In order to make barium titanate ceramics having high dielectric constant by a process of crystallization of glass, a series of glasses with the general composition (100-x-y) BaO⋅TiO₂+xSiO₂+yAl₂O₃, in which x and y are within the range of 10 to 60 and 0 to 20 mole per cent, respectively, was investigated, especially with respect to their glass formation tendencies, crystallization behaviors during reheating and dielectric properties of their crystallized products.
The glasses were melted, in all cases, in platinum crucibles at 1450°C for one hour and formed into plate of about 2mm thick. They were crystallized by reheating up to 1100°C at a rate of 5°C/min.
The results of the experiments are summerized as follows:
1) A glass formation region was determined (cf. Fig. 1).
2) In general, crystallization of the glasses in this region starts at about 850°C, and is almost completed at 1100°C (cf. Fig. 3 and 5). The resulting crystallized products show flaws such as cracks and surface unevenness when the parent glasses contain BaO⋅TiO₂ or Al₂O₃ in excess (cf. Fig. 2).
3) Major constituents of the crystallized products are BaTiO₃ (tetragonal), BaAl₂Si₂O₈ (hexagonal), BaTiSiO₅ and some unidentified crystalline phases (cf. Table 2).
4) For the crystallized products with Al₂O₃ held constant, their dielectric constant decreases with increasing the SiO₂/BaO⋅TiO₂ mole ratio. For those with BaO⋅TiO₂ held constant, it reaches a maximum when the Al₂O₃/SiO₂ mole ratio is set to 35/65 (cf. Fig. 7). Generally, it increases with increasing the amount of barium titanate crystals separated out in the crystallized products. The composition of the crystallized product which shows a maximum dielectric constant (about 500 at 10⁶c) in the present experiments is BaO⋅TiO₂ 60, SiO₂ 26, Al₂O₃ 14 mole per cent.
5) On the basis of Bruggeman's theory the crystallized products obtained were assumed to have the structure of prophyritic mixture in which lamellar barium titanate crystals are dispersed in matrix consisting of glass phase and crystal phases of BaAl₂Si₂O₈ and BaTiSiO₅ etc. (cf. Fig. 8).