Research On Thermal Effects And Growth Mechanism Of SiC Prepared By Microwave Heating

SiC is an excellent material,whose structure and function is interrelated.However,the present synthetic methods has high energy consumption and high pollution,which cannot meet the needs of high-end products.Microwave heating technology was used in this dissertation,which is energy saving and high efficiency.Different C raw materials are chosen to develop the synthesis of SiC.The main research contents and results are as follows.Three kinds of C raw materials including industrial coal particles,activated carbon particles and graphite sheets are used,orthosilicate?TEOS?is Si source,C raw materials were treat by different treatments and mixed with different raw materials,synthetic materials with different mixed structures were obtained.The heating rate was controlled by input power and the temperature was 1000-1700 ? and holding time is 0-120 min.The samples are characterized by XRD,Raman,SEM,TEM.The dielectric properties and the wave absorption performances were analyzed.Experiments showed that there were roughly four stages in the heating process of microwave synthesis of SiC:?1?heat accumulation at initial stage?<600 ??,?2?thermal upheaval at rapid heating stage?600?900 ??,?3?high temperature growth stage?900?1100 ??and?4?temperature preservation stage.Different microwave heating behaviors were caused by different C raw material samples,respectively expressed as:different C raw material caused different heating rates and different thermal upheaval phenomena.At the same input power level,industrial coal particles,activated carbon particles and graphite sheets are used as carbon sources.The actual reflected power had different trends,it cost about 31 min,6 min and 9 min to 600 ? for different carbon sources.The temperature of the beginning and end of thermal runaway is 600-900 ?,600-1100 ? and 600-1100 ?.The mass of the synthetic raw materials,the size of the raw material particles,the addition of high dielectric loss SiC as seed crystals and Al N heating aids would change the heating behavior,such as heating time and thermal upheaval.These differences in heating behavior were related with microwave coupling effect,microwave plasma effect,local electric field enhancement effect,etc.The mechanisms of SiC crystal growth are altered by different microwave heating behaviors and microwave heating effects which finally caused the changes of SiC sample composition,structure and morphology.Normally,SiC synthesized by microwave were in the form of particles,whiskers,etc.,which respectively correspond to different nucleation and growth mechanisms,which related to the microwave heating effect.The microwave coupling effect at the low temperature stage of the heating caused heat accumulation in the synthetic raw materials.At a certain temperature,the reactions on C@Si O2 interface were excited and generated Si O and CO gas,a large amount of gas accumulation excited microwave plasma,caused local instantaneous high temperature,prompted the gas phase reaction of Si O and CO to form SiC crystal nuclei and O₂ gas.On the one hand,SiC crystal nuclei were deposited and grown on the surface of the added seed crystals to formed large particles.On the other hand,the newly generated O₂ diffused along the C@Si O2interface,and it drove the SiC crystal nuclei to aligned in the diffusion direction and continued to grow to form SiC whiskers.Al N generated gas during the heating progress and induced synthesis of SiC whiskers.SiC whiskers had better dielectric properties and EM absorbing properties than SiC particles.The results show that the optimal synthesis process of SiC with different carbon sources is slightly different,referring to the absorbing properties of SiC synthetic samples.The optimal synthesis temperature is about 1100 ? for 20 min when using industrial coal as carbon source.When using activated carbon and adding 10%Al N crystal seed,the best synthesis process is 1100 ? for 10 min.When the graphite sheets as the carbon source,the optimal synthesis process is 1300 ? for 10 min.