Research On The Preparation Of SIC Whiskers In Nitrogen Atmosphere By Carbon And Silicon Dioxide From Rice Husks

First, lignin and silica were separated from rice husks together by alkaline extraction and acid precipitation method, then treated properly for preparation of SiC whiskers.By single factor alteration method and orthogonal experiments, not only the relationship between the concentra-tion of alkaline extraction, alkaline extraction liquid ratio, alkaline extrac-tion time and the lignin/silica mixture yield was researched, but also the relationship between acid precipitation temperature, pH, holding time and the lignin/silica mixture yield was studied. The lignin/silica mixture was carbonized and afterwards the silicon-carbon ratio was regulated. The SiC whiskers were prepared using a simple carbon thermal reduction. SiC whiskers were eventually synthesized with Fe powder catalyst by research the effects of temperature, holding time, the silicon and carbon ratio, catalyst, heating methods, and the reaction mechanism was discussed. The phase structure of products was studied by XRD analysis, and the morphology of samples was ovserved through SEM. On the basis of the preparation of SiC whiskers, the preparation of the dumbbell-shaped SiC whiskers bionic model was researched, and the synthesis condition was determined. The main contents of this paper are as follows:(1) The pretreatment of raw materialsBy single factor alteration method and orthogonal experiments, the optimum cooking conditions and acid precipitation conditions were determined:the alkali extraction concentration was11%, the alkaline extraction liquid ratio was1:40, the extraction time was11h; the acid precipitation pH was0.5,the holding time was5h, the holding temperature is55℃. In this combination of parameters the extraction rate of lignin and silica was above91%. The carbonized lignin and silica mixt-ure contained silica(67%) and carbide(33%), then the silicon-carbon ratio was regulated for the preparation of SiC whiskers.(2) The synthesis of SiC whiskersThe SiC whiskers were synthesized by a simple carbon thermal reduction method. It was showed that the product with3%Fe powder as catalyst, ωt(SiO2)=58.5%, first heating to1600℃and then naturally cooling to1500℃, holding3h had a better quality. And the whiskers prepared had a high straight crystal rate, diameter range from0.2μm to1.2μm. The XRD analysis indicated that the crystal form was β-type.(3) The postprocessing of crude productsThe crude products were obtained after high temperature reaction. And the postprocessing included heat treatment and acid immersion in order to removal of residual carbon and SiO2. The Optimum temperature of heat treatment was700℃without causing damage to the whisker surface structure. The best time of using hydrochloric acid and hydrofluoric acid soaking was24h, SiO2was completely removed after treatment.(4) The generation mechanism The generation mechanism of SiCw was determined by the growth areas and the ellipsoidal-shaped protrusion of whiskers. The3%Fe-catalyzed SiCw growed mainly by V-L-S mechanism, only a few of whiskers generated by the gas phase reactions.(5) The synthesis of dumbbell-shaped SiC whiskers bionic modelOn the basis of the preparation of SiC whiskers, the effects of tem-perature, holding time, silicon-carbon ratio, catalyst and heating methods on the prepration of the dumbbell-shaped SiC whiskers bionic model was researched, and the synthesis condition was determined. The experimenta-1results showed that the optional synthesis condition is with adding3%Fe+2%NaF powder as catalyst, ωt(SiO2)=58.5%, firstly directly heating to nucleation temperature (1600℃) and then naturally dropping to1500℃and keeping3h; The crystal of the whiskers is identified as β-SiC by Xray diffraction method. The acquisition rate of the dumbbell-shaped SiC whiskers is more than90%according to the results of SEM.