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Data-driven machine learning techniques can be useful for the rapid evaluation of material properties in extreme environments, particularly in cases where direct access to the materials is not possible. Such problems occur in high-throughput material screening and material design approaches where many candidates may not be amenable to direct experimental examination. In this paper, we perform an exhaustive examination of the applicability of machine learning for the estimation of isothermal shock compression properties, specifically the shock Hugoniot, for diverse material systems. A comprehensive analysis is conducted where effects of scarce data, variances in source data, feature choices, and model choices are systematically explored. New modeling strategies are introduced based on feature engineering, including a feature augmentation approach, to mitigate the effects of scarce data. The findings show significant promise of machine learning techniques for design and discovery of materials suited for shock compression applications.

Chemical energy ferroelectrics are generally solid macromolecules showing spontaneous polarization and chemical bonding energy. These materials still suffer drawbacks, including the limited control of energy release rate, and thermal decomposition energy well below total chemical energy. To overcome these drawbacks, we report the integrated molecular ferroelectric and energetic material from machine learning-directed additive manufacturing coupled with the ice-templating assembly. The resultant aligned porous architecture shows a low density of 0.35 g cm−3, polarization-controlled energy release, and an anisotropic thermal conductivity ratio of 15. Thermal analysis suggests that the chlorine radicals react with macromolecules enabling a large exothermic enthalpy of reaction (6180 kJ kg−1). In addition, the estimated detonation velocity of molecular ferroelectrics can be tuned from 6.69 ± 0.21 to 7.79 ± 0.25 km s−1 by switching the polarization state. These results provide a pathway toward spatially programmed energetic ferroelectrics for controlled energy release rates.

Summary This paper presents an all-atom molecular dynamics (MD) simulation of highly charged and densely grafted polyelectrolyte (PE) brushes on a flat surface. Simulation findings for the variation of brush height with polymer size and grafting density are explained by the scaling laws for brushes in the non-linear osmotic regime. More importantly, this study establishes the triggering of an ultraconfinement effect by the densely grafted brushes. This effect leads to significant changes in the distribution, structure, and properties as well as a massive mobility reduction of both the counterions and water. Furthermore, the interplay of the ultraconfinement effect and the large counterion-PE electrostatic interactions trigger a “water-in-salt”-like scenario (witnessed in highly concentrated aqueous electrolyte solutions), which is characterized by the counterion-PE-functional group serving as the “salt” with the “salt” overwhelming the water in both mass and volume and affecting the solvation structure of the counterions by replacing the solvation water with the PE-functional group.

In this paper, we determine the degree to which changes can be induced in the equilibrium thermal diffusivity and conductivity of a material via a selective nonequilibrium infrared stimulation mechanism for phonons. Using the molecular crystal RDX, we use detailed momentum-dependent coupling information across the entire Brillouin zone and the phonon gas model to show that stimulating selected modes in the spectrum of a target material can induce substantial changes in the overall thermal transport properties. Specifically in the case of RDX, stimulating modes at ∼22.74 cm

Purpose The role of chemotherapy (CT) and radiotherapy (RT) for management of extraskeletal osteosarcoma (ESOS) remains controversial. We examined disease outcomes for ESOS patients and investigated the association between CT/RT with recurrence and survival. Patients and methods Retrospective review at 25 international sarcoma centers identified patients ≥18 years old treated for ESOS from 1971 to 2016. Patient/tumour characteristics, treatment, local/systemic recurrence, and survival data were collected. Kaplan–Meier survival and Cox proportional-hazards regression and cumulative incidence competing risks analysis were performed. Results 370 patients with localized ESOS treated definitively with surgery presented with mainly deep tumours (n = 294, 80%). 122 patients underwent surgical resection alone, 96 (26%) also received CT, 70 (19%) RT and 82 (22%) both adjuvants. Five-year survival for patients with localized ESOS was 56% (95% CI 51%–62%). Almost half of patients (n = 173, 47%) developed recurrence: local 9% (35/370), distant 28% (102/370) or both 10% (36/370). Considering death as a competing event, there was no significant difference in cumulative incidence of local or systemic recurrence between patients who received CT, RT, both or neither (local p = 0.50, systemic p = 0.69). Multiple regression Cox analysis showed a significant association between RT and decreased local recurrence (HR 0.46 [95% CI 0.26–0.80], p = 0.01). Conclusion Although the use of RT significantly decreased local recurrences, CT did not decrease the risk of systemic recurrence, and neither CT, nor RT nor both were associated with improved survival in patients with localized ESOS. Our results do not support the use of CT; however, adjuvant RT demonstrates benefit in patients with locally resectable ESOS.

