High-efficiency microwave absorbers with broadband absorption are strongly desired for electromag-netic protection.Herein,we successfully synthesized a hybrid microwave absorbing material with two-dimensional layered ...High-efficiency microwave absorbers with broadband absorption are strongly desired for electromag-netic protection.Herein,we successfully synthesized a hybrid microwave absorbing material with two-dimensional layered structure,which consisted of expanded graphite(EG)and boron nitride(BN).The introduction of BN is to regulate the conductivity of EG and also to improve the thermal stability of the composite material.The ultrathin BN nano-sheets were uniformly wrapped on the EG sheets via chemi-cal vapor deposition.Attributed to the dielectric loss and conductive loss,the as-prepared hybrid material exhibited high performance for microwave absorption.The effective absorbing bandwidth(reflection loss value<-10 dB)was achieved up to 9.37 GHz with the thickness of 2.4 mm in the frequency range of 5.75-6.98 GHz and 9.86-18 GHz.And the minimum reflection loss was-51.58 dB at the thickness of 3 mm.The excellent performance of microwave absorption was attributed to the dielectric loss,interfacial polarization loss and optimized impedance matching.Moreover,compared to pure EG,the thermal de-composition temperature of EG/BN composite materials had increased by about 100℃which was up to 714℃.Taking the advantages of high thermal stability of BN as well as the fine microwave absorbability of EG,the EG/BN composites would be applied in high-temperature microwave absorbing fields.展开更多
Photopolymerization of vinyl-containing liquid silizane preceramic monomers copolymerized with thiol monomers,based on a step-growth radical polymerization mechanism,is a novel,rapid,inexpensive and simple tech-nique ...Photopolymerization of vinyl-containing liquid silizane preceramic monomers copolymerized with thiol monomers,based on a step-growth radical polymerization mechanism,is a novel,rapid,inexpensive and simple tech-nique for producing preceramic structures from liquid pre-cursors.The kinetics of alkyl 3-mercptopropionate-vinyl silizane under UV irradiation is investigated by using real-time Fourier transform infrarcd(FT-IR)and photo-dif-ferential scanning calorimetry(photo-DSC).The experi-mental results show preliminarily that:(1)about 80%conversion of vinyl group has been achieved in the pres-ence of a low concentration photoinitiator under UV irra-diation;(2)by increasing the functionalities of the thiol group,the peak rate of copolymerization increases and the final conversion of the vinyl group decreases;(3)the copolymerization is primarily a bimolecular radical ter-mination process;(4)the copolymerization is first-order,i.e.,its rate is proportional to the vinyl group concentra-tion and independent of the concentration of thiol group.展开更多
文摘High-efficiency microwave absorbers with broadband absorption are strongly desired for electromag-netic protection.Herein,we successfully synthesized a hybrid microwave absorbing material with two-dimensional layered structure,which consisted of expanded graphite(EG)and boron nitride(BN).The introduction of BN is to regulate the conductivity of EG and also to improve the thermal stability of the composite material.The ultrathin BN nano-sheets were uniformly wrapped on the EG sheets via chemi-cal vapor deposition.Attributed to the dielectric loss and conductive loss,the as-prepared hybrid material exhibited high performance for microwave absorption.The effective absorbing bandwidth(reflection loss value<-10 dB)was achieved up to 9.37 GHz with the thickness of 2.4 mm in the frequency range of 5.75-6.98 GHz and 9.86-18 GHz.And the minimum reflection loss was-51.58 dB at the thickness of 3 mm.The excellent performance of microwave absorption was attributed to the dielectric loss,interfacial polarization loss and optimized impedance matching.Moreover,compared to pure EG,the thermal de-composition temperature of EG/BN composite materials had increased by about 100℃which was up to 714℃.Taking the advantages of high thermal stability of BN as well as the fine microwave absorbability of EG,the EG/BN composites would be applied in high-temperature microwave absorbing fields.
基金supported by the National Natural Science Foundation of China(Grant No.20574056)Military Equipment Foundation of Defence(No.9140A12070106HK0338)the Doctorate Foundation of Northwestern Polytechnical University(No.CX200614).
文摘Photopolymerization of vinyl-containing liquid silizane preceramic monomers copolymerized with thiol monomers,based on a step-growth radical polymerization mechanism,is a novel,rapid,inexpensive and simple tech-nique for producing preceramic structures from liquid pre-cursors.The kinetics of alkyl 3-mercptopropionate-vinyl silizane under UV irradiation is investigated by using real-time Fourier transform infrarcd(FT-IR)and photo-dif-ferential scanning calorimetry(photo-DSC).The experi-mental results show preliminarily that:(1)about 80%conversion of vinyl group has been achieved in the pres-ence of a low concentration photoinitiator under UV irra-diation;(2)by increasing the functionalities of the thiol group,the peak rate of copolymerization increases and the final conversion of the vinyl group decreases;(3)the copolymerization is primarily a bimolecular radical ter-mination process;(4)the copolymerization is first-order,i.e.,its rate is proportional to the vinyl group concentra-tion and independent of the concentration of thiol group.
