Structure, energy, and structural transformations of graphene grain boundaries from atomistic simulations

Domain/grain boundaries are often introduced into graphene during chemical vapor deposition growth processes. Here, we performed a series of hybrid molecular dynamics simulations to study the structures, energies, and structural transformations of symmetric tilt grain boundaries of graphene. The grain boundary comprises an array of edge dislocations, with the dislocation density increasing upon increasing the grain boundary misorientation angle. The dislocation in the zigzag-oriented grain boundary contains an edge-sharing pentagon/heptagon defect, whereas the dislocation in the armchair-oriented grain boundary contains two paired pentagon/heptagon defects. In some grain boundaries, out-of-plane buckling exists due to the presence of dislocations. In the transition region (the region between the zigzag- and armchair-oriented grain boundaries), the grain boundary structures feature complex mixtures of both zigzag and armchair grain boundaries. We also discuss the grain boundary transformations and migrations that occur upon adding or removing carbon atoms at the grain boundaries for all of our investigated types of grain boundaries.     

Evaluation of the elastic constants of nanoparticles from atomistic simulations

We present an approach to estimate the elastic constants of molecules and nanoparticles, based on the analysis of thermal fluctuations from Monte Carlo (MC) or molecular-dynamics (MD) atomistic simulations. The method and the force-field used for these calculations have been tested by the calculation of Young's modulus of a graphite sample along the basal plane; the calculated value was found to be 1.07 TPa, in very good agreement with the experimentally determined one of 1.02 TPa.The results on a carbon-based nanotube indicate that for the longitudinal direction of the particle, the value of the elastic constant is on the order of 400 GPa. The elastic constant of the considered nanotube in the radial direction is significantly lower, the predicted values being in the range 4-7 GPa.The method was also applied to the elastic constants of a type of siloxane-based nanostructure, whose longitudinal elastic constant (30 GPa) is an order of magnitude lower than the corresponding value for the carbon-based nanotube.     

Elastic and thermal parameters of lanthanide-orthophosphate (LnPO4) ceramics from atomistic simulations

We performed extensive and accurate atomistic simulations of elastic and heat transport properties of series of rare-earth orthophosphate ceramics LnPO₄ (Ln = La, …, Lu and Y) in monazite and xenotime structures. The results show clear trends in the elastic moduli along the lanthanide-series, which complement the existing experimental data on these materials. We found that the thermal conductivities of xenotimes are about two times larger than those of monazite, which is in agreement with the experimental measurements and explained by sizes of the primitive cells. Large sets of data allowed assessment of the validity of Slack's model as well as accuracy of molecular dynamics simulations of heat flow for prediction of thermal conductivity. Last, but not least, the separation of the intrinsic and extrinsic contribution to the measured thermal diffusivities allowed for a detailed analysis of the phonon mean free paths in the considered materials.     

Effect of carbon content on structural and mechanical properties of SiCN by atomistic simulations

Silicon carbonitride (SiCN) presents good performance on thermal stability and mechanical properties at high temperature. However, experiments still have problems to investigate the chemical structure of nanodomains and high temperature mechanical properties for SiCN. In this paper, atomistic simulations were used to generate amorphous SiCN with different carbon contents, the resulting structures show a tendency to include a “free carbon” phase when the carbon content increases. The calculated pair distributions, angular distributions and structure factor are comparable with experiments. Particularly, the first peaks of C-C and Si-C distributions become more significant when C content decreases, this is related to the variations of Si-C bonds near the graphene regions when the sizes of carbon phases change. The calculated Young's moduli are close to the experimental data and increase with increasing carbon content. The proposed atomic model can be used to predict the structural and mechanical properties of SiCN at different compositions.     

Effects of grain size and temperature on mechanical and failure properties of ultrananocrystalline diamond

Molecular dynamics simulations of ultrananocrystalline diamond (UNCD), with random distribution of grain sizes and grain boundaries (GBs), are performed to investigate the effect of grain size and temperature on the mechanical properties and failure mechanisms under tensile loading. Results show that when the grain size of UNCDs decreases from 4.1 nm to 2.26 nm, the Young's modulus decreases from 891 GPa to 840 GPa, while the obtained intrinsic fracture strength, 113 GPa, is insensitive to the grain size. Elastic softening is attributed to the increased volume fraction of amorphous-like atoms. Our analysis reveals that at room temperature, UNCD fails via sliding along a grain boundary with a large shear stress. Such sliding triggers crack initiation at an adjacent triple junction and subsequent propagation along an adjacent grain boundary with a large normal stress. With increasing temperature, a crossover from grain sliding to a direct intergranular fracture is observed. The crossover is caused by a different dependence of GB shear and tensile strength on temperature. The present work provides information that may be useful to the design and optimization of the mechanical properties and failure behavior of UNCDs.     

