Study on Aging of talc ceramics – Transformation between Magnesium Metasilicate Crystals

Extensive research has been conducted on the conversion between crystal forms of magnesium metasilicate. However, at present, it cannot be considered that there is a completely unified and definite understanding of this issue. However, the results obtained by J.F. Sarver et al. in 1962 can be used as MgSiO3 at temperatures below 1400 ℃ basic information on the laws of crystal transformation. Saville et al. provided MgSiO3 The T-P phase diagram shows that the main crystalline phase formed during the firing of talc ceramics is original enstatite, which is cooled to 1042 ℃ (measured by quenching method). The result obtained by high-temperature X-ray diffraction method is 1035 ℃, and there is a tendency to transform into enstatite between 865 ℃, and below 865 ℃, there is a tendency to transform into metastable crystalline clinopyroxene. Below 865 ℃, metastable crystalline clinopyroxene can exist for a long time and will not be converted into thermodynamically stable clinopyroxene. The aging or pulverization of talc ceramics is caused by the transformation of high-temperature crystalline phase primary enstatite into enstatite or clinopyroxene during cooling, placement, and use.

According to the theoretical density calculated from lattice parameters, it can be seen that the conversion of primary enstatite to clinopyroxene at room temperature is accompanied by a volume change of about 2.8%, and the volume change when primary enstatite is converted to enstatite is even greater. If analyzed from lattice parameters, as the volume change is the total result of crystal c-axis shortening and b-axis elongation, the impact of grain length change during the conversion process is inevitably more prominent than that reflected by the volume change value. The large strain accompanying the transformation process inevitably generates large stress, and the effect of this phase change stress is the more direct reason for the aging or pulverization of talcum china. Original enstatite is the main crystalline phase of talcum china, and the excellent performance of talc ceramics is directly related to the original enstatite. In this way, solving the aging or pulverization problem of talcum china becomes the problem of how to stabilize the original enstatite so that it does not transform into enstatite or clinopyroxene during cooling, placement, and use.