Sv Silvia Nedea

Parameterization of a reactive force field using a Monte Carlo algorithm

Parameterization of a molecular dynamics force field is essential in realistically modeling the physicochemical processes involved in a molecular system. This step is often challenging when the equations involved in describing the force field are complicated as well as when the parameters are mostly empirical. ReaxFF is one such reactive force field which uses hundreds of parameters to describe the interactions between atoms. The optimization of the parameters in ReaxFF is done such that the properties predicted by ReaxFF matches with a set of quantum chemical or experimental data. Usually, the optimization of the parameters is done by an inefficient single‐parameter parabolic‐search algorithm. In this study, we use a robust metropolis Monte‐Carlo algorithm with simulated annealing to search for the optimum parameters for the ReaxFF force field in a high‐dimensional parameter space. The optimization is done against a set of quantum chemical data for MgSO4 hydrates. The optimized force field reproduced the chemical structures, the equations of state, and the water binding curves of MgSO4 hydrates. The transferability test of the ReaxFF force field shows the extend of transferability for a particular molecular system. This study points out that the ReaxFF force field is not indefinitely transferable.

Heat transfer predictions for micro-/nanochannels at the atomistic level using combined molecular dynamics and Monte Carlo techniques

The thermal behavior of a gas confined between two parallel walls is investigated. Wall effects such as hydrophobic or hydrophilic wall interactions are studied, and the effect on the heat flux and other characteristic parameters such as density and temperature is shown. For a dilute gas, the dependence on gas-wall interactions of the temperature profile between the walls for the incident and reflected molecules is obtained using molecular dynamics (MD). From these profiles, the effective accommodation coefficients for different interactions and different mass fluid/wall ratio are derived. We show that Monte Carlo (MC) with Maxwell boundary conditions based on the accommodation coefficient gives good results for heat flux predictions when compared with pure molecular dynamics simulations. We use these effective coefficients to compute the heat flux predictions for a dense gas using MD and MC with Maxwell-like boundary conditions.

Steady-state properties of single-file systems with conversion

We have used Monte-Carlo methods and analytical techniques to investigate the influence of the characteristic parameters, such as pipe length, diffusion, adsorption, desorption and reaction rate constants on the steady-state properties of Single-File Systems with a reaction. We looked at cases when all the sites are reactive and when only some of them are reactive. Comparisons between Mean-Field predictions and Monte-Carlo simulations for the occupancy profiles and reactivity are made. Substantial differences between Mean-Field and the simulations are found when rates of diffusion are high. Mean-Field results only include Single-File behavior by changing the diffusion rate constant, but it effectively allows passing of particles. Reactivity converges to a limit value if more reactive sites are added: sites in the middle of the system have little or no effect on the kinetics. Occupancy profiles show approximately exponential behavior from the ends to the middle of the system.

Infinitely fast diffusion in single-file systems.

We have used dynamic Monte Carlo(DMC) methods and analytical techniques to analyze single-file systems for which diffusion is infinitely fast. We have simplified the master equation removing the fast reactions, and we have introduced a DMC algorithm for infinitely fast diffusion. The DMC method for fast diffusion give similar results as the standard DMC with high diffusion rates. We have investigated the influence of characteristic parameters, such as pipe length, adsorption, desorption, and conversion rate constants on the steady-state properties of single-file systems with a reaction, looking at cases when all the sites are reactive and when only some of them are reactive. We find that the effect of fast diffusion on single-file properties of the system is absent even when diffusion is infinitely fast. Diffusion is not important in these systems. Smaller systems are less reactive and the occupancy profiles for infinitely long systems show an exponential behavior.

Molecular dynamics study on thermal dehydration process of epsomite (MgSO4·7H2O)

Water vapour sorption in salt hydrates is one of the most promising means of compact, low loss and long-term solar heat storage in the built environment. Among all, epsomite (MgSO4·7H2O) excels for its high-energy storage density and vast availability. However, in practical applications, the slow kinetics and evident structural changes during hydration and dehydration significantly jeopardise the heat storage/recovery rate. A molecular dynamics (MD) study is carried out to investigate the thermal properties and structural changes in the thermal dehydration process of the epsomite. The MD simulation is carried out at 450 K and a vapour pressure of 20 mbar, in accordance with experimental heat storage conditions. The study identifies the dehydration as multiple stages from the initial quick water loss and collapse of the crystal framework to the adsorption of water molecules, which inhibits complete dehydration. Further, the anisotropic diffusion behaviour supports the important role of the porous matrix structure in the heat and mass transfer process. The enthalpy changes, partial densities, mass diffusion coefficients of water and radial distribution functions are calculated and compared with corresponding experimental data to support the conclusions.

A DFT based equilibrium study on the hydrolysis and the dehydration reactions of MgCl2 hydrates.

