Preparation And Dielectric Properties Of Boron Nitride Nanosheets Reinforced Silica Composites

With the rapid development of high Mach vehicle,it is of great significance to study wavetransmitting materials with better performance.Silicon dioxide（SiO₂）ceramics have a high melting point,low thermal expansion coefficient,outstanding thermal shock resistance,chemical stability and excellent dielectric properties.Thus,SiO₂ ceramics can be used as candidate materials for radomes and aircraft signal receivers.However,the poor mechanical properties of SiO₂ ceramics and the difficulty of densification during sintering limit their further application in radome materials.Therefore,the study and prepare of SiO₂ composites with excellent mechanical properties and wave-transmitting properties are demanding.In this paper,boron nitride nanosheets（BNNSs）were fabricated by a simple ultrasoundassisted liquid phase exfoliation method.Pure SiO₂ and BNNSs/SiO₂ composites were prepared by hot pressing sintering in a multifunctional high temperature sintering furnace.The effects o f sintering temperature,holding time and BNNSs content on the phase transformation,microstructure,mechanical properties and dielectric properties of the composites were analyzed,and the related mechanisms were discussed in detail.The main research contents of this paper are as follows:（1）Pure SiO₂ and 0.5 wt.%BNNSs/SiO₂ composites were prepared by hot pressing sintering at 1250,1300,1350,1400 and 1450℃,respectively.The effects of sintering temperature and BNNSs on properties of SiO₂ ceramics were investigated.The conclusions show that the sintering temperature plays an extraordinary role in the phase transformation,microscopic morphology,mechanical and dielectric properties of ceramics,while the influence of BNNSs on properties of the ceramics is more complicated.In particular,the BNNSs/SiO₂ composite prepared at 1350℃ has the most excellent comprehensive properties.The flexural strength and fracture toughness reach 100.11 MPa and 1.54 MPa m1/2,separately.The dielectric properties are excellent,the dielectric constant is approximately 3.5,and the dielectric loss is between 0 and 0.01.（2）Pure SiO₂ and 0.5 wt.%BNNSs/SiO₂ composites were prepared by hot pressing sintering at 1350℃ for 0.5,1 and 1.5 h,respectively,and their properties were studied.The results show that the 0.5 wt.%BNNSs/SiO₂ composite prepared by holding for 1.5 h has the most outstanding properties.The flexural strength and fracture toughness are 152.31 MPa and 1.71 MPa·m1/2,separately,which are 41.6%and 14%higher than those of pure SiO₂ ceramic prepared under the same condition.The dielectric constant is about 3.5 and the dielectric loss is in the range of 0 and 0.02.（3）Composites with BNNSs content of 0,0.5,1,1.5 and 2 wt.%were prepared by hot pressing sintering at 1350℃ for 1.5 h.The properties of the composites were studied in detail.The results show that the addition of a small amount of BNNSs is beneficial to inhibition of the phase transformation of SiO₂ and reducution of the precipitation of cristobalite phase.The flexural strength of 0.5 wt.%BNNSs/SiO₂ composite reaches the highest of 152.3 1 MPa,which is 41.6%higher than that of pure SiO₂ ceramics,and the lowest dielectric constant is about 3.45,which indicates that the addition of BNNSs improves the dielectric properties of the composites to a certain extent.When the content of BNNSs is 1 wt.%,its bulk density and relative density are the lowest,which are 1.99 g/cm3 and 77.87%,respectively.And the fracture toughness is the highest,reaching 1.82 MPa·m 1/2,which is 21.3%higher than that of pure SiO₂ ceramic.The toughening mechanism of BNNSs in SiO₂ ceramic matrix is mainly the pull-out of nanosheets and the tearing of BNNSs.