bluuebunny/crossref_metadata_2025 huggingface dataset:
N-alkyldithiocarbamato complexes [Cd(S2CNHR)2] (R = C2H5, C4H9, C6H13, C12H25); Synthesis, Characterisation and Deposition of II/VI Nanoparticles.
Authors: Memon A. A. , Malik M. A. , Afzaal M. , O'Brien P.
Published: June 2019

A series of N-alkyldithiocarbamato complexes of cadmium have been synthesised and characterized. [Cd(S2CNHC12H25)2] complex was used as a single source precursor to synthesize CdS nanoparticles. Synthesis of CdS nanoparticles was carried out at various thermolysis temperatures and concentration of the precursor to observe the morphological changes. At moderate or high temperature the 1D nanostructures (single-rods, bend rods or multi-rods) are generated. The shape of CdS nanocrystals varied from longer rods to shorter rods with increased width (average size: 4nm wide and 27nm long vs. 9nm wide and 27nm long) by simply increasing the quantity of precursor in the bulk solution from 0.05g/8ml HDA to 1g/8ml HDA at 250°C.
Mechanical Behavior of a Tri-modal Al Matrix Composite
Authors: Ye Jichun , Han Bing Q. , Tang Feng , Schoenung Julie M.
Published: June 2019

Mechanical milling at cryogenic temperatures (cryomilling) was applied to fabricate a composite powder with 20 wt. % B4C (submicron-to-several microns in size) in a nanocrystalline (NC) 5083 Al matrix. A uniformly blended powder with 50 wt. % cryomilled composite powder and 50 wt. % coarse-grained (CG) 5083 Al powder was degassed, cold isostatic pressed (CIPped) and extruded to form a composite with 10 wt. % B4C, 50 wt. % CG 5083 Al and balance NC 5083 Al. This tri-modal material was then tested for mechanical behavior under compressive and tensile load conditions at various temperatures. The composite exhibited an extremely high yield stress at room temperature, but limited ductility. Although the composite lost its strength at elevated testing temperatures rapidly, the retained strength was still much higher than that of the conventional 5083 Al. The composite exhibits its highest ductility of 26% at 200°C under tensile load. In compression, it plastically deformed uniformly at all the elevated temperatures (≥373 K) and did not fracture even when the deformation exceeded 30%. The microstructure of this composite, including the distribution of each phase, the grain sizes of the Al matrix, the interfaces between these three phases, and the fracture surfaces were characterized using transmission electron microscopy (TEM) and optical microscopy (OM) techniques. The relationship between the microstructures and mechanical properties was discussed.
Nanoconfinement Effect on the Mechanical Behavior of Polymer Thin Films
Authors: Zhou J. , Komvopoulos K.
Published: June 2019

Nanoimprint is one of the most promising fabrication techniques for nanoelectronics. The main feature of this process is the compression of a thin film of a polymer resist by a rigid mold to produce a surface pattern. Hence, nanoimprint is essentially a mechanical forming process that depends greatly on the nanoscale mechanical behavior of the plastically deformed polymer film. Consequently, basic understanding of nanoimprint mechanics is imperative for improving pattern quality, reproducibility, and automation. The objective of this study was to elucidate the mechanical response of thin polymer films subjected to nanoindentation loading. Three deformation regimes were identified in the experiments performed with poly(methyl methacrylate) (PMMA) films of thickness in the range of 200-400 nm with a Berkovich tip of nominal radius of curvature equal to 100 nm. A three-layer model consisting of surface, intermediate, and interface layers was introduced to explain the mechanical response of the indented PMMA films. The spatial constraints imposed to the plastic flow of the interface layer by the rigid indenter and substrate surfaces produce a dynamic effect, demonstrated by the loading rate dependence of the deformation response. This phenomenon is of great importance to polymer plastic flow in nanoimprinting.
Germanium Nanocrystals Embedded in Sapphire
Authors: Xu Q. , Sharp I.D. , Liao C.Y. , Yi D. O. , Ager J.W. , Beeman J.W. , Yu K.M. , Chrzan D. C. , Haller E.E.
Published: June 2019

