Deformation, stress relaxation, and crystallization of lithium silicate glass fibers below the glass transition temperature

Author： Reis S. T. Kim Cheol-Woon Brow R. K. Ray C. S.

Publisher： Springer Publishing Company

ISSN： 0022-2461

Source： Journal of Materials Science, Vol.39, Iss.21, 2004-11, pp. : 6539-6549

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Abstract

The deformation and crystallization of Li₂O·2SiO₂ and Li₂O·1.6SiO₂ glass fibers subjected to a bending stress were measured as a function of time over the temperature range ∼50 to ∼150°C below the glass transition temperature (T_g). The glass fibers can be permanently deformed at temperatures about 100°C below T_g, and they crystallize significantly at temperatures close to, but below T_g, about 150°C lower than the onset temperature for crystallization for these glasses in the no-stress condition. The crystallization was found to occur only on the surface of the glass fibers with no detectable difference in the extent of crystallization in tensile and compressive stress regions. The relaxation mechanism for fiber deformation can be best described by a stretched exponential (Kohlrausch-Williams- Watt (KWW) approximation), rather than a single exponential model.The activation energy for stress relaxation, E_s, for the glass fibers ranges between 175 and 195 kJ/mol, which is considerably smaller than the activation energy for viscous flow, E_η (∼400 kJ/mol) near T_g for these glasses at normal, stress-free condition. It is suspected that a viscosity relaxation mechanism could be responsible for permanent deformation and crystallization of the glass fibers below T_g.