Study On Preparation And Temperature And Oxidation Resistance Properties Of Broadband Microwave Absorbing Coatings Based On Carbonyl Iron And Graphene

In recent years,the rapid development of electronic information technology,wireless communication facilities and radar detection systems has led to the widespread application of electromagnetic waves in civil and military fields.At the same time.the ubiquitous electromagnetic waves have caused serious harm to human health and information security,and threaten the survivability and defense of weapon equipments.Therefore,it is very necessary to develop efficient microwave absorbing materials for eliminating electromagnetic interference and pollution,and improving the survival and defense capabilities of weapon equipments.In addition,for highly integrated precision electronic equipments or special parts of weapon equipments,such as missile casing,engine tail nozzle,tank exhaust pipe,etc.,large amounts of thermal radiation are often generated during operation.So the microwave absorbing materials not only possess the properties of wide bandwidth,thin thickness,strong absorption and low density,but also have excellent temperature and oxidation resistance to meet the requirements of short-term use or long-term service at low and moderate temperatures.As a typical microwave absorbing material,carbonyl iron has the advantages of low cost,good impedance matching,high magnetic saturation intensity,high permeability and high Curie temperature.However,carbonyl iron has some deficiencies such as single electromagnetic loss mechanism.narrow absorbing bandwidth,poor temperature and oxidation resistance,which limit their practical applications.This paper aims to broaden the absorbing bandwidth of carbonyl ironbased coatings and improve the temperature and oxidation resistance of carbonyl iron.From the view of practical application,the microwave absorbing coatings were designed to increase the electromagnetic wave loss ability and broaden the absorbing bandwidth by the introduction of the different morphologies of carbonyl iron powder and graphene.And their service temperature range and the failure reason of the microwave absorbing coatings were also investigated in detail.Carbonyl iron absorbers coated by different inorganic oxide were prepared to significantly improve the temperature and oxidation resistance of carbonyl iron.and combined with modified graphene powder to achieve the preparation of microwave absorbing coatings with wide bandwidth,thin thickness,excellent temperature and oxidation resistance.Firstly,through physical blending and multilayer composite methods,single-layer coatings and multilayer coatings(FCI/SCI/PU)were prepared by the incorporation of flaky carbonyl iron(FCI)and spherical carbonyl iron(SCI)with different electromagnetic wave loss characteristics into the polyurethane(PU)matrix.The effects of FCI and SCI content,filler ratio and coating structure on the microwave absorbing properties of the coatings were investigated.Then,after optimization and regulation,the optimal bandwidth of reflection loss(RL)values exceeding-5dB for single-layer and double-layer coatings reached 15.0GHz(3.018.0GHz)and 15.6GHz(2.4-18.0GHz),respectively.In order to investigate the short-term use temperature range of carbonyl iron-based microwave absorbing coatings,we adopt the silicone resin(SI)as the matrix to prepare the single-layer temperature-resistant coatings(FCI/SCI/SI).The study results showed that with the increaing of test temperature,the microwave absorbing properties of the carbonyl iron coatings decrease,especially when the temperature is higher than 300?.This is mainly due to the oxidation of carbonyl iron and the formation of mixture composed of Fe₃O₄ and Fe₂O₃,which have reduced the magnetic loss of carbonyl iron.When the mass ratio of SCI:FCI:SI is 0.75:0.75:1,the coating shows the best absorption performance in the temperature range of 25-300?.which reaches the radar stealth requirement of missile weapons.Secondly,the strong dielectric loss and light weight of graphene(G)was introduced into the carbonyl iron microwave absorbing coatings,and the preparation of coatings with thin thickness and