Study Of Fluosilicate Glass-ceramics

Compositions, crystallization mechanisms and properties of the fluosilicate glass ceramics were investigated in this paper. The appropriate composition of the glass was determined using the orthogonal design method, the results of DTA and mechanical properties showed that TiO₂ and Y₂O₃ were appropriate nucleating agent in this fluosilicate glass. The type of MF₂(M=Mg, Ca, Ba)in the parent glass had significant effect on the crystallization of the glass, CaF₂ promoted the crystallization of glass, while BaF₂ inhibited it. The nucleating agents could change the viscosity of the glass and they had also prominent effect on the crystallization of the glass.Microstructure analysis of the glass by SEM and TEM showed that the change of the parent glass compositions and the nucleating agents caused different phase separation behavior of the glass, they can influence its crystallization. It is proved that phase separation was a necessary stage for phase transformation from glass to crystal. Nuclei formed first inside the phase separated droplets, then they grew at higher temperature through the accumulation of the droplets and the adhesion of small crystals. If these droplets did not accumulate or even decreased in size as a result of secondary phase separation, the small crystals first formed could not grow. The microstructure analysis also revealed that phase separation accompanied the crystallization below certain temperature, so the number of nuclei changed in the process of crystallization.X Ray diffraction studies indicated that the first appeared crystals in the fluosilicate glass were tetrasilicic-mica (KMg_2.5Si₄O₁₀F₂) in spite of the difference of the parent glass compositions and the type of the nucleating agents. The crystallization sequences were also the same. Change of the parent glass compositions and the nucleating agents modified the temperature range of the crystals appeared and the amounts of certain crystals or their morphology. These factors may change the properties of the glass ceramics.