Preparation And Electromagnetic Absorption Properties Of Boron Nitride Composites

Owing to severe electromagnetic environment and enhanced technical requirements in modern military aviation,the application of conventional wave absorbent materials is limited due to their temperature instability and inefficient absorption,which is hard to meet the performance requirements in high temperature and ultracold environmental conditions.Therefore,looking for new absorbing materials is expected to overcome the shortcomings of traditional wave absorbing materials and provide new material options,and to study new ideas for developing wave absorbing materials.Boron nitride(BN)has advantages of high temperature and corrosion resistance and high thermal conductivity.It is a high-performance wave permeable material.Combined with traditional absorbents,BN is expected to improve the environmental stability,impedancematchingandenhancedabsorptionperformanceof electromagnetic wave absorbers.In addition,it is considered that in-situ compounding process can improve the dispersion properties of the absorber and impart additional interfacial polarization mechanisms to improve the overall performance of the absorbing material.Recent studies show that the introduction of microscopic defects can achieve the transformation of BN electrical and magnetic properties by doping heterogeneous elements.But so far,there are still no reports on BN based electromagnetic loss performance.Based on the above considerations,we studied BN with preparing BN composite materials and doped BN nanomaterials.The absorption performance and loss mechanism were studied from the perspectives of electromagnetic wave transmission and absorption.The main research results are shown below:1.Boron nitride/nickel(BN/Ni)composites were prepared by in-situ composite method,in which the control of nanoparticles size was achieved to ensure the effective interface between BN and Ni and gave the material enhanced electromagnetic loss mechanism.The results show that the BN/Ni composites have higher mutual dispersion properties and smaller particle sizes.The electromagnetic properties of the paraffin-based composite were prepared using BN/Ni as filler.The results show that the dielectric loss performance of the composite increased with increase of the filler content,but the impedance matching performance decreased during this process.When the filler content was 48%by weight,the minimum reflection loss(RL)reached-38.5 dB(at absorbent thickness of 2.5 mm),and the effective absorption band reached 3.3 GHz(RL<-10 dB,13-16.3 GHz),which demonstrate the improved electromagnetic wave absorption performance.In order to further improve the dispersibility of the components in the BN/Ni composite and make the material interface more uneven,we prepared the BN/Ni microspheres by simple liquid phase reduction method.The BN/Ni microspheres are uniform in size with diameters between 1 and 2?m,and the thermal stability of the composite was improved as well in comparison to pure nickel particulates.The electromagnetic properties of the BN/Ni microspheres were investigated.The results show that when the filler content was 60%by weight,the minimum reflection loss of the composite reached-46.7 dB(at absorber thickness of 6.5 mm)and the absorption bandwidth was3.9 GHz(RL<-10 dB,11.6-15.5 GHz,at absorber thickness of 2.5 mm).Our studies show that the in-situ recombination of BN and Ni nanoparticles can effectively optimize the electromagnetic loss performance and impedance matching of materials,improve the high temperature stability of magnetic metals,and expand its application in high temperature applications.2.Nano titanium nitride(TiN)has a unique electromagnetic loss mechanism and is a potential lightweight high temperature absorbing material.In order to impart more electromagnetic loss mechanism and improve the impedance matching performance of TiN,BN/TiN and BN/TiN/Fe_x N composite fibers were prepared by electrospinning and in-situ thermal nitridation.The electromagnetic properties of the above composite fibers were investigated.The results show that as the filler content increased,the dielectric loss performance of the fiber increased.When the filler content was 15%by weight,the minimum reflection loss of the BN/TiN fiber reached-30 dB.The BN/TiN/Fe_xN fiber has a minimum reflection loss of-42.33 dB,which exhibits enhanced absorption performance and reduced density compared to TiN nanoparticles only.Considering the loss mechanism and impedance matching performance of the material,the BN/TiN composite fibers are easy to overlap each other to form conductive channels,and the in-situ composite material imparts rich and uneven interface within the material,thereby makes it exhibiting strong performance.The material has strong electromagnetic wave absorption performance,and has the advantages of low density,low filler content and high temperature resistance.3.To date,theoretical and experimental studies show that the introduction of microscopic defects is expected to achieve tremendous changes in BN electrical and magnetic properties.In view of the close relationship between the electromagnetic wave absorption characteristics of materials and their electromagnetic properties,we proposed a new idea,namely,the electromagnetic properties generated by microscopic defects,and used to study and design the electromagnetic wave absorption mechanism of materials.Based on such consideration,the precursor was prepared from boric acid and hexamethylenetetramine.The carbon-oxygen co-doped BN was prepared by high temperature nitridation treatment,and the electromagnetic loss mechanism of boron-doped BN was studied.The results show that the prepared BN contains carbon and oxygen,which is a simple method to prepare the doped BN material.The electromagnetic performance results show that the doped BN has electromagnetic wave loss capability,and its electromagnetic wave absorption performance is positively correlated with the hexamethylenetetramine content,which shows a magnetic loss mechanism.The research on the mechanism of electromagnetic loss caused by this defect is expected to shed new light on the understanding of the mechanism and performance optimization of the electromagnetic functional materials.