Thermodynamic Analysis And Experiment Research In Synthesization Process Of Si₃N₄, Fe-Si₃N₄

Fe-Si₃N₄ is synthesized using silicon ferrous alloy powder as starting materials , and a new synthesis method is put forward that Si₃N₄ would be formed from the melted alloy containing silicon while Fe remains in melt. At the same time, by thermodynamic calculation, the influence of the reaction temperature and atmosphere on the characteristic of the product is analyzed;the nitridation mechanism of ferro-silicon and the influence of Fe on it are expounded. In addition, the relationship between â–³G° and α _Si is analyzed. The influence of particle size of the starting materials, reaction temperature and holding time are studied experimentally. The phase composition and microstructure of the product are analyzed by means of XRD SEM and EDS.The thermodynamic analysis shows that the high pure nitrogen is required in nitridation process. Silicon nitride can be made in a low partial pressure of nitrogen, and the decomposition of silicon nitride can be prevented by fine control of the reaction temperature. The compositions of the products can be adjusted by changing the concentration of the alloy containing silicon.The nitridation mechanism of ferro-silicon alloy and the effect of Fe on it are expounded by thermodynamic analysis. The results show that besides that of 3Si+2N₂=Si₃N₄, the reaction of 3SiO+2N₂=Si₃N₄+3/2O₂ and 9/5FeSi₂+2N2=Si₃N₄+3/5Fe₃Si occur possibly. Firstly, Fe reacts with O₂ to produce FeO. Then the formed FeO reacts with Si to produce SiO. Finally SiO reacts with N₂ to produce Si₃N₄. Therefore, Fe can improve the nitridation rates.The experiment results show that specimen with an average particle size of 5.229μm and that of 8.023μm can not be nitrified sufficiently, but that of 13.410μm can be sufficiently. Thefibrous and columnar Si3N4 crystals are formed in fine ferro-silicon particle while the spherical aggregates and columnar crystals are in coarse ones. There are more P-SisN4 on the condition of finer particle size, higher reaction temperature and longer holding time. Proportional distributing ferroalloys remain in the prepared ferro-silicon nitride.