Cooling-Rate Computer Simulations for the Description of Crystallization of Organic Phase-Change Materials

Abstract

A molecular-level insight into phase transformations is in great demand for many molecular systems. It can be gained through computer simulations in which cooling is applied to a system at a constant rate. However, the impact of the cooling rate on the crystallization process is largely unknown. To this end, here we performed atomic-scale molecular dynamics simulations of organic phase-change materials (paraffins), in which the cooling rate was varied over four orders of magnitude. Our computational results clearly show that a certain threshold (1.2 × 1011 K/min) in the values of cooling rates exists. When cooling is slower than the threshold, the simulations qualitatively reproduce an experimentally observed abrupt change in the temperature dependence of the density, enthalpy, and thermal conductivity of paraffins upon crystallization. Beyond this threshold, when cooling is too fast, the paraffin’s properties in simulations start to deviate considerably from experimental data: the faster the cooling, the larger part of the system is trapped in the supercooled liquid state. Thus, a proper choice of a cooling rate is of tremendous importance in computer simulations of organic phase-change materials, which are of great promise for use in domestic heat storage devices.

A Comprehensive Investigation into the Crystallology, Molecule, and Quantum Chemistry Properties of Two New Hydrous Long-Chain Dibasic Ammonium Salts CnH2n+8N2O6 (n = 35 and 37)

“…Due to their carboxyl group, long-chain saturated fatty acids can undergo an esterification reaction [ 5 , 6 ], acylation reaction [ 7 , 8 ], salt formation reaction [ 9 ], oxidation-reduction reactions [ 10 ], and decarboxylation reactions [ 11 , 12 , 13 ]. In recent years, in addition to being widely studied in the field of biochemistry, long-chain saturated fatty acids have also drawn much attention in the field of thermodynamics [ 14 , 15 ], especially in the area of phase change energy storage [ 16 , 17 , 18 , 19 , 20 , 21 , 22 ], where they are becoming increasingly popular.…”

Size matters: asphaltenes with enlarged aromatic cores promote heat transfer in organic phase-change materials

“…While computationally demanding, a cooling rate of this magnitude has been previously demonstrated as being necessary to accurately model the temperature-dependent changes in mass density and thermal conductivity that occur in paraffin during crystallization. 46…”

“…While computationally demanding, a cooling rate of this magnitude has been previously demonstrated as being necessary to accurately model the temperature-dependent changes in mass density and thermal conductivity that occur in paraffin during crystallization. 46 All simulations were performed using the all-atom CHARMM36 force field. 47,48 The aggregation of asphaltenes stems from p-p interactions.…”

Solubility of Gases and Free Volume Evolution in R-BAPB Polyimide: Molecular Dynamics Simulations and Analytical Theory Insights into Cooling Velocity Effect

“…Most schemes for preparing a glassy sample include a cooling stage with cooling velocities many orders of magnitude higher than typical standard experimental values. However, it is the thermal history, directly related to the cooling velocity upon passing through the glass transition or crystallization region, that can significantly affect the values of all physical properties, including the density, Young’s modulus, and the thermal conductivity. − Thus, the preparation stage of polymer membranes (both in simulations and experiments) may be a key factor when comparing the results of computer modeling with real measurements.…”

“…Previous studies show that MD simulations allow us to capture the impact of the cooling velocity on several macroscopic properties of polymers and oligomers, such as density (specific volume), glass transition temperature, Young’s modulus, etc. − ,− Moreover, for such fundamental polymer properties as the glass transition temperature, MD simulations have reproduced the experimentally known empirical logarithmic dependence on the cooling velocity. ,− Recently, Khare and Phelan have proposed a methodology for extrapolating MD simulation results for the temperature dependence of the polymer density to allow a direct comparison with the corresponding experimental curves by considering epoxy resins . Inspired by these results, we attempt a step toward a more comprehensive investigation of the polymer transport properties in relation to the thermal history of polymer samples in MD simulations.…”