74Ge nanocrystals are formed in a sapphire matrix by ion implantation followed by thermal annealing. Transmission electron microscopy (TEM) of as-grown samples reveals that the nanocrystals are faceted and have a bi-modal size distribution. Notably, the matrix remains crystalline despite the large implantation dose and corresponding damage. Embedded nanocrystals experience large compressive stress relative to bulk, as measured by Raman spectroscopy of the zone center optical phonon. In contrast, ion-beam-synthesized nanocrystals embedded in silica are observed to be spherical and experience considerably lower stresses. Also, in situ TEM reveals that nanocrystals embedded in sapphire melt very close to the bulk melting point (Tm= 936 °C) whereas those embedded in silica exhibit a significant melting point hysteresis around Tm.
Multiscale Modeling of Strengthening Mechanism in Aluminum-Based Amorphous Nanocomposites
Authors: Liu H. T. , Sun L. Z.
Published: June 2019

In this work we focus upon theoretical exploration of the mechanical constitutive behavior of amorphous nanocomposites in terms of a multi-scale approach starting from the nanostructure. Local heterogeneous stress field and deformation are calculated based on the concept of eigenstrain and equivalent inclusion method. The overall elastoplastic constitutive model for amorphous nanocomposites is developed through homogenization averaging procedures. Explicit expressions of the effective elastic stiffness and yield strength of amorphous nanocomposites in terms of the constituents' properties and nanostructures are obtained. An interlayer phase between nanoparticles and the amorphous matrix is experimentally observed and incorporated in the proposed model. The interlayer thickness is treated as a characteristic length scale. Thus, the particle size effect on the nanocomposite properties is particularly investigated within continuum nanomechanics framework. It provides direct determination of the intrinsic mechanisms of the nanocomposite structure-property relationship at the nanoscale.
Determination of the Modulus and Hardness of Spin-on Zeolite Low-K Thin Films
Authors: Johnson Mark , Li Zijian , Yan Yushan , Wang Junlan
Published: June 2019

With the semiconductor technologies continuously pushing the miniaturization limits, there is a growing interest in developing novel low dielectric constant (low-k) materials to replace traditional dense SiO2 based insulators. In order to survive the multi-step integration process and provide reliable material and structure for the desired integrated circuit (IC) functions, the new low-k materials have to be mechanically strong and stable. Thus the material selection and mechanical characterization are vital in the successful development of next generation low-k dielectrics. A new class of low-k dielectric materials, nanoporous pure-silica zeolite, is prepared in thin films using IC compatible spin coating process and characterized using depth sensing nanoindentation technique. The elastic modulus measurements of the zeolite thin films are found to be significantly higher than that of other porous silicates with similar porosity and dielectric constants. Correlations of the mechanical, microstructural and electrical properties are discussed in detail.
Effect of Pile-up in Nanoindentation of Micro and Nano crystalline Ni using FEM
Authors: Pothapragada Raja Mahesh , Mirshams Reza A. , Vadlakonda Suman
Published: June 2019

A much better understanding of the contact mechanics can be obtained through finite element modeling. The experimentally determined tip shape function was included to reproduce the same contact area for a given contact depth as in the experiment. The fundamental material properties affecting pile-up are the ratio of the effective modulus to yield stress Eeff/Y and the work hardening rate.
Deformation behavior and strain rate sensitivity of nanostructured materials at moderate temperatures
Authors: Duhamel Cécilie , Guérin Sandrine , Hÿtch Martin , Champion Yannick
Published: June 2019

Strain-rate jump tests in compression are carried out on nanostructured copper (grain size = 90 nm) at moderate temperatures (353K - 393K). Strain-rate sensitivity m is measured as a function of temperature, T, and strain rate, έ. Increasing temperature or decreasing strain rate induces an increase in the strain-rate sensitivity. For (έ, T) = (1×10-5 s-1, 393K), m is equal to 0.17 which is the highest value reported for nanocrystalline copper. These results of enhanced m are encouraging in terms of gain in ductility. The measurements emphasize the existence of a thermally activated mechanism different from the normal rate-controlling process observed for microcrystalline fcc metals.
Fabrication and Characterization of Cold Rolled Ni/Al Multilayer Foils
Authors: Qiu Xiaotun , Graeter Jesse Harris , Kecskes Laszlo , Wang Jiaping
Published: June 2019

