Controlled Preparation And Properties Of BN/SiO₂Composites

With the rapid development of modern military technology in recent years, hypersonic missiles, as one of the focus of national military, have more rigorous requirements to missile radome materials. So, the waves through material of the device must have high temperature resistance, erosion resistance, wipers, thermal shock resistance, good mechanical properties, thermal properties and dielectric properties. Silica composite ceramics with good mechanical properties, thermal properties and excellent dielectric propertie, is an excellent candidate material of making missile radome materials of waves through.This paper will add boron nitride particles, boron nitride nanotubes and nano-alumina to silica matrix composites, and prepare three different system SiO2silica ceramic composite materials of BNp/SiO2, BNNTs/SiO2and Al2O3-BNp-SiO2by hot pressing. We study of the composite material of the mechanical, thermal and dielectric properties, and analyze its performance relative to the added. The main contents are as follows:(1) Successfully prepared BNp/SiO2composite ceramics adding different levels of BNp in SiO2matrix by hot pressing. Flexural strength and fracture toughness of ceramic composites improved by the addition of BNp.7wt%BNp/SiO2has the maximum flexural strength of the composite material, the flexural strength and fracture toughness than pure SiO2ceramics were improved by239%and129%, reaching123MPa and1.60MPa·m1/2. The high-temperature bending strength reaches a maximum144.4MPa at1000℃, and decreases not only before but also after it. Mainly because of the production of B2O3oxidized by BN and cause to glass phase in the SiO2grain boundary, and softening the SiO2grain boundary. The thermal conductivity of7wt%BNp/SiO2composite is3.82W/(m·K) at the temperature of292K, and the specific heat capacity of it is0.382kJ/(kg-K). The coefficient of thermal expansion in the range0～900℃of7wt%BNp/SiO2composite isl.8×10-5K-1, and it residual bending strength decline after thermal shock when the temperature exceed200℃, the mass loss rate is1.0×10-3g/s after the flame ablation at about1700～1900℃. The dielectric constants of the composite material keep stable in the26.5～30GHz frequency and the average dielectric constant of the composite7wt%BNp/SiO2are about3.44. (2) Successfully prepared BNNTs/SiO2composite ceramics adding different levels of BNNTs in SiO2matrix by hot pressing. Flexural strength and fracture toughness of ceramic composites improved by the addition of BNNTs.4wt%BNNTs/SiO2has the maximum flexural strength of the composite material, the flexural strength and fracture toughness than pure SiO2ceramics were improved by271%and187%, reaching134MPa and2.01MPa-m1/2. The high-temperature bending strength reaches a maximum134MPa at1000℃, and decreases not only before but also after it. It reaches the point of change of brittle-ductility at1200℃. Mainly because of the production of B2O3oxidized by BNNTs and cause to glass phase in the SiO2grain boundary, and softening the SiO2grain boundary that cause grains slip. The thermal conductivity of4wt%BNNTs/SiO2composite is2.27W/(m-K) at the temperature of292K, and the specific heat capacity of it is0.27kJ/(kg-K). The coefficient of thermal expansion in the range0～900℃of7wt%BNNTs/SiO2composite is1.1×10-5K-1. The dielectric constants of the composite materials most keep stable in the26.5～30GHz frequency and the average dielectric constant of the composite4wt%BNNTs/SiO2is3.43.(3) Successfully prepared Al2O3-BNp-SiO2by hot pressing. Flexural strength and fracture toughness of ceramic composites improved by the addition, and the composite3a7b, added7wt%BNP and3wt%Al2O3has the maximum fracture toughness of the composite material, the flexural strength and fracture toughness than pure SiO2ceramics were improved by265%and310%, reaching135.8MPa and2.87MPa·m1/2. The high-temperature bending strength reaches a maximum146.9MPa at1000℃, and decreases not only before but also after it. It reaches the point of change of brittle-ductility at1200℃. Mainly because of the production of B2O3oxidized by BN and cause to glass phase with Al2O3in the SiO2grain boundary, and softening the SiO2grain boundary that cause grains slip. The thermal conductivity of3a7b composite is4.57W/(m-K) at the temperature of292K, and the specific heat capacity of it is0.457kJ/(kg-K). The coefficient of thermal expansion in the range0～900℃of3a7b composite is1.0×10-5K-1, and it residual bending strength decline after thermal shock when the temperature exceed300℃, the mass loss rate is3.1x10-3g/s after the flame ablation at about1700～1900℃. The dielectric constants of the composite materials are about stable in the26.5～30GHz frequency and the average dielectric constant of the composite3a7b is about3.44. (4)This study not only achieved preparation a high strength, good dielectric properties of SiO2composite ceramics. And on the basis of systems SiO2composite ceramics, clarified that Al2O3, BNp and BNNTs composite ceramics mechanical, thermal and dielectric properties of impact. Give for future high-performance SiO2composite design and fabrication of ceramics with technical and theoretical basis.