Natural The Al <sub> 2 </ Sub> O3-sio <sub> 2 </ Sub> Department Of Raw Materials By Carbothermal Reduction Nitridation Process In The Phase Transition Characteristics And Influencing Factors

Investigations on phase transformations during the synthesis process of Sialon from natural iO2 system raw materials by carbothermal reduction-nitridation have been carried out. The influence of technological conditions (including holding time, reductives and additives) are also researched. The phase components and microstructure have been determined by XRD and SEM. The results show that there are no nitridation product before 1300?, except for the samples with low A12O3 content (21%~25%, wt.%) and high A12O3 content(84%~88%), whose temperature range are before 1400 ? and before 1350癈 respectively.Most of initial nitridation begin at 1300 癈. Carbothermal reduction can be divided into three stages that are initial nitridation, transition and full nitridation. At first, in the initial nitridation stage, A21 and A25 begin their initial nitridation at 1400 ? and the A84 and A88 start at 1350 ?. The nitridation product of pyrophylite systems and kaolinite systems is SieAlOeNs, bauxite systems are 8-Sialon (Z=3) and O'-Sialon.The temperature of transition of pyrophylite system ranges from 1400 to 1450 癈 and kaolinite system ranges from 1350 to 1400?. With the temperature rises, the nitridation products are transformed from Si6AlO6N5 to 8-Sialon (Z=3), O'-Sialon and X-phase, quartz, cristobalite and mullite disappear during this stage, the B-Sialon crystal transforms from irregular grain to irregular short column.The full nitridation stage of pyrophylite system ranges from 1500 to 1550癈. The nitridation products are 6-Sialon and O'-Sialon at 1500癈 and 8-Sialon and 15R at 1550癈 respectively. The full nitridation stage of kaolinite system ranges from 1450 to 1550 "C and 8-Sialon is the only nitridation product at 1450癈,from 1500 to 1550 癈, the nitridation products are 8-Sialon and 15R. For bauxite system, the temperature range is 1400~1550癈 , from 1450 to 1500 癈, the nitridation products are 8-Sialon and 15R and 15R is the only nitridation product at 1550 癈. During this stage, the B-Sialon crystal transforms from irregular short column to irregular short and long column andthe 15R crystal is bar shape.The results show that the influence of holding time is important factor for the synthesis process of Sialon according to the pyrophylite and kaolinite system at 1450癈. For pyrophylite system, 9 hours is optimum holding time to form single Sialon and for kaolinite system, is 6 to 9 hours for nitridation completely, holding 6h is favorable to form single 6-Sialon and holding 9h is favorable to form 8-Sialon with some 15R.Reductives and addictives also take an important role during the whole process. The results show that there is no nitridation product without reductives. The temperature of initial nitridation is 1300癈 with reductives. Carbon black and anthracite are favorable to form B-Sialon, graphite and activated carbon are favorable to form X-phase, silicon and Silicon and black carbon in 1:1 ration are favorable to form-Sialon.The additives use in the experiments are MnO2, Fe2O3, TiO2, Cr2O3 and Ni(OH)2 and the results show that at 1450, MnO2and Fe2O3 are favorable to form X-phase and O'-Sialon, TiO and CrO are favorable to form B-Sialon and X-phase, Ni(OH) is favorable to form 8-Sialon and O'-Sialon. Compared with the samples without additive that is favorable to form B-Sialon.