Study On The Mechanism Of Aluminum Nitride Prepared By Carbothermic Reduction And Nitridation Of Alumina

At present,the method of alumina carbothermal reduction nitridation prepared aluminum nitride powder is used in industry,however,it still exists some drawbacks such as high reaction temperature,long processing time.Although the purity Al N powder have been obtained by alumina carbothermal reduction nitridation under vacuum at 1923K for 2.5hours in a nitrogen gas pressure of 300Pa in our previous investigation.In order to further optimize the parameters of the method of aluminium nitride by alumina carbothermal reduction nitridation under vacuum.Firstly,the thermaldynamics of the relevant reactions were calculated in Factsage software,and then nitridation mechanism of the intermediate products such as Al₂CO,Al₄C₃,and Al₆C₃N₂,which were discovered in the previous experiment,were studied by the first principle method.According to the results of thermodynamic calculations and first principle researches,the experiments were carried out and a more optimized method of alumina carbothermal nitridation under vacuum has been obtained.In this paper,the thermodynamics of the revalent reactions in the process of preparing aluminum nitride by carbothermal reduction nitridation of alumina under vacuum were firstly calculated under the pressure of 100k Pa and 300Pa.The calculation results show that,the reaction temperature of Al N,Al₂CO,Al₂O?g?,Al?g?and Al₄C₃ will reduced from 1963.3K,2208.4K,2357.0K,2447.5K and 2257.8K to1545.9K,1723.2K,1804.0K,1818.2K,and 1775.5K,respectively,when the pressure decreased from 100k Pa to 300Pa.And the order for the reaction start temperature with increasing is:Al N<Al₂CO<Al₄C₃<Al₂O?g?<Al?g?.All possible intermediates will react with N₂ to form Al N at the end.In order to investigate the nitriding mechanism of the intermediate products such as Al₂CO,Al₄C₃,and Al₆C₃N₂,the ab initial dynamic simulation has been adopted.The results of the ab initial dynamic simulation show that for Al₂CO the C atoms are easier to aggregate,and it is easier to generate Al₆C₃N₂ after carbothermal reduction nitridation.For the system of Al₄C₃-C-N₂,it was easier to nitride of Al₄C₃ when the system exist external C and the temperature is 1923K,the structure of Al-C-N will form at the end.Based on the results of ab initial dynamic simulation,it shows that the C atoms in the lattice of Al₄C₃ will grather in the process of nitriding Al₄C₃.To analysis the results of Al₆C₃N₂ ab initial dynamic simulation,it shows the nitride reaction can be easier to occer during alumia carbothermal reduction nitridation,and the C atoms in the lattice exsit the phenomenon aggregation.Based on the results of the calculation about Al₆C₃N₂,it can be concluded that the nitridation reaction of Al₆C₃N₂ is Al₆C₃N₂+2N₂?6Al N+3C.According to the nitriding mechanism of the intermediates,the intermediate of Al₂CO is the most difficult to nitride,followed by Al₄C₃ and Al₆C₃N₂ is most easily to nitride.After thermaldynamic calculating and ab initial dynamic simulating,the experiments of alumina carbothermal reduction nitridation have been investigated in a double temperature zones vacuum furnace.The ultrafine Al N powder,which the size is under 5?m can be obtained,when the molar ratio of Al₂O₃ and C is 1:3.25 at temperature of 1923K in the pressure of 300Pa for 1.5 hours.Compared with the sigle temperature zone vacuum furnace,the soaking time has been greatly shorted.Combining the results of experimental and theoretical calculations,it shows that the Al?g?and Al₂O?g?are not exist in the progress of Al N preparing by alumina carbothermal reduction nitridation and combine the investigation results of thermaldynamics,reaction mechanism and the experimentals,the react pathways of this method can be concluded as Al₂O₃?Al₂CO/Al₄C₃?Al₆C₃N₂?Al N.According to the results of experimentals investigation,the nano Al N powder have the opportunity to be obtained by the method of alumina carbothermal reduction nitridation under vacuum.Because of that the mechanism of Al_nN_n?n=2-9?cluster growth has been investigated.Besed on the results,the reaction ways of Al_nN_n?n=2-9?cluster can be concluded as Al N?plane??Al₂N₂?plane??Al₃N₃?plane??Al₄N₄?plane??Al₅N₅?bulk??Al₆N₆?bulk??Al₇N₇?bulk??Al₈N₈?bulk??Al₉N₉?bulk?.The pathways show that there exsit a transfer point from two-dimension to three-dimension between the cluster of Al₄N₄ and the cluster of Al₅N₅,and the stable structures of the clusters are all three-dimensional phase structure after the transfer point.