Rindone found that a small amount of platinum (0.01%) introduced into a glass of the composition Li₂O⋅4SiO₂ acts as a nucleating agent of repeating, converting the whole mass into an assembly consisting of extremely small crystals (G. E. Rindone, J. Am. Ceram. Soc., 41, 41 (1958)).
This paper presents the results of the investigation of authors which covers the nucleation by platinum for glasses containing Li₂O, MgO, Al₂O₃, and SiO₂. The bending strength was used for the evaluation of the effect of the nucleating agent.
(1) Optimum amount of platinum The glasses of the composition, Li₂O 12.5, K₂O 2.5, Al₂O₃ 4, SiO₂ 81% by weight, added, respectively, with 0, 0.001, 0.01, 0.1% of platinum were formed into the specimens of the size 50×5×2.5mm. Taking the density increase as a reference the effect of the concentration of platinum on the devitrification of the specimens under a stepwise heat treatment was investigated. It was found out that 0.01% was sufficient for the completion of devtirification (Fig. 1).
The bending strength of the devitrified specimen increased with increasing platinum content (Fig. 2). Taking into consideration of the cost of platinum the authors concluded that 0.01% is the optimum amount for nucleation from practical stand point.
(2) The influence of lithium content The glass of the composition, MgO 15, Al₂O₃ 23, SiO₂ 62=100+XLi₂O, whereX=4, 6, 8, 12, by weight, was melted with and without addition of 0.01% platinum. The specimens of the same size as above were heated from room temperature to 1050-1100°C with the rate of 5°C/min., and then kept constant for one hour. The bending strength of platinum containing glasses increased remarkably when their lithium content was high (12%), whereas only a negligibly small increase wasfound in the glasses of low lithium content below 6% (Fig 7). This indicates clearly that the function of platinum was influenced by the amount of lithium in glass.
It was also conformed that the glasses of tow lithium content could be converted into a polycrystalline material of high mechanical strength even when no platinum was added (Fig. 7). The investigation of this interesting phenomenon is now going on.
(3) The optimum composition range in the system Li₂O-MgO-Al₂O₃-SiO₂. Keeping Li₂O at a constant value of 12% other components were changed as; MgO x, Al₂O₃ y, SiO₂ z, where x, y, z are mole ratio by weight and x+y+z=100, the range in which; the two conditions, (a) glass may be obtained at a melting temperature lower than 1400°C, (b) the glass converts into the polycrystalline material without any noticeable deformation during the reheating will be satisfied. The range in which the condition (a) was satisfied was x: 0-30, y: 0-30, z:60-100, (Fig. 8), and that of satisfying (b) was x: 0-20, y: 0-30 and z: 60-100 (Fig. 9 and Table 1).