Ni/Al reactive multilayer foils were fabricated by cold rolling method and self-propagating reactions in these foils were investigated. A two-stage phase formation process was observed in the ignition experiment. The first step is the lateral growth of Al3Ni phase from isolated nucleation sites and the following coalescence into a continuous layer; the second step is the growth of such Al3Ni layers in the perpendicular direction of the interface until all Al is consumed. As there is still Ni available, Al3Ni will continuously react with Ni until no Ni is left, and the final reaction product turns out to be ordered B2 AlNi compound. X-ray diffraction (XRD) experiments showed that the reaction product of the cold rolled foil was the same as the physical vapor deposition (PVD) foil. The reaction process was studied by differential scanning calorimetry (DSC). Three peaks can be identified from the DSC curve. Cold rolled foils were heated to different peak temperatures obtained from DSC curve with the same heating rate as DSC. XRD results for such foils showed that the first two peaks were the exothermic formation of Al3Ni, while the last one was for the formation of AlNi. The enthalpy of the reaction for the cold rolled foil was calculated to be -57.5 KJ/mol, which was in good agreement with the formation enthalpy of AlNi (-59 KJ/mol). The reaction velocities of the first formation stage were measured to be 7mm/s for cold rolled foils, which were much smaller than the reaction velocities of PVD foils (which range between 1-30 m/s).
Controlling Residual Stress in Metal Matrix Ceramic Fiber Composite
Authors: Al-Haik Marwan , Garmestani Hamid , Haik Yousef
Published: June 2019

In metal matrix composites the nature of the reinforcement can influence the development of residual stresses not only as a result of the mismatch in the thermal expansion coefficient between the fiber and the matrix but also caused by the interface of the materials during the processing cycles. The residual stress can be minimized through controlling the processing path and the thermal environment. We studied the residual stress formation and evolution in gamma titanium aluminide (Ti-47AL-2Ta) matrix. The matrix was reinforced with three different types of fibers: Alumina, Sapphikon, and Tiboride through hot isostatic pressing. The composite was heat treated for various combinations of time: 100, 200 and 500 hours; and temperature: 590C, 815C and 982 C respectively. Residual stresses were measured in the gamma phase of the matrix using X-Ray diffraction
Processing-Microstructure-Property Relations in Anisotropic Thermal Sprayed Composites
Authors: Chi Weiguang , Srinivasan Vasudevan , Sharma Atin , Sampath Sanjay , Gambino Richard
Published: June 2019

Thermal spray is a significantly advanced but inherently complex deposition process that involves successive impingement of molten droplets on a substrate to form coating with a ¡°brick-wall¡± layered structure. The anisotropic microstructure of coatings is very sensitive to processing conditions and has significant influence on the properties. This study aims to understand the processing-microstructure-thermal property correlation of thermally sprayed coatings. Thermal transport properties of three coating systems forming composites with pores (yttria stabilized zirconia (YSZ) -Air), a second phase (Mo-Mo2C) and a graded material (YSZ-NiCrAlY) are interpreted from the point of view of microstructure and chemical composition. In the case of YSZ-Air composite, results indicate that porosity contribution from 20-35% decreases the thermal conductivity by 50-70% of the bulk value. For the intrinsic composite of Mo and Mo2C, which coexist as stable phases, thermal conductivity increases significantly with 1.75wt% carbon addition since it reduces formation of MoO2 during processing, but decreases with 3.5wt% carbon addition. This is attributed to larger carbide retention in the latter. For the discrete layered and graded composites of YSZ-NiCrAlY, which are made up of varying fractions of these two constituents, thermal conductivity decreases sharply up to 40wt% YSZ and then more gradually with increasing YSZ content. This paper examines these experimental findings by treating the these complex coatings as multiphase composites.
Properties of Transparent Conductive Oxide Films on Flexible Substrates
Authors: Hsiao Shih Hsiu , Tanaka Yoshikazu , Ide-Ektessabi Ari
Published: June 2019

Transparent conductive oxide (TCO) thin films are extensively used in display industry and they can be utilized for flexible displays. The polymer and the plastic materials used as flexible substrates are more bendable and lighter weight compared to glass substrates. However, its mechanical and surface properties differed from glass substrates result in different quality of TCO layers deposited on it. In this study, Polyethylene Terephthalate (PET) and glass were used as substrates. Indium Tin Oxide (ITO), Zinc Oxide (ZnO), Mg-doped ZnO (MZO), Al-doped ZnO (AZO), Ga-doped ZnO (GZO), Al-doped MZO (AMZO), Ga-doped MZO (GMZO) were used as TCO materials deposited by RF sputtering. Rutherford Backscatter Spectroscopy (RBS) and X-Ray Diffraction (XRD) were used to analyze the chemical composition and crystal structure of TCO thin films. Light transmittance and surface resistivity were measured after the different deposited conditions. , Mg-, Al-, Ga- doped ZnO indeed modified the optical properties of ZnO and better than ITO. However, the electrical conductivity was not improved obviously compared to ITO when they deposited on PET substrate at room temperature.
Molecular Dynamics Simulations of the Mechanical Response of Metallic glass/BCC Nanocomposite Materials
Authors: Shi Yunfeng , Falk Michael L
Published: June 2019

