CARACTERIZAREA STRUCTURALA LA SCALA SUB-NANOMETRICĂ A STICLELOR ALCALINO - SILICATICE

In SiO 2 glass, oxygen has a very stable coordination number of 4 in regard to silicon. The corresponding coordination polyhedra are the SiO 2 tetrahedra, which are interconnected through the tips. By introducing alkali oxides in the Si0 2 glass, some of the Si - O - Si bridges are broken, thus appearing a series of structural units having different geometries and structural compositions. These consist of structural entities of type Q n , having the formula Si(O b ) n (O nb ) 4-n , in which O b represents a bridged oxygen atom and O nb a non-bridged oxygen. The size of the Q n structural units is below 1 nm. Herein is presented the distribution of these structural entities for glasses having the composition of disilicate from the alkali-silica systems studied. The distribution of the considered structural entities was computed using a thermodynamic model, in correlation with some chemical and structural particularities of alkali oxides. The results were compared to experimental data obtained using the 29 Si MAS-NMR methods.