| At present,the developments of information transmission,industrial equipment and military equipment are towards intellectualization.The 5G network under construction will develop information intellectrialization deeply and bring tremendous changes to production and human life.The innovation of military equipment and the modem war against the background of electromagnetic warfare in complex environment need not only the equipment with stealth function,but also the cover of the electromagnetic interference domain in the air.Generally,the smoke containing micro/nanoparticles,generated by explosion or combustion of smoke bombs,could affect the detection of radar,infrared and other equipment in the air electromagnetic interference domain.Detonation method,a dynamic method for preparing nanomaterials,can fabricate nanomaterials in a rapid short time and form a dynamic smoke screen consisted of nanoparticles in the air.The smoke screen could generate instantaneous electromagnetic interference/shielding area and obtain valuable time for transfer.As part of the study of spatial dynamic absorption,the purpose of this paper is to prepare electromagnetic(EM)wave absorbing materials by detonation method,and study the EM wave absorption properties of the materials.On this basis,the target microwave absorbing materials is selected to provide the selection scheme of absorbing materials for the study of spatial dynamic wave absorption.Fe@C nanoparticels,Co@C nanoparticles and CNTs have excellent dielectric and magnetic properties,as well as good EM wave absorbing ability,and they are always used in the design of wave absorbers.In the present paper,three kinds of carbon-based magnetic composites,Fe@C nanoparticels,Co@C nanoparticles and CNTs,were prepared by the gaseous detonation method.The influence of various types of explosion sources and the molar ratio of flammable gas in explosion source gas on the phases,morphologies and magnetic properties of carbon-based magnetic composites were studied.The electromagnetic properties and wave absorbing ability were researched with the frequency ranging from 1 to 18 GHz.The conclusions are as follows:The effects of the type of explosion source and the molar ratio of flammable gas in the explosion source on the morphology and phases of Fe@C nanoparticles were studied,and the EM wave absorbing properties of Fe@C nanoparticles in the frequency range of 1～18 GHz were discussed.The type of the explosion source had a significant effect on the morphology of Fe@C nanoparticles;the molar ratio of explosion source gas had relatively obvious influence on the morphology and phase of Fe@C nanoparticles.On the condition of zero oxygen balance,the products were Fe@C nanoparticles,with iron/iron carbide core and partially graphitized amorphous carbon shell.The size of Fe@C particles prepared by high combustion heat explosion source was larger,and the degree of crystallinity of metal core and carbon layer was relatively higher.Some iron would be oxidized to ferrous oxide or magnetite when the proportion of hydrogen was reduced under the condition of positive hydrogen-oxygen balance.Meanwhile,the particle size increased and the dispersibility increased slightly.When the methane-oxygen reaction was negative oxygen balance,with the increase of the molar ratio of methane,the morphology of detonation products would change from core-shell nanoparticles to core-shell-carbon nanotubes,and the proportion of iron carbide phase increased gradually.However,on the benzene-oxygen negative oxygen balance condition,the ’carbon capsule’appeared in the product and the carbon shell thickness increased gradually with the increase of benzene proportion.However,the type and the molar ratio of explosion source gas had little influence on the degree of graphitization.The EM wave absorbing property of initial Fe@C nanoparitcles was low,while after 700 ℃ heat-treatment with the protection of Ar could significantly improve the EM wave absorbing property of Fe@C nanoparticles.The RL of initial Fe@C nanoparticles was larger than-2 dB,and the EM wave absorbing property was obviously enhanced after heat-treatment,the minimum RL was-17.1 dB with the frequency at 5.4 GHz and thickness of 3 mm.The effective absorption frequency ranged from 2.7 GHz to 9.8 GHz with the thickness in range of 2～5 mm.Co@C nanoparticels were prepared by the gaseous detonation method for the first time,and its EM wave absorption properties were discussed.The effects of explosion source on the morphology,phase and degree of carbon graphitization of the products were studied with cobalt(III)acetylacetonate as precursor and hydrogen-oxygen,methane-oxygen and benzene-oxygen as explosion source.The results show that the morphology of cobalt-carbon materials was greatly affected by explosion source.At zero-oxygen balance,the products of hydrogen-oxygen explosion source(low combustion heat)were consisted of Co@C-CNTs composite materials,while those of benzene-oxygen explosion source(high combustion heat)were Co@C particles,and the high combustion heat resulted in larger Co@C particle.When the explosion source of methane-oxygen changed from zero-oxygen balance to negative-oxygen balance,the morphology of products gradually transformed from Co@C nanoparticles to Co@C-CNTs co-existing composite materials.The minimum reflection loss of initial Co@C nanoparticles was-2.8 dB at 11.88 GHz with the thickness of 1 mm.After 700 ℃heat-treatment with Ar protection,the EM wave absorbing property of Co@C nanoparticles improved to-17.5 dB,and the corresponding frequency was 16.98 GHz with sample thickness of 5 mm.The influence factors of gaseous detonation preparing CNTs were studied using hydrogen-oxygen,methane-oxygen as explosion source and frozen as a precursor,and the EM wave absorbing properties of CNTs were analyzed.Under the negative-oxygen balance of hydrogen-oxygen explosion source,the molar ratio of hydrogen to oxygen had a significant effect on the morphology of the product.With the increase of hydrogen proportion,the morphology was changed from core-shell structure(Fe@C)to core-shell-CNTs(Fe@C-CNTs),and the proportion of CNTs was increased gradually,and the size of Fe@C particles gradually became uniform.When the molar ratio of methane to oxygen was 1:1 and the mass of ferrocene was 2 g,it can be fabricated multi-walled CNTs with a length of several micrometers,an outer diameter of 30 nm and an inner diameter of 15 nm.The degree of graphitization of CNTs was low,and the saturation magnetization was 77.4 emu/g.The growth mechanism of CNTs prepared by gaseous detonation was proposed through the V-L-S growth model and the ZND model of detonation process.The EM wave absorbing properties of CNTs were poor,and the minimum RL was-6.1 dB with frequency at 11.2 GHz and thickness of 2 mm.With the increase of sample thickness,the minimum reflection loss shifted to low frequency.The reasons of the low EM wave absorbing properties of initial carbon-based magnetic composites were explored,and it was pointed out that the core-shell structure of carbon-based magnetic composites was more suitable for the study of air dynamic EM wave absorbing.Impedance mismatch was the main reason for the low EM wave absorbing property of the initial samples.A large amount of amorphous carbon and low crystallinity of metal were in the initial carbon-based magnetic composites,which resulted in the high value of complex permittivity,and then the maximum impedance matching was less than 0.4.As a result,EM waves were reflected at the absorber-air interface and cannot enter the absorber.After 700℃heat treatment,the phase and microstructure of Fe@C and Co@C changed in varying degree,and both the complex permittivity and complex permeability also changed in different degree,then the impedance matching had been greatly improved.The EM wave could enter the sample and the electromagnetic energy was effectively absorbed by the cooperation of dielectric loss and magnetic loss.Because the wall of CNTs is easy to break in detonation reaction,which is disadvantageous to EM wave absorbing performance.While,the core-shell structure of carbon-encapsulated metal nanoparticles are more stable,and the EM wave absorbing ability of Fe@C and Co@C nanoparticles has been significantly improved after heat treatment with the protection of inert gas.Therefore,considering the morphology,structure,stability of physical and chemical characteristics and electromagnetic wave absorption properties of detonation products,nano-carbon-based magnetic metal with core-shell structure is a relatively suitable candidate material in the study of dynamic electromagnetic wave absorption in air detonation. |
