Preparation And Mechanisms Of SiC/CNTs Ceramic Composites And One-dimensional SiO₂/C Nanomaterials

The special structures and properties of one-dimensional nanomaterials,such as carbon nanotubes（CNTs）,nanofibers and nanowires enable them to perform well in the preparation of nano ceramic composites and optoelectronic devices.However,there are some problems when it comes to practical applications.The adverse interfacial reactions would deteriorate the toughening ability,as well as the mechanical properties of as-synthesized ceramic composites.Besides,the precisely controlled and convenient synthesis processing of nanofibers and nanowires remain to be developed.In view of the adverse interfacial reactions between the CNTs and the residual SiO₂impurity in SiC powder when fabricating SiC/CNTs ceramic composite,a simple and novel HF treatment is proposed to remove the SiO₂ impurity.Under the same experimental conditions,two 10 vol%CNTs were introduced into HF-treated and untreated SiC powders,respectively.After spark plasma sintering（SPS）at 1850℃,two SiC/CNTs ceramic composites were obtained.Field emission scanning electron microscope（FESEM）and Raman spectrometer（RS）were utilized to characterize the as-prepared composites.The bulk density,thermal diffusivity and fracture toughness were measured and some thermodynamic calculations were carried out.The results showed that the interfacial reactions did occurred during sintering,which is consistent with the thermodynamic calculations and dynamic conditions.The structural integrity of the CNTs was improved after HF treatment and the bulk densities,thermal diffusivity were also increased to a certain extent.Especially,the fracture toughness tested by indentation method of the composites increased from 4.4±0.4 MPa?m^1/2to 5.4±0.5 MPa?m^1/2.The precisely controlled and convenient synthesis of nanofibers and nanowires remains as a challenge.In the present work,a carbon fiber was also prepared using H₂,CH₄ and Ar as the gaseous materials without a catalyst at 1100℃by chemical vapor deposition（CVD）.The lengths of the fibers ranged from several milimeters to several centimeters and the diameters and lengths increased with the holding time.The electrical resistivity of a single carbon fiber was measured to be 10^-5Ω?m order of magnitude with voltammetry.Based on the experimental results and theoretical analyses,the growth mechanism of the carbon nanofiber was proposed.In addition,in-situ synthesis of SiO₂ nanowires was also realized using SiC powder as the raw material and Fe（NO₃）₃ as a catalyst under the same experimental system.The nanowires were several tens of micrometers in length with diameters of 20-25 nm.After a second treatment to the nanowires by introducing CH₄,a new carbon beaded SiO₂ nanowires（SiO₂/C nanochains）was fabricated.A VLS growth mechanism and a Plateau-Rayleigh instability induced growth mode for the SiO₂ nanowires and the nanochains were proposed,respectively.