STRUCTURE, PROPERTIES AND APPLICATION OF OXYNITRIDE GLASSES

The structure and properties of nitrided silicate glasses have been reviewed mainly on the basis of the works made by the author's group. Neutron diffraction analysis and Si-29 and Al-27 nuclear magnetic resonance spectroscopy have shown that in Na-Si-O-N and Y-Al-Si-O-N oxynitride glasses there exist N atoms bonded to two Si atoms as well as N atoms bonded to three Si atoms. It is also shown that N atoms hardly make bonds with Al atoms in glasses containing both Si and Al atoms and that N atoms do not enter into the glass network but form clusters consisting of AlN4 units in aluminate glasses. The elastic moduli and hardness markedly increase with increasing N content. The increase in elastic moduli with increasing nitrogen content is attributed to the presence of N atoms bonded to three Si atoms. This has been confirmed by an ab initio molecular orbital calculation, which shows that the bending force constant for model molecule N[Si(OH)(3)](3) is very large compared with that for O[Si(OH)(3)](2). It is shown that a drastic improvement in chemical durability by nitriding is seen in some glasses but not in others. The ionic conductivity of alkali silicate glasses increases with increasing nitrogen content. Finally, a brief explanation is made on the attempts in Japan for applying oxynitride glasses as industrial products.