Surface Structure of Silica Glasses by Molecular Dynamics Simulations

1985 ◽  
Vol 61 ◽  
Author(s):  
S. M. Levine ◽  
S. H. Garofalini
Keyword(s):  
Molecular Dynamics ◽  
Silica Glass ◽  
Surface Region ◽  
Surface Relaxation ◽  
Silica Glasses ◽  
Pure Silica ◽  
New Information ◽  
Outermost Surface ◽  
Dynamics Simulations ◽  
Glass Surfaces

ABSTRACTMolecular dynamics computer simulations were used to study surfaces of pure silica glass. The potentials used here were those previously established to model bulk silica and have been extended to study surface relaxation in a perfect vacuum. A large number of surfaces were made using different starting configurations; system sizes, and cooling procedures. Following “fracture”, many broken bonds rearranged in response to the changes in the net forces in the surface region. After this reconstruction, the simulations showed the expected general features observed experimentally, such as a prevalence of oxygen atoms at the outermost surface, non-bridging oxygens, and strained siloxane bonds. Three fold silicons (similar to e’ centers) were initially present in the “fractured” surfaces but most often were incorporated into the network tetrahedrally after reconstruction. Other defects produced during the reconstruction were five coordinated silicons and more importantly, edge sharing tetrahedra, forming the strained siloxane bonds. Bond angles and bond lengths for each defect were determined, showing good agreement with previously published results as well as providing new information. Finally, estimations for silanol concentrations were made which compare well with experimentally determined coverages. The computer simulation technique used here adequately reproduces many of the structural and dynamic characteristics of silica glass surfaces.

Absorption of Mechanical Energy via Formation of Ice Nanotubes in Zeolite

2021 ◽  
Author(s):  
Kenji Mochizuki
Keyword(s):  
Molecular Dynamics ◽  
Molecular Dynamics Simulations ◽  
Heterogeneous System ◽  
Mechanical Energy ◽  
Bulk Water ◽  
Pure Silica ◽  
Dynamics Simulations ◽  
First Time

Abstract Molecular dynamics simulations are carried out for a heterogeneous system composed of bulk water and pure-silica zeolites of the AFI type. Our simulations show, for the first time, the...

scholarly journals Interface Characteristics of Neat Melts and Binary Mixtures of Polyethylenes from Atomistic Molecular Dynamics Simulations

Polymers ◽  
2020 ◽  
Vol 12 (5) ◽  
pp. 1059
Author(s):  
Sanghun Lee ◽  
Curtis W. Frank ◽  
Do Y. Yoon
Keyword(s):  
Molecular Dynamics ◽  
Surface Tension ◽  
Molecular Dynamics Simulations ◽  
Binary Mixtures ◽  
Surface Segregation ◽  
Surface Region ◽  
Polymer Chains ◽  
Methyl Groups ◽  
Free Standing ◽  
Dynamics Simulations

Molecular dynamics simulations of free-standing thin films of neat melts of polyethylene (PE) chains up to C150H302 and their binary mixtures with n-C13H28 are performed employing a united atom model. We estimate the surface tension values of PE melts from the atomic virial tensor over a range of temperatures, which are in good agreement with experimental results. Compared with short n-alkane systems, there is an enhanced surface segregation of methyl chain ends in longer PE chains. Moreover, the methyl groups become more segregated in the surface region with decreasing temperature, leading to the conclusion that the surface-segregation of methyl chain ends mainly arises from the enthalpic origin attributed to the lower cohesive energy density of terminal methyl groups. In the mixtures of two different chain lengths, the shorter chains are more likely to be found in the surface region, and this molecular segregation in moderately asymmetric mixtures in the chain length (C13H28 + C44H90) is dominated by the enthalpic effect of methyl chain ends. Such molecular segregation is further enhanced and dominated by the entropic effect of conformational constraints in the surface for the highly asymmetric mixtures containing long polymer chains (C13H28 + C150H3020). The estimated surface tension values of the mixtures are consistent with the observed molecular segregation characteristics. Despite this molecular segregation, the normalized density of methyl chain ends of the longer chain is more strongly enhanced, as compared with the all-segment density of the longer chain itself, in the surface region of melt mixtures. In addition, the molecular segregation results in higher order parameter of the shorter-chain segments at the surface and deeper persistence of surface-induced segmental order into the film for the longer chains, as compared with those in neat melt films.

