Studies On Syntheses,Characterization And Ceramic-making Verification Of High Performance AlN Powders

With the rapid development of modern electronic technologies,microelectronic and power electronic devices are developing towards miniaturization,integration,high power density,high reliability and so on.It puts forward higher and higher requirement of heat dissipation capability of the devices.AlN ceramics have many advantages,such as high thermal conductivity,low dielectric loss and dielectric constant,good insulation performance,thermal expansion coefficient matching with Si,Ga As and other semiconductor materials,excellent mechanical properties,safety and non-toxicity.It have become one of the most widely used electronic packaging materials in power electronic devices.Of course,high-quality AlN powder is an essential material for preparing high-performance AlN ceramic substrates.Research and development of such high-quality AlN powder is extremely valuable,theoretically and practically.In this study,the AlN powder were synthesized using ultrafine Al₂O₃powder and nanometre carbon black as main raw materials via a carbothermal reduction nitridation（Carbothermal reduction nitriding process,referred to as CRN）technique.Characteristics of precursors,calcining conditions and decarbonization of the CRN process were experimentally and thermodynamically investigated.Mesophase product and its formation mechanism during synthesizing the AlN powder were also clarified.And thus,the optimized CRN processing parameters were put forward.Properties and microstructures of the AlN ceramic substrates prepared using the AlN powders with different O and C contents were studied comparatively to indicate the effects of the O,C contents for the AlN ceramics.In the CRN process,porous precursors were prepared by discharging at 360℃for 2h.The pores are conducive to gas exchange and transportation in the precursors,and therefore promote the synthesis reactions.When the calcination temperature is above1650℃,the synthesis reactions are favored thermodynamically.As the temperature risen to 1700℃,the thermodynamic driving force of the reactions increases to accelerate the CRN process.It was determined by the orthogonal test that the temperature had the greatest effect on decarbonization of the AlN powder,and the feed rate was in the second rate.After the porous precursors calcined at 1700℃for 12 h and then decarburized at 700℃under the conditions of the air to N₂ratio of 1:0 with the feed rate of 4 kg/h,the AlN powder with a composition of 0.77 wt%O,33.42 wt%N and 323 ppm C and an average particle size of 1.36μm was obtained.The synthesizing process of the AlN powder includes two subprocesses,i.e.,the reduction of Al₂O₃at the CO reduction atmosphere and then the reaction of the reduction products with N₂to form AlN.The formation of mesophase product of Al₂CO likely takes place.It is the main factor resulting in the increase of the contents of O and C impurities of the AlN powder.Based on the thermodynamic calculations,the partial pressure functions of P_COunder the varying equilibrium conditions were deduced,and thus the phase equilibrium state diagram was drawn at 1700℃.The formation mechanism of the mesophase Al₂CO was therefore determined.As a result,the AlN synthesizing reaction was promoted and the formation of mesophase product of Al₂CO was inhibited through enlarging the flow rate of N₂.The AlN ceramic substrates were prepared via a pressureless sintering technology using the AlN powders of different qualities synthesized by the CRN method as raw materials.The AlN grains grow well and contact tightly with each other.The secondary phase such as Y₃Al₅O₁₂（YAG）distribute randomly at the AlN grain boundaries.With the increase of the O and C contents in the AlN powders,the AlN grain size/the secondary phases of the AlN ceramic substrates decreases/increases,and also,the thermal conductivity/the flexural strength of the AlN ceramic substrates decreases/increases correspondingly.The thermal conductivity of the AlN ceramic substrates arrives at 176.3 W/（m·K）,which reaches the international advanced level,using the AlN powder of 33.6 wt%N,0.85 wt%O and 463 ppm C as raw material.