Assuming five simple and reasonable rules concerning the formation of atomic groups in borosilicate glasses, the author was able to interpret satisfactorily, by the way different from those used by others in their attempts, not only the boric oxide anomaly in the glasses of the ternary system Na
2O-B
2O
3-SiO
2 but also various properties of borosilicate glasses in general, and further succeeded in improving the properties of these glasses by the method based on these considerations.
In the first place, by representing, for simplicity, the BO
4 tetrahedron by x and the BO
3 triangle by y, the following two rules were assumed concerning the atomic arrangement in the glass of the composition corresponding to the maximum number of x in Na
2O-B
2O
3 system: Rule 1. One x is bonded to four y's. Rule 2. One y is bonded to two y's and one x. By satisfying these two rules, x's and y's must be bonded to each other forming the unit group xy
4 in the network of glass, i.e., to one BO
4 tetrahedron four BO
3 triangles are bonded each of which sharing only one corner with the tetrahedron.
Then the large network of glass should be built up of these unit groups by the following rule: Rule 3. “xy
4-groups” share corners with each other not edges or faces. These “xy
4-groups” may exist in all glasses of Na
2O-B
2O
3 system, Na
2O-B
2O
3-SiO
2 system, and even in borosilicate glasses in general by the following rule: Rule 4. “xy
4-groups” and BO
3 triangles or SiO
4 tetrabedra share only corers with each other. These rules, however, prescribe only the manner in which several unit groups are bonded, and the selection of the associate group should be considered by other rules based on various experimental facts. One of them may be as follows: Rule 5. “xy
4-groups” are formed only in a certain specific temperature range between the softening temperature, and are decomposed to y's at higher temperatures.
By these assumptions concerning “xy
4-group” formation, the manner of the variation of the expansion coefficient of the glasses in the ternary system Na
2O-B
2O
3-SiO
2 determined by Gooding and Turner as well as the limit of glass formation tn this system was very satisfactorily interpreted, and it was also deduced that for the domain of composition where the mol ratio of SiO
2 to B
2O
3 is more than 3/2, the “xy
4-group” formation is accompanied by the separation of the glass into two phases, one is rich in SiO
2 and the other in B
2O
3 and Na
2O.
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