Studies of the Glass-formation Range of Silicate Systems

Following a previous report “Studies of the Glass-formation Range of Borate Systems” (This Journal, 69, 282 (1961)), we studied the glass-formation range of silicate systems. Although silicate glasses have long been used, sufficient systematic studies of the glassformation ranges of silicate systems have not yet been made.In this experiment, 1/80 mole (about 1g.) of materials were melted in crucibles made of pure platinum or platinum containing 20% rhodium at temperatures from 1400° to 1750°C. The oxides used besides SiO2 were 16 kinds of the oxides of a-group elements namely Li, Na, K, Be, Mg, Ca, Sr, Ba, Al, La, Ti, Zr, Th, Nb, Ta and W.The glass-formation ranges of binary silicate systems are shown in Table 1. These ranges differ in some points from those of borate systems. For instance, the La-silicate system has no vitrified range, while the Mg-silicate system has a wider glass-formation range than the borate. Whether a component ion enters into the glass structure as a modifier or as a network-former depends on the acidity of the glass-former and on the electronegativity of the component ion. In silicate systems, however, the actual range of glass-formation equals the difference between the glass-formation range and the immiscible range. If the latter is equal to the former or somewhat wider, the vitrified range will disappear.The glass-formation ranges of ternary systems are shown in Fig. 1-34. The whole number of the studied systems reached about one hundred. The Experimental results show that the actual glass-formation ranges agree with the range (hatched areas in the figures) to be expected from the “Conditions of Glass-formation” (This Journaj, 67, 364 (1959)). Among these systems, the systems containing TiO2 (cf. Figs. 12 and 13) are remarkably different from the corresponding borate systems. In the borate systems, a glass-fromation range spreads on the right side of the SiO2-D line (cf. Fig. 10), and therefore it has been estimated that the co-ordination number of Ti4+ is 6. (In order for the Ti4+ ion to take 6-co-ordination as a network-former, the modifier of divalency must also be present, therefor, in the area on the left of the SiO2-D line glass-formation is impossible.) However, in the silicate system the limited line of SiO2-D is lacking. Consequently, it is concluded that the Ti4+ ion as well as the Si4+ ion takes the 4-co-ordination. The glass-formation range of the TiO2-containing systems are shown in the hatched areas of Figs. 12 and 13, which are limited by the AD line.The WO3-containing silicate systems have a remarkably narrow glass-formation range compared with borate systems. The vitrified range of the B2O3-WO3-alkali oxide system has two feet, but the silicate system, we suppose, lacks the left foot. Moreover, it is considered that the left foot consists of WO3 and alkali borate in borate systems, but that this part becomes immiscible in silicate systems.According to the devitrification of the binary system, La2O3 systems are classified as C-type ternary systems, and their glass-formation ranges are narrower than those of borate systems. The vitrified range of the SiO2-Al2O3-La2O3 system (cf. Fig. 29) agrees with that of the corresponding borate system, but the limiting line, kl, of the immiscible range is lower than that of the borate and, accordingly the glass-formation range of the former is narrower than that of the latter. Other La-silicate systems are similar. Among the C-type ternary systems containing ThO2 or Al2O3, which have vitrified ranges in case of the corresponding borate systems, we found the glassy state only in the SiO2