Sialon is the general name used to described the family of Si-Al-O-N phases and related systems that were discovered independently by Oyama and Kamigatito (1971) in Japan and Jack and Wilson (1972) in England. Sialon ceramics, which are well known as excellent heat and corrosion-resistant materials, have been studied for about 30 years. Sialon can be produced by reaction sintering of a mixture of Si3N4, AIN and Al2O3 or synthesised from clay minerals by the carbothermal reduction nitridation (CRN) method.
Carbothermal Reduction Nitridation
The carbothermal reduction nitridation method offers the advantage of using low-cost starting materials. Since Lee and Culter first successfully synthesized β-sialon from clay, several investigators have studied various aspects of this synthesis method for sialon powder using kaolinite. Others have studied silicate or aluminosilicate minerals such as halloysite, montmorillonite-polycrylonitrile and bentonite
Broadly speaking the reaction proceeds according to following equation:
According to this equation, the reaction appears to produce very simple final phases, but in practice the carbothermal reduction nitridation method includes a series of complicated reactions and results in SiC, AlN, and other sialon phases being formed in the carbothermal reduction nitridation process.
Raw Materials for SiAlON Production by Carbothermal Reduction Nitridation
Currently, some researchers prepare α-sialon ceramics from halloysite and pure Y2O3 doped SiO2 or silicon. However, neither powder of α-sialon synthesized by carbothermal reduction nitridation using only minerals nor Mg- α-sialon ceramics successfully prepared by reaction sintering has been reported.
Alternative Raw Materials
In our studies, we used talc ((Mg3(Si2O5)2(OH)2) because of the high ratio of MgO/SiO2 itself. It seemed that this metal Mg could be incorporated into sialon and formed Mg-α-sialon.
For the above purposes, the present research was undertaken to explore the possibility of synthesising Mg-α-sialon from talc and halloysite by carbothermal reduction nitridation. Raw materials employed were New Zealand Halloysite clay, talc, silicon nitride powder and carbon black.
The carbothermal reduction nitridation technique offers a method for obtaining low-cost Mg- α-sialon powder. α-sialon formation depended on the ratios of talc to halloysite, as well as the control of the reaction temperature and holding time The content of Mg-α-sialon/ß-sialon was only about 30% at the lower ratios of talc to halloysite, and ß-sialon was the main phase. In contrast, the higher ratio compositions caused much glassy phase to remain in final products. The optimal ratio of talc to halloysite was 1.5:1.0, which extra amount of magnesium is necessary to compensate the evaporation of Mg and reduces ß-sialon, AlN and 15R phases in final compositions. Impurities in talc and halloysite materials not only enhance ß-sialon and produce SiC, AIN and 15R, but also influence synthesized phases and the rates of reaction in the Si-Al-Mg-C-O-N system during the carbothermal reduction nitridation process. The formed MgSiN2 disappeared with prolonged holding time. The key roles of MgSiN2 and SiC intermediation were also confirmed. SiC phase first appeared a little earlier than the sialon phase, reached its maximum after about 30 min at 1480°C. It then decreased with increasing holding time and was detected in final phases. The optimum for synthesis of Mg-α-sialon is 4h at 1480°C with a talc to halloysite ratio of 1.5:1.0. The highest synthesized amount of Mg-α-sialon was 90wt%, ß-sialon, SiC, AlN and 15R were also produced.