wide broadband microwave absorption by optimizing filler composition and ratio.In addition,the microwave absorbing properties of the temperature-resistant coatings(G/SCI/SI)have been investigated at different temperatures.The results show that the introduction of graphene can significantly improve the microwave absorbing properties of the coatings in the low frequency region,and is beneficial to broaden the absorbing bandwidth of RL<-5dB.It's due to the formation of a large number of graphene-carbonyl iron-matrix heterogeneous interfaces and complete conductive network in the coatings after the addition of graphene,thus improving the dielectric loss capability of the coatings.However,the high conductivity of graphene added to the coatings can exacerbate the reflection of electromagnetic waves on the coating surface,and the bandwidth of RL<-10dB was narrowed.In addition,the G/SCI/SI coatings can be used for short-term broadband microwave absorbing in the temperature range of 25-200?.When the mass ratio of G:SCI:SI is 0.02:3:1.the coating possesses the best microwave absorbing properties,and the bandwidth of RL<-5dB at 25?.,100? and 200?has all reached above 15.0GHz.Furthermore,in order to extend the service life and application range of carbonyl iron,we have coated carbonyl iron with the SiO₂ and BaTiO₃ by the Stober method and sol method to improve the temperature and oxidation resistance of carbonyl iron.The effects of long-term heat treatment on microwave absorbing properties,phase and morphology of the coated carbonyl iron at different temperatures were studied in detail.The results show that after heat treatment at 200? for 100h,the real and imaginary parts of the permittivity for uncoated carbonyl iron increase greatly.while the permeability decreases,resulting in a sharp drop in impedance match and microwave absorbing performance.It is mainly related to the increase in particle size and oxidation degree of carbonyl iron during long-term heat treatment.In contrast.the coating treatment with SiO₂ or BaTiO₃ can significantly reduce the degree of oxidation of carbonyl iron.After heat treatment at 200? and 250? for 100h each,the bandwidth of RL<-10dB for FCI@SiO₂ is still as high as 5.4GHz(9.5-14.9GHz)at a thickness of 1.4mm.For the SCI@BaTi0₃@SiO₂,after heat treatment at 200?,250?,300? and 350? for 100h each,the bandwidth of RL<-10dB is still 4.8GHz(12.4-17.2GHz)at a thickness of 1.8 mm.In addition,the SCI@SiO₂/SI coating with temperature resistance was prepared,and the coating was heat treated at 300? for 100 h.The results showed that the effect of heat treatment have little impact on the microwave absorbing properties of the coating,after heat treatment the bandwidth of RL<-5dB and-10dB for this coating is still 11.2GHz(6.8-18.0GHz)and 5.8GHz(9.0-14.8GHz),respectively.Finally,the poly(phenyl-carborane)(PPB polymers)modified graphene oxide(GO-PPB)was designed and synthesised,which optimize the microwave absorbing properties of the SCI@SiO₂/SI coatings according to the mechanism of dielectric-magnetic synergistic effect.The results show that the introduction of PPB polymers on the GO surface greatly improves the dielectric properties and the microwave loss ability of the GO,thus the bandwidth of RL<-10dB for the GO-PPB composite with a low filler loading(20 wt.%)reaches 3.6GHz(14.2-17.8GHz)at a thickness of only 1.1 mm.Moreover,the GO-PPB composite has excellent temperature and oxidation resistance.After heat treatment at 200?,250? and 300? for 100 h each,the permittivity and the microwave loss ability of GO-PPB have been improved to a certain extent.It is caused by the increase in the degree of reduction and order of GO after longterm heat treatment.In addition,the absorbing bandwidth of the SCI@SiO₂/SI coating has successfully broadened by the introduction of GO-PPB.When the mass ratio of SCI@Si0₂:SI:GO-PPB is 2.5:1:0.01,the bandwidth of RL<-5dB and RL<-10dB for the coating is as high as 14.9GHz(3.1-18.0GHz)and 9.7GHz(4.4-9.9GHz and 13.3-17.5GHz),respectively.And the microwave absorbing properties of this coating has stayed almost stable after heat treatment at 300? for 100h,which possesses excellent temperature and oxidation resistance.