scholarly journals Study of the Lamellar and Micellar Phases of Pluronic F127: A Molecular Dynamics Approach

Processes ◽  
2019 ◽  
Vol 7 (9) ◽  
pp. 606 ◽  
Author(s):  
Juan Albano ◽  
Damian Grillo ◽  
Julio Facelli ◽  
Marta Ferraro ◽  
Mónica Pickholz
Keyword(s):  
Molecular Dynamics ◽  
Amorphous Structure ◽  
Pluronic F127 ◽  
Coarse Grain ◽  
Lamellar Thickness ◽  
Initial Polymer ◽  
Lamellar Phases ◽  
New Information ◽  
Order Of Magnitude ◽  
Dynamics Simulations

In this work, we analyzed the behavior of Pluronic F127 through molecular dynamics simulations at the coarse-grain level, focusing on the micellar and lamellar phases. To this aim, two initial polymer conformations were considered, S-shape and U-shape, for both simulated phases. Through the simulations, we were able to examine the structural and mechanical properties that are difficult to access through experiments. Since no transition between S and U shapes was observed in our simulations, we inferred that all single co-polymers had memory of their initial configuration. Nevertheless, most copolymers had a more complex amorphous structure, where hydrophilic beads were part of the lamellar-like core. Finally, an overall comparison of the micellar a lamellar phases showed that the lamellar thickness was in the same order of magnitude as the micelle diameter (approx. 30 nm). Therefore, high micelle concentration could lead to lamellar formation. With this new information, we could understand lamellae as orderly packed micelles.

Nucleation Process During Excimer Laser Annealing of Amorphous Silicon Thin Films on Glass: A Molecular-dynamics Study

2006 ◽  
Vol 958 ◽  
Author(s):  
Shinji Munetoh ◽  
Takanori Mitani ◽  
Takahide Kuranaga ◽  
Teruaki Motooka
Keyword(s):  
Thin Films ◽  
Molecular Dynamics ◽  
Amorphous Silicon ◽  
Excimer Laser ◽  
Laser Annealing ◽  
Surface Region ◽  
Excimer Laser Annealing ◽  
Silicon Thin Films ◽  
Steady State Temperature ◽  
Dynamics Simulations

ABSTRACTWe have performed molecular-dynamics simulations of heating, melting and recrystallization processes in amorphous silicon (a-Si) thin films deposited on glass during excimer laser annealing. By partially heating the a-Si surface region with 2 nm depth and removing thermal energy from the bottom of the glass substrate, a steady-state temperature profile was obtained in the a-Si layer with the thickness of 15 nm and only the surface region was melted. It was found that nucleation predominantly occurred in the a-Si region as judged by the coordination numbers and diffusion constants of atoms in the region. The results suggest that nucleation occurs in unmelted residual a-Si region during the laser irradiation and then crystal growth proceeds toward liquid Si region under the near-complete melting condition.

Dissolution Dynamics of Sub-Critical Clusters in Liquid Silicon

1990 ◽  
Vol 205 ◽  
Author(s):  
Michael J. Uttormark ◽  
Stephen J. Cook ◽  
Michael O. Thompson ◽  
Paulette Clancy
Keyword(s):  
Molecular Dynamics ◽  
System Size ◽  
Atomic Processes ◽  
Spontaneous Nucleation ◽  
New Information ◽  
Different Temperatures ◽  
Transient Nucleation ◽  
Dynamics Simulations ◽  
Small Clusters ◽  
Kinetics Of

AbstractPrevious attempts to simulate by Molecular Dynamics the spontaneous nucleation and growth of a crystalline Stillinger-Weber ‘silicon’ from the liquid have been essentially impossible because of constraints on system size and time scales. We have overcome these limitations by studying the related problem of the disintegration of crystalline ‘embryos’ into the liquid phase at temperatures slightly above the melting point. Molecular Dynamics simulations using the Stillinger-Weber potential were performed by embedding crystallites of 400 atoms in a liquid consisting of approximately 3600 atoms. During each simulation, the time-evolution of the size and shape of the embryo was followed until it became indistinguishable from the liquid. These simulations provide intriguing new information on the atomic processes involved in dissolution and on the macroscopic kinetics of small clusters. Comparisons of results at different temperatures, system sizes and initial configurations are shown and the implications of these cluster dynamics for crystal growth in supercooled liquids, homogeneous nucleation, and transient nucleation are discussed.

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