X-ray-induced fluidization: a non-equilibrium pathway to reach glasses at the extremes of their stability range.

Informazioni relative al progetto

GLAXES

ID dell’accordo di sovvenzione: 101053167

DOI

10.3030/101053167

Data della firma CE 16 Giugno 2022

Data di avvio 1 Gennaio 2023

Data di completamento 31 Dicembre 2027

Finanziato da

• European Research Council (ERC)

Costo totale

€ 2 499 876,25

Contributo UE

€ 2 499 876,00

Coordinato da

UNIVERSITA DEGLI STUDI DI PADOVA
Italy

Periodic Reporting for period 1 - GLAXES (X-ray-induced fluidization: a non-equilibrium pathway to reach glasses at the extremes of their stability range.)

Periodo di rendicontazione: 2023-01-01 al 2025-06-30

Sintesi del contesto e degli obiettivi generali del progetto

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.

Lavoro eseguito dall’inizio del progetto fino alla fine del periodo coperto dalla relazione e principali risultati finora ottenuti

The activities of the project GLAXES are focused on the characterization of the physical properties of inorganic glasses upon irradiation with hard X-rays. To this aim, a new setup has been developed to combine thermal, structural and dynamical measurements of glasses during x-ray irradiation. This setup has been realized integrating a flash-calorimeter in a beamline for coherent x-ray scattering in a synchrotron radiation center, see figure. In fact, synchrotron radiation sources are brilliant enough to allow reaching the high doses (energy absorbed per unit mass) required to significantly change the properties of glasses.
A number of inorganic glasses, including silicates, borates and chalcogenides, have been screened within GLAXES and they have all been found to change upon x-ray irradiation in a qualitatively similar way. The effects of x-ray irradiation start, in a first stage, with the creation of defects in the glass structure. At low doses the defects are few and sparse in the glass matrix that, from a mechanical point of view, behaves essentially as the pristine glass. At this stage, the structure is very little changed but the enthalpy of the glass is higher than for the pristine glass, which means that the glass is being rejuvenated by irradiation. This is similar (though not identical) to a glass state produced at higher quenching rate.
As the irradiation proceeds, the number of defects in the glass structure continue to increase until, at one point, they reach a maximum in a stationary state, a state that does not evolve any longer upon further x-ray irradiation: an equilibrium is reached between the number of defects that are continuously produced by x-ray irradiation and those that decay to the ground state. We describe such state as being defect saturated. In this stationary state the thermal properties of the glass are stable, as are the structural and dynamical ones. We describe this state as a state of x-ray induced yielding as the glass flows upon irradiation. The flow observed in this state is however different from that of a liquid: it is a-thermal, i.e. fully dependent on the irradiation conditions (dose and dose-rate) and independent of the temperature of the glass.

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The results obtained by GLAXES until now establish a clear parallel between the changes that take place in glasses upon x-ray irradiation and the changes that take place upon mechanical solicitation. It is textbook knowledge, in fact, that a ductile solid mechanically loaded beyond the yield stress loses its elastic properties and becomes plastic until, eventually, yielding. From a macroscopic point of view, this corresponds to the condition where plastic regions become so densely packed that they give rise to system-spanning structures. Upon x-ray irradiation, the yield point is reached by the alternative approach of increasing the density of point defects generated during x-ray irradiation. From a qualitative point of view, the mechanical defects generated upon loading a glass seem to play the same role as the x-ray induced defects, though this observation calls for more investigations.
The results obtained until now open up the possibility to study yielding using a different approach than the traditional mechanical one. From a technical point of view, the yielding transition reached upon mechanical loading is, for brittle materials as most glasses are, a transient state that precedes fracture, and is thus difficult to characterize. Upon x-ray irradiation, instead, yielding is a stationary state prone to detailed characterization studies. This can be of high value as the yielding transition in amorphous materials remains an open problem of high scientific relevance, but also with obvious practical consequences: understanding and maybe predicting the location of the yielding transition in amorphous materials can be crucial, e.g. for many applications in mechanical engineering.
The results obtained until now also show that while the glass at yield has higher enthalpy than the annealed glass, its properties are not extreme as one could expect: they rather match those of a glass instantaneously quenched from a temperature about 20% higher than the glass-transition temperature. This is a well-known, key temperature for glass-forming liquids that marks the location of a dynamical transition, and it is remarkable that glasses prepared using different protocols (and thus at different enthalpy levels) head all there upon irradiation.
Finally, the results obtained until now clearly suggest that possible applications of the glass state reached upon sufficient irradiation are related to its stationarity: a glass in this state could be the ideal solution in radiation-hostile environments as, e.g. in space.

A sketch of the setup used for x-ray irradiation and for structural and dynamical measurements.