Investigation On Seeds-induction Effect During The Preparation Of SiC Crystals By Microwave Heating

The microwave heating method is the most likely to achieve stable heating and mass production because of its unique advantages in the many synthesis methods of SiC.However,the relative theory and the technological stability are not enough,so it is urgent to research systematic theory.SiC crystals were synthesized with seed-induction by the traditional synthesis method which could reduce the heating temperature,improve the purity and reduce the total synthesis time.In the world,the method of microwave heating with seed-induction for preparation of SiC has not been found.Just under such a background,?-SiC particles(0.6?m)were used as heating seeds.And the coal mineral particles(?0.15mm)were commercially available as carbon source,tetraethoxysilane as silicon source.Above all,a series of effect mechanisms and the suitable heating process were investigated during the microwave heating without seeds.Afterwards the preparation process of SiC with seeds,seed-induction effect and SiC crystal growth mode were investigated.The preparation of SiC with seeds by microwave heating was expected to become a new method to further improve the theoretical system research of microwave heating.Firstly,SiC was synthesized during the preparation of SiC without seeds at-600?,-900?,?1100?,and?1100? with holding time of 10 min,respectively.The perfect SiC crystal was synthesized at 1100? for 10min which could ensure enough temperature and reaction time for SiC nucleation and growth.Four consecutive heating stages can be distinguished according to the different chemical or physical changes,i.e.,stages of heat accumulation,thermal runaway,complex reactions and grain growth.Then a-SiC particles(?0,6?m)were used as heating seeds to prepare SiC from coal minerals with microwave heating method at 1100? for 10min.Coating technique was carried out to prepare composite raw powders in three different methods.It was found that different distribution between C and a-SiC particles from different mixing method leads to different microwave heating behavior and growth mechanisms.V-V reaction leads to in situ nucleation and grain growth on the surface of a-SiC seeds which contact with C particles.Well-grown P-SiC particulates appear.Hybrid V-V reaction V-L reaction lead to local microwave plasma and diffusion-precipitation with a very thin layer of SiO2 between raw C particles and a-SiC seeds.Local fine P-SiC whiskers and particulates on the surface of a-SiC seeds co-exist.Primary V-V reaction leads to nucleation and grain growth along reacted C-SiO2 interface with very thick layer of SiO2 between raw C and a-SiC seeds.Substantial P-SiC whiskers appear.So the seed-induction effect is obvious for mixing method I.Moreover,different temperatures and holding time will also affect the growth process of SiC crystals.P-SiC and a-SiC particles were synthesized at 1100? for 10min under the method I.With duration of holding time,the content of whisker increased.The crystal growth mode was mainly V-V reaction to form SiC crystallites on the surface of a-SiC seeds by in situ nucleation and grain growth.Finally,different contents of seeds also affect the properties of SiC and thermal effects in the process of seed-induction synthesis of SiC crystals.It was found that different impetence matching between coal minerals and a-SiC seeds showed crucial influences on power redistribution,which caused different heating rate,phase and morphology during the microwave heating.And 15(wt.%)seeds content was the critical value for both microwave coupling effects and power redistribution.