Computer modelling of the surface tension of the gas-liquid and liquid-liquid interface

This review presents the state of the art in molecular simulations of interfacial systems and of the calculation of the surface tension from the underlying intermolecular potential. We provide a short account of different methodological factors (size-effects, truncation procedures, long-range corrections and potential models) that can affect the results of the simulations. Accurate calculations are presented for the calculation of the surface tension as a function of the temperature, pressure and composition by considering the planar gas-liquid interface of a range of molecular fluids. In particular, we consider the challenging problems of reproducing the interfacial tension of salt solutions as a function of the salt molality; the simulations of spherical interfaces including the calculation of the sign and size of the Tolman length for a spherical droplet; the use of coarse-grained models in the calculation of the interfacial tension of liquid-liquid surfaces and the mesoscopic simulations of oil-water-surfactant interfacial systems.

Molecular Dynamics Simulations of Helium Atoms Clustering in bcc Iron

Ning Gao

Helium atoms are known to have a significant impact on materials used in fission and fusion reactors. In particular, the presence of helium atoms can change the mechanical properties and degrade the lifetime of reactors. In order to develop the helium-resistance materials, the underlying interactions between helium atoms and matrix atoms have to be understood. In the past, atomistic simulations have contributed a lot to the basic energy states of helium atoms in different defects of materials. Especially, insights into the formation energies of helium atoms at simple defects have been calculated by ab initio and molecular dynamics. The substitional helium has lower formation energy than the tetrahedral and octahedral interstitial helium atom. Additionally, the binding energies of helium with small HenVm cluster have also been found to be all positive. However, all these simulations have been performed without showing the kinetic formation process of such helium defects. The interactions of defect with larger number of helium atoms are also not explored in detail. In the first part of this thesis, a new relaxation-fitting method has been developed for cross potential of Fe-He system based on more energy data of helium-defect clusters from ab initio calculations and by including the migration energy of single free helium atom in the fitting process. Besides, the recently fitted magnetic potentials of Fe have been used to describe the Fe-Fe interactions for such cross potential. The new fitted Fe-He potentials have predicted well not only the properties of simple helium defect, but also the properties of helium clusters and diffusion process of helium atoms. Based on the new Fe-He potentials, the interactions between helium atoms and Fe with the effect of single vacancy have been simulated utilizing the classical molecular dynamics (MD) method in the single bcc Fe lattice at room temperatures by helium atoms up to 18. The binding energies of helium atom with HenV cluster are computed. By increasing number of helium atoms, the binding energy decreases first, then slowly increases up to n = 4. It is then almost constant between n = 5 to 15 but increases strongly at n = 16 to form a peak. This last increase is related to the athermal self-interstitial atom (SIA) emission in the form of dumbbell which relaxes the restrained system to He16V2 cluster. The second binding energy peak occurs at n = 18 by the formation of the second SIA to form He18V3 cluster. Close inspection of the local configuration shows the formed SIA combines with the helium-defect cluster. Calculation of the pressure of the helium-defect cluster shows local peak normal stress and shear stress values up to 9 GPa and 4 GPa, respectively. The local configurations of helium-defect cluster suggest that with increasing helium content, some symmetrical structures can be formed. By increasing the number of helium atoms inserted in single bcc Fe lattice, the clustering process of the formed SIAs with presence of helium cluster has been simulated with MD methods at room temperatures with the new fitted Fe-He potential. The first SIA is formed after 6 He atoms being inserted without vacancy. The simulations indicate that the SIA has mainly two configurations as either a dumbbell or a crowdion, which can be exchanged in the interface between helium bubble and matrix atoms. The SIA cluster can change direction and absorb more SIAs to grow. Small dislocation loops have been observed to form with Burgers vector b = 1/2 a0 on a {111} habit plane. The kinetic process of formation and growth of dislocation loop explored by MD simulations includes: (1) the formation of a self-interstitial atom by the high pressure of the helium cluster/bubble; (2) the diffusion of these self-interstitial atoms on the interface between helium bubble and matrix; (3) the clustering of the self-interstitial atoms in the form of a dislocation loop and its diffusion away from bubble when the number of SIAs in loop reaches a critical number. Moreover, the effect of grain boundary (GB) on the formation of helium clusters has been explored with MD simulations in symmetrical GBs and normal nano-grain boundary samples. Helium atoms can be trapped by GBs at free spaces. The local atomic excess free volume (LAEFV) is found to the one of the main parameters not only related with the energy state of GB but also with the nucleation and growth of He atoms in the GB.

