Enhanced microwave absorption property of silver decorated biomass ordered porous carbon composite materials with frequency selective surface incorporation

Porous carbon(PC)is a promising electromagnetic(EM)wave absorbing material thanks to its light weight,large specific surface area as well as good dissipating capacity.To further improve its microwave absorbing performance,silver coated porous carbon(Ag@PC)is synthesized by one-step hydro-thermal synthesis process making use of fir as a biomass formwork.Phase compositions,morphological structure,and microwave absorption capability of the Ag@PC has been explored.Research results show that the metallic Ag was successfully reduced and the particles are evenly distributed inward the pores of the carbon formwork,which accelerates graphitization process of the amorphous carbon.The Ag@PC composite without adding polyvinyl pyrrolidone(PVP)exhibits higher dielectric constant and better EM wave dissipating capability.This is because the larger particles of Ag give rise to higher electric conductivity.After combing with frequency selective surface(FSS),the EM wave absorbing performance is further improved and the frequency region below-10 d B is located in8.20-11.75 GHz,and the minimal reflection loss value is-22.5 dB.This work indicates that incorporating metallic Ag particles and FSS provides a valid way to strengthen EM wave absorbing capacity of PC material.

From binary to ternary and back to binary:Transition of electromagnetic wave shielding to absorption among MAB phase Ni_(3)ZnB_(2)and corresponding binary borides Nin+1Bn(n=1,3)

Due to chemical inertness of nickel and boron,the preparation of nickel borides and corresponding layered ternary transition metal borides Ni_(3)ZnB_(2)(MAB phase)has always required high-temperature and/or high-pressure conditions.Yet,an innovative and efficient approach to preparing Ni_(3)ZnB_(2)at only 600℃and without applied pressure is presented in this study.It is discovered that by simply adjusting the temperature,a phase transition from Ni_(3)ZnB_(2)to Ni4B3 with a layered structure could be induced.This transition between the binary-component and the ternary-component brings about significant variation in electromagnetic wave(EMW)shielding/absorption performance of prepared borides.For instance,Ni2B has good EMW shielding performance(42.54 dB in X band)and Ni_(3)ZnB_(2)is of weak EMW shielding(13.43 dB in X band);Ni_(3)ZnB_(2)has poor EMW absorption performance(−5 dB)while Ni4B3 has excellent EMW absorption performance(−45.19 dB)at a thickness of 2.7 mm with effective absorption bandwidth(10.4 GHz).

One-step synthesis and electromagnetic absorption properties of high entropy rare earth hexaborides(HE REB6)and high entropy rare earth hexaborides/borates(HE REB6/HE REB03)composite powders

Considering the emergence of severe electromagnetic interference problems,it is vital to develop electromagnetic(EM)wave absorbing materials with high dielectric,magnetic loss and optimized impedance matching.However,realizing the synergistic dielectric and magnetic losses in a single phase material is still a challenge.Herein,high entropy(HE)rare earth hexaborides(REB6)powders with coupling of dielectric and magnetic losses were designed and successfully synthesized through a facial one-step boron carbide reduction method,and the effects of high entropy borates intermedia phases on the EM wave absorption properties were investigated.Five HE REB6 ceramics including(Ce0.2Y0.2Sm0.2Er0.2Yb0.2)B6,(Ce0.2Hu0.2Sm0.2Er0.2Yb0.2)B6,(Ce0.2Y0.2Eu0.2Er0.2Yb0.2)B6,(Ce0.2Ya2Sm0.2Eu0.2Yb0.2)B6,and(Nd0.2Y0.2Sm0.2Eu0.2Yb0.2)B6 possess CsCl-type cubic crystal structure,and their theoretical densities range from 4.84 to 5.25 g/cm^(3).(Ce02Y0.2Sm0.2Er0.2Yb02)B6 powders with the average particle size of 1.86 jim were found to possess the best EM wave absorption properties among these hexaborides.The RLmin value of(Ce0.2Y0.2Sm0.2Er0.2Yb0.2)B6 reaches-33.4 dB at 11.5 GHz at thickness of 2 mm;meanwhile,the optimized effective absorption bandwidth(EAB)is 3.9 GHz from 13.6 to 17.5 GHz with a thickness of 1.5 mm.The introduction of HE REB03(RE=Ce,Y,Sm,Eu,Er,Yb)as intermediate phase will give rise to the mismatching impedance,which will further lead to the reduction of reflection loss.Intriguingly,the HEREB6/HEREB03 still possess wide effective absorption bandwidth of 4.1 GHz with the relative low thickness of 1.7 mm.Considering the better stability,low density,and good EM wave absorption properties,HE REB6 ceramics are promising as a new type of EM wave absorbing materials.