Growth Mechanism And Manipulation Of Self-propagating High-temperature Synthesis Of Si₃N₄

Particle size and phase composition are two main quality index of synthesis of silicon nitride.Self-Propagating High-temperature Synthesis is one of main method of Si₃N₄ powder with advantages of low energy consumption,non-pollution,high purity and low-cost.Synthesis of silicon nitride powder exist accumulation growth phenomena,for the diluents in the raw materials is added to silicon nitride.Ascertainment the way of silicon nitride grains grown up has great theoretical and practical significance,which can control powder growth,reduce late grinding costs and reduce impurity mixed.At the same time,self-propagating high-temperature synthesis of silicon nitride has the characteristics of high temperature and fast reaction,how to effectively control its phase composition is the other important technology issues.The silicon nitride powder synthesized by SHS contains alpha and beta phase.By adjusting the ratio of raw materials,particle size, reaction pressure can controlαandβphase ratio changes in silicon nitride powder.In this paper,it is a primary goal to research the growth mechanism of Si₃N₄ powder by SHS and the formation causes of some grains of particular shape.Through designing the cumulative experiments,placement in layers experiments and adding silicon dioxide experiments,the Si₃N₄ powder is obtained;through using SEM and XRD,the composition and content of alpha and beta phase are obtained;and the formation mechanism and varying rule of alpha and beta phase are analyzed.By using placement in layer change NH₄Cl content of layers and add SiO₂ powder changed the response system's temperature, can reduce the particle size and increased alpha phase content.The results show that the beta phase silicon nitride synthesized by SHS grows on silicon nitride diluents by the(?)action of silicon liquid phase and nitrogen.Alpha phase synthesize by gas growth.High temperature promotedα-Si₃N₄ intoβ-Si₃N₄ and the growth of grains rapidly.The placement in layers effectively reduced the response system's temperature;hampered powder grew up,and improved alpha phase powder content.When SiO₂ content in the middle layer was 5%,and contents of above and bottom layers were 2.5%,content of alpha phase reached 18.73%.SiO₂ increases the impurity oxygen content and reduced liquid Si formation temperature decreased the content of gas Si,reduced alpha content.In high temperature to added SiO₂ unable to impede rapid growth and improved powder alpha content by absorbing heat and reducing the temperature of the system.