A study has been made for obtaining informations on thermal properties and crystallization process of magnesium phosphate glass with the molar composition MgO⋅P2O5. The thermal properties such as linear expansion, DTA figure and specific volume change by heat-treatment of the glass were tested. The crystallization temperature was determined from the DTA, and the crystal growth in the glass specimen heat-treated was observed through a microscope.From the thermal expansion curve, transformation temperature (glass transition point) (Tg) and softening temperature of the phosphate glass were determined to be 563°C and 595°C, respectively. Linear coefficients of the thermal expansion below Tg(αl) and above Tg(αh) were 98×10-7 and 99×10-6cm/cm⋅°C, respectively. A wide difference between αl and αh can be interpreted in connection with the characteristics of this glass having an open structure.The DTA curve of the fine powdered glass showed that the crystallization started at 630°C and finished at 698±5°C, which points were determined from the sharp exothermic peak. In the case of the bulky glass sample, it was indicated that the crystallization initiated at about 730°C and completed above 800°C, which temperatures were pointed out by deviation from the basic line in the DTA chart and by the broad exothermic peak, respectively.The crystallization of the phosphate glass specimen began at its surface. The crystals formed at the surface were observed to be fine granular particles and to be smaller than several microns in size. The size of the crystallite was affected by keeping conditions of the sample, i.e., temperature and time. During the crystallization process at higher temperatures than about 660°C, after the whole surface of the specimen had been filled with fine crystal particles, the crystal growth proceeded inside of the specimen. The X-ray diffraction patterns of both the surface and the inside crystals coincided with that of Mg(PO3)2 which is mentioned in ASTM X-ray powder data. The crystals observed inside of the specimen were oriented vertically to the surface, and they were rod-and/or needlelike polycrystals.The rate of crystal growth proceeding from the surface of the specimen toward its inside in a steady state increased linearly with increasing temperature within the range of 600°C to 700°C. The activation energy of the crystal growth was determined to be 80kcal/mol in the above temperature range.
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