Abstract
AbstractUltrastable glasses (mostly prepared from the vapor phase under optimized deposition conditions) represent a unique class of materials with low enthalpies and high kinetic stabilities. These highly stable and dense glasses show unique physicochemical properties, such as high thermal stability, improved mechanical properties or anomalous transitions into the supercooled liquid, offering unprecedented opportunities to understand many aspects of the glassy state. Their improved properties with respect to liquid-cooled glasses also open new prospects to their use in applications where liquid-cooled glasses failed or where not considered as usable materials. In this review article we summarize the state of the art of vapor-deposited (and other) ultrastable glasses with a focus on the mechanism of equilibration, the transformation to the liquid state and the low temperature properties. The review contains information on organic, metallic, polymeric and chalcogenide glasses and an updated list with relevant properties of all materials known today to form a stable glass.
Highlights
Glasses can be broadly defined as non-equilibrium solids lacking long-range order
In view of the above, though ultrastability is a tunable quantity, an ultrastable glass could be defined as a glass that simultaneously exhibits: (i) transformation times when annealed above the heat capacity on cooling (T g) that are orders of magnitude larger than the alpha relaxation times at that temperature or equivalently shifts in the onset of devitrification (T on) by some important fraction relative to T g
Equation 4 reduces to the expression for ti of Eq 3 when considering a propagation of the fast region in one-dimension, as it is the case of the parallel front, an equivalency between tR and ti, τ (TFfin, T ) and τ, τ (TFin, T ) and tbulk, and a beta close to one, implying a relatively sharp distribution of relaxation times in the glass, as is expected for ultrastable glasses [106] (An estimation of the dispersion in front velocity, and in relaxation time according to Eq 4, in ultrastable glasses yields distributions with a full-width half-maximum lower than a 25% of total deviation [106])
Summary
Glasses can be broadly defined as non-equilibrium solids lacking long-range order. The absence of periodicity provides exceptional attributes to these materials. Organic glasses show low enthalpic fictive temperatures while in metallic glasses, due to the difficulty of measuring in a sufficiently large temperature interval the heat capacity of the liquid because of crystallization, it is best to correlate the stability with respect to the mechanical properties At this point, we introduce the potential energy landscape (PEL) paradigm, which is so widely employed to visualize and discuss the dynamic and thermodynamic behavior of glasses and viscous liquids. A vapor deposited ultrastable glass can access low energy states, close to the ideal glass many authors have speculated on the possible existence of an “ideal glass” which should correspond to the best and most stable possible glass achievable, associated with the lowest relative minimum This ideal glass would have zero configurational entropy, equal to that of crystals, and has been associated to the possible existence of an underlying, likely second order, thermodynamic glass transition occurring at the so-called Kauzmann temperature T K [22]. The lower the value of T f, the higher is the stability of the glass or, to put it another way, the lower is the position of the glass in the PEL
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