Preparation And Dielectric Properties Of BNNSs Reinforced Ceramic Based Radome Composites

With the continuous improvement of the Mach number of aerospace and spacecraft,the requirements for various properties of radome materials are also continuously improved,such as higher strength and toughness,more excellent high temperature resistance and electromagnetic wave permeability.Ceramic matrix composites such as MgO and Si3N4 are the most widely used and promising materials in radome(window)materials for aerospace and spacecraft at present.In this paper,boron nitride nanosheets(BNNSs)with a thickness of less than 10 nm and a transverse size of micron were prepared by ultrasonic-assisted liquid-phase stripping.BNNSs/MgO and BNNSs/Si3N4 radome composites were prepared by adding BNNSs into MgO and Si3N4 ceramic matrix which have excellent wave permeable and mechanical properties,respectively.The strength and toughness of the matrices were further improved without damaging the electromagnetic wave permeability and high temperature resistance of the matrices.The mechanical properties and dielectric properties of composites and toughening mechanism of BNNSs were studied.(1)The BNNSs/MgO composites were prepared by hot pressing sintering after homogeneous mixing of BNNSs and MgO powder by mechanical stirring assisted flocculation.The mechanical properties of MgO ceramics were improved by adding BNNSs.Compared with monolithic MgO ceramics,the bending strength of BNNSs/MgO composites with the addition of 1 wt.%BNNSs is increased by about 85%,reaching 411 MPa.With the addition of 1.5 wt.%BNNSs,the fracture toughness of BNNSs/MgO is increased by 34%,reaching 3.64 MPa·m1/2.The microstructure analysis of the composites shows that toughening mechanisms of BNNSs in MgO matrix are the joint action of pull out and bridging of BNNSs,crack deflection and crack bypass mechanism.By analyzing the dielectric properties of the composite with the addition of 1 wt.%BNNSs,it is finds that the addition of a small amount of BNNSs does not damage the excellent electromagnetic wave permeability of MgO matrix.(2)In order to disperse BNNSs more easily and uniformly in ceramic matrix composites,two active groups,hydroxyl and amino,were modified on the surface of BNNSs by covalent bonding.After the surface hydroxyl modification of BNNSs,more hydroxyl groups were obtained on the surface of BNNSs.The absolute value of zeta potential on the surface of BNNSs increased.The surface zeta potential of BNNSs was changed from negative to positive by modifying BNNSs with amino on the basis of hydroxyl.The positive and negative regulation of zeta potential on BNNSs surface was realized by modifying BNNSs surface with amino group.The results provide a basis for the uniform dispersion of BNNSs in ceramic matrix composites and make them more suitable for solution-based applications and reprocessing.(3)The amino modified BNNSs(NH2-BNNSs)with positive zeta potential and Si3N4 powder with negative zeta potential were uniformly mixed by surface electrostatic flocculation.BNNSs/Si3N4 composites were prepared by SPS sintering.The bending strength and fracture toughness of BNNSs/Si3N4 composites with the addition of 0.75 wt.%BNNSs reached 778.2 MPa and 7.97 MPa·m1/2 which increased by 28%and 34%,respectively,compared with the monolithic Si3N4 ceramics.Through microstructure analysis,the toughening mechanisms of BNNSs are the synergistic effect of pulling out,bridging and tearing of BNNSs and crack deflection.Through the analysis of the dielectric constant and dielectric loss of the composites,it is finds that the electromagnetic wave permeability of Si3N4 ceramics is improved to some extent by adding NH2-BNNSs.