Polymeric Nature of Glass Melts

Abstract

X-ray diffraction patterns from SiO2 and B2O 3 in the glassy and molten states are very similar. In fused silica the basic building units are SiO4 tetrahedra which share corners with one another; glassy B2O 3 is built up of BO3 triangles sharing corners. The radial distribution functions (RDF) for fused silica, fused boric oxide, and three glasses of molar compositions 34 Na2O.66 SiO2, 34 Na2O.66 B2O3 and 43 CaO.57 P2OS are shown in Figure 8.1(a) and (b). In the case of fused silica the first peak occurs at 1.62 A which corresponds closely to the silicon-oxygen distance in many silicate crystals. From the area under this first peak, a coordination number of 4.3 ± 10 per cent oxygen atoms about each silicon has been estimated [1], so establishing that the SiO4 tetrahedra which exist in the crystalline form of silica persist in the glass. In the RDF for silica the atomic density falls to almost zero at 2.2 A, indicating that this zone between silicon and oxygenoatoms is not atomically populated. Thus oxygen atoms within a sphere of 2.2 A radius are permanently associated with each silicon atom, because if they were to move with any significant frequency beyond this boundary, the time-average atomic density (RDF) could not be zero. Such a completely prohibited distance does not occur with molten salts (where the liquid is predominantly ionic), and arises because there are strong covalent bonds between silicon and oxygen. The RDF curve for silica immediately after 2.2 A comlists of two overlapping peaks — one for the distance between two oxygens (2.65 A) and another for the distance between two silicons (3.2 A).