硼珪酸ガラスにおける原子集団の形成〔その1〕
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概要
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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 Na2O-B2O3-SiO2 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 BO4 tetrahedron by x and the BO3 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 Na2O-B2O3 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 xy4 in the network of glass, i.e., to one BO4 tetrahedron four BO3 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. "xy4-groups" share corners with each other not edges or faces. These "xy4-groups" may exist in all glasses of Na2O-B2O3 system, Na2O-B2O3-SiO2 system, and even in borosilicate glasses in general by the following rule: Rule 4. "xy4-groups" and BO3 triangles or SiO4 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. "xy4-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 "xy4-group" formation, the manner of the variation of the expansion coefficient of the glasses in the ternary system Na2O-B2O3-SiO2 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 SiO2 to B2O3 is more than 3/2, the "xy4-group" formation is accompanied by the separation of the glass into two phases, one is rich in SiO2 and the other in B2O3 and Na2O.
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