Study On The Transformation Mechanism In Microwave Reduction-nitridation Sintered Micro-crystal Muscovites

In this paper, natural micro-crystal muscovite and carbon black or aluminium were used as raw materials and the reducing agent, respectively. Five different samples were sintered through microwave hybrid heating in nitrogen atmosphere. The temperature-raising characteristics in the microwave field and the phase change characteristics of the product in sintering process were studied. Based on experimental results, the reaction mechanism of the sintering process was discussed. The study of micro-crystal muscovite applied for refractory material provides experimental data. This has important practical significances for expanding the field of micro-crystal muscovite applications and developing new products.The microwave heating systems include: System 1: Natural micro-crystal muscovite and carbon black in Corundum crucible; System 2: Natural micro-crystal muscovite and carbon black in graphite crucible; System 3: Natural micro-crystal muscovite, silicon carbon rod (SiC) and carbon black in corundum crucible; System 4: Natural micro-crystal muscovite, SiC powder (less the amount) and aluminum in corundum crucible; System 5: Natural micro-crystal muscovite, SiC powder (larger amount) and aluminum in corundum cruciblewere.For the system 1, 2, 3, 4 and 5, the heating temperature, the average heating rate, average power and sintering time are 1000℃, 14.5℃·min-1, 3.5 kW, 83 min; 1300℃, 37℃·min-1, 3.7 kW, 85 min; 1420℃, 32℃·min-1, 1.3 kW, 78 min; 1030℃, 11.6℃·min-1, 3.4 kW, 178 min; 1300℃, 13.4℃·min-1, 1.7 kW, 190 min, respectively. After sintering, the resulted products of the heating systems from system 1 to system 5 areα-SiO2, tridymite, Al2O3; Si6AlO6N5, mullite, Al2O3, tridymite; O?-Sialon, mullite, Al2O3,β?-Sialon; Al2O3, KAlSi2O6, tridymite, Si; Al2O3, KAlSi2O6, Si2N2O, Si, AlN, respectively. Compared with the above-mentioned characteristics of heating systems, we can see that the heating rate of samples with carbon black is faster than that of the samples with aluminum as reductant. The time required to reach the same temperature is also relatively shorter. As a pre-body heater, SiC can promote the microwave heating of the aove heating systems.Through the phase analysis, we can see that the carbothermal reduction nitridation sintering of muscovite below 1000℃is mainly related to the transformation process of muscovite, that is, theα-quartz in the mineral is transformed intoα-tridymite and the muscovite is partly decomposed into Al2O3. When the heating temperature increased to 1300℃, the micro-crystal muscovite is transformed into mullite and most of it is then further transformed into Si6AlO6N5. At 1420℃, the sintering products are O?-Sialon,β?-Sialon, mullite and Al2O3. For the sintering system of aluminum thermal reduction nitridation below 1030℃, micro-crystal muscovite is transformed into leucite while theα- quartz is transformed intoα- tridymite. Al2O3 and Si are also formed because of the reduction reaction of Al with the SiO2. At 1300℃, the leucite, Al2O3 and Si still exist but the nitride products Si2N2O and AlN are also determined by XRD.