Mechanical properties of sintered meso-porous silicon: a numerical model

Because of its optical and electrical properties, large surfaces, and compatibility with standard silicon processes, porous silicon is a very interesting material in photovoltaic and microelectromechanical systems technology. In some applications, porous silicon is annealed at high temperature and, consequently, the cylindrical pores that are generated by anodization or stain etching reorganize into randomly distributed closed sphere-like pores. Although the design of devices which involve this material needs an accurate evaluation of its mechanical properties, only few researchers have studied the mechanical properties of porous silicon, and no data are nowadays available on the mechanical properties of sintered porous silicon. In this work we propose a finite element model to estimate the mechanical properties of sintered meso-porous silicon. The model has been employed to study the dependence of the Young’s modulus and the shear modulus (upper and lower bounds) on the porosity for porosities between 0% to 40%. Interpolation functions for the Young’s modulus and shear modulus have been obtained, and the results show good agreement with the data reported for other porous media. A Monte Carlo simulation has also been employed to study the effect of the actual microstructure on the mechanical properties.     

Computer simulation of the damage evolution of fiber‐reinforced polypropylene‐matrix composites with matrix defects

The damage evolution of fiber‐reinforced polypropylene‐matrix composites with matrix defects was studied via a Monte Carlo technique combined with a finite element method. A finite element model was constructed to predict the effects of various matrix defect shapes on the stress distributions. The results indicated that a small matrix defect had almost no effect on fiber stress distributions other than interfacial shear stress distributions. Then, a finite element model with a statistical distribution of the fiber strength was constructed to investigate the influences of the spatial distribution and the volume fraction of matrix defects on composite failure. The results showed that it was accurate to use the shear‐lag models and Green's function methods to predict the tensile strength of composites even though the axial stresses in the matrix were neglected. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 64–71, 2007     

Thermal conductivity and tensile response of defective graphene: A molecular dynamics study

In this study, effects of point vacancy, Stone–Wales and bivacancy defects on thermal conductivity and tensile response of single-layer graphene sheets are studied using classical molecular dynamics (MD) simulations. Using non-equilibrium molecular dynamics (NEMD) method, we found that thermal conductivity of graphene is considerably sensitive to existence of defects. It was observed that only 0.25% concentration of defects in graphene lead to significant reduction of graphene thermal conductivity by around 50%. By applying uniaxial tensile loading, we studied the deformation process of graphene. We found that elastic modulus, tensile strength and strain at failure of graphene decrease by increase of defects concentrations. Obtained results suggest that thermal conduction in graphene is much more vulnerable to defects in comparison with mechanical properties. Reported results by this work provide an overall viewpoint concerning the intensity of defects’ effects on the graphene thermal and mechanical response.     

Influence of the curvature of deformed graphene nanoribbons on their electronic and adsorptive properties: theoretical investigation based on the analysis of the local stress field for an atomic grid

Electronic and adsorptive properties of deformed graphene are investigated in the current work. Armchair and zigzag nanoribbons are the subject of the study. The axial compression was a deforming load. A calculation method for the local stress field was developed. This method was based on the quantum model of the finite graphene nanoribbon and empirical calculation method of the single atom energy. The stress field of the deformed ribbon was calculated by means of the suggested methodology. The effects of the atomic grid curvature on the adsorptive capacity of graphene and the hydrogenation process was investigated by means of the developed method. The prediction of the appearance of defects on covalent C-C bond breakdown is also performed.     

Effect of vacancy defects on phonon properties of hydrogen passivated graphene nanoribbons

The phonon properties of hydrogen-passivated armchair graphene nanoribbons (AGNRs) with different vacancy concentrations are investigated theoretically. We calculate the change in the phonon density of states (PDOSs) due to a broad range of vacancies and hydrogen passivation effects using forced vibrational method. A large downshift of prominent Raman active Г point LO mode phonons with an increase of vacancy concentration or decrease of ribbon widths are observed. We find an increasing peak intensities for the C–H stretching mode with the decrease of ribbon width or the increase of defect density. An inserted vacancy concentration of 10% and higher induce the broadening and distorting of the PDOS peaks significantly. The localization properties of phonon due to defects were also studied. The typical mode pattern of K point iTO mode phonons show the spatial localized vibrations persuaded by armchair edges or vacancies, which are in conceptually good agreement with the large D band of the Raman spectra comes from the armchair-edges or the imperfections of crystal. The typical displacement pattern for C–H stretching mode shows a random displacement of H atoms in contrast to C atoms. Our simulation results show the significant impact of vacancy defects on the vibrational properties of GNRs.     

