Preparation Of SiC Nanowires By Polymer Pyrolysis And Study Of SiCNW Toughing Ceramics

Silicon carbide nanowires?SiCNW?have lots of excellent properties,such as high temperature resistance and outstanding mechanical properties.SiC nanowires have become the focus of intensive research.SiC nanowires show potential for fruitful applications in many fields,and are the important materials for toughening phase in ceramic and metal composites.In this paper,SiCNW was successfully synthesized by polymer precursor pyrolysis with polycarbosilane?PCS?as precursor and ferrocene as catalyst.The effects of different synthesis temperatures,catalyst concentrations and substrates materials on the morphology and structure of SiC nanowires were investigated.The ZrB₂/SiC-SiCNW ceramic composites was prepared under pressureless sintering by adding different contents of SiCNW into the ZrB₂/Si C ceramic composites.The effects of the content of SiCNW on the properties of the composites were measured by various testing and characterization methods.The toughening mechanism and the high temperature oxidation resistance were analysed.The results are as follows:SiCNW was prepared by polymer precursor pyrolysis with mixed powders of PCS and ferrocene.The results show that with the synthesis temperature increasing,the morphology of the nanowires changes from linear-shaped to spindle-shaped and the diameter of the nanowires also increases.With the catalyst concentration increasing,the yield of SiCNW increases,and the length-diameter ratio of the nanowires becomes larger.With activated carbon adding as adsorbent,the length-diameter ratio and yield of SiCNW significantly increases,but there are many particles of SiC.SiCNW is a face-centered cubic single crystal structure with a lattice constant of 4.357?,a diameter of about 400 nm and a length of more than 100?m.The outer layer is coated with a 70 nm thin amorphous SiO₂ shell.The growth mechanism of SiCNW can be explained by the VLS process.SiCNW was prepared by the mixed liquid impregnation method with the impregnated substrate.With the synthesis temperature increasing,the morphology of nanowires changes from curved to linear-shaped and chainlike,and the diameter of nanowires increases.With the catalyst concentration increasing,the nanowires yield increases.Once the catalyst is too many,the yield decreases sharply and the product are chainlike nanowires mostly.When the substrate is carbon fiber cloth,the morphology of the nanowires and yield is better.When the substrate is graphite flake,yield of nanowires decreases.When the substrate is carbon felt,a large amount of nanowires and nanocrystals are simultaneously synthesized,which is hard to separate.SiCNW has differert morphologies,such as bamboo-like nanowires and branched nanowires.It is believed that the SiC nanowires growth is governed by VLS mechanism and VS mechanism.It indicated that the product is the cubic structure of?-SiC nanowires curved growth,and a diameter of about 400 nm with 40 nm SiO₂ shell,which has a good length-diameter ratio.The photoluminescence spectra of SiCNW displays two strong peaks between 460 nm to 490nm,which is the blue zone emission zone.And the PL peaks of nanowires are blue-shifted,which can be explained by the quantum size effect and the SiO₂ shell.ZrB₂/SiC composites with different content of SiCNW were successfully prepared by pressureless sintering.It is found that the addition of SiCNW can obviously increase the density of the material,improve the mechanical properties of the material and enhance the toughening effect.When the content of SiCNW is 20 wt.%,the relative density of the product is 90.8%,the Vickers hardness increases to 18.37 GPa and the fracture toughness was 4.18 MPa·M^1/2.The fracture mode is a mixture of transgranular fracture and intransgranular fracture.At 1500°C,the intestigation of oxidation behavior of ZrB₂/SiC composites indicates that the composite with SiCNW content of 20 wt.%has better high temperature oxidation resistance,which is mainly attributed to the generation of the SiO₂ and B₂O₃ glass phases on the surface of product and the toughening of nanowires.