Bulk metallic glass (BMG) has been of intense interest recently because of its unique combination of excellent mechanical properties including high strength. One of the major drawbacks of monolithic BMG materials is the limited ductility due to strain localization. Except a few recent reports on monolithic ductile BMG, the most popular method to enhance the ductility of the BMG samples is to introduce a crystalline or quasicrystalline phase by precipitation or mixing. The composite material usually exhibits some degree of strain hardening along with significantly higher impact resistance and fracture toughness. However, the mechanism of this strain hardening is not well understood. We present simulated uniaxial compression tests on a monatomic model amorphous system embedded with body-center cubic (BCC) nanocrystals. The advantage of this model system is that intimate amorphous-crystal interfaces can be obtained. We observe that when comparing to monolithic glassy samples where a single shear band normally dominates, multiple shear bands appear in the BCC-amorphous composite samples. The plastic deformation initiates at the interfaces between nanocrystals and the glassy phase due to stress concentration. Furthermore, we demonstrate that shear along the bands results in growth of the nanocrystals.
Effect of Crystallinity on the Friction Behavior of Ultra-high-molecular-weight-polyethylene
Authors: Kanaga Subramanian Kanaga Karuppiah , Bruck Angela L , Sundararajan Sriram , Lin Zhiqun
Published: June 2019

In this study we evaluate the interfacial shear strength and scratch resistance of medical grade ultra-high molecular weight polyethylene (UHMWPE) (GUR 1050 resin) as a function of polymer crystallinity. Crystallinity was controlled by heating UHMWPE samples to a temperature above its melting point and varying the hold time and cooling rates. Degree of crystallinity of the samples was evaluated using differential scanning calorimetry (DSC). Quantitative nanoscale friction experiments were conducted using an atomic force microscope with commercially available Si3N4 probes under dry conditions. A higher crystallinity resulted in lower friction force and lower interfacial shear strength as well as increased scratch resistance. The trend in friction response was observed in microscale friction measurements.
X-ray Diffraction Study of the Mechanical Elastic Properties of Nanometric W/Cu Multilayers
Authors: Villain Pascale , Girault Baptiste , Renault Pierre-Olivier , Le Bourhis Eric , Goudeau Philippe , Badawi Frederic
Published: June 2019

The mechanical behavior of W/Cu multilayers with a period of 24 nm and a 1/3 W/Cu thickness ratio prepared by magnetron sputtering was analyzed using a method combining X-ray diffraction and tensile testing. Tests were performed both with a conventional and a synchrotron light source to analyze the elastic response of the system. Comparison between the strain-load curves obtained in both experimental conditions and estimated curves clearly shows that high quality synchrotron measurements are a preliminary condition for size-effect studies. Moreover, cyclic tests were used to determine the elastic domain of each material and compare their mechanical responses. Plastic strain was observed in copper while tungsten layers were still elastically strained until cracks appeared.
Strength of Coherently Strained Nanolayers Under High Temperature Nanoindentation
Authors: P'ng Mok Yew , Hou X. D. , Dunstan D. J. , Bushby A. J.
Published: June 2019

Semiconductor strained layer superlattices are an ideal model material to study the effects of coherency strain in plasticity, due to the fine control of nanolayer thickness and internal strain afforded by MBE deposition. Previously, nanoindentation of bulk InGaAs at 300K gave a yield pressure of 6GPa (Jayawera et al Proc. Roy Soc, A459, 2049, 2003) while bending at 500 centigrade gave a yield value of 30MPa (Pp’ ng et al Phil. Mag. 85, 4429, 2005). In contrast, coherently strained InGaAs superlattices gave nanoindentation values of 3GPa at room temperature and bending at 500oC gave a yield value also around 3GPa. It appears that the coherency strain can impart an athermal strengthening to the superlattice. It is clearly necessary to do mechanical testing over the range 300-800K that will be able to link the room temperature nanoindentation with the results from the high temperature bending experiment and to determine the relationship between strength, coherency strain and temperature. Preliminary experiments on these samples at elevated temperatures using a hot stage and the UMIS nanoindentation system is difficult but feasible with the help of AFM to verify the contact area.
Glass Transition Temperature in Confined Polymers
Authors: Ozisik Rahmi , Liu Tong , Siegel Richard W.
Published: June 2019

