Conclusion

The sintering properties of Si3N4-Al2O3-CaO composites are associated with the composition of the specimens and their sintering temperatures. When the sintering temperature is heightened from 1500℃ to 1600℃, the bulk densities of the specimens increase and their apparent porosities decrease; after the sintering temperature is further heightened to 1650℃, in contrast, the bulk densities of the specimens decrease and the porosities increase. At the same temperature, along with the increment of Si3N4 content, the bulk densities of the specimens decrease, and their porosities increase.

After being fired at 1500℃ ~ 1650℃, the main phases of the Si3N4-Al2O3-CaO composites are corundum phase, β- Si3N4 phase and α- Si3N4 phase, and the content levels of these phases are associated with the content levels of Si3N4 and Al2O3 that were initially introduced into the composite system. Meanwhile, phase transformations are observed with the specimens during their firing process: At 1500℃, the CaAl2Si2O3 appears in the specimens; when the temperature is heighten to 1600℃, the CaAl2Si2O3 disappears, and Ca-α-Sialon and β-Sialon are found in the specimens; when the temperature is further heightened to 1650℃, Ca-α-Sialon disappears, and a great amount of β-Sialon is found in some of the specimens.

When firing the Si3N4-Al2O3-CaO composites within the temperature range of 1500℃~1650℃, phenomenon of mass increment or mass loss is observed. According to thermodynamic analysis, these changes on masses are caused by the complicated chemical reactions taking place within the specimen during the sintering process. When the temperature is lower than 1500℃, the reaction Si3N4(s)+3/2CO(g)=3/2Si2N2O(s)+1/2N2(g)+3/2C(s) is the main mechanism leading to the mass increment of the specimens; when the temperature is higher than 1500℃, the reaction Si3N4(s)+3/2CO(g)=3/2SiC(s)+3/2SiO(s)+2N2(g) is the main mechanism leading to the mass loss of the specimens.

References:

Jiang Dongliang: Fine Ceramic Materials, China Geological Publishing House, Beijing, 2000: 87-118
Roy W brow: A Report for Blast Furnace Carborundum Refractories in 90’s, Overseas Steel & Iron Industry, 1991, 11.6-15
Trigg H B,Ellson D B,Sinclair W: Corrosion of Selected Non-exide Ceramics in Liquid Steel. Br Ceram Trans, 1988,87:153-157
Kazuhiro Kometani, Keishi Ⅱ Zuka, Tetsuo Kaga: Behavior of ferro-Si3N4 in blast furnace taphole mod. Taikabutsu, 1998, 50(6):326-330.
Li Yawei ,Zhang Xin, Jin Shengli, etc: al Corundum castables containing nitrogen for purging plug in ladle. Proceedings of 44th International colloquium on Refectories, 26-27, 9, 2001, Aschen,Germany
Li Yawei, Li Nan, Wang Binyao: Researches on Refractories of β-Sialon/Corundum Complex Phases, Transaction of Inorganic Materials, 2000, 15(4): 612-617
(7 and 8 missing out of the bottom of the page)
Zeng Qiang, Wang Fuming, Li Wenchao: Estimation & Assessment of The Thermodynamic Properties of Sialon System. Rare Metal, 1999, 23(4): 254-257
Shankoumingliang (a direct translation of a Japanese name from Chinese): Practical Thermodynamics And Its Application in High Temperature Ceramic, translated by Zhang Wenjie, Wuhan Industrial University Publication House, Wuhan, 1993.
Kazushige Yokoyama ,Shigetuka Wada: Solid -gas reaction during sintering of Si3N4 ceramics (Part 3). Journal of the Ceramic Society of Japan ,2000,108(3):230～235.
Anatoly Rosenflans ,I-wei Chen: Phase relationships and stability ofα-Sialon . J Am Ceram Soc, 1999 ,82(4):1025～1036.