Research On Synthesies,Sintering And Properties Of Nanosized Aluminum Nitride Powders

Due to the excellent properties,aluminum nitride(AIN)has a very wide ap-plication and great potential in the fields of integrated circuits,semiconductors,microwave devices,infrared windows,etc.The existing production process of AIN powder has serious problems,such as high reaction temperature,difficulty in uniformly mixing raw materials and obtaining raw materials,and high energy consumption.Solution combustion synthesis(SCS)is developed to an effective method for preparing ultrafine nano powders in recent years,which has many ad-vantages such as high powder activity,uniform composition,easy access to raw material and lower energy consumption.This paper will use this method to syn-thesize AlN nanopowders.The research mainly includes the following aspects:(1)The amorphous precursor of Al2O3+C was prepared by SCS.The NH3 was used as the reaction gas.The ultrafine AIN nanopowders were successfully prepared.The effects of combustion atmosphere on the particle size morphology,specific surface area and carbon content of the combustion synthesis precursors have been studied in detail.The differences of the carbothermal reduction process of the precursors combusted in different atmospheres have been compared.The precursor combusted in Ar has a specific surface area of 7.4m2/g and a carbon to aluminum atomic ratio of 4.4:1.The AIN is prepared by calcining the precursor at 1000?.Complete nitridation is achieved after nitriding at 1300? for 2 h.Ul-trafine AIN powders is successfully prepared with particle size of 20 to 30nm.The density is 98.5%after sintering at 1600? for 4 h with additive free.The thermo-dynamic calculation results show that the theoretical initial nitriding temperature between amorphous Al2O3 and NH3 is 1068?.The initial nitriding temperature of the amorphous Al2O3 is 50? lower than that of y-A1203.(2)The phase changes of the precursors in the low temperature nitriding,the effects of C on the phase transition and nitridation of Al2O3 and the nucleation and growth of AIN particles have been studied.With N2 as reaction gas,the initial ni-triding temperature of the SCS precursor is 1300?,which is 200?lower than that of the mechanically mixed precursor.The SCS precursor achieves complete nitridation by nitriding at 1500?for 2 h,and AIN nanopowders are obtained with particle size of 80?100 nm.During the carbothermal reduction reaction,the C particles not only play the role of reducing the Al2O3,but also inhibit the conver-sion of Al2O3 from the high-activity crystal form to the stable crystal form.In the process of carbonothermic reduction reaction of SCS precursor,amorphous Al2O3 gives priority to carbonothermic reduction reaction with C particles to form AIN before transforming into crystallized Al2O3,which will undoubtedly accelerate the nitriding reaction The crystallization process of the amorphous AIN is uneven.The formation of AIN grains is mainly divided into the following stages.In the initial stage of nitridation reaction,the amorphous AIN firstly nucleates uniformly on the flake particles of the SCS precursor,and then the amorphous AlN in the local region takes priority in crystallization transformation.With the development of nitriding reaction,nucleation,crystallization and growth of AIN occur contin-uously.Finally,when all the Al2O3 transformes into AlN,complete nitriding is achieved.(3)The sintering densification of as-synthesized AIN nanopowder by SCS have been studied.The effects of sintering aid,pressing pressure and pretreatment on relative dentsity,micro structure and ceramic property have been researched.The relative density is 98.4%of the additive-free AlN nanopowder with pressure-less sintering at 1700? for 4 h.The Vickers hardness and flexural strength of the AIN ceramic are 1040HV0.3 and 277.41MPa,respectively.When 3%Y2O3 is added into AlN nanopowder as sintering aid,the relative density is 99.5%with pressureless sintering at 1600? for 4 h.The Vickers hardness and flexural strength of the AlN ceramic containing 3%Y2O3 reaches 1310HV0.3 350.34MPa,respectively.High relative density of AIN ceramic can be obtained under low pressing pressure.However,if the pressing pressure is too high,elastic expansion pores and cracks in the green body will develop after pressure release.Thus the dentsity and mechanical property of the AIN ceramic will be decreased.The pre-treatment reduced the oxygen as well as carbon content of the degreased body.And with the decrease of oxygen content of AIN ceramics,the second phase changes from aluminum-rich salt to yttrium-rich salt.The pretreatment process improves the flexural strength and thermal conductivity of the AIN ceramics.Among the pretreated sintering samples,the bending strength of the sintered ce-ramic pretreated at 1300? for 5h reaches the highest value of 549.28mpa,which is 59.0%higher than that of the untreated AIN ceramic.The thermal conductivity of the AIN ceramic pretreated at 1500?for 10h reaches the highest value of 144.98w/m·K,which is 36.5%higher than that of the untreated AIN ceramic.