Study On The Preparation Of Aluminum Nitride By Carbon Thermal Nitriding Of Aluminum Oxide Under Vacuum

Although alumina carbon thermal nitriding reduction has been industrialized,which need a relatively high temperature and a long preparation time.However,the high remperature and long preparation time results in low production efficiency and high production cost.Therefore,for theese two prominent problems,this paper adopts ab initio molecular dynamics simulations?AIMD?to investigate the mechanism of carbon thermal reduction nitridation.Based on the results of AIMD,a large number of experimental have been carried out in order to provide a theoretical and experimental basis for the preparation of aluminum nitride by alumina carbon thermal reduction and nitridation under vacuum.In this paper,the thermodynamic calculations of mian reactions that may be involved in the nitridation of alumina carbon thermal products are carried out under atmosphere pressure and 300 Pa.It was found that reducing the pressure is not only beneficial to the formation of AlN but also to the formation of by-products such as Al₂CO,Al₄C₃ and Al₂O?g?,and these by-products are easily to be converted into aluminum nitride under a nitrogen atmosphere and 300Pa.Then,the AIMD was used to investigate the Al₂O₃-N₂-C system.The results showed that the process of reduction and nitridation occur simultaneously during the process of preparing aluminum nitride by alumina carbon thermal reduction and nitridation and the reaction of carbon thermal reduction for alumina is Al₂O₃+3C=2AlN+3CO?g?.The results of experimental investigation on the alumina carbon thermal nitridation under vacuum showed that the optimum conditions for the synthesis of aluminum nitride were temperature of 1550?,pressure of 300 Pa,and holding time of 2.5 h.When the molar ratio of Al₂O₃/C is 1:3¹:4,the carbon ratio has little effect on the reaction.2g 200 mesh raw material which with molar ratio of Al₂O₃/C of 1:3.25 can be completely converted into aluminum nitride at temperature of 1550?for2.5 h and pressure of 300Pa.Compared 1g raw material preparation at atmosphere?1600?,5 h?with that at pressure of 300Pa,the preparation of aluminum nitride under 300Pa can effectively reduce the preparation temperature and the preparation time.Combined with the results of experimental investigation,thermodynamic calculation and AIMD simulation,the possible reaction mechanism of alumina carbon thermal reduction nitridation under pressure of 300Pa was speculated as a gas-solid phase reaction and the possible reaction mechanism is:at pressure of 300Pa,when the reaction temperature range from 1273?to 1501?,N₂ can react with Al₂O₃ and C and undergo carbothermal reduction nitridation reaction in the area where N₂ can reach,and the reaction equation is Al₂O₃+3C+N₂=2AlN+3CO?g?,however,the reaction rate of this reaction is very slow.When the temperature exceeds 1501?,since Al₂O₃ can react with C to form Al₄C₃ or Al₂CO at this temperature,and Al₄C₃ or Al₂CO can easily react with N₂ to form AlN and C or CO?g?once N₂ diffused to its surface.The nitridation reaction of gaseous Al does not occur during the thermal nitridation of alumina carbon under the experimental conditions of this paper.The effects of CaO and CaF₂ on the alumina carbon thermal reduction and nitridation in 300Pa were also investigated.The results showed that after adding10wt%CaF₂ and CaO respectively,the alumina can be completely transformed into aluminum nitride at 1650?for 2 h and 300Pa.This reduces the holding time by 0.5 h compared to the nitriding process in which no reaction aid is added under the same conditions.It means that the addition of CaF₂ or CaO is beneficial for alumina carbothermal reduction nitridation reaction,perhaps because the addition of CaF₂ or CaO converts the alumina carbothermal reduction nitridation reaction from a gas-solid reaction into a gas-liquid-solid reaction.