Glasses can be described as liquids no longer able to flow and, in fact, are usually prepared by fast quench of the melt to below the glass transition temperature. As the temperature decreases, the characteristic timescale of the molecular motions increases by many orders of magnitude, and the liquid becomes more and more viscous. The glass transition temperature is conventionally defined as the temperature where the molecular motions are so slow that the liquid is no longer able to equilibrate on the typical experimental timescale, and appears macroscopically as a solid.
Many materials around us are in equilibrium conditions, which implies that their properties depend only on the thermodynamic state variables, e.g. temperature and pressure. A glass, instead, is an out-of-equilibrium material, and its properties also depend on the protocol used to prepare it. This provides a quite large flexibility in choosing the properties of these materials and is key to their widespread use. In fact, many applications of glasses, e.g. in optics, require to optimize their transparency and homogeneity, and to this aim glasses are typically annealed (aged). Other applications, e.g. for bendable covers of self-phones, require improved ductility, which is usually achieved by rejuvenating the glass, i.e. bringing it into a higher enthalpy state.
The most common way of changing the properties of a glass in a given thermodynamic state is provided by the choice of the cooling rate used to quench the glass from the melt. A very slow cooling rate (or annealing below the glass transition temperature) can produce a very stable (low enthalpy) glass, while fast quenching will produce an unstable glass. This approach has however practical limitations.
The project GLAXES tackles the challenge of developing an approach based on non-thermal, x-ray irradiation to produce glasses at the extremes of their stability range: ultra-stable and ultra-unstable glasses. There are many applications where such extreme glasses might be useful for. Ultra-stable glasses could be key to overcome mechanical thermal noise limitations in highly technological applications, with examples in opto-mechanics and nanomechanical devices. Ultra-unstable glasses are candidates to display a ductile response, which would open glasses to new applications, e.g. in mechanical engineering.