An admittance spectroscopy study of grain and grain boundary of diamond

AC electrical response of polycrystalline diamond films, prepared by hot-filament assisted chemical vapor deposition technique, was studied by admittance spectroscopy. Temperature dependent admittance evidenced two main exponential regimes associated with distributions of traps within diamond grains and at grain boundaries, respectively. Activation energies of the low-frequency conductance and of the characteristic relaxation frequency from Jonscher equation also evidence two trap levels associated to grain and grain boundary. This picture is supported by capacitive contributions obtained from imaginary part of electric modulus spectra, furthermore suggesting the presence of charge carriers tunneling at the Fermi level. Results are discussed in terms of a schematic band energy diagram.     

An atomistic study of the abrasive wear and failure of graphene sheets when used as a solid lubricant and a comparison to diamond-like-carbon coatings

The failure mechanisms of graphene under nanoscale sliding conditions are examined using atomistic simulations to evaluate its use as a solid lubricant and to simultaneously answer principal questions regarding wear of lamellar films comprised of atomically-thin sheets. To determine the failure behavior of graphene and the impact of adhesion on wear and failure, an asperity is slid over a substrate-supported graphene film with various adhesion strengths. For a purely-repulsive asperity, the graphene never delaminates and lower substrate-membrane adhesion appears to reduce the overall damage to the graphene layer and permits the recovery of more of the load-bearing capability of the graphene post-tearing. When tri-layer graphene is benchmarked with a 2 nm repulsive asperity against an 86% sp³ content diamond-like-carbon (DLC) coating of the same thickness (1.0 nm), the graphene supports up to 8.5 times the normal load of DLC during indentation, and up to twice the normal load of DLC during sliding even after failure of one or more layers. The preliminary results indicate that graphene has promise as a solid lubricant with thickness on the order of nanometers due to its atomically-thin configuration and high load carrying capacity.     

The influence of CaO on alumina grain boundary mobility

The influence of CaO on the evolving microstructure of alumina has been studied in a range of concentrations below the solubility limit. The amount of Ca in the alumina was determined by conducting fully standardized wavelength dispersive spectroscopy, and the change in grain boundary mobility as a function of the amount of dopant was characterized using scanning electron microscopy. Unlike segregating dopants which reduce grain boundary mobility by solute-drag, CaO increases the rate of grain growth, and a trend of increased mobility with increasing dopant level was shown. The increased mobility with Ca segregation is believed to be due to an increase in vacancy concentration in the vicinity of the grain boundaries, thus facilitating faster grain boundary motion.     

Role of cone angle on the mechanical behavior of cup-stacked carbon nanofibers studied by atomistic simulations

Classical molecular dynamics simulations are used to study the effects of cone angle on mechanical properties and failure mechanisms in thermally-treated cup-stacked CNFs. We find a 22-fold reduction in elastic modulus and 4-fold decrease in tensile strength of cup-stacked CNFs with a wide range of cone angles between 19.2° and 180°. Our results show significant elastic stiffening for intermediate angles between 38.9° and 112.9°, as well as a minimum in tensile strength at a critical cone angle, due to the competition between weak van-der-Waals forces between layers and strong strengthening mechanisms from surface bonds introduced during thermal treatment. Different failure modes in CNFs subjected to tensile deformation are also predicted as a function of cone angle. This study constitutes an important step toward understanding the origin of strength dispersions observed experimentally in CNFs, and suggests that the design of high-strength CNFs can be optimized structurally by appropriately tuning the cone angle.     

