article posted 22 March 2016
Ondrej Gedeon Having graduated in Mathematics and Physics and obtained PhD in Chemistry he now heads a research group at University of Chemical Technology, Department of Glass and Ceramics, in Prague, Czech Republic. His scientific interest is parted into atomistic simulations, mostly Molecular Dynamics of silicate systems, interaction of glass with electron beam, and characterization of materials by means of spectroscopic and microscopic methods, among them mainly SEM, AFM, EPMA, and XPS. Current projects include i) MD simulation of bulk and surface of silicate systems, ii) modification of glass surface by electron beam, and iii) characterization of corroded glass surfaces
Configuration entropy and glass transition in Molecular dynamics
University of Chemical Technology, Technicka 5, 166 28 Prague 6, Czechia
Glass transition, despite of many decades lasting effort, resists of any simple, unique and widespread acceptable explanation. The transition is often presented as a set of a few particular effects, not always clear how much they are essential for the transformation of melt into glass solid.
Molecular dynamics (MD) offers a powerful tool that provides, in principle, a complete picture of ensemble evolution. Therefore, observation of cooled-down melt should comprise the detailed description of glass transition. Even though MD suffers from the extremely short modelling time resulting in the very high cooling rate, the temperature evolution of the system reveals presence of glass transition and the obtained glass structure closely resembles that found experimentally. Hence, each theoretical model may be directly confronted with microscopic picture of MD glass at hand.
Medium range order (MRO) seems to be crucial for the glass substance. Rings were identified as natural structural units but no unanimous definition is utilised. Commonly used primitive rings do not unambiguously decompose glass structure into rings and therefore another type of ring, so called basic ring, is introduced to uniquely factorize the structure. Consequently, configurational entropy (CE) is evaluated by means of ideal mixing of structural units from the factorized structure. The method was applied to vitreous silica, archetype of silicate glasses, so that temperature evolution of CE may be delivered.
The figure demonstrate the match of the temperature evolution of configuration entropy evaluated from MD and the relaxation model. The inset figure visualises setting of Tool’s temperature to 1474 K, the value close to the experimental one, for the experimental cooling rate of 20 K/min.
On the other hand, relaxation of CE was assumed in the form where is equilibrium value corresponding to the actual microscopic state zeta, and tau is the relaxation time with Arrhenius-like temperature dependence . Solving explicitly the differential equation one can directly compare the theoretical model with MD results. The presented Figure shows not only the match of both approaches but the inset proves the relaxation model yields the correct value of glass transition temperature for the experimental cooling rate.