A long-held belief is that shock energy induces initiation of an energetic material through an indirect energy up-pumping mechanism involving phonon scattering through doorway modes. In this paper, a 3-phonon theoretical analysis of energy up-pumping in RDX is presented that involves both direct and indirect pathways where the direct energy transfer dominates. The calculation considers individual phonon modes which are then analyzed in bands. Scattering is handled up to the third order term in the Hamiltonian based on Fermi's Golden Rule. On average, modes with frequencies up to 90 cm-1 scatter quickly and redistribute the energy to all the modes. This direct stimulation occurs rapidly, within 0.16 ps, and involves distortions to NN bonds. Modes from 90 to 1839 cm-1 further up-pump the energy to NN bond distortion modes through an indirect route within 5.6 ps. The highest frequency modes have the lowest contribution to energy transfer due to their lower participation in phonon-phonon scattering. The modes stimulated directly by the shock with frequencies up to 90 cm-1 are estimated to account for 52 to 89\% of the total energy transfer to various NN bond distorting modes., There is a significant error in this paper. Please refer to the corrected version that is now available in the corresponding journal article https://pubs.acs.org/doi/10.1021/acs.jpca.1c03225

This innovative approach to teaching the finite element method blends theoretical, textbook-based learning with practical application using online and video resources. This hybrid teaching package features computational software such as MATLAB®, and tutorials presenting software applications such as PTC Creo Parametric, ANSYS APDL, ANSYS Workbench and SolidWorks, complete with detailed annotations and instructions so students can confidently develop hands-on experience. Suitable for senior undergraduate and graduate level classes, students will transition seamlessly between mathematical models and practical commercial software problems, empowering them to advance from basic differential equations to industry-standard modelling and analysis. Complete with over 120 end-of chapter problems and over 200 illustrations, this accessible reference will equip students with the tools they need to succeed in the workplace.

Deep learning has shown great potential for generating molecules with desired properties. But the cost and time required to obtain relevant property data have limited study to only a few classes of materials for which extensive data have already been collected. We develop a deep learning method that combines a generative model with a property prediction model to fuse small data of one class of molecules with larger data in another class. Common low-level physicochemical properties are jointly embedded into a latent space that can be used to design molecules in the smaller class. The chemical space around the molecules in the training set is explored through local gradient ascent optimization. Based on nine molecules from the original training set, nine new molecules are found to have improved properties while remaining structurally similar to the training molecules thereby easing requirements for entirely new synthesis routes. Validation is performed using an equilibrium thermochemistry code to verify the molecules and target properties. A specific example targeting the Chapman-Jouguet velocity and small data for nitrogen-rich molecules is shown. Despite the relative lack of nitrogen-rich molecule data, the results demonstrate that fusing and joint embedding with plentiful low nitrogen molecular data can produce higher generative performance than using the scarce data alone.

Due to its high computational speed and accuracy compared to ab-initio quantum chemistry and forcefield modeling, the prediction of molecular properties using machine learning has received great attention in the fields of materials design and drug discovery. A main ingredient required for machine learning is a training dataset consisting of molecular features—for example fingerprint bits, chemical descriptors, etc. that adequately characterize the corresponding molecules. However, choosing features for any application is highly non-trivial, since no "universal" method for feature selection exists. In this work, we propose a data fusion framework that uses Independent Vector Analysis to uncover underlying complementary information contained in different molecular featurization methods. Our approach takes an arbitrary number of individual feature vectors and generates a low dimensional set of features—molecular signatures—that can be used for the prediction of molecular properties and for knowledge discovery. We demonstrate this on a small and diverse dataset consisting of energetic compounds for the prediction of several energetic properties as well as for demonstrating how to provide insights onto the relationships between molecular structures and properties.

