| β-Si3N4whisker is a rod-shaped single crystal synthesized under artificially controlled conditions.Because of its few internal defects,excellent mechanical and dielectric properties,it can give the composite material better performance.Theβ-Si3N4whisker with high aspect ratio is an ideal reinforcing phase because of its crack deflection,crack bridging and grain pull-out mechanism in the composites,which can greatly improve the fracture toughness of the composites.However,β-Si3N4whiskers with high aspect ratio require extremely high heat treatment temperature or high content of additives to prepare,which greatly increases the industrial production cost.In this paper,the most commonα-Si3N4powder was used as the main raw material,and metal oxides were introduced as additives.Theα/βphase transition process of Si3N4was studied by adjusting the experimental temperature,holding time,additive content and other factors.The growth mechanism ofβ-Si3N4whiskers was analyzed,and the experimental factors were optimized to obtain the heat treatment process for preparing the best morphology ofβ-Si3N4whiskers.The effects of various additives on the phase transformation process of Si3N4and the formation process of whiskers were discussed.Several methods for preparingβ-Si3N4whiskers with high aspect ratio were proposed.The main contents of this paper are as follows:（1）In this paper,α-Si3N4was selected as the main raw material,Yttrium oxide（Y2O3）,lanthanum oxide（La2O3）,samarium oxide（Sm2O3）,dysprosium oxide（Dy2O3）,lutetium oxide（Lu2O3）,alumina（Al2O3）,magnesium oxide（Mg O）,calcium oxide（Ca O）as additive.In the sample preparation process,the ball mill was used to mix the materials well,the slurry was dried in a drying oven,and finally the tube furnace was used for heat treatment.The test characterization part mainly analyzes the phase composition and microstructure.（2）Theα-Si3N4powder was used as the main raw material,different rare earth oxides（Re2O3）were introduced as additives,and the protective gas of the heat treatment process was set to different gases.Through the analysis of the microstructure and phase composition of the whiskers,the phase transition process of Si3N4and the formation process of whiskers were discussed,so as to explore the most suitable reaction conditions for the preparation ofβ-Si3N4whiskers.The results show that high purity nitrogen is the most suitable protective gas for the heat treatment process,which can inhibit the decomposition of Si3N4at high temperature.Y2O3is more suitable for the preparation of high aspect ratio whiskers than other Re2O3.（3）In this paper,α-Si3N4powder was used as the main raw material,and Y2O3was introduced as an additive.By adjusting the temperature,additive content and holding time of the heat treatment process,the effects of various factors on the phase transition process of Si3N4and the formation process ofβ-Si3N4whiskers were analyzed,so as to explore the most suitable reaction conditions for the preparation ofβ-Si3N4whiskers and realize the controllable synthesis of whiskers.The results show that increasing the heat treatment temperature,prolonging the holding time and increasing the additive content can effectively promote the phase transformation of Si3N4and the formation ofβ-Si3N4whiskers.Increasing the heat treatment temperature or increasing the additive content can effectively increase the aspect ratio of whiskers,and prolonging the holding time is more effective in increasing the particle size of whiskers.（4）Y2O3was introduced as the first phase additive,and Al2O3,Mg O and Ca O were introduced as the second phase additives,respectively.β-Si3N4whiskers were prepared by direct heat treatment ofα-Si3N4powder.By changing the proportion of the second phase additive in the composite additive and the temperature and holding time of the heat treatment process,and analyzing the SEM images and XRD results,the effect of the introduction of the second phase additive on the phase transformation process and whisker formation process of Si3N4was discussed.The results show that the introduction of Al2O3can reduce the phase transition temperature by about 100°C.The role of Y2O3is to orient the growth direction ofβ-Si3N4whiskers.The role of Al2O3is to reduce the liquid viscosity,accelerate mass transfer and promote whisker growth.The addition of Mg O can form a liquid phase with lower temperature or viscosity,which can effectively promote the formation ofβ-Si3N4whiskers and the phase transition fromα-Si3N4toβ-Si3N4.Theβ-Si3N4whiskers after phase transformation still grow in the form of Oswald ripening.Ca O can effectively promote the growth process of whiskers and improve the effect of rare earth atoms（Re）on inhibiting the radial growth of whiskers.Ca O can make Si3N4rapidly formβ-Si3N4crystal nucleus,which is beneficial to the preparation of abnormally grownβ-Si3N4whiskers. |
