Preferential bonding in low alkali borosilicate glasses

摘要

In an earlier review((1)) we discussed the connectivity in borosilicate glasses and compared our experimental findings by NMR, infrared and Raman spectroscopies with older structural models. We could show, contrary to the often cited reedmergnerite type model, that a significant preference exists in low alkali borosilicate glasses for trigonal rather than tetrahedral borate groups to link to silicate entities. Another often cited misconception is the application of the Loewenstein rule to borate tetrahedra. While linking of two "[Alempty set(4)](-) tetrahedra is disadvantageous compared to higher coordinated aluminate polyhedra, the borate tetrahedral units represents already the alternate higher coordination state and accordingly, many examples of linked [Bempty set(4)](-) tetrahedra are known to exist in glasses as well as in crystalline compounds of boron at normal pressure conditions. We now present more NMR data on three different low alkali borosilicate glasses with Na2O:B2O3=02 to 035 and decreasing SiO2 fractions (74 to 43 mol% SiO2), for which we compare variations in the near and intermediate range structure of quenched and slowly annealed samples. None of the studied glasses showed a significantly higher fraction of trigonal Bempty set(3) groups in the quenched than in the annealed samples, even though borate in the three-coordinated state is the preferred metaborate unit in the melt. However, for the two silicate rich glasses (including NBS 2), we observe at low temperatures a deviation of the viscosity-temperature plot from the ideal VFT-fit, that is from T-g at circa 440 to the expected 600 degrees C. For samples prepared at any annealing temperature below 600 degrees C, structural variations with thermal history are apparent, and are also reflected in many glass properties including density, fracture probability, or refractive index. Even though the glass NBS 2 shows no visible phase separation, DSC measurements indicate the presence of two different T-g values corresponding to the borate and the silica rich subnetworks. The values of the two T-g events shift with different cooling rates: for fast quenched glasses the two T-g values are closer together than for slowly annealed glasses. The study of these low alkali borosilicate glasses is ongoing, as we understand better how structural variations with changing thermal history impact the glasses' properties. Analogously, we can apply the same techniques to follow structural variations under external forces, such as irradiation, laser modification, pressure and mechanical impact.