At nanometer to micron length scales, there exists a strong competition between intrinsic and extrinsic scattering mechanisms that can curtail the free flight of phonons and ultimately affect the thermal transport. Despite significant progress in showing the ability to reach behaviors significantly below the Casimir limit, little appears to be understood about the competition between these scattering sources. In this investigation, we propose a simple one-parameter geometry that simultaneously modulates backscattering and trapping effects to enable directed study of these different means of controlling phonons. The geometry is a simple sequence of chambers offset from one another by a defined distance. We use the geometry to study the effects of phonon backscatter, trapping, and corner-turning on the thermal conductance in Si nanowires (NWs). We employ a full Brillouin zone Boltzmann Transport Equation (BTE) method to determine spatially-varying phonon densities in the geometry. Significantly greater impact is seen due to backscatter than any other means of arresting phonon flow. By creating a geometry that maximizes backscatter, a roughly 8-fold reduction in thermal conductance below the Casimir limit can be achieved at room temperature which is a factor of four smaller than the nearest reported value in the literature. The geometry is also useful for systematic investigation of other means of controlling phonons and affecting thermal transport; particularly, we investigate diffuse versus specular boundary scattering and the induced misalignment between the phonon flow and thermal flux due to the shape of the geometry. These effects combine to offer new insights into fundamental phonon behaviors and possible routes to phonon control.

A unique feature of transition metal dichalcogenides is their single-layer form, which enables folding. Although folding has been found to significantly affect the photoluminescence spectrum and some in-plane properties, only limited insight has been gained on how to modulate those properties. In this report, we examine the structure of folds of a single sheet of MoS2 and the dependence of the ground-state electronic and phonon transport properties on the wrapping length. As the folded structure is effectively a bilayer that terminates in a loop, the wrapping length modulates the relative size of the bilayer region to the closed loop along the edge. A combination of computational methods, including approaches based on variational mechanics, classical potentials, and density functional theory, are employed. Highly accurate calculations of the reference folded structure are first carried out to show that the folded structure is largely insensitive to the wrapping length. The folded structures are subsequently used to estimate the electronic band gap, which is found to vary significantly as a function of the wrapping length, and converges from below to the limit value corresponding to an infinite bilayer. The gap values range from 0.43 to 1.09 eV, with a crossover to an indirect gap, which suggests that the transitions must be lattice-assisted, similar to the transitions in the bilayer and bulk forms. However, the phonons, while affected by the formation of the folded structure, are insensitive to the wrapping length. In fact, the overall thermal transport behavior along the folding axis is unchanged. The possibility of modulating the gap value while keeping the thermal properties unchanged opens up new exciting avenues for further applications of this emerging material.

Broad-based interest in microscale heat transport in novel materials, engineered phononic materials, metamaterials, and their relevant systems has created significant demand for computational approaches to aid in investigation and design of materials that support phonons. This review describes the significant improvements that have been made and new needs that have emerged for capabilities associated with the computability of phonons. The technical scope encompasses issues, especially relevant to bulk, interface, and surface effects. Traditional approaches such as molecular dynamics, lattice dynamics, and Boltzmann transport equation continue to advance the field but are frequently extended to the limits of their physical or numerical validity. New materials beyond traditional group-IV, III–V, and II–VI semiconductors, phenomena that critically depend on scattering, such as in low-dimensional nanostructures, materials with interior surfaces and defects, and in high-temperature environments, continue to push these limits. The basis for the traditional calculation methods shares their origins with the earliest theories for thermal transport, acoustic waves in solids, spectroscopy and dynamical crystal lattices. These will remain in wide use in the future. But computing methods and the accompanying advances in microprocessor technologies have enabled growth of phonon computing models and methods in sophistication, accuracy, fidelity and complexity that will lead to fundamental impacts beyond the classic types of problems for which they were developed. With their increasingly integrated use for design and research, the myriad developments that presently exist must be understood for their suitability for certain applications and their ability to aid in the pursuit of new technologies.

Hydrophobic graphene marries the usefulness of graphene with the benefits of a hydrophobic surface leading to applications in oil–water separation, wettability switching, electromagnetic radiation shielding, fabrication of smart actuators and wearable electronics, tissue engineering, and many more. In current practices, such hydrophobicity in graphene or graphene-based materials or GBMs (e.g., graphene oxide) has been introduced by either chemical treatment or by enforcing graphene or GBM into a physical structure that triggers a hierarchical roughness enforcing the water drop into a Cassie–Baxter (CB) state. Here, through molecular dynamics (MD) simulations, we describe a new route to graphene-based hydrophobicity. We demonstrate that the wetting translucency property of graphene ensures that the surface-periodic features such as nanopillars can trigger a roughness-induced chemical heterogeneity. Therefore, an uneven coating of graphene on a hydrophilic surface (e.g., gold) will lead to a chemical hetero...

