Study On The Synthesis Of Slicon Nitride Powder And Preparation Of Translucent Ceramic Block

Silicon nitride ceramics are usually used as a structural material with high strength at high temperature,well wear resistance and excellent chemical stability.However,its synthetic process of powder and the special performance,such as transparent performance,have been attracted more attentions.In order to study the effect of the process on the preparation of the silicon nitride powder,two synthetic processes were discussed in this paper,one is thermal reduction method and the other is self-propagating method.Transluent silicon nitride ceramics were prepared by SPS technology and the effects of the amount of the sintering aids,temperature on phase composition and transparency were studied.The phase composition and morphology of composite Si₃N₄ powder and ceramics were characterized by XRD,SEM and TEM,respectively.Si₃N₄ powder was prepared by carbon thermal reduction method using SiO₂ and carbon as raw materials and N₂ as high temperature reaction gas.The effects of reaction temperature,nitrogen flow,the molar ratio of carbon and silica on the Si₃N₄ powders have been investigated.The results indicates that:Si₃N₄ powders were successfully synthesized at the temperature of 1600?,nitrogen flow rate of 1 L/min and C:SiO₂ molar ratio of 3.5.The obtained Si₃N₄ powders are composited by 58 wt.% ?-Si₃N₄ and 42 wt.% ?-Si₃N₄.The ?-Si₃N₄ is a particle with an average diameter of 0.2 ?m,and the?-Si₃N₄ is a rod shape with a diameter of 0.5 to 1 ?m.The results of self-propagating reaction show that: when the amount of sintering aids is 2.4wt.% and N₂ pressure of 10 MPa,Si₃N₄ powder mixtured of ?-Si₃N₄ and ?-Si₃N₄ would be obtained,mainly for needle-like ?-Si₃N₄ crystal with particle size of several hundred nanometers and more than ten nanometers in diameter.Two different SiO₂ powder do not cause significantly influence on the product crystal in this paper.The translucent block ceramics have been sintered by SPS using ?-Si₃N₄ powder as the raw material and AlN/MgO/(Y2O³⁺La₂O₃)as sintering additives respectively.The results show that: at the 1700?,Si₃N₄ ceramics samples with the density and Vickers hardness are more than 99% and 22 GPa have been prepared.The relationship between the content of ?-Si₃N₄ and the amount of sintering aid is described as y=0.03162-0.0316x+0.00896x2.When phase composition of ? and ? phase content is close to 1:1,the Si₃N₄ ceramic transparency was the best.When the amount of sintering aids is 6wt.% and the sintering temperature is 1650?,the linear transmittance at 800 nm of 0.2 mm thick polycrystalline Si₃N₄ ceramics is about 29.7%.