Showing papers by "Sumio Sakka published in 1966"

Crystallization of Glasses Containing BaO and TiO2

Abstract: 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).