A model and associated numerical method are presented for simulation of heat transport at the microscale via the solution of the three dimensional phonon Boltzmann Transport Equation (BTE). In small domains, the full Brillouin Zone has a finite number of vibrational modes, as determined by Born von-Karman boundary conditions. As a result of this discreteness, the present method allows for general crystal anisotropy and finite dimensional effects that naturally permit anisotropic thermal transport and energy flow. The method is shown and verified using analytical solutions for isotropic flows. Then numerical experiments are performed to calculate temperature and energy in a fin field effect transistor made of a cubic crystalline material. The anisotropic thermal conductivity and the consequences on thermal fields are calculated. The differences between an isotropic solution and the anisotropic model are shown to be significant with differences in the temperatures approximately 10%. At larger scales, where scattering effects dominate, differences in the solutions become smaller and macroscopic isotropy is recovered due to the cubic symmetry of the material.

In this paper, we develop a calculation method for determining the phonon or vibration spectra of lattices with defects. The dynamical matrices of lattices containing defects are calculated by introducing defects systematically into the dynamical matrices of pristine, defect-free lattices using linear operators. Each operation effectively modifies or removes an individual bond or mass. Then, complex defect configurations can be constructed through reiterative application of the operators. The proposed method may be applied to systems containing any interaction type or bond order, is amenable to parameterization with continuous variables, and is suited to the study of periodic structures and atomic lattices. For verification, the method is used to calculate the dilute limit in the optical mode of a point vacancy and the periodic-to-aperiodic convergence rate as a function of the increasing unit cell size. Additional demonstrations are then shown using two dimensional lattices containing a vacancy point defect and a tri-vacancy defect cluster.

Materials with multiple structural phases can be highly desirable because the transition can drive properties between phases. Low enthalpy transitions can particularly enable reconfigurable behaviors due to their low energy cost. In this work, a reversible thermally induced phase transformation is found near room-temperature between centrosymmetric and a previously unobserved non-centrosymmetric hafnium disulfide polytype. The symmetry-breaking mechanism requires no diffusion, intercalation, or charge transfer. A combination of experiments and simulations is used to determine that anharmonic phonons, weaker interlayer interactions, and variable metal atom coordination play important mediating roles in creating a pathway accessible to a lower free energy configuration. The transition occurs at 300 K and is marked by stacked AAA planes shifting into an ABC stacking and the shift is reversible in repeated cool-down experiments. The results demonstrate a reversible a phase transition that offers a unique mechanism for reconfigurability in a two dimensional material.

The heat transfer properties of the organic molecular crystal ${\alpha}$-RDX were studied using three phonon-based thermal conductivity models. It was found that the widely used Peierls-Boltzmann model for thermal transport in crystalline materials breaks down for ${\alpha}$-RDX. We show this breakdown is due to a large degree of anharmonicity that leads to a dominance of diffusive-like carriers. Despite being developed for disordered systems, the Allen-Feldman theory for thermal conductivity actually gives the best description of thermal transport. This is likely because diffusive carriers contribute to over 95% of the thermal conductivity in ${\alpha}$-RDX. The dominance of diffusive carriers is larger than previously observed in other fully ordered crystalline systems. These results indicate than van-der Waals bonded organic crystalline solids conduct heat in a manner more akin to amorphous materials than simple atomic crystals., Comment: Presented at 2019 MRS Spring Meeting & Exhibit, Phoenix Arizona, USA Session QN04.08.02, https://www.mrs.org/spring2019/symposium-sessions/symposium-sessions-detail?code=QN04 8 pages, 3 figures

In the space of only a few years, deep generative modeling has revolutionized how we think of artificial creativity, yielding autonomous systems which produce original images, music, and text. Inspired by these successes, researchers are now applying deep generative modeling techniques to the generation and optimization of molecules - in our review we found 45 papers on the subject published in the past two years. These works point to a future where such systems will be used to generate lead molecules, greatly reducing resources spent downstream synthesizing and characterizing bad leads in the lab. In this review we survey the increasingly complex landscape of models and representation schemes that have been proposed. The four classes of techniques we describe are recursive neural networks, autoencoders, generative adversarial networks, and reinforcement learning. After first discussing some of the mathematical fundamentals of each technique, we draw high level connections and comparisons with other techniques and expose the pros and cons of each. Several important high level themes emerge as a result of this work, including the shift away from the SMILES string representation of molecules towards more sophisticated representations such as graph grammars and 3D representations, the importance of reward function design, the need for better standards for benchmarking and testing, and the benefits of adversarial training and reinforcement learning over maximum likelihood based training., Comment: 24 pages, new title, published in RSC MSDE

