Preparation And Property Regulation Of Rice Husk Carbon-based Composite Microwave Absorbers

With the rapid development of electronic science and technology,high-frequency electronic devices and equipment have been widely used in civil and military fields,which also brings electromagnetic radiation,posing a threat to the normal work of electronic instruments and human health.In the face of increasingly serious electromagnetic pollution,it is urgent to develop new high-performance absorbing materials with characteristics of"thin coating,low density,wide bandwidth and strong reflection loss".As a kind of renewable carbon source,biomass carbon has the advantages of strong dielectric loss,low density,low cost,green environmental protection,stable performance,etc.,which has been widely studied in the field of electromagnetic wave absorption.However,like other carbon materials,there are problems such as narrow effective absorption frequency band and low absorption intensity when only using biomass carbon as the absorbsent due to serious impedance mismatch,which hinders its practical application.Currently,the combination with magnetic materials and the introduction of low dielectric materials are considered to be effective strategies to improve the impedance matching level of carbon-based materials.In this thesis,Ni_xCo_y/PRC,NiCo₂S₄/CoS₂/PRC and NiCo₂/PRC/ZnO composites were prepared using porous rice husk carbon（PRC）as a carbon source.The preparation method involved impregnation combined with hydrogen thermal reduction,vulcanization and atomic layer deposition.The effects of alloy content,alloy proportion,sulfide content,oxide（ZnO）thickness and filling loading on electromagnetic and wave-absorbing properties of the corresponding absorbing coatings and the regulation mechanism were investigated.The main research contents and results are as follows:（1）By immersing the wasted rice husk carbon from biomass power generation in hydrofluoric acid solution,porous rice husk carbon（PRC）was obtained.Although the absorption performance of etched porous rice husk carbon is improved,its wave absorption performance is still poor due to its poor impedance matching.At a matching thickness of 1.5mm,the minimum reflection loss(RL_min)reaches-13.8 dB at 14.4 GHz,and the effective absorption bandwidth（EAB）of RL<-10 dB is 3.04 GHz（12.48～15.52 GHz）.（2）A series of Ni_xCo_y/PRC（x:y=1:0,2:1,1:1,1:2,0:1）composites were obtained through impregnation combined with hydrogen heat treatment.This method not only maintains the original carbon material structure but also increases the magnetic properties of the material and effectively improves the impedance matching.When the carbonation temperature is 600℃and the NiCo₂ content is 27.5%,the NiCo₂/PRC shows the best microwave absorption performance at 30%filling amount.When the matching thickness is3.57 mm,the RL_min reaches-55.62 dB at 6.32 GHz;the EAB is 3.68 GHz（10.64～14.32 GHz）with a matching thickness of 2.0 mm.The position of the absorption peak of the Ni_xCo_y/PRC composite moves slightly to high frequency with the decrease of the Ni/Co ratio,and the absorbing performance first increases and then weakens.Among them,the NiCo₂/PRC composite has the best comprehensive absorbing performance.（3）NiCo₂/PRC was further sulfurized to prepare NiCo₂S₄/CoS₂/PRC composite material.The presence of multiphase particles can enhance the interface polarization relaxation of the material and further improve the dielectric loss ability.Compared with PRC,the absorption performance of NiCo₂S₄/CoS₂/PRC is greatly improved.But as the content of NiCo₂S₄/CoS₂increases,the absorption intensity gradually decreases.At 1.6 mm matching thickness,the RL_min value of NiCo₂S₄/CoS₂/PRC-1 is-38.3 dB,and the corresponding EAB value is 4.56GHz（12.32～16.88 GHz）.The frequency range of RL≤-20 dB covers 5-15 GHz when the thickness varies between 1.6 and 5.0 mm.（4）ZnO was deposited on the surface of NiCo₂/PRC by atomic layer deposition technology（ALD）to study the effect of different ZnO deposition numbers on wave absorption performance.It was found that when the number of deposition cycles reaches 150,the wave absorption performance of the sample is significantly improved.When the number of deposition turns is 150,the RL_min value of NiCo₂/PRC/ZnO-150 at 11.76 GHz is-46.41 dB at a matching thickness of 1.9 mm,and the EAB value is 4.48 GHz（13.36～17.84 GHz）when the thickness is 1.5 mm.The RL_min value of NiCo₂/PRC/ZnO-200 at 1.4 mm thickness is-43.17 dB,and the EAB value is 3.44 GHz（9.60～13.04 GHz）when the thickness is 2.0 mm.