Evidence of a fragility window that correlates with the reversibility window in (Na₂O)_x (B₂O₃)_100-x glasses

Evidence for a reversibility window in titled glasses was reported¹ recently and permitted fixing the Topological phases. The reversibility window permitted one to identify glasses at x < 20% to be in the Stressed-rigid phase those at x > 40% to be in the flexible phase and those in between with the Intermediate Phase. We now report on melt fragility index using mDSC. The imaginary part of C_p, examined in the cool down mode, shows a Gaussian-like peak that shifts to higher T as the modulation frequency is increased. By tracking the peak shift in C_p^imag with T, we establish how τ , the enthalpic relaxation time with T and deduce the fragility index (m). Our results show that m(x) also displays a square-well like variation with m< 20 in the 20 < x < 40% range of soda, coinciding with the reversibility window. Remarkably if the glasses are not dry the variation of m(x) changes qualitatively²; m(x) increases quadratically in x underscoring that Na⁺ ions now diffuse more rapidly. Glass synthesis conditions can qualitatively alter melt dynamics.
1. K. Vignarooban et al 2014 EPL 108 56001
2. Yasuteru Fukawa, Determination of complex-specific heat and fragility of sodium borate glasses by temperature-modulated DSC, J Therm Anal Calorim, 2010 99:39–44