZIF-8 derivative ZnO@N-C/RGO composites were regulated and controlled by the molar ratios of GO and ZIF-8,and the electromagnetic wave absorption properties of ZnO@N-C/RGO composites were studied.The maximum reflection loss(RL)of ZnO@N-C/RGO composites can reach up to-37.12 dB,the coating thickness is 3.5 mm,and the effective absorption bandwidth of RL≤-10 dB is 1.92 GHz(from 5.28 to 7.2 GHz).(3)ZIF-67 derivative Co3O4@N-C/RGO composite was prepared by solvothermal method and carbonization process(500℃ calcination)using ZIF-67 and GO as precursors.By controlling the molar ratios of GO and ZIF-67 and analyzing the effect of GO ratio on the electromagnetic wave absorption performance of the composite,the optimal molar ratio of GO and ZIF-67 in the composite materials was finally proposed.For the samples with the best molar ratio of GO,the composition,morphological characterization and structural characterization of the composites formed by calcination at different annealing temperatures(500℃,600℃and 700℃)were studied and investigated,and the electromagnetic wave absorption properties were explored.The results suggest that the variation of calcination temperature for preparing ZIF-67 derivative/RGO composites leads to different electromagnetic wave absorption properties.The composite of 500-3,600-3 and 700-3 samples obtained at the temperatures of 500℃,600℃ and 700℃ are Co304@N-C/RGO,Co3O4/Co@N-C/RGO and Co@N-C/RGO,respectively.The electromagnetic wave absorption property of the ZIF-67 derivative Co3O4/Co@N-C/RGO composite obtained by calcination at 600℃ is the best,of which the maximum reflection loss(RL)value is-43.52 dB and the coating thickness is only 2.0 mm.Remarkably,the effective absorption bandwidth of RL≤-10 dB can reach 4.81 GHz(from 11.08 to 15.89 GHz)for the Co3O4/CO@N-C/RGO composite.(4)By using solvothermal method,a certain ratio of ZIF-67 and ZIF-8 are mixed to prepare Coo.5Zno.5ZIF,which was used as precursor to synthesize the Co0.5Zn0.5ZIF/GO complex.After that,the Co0.5Zn0.5ZIF derivative and graphene composite material were obtained by pyrolysis at high temperature,and the final product was Co/ZnO@N-C/RGO composites.By adjusting the ratio of precursor(Coo.5Zno.5ZIF and GO)and annealing temperature,the electromagnetic wave absorption performance of the composite was explored,and the optimal or most suitable precursor ratio and annealing temperature are determined.Additionally,the phase,morphology and characteristics of the final product are characterized and analyzed.Our studies find that the Coo.5Zno.5ZIF derivative Co/ZnO@N-C/RGO composite obtained by calcination at 600℃ has the excellent electromagnetic wave absorption performance.The reflection loss value(RL)of this sample is as high as-52.2 dB with a thickness of 3.5 mm,and the effective absorption bandwidth of RL≤-10 dB can reach up to 5.6 GHz(from 8.72 to 14.32 GHz).