Finite element analysis of fracture statistics of ceramics: Effects of grain size and pore size distributions

A novel numerical simulation method based on finite element analysis (FEA), which can evaluate the fracture probability caused by the characteristics of flaw distribution, is considered an effective tool to facilitate and increase the use of ceramics in components and members. In this study, we propose an FEA methodology to predict the scatter of ceramic strength. Specifically, the data on the microstructure distribution (i.e., relative density, size and aspect ratio of pore, and grain size) are taken as the input values and reflected onto the parameters of a continuum damage model via a fracture mechanical model based on the circumferential circular crack emanating from an oval spherical pore. In addition, we numerically create a Weibull distribution based on multiple FEA results of a three‐point bending test. Its validity is confirmed by a quantitative comparison with the actual test results. The results suggest that the proposed FEA methodology can be applied to the analysis of the fracture probability of ceramics.     

The mechanical responses of tilted and non-tilted grain boundaries in graphene

Various mechanical characteristics of tilted and non-tilted grain boundaries in graphene were investigated under tension and compression in directions perpendicular and parallel to the grain boundaries using molecular dynamics simulation. In contrast to the non-tilted grain boundary and the pristine graphene, the mechanical response of tilted grain boundary was observed to be quite unique under perpendicular tension, exhibiting distinct crack propagation prior to tensile failure and the subsequent pattern of incomplete fracture. These features are manifested as a remarkable decrease in the slope and a rugged pattern in the stress–strain curves. The characteristic of incomplete fracture was striking especially for large misorientation angles with formation of long monoatomic carbon chains, suggesting a methodology for feasible production of the monoatomic carbon chains that have been difficult to synthesize and extract. Under perpendicular compression, the folding of the sheet occurred consistently along grain boundaries during the entire process, indicating a tunable folding, while the folding line wandered extensively for pristine graphene. Under parallel compression, we found that folding along grain boundaries disturbed the bending of the graphene substantially for intrinsic reinforcement.     

Effects of organo-modified montmorillonite on strengths and permeability of cement mortars

Organo-modified montmorillonites (OMMT) which have been widely used in polymer/clay nano-composites are employed here as fillers and reinforcements in cement mortars. The ratio of quartz sand and cement is 2.75 while the water/cement ratios of 0.40, 0.485 and 0.55 are considered for the cement mortars we studied. Experimental results indicate that the coefficients of permeability of cement mortars could be 100 times lower if a lower dosage of OMMT micro-particles is added. At the same time, the compressive and flexural strengths of cement mortars can be even increased up to 40% and 10%, respectively. It is also found that the optimal dosage of OMMT micro-particles to give higher compressive and flexural strengths and a lower coefficient of permeability for cement mortars is less than 1%. Meanwhile, the microstructure of cement mortars is characterized by using SEM, EDS and MIP to evaluate the effects of OMMT micro-particles on the improvements of strengths and permeability of cement mortars.     

Effects of temperature on the flow and tensile strengths of powders

Measurements have been made of the flow rates and tensile strengths of a variety of powders — magnesia, lactose, fatty acids, organic drugs — over a range of temperatures from ?20° to 200 °C. The results have been explained in terms of the effects of temperature on the hardness and elasticities of the materials concerned, and the distances between the particles.Under compression, the asperities on their surfaces deform plastically and may melt, if the temperature is raised above about 0.9 of the conventional melting point in K, to form welded bonds.The activation energy of bonding for the different materials is between about 8 and 11 kJ mole^?1.     

Influence of grain boundary sliding on diffusion in yttria doped zirconia

Grain boundary sliding during high temperature deformation can lead to stress concentrations and an enhancement of diffusion in mobile boundaries. Experiments were conducted on a fine grained 3 mol% yttria stabilized tetragonal zirconia, under conditions associated with superplastic flow involving grain boundary sliding. Tracer diffusion studies under creep conditions and without load indicate that there is no enhancement in either the lattice or grain boundary diffusivities. The experimental creep data are consistent with an interface controlled diffusion creep mechanism.     

Bulk and grain boundary diffusion of C in tungsten hemicarbide

Dense tungsten hemicarbide specimens were used to study the bulk and grain boundary diffusion of ¹⁴C into W₂C at temperatures of 1200 to 2000°C. The bulk diffusion coefficient is given by:

$\text{D}{}_{\mathrm{v}}\text{=18.3}\text{exp}\text{?}\text{91,500}\text{RT}\text{cm}{}^2\text{s}{}^{\mathrm{?1}}$

The grain boundary diffusion coefficient is represented by the expression:

$\text{P}{}_{\mathrm{GB}}\text{=1.8 10}{}^{\mathrm{?4}}\text{exp}\text{?}\text{68,880}\text{RT}\text{cm}{}^2\text{s}{}^{\mathrm{?1}}$

A comparison is give with preceding results on other carbides.     

