Crystallization process and some properties of the calcium magnesium silico-phosphate glass containing small amount of fluorine have been studied. Calcium fluoride was added to a base glass with the molar composition of 3.5CaO-3.5MgO-P
2O
5-3.5SiO
2, which belongs to the high oxygen type silico-phosphate glass (
cf. T. Kanazawa,
Yogyo-Kyokai-Shi 72, 27 (1964); T. Kanazawa, A. Nakai, H. Kawazoe,
Kogyo Kagaku Zasshi 72, 2211 (1969)), in expectation of preparing glasses containing 0.1, 0.5, 1.0 and 1.5wt% of fluorine. On these four glass samples heat-treated and untreated, DTA, electron microscopic observation, EPMA and measurements of thermal expansion, viscosity, acid solubility and others were carried out.
In the DTA profile, the temperature (
Tp) of the firstly exothermic peak, which appearance is due to the formation of fluorapatite (FAp) crystals, was reduced with increasing the fluorine content in the glass. It was seen that the introduction of fluorine resulted in a decrease of the viscosity of every glass specimen in its transformation range. These facts show that the change of
Tp is considered to be related mainly to the viscosity change by varying the fluorine content.
In the glasses containing fluorine above 0.4wt%, the crystallization of FAp was observed to begin at both the surface and the inside of the test piece simultaneously. The electron micrographs by the transmission method proved the formation of droplets with 200-400 Å in size. These droplets were recognized in the untreated glass similar to the heat-treated. The fine crystals of FAp seemed to form at the interfaces between the droplets and the surrounding matrix region. The composition of these droplets is considered to be rich in magnesium. In comparison of the above-stated results with those of the glass samples containing fluorine below 0.1wt%, it was suggested that the homogeneity of the crystallization of FAp was attributed to the homogeneous dispersion of these droplets in the glass specimen.
During the growth of FAp crystals, the glass specimen with fluorine content of above 0.4wt% was found to have a two-phase structure with Ca-rich and Mg-rich phases. As the Ca-rich phase contained numerous FAp crystals, which solubility to the acid solvent was considerably lower than that of the starting glass, the dissolving process of the fluorine containing test piece to N/2 hydrochloric acid was considered to be different in appearance from that of the glasses with fluorine content of below 0.1wt%.
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