Glasses, supercooled liquids and stability: probing the crystallization kinetics with neutrons

by Dr Marco Zanatta (Department of Computer Science, University of Verona, Verona, Italy)

Monday 17 Dec 2018, 14:30 → 15:30 Europe/Berlin

HS 3 (Physics Department)

Glasses are ubiquitous in everyday life and we usually consider them as fully understood materials. Conversely, despite an increasing use in technological devices, our scientific knowledge is still far from a full comprehension. The glassy state can be seen as a metastable condensed state of matter that couples a liquid-like disordered structure with a solid-like atomic dynamics. The mechanism itself of the glass formation, the glass transition, and many properties of the glassy state represent open and debated topics. Moreover, the same phenomenology is common also to polymers, gels, colloids, and biological materials like proteins and DNA.

In this seminar I will briefly review some properties of the glassy state and some of the open questions that characterize the physics of these systems below the glass transition temperature. I will then focus on glass stability and crystallization. Despite their apparent stability, the ultimate fate of glasses is indeed to crystallize. I will then “heat up” the temperature slightly above the glass transition, accessing the supercooled liquid region, showing the results of a neutron diffraction investigation of the isothermal crystallization process in a protypical glassformer such as GeO2. Experimental data allow to observe the real-time evolution of the process, showing the continuous reorganization of the amorphous structure towards a crystalline phase. In a timescale of days, the final material results as composed by crystalline domains plunged into a low-density, residual amorphous matrix. An empirical model was then developed to provide a description of the experimental data. This approach identifies a predator-prey-like interplay between crystal and amorphous, where the density variation acts as a blocking barrier. In addition to a fundamental interest, the study of this kind of phenomena could have interesting application. For example the crystallization of volcanic magmas strongly affects the eruptive style of volcanoes and thus their associated risks.