Bauxite-based Ca-alpha-sialon Nitridation Reaction Study Of The Synthesis Process

Using Al₂O₃, bauxite, Si , Al and CaCO₃ as main starting materials, with minor additive(Y₂O₃, TiO₂, Fe₂O, NH₄Cl, NH₄F) and crystal seeds( α -Si₃N₄, α-Sialon) added, Ca-α-Sialon powders have been synthesized by nitridation way. The effects of some factors on the synthesis of Ca-α-Sialon have been studied. The thermal stability of Ca-α-Sialon has been studied. Oxidation of Ca- α/β -Sialon powder was also investigated. And the Al₂O₃ based Ca- α/β-Sialon bonded corundum composites are prepared.The results of Ca-α-Sialon's synthesis indicate: (1)The synthesized powders of the Al₂O₃ based specimens would contain up to 72% Ca-α-Sialon after nitriding at 1550℃ for 12h, as to bauxite based specimens it's values are 42%, the other crystal phase is P -Sialon. (2)The relative content of Ca-α-Sialon could be obviously increased with the content of CaCO₃ increased. (3)The additive of minor amount of Y₂O₃, Fe₂O₃ and TiO₂ as promoters would lead to increase in Ca-α-Sialon synthesized. The effect of Y2O3 is most important, and TiO₂ takes second place. (4) α -Si₃N₄ for seeding would lead to obvious increase in Ca-α-Sialon synthesized. (5)The type of Al₂O₃ used and the amount of Si and Al powder added in the starting materials have little effect on Ca-α-Sialon synthesis. (6)SEM result indicates, Ca-α-Sialon crystals are observed to be elongated columnar in morphology, with aspect ratios ranging from 3 to 10. (7)The synthesis temperature of Ca-α-Sialon would be lowered by microwave nitridation method, and the react time can be saved to a great extent. (8)BP artificial neural network was used to investigate the relationship between amount of Si, Al addition, α-Si₃N₄ and amount of Ca-α-Sialon ,a non-linear model with well prediction performance is set up.Result of thermal stability of Ca-α-Sialon shows that, Ca-α-Sialon synthesized at 1800 ℃ is just stably at 1300¹500℃, it would convert to β -Sialon at 1550℃. About 95 percent Ca-α-Sialon are changed to β-Sialon after 20 hours' heat treatment at 1550℃. Ca- α/β-Sialon prepared at 1550℃ using nitridation-reaction method have better thermal stability than that of Ca-α-Sialon synthesized at high temperature(1800℃) by reaction sintering method, only little α -Sialon is converted to β-Sialon after heat treatment at 1550...