Preparation And Optical Properties Of Aluminum Oxynitride Transparent Ceramic

Aluminum oxynitride（AlON）transparent ceramic is a new transmission window material developed in recent decades.It is optically isotropic,and its mechanical properties are similar to sapphire.But the manufacturing cost of AlON transparent ceramic is lower than sapphire,and it’s easier to fabricate in a large size.In order to prepare highly transparent AlON transparent ceramics,hot isostatic pressing sintering method is widely used at home and abroad.Hot isostatic pressing has strong sintering driving force,which makes it easier to prepare highly transparent AlON transparent ceramics.However,the high cost of hot isostatic pressing equipment and its use and maintenance are important factors that pushed up the cost of AlON transparent ceramics.Pressureless sintering is an effective method to realize low-cost preparation of AlON,but the driving force of pressureless sintering is weak,so it is difficult to prepare highly transparent AlON transparent ceramics by pressureless sintering.In view of the above problems,this paper comprehensively optimized the preparation process of AlON transparent ceramics based on pressureless sintering from the three processes of powder preparation,forming and sintering.The method of rapidly synthesis of high purity and high activity AlON transparent ceramic powders at low temperature was systematically studied.The effects of spray granulation and cold isostatic pressing on the microstructures and properties of AlON green body and ceramic were analyzed.And the characteristics and advantages of cold isostatic pressing,tape casting and slip casting technologies in forming AlON green body were compared.The effects of sintering parameters and sintering additives on the microstructures and properties of AlON ceramics were discussed,and the evolution rules of microstructure during sintering of AlON ceramics were analyzed.The optical,mechanical and thermal properties of AlON transparent ceramics were systematically tested and analyzed.The main research works are as follow:1)Al₂O₃ and AlN raw powders were mixed by two-step ball milling method,which improved the reaction activity of AlN raw powders,realized the uniform mixing of Al₂O₃and AlN raw powders in submicron scale,and avoided the hydrolysis of AlN raw powders.When the calcination temperature was 1700℃for 2h,the molar content of AlN corresponding to single-phase AlON region was 26%～28%,which provided a reference for finding components more suitable for low temperature rapid synthesis of AlON powders.For an AlN molar content of 27%,single-phase AlON powders can be obtained by calcining at above 1660℃for 2h.When the calcination temperature was 1680℃,single-phase AlON powders can be prepared by holding for20min.The median particle size of AlON powder was 410nm after ball milling at250rpm for 24h.2)The spheroidization of AlON powder was carried out by spray drying.When the solid contents were in the range of 30～50wt%,the diameter and density of AlON sprayed granules improved significantly with the increase of solid content.The AlON green bodies were fabricated by cold isostatic pressing of sprayed granules.Within a certain range,the density and uniformity of green body were improved,and the average pore diameter of green body was reduced with the increase of forming pressure.The results showed that when cold isostatic pressing pressure was greater than 200MPa,the density and average pore diameter of the green body remained unchanged and the density and transmittance of AlON transparent ceramic was not significantly affected.It was found that AlON green body formed by slip casting had poor compactness and uniformity,wide pore size distribution and large pore size.However,AlON green bodies formed by cold isostatic pressing and tape casting were more compact and uniform,the pore size distributions were narrower and the average pore sizes were smaller.3)The shrinkage and density of AlON ceramics increase with the increase of sintering temperature.When the sintering temperature reached 1800℃,the green body was basically dense and the relative density reached more than 98%.When the sintering temperature was 1700℃and over,AlON ceramic was in the middle stage of sintering.When the sintering temperature was 1750℃and over,the sintering of AlON ceramic entered the final stage of sintering.According to the change rule of AlON microstructure in the final stage of sintering,the pores remove modes were analyzed:1.Pores were removed during grain boundary migration.2.Pores gradually migrated to grain boundaries through lattice diffusion and removed.With the increase of temperature the grain size increase and the diffusion path of pores was prolonged,which was unfavorable to the elimination of pores.It is necessary to control the sintering temperature and time to control the overgrowth of grains.In terms of sintering additives,the addition amount of La₂O₃and Y₂O₃should moderate.The optimal addition amount of La₂O₃was 0.03wt%,and the optimal addition amount of Y₂O₃was 0.4wt%.4)The effect of particle size on the properties of AlON transparent ceramics was studied.The result showed that the particle size should be matched with the sintering system to make the migration rate of grain boundary and pore fit in the process of sintering densification,so as to reduce the formation of pores in grains.The pores of the greenbody formed by CIP and tape-casting are easily discharged when sintering,and the transmittance of the transparent ceramics is more than 80%.The effect of pore size on the density sintering of transparent ceramics is relatively weak,but the effect of pore size distribution uniformity is more obvious.5)AlON transparent ceramics with a diameter ofΦ170 mm plate andΦ110 mm dome were fabricated by pressureless sintering.The average transmittance of AlON transparent ceramic was>82%in the visible band and>84%in the mid infrared band,which were close to the theoretical transmittance The optical properties of AlON transparent ceramics such as transmittance,refractive index,haze were tested and analyzed.At the same time,the mechanical and thermal properties of AlON transparent ceramics were tested.