EPFL2011

Molecular Simulation Study of the Competitive Adsorption of H2O and CO2 in Zeolite 13X

Lennart Joos, Berend Smit

The presence of H2O in postcombustion gas streams is an important technical issue for deploying CO2-selective adsorbents. Because of its permanent dipole, H2O can interact strongly with materials where the selectivity for CO2 is a consequence of its quadrupole interacting with charges in the material. We performed molecular simulations to model the adsorption of pure H2O and CO2 as well as H2O/CO2 mixtures in 13X, a popular zeolite for CO2 capture processes that is commercially available. The simulations show that H2O reduces the capacity of these materials for adsorbing CO2 by an order of magnitude and that at the partial pressures of H2O relevant for postcombustion capture, 13X will be essentially saturated with H2O. © 2013 American Chemical Society.

2013

Molecular dynamics simulations of nucleation and phase transition in halide perovskites

Paramvir Ahlawat

Perovskite solar cells have emerged as the most promising cheaper photovoltaic technology. Besides solar cells, halide perovskites have a wide range of applications due to their remarkable optoelectronic properties. Starting from 3.8% in 2009, solar to the power conversion efficiency of perovskite solar cells is now >25%, exceeding market leader polycrystalline silicon. These advancements are mainly due to the improvement in their manufacturing process targeted to make better morphology. To this objective, tens of thousands of experimental studies are conducted where a wide range of additives, compositions, and processing conditions are optimized based on a trial and error methodology. Although this approach increased the efficiency, however, perovskite electronics suffer from the problem of stability and reproducibility. Therefore, to industrialize highly efficient perovskite electronics, one needs to understand better and rationalize their fabrication process, i.e., crystallization of halide perovskites. In this thesis, I perform molecular simulations to understand the formation process of halide perovskites. First, I try to lay out the atomic details of the nucleation process of perovskite from the solution. I find that nucleation proceeds in a two-step manner where an intermediate phase forms before the emergence of perovskite crystals from the solution. I observe that monovalent cations can play an essential role in the nucleation process. On this information, simulation-driven experiments are performed by experimental collaborators to demonstrate that the grain size and crystallinity can be tuned by only varying the concentration of monovalent cations in solution. I also attempted to study the effects of different solvents and pseudo-halide additives used to make high-efficiency solar cells. I calculated that by modulating the solute-solvent interactions, one could minimize raw material losses during the coating process. In comparison, pseudo-halide additives are found to passivate the ionic defects and may slow down the growth process that helps to improve the final morphology. Second, I study the solid-solid phase transition in the popular two-step process. I discover a new path to the low-temperature formation of highly efficient metastable cubic phase of formamidinium lead iodide perovskite. I first validate my simulations with in-situ experiments performed by experimental collaborators. Then simulations are employed to design a practical methodology to successfully make thin films of phase pure cubic formamidinium lead iodide at lower temperatures. Moreover, I study the intrinsic stability of this material. I calculate the energy barrier between the metastable photo-active phase and its photo-inactive thermodynamically stable hexagonal phase. I find that once formed; the photo-active phase might be kinetically stable. Additionally, I identify new polymorphs (perovskitoids) of halide perovskites and also study the structural insights of low-dimensional halide perovskites. Overall, I present a fundamental understanding of the crystallization of perovskites which is helpful to design better experiments to make high efficiency and stable perovskite electronics.