Magnesium chloride hydrates are characterized as promising energy storage materials in the built-environment. During the dehydration of these materials, there are chances for the release of harmful HCl gas, which can potentially damage the material as well as the equipment. Hydrolysis reactions in magnesium chloride hydrates are subject of study for industrial applications. However, the information about the possibility of hydrolysis reaction, and its preference over dehydration in energy storage systems is still ambiguous at the operating conditions in a seasonal heat storage system. A density functional theory level study is performed to determine molecular structures, charges, and harmonic frequencies in order to identify the formation of HCl at the operating temperatures in an energy storage system. The preference of hydrolysis over dehydration is quantified by applying thermodynamic equilibrium principles by calculating Gibbs free energies of the hydrated magnesium chloride molecules. The molecular structures of the hydrates (n = 0, 1, 2, 4, and 6) of MgCl2 are investigated to understand the stability and symmetry of these molecules. The structures are found to be noncomplex with almost no meta-stable isomers, which may be related to the faster kinetics observed in the hydration of chlorides compared to sulfates. Also, the frequency spectra of these molecules are calculated, which in turn are used to calculate the changes in Gibbs free energy of dehydration and hydrolysis reactions. From these calculations, it is found that the probability for hydrolysis to occur is larger for lower hydrates. Hydrolysis occurring from the hexa-, tetra-, and di-hydrate is only possible when the temperature is increased too fast to a very high value. In the case of the mono-hydrate, hydrolysis may become favorable at high water vapor pressure and at low HCl pressure.

Rebuilding of Rothe's nozzle measurements with OpenFOAM software

In this paper the dsmcFoam solver is tested and validated for the the three main solver functionalities of 1) free-stream boundary conditions, 2) kinetic intermolecular collision including internal degrees of freedom and 3) gas/surface interactions. The free-stream utility was improved such that a spatially uniform field of particles gets inserted now yielding reliable results for the cells located close to these patches. Implementation of the collision models were validated for two test cases (monatomic gas mixtures and diatomic gas) by observing the equilibration of both the kinetic and internal energies. It was found that the present code had good agreement to the independent codes of HAWK and SMILE as well as to results by G. Bird. The validation of the present codes treatment for the gas/surface interactions was evaluated using the benchmark case of Rothes nozzle measurements. Results show that the present version of dsmcFoam obtained good agreements for this case compared to the measurements of Rothe for density and temperature. It was also found that the Navier-Stokes solver of OpenFOAM produced reasonable results, even though the local Knudsen number of the flow exceeds the range of applicability for this method, Kn=0.1.

Transient behavior in single-file systems

We have used Monte Carlo methods and analytical techniques to investigate the influence of the characteristics, such as pipe length, diffusion, adsorption, desorption, and reaction rates on the transient properties of single-file systems. The transient or the relaxation regime is the period in which the system is evolving to equilibrium. We have studied the system when all the sites are reactive and also when only some of them are reactive. Comparisons between mean-field predictions, cluster approximation predictions, and Monte Carlo simulations for the relaxation time of the system are shown. We outline the cases where the mean-field analysis gives good results compared to dynamic Monte Carlo results. For some specific cases we can analytically derive the relaxation time. Occupancy profiles for different distributions of the sites both for the mean field and simulations are compared. Different results for slow and fast reaction systems and different distributions of reactive sites are discussed.

Implicit particle wall boundary condition in molecular dynamics

Thin film and nano-tube manufacturing, micro-channel cooling, and many other similar interesting techniques demand the prediction of heat transfer characteristics at the nanometre scale. In this respect, the transport properties at gas—solid and liquid—solid interfaces are very important. The processes at these interfaces can be studied in detail with molecular dynamics (MD) simulations. However, the computational cost involved in simulating the solid wall currently restrains the size of channels, which can be simulated. Therefore, the solid wall is sometimes replaced by boundary conditions, which often compromise on macroscopic quantities, such as density, temperature, pressure, and heat flux. In the current paper, a new particle wall boundary condition is presented, which is in good agreement with existing boundary conditions, but allows for the pressure calculation. This new boundary condition is based on averaging the contributions of an explicit solid wall and is derived using knowledge on common practices in MD algorithms, such as truncation and shifting. Moreover, it allows for different crystal lattices to be included in the new potential. The applicability of the new method is demonstrated by MD simulations of a gas between two parallel plates at different temperatures and densities. Furthermore, these simulations are compared with explicit wall simulations and existing boundary conditions.

Phase Change Materials and Thermochemical Materials for Large-Scale Energy Storage

Replacing the use of fossil fuels by renewables is of paramount importance not so much because of declining reserves (fossil fuel reserves are estimated abundant for at least over a century) but because of increasing CO\(_2\) emissions which cause irreversible climate changes. To overcome the mismatch between supply and demand of solar heat and electricity, smart combinations of heat pumps and heat storage are currently investigated. A reliable method for heat storage is to use thermochemical (TCM) and phase change materials (PCM). These materials should be tested for energy density, temperature range, corrosion, toxicity, (dis)charge time and longevity. A prototype TCM reactor is built and tested for hot water generation. Using zeolite 13X as TCM, it is shown that tap water temperatures of 45 \(^\circ \mathrm{C}\) can be obtained. Using optical microscopy, the hydration and dehydration process of TCM material can be observed, as well as the phase transitions of PCMs. It is also argued that computational molecular modelling methods provide a powerful tool for both TCM and PCM material synthesis.