Machine learning of carbon vacancy formation energy in high-entropy carbides

Carbon vacancies as intrinsic defects in high-entropy carbides (HECs) strongly affect their physical and chemical properties, it is important to understand the details of vacancy formation at an atomic level. In this work, 1280 carbon vacancies are investigated via high-throughput density functional theory (HT-DFT) calculations and machine learning (ML) models on their formation energies. We found that more than half of the carbon vacancies are preferentially surrounded by three types of cations, with Hf less likely to be present. The role of metal cations in promoting the formation of carbon vacancies is: Nb＞Ta＞Zr＞Hf, indicating the possibility to modify the carbon vacancy concentration by change the cation elemental ratios. A random forest model was trained on DFT results to predict vacancy formation energies with a mean absolute error of 54 meV. The feature importance analysis is consistent with the analysis of the DFT results.     

Characterization of Cabot non-graphitized carbon blacks with a defective surface model: Adsorption of argon and nitrogen

A grand canonical Monte Carlo simulation (GCMC) is used to study the adsorption of argon and nitrogen on non-graphitized carbon black. The surface is assumed to be finite in length and composed of three graphene layers, the top layer of which contains defects. The isotherm obtained for the non-graphitized carbon shows a smooth S-shaped type while that obtained for the perfect graphene layer shows a wavy type. The isosteric heat is also affected by the defect; its behaviour versus loading exhibits a decrease at the beginning and then slightly increases once the first layer has been formed. The decreasing behaviour of isosteric heat at low loadings is not observed in the case of graphitized carbon black. The simulated results are compared against the experimental data of argon and nitrogen at 77 and 87.3 K on the Cabot carbon black BP 280, 460 and 2000. It is found that the defected finite surface describes well the data of these blacks. For the case of BP 2000 we have found that besides the defects of the surface, this sample contains a small population of small micropores having a width of 8.2 Å and its specific pore volume of 0.08 cm³/g.     

Atomic hydrogen adsorption on lithium-doped graphite surfaces

The effects of lithium doping of pristine and defective graphite surfaces on hydrogen adsorption are studied by the first-principles Plane-Wave Density Functional Theory. The surface defects are simulated by a single atomic vacancy. The DFT calculation is corrected for long-range effects through semi-empirical London terms for each constituent of the system. The lithium doping of the graphite surfaces notably reinforces hydrogen atom binding. Qualitative comparison with experimental results is given using the lithium 1s energy level shifts induced by the atomic vacancy and/or hydrogen trapping.     

Relationship between electrochemical behavior and Li/vacancy arrangement in ramsdellite type Li2+xTi3O7

The changes of Li⁺/vacancy arrangement in Li_2+xTi₃O₇ with a ramsdellite-type structure upon topo-electrochemical Li⁺ insertion were investigated by the entropy measurement of reaction combined with the Monte Carlo simulation. The experimental entropy measurement was conducted by potentiometric and calorimetrical methods. The obtained experimental data were in good accordance with simulated results.The results indicated that the ordered Li⁺/vacancy arrangement appeared at the compositions of x ∼ 0.45 and ∼1.20, where the observed entropy of reaction humped. The ordering of Li/vacancy were also indicated at the composition x ∼ 0.24 and 1.16 in Li_2+xTi₃O₇ by the Monte Carlo simulation which considers the most stable Li/vacancy arrangement in terms of Coulombic interaction. This substantial agreement between electrochemical behaviors and computational results confirmed that the formation of superstructure arising from Li/vacancy arrangement during the electrochemical reaction deeply related to the atomic level Coulombic interactions.     

