Energy Release in Isothermally Stretched Silicate Glass fibers

Abstract

Three types of silicate glass fibers are annealed, simultaneously stretched in the glass transition region for certain time lengths, then slowly cooled to room temperature under load, and subsequently scanned by differential scanning calorimetry (DSC). During the DSC scanning, a broad exothermic peak (representing energy release) occurs in the stretched fibers well below the glass transition temperature, while it does not occur in the non‐stretched fibers. The peak indicates that mechanical stretching can result in an energy enhancement in the fibers. It also confirms that the energy released during reheating of the fibers formed using an industrial continuous fiber drawing process originates not only from thermal quenching but also from mechanical stretching. However, the mechanical stretching‐induced energy is much lower than the thermal hyperquenching‐induced energy in glass fibers. The effect of annealing temperature and time on the energy release behavior is discussed in terms of viscoelasticity.

Anomalous Enthalpy Relaxation in Vitreous Silica

“…The effect of the surface absorbed water on the enthalpy relaxation pattern could be ruled out since similar relaxation patterns are also observed in HQ metallic glasses that do not have chemically bonded surface water (Hu et al, 2011). Furthermore, the enthalpy enhancement in glass fibers induced by mechanical stretching is much weaker than that induced by hyperquenching, and this is verified by the sub-Tg relaxation studies (Martin et al, 2005;Hornbøll et al, 2006). All this implies that the enthalpy relaxation peaks of HQSiO2 (Figures 5C and 6C) arise mainly from the release of the excessive enthalpy trapped in the samples by hyperquenching, but neither from surface relaxation nor from the stretching stress relaxation.…”

“…The finding of such features is of both scientific importance and technological importance. The optical and mechanical properties of SiO2 fibers are strongly influenced by their thermal history (e.g., cooling rate and the degree of annealing), the repeated heating-cooling cycles and mechanical history (e.g., drawing stress) (Martin et al, 2005;Hornbøll et al, 2006;Ya et al, 2008). The outcome of this work is useful for optimizing both forming conditions and annealing process of vitreous silica fibers with respect to their optical and mechanical performances.…”

Entropic shrinkage of an oxide glass

“…6). Although the exotherm below T g for glass fibres [26][27][28] is reported to be attributed to the simultaneous relaxation of both frozen-in liquid structures at the fictive temperature and frozen-in local structures within the glass fibres due to axial stress exerted by the drawing process, the endotherm above T g , observed only in the anisotropic glass, has not previously been reported in oxide glasses. Figure 5b shows the difference between φ aniso and φ iso , and the shrinkage rate (% K −1 ) of an anisotropic fibre with a birefringence of 0.008.…”

Enthalpy relaxation in hyperquenched glasses of different fragility

“…Both methods are expected to introduce some amount of anisotropy and thereby mechanical structure change in the samples. Such effects have been ignored because of suggestions that the enthalpy excitation with this origin is very small compared to the amount from thermal quenching [16,27].…”