Study On Preparation Of Silicon Nitride And Silicon Oxynitride By Rapid Reaction Synthesis

Silicon nitride material has excellent wear resistance,high temperature resistance,corrosion resistance and thermal shock resistance.It is widely used in the fields of chemical,energy and aerospace.The silicon nitride material has high toughness and higher thermal shock resistance.It is a new material with the potential of application and development.There are two structures of silicon nitride:alpha phase and beta phase.Generally speaking,the main phase is alpha phase.At present,the main technology of silicon nitride powder production is silicon powder direct nitridation,that is to say,under certain temperature,silicon powder and nitrogen or ammonia gas react with solid gas.This method has a difficult point.When the reaction is too fast,the proportion of the beta phase will be increased,so the reaction rate has to be strictly controlled in production,which makes this method have the problem of time consuming and electricity consumption.It usually takes 100-200hours to complete a furnace.Silicon oxynitride has not yet formed mature industrial production.The laboratory mainly adopts the direct nitridation method of silica fume and quartz powder.This reaction involves two solid phase reactions,which takes longer time,and is often caused by two solid phase contacts.To solve these problems,we conducted an experimental study using doped Ca F₂to accelerate silicon direct nitridation and alpha phase formation.The results show that in the temperature range from 1200℃to 1300℃,the conversion rate and the alpha phase of silicon powder increase with the increase of reaction temperature,and the initial nitriding temperature needs to be controlled at 1300℃.When the ammonia is the reaction atmosphere,the nitriding reaction is more sufficient,and there is a higher content of alpha phase and the more homogeneous in each part;with Ca F₂With the increase of proportion,more and more spherical granular silicon nitride is in the shape of the product,and the proportion of alpha phase is higher,but the phase transition process will occur from alpha phase to beta phase after the ratio is over 4%.With the prolongation of the heat preservation time,the silicon powder has a higher conversion rate.Compared with the middle and later stage,the prolongation of initial holding time has greater influence on the rapid nitridation of silicon powder and the synthesis of alpha phase.In this paper,1300℃ball milling with 4%Ca F₂ is used for4 hours heat preservation.Compared with the current method,the synthesis period is greatly reduced,and the product is submicron regular high alpha phase silicon nitride.In the rapid reaction synthesis of silicon oxynitride,we have proposed and studied a method of thermal explosion synthesis,that is,the rapid temperature rise by the mixture of submicron silicon powder and ten micron grade quartz powder in nitrogen gas.The rapid reaction is realized by the strong exothermic characteristics of the system and the melting of silicon.The results show that the reaction time of the thermal explosion is more common with the furnace reaction,which greatly reduces the reaction time and improves the synthesis efficiency.With the increase of the preheating temperature,the amount of residual silicon and quartz is reduced,the amount of silicon oxynitride is gradually increased,and the amount of residual silicon is reduced and the nitrogen oxidation is reduced as the time of thermal explosion is prolonged.With the increase of silicon and the increase of cell volume,the recovery and oxygen content of silicon oxynitride decreased with the increase of water deposition time.The suspended particles after water deposition all obtained a single silicon oxide diffraction peak.After dispersing 5min and settling 120s through ultra pure water,the residual quartz in the nitrogen oxidation product can be removed.The purity of the nitrogen oxide in the product is high,and the oxygen content is 16.2%,which is close to the theoretical oxygen content of the silicon oxide.