透明結晶化ガラスの生成過程中に起こる構造変化

抄録

A glass containing Li₂O, Al₂O₃ and SiO₂ as the principal components and P₂O₂ of about 2 wt% and TiO₂+ZrO₂ of about 4 wt% as the so-called nucleating agents can be converted by suitable heat treatment to a transparent glass ceramic material whose average transmission of 5mm thick plate is 80% or over in visible region. It is the purpose of present paper to examine the structural change of the glass during the heat treatment putting the stress on the nucleation phenomenon.
The study was made by electron microscope observation (mainly direct transmission method), electron and X-ray diffraction methods, differential thermal analysis and measurement of thermal expansion coefficient. The results are summarized as follows:
1) The tendency toward phase separation of glass was increased by the addition of nucleating agents.
2) The crystalline nuclei whose mean diameter was 30-80 Å were confirmed to precipitate before the main crystals-β-Quartz solid solution-crystallize out. The position of diffraction peak due to the nuclei in both electron and X-ray diffraction patterns agreed with that of the strongest one of ZrO₂ (cubic system) or ZrTiO₄.
3) It was found that some incubation period (about 40 min at 750°C) was required for the nuclei to precipitate and thereafter the nuclei tended to grow by the diffusion controlled mechanism and to approach to a certain constant size. The higher the temperature at which the glass was heat treated, the larger was the mean size of nuclei.
4) The approximate number of nuclei per unit volume was of the order of 10¹⁶-10¹⁷ and the volume fraction seemed to be less than 2%.
5) The mean size of main crystals which was estimated from the broadening of X-ray diffraction peak increased with increasing heat treatment temperature and time, depending more strongly upon the temperature.
6) The degree of crystallinity which was calculated using an equation proposed by Ohlberg et al. increased in S-shape with increasing heat treatment temperature and time. This change was similar to that in thermal expansion coefficient.
7) A probable mechanism for formation of main crystal was suggested, that is, the region which is close to main crystal in composition but still remains in amorphous state is formed first around nucleus and then the region transforms rapidly to crystalline state after it grew to fairly large size in comparison with nucleus.
8) It was also elucidated that probably one main crystal was formed from one nucleus by the analysis of the relationship between the number of nuclei per unit volume, the mean size of main crystals and the degree of crystallinity which were determined independently each other. This seems to be supported by the direct observation of thin glass films by electron microscope.