硅酸盐学报

以自制AlON粉体为原料、莫来石为烧结助剂，结合无压烧结与热等静压烧结在相对较低的温度下(分别为1 880℃和1 800℃)制备了AlON透明陶瓷，研究了莫来石含量对无压烧结和热等静压烧结AlON陶瓷的显微结构以及光学性能的影响。结果表明，莫来石含量在0.02%～0.20%(质量分数)时，无压烧结AlON陶瓷为纯AlON相，其Raman光谱相似。随着莫来石含量增加，AlON陶瓷无压预烧体的致密度先增加后减小，而其晶粒尺寸逐渐变大。热等静压烧结后，所有样品的致密度均高于理论密度的99.2%且晶粒长大；其致密度、晶粒和透过率随莫来石含量增加呈现出先增大后减小的趋势。最终，0.05%莫来石AlON透明陶瓷致密度为3.67 g/cm³，平均晶粒尺寸为80.4μm，其透过率最高，4 mm厚样品在2 000 nm处达到81.9%。

Introduction g-aluminum oxynitride (g-AlON) is widely used for optical lens,transparent arms,and infrared windows due to its high transparency from the ultraviolet to mid-infrared range.To obtain a high-performance AlON transparent ceramic,each stage of its manufacture process should be controlled with cares,i.e.,powder synthesis,forming,sintering and annealing.Sintering aid (i.e.,Y_2O₃,CaCO₃,Y_2O₃–La_2O₃,etc.) is also used to improve the property of AlON transparent ceramic.However,the sintering temperature is still rather high,resulting in a high production cost.Recently,SiO₂ is used in AlON system,leading to a decreased temperature of hot isostatic pressing (HIP)(i.e.,1 810℃).Unfortunately,however,the evaporation of SiO decomposed from SiO_2could increase the micro-pores in pre-sintered bodies,leading to the density variations and then optical inhomogeneity after HIP.It is thus necessary to explore new Si-contained additive that is stable at a high temperature for the preparation of high-performance AlON transparent ceramics.Mullite (3Al_2O₃·2Si O₂) as a main phase of AlON has a superior high-temperature thermal stability,which is a promising application in AlON system.In this paper,AlON transparent ceramics were fabricated via pressureless sintering and hot isostatic pressing.In addition,the effect of mullite content on the microstructure of pre-sintered and hot isostatic pressed AlON ceramics and their corresponding optical property was also investigated.Materials and method An γ-AlON powder with 0.02%–0.20%mullite in alcohol was ground by a planetary mill with high-purity alumina balls.After drying,sieving and then calcining,the mixed powder was pressed into pellets and then cold isostatic pressed to obtain AlON green bodies.The green bodies were pressureless-sintered in a flowing N₂ atmosphere at 1 880℃for 4 h,and further hot isostatic pressed in a flowing Ar atmosphere at 1 800℃and 200 MPa for 6 h to obtain AlON transparent ceramics.The phase analysis of specimens was identified by a model D8 Advance X-ray diffractometer (XRD,Bruker Co.,Germany)using nickel-filtered Cu Kα radiation (λ=1.540 6?).The bulk density of AlON ceramics was determined by a water immersion method based on Archimedes’principle.The chemically-etched and fracture surfaces of AlON ceramics were characterized by a model TM3000 scanning electron microscope (SEM,Hitachi Co.,Japan).The average grain size was determined by a linear intercept line method with a correction factor of 1.56 via a software named Nano Measurer.The Raman spectra were recorded by a model XploRA One-532 Raman spectrometer (Horiba Co.,Japan) using a 532 nm Ar~+laser.The optical in-line transmittance spectra of HIPed Al ON ceramics were measured by a model V-770 UV-Vis-NIR spectrophotometer (JASCO Co.,Japan) and a model FT/IR-4600 Fourier transform infrared spectrometer (FTIR,JASCO Co.,Japan).Results and discussion The ground AlON powder mixed with mullite has a homogeneous distribution and the particle sizes are in the range of 0.2–1.6μm with an average value of 0.7μm.After pressureless sintering,all the XRD patterns of AlON ceramics with0.02%–0.20%mullite match well with the standard cubic AlON phase (JCPDS 48-0686),and no extra peaks appear,indicating the dissolution of mullite in AlON lattice.The Raman spectraindicate the effect of the mullite content on the structure of the pre-sintered Al ON ceramics.The phonon modes located at 316,372,628,764 cm^–1 and 920 cm^–1 appear in all the samples,which are corresponded to the 3T_2g,E_g and A_1g modes of spinel.No shift of the Raman bands occurs and new bands do not appear,indicating the same structure and phases in AlON with different mullite contents.All the pre-sintered AlON ceramics have a milk-white color and are opaque.The microstructure of fracture surfaces indicates that pores easily appear in all the samples.Large pores appear and density decreases in AlON ceramics with 0.10%–0.20%mullite.The grain size increases steadily with increasing mullite content.For the low content and high-temperature stability of mullite,a reaction of solid-state sintering is proposed as■.Furthermore,for an oxygen-richAlON phase in the formation of AlON,the covalent bond strength of Al-N is higher than that of Al-O,and the diffusivity of oxygen is faster than that of nitrogen.AlON with a higher Al_2O₃ content is easy to sintering,leading to the grain growth.Al_2O₃ content increases and grains grow with increasing mullite content.The grains grow fast at a high mullite content,resulting in some trapped pores within the grains.All the hot isostatic pressed AlON ceramics have a density of>99.2%of the theoretical value and a high transparency.The density,grain size and transmittance of hot isostatic pressed AlON ceramic firstly increase and then decrease with increasing mullite content.AlON transparent ceramic doped with 0.05%mullite exhibits the maximum transmittance of 81.9%(4 mm thickness) at 2 000 nm,with a density of 3.67 g/cm³ and an average grain size of 80.4μm.Some pores in the grains appear in pre-sintered bodies at a higher mullite content,inhibiting the grain growth during the HIP process and desceasing the grain size.Also,some pores in the grains cannot eliminate and remain after HIP,degrading the transmittance.Conclusions AlON transparent ceramics were fabricated via pressureless sintering at 1 880℃and hot isostatic pressing (HIP) at1 800℃,respectively,with AlON powder and mullite.,The same structure of AlON with different mullite contents of 0.02%–0.20%could be obtained.AlON transparent ceramic with 0.05%mullite exhibited the maximum transmittance of 81.9%(4 mm thickness) at2 000 nm.At a mullite content of>0.10%,some pores in grains appeared in pre-sintered bodies,inhibiting the grain growth in the HIP process.