The Effects Of Pocessing Parameters On The SiC Whisker Formation In The SiO₂-C-Na₃AlF₆ System

A whisker is usually defined as a single crystal of very high aspect ratio,i.e.the ratio of length to diameter,with 1/d(?)10 and the diameter in the range of 0.1-10um. Inherent to their chemically pure nature and highly ordered structure are strengths that approach almost the inter-atomic bonding forces.Among the ceramic ones,SiC whisker is expected to be the reinforcing constituent in ceramic and metal matrix composites.So considerable attentions have been taken to the research for SiC whiskers.It is the intention of this study to prepare SiC whiskers with tailored characteristics by a method of relatively low cost,and to undestand the effects of processing parameters on SiC whisker characteristics.SiC whiskers have been synthesized with well-controlled pocessing parameters such as starting materials,reaction temperature,atmosphere.X-ray diffraction(XRD) and scanning electron microscopy(SEM) are employed to analyze the composition and morphology of the synthesized whiskers.The results show that elkem-SiO₂ and graphite are more fit to the whisker formation than active carbon and other SiO₂ sources.Graphite with particle-size of 100 meshes minus,is most suitable for the whisker formation.The synthesized whiskers at 1500℃are better than those at 1400℃and at 1300℃.The whiskers are larger in diameter and longer in length with longer reaction time.The optimum Si/Al mole ratio is 4:1.And the higher the Si/Al mole ratio,the smaller in whisker diameter.The CO and SiO partial pressure was calculated thermodynamically.The results indicate that the partial pressure of SiO should be in the range of 10^-2atm～1atm for the stable existance of SiC phase at the thermodynamical equilibrium state.The starting temperature for SiC formation reduced with the lower partial pressure of CO.A certain degree of supersaturation in the gas phase is required for growing whiskers.The supersaturation is increased with an elevated partial pressure of SiO.When SiO partial pressure is lowered to 10^-2atm,the partial pressure of CO should be smaller than 10^-1atm,in order that the supersaturation become larger than 10.Experimental results in the present study suggested that the whisker growth was more likely through a two-dimensional vapor-solid(VS) nucleation mechanism.