Читать книгу Encyclopedia of Glass Science, Technology, History, and Culture - Группа авторов - Страница 174

Early in its study, the structure of silica glass was described in terms of the so‐called microcrystalline model [13], and hence it is useful to mention it briefly here. Its starting point is that, although crystals have diffraction peaks that are much narrower than for glasses (Figure 5a), significant broadening is observed if crystallites are very small, in accordance with the Scherrer equation, ΔQ = 2πK/L, which relates the width of a Bragg peak ∆Q to the crystallite dimension L and to a shape factor K (~1).

To account for its broad diffraction peaks, one might thus describe a glass in terms of very small crystallites. However, a problem with the model is that to explain the large widths of the observed glass diffraction peaks, the crystallite size should typically be on order of 5 Å, a value similar to unit‐cell dimensions. Philosophically, it makes no sense to consider crystallites as ordered entities if they contain only one unit cell, since there would be no translational symmetry. Furthermore, with such small crystallites, a crystalline powder would be composed almost entirely of grain boundary material, which by definition differs structurally from the bulk. Hence, microcrystalline models cannot provide a description of the structure of most of the material in the glass. For these two reasons, they need not be considered further.