Synthesis Of Boron Nitride Nanotubes And The Preparation And Performance Of BNNTs/Si₃N₄ Wave Transparent Composites

The development of high mach number missiles and hypersonic vehicles has put forward a series of fundamental requirements on the performance of radome materials, such as long-time dynamic loading bearing ability at high temperatures, superior thermal shock resistance and excellent dielectric properties. Silicon nitride?Si₃N₄? ceramic is one of the most promising high-temperature wave transparent materials since it possesses excellent high-temperature mechanical properties and thermal stability. To avoid the high density and dielectric constant of dense Si₃N₄ ceramic, it is often fabricated as a porous material or Si₃N₄ based composite. Meanwhile, due to the high sintering temperature of silicon nitride ceramics, it is essential for the reinforcements to maintain their good mechanical properties and phase stability at high temperatures. Boron nitride nanotubes?BNNTs? has a low density, a high modulus as well as prominent heat, corrosion and oxidation resistances, and such is recognized as a prospective reinforcing material for Si₃N₄ ceramic.In this dissertation, BNNTs were prepared by a chemical vapor deposition method using carbon nanotubes?CNTs? as templates. The effects of different processing parameters on the growth rate, crystalline degree and morphology features of the obtained BNNTs were investigated. The optimized processing parameters include a deposition temperature of 1300 ?, a deposition time of 30 min, and a deposition zone pressure of 1000 Pa.In addition, BNNTs/Si₃N₄ composites were fabricated by die-pressing and pressureless sintering and the influences of sintering temperature and BNNTs content on the mechanical properties and morphology of the composites were studied. The results show that the flexural strength of the BNNTs/Si₃N₄ composites increases with the increasing of sintering temperature. When sintered at 1750 ?, the composite containing 0.5 wt.% BNNTs displays an average bending strength of 231.8 MPa and an average elastic modulus of 62.04 GPa. Besides, when using a sintering temperature of 1700 ?, the mechanical properties of the BNNTs/Si₃N₄ composites increase with the increasing of BNNTs content at first, which then decrease rapidly as the BNNTs content exceeds 0.7 wt.%. With a BNNTs content of 0.7 wt.%, the obtained flexural strength and fracture toughness of the BNNTs/Si₃N₄ composites are 180 MPa and 2.85 MPa·m1/2, respectively.The thermophysical and dielectric properties of the BNNTs/Si₃N₄ composites were also characterized. The coefficient of thermal expansion?CTE? of the 1700 ? fabricated BNNTs/Si₃N₄ composite increases gradually with the elevating of testing temperature, which reaches 3.69×10^-6K^-1 at 1000 ?. For the dielectric properties, the dielectric constant and dielectric loss tangent of the 1700 ? fabricated BNNTs/Si₃N₄ composites are 4.25 and 3.3 ×10^-3, respectively.