Preparation And Properties Of Cellulose-derived Carbon-Based Absorbing Materials

With the deteriorating electromagnetic environment and the continuous development of various electronic equipment,the development trend of a new generation of electromagnetic absorbing materials is summarized as "thin,light,wide and strong".Biomass,as a real green resource,has received wide attention due to its various unique natural microstructure and chemical components.However,large amounts of biomass has not yet been fully exploited and utilized.Biomass residues can be used as precursors for the preparation of light and efficient carbon-derived microwave absorbing materials which have considerable development prospects.Among them,cellulose,as the most abundant natural polymer material,widely exists in most plants and can be used as precursor for carbon nanofibers.Cellulose-derived carbon nanofibers have great development and application in the field of electromagnetic absorption.However,due to t the existence of a lot of polar groups on the surface of cellulose molecules,the dispersibility of cellulose nanofibers needs to be further solved.Hence,the oilin-water(Oil/water)emulsion of cellulose nanofibers was prepared,the problem of agglomeration between cellulose nanofibers can be solved by the emulsification of organic phase,and the porous structure was constructed in cellulose aerogels by freeze-drying.Subsequently,cellulose-derived carbon nanofibers(CNFs)were prepared by hightemperature carbonization using cellulose aerogels as precursors.Phosphorus(P)and nitrogen(N)atoms were selected as heteroatoms for doping to prepare a series of CNFs with threedimensional porous network structures.The microscopic morphology and chemical composition of CNFs were studied by field emission scanning electron microscope,Raman spectrometer,X-ray photoelectron spectroscopy and other material characterization methods.And the electromagnetic parameters of the material were measured by a vector network analyzer using the coaxial transmission line method.The main research contents of this paper are as follows:(1)To solve the dispersibility problem of cellulose nanofibers,a uniform oil-in-water(O/W)emulsion was prepared by adding a certain volume ratio of cyclohexane(CHE)to cellulose dispersion and ultized the ultrasonic emulsification of CHE.The method can effectively improve the dispersibility of cellulose nanofibers and combine with freeze-drying to constructe a porous network microstructure.Cellulose-derived carbon nanofibers(CNFs)were then prepared by high-temperature carbonization using cellulose aerogel as precursor.The microstructures of cellulose aerogels and CNFs were optimized by adjusting the concentration of cellulose dispersion and the volume ratio of CHE,and CNFs with fibrous cross-linked network and graphite-like platelets were successfully prepared.The abundant carbon lattic defects and few oxygen-containing groups promoted the polarization loss,the conductive network promoted the conductance loss,and the three-dimensional porous network structure facilitated impedance matching and multiple reflections of electromagnetic waves.Therefore,CNFs have excellent impedance matching characteristics and good electromagnetic attenuation ability.Among,CNF1-25 reachs a minimum reflection oss of-44.95 dB at 16 GHz when the thickness is 2 mm,and the effective absorption width is 5 GHz under a low fill load(5 wt%).GHz(13.8-18 GHz).(2)On the basis of preparing CNFs,using sodium hypophosphite(Na2H2PO2)as phosphorus source,phosphorus-doped cellulose-derived carbon nanofibers(CNF-P)were prepared by dry treatment method.There investigated the effects of P atom doping on the microstructure,chemical compositions and electromagnetic absorption properties of CNFs.Due to the large atomic radius of P atoms,the abundant oxygen-containing groups and carbon lattice defects of CNFs facilitate the doping of P atoms into carbon lattices.The impurity groups introduced by P atom lead to the large deformation of the carbon lattice.CPO3,C2PO2 and C3PO inhibited the transmission of electrons and contribute to optimize the impedance matching.The high electron cloud density of C-OPO3 was beneficial to promote electron migration and increase polarization loss.Among,under low fill loading of 4wt%,CNF-P30 reachs a minimum reflection loss of-45.59 dB when the thickness is 2.5 mm,and the effective absorption width is 5.76 GHz which from 10.64 GHz to 16.4 GHz).(3)On the basis of previous experiments,nitrogen-doped cellulose-derived carbon nanofibers(CNF-N)and nitrogen-phosphorus co-doped cellulose-derived carbon nanofibers(CNF-PN).The polarity of N atom is opposite to P atom,and the C-N bond actually acts as an electron donor,which is beneficial to improve the conductivity of CNFs and promote conduction loss and polarization loss.Among,CNF-N2 exhibited the minimum reflection loss of-51.94 dB when the thickness is 2.3 mm under low loading of 4 wt%,and the effective absorption width reached 4.8GHz.The cellulose-derived carbon nanofibers co-doped with nitrogen and phosphorus diatoms were mainly affected by the first-step doping of atoms.When P atoms were doped at first,a large number of defects were introduced and the carbon lattice was large deformed.When N atoms were first doped,it would form a stable structure by replacing C atoms which increases the difficulty of doping P atoms.Combined with various loss mechanisms,CNF-PN exhibited excellent electromagnetic absorption performance.At a low loading of 4 wt%,the minimum reflection loss at a thickness of 2.3 mm reached-55.02 dB,and the effectively absorbing width reached 6.4GHz which covered all Ku-band.Compared with single-atom doping,phosphorus-nitrogen diatomic co-doping enabled the samples to have richer attenuation loss mechanisms and better impedance matching properties.