| INTERMEDIATE PHASES, REVERSIBILITY WINDOWS, STRESS-FREE AND NON-AGING NETWORKS, AND STRONG LIQUIDS (2008) | |||||||||||||||
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| Network glasses usually display glass transitions that are hysteretic, i.e., a second scan across a glass transition endotherm usually does not replicate the first one. But in recent years, examination of glass transition endotherms of several inorganic systems [1-4] reveal a different pattern: one observes compositional windows across which glass transitions become almost completely non-hysteretic. Specifically, the non-reversing enthalpy ( Hnr) associated with the glass transition (Tg) accessed from modulated Differential Scanning Calorimetry is found to nearly vanish [1-4]. These compositional windows, also called reversibility windows, usually occur [5] in the 2.29 < r < 2.52 range, where r represents the mean coordination number, a measure of network connectivity. Glasses in reversibility windows form ideal stress-free networks. The ideality derives from the optimization of glass forming tendency. The stress-free character of binary GexSe1-x glasses in the reversibility window, 0.20 < x < 0.25, was recently demonstrated [5] in Raman pressure measurements. In these experiments one found that select vibrational modes blue-shift as a function of external pressure (P), but only once P exceeds a threshold value (Pc). The threshold pressures, Pc, serve as a measure of network internal stress. Chemical trends in Pc(x) are found [5] to closely track | |||||||||||||||
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