Study On Controlled Synthesis Of Aluminum Oxynitride Powders And Properties Of Transparent Ceramics

Aluminum oxynitride（AlON）is a solid-solution formed in pseudobinary Al N–Al₂O₃systems.AlON transparent ceramics have the advantages of high hardness,high strength,corrosion resistance,high temperature resistance,wear resistance,and excellent light transmittance in the near ultraviolet to mid-infrared band（0.2～6μm）,which makes it have important application prospects in missile seeker,imaging device,airborne photoelectric pod and lightweight transparent armor.The research on AlON transparent ceramics in China has been accumulated for more than ten years,but the large size AlON transparent ceramics with high optical quality has not been prepared at present.The main reason is that the accuracy control of AlON components has not been realized.The synthesis of AlON powders is a kind of gas-solid-solid three-phase reaction,which is easy to cause surface volatilization and insufficient reaction inside,leading to the heterogeneous of AlON powders,and finally causing the decline of optical properties of AlON transparent ceramics,which cannot meet practical applications.In this paper,the influence of composition on the densification behavior and properties of AlON transparent ceramics and the preparation of AlON transparent ceramics with large size and excellent optical properties were studied,and by optimizing the ball milling process of AlON powder and sintering process of AlON transparent ceramics,excellent optical properties AlON transparent ceramics with a size of 200 mm×160 mm×10 mm were prepared.All these results will make contributions to making larger size AlON transparent ceramics and realizing its practical applications.Specific research contents and results are as follows:（1）Accuracy control of AlON componentsAlON powders were prepared by two-step carbothermal reduction nitriding method,and the fine control of AlON powders was realized by optimizing raw materials and reaction conditions:First,selectin of Al₂O₃ raw materials.Four kinds of Al₂O₃powders were selected to prepare AlON transparent ceramics respectively.Combined with the mixing effect of raw materials,the adequacy and stability of the synthesis reaction of AlON powders and the optical quality of AlON transparent ceramics,suitable Al₂O₃ powder was determined as raw materials.Secondly,two-step method was employed to prepare AlON powders.It includes carbothermal reduction reaction and solid solution reaction.During the carbothermal reduction reaction,the reaction conditions such as nitrogen concentration and holding time were optimized by real-time monitoring of CO concentration to ensure adequate reaction.Residual carbon in the intermediate product is only about 0.1 wt%,indicating that the reaction is sufficient.During the solid solution reaction,the intermediate product is placed into a closed crucible for reaction to ensure the stability of atmosphere and effectively prevent the volatilization of components.The lattice parameters fluctuation of the prepared AlON powder is less than 0.04%,and AlON component is accurately controlled.Third,after realizing the control of AlON components,six kinds of AlON powders were prepared with the content of Al N in the range of 21.6～33.8 mol%and the lattice parameters in the range of 7.941～7.951（?）.（2）Effect of composition on densification behavior and properties of AlON transparent ceramicsOn the basis of realizing AlON components accurately controlled,the effects of composition on AlON powders and the properties of AlON transparent ceramics were studied.The results show that:The composition has no obvious effect on the morphology and particle size distribution of AlON powder.When the sintering temperature is lower than 1650°C,part of AlON decompose into Al N andα-Al₂O₃phase,and then Al N andα-Al₂O₃ phase are resynthesized into AlON phase,and become pure AlON phase at 1850°C.In the process of resynthesizing AlON phase,the volume change of ceramic compact is related to the content of Al N.The volume of ceramic compact contracted with Al N content in the range of 21.6～27.5 mol%,but the shrinkage rate slowed down,while the volume of ceramic expanded with Al N content in the range of 29.7～33.8 mol%.With the increase of Al N content,the density,Vickers hardness,fracture toughness,bending strength,dielectric constant and refractive index of AlON transparent ceramics all increased,and the grain size of AlON transparent ceramics decreased.When the content of Al N is in the range of 21.6～29.7 mol%,AlON transparent ceramics with high optical transmittance（82～85%transmittance at3600 nm for 4 mm thick samples）can be obtained by pressureless sintering at 1950°C for 30 h.（3）Preparation of AlON transparent ceramics with large size and high optical qualityOn the basis of realizing the control of AlON composition and optimizing the preparation process of AlON transparent ceramics,the AlON powder containing 27.5mol%Al N shaped by dry pressing and presureless sintered at 1950°C for 30 h and combined with HIP（1800°C×3 h）.AlON transparent ceramics with a size of 200 mm×160 mm×10 mm with high optical quality was successfully obtained.