In a recent Journal of Physics: Condensed Matter paper, C Desgranges, P W Anderson and J Delhommelle used molecular simulation to determine the critical properties of Si, revealing the loci of several remarkable thermodynamic contours spanning the supercritical region of the phase diagram. Read on to find out more from the authors themselves.
Supercritical fluids have drawn considerable interest in recent years, as their unusual physical properties has led to the development of new methods to process materials. However, significant gaps remain in our understanding of supercritical fluid thermodynamics. For instance, the thermodynamic properties of fluids at temperatures below and up to the critical point often exhibit extensive similarities, as summarized by the law of corresponding states. On the other hand, it was only recently that remarkable thermodynamic loci, such as the existence of a straight Zeno line and the curve of ideal enthalpy for the van der Waals fluid, have started to be identified in the supercritical region of the phase diagram.
While this hints at how a map of the supercritical region of the phase diagram could be drawn, this recent line of inquiry has also prompted the emergence of a new set of questions regarding the sensitivity of the features of these thermodynamic loci on the interatomic interactions. In this work, we carry out Expanded Wang-Landau simulations on classical and quantum (tight-binding) many-body force fields of Si to determine the thermodynamic loci for several ideality contours, including the Zeno line (along which the compressibility factor of the fluid is 1) and the H line (along which the enthalpy of the fluid is the same as an ideal gas). Regardless of the strategy used to model the interactions between Si atoms, we find that Zeno and H lines remain remarkably linear across a large temperature interval spanning the supercritical region of Si , as shown in the figure. This suggests that the features of these ideality contours are robust and hold for a wide range of elements and compounds, providing new ways to establish a correspondence between the thermodynamic properties of different supercritical fluids.
About the authors
This study was carried out in the ‘Molecular Simulation of NonEquilibrium Systems’ (MSNEP) group in the Chemistry Department at the University of North Dakota. Partial funding for this research was provided by the National Science Foundation (Division of Materials Research) through a CAREER award (DMR-1052808) and by the Donors of the American Chemical Society Petroleum Research Fund through a New Directions grant (ND548002-ND10).
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