11B, 23Na, 27Al, and 19F NMR study of solid and molten Na3AlF6-Na2B4O7.
High temperature electrochemical synthesis is a very promising method for the production of refractory boride powders and coatings. The Na(2)B(4)O(7)-Na(3)AlF(6) binary system is part of the more complex NaCl-Na(3)AlF(6)-Na(2)B(4)O(7)-TiO(2) electrolyte proposed for TiB(2) electrosynthesis. A new approach by high temperature and high resolution solid state nuclear magnetic resonance was employed for the structural description of this system, from the solid at room temperature to the liquid at 1100 degrees C. The change in (27)Al, (23)Na, (11)B, and (19)F NMR spectra with composition give evidence for the insertion of aluminum into the glassy network of Na(2)B(4)O(7) as [AlO(4)] and diluted [AlF(6)] units as well as formation of [BOF(2)] groups into solidified mixtures. A devitrification mechanism of glassy Na(2)B(4)O(7) under influence of Na(3)AlF(6) addition and temperature was discussed in terms of lowering boron coordination and obtaining oxyfluoride species.
Bessada
Catherine
C
CNRS, Centre de Recherche sur les Matériaux à Hautes Températures, 1D, Avenue de la Recherche Scientifique, 45071 Orléans Cedex 2, France. bessada@cnrs-orleans.fr
Anghel
Elena M
EM