Research on the mechanical properties prediction of carbon/epoxy composite laminates with different void contents

The influence of the porosity on the static mechanical strength of the carbon fiber fabric reinforced epoxy composites laminates was investigated. The tensile, compressive, bending, and interlaminar strength test on the CFRP laminates with porosity of 0.33% and 1.50% were conducted and simulated by a finite element analysis model. The article proposes the failure criterion of the static mechanical strength of the fabric fiber reinforced composites based on the improved Hashin failure criterion that is suitable for the undirectional composite laminates. The basic composite strength parameters are used to evaluate the mechanical properties of CFRP laminates with different porosities. A finite element analysis model is established by using software ABAQUS™ combined with the sudden stiffness degradation model. The experiment results show that the tensile, compressive, bending, and interlaminar strength decrease with the increasing porosities. The tensile, compressive, bending, and interlaminar strength of the fabric carbon fiber reinforced epoxy composites laminates are simulated accurately by the finite element model. POLYM. COMPOS., 14–20, 2016. © 2014 Society of Plastics Engineers     

Effects of Lanthanide Dopants on Oxygen Diffusion in Yttria-Stabilized Zirconia

The effects of lanthanide co-dopants on oxygen diffusion in yttria-stabilized zirconia (YSZ) are studied using a combined first principles density functional theory (DFT)/kinetic Monte Carlo (kMC) modeling approach. DFT methods are used to calculate barrier energies for oxygen migration in different local cation environments, which are then input into kMC simulations to obtain long-time oxygen diffusivities and activation energies. Simulation results show a substantial increase in the maximum value of the oxygen diffusivity upon co-doping and in the dopant content at which this value is obtained for Lu-co-doped YSZ; while relatively little change is seen for Gd-co-doped YSZ. Examination of the DFT barrier energies reveals a linear scaling of barrier heights with the size of cations at the diffusion transition state. Using this strong correlation, oxygen diffusivity is examined in YSZ co-doped with several lanthanide elements. The oxygen diffusivity decreases with dopant atomic number (and decreasing dopant ion size) for co-dopants smaller than Y, and changes relatively little when Y is replaced by co-dopants larger than it. These results are broadly consistent with experiment, and are explained in terms of cation-dopant and vacancy concentration-dependent correlation effects, with the aid of a simple analytical model.     

Current-voltage characteristics of nanoplatelet-based conductive nanocomposites

In this study, a numerical modeling approach was used to investigate the current-voltage behavior of conductive nanoplatelet-based nanocomposites. A three-dimensional continuum Monte Carlo model was employed to randomly disperse the nanoplatelets in a cubic representative volume element. A nonlinear finite element-based model was developed to evaluate the electrical behavior of the nanocomposite for different levels of the applied electric field. Also, the effect of filler loading on nonlinear conductivity behavior of nanocomposites was investigated. The validity of the developed model was verified through qualitative comparison of the simulation results with results obtained from experimental works.     

Controlled generation of atomic vacancies in chemical vapor deposited graphene by microwave oxygen plasma

The introduction of atomic-scale defects in a controllable manner and the understanding of their effect on the characteristics of graphene are essential to develop many applications based on this two-dimensional material. Here, we investigate the use of microwave-induced oxygen plasma towards the generation of small-sized atomic vacancies (holes) in graphene grown by chemical vapor deposition. Scanning tunneling microscopy revealed that tunable vacancy densities in the 10³–10⁵ μm⁻² range could be attained with proper plasma parameters. Transport measurements and Raman spectroscopy revealed p-type doping and a decrease in charge carrier mobility for the vacancy-decorated samples. This plasma-modified graphene could find use in, e.g., gas or liquid separation, or molecular sensing.     

Raman study of damage extent in graphene nanostructures carved by high energy helium ion beam

We performed spatial Raman mapping on supported monolayer graphene carved by 30 keV He⁺ beam. A tilted beam was introduced to effectively eliminate the substrate swelling. The ratio between D and G peak intensities shows that Stage 1 and Stage 2 disorder are introduced over a wider range on both sides of the 35 nm etched line. The mean defect distance L_D was estimated in these regions using the local activation model. Vacancies and amorphisations are dominant types of defects as suggested by the ratio of D and D′ peak intensities. Monte Carlo simulation on stopping range of ions was accomplished to explain the asymmetric defect formation in graphene.     