Investigation of grain boundary diffusion and grain growth of lithium zinc ferrites with low activation energy

Activation energy and diffusion kinetics are important factors for grain growth and densification. Here, Bi₂O₃ was introduced into Li_0.43Zn_0.27Ti_0.13Fe_2.17O₄ ferrite ceramics via a presintered process to lower the reaction activation energy and to achieve low temperature sintering. Interestingly, Bi³⁺ ions entered the lattice and substituted for Fe³⁺ in the B‐site (i.e., a pure LiZn spinel ferrite). Also, SEM image results show that Bi₂O₃‐substituted LiZn ferrite ceramics have low critical temperature for grain growth (920°C), which is very advantageous for LTCC technology. This indicates that Bi₂O₃ is an excellent dopant for ceramics. Furthermore, to promote normal grain growth of the ceramics at low temperatures, different volumes of V₂O₅ additive were added at the final sintering stage. Results indicate that an optimal volume of V₂O₅ additive promotes grain growth (with no abnormal grains) and enhances magnetic performances of the ceramics at low sintering temperature. Finally, adding the optimal volume of V₂O₅ additive resulted in a homogeneous and compact LiZnTiBi ferrite ceramic with larger grains (average size of ~8 μm), high 4πM_s (~4100 gauss), and low ΔH (~190 Oe) obtained (at 900°C). Moreover, the doping method reported in this study also provides a reference for other low temperature sintered ceramics.     

Discrete element simulations of stress distributions in silos: crossover from two to three dimensions

The transition from two-dimensional (2D) to three-dimensional (3D) granular packings is studied using large-scale discrete element computer simulations. We focus on vertical stress profiles and examine how they change with dimensionality. We compare results for packings in 2D, quasi-2D packings between flat plates, and 3D packings. Analysis of these packings suggests that the Janssen theory does not fully describe these packings, especially at the top of the piles, where a hydrostaticlike region of vertical stress is visible in all cases. We find that the interior of the packing is far from incipient failure, while, in general, the forces at the walls are close to incipient failure.     

Dynameomics: mass annotation of protein dynamics and unfolding in water by high-throughput atomistic molecular dynamics simulations

The goal of Dynameomics is to perform atomistic molecular dynamics (MD) simulations of representative proteins from all known folds in explicit water in their native state and along their thermal unfolding pathways. Here we present 188-fold representatives and their native state simulations and analyses. These 188 targets represent 67% of all the structures in the Protein Data Bank. The behavior of several specific targets is highlighted to illustrate general properties in the full dataset and to demonstrate the role of MD in understanding protein function and stability. As an example of what can be learned from mining the Dynameomics database, we identified a protein fold with heightened localized dynamics. In one member of this fold family, the motion affects the exposure of its phosphorylation site and acts as an entropy sink to offset another portion of the protein that is relatively immobile in order to present a consistent interface for protein docking. In another member of this family, a polymorphism in the highly mobile region leads to a host of disease phenotypes. We have constructed a web site to provide access to a novel hybrid relational/multidimensional database (described in the succeeding two papers) to view and interrogate simulations of the top 30 targets: http://www.dynameomics.org. The Dynameomics database, currently the largest collection of protein simulations and protein structures in the world, should also be useful for determining the rules governing protein folding and kinetic stability, which should aid in deciphering genomic information and for protein engineering and design.     

The effect of rare earth dopants on grain boundary cohesion in alumina

The effect of rare earth (RE) grain boundary segregation on mode of fracture in alumina has been investigated. In order to isolate the effects of microstructure (i.e. grain size and residual porosity) from those due to grain boundary chemistry, the fracture behaviour of virtually pore-free (i.e. nearly transparent) undoped alumina has also been studied. This showed that mode of fracture becomes increasingly transgranular as grain size is reduced, a trend which has been explained by thermal expansion anisotropy effects.The addition of RE (i.e. Yb, Gd or La) dopants to alumina resulted in a substantial increase in the proportion of intergranular fracture relative to the undoped material of similar grain size. This can be explained by the significant reduction in the free surface energy that results from RE segregation at grain boundaries, which reduces the work of fracture for intergranular failure. This is expected to lead to a reduction in strength compared to undoped aluminas with equivalent microstructures, although this is often more than offset by the improved microstructures that using a RE dopant can provide.