We present a proof of concept that machine learning techniques can be used to predict the properties of CNOHF energetic molecules from their molecular structures. We focus on a small but diverse dataset consisting of 109 molecular structures spread across ten compound classes. Up until now, candidate molecules for energetic materials have been screened using predictions from expensive quantum simulations and thermochemical codes. We present a comprehensive comparison of machine learning models and several molecular featurization methods - sum over bonds, custom descriptors, Coulomb matrices, bag of bonds, and fingerprints. The best featurization was sum over bonds (bond counting), and the best model was kernel ridge regression. Despite having a small data set, we obtain acceptable errors and Pearson correlations for the prediction of detonation pressure, detonation velocity, explosive energy, heat of formation, density, and other properties out of sample. By including another dataset with 309 additional molecules in our training we show how the error can be pushed lower, although the convergence with number of molecules is slow. Our work paves the way for future applications of machine learning in this domain, including automated lead generation and interpreting machine learning models to obtain novel chemical insights., published in Scientific Reports

Through a systematic study of HfS 2 based on density functional theory calculations using the quasiharmonic approximation, we show that out-of-plane ZA phonons alone are responsible for 80% of the thermal transport—both in-plane and out-of-plane. The calculations determine the temperature-dependent structural, phonon, and thermal properties in HfS 2. The cause is the uncommonly strong interlayer van der Waals interaction relative to its in-plane interaction. The highly anisotropic interactions also give rise to a combination of effects including anisotropic phonon group velocities, a large ZA-optical phonon gap, and a vanishing Gruneisen parameter in ZA modes that leads to large phonon lifetimes. In turn, these enable other unusual properties. HfS 2 can have either a negative ( T 40 K) temperature-dependent thermal expansion coefficient and an exceptionally large LO-TO splitting.

This work explores possible cross-slip mechanisms of gliding type = a /3[1 –2 1 0] dislocation loops in wurtzite gallium nitride (GaN) as a function of slip plane. A modified form of the dislocation dynamics code ParaDiS was employed using isotropic linear elasticity and dislocation mobilities estimated in part from molecular dynamics (MD) simulations. Under an externally applied uniform stress, the occurrence of cross slip events is highly dependent on the initial dislocation slip plane. The basal plane is the preferred active plane, owing to the greater mobility of type segments on that plane, over the other planes considered including the prismatic (–1 0 1 0) and two equivalent pyramidal planes (–1 0 1 1) and (1 0 –1 1). For an applied stress state, cross slip processes are more readily seen from the prismatic-to-basal planes or the pyramidal-to-basal planes, and neither is found to occur in reverse. Cross slip by climb is not presently considered. In all cases, cross-slip events occur after the loop expands until a greater number of screw-oriented segments are able to access the cross slip plane and the resolved stresses on the plane become sufficiently large. In comparison to dislocations found in GaN previously, the calculations suggest that some threading dislocations along the [0001] direction that have edge character may have been formed from loops whose screw segments slip and escape on basal planes leaving only the edge segments.

The method and the advantages of an evolutionary computing based approach using a steady state genetic algorithm (GA) for the parameterization of interatomic potentials for metal oxides within the shell model framework are developed and described. We show that the GA based methodology for the parameterization of interatomic force field functions is capable of (a) simultaneous optimization of the multiple phases or properties of a material in a single run, (b) facilitates the incremental re-optimization of the whole system as more data is made available for either additional phases or material properties not included in previous runs, and (c) successful global optimization in the presence of multiple local minima in the parameter space. As an example, we apply the method towards simultaneous optimization of four distinct crystalline phases of Barium Titanate (BaTiO3 or BTO) using an ab initio density functional theory (DFT) based reference dataset. We find that the optimized force field function is capable of the prediction of the two phases not used in the optimization procedure, and that many derived physical properties such as the equilibrium lattice constants, unit cell volume, elastic properties, coefficient of thermal expansion, and average electronic polarization are in good agreement with the experimental results available from the literature.

Classical molecular dynamics simulations of wurtzite gallium nitride are performed to calculate the velocities of the three types of a-type edge dislocations at 1300 K. Slips in the basal, prismatic and pyramidal planes are studied, and the anisotropic glide mechanisms are examined at both low and high stresses. The structures of the dislocation cores are described and the mechanisms of dislocation glide are discussed in the context of their potential motion in thin film growth.