Linear displacement of a wetting fluid by an immiscible non-wetting fluid in a porous medium: A predictive algorithm

The linear displacement of a wetting fluid by an immiscible non-wetting fluid in a two-dimensional porous medium composed of a network of sites multi-connected by bonds has been simulated mathematically. The algorithm involves Monte Carlo decision making, random walks and principles of the percolation theory. The algorithm described in the present work successfully predicts the three distinct behaviours of immiscible displacement in porous media. This algorithm is tested against experiments available in the literature for two-dimensional porous media. The agreement between the numerical results and the experiments is very good.     

Numerical analysis for the dynamics of the oxidation-induced stacking fault in czochralski-grown silicon crystals

The continuum model of point defect dynamics to predict the concentration of interstitial and vacancy is established by estimating expressions for the thermophysical properties of point defects and the point defect distribution in a silicon crystal and the position of oxidation-induced stacking fault ring (R-OiSF) created during the cooling of crystals in Czochralski silicon growth process are calculated by using the finite element analysis. Temperature distributions in the silicon crystal in an industrial Czochralski growth configuration are measured and compared with finite volume simulation results. These temperature fields obtained from finite volume analysis are used as input data for the calculation of point defect distribution and R-OiSF position. Calculations of continuum point defect distributions predict the transition between vacancy and interstitial dominated precipitations of microdefects as a function of crystal pull rate (V). The dependence of the radius of R-OiSF (R_OiSF) on the crystal length with fixed growth rate for a given hot zone configuration is examined. The R_OiSF is increased with the increase of crystal length. These predictions from point defect dynamics are well agreed with experiments and empirical V/G correlation qualitatively, where G. is the axial temperature gradient at the melt/crystal interface.     

A multi-scale model for predicting the thermal shock resistance of porous ceramics with temperature-dependent material properties

This paper develops a novel multi-scale thermal/mechanical analysis model which not only can efficiently measure the thermal shock response but also highly reflects the effects of diversiform micro-structures of porous ceramics. Knowledge of the temperature distribution and time-varied thermal stress intensity factors (SIF) is derived by finite element/finite difference method and the weight function method in the macro continuum model. The finite element analysis employs a micro-mechanical model in conjunction with the macro model for the purpose of relating the SIF to the thermal stress in the struts of the porous ceramics. The micro model around the crack tip was established by using Voronoi lattices to accurately explore the micro-architectural features of porous ceramics. Hot shock induced center crack and cold shock induced edge crack are both considered. Effects of relative density and pore size on the thermal shock resistance are investigated and the results are well coincident with the experimental tests. The influence of cell regularity and cross section shape of the cell struts is discussed and the corresponding explanations are provided. The importance of incorporating temperature-dependent material properties on the thermal shock resistance prediction is quantitatively represented. These multi-faceted models and results provide a significant guide to the design and selection of porous ceramics against the thermal shock fracture failure for the future thermal protection system of space shuttle.     

A Monte Carlo method for simulating dispersion and transport through horizontal rotating cylinders

Monte Carlo simulation is used to extend the simple model originally derived by Vahl and Kingma for the flow of powder through a rotating cylinder to include dispersion. The concept of a random normally distributed angle of descent along the powder bed surface is employed in the Monte Carlo simulation to provide an adequate amount of dispersion. An empirical relationship between the standard deviation of the normal distribution and the fractional filling of the rotating cylinder is given. Results predicted by the Monte Carlo model are shown to accurately reproduce experimental results and the solution to the one-dimensional axial dispersion model.     

Increased H2 gravimetric storage capacity in truncated carbon slit pores modeled by Grand Canonical Monte Carlo

Hydrogen adsorption in slit shaped pores built up from truncated graphene fragments has been simulated using Grand Canonical Monte Carlo technique and the influence of pore wall edges on hydrogen storage by physisorption has been analyzed. We show that due to the additional gas adsorption at the pore edges the adsorbed gravimetric amount significantly increases (by a factor of two) with respect to models of pores with infinite graphene walls. The contribution of the edges’ adsorption to the total hydrogen uptake is independent of the pore wall shape but it depends on its surface. We also show that the maximum of the excess adsorption shifts towards higher pressures when the edge contribution increases. This information can be used to characterize experimentally structures of porous adsorbents and complement pore size distribution analysis usually performed with gases others than hydrogen. We suggest that porous carbons built from polycyclic hydrocarbons can achieve storage performances required for practical applications.