Purification Of Aluminum Nitride Ceramics By Pickling And Thermal Treatment Processes

Aluminum nitride（AlN）ceramic is an ideal encapsulating and heat dispersing material with excellent properties and widely used in large-scale integrated circuits and high-power electronic devices.However,its application and development are limited by its characteristics of easily becoming oxidized and being difficult to sinter.In order to obtain high purity and high performance AlN ceramics,the AlN powders are purified by means of acid-pickling,and plasma activated sintering（PAS）is used to densify the acid-pickled AlN powders and thermal treatment is used to remove impurities and improve performance after sintering.Firstly,using AlN nanopowder as raw material,the effect of pickling process（acid type,acid ratio,pickling time,pickling temperature,etc.）on its impurity content,phase composition and microstructure was systematically studied.The results show that inorganic acids such as H₃PO₄,HNO₃,HF,and HCl can significantly remove metal impurities.HF can also reduce the oxygen content in the powder by reacting with Al₂O₃,which is mainly oxygen-containing impurity.Using the HF:HNO₃=2:1 ratio to arrange the pickling solution to pickle the AlN raw material powder at 50°C for 5 hours,which can significantly reduce the metal impurity content in the AlN powder and effectively remove oxygen impurities.After pickling,the content of metal impurities dropped below 100 ppm and the oxygen content was only 1.2wt%.Then,using acid-pickled AlN powders as raw material,the effects of PAS process（sintering temperature,axial pressure,holding time,etc.）on the sintering process,density,phase composition and microstructure of the sample were investigated.The results show that increasing the sintering temperature,increasing the axial pressure,prolonging the holding time,are all favorable for obtaining dense AlN ceramics.The PAS process suiTab.for acid-pickled AlN powders was determined:1800°C-45MPa-10min.Under this condition,AlN ceramics with high purity（oxygen content<0.4wt%）and denseness（>99.8%）were obtained.Finally,the sintered AlN ceramics are subjected to high temperature thermal treatment.The effects of thermal treatment（temperature and holding time）on the impurity content,phase composition,microstructure and thermal conductivity of the ceramics are studied.Through research,it has been found that thermal treatment can effectively reduce oxygen impurities in AlN ceramics,purify grain boundaries,promote grain growth and development,thereby enhancing the purity of ceramics and improving their thermal conductivity.AlN ceramics have been thermal-treated at high temperatures（2200°C）for a long time（20h）,their oxygen content has dropped below220ppm,and their thermal conductivity has increased by 28%.