Dislocation mediated plasticity in the α phase of the energetic molecular crystal cyclotrimethylene trinitramine (RDX) was investigated using a combination of atomistic simulations and the Peierls-Nabarro (PN) model. A detailed investigation of core structures and dislocation Peierls stress was conducted using athermal atomistic simulations at atmospheric pressure to determine the active slip systems. Generalized stacking fault energy surfaces calculated using atomistic simulations were used in the PN model to also estimate the critical shear stress for dislocation motion. The primary slip plane is found to be (010) in agreement with experimental observations, with the (010)[100] slip systems having the lowest Peierls stress. In addition, atomistic simulations predict the (021)[01[overline]2], (021)[100], (011)[100], (001)[100], and (001)[010] slip systems to have Peierls stress values small enough to allow plastic activity. However, there are less than five independent slip systems in this material in all situations. The ranking of slip systems based on the Peierls stress values is provided, and implications are discussed in relation to experimental data from nanoindentation and shock-induced plastic deformation.

A comparative study of non-coplanar binary dislocation junctions in magnesium (Mg) and beryllium (Be) is presented to examine the effects of elastic properties and active Burgers vectors on junction formation and destruction in hexagonal close-packed (hcp) crystals via discrete dislocation dynamics simulations. Two junction configurations formed at intersecting prismatic ( 0 1 1 ¯ 0 ) / basal ( 0 0 0 1 ) planes and type-II pyramidal ( 2 ¯ 1 1 2 ) / prismatic ( 0 1 1 ¯ 0 ) planes are studied using Burgers vectors of varying magnitudes. The equilibrium junctions are created from two intersecting straight gliding dislocations, and their subsequent strengths are evaluated under uniform applied stresses. The relative junction strengths between Mg and Be are consistent with their relative elastic stiffness, i.e., the modulus of elasticity for Mg is approximately one order of magnitude smaller than that of Be, and their junction strengths are similarly one order of magnitude apart. In general, the yield surfaces for junctions in Be are larger than those in Mg after normalization with the respective elastic moduli and Poisson’s ratios. All yield surfaces exhibit a strong symmetry. However, the size and shape of the yield surfaces depend on the slip systems, especially the active Burgers vectors. The yield surfaces of hcp crystals can resemble those of face-centered cubic or body-centered cubic crystals when the active Burgers vectors of the dislocations involved in the junction are of type 〈a〉, namely 1 3 〈 1 1 2 ¯ 0 〉 , and are distinct when Burgers vectors of different types are used: for instance, a type 〈a + c〉, namely 1 3 〈 1 1 2 ¯ 3 ¯ 〉 , interacting with a type 〈a〉. It was also found that junctions with more edge part exhibit more elongated yield surfaces than those with more screw part, and slip systems involved with 〈a〉 Burgers vectors result in smaller yield surfaces. These results demonstrate that junction strengths for hcp crystals are largely determined by elastic properties and Burgers vectors. The work aims to assess the effects of intrinsic material properties and dislocation slip systems on the strength of different binary dislocation junctions for general hcp structures.

Water–graphene wetting interactions are central to several applications such as desalination, water filtration, electricity generation, biochemical sensing, fabrication of fuel cells, and many more. While substantial attention has been devoted to probe the wetting statics of a water drop on graphene, unraveling the possible wetting translucency nature of graphene, very little research has been done on the dynamics of wetting of water drops on graphene-coated solids or free-standing graphene layers. In this paper, we employ molecular dynamics (MD) simulations to study the contact and the spreading of a water nanodrop, quantifying its wetting dynamics, on supported and free-standing graphene. We demonstrate that nanoscale water drops establish contact with graphene by forming patches on graphene, and this patch formation is hastened for graphene layer(s) supported on hydrophilic solids. More importantly, our results demonstrate that the nanodrop spreading dynamics, regardless of the number of graphene layers or the nature of the underlying solid, obey the half-power law, i.e., r ∼ t1/2 (where r is the wetting contact radius and t is the spreading time) for the entire timespan of spreading except towards the very end of the spreading lifetime when the spreading stops. Such a spreading behavior is exactly analogous to the spreading dynamics of nanodroplets for standard solids – this is in sharp contrast to the wetting statics of graphene where the wetting translucency effect makes graphene different from other standard solids.

