1.1 Research on sintering mechanism
1.1.1 DSC differential thermal analysis
Figure 4-1 shows the measured DSC-TG curve. The exothermic peaks in the figure are 146°C, 570°C, 663°C, and 388°C, and the endothermic peaks are 614°C and 1119°C. 146°C is the temperature at which the polyurethane foam begins to soften: 570°C is the temperature at which the polyurethane foam is coked and volatilized; the endothermic valley at 1119°C shows that adding 4wt% CeO2 as a sintering aid makes the slurry produce a liquid phase at this temperature; At 1388 °C, the liquid phase precipitated into the porous ceramic filter solid phase, resulting in an exothermic peak. The most prominent problem in the production of cordierite is the narrow firing range. The formation rate of cordierite is very slow at 1300-1400 °C, and when the temperature reaches 1450 °C, cordierite is melted and decomposed into mullite and glass body. It can be seen that adding 4wt% ceO2 can effectively reduce the sintering temperature and expand the sintering range.
The theoretical composition of cordierite is 2MgO2Al2O35siO2 (Mg2 Algsisc13), of which MgO138wt%, Al2O334.9wt%, SiO251.3w%. Its chemical composition is often different from the theoretical composition due to the replacement of quasi-in-phase.
The position of cordierite in the phase diagram circle of the MgO-Al2O3-SiO2 system is shown in Figures 4-2. The melting point of cordierite is around 1465 °C, and it is not uniformly melted and decomposed into mullite and liquid phase when heated. Therefore, the cordierite ceramic material cannot be used at too high a temperature. Cordierite has two homogeneous multiphase variants, namely low-temperature cordierite and high-temperature cordierite. High-temperature cordierite, also known as Indian stone, belongs to the hexagonal crystal system, and low-temperature cordierite is commonly known as Dong Qingshi. Belongs to the orthorhombic system. It can also be seen from the phase diagram of the MgO-Ab2O3-SiO2 system that the cordierite sintering temperature range is very narrow, and the biggest difficulty in artificial synthesis is how to control the intermediate phases (quartz phase, spinel phase) to obtain high-purity cordierite ceramics.
It can be seen from Figures 4-3 that there are a small number of intermediate phases such as quartz and spinel in the sintered product without adding ceO2, but after adding 4wt% ceO2, the content of the intermediate phase is significantly reduced, and almost disappears, which is ideal. The sintering effect shows that adding a certain amount of ceO3 as a sintering aid can significantly improve the sintering effect.