Synthesis Of Silicon Nitride Nanowires And The Preparation Of SNNWs/Si₃N₄ Wave Transparent Composites

For the rapid development of missile and aircraft,especially when the high mach number missles and hypersonic aircraft appear,the radomes,which possess the function of load bearing,wave transparent,anti-corrosion and heat proof,has to face more harsh application environment.The radome materials are asked to have the ability of load bearing for longer time,superior shock resistance and excellent wave transparent properties to ensure the high-speed flight and precision guidance.As the most promising candidate of the next generation radome materials,silicon nitride possesses excellent high-temperature mechanical properties,superior heat proof ability and fair enough shock resistance.However,as ceramic material,the inherent brittleness of silicon nitride acts as an obstacle for its further application.To improve the toughness has become the core issue of silicon nitride research.In this dissertation,silicon nitride nanowires（SNNWs）were prepared by a Vapor-Solid（VS）method and used as the reinforcement materials.For the fabrication of SNNWs/Si₃N₄ composites,further characterization and analysis of silicon nitride nanowires and SNNWs/Si₃N₄ were also done.The silicon nitride nanowires were synthesized by VS method.Al₂O₃ and graphite plates were used as substrates.The influences of reaction temperature and holding time on the morphology of silicon nitride nanowireswerewere also studied.The analysis and characterization of the synthesized silicon nitride nanowires were also done.The growth mechanism,surface element composition and oxiadation resistance at high temperature were also investigated.The results reveal that the silicon nitride nanowiressynthesized at1500℃possesse a diameter about 200nm,a length about several hundreds micrometer and a high yield.The growth mechanism of silicon nitride nanowires is VS model and the surface of silicon nitride nanowires has an amorphous layer of Si Ox and the elements are mainly Si,N and O.In the test of TG-DSC,silicon nitride nanowires were oxidized sharply upon the temperature of 1135℃.The SNNWs/Si₃N₄ composites were fabricated by die-pressing and pressureless sintering.The effects of the sintering temperature and contents of silicon nitride nanowires on micro physical properties,fracture morphology,mechanical properties and dielectric properties were investigated.The transformation of silicon nitride nanowires in SNNWs/Si₃N₄ composites at different sintering temperature and the toughening mechanism were also studied.The results show that the density and porosity of SNNWs/Si₃N₄ composites are between 1.5².3g/cm³ and 27.5%⁵0.0%,respectively.Higher sintering temperature and highcontents of silicon nitride nanowires lead to a more thoroughly transformation of phase in SNNWs/Si₃N₄ composites and theα-Si₃N₄are almost all transformed intoβ-Si₃N₄ at 1750℃.The bending strength of SNNWs/Si₃N₄ composites increases with higher sintering temperature and decreases with higher contents of silicon nitride nanowires.The bending strength are between15MPa²10MPa.The fracture toughness of SNNWs/Si₃N₄ roughly increaseswith higher sintering temperature.With the increasing contents of silicon nitride nanowires,the fracture toughness decreases at 1700℃¹750℃,a little change at 1500℃¹550℃and increases first and then decreases at 1600℃¹650℃.The highest fracture toughness value of SNNWs/Si₃N₄ composites at 1650℃is 1.21 MPa·m^1/2 and 0.86MPa·m^1/2 at1600℃.The fracture mode of SNNWs/Si₃N₄ is ductile fracture and“Load holding”emerges in load-displacement curves.The dielectric constant and dielectric loss tangent of SNNWs/Si₃N₄ composites are among 3.08³.91 and 0.0018⁰.0052 respectively.The toughening phenomenons including crack deflection,silicon nitride nanowires pull-out,silicon nitride nanowires-bridged,silicon nitride nanowires breakage and so on appear in SNNWs/Si₃N₄ composites.