The primary objective of this study was to estimate the change in health-related quality of life (HRQL) 1 year following treatment for extremity soft tissue sarcoma (STS), measured by the EQ-5D. Secondary objectives included determining clinical variables associated with HRQL at 1 year, estimating the proportion with a clinically important difference (CID) in HRQL, and evaluating variability within EQ-5D domains.Patients over the age of 16 years, treated for a localized extremity STS, were included. The EQ-5D change score from pre-treatment to 1-year follow-up was determined. The association of clinical variables with EQ-5D scores was estimated using a linear regression model. The proportion of patients with a CID in HRQL score was determined. A vector analysis of the EQ-5D domains was undertaken.The mean EQ-5D change score was 0.02. Age, sex, disease status, and initial EQ-5D score were associated with EQ-5D score at 1 year. There was a CID improvement in 32% and a deterioration in 24%. The anxiety and depression domain demonstrated the most change between baseline and 1 year after treatment.Most patients maintain a high level of HRQL following treatment for extremity STS. J. Surg. Oncol. 2016;114:821-827. © 2016 2016 Wiley Periodicals, Inc.

The generalized stacking fault (GSF) energy surfaces in the organic energetic molecular crystal, hexahydro-1,3,5-trinitro-s-triazine (RDX), were studied through atomistic simulations. Using a fully flexible molecular potential in highly damped molecular dynamics simulations, we determined quenched 0 K GSF energy surfaces and structures for a set of planes in the α-polymorph RDX crystal and subsequently compare predictions of slip or cleavage with available experimental observations. To account for the steric contributions and elastic shearing due to the presence of flexible molecules, a modified calculation procedure for the GSF energy surface is proposed that enables the distinction of elastic shear energy from the energy associated with the interfacial displacement discontinuity at the slip plane. Comparisons of the unstable stacking fault energy with the surface energy are used to differentiate cleavage planes from likely slip planes, and the calculations are found to be largely in agreement with avail...

The direct estimation of thermal conductivity in solids using the constant flux non-equilibrium molecular dynamics (NEMD) simulation method has proven to be difficult with significant differences still existing for various, even simple, systems. The application of NEMD to study the thermal transport in molecular solids was rarely attempted. We report the first application of NEMD based on the Ikeshoji and Hafskjold (NEMD-IH) scheme to study the thermal conductivity of crystalline hexahydro-1,3,5-trinitro-s-triazine (RDX), an important energetic material, at temperatures T = 230, 275, and 300 K for different sample sizes and different crystallographic directions. We found that the size effects arising from a finite distance between sources of heat in the NEMD-IH simulation show a well-defined linear dependence on the inverse of the sample length within the errors of simulation. The thermal conductivity exhibits anisotropy with the largest value found along the [0 0 1] crystallographic direction and the smallest value along the [0 1 0] direction. The extrapolated value of the directionally averaged thermal conductivity coefficient κ ¯ to the sample of infinite length (e.g. 0.355 W/Km at T = 300 K ) is higher, but still in reasonable agreement with existing experimental data for powdered samples. The NEMD-IH heat transport properties were reproduced in a broad range of values of heat flux and sizes of heat source regions. Surprisingly, the thermal conductivity coefficient at T = 275 K is found to be higher by about 13% than at T = 300 K and 230 K, which may suggest a location of the temperature maximum above 230 K. A similar location of the low temperature peak was reported from experiments on powdered samples of RDX. Through the use of additional calculations based on the Green–Kubo approach, we discuss the possible origins of such a behavior in the NEMD-IH approach.

The binding energy, density, and solubility of functionalized gold nanoparticles in a vacuum are computed using molecular dynamics simulations. Numerous parameters including surface coverage fraction, functional group (-CH(3), -OH, -NH(2)), and nanoparticle orientation are considered. The analysis includes computation of minimum interparticle binding distances and energies and an analysis of mechanisms that may contribute to changes in system potential energy. A number of interesting trends and results are observed, such as increasing binding distance with higher terminal group electronegativity and a minimum particle-particle binding energy (solubility parameter) based upon surface coverage. These results provide a fundamental understanding of ligand-coated nanoparticle interactions required for the design and processing of high-density polymer composites. The computational model and results are presented as support for these conclusions.

We simulate in three dimensions molecular beam epitaxial (MBE) growth of CdTe/ZnTe/Si using classical molecular dynamics. Atomic interactions are simulated with Stillinger–Weber potentials, whose parameters are obtained by fitting to experimental data or density function theory-calculated distortion energies of the component crystals. The effects of substrate temperature and atomic species flux ratios on epilayer morphology are investigated. The agreement between simulations and experiments suggests that this model has reasonable ability to predict the microstructures of CdTe/ZnTe/Si grown by MBE.

