Synthesis Mechanism Of Sic Nanowires And Application In C_f/SiC Composites

With indepth researches into nanomaterials,varieties of unique outstanding properties of nanomaterials have been found,and structural and functional nanodevices have been exploited over three decades.In order to carry out precise control of the growth of nanomaterials,the forming mechanism of nanomaterials have attracted considerable interest.Furthermore,introducing nanomaterials by rational design of constituent and structure into composites,in which nanomaterials can take the synergistic effects of performance complementary,has been the important development direction of composite.In this thesis,SiC nanowires were synthesized on carbon fabric by chemical vapor reaction?CVR?method.The influences of annealing temperature,fabric structure and molar ratios of silicon and silica on morphologies and structure of SiC nanowires were studied systematically.The results showed that substantial nanowires with²5 nm mean diameter and narrow diameter distribution were fabricated on the surface of carbon fiber at a 2:1 molar ratio of silicon and silica and 1350?C and 1400?C for 30min,respectively.According to analysis of the nanostructure obtained by different holding time at 1300?C,a growth mechanism of SiC nanowires is proposed as followed:SiO vapor is formed by the reaction of the mixture powders of Si and SiO₂;SiO vapor reacts to form nanoparticles,in which Si is as core and SiO_x is as shell;The nanoparticles subsequently self-assemblie into Si-SiO_x nanorods;carbon fiber reacted with residual oxygen to form CO gas and above nanorods reacted with CO gas to transform chemically into SiC nanowires.The interaction between nanoparticles to fabricate nanorods is attributed to electrical dipole-dipole interaction and van der Waals force.The electrical dipole-dipole interaction originals from the interfaces between Si and SiO_x.Furthermore,the SiC nanowires converted from nanorods was of amorphous structure at the beginning.The nanowires crystallized in different areas and the structure consisted of the mixture of amorphous form and crystal or inconsistent orientation of crystal planes.The structure evolved into crystal structure with irregular defects.The nanowires continued to grow via vapor-solid?VS?mechanism and near-periodical twins were formed in SiC nanowires.The photoluminescence and electrochemical capacitance properties of SiC nanowires were studied.The results indicated that SiC nanowires are of blue-shifted emission bands in photoluminescence spectrum.SiC nanowires are of intensive emission peaks at 420 nm and 468 nm and can be promising candidates for applications in violet-blue light emitters.The electrochemical characterization manifests that the gravimetric capacitance of carbon fabric with SiC nanowires is 3 F/g level,which is approach to that of the SiC nanowires that have been reported.This is because of substantial thin SiC nanowires with high surface areas and electric double layer areas,and carbon fibers as electrode.SiC nanowires exhibit an almost ideal capacitive behavior and robust stability.These properties demonstrate SiC NWs on carbon fabric as promising candidates for high-performance supercapacitors.C_f/SiC composites were fabricated by combining chemical vapor infiltration?CVI?and polymer-infiltration-pyrolysis?PIP?process with methyltrichlorosilane?MTS?and polycarbosilane?PCS?as raw materials.In CVI process,when the ratio of MTS and H₂is 1/8,a uniform coating with⁰.1?m thickness coated on carbon fiber and nanowires with⁴00 nm diameter in the interval of fibers are synthesized.While the ratio of MTS and H₂ is 1,a coating with⁰.8?m thickness coated on carbon fiber.As-prepared C_f/SiC composite is of high flexural strength?248 MPa?and favorable toughness,owing to its high density,substantial nanowires as well as weak interfaces between matrix and fibers.C_f-SiC_NW/SiC composites were obtained by several PIP processes with carbon fabrics loading SiC nanowires synthesized by CVR as reinforcements.The results showed that the highest flexural strength of composite with SiC nanowires is 328 MPa.However,all composites exhibited brittle properties due to the damage of carbon fibers.SiC coatings on fiber were prepared by CVI process to protect fiber in advance.SiC nanowires were subsequently fabricated on carbon fiber by CVR and SiC matrix were prepared by PIP process.Thus,SiC@C_f-SiC_NW/SiC composites were made.The results revealed that these composites are of excellent toughness.The coating can protect carbon fiber,weaken the interface between carbon fiber and matrix as well as improve the mechanical property of composite.