Developed in this note is a theoretical model describing the mobility of a misfit screw dislocation dipole in a wire composite consisting of a stiff cylindrical substrate covered by a soft co-axial cylindrical film. A critical value of the film thickness, which is a function of the parameter measuring the stiffness of the film with respect to the substrate, is identified. It is observed that: (i) there exist two equilibrium positions of the misfit dislocation dipole (one stable and the other one unstable) when the film is thicker than the critical value; (ii) the two equilibrium positions of the misfit dislocation dipole converge to a single saddle point equilibrium position which is neither stable nor unstable when the thickness of the film is at the critical value; (iii) there exists no equilibrium position of the misfit dislocation dipole when the thickness of the film is below the critical value. These features could be useful to the design of wire composites and to the dislocation-related plasticity analysis.

Discrete dislocation dynamics simulations were employed to study the primary mechanisms by which superlattices (SLs) block threading dislocations (TDs) from reaching the surface of a composite CdZnTe/SL/Si substrate. We found that the difference in strain (composition) across layer boundaries played a key role as to whether a TD might veer away or continue across. We also found that, while the distance between interfaces had little effect on the motion of a single TD, it could have an impact on the dislocation tangle as a whole and should be considered carefully in SL design. We discuss our results in relation to the design of SLs for TD blocking.

This research is devoted to the study of anisotropic bimaterials with Kelvin-type viscoelastic interface under antiplane deformations. First we derive the Green’s function for a bimaterial with a Kelvin-type viscoelastic interface subjected to an antiplane force and a screw dislocation by means of the complex variable method. Explicit expressions are derived for the time-dependent stress field induced by the antiplane force and screw dislocation. Also presented is the time-dependent image force acting on the screw dislocation due to its interaction with the Kelvin-type viscoelastic interface. Second we investigate a rectangular inclusion with uniform antiplane eigenstrains embedded in one of the two bonded anisotropic half-planes by virtue of the derived Green’s function for a line force. The explicit expressions for the time-dependent stress field induced by the rectangular inclusion are obtained in terms of the simple logarithmic and exponential integral functions. It is observed that in general the stresses exhibit the logarithmic singularity at the four corners of the rectangular inclusion. Our results also show that when one side of the rectangular inclusion lies on the viscoelastic interface, the interfacial tractions are still regular at the two corners of the inclusion which are located on the interface. Last we address a finite Griffith crack normal to the viscoelastic interface by means of the obtained Green’s function for a screw dislocation. The crack problem is formulated in terms of a resulting singular integral equation which is solved numerically. The time-dependent stress intensity factors at the two crack tips are obtained and some interesting features are discussed.

We investigate a semi-infinite crack penetrating a piezoelectric circular inhomogeneity bonded to an infinite piezoelectric matrix through a linear viscous interface. The tip of the crack is at the center of the circular inhomogeneity. By means of the complex variable and conformal mapping methods, exact closed-form solutions in terms of elementary functions are derived for the following three loading cases: (i) nominal Mode-III stress and electric displacement intensity factors at infinity; (ii) a piezoelectric screw dislocation located in the unbounded matrix; and (iii) a piezoelectric screw dislocation located in the inhomogeneity. The time-dependent electroelastic field in the cracked composite system is obtained. Particularly the time-dependent stress and electric displacement intensity factors at the crack tip, jumps in the displacement and electric potential across the crack surfaces, displacement jump across the viscous interface, and image force acting on the piezoelectric screw dislocation are all derived. It is found that the value of the relaxation (or characteristic) time for this cracked composite system is just twice as that for the same fibrous composite system without crack. Finally, we extend the methods to the more general scenario where a semi-infinite wedge crack is within the inhomogeneity/matrix composite system with a viscous interface.

In this paper, we propose a bimaterial boundary element method (BEM) for the calculation of the strain energy density and the relative strain energy in quantum wire (QWR) nanostructures. We first derive the bimaterial Green's functions in terms of the Stroh formalism. We then discretize the boundary of the problem with constant elements for which the involved Green's function kernels can be exactly integrated. Our bimaterial BEM program is finally applied to calculate the strain energy density and relative strain energy in free-standing/embedded InAs QWR on/in GaAs substrate. For a square InAs (1 1 1) QWR, it is found that the magnitude of the relative strain energy increases with increasing depth of the QWR with respect to the surface of the GaAs (1 1 1) substrate. For an isosceles triangle of InAs (1 1 1) QWR free-standing on the surface of the GaAs (1 1 1) substrate, we found that the magnitude of the relative strain energy increases with increasing base angle of the triangle QWR. Strain energy density inside the InAs QWR is also plotted to show its strong dependence on the QWR shape. These results could be useful to the control of the QWR shape and size in epitaxial growth