Glass transition temperature of polyetherimide (PEI) thin films and nanoporous PEI samples was investigated using differential scanning calorimetry. In both of these systems, the glass transition temperature decreased with respect to the bulk value. In the nanoporous system, scanning electron microscope images were used to characterize pore size distribution, and Monte Carlo simulations were performed to calculate the nearest neighbor pore-to-pore distances. Pore-to-pore distances and thin film thickness values were used to establish a quantitative analogy between thin films and nanoporous system.
High Temperature Clay Filled Epoxy Composites
Authors: Raghavan Dharmaraj , Langat Joseph Ktoo , Zammarano Mauro , Gilman Jeffrey
Published: June 2019

The primary objective of this study is to improve the thermal stability of clay and clay filled composite. An epoxy-clay composite has been prepared by dispersing 1-hexadecyl-3-(6-hydroxyhexyl)-2-methylimidazolium modified clay in an epoxy resin and cured with metaphenylene diamine (m-PDA) at 110.0 °C for 7 h and post cured at 140°C for 4 h. The thermal stability of the modified clay and clay filled epoxy composite was characterized via thermogravimetric analysis (TGA). The onset decomposition temperature of the imidazolium functionalized clay was 360°C. Transmission Electron Microscopy (TEM) of the composite showed mixed morphology with predominant fraction of intercalated clay platelets in the epoxy matrix. The onset decomposition temperature of the modified clay filled epoxy composite was found to be higher than that of pristine epoxy.
Using Elasticity to Correct for Boundary Effects in Calculations of Stress-Diffusion Coupling Parameters
Authors: Puchala Brian , Falk Michael , Garikipati Krishna
Published: June 2019

The effect of stress on diffusion during semiconductor processing becomes important as device dimensions shrink from microns to nanometers. Simulating these effects requires accurate parameterization of the formation and migration volume tensors of the defects that mediate diffusion on the atomistic scale. We investigate the effect of boundary conditions on the accuracy of atomistic calculations of defect formation energies and formation volume tensors. Linear elasticity provides a correction to the effect of the boundaries on the resulting relaxation volume tensor. By a formal proof we show that the correction term is zero for free boundaries and for periodic boundary conditions with zero mean boundary stress. This is demonstrated in the far field for periodic and free boundary conditions for an isotropic (vacancy) and an anisotropic (<110> intersitial) defect in Stillinger-Weber silicon. For periodic boundary conditions, formation volume tensor components converge to within 5% in a 216 atom simulation cell. For free boundary conditions, slow convergence of elastic constants results in slow convergence of formation volumes. Most significantly, this provides a new method to calculate the formation volume from constant volume calculations. This removes the need for relaxing boundaries, allowing for simpler and more efficient algorithms. We apply this method to both the vacancy and the <110> interstitial in Stillinger-Weber silicon.
A method for analyzing nanomechanical deformation of nanocrystalline Ni at higher timesteps than is possible in classical molecular dynamics
Authors: Tomar Vikas
Published: June 2019

A majority of computational mechanical analyses of nanocrystalline materials have been carried out using classical molecular dynamics (MD). Due to the fundamental reason that the MD simulations must resolve atomic level vibrations, they cannot be carried out at the timescale of the order of microseconds. Additionally, MD simulations have to be carried out at very high loading rates (∼108 s−1) rarely observed in experiments. In this investigation a modified Hybrid Monte Carlo (HMC) method that can be used to analyze time-dependent (strain rate dependent) atomistic mechanical deformation of nanocrystalline structures at higher timescales than currently possible using MD is established. In this method there is no restriction on the size of MD timestep except that it must be such that to ensure a reasonable acceptance rate between consecutive Monte-Carlo (MC) time-steps. For the purpose of comparison HMC analyses of a nanocrystalline Ni sample at a strain rate of 109 s−1 with three different timesteps, viz. 2 fs, 4fs, and 8 fs, are compared with the analyses based on MD simulations at the same strain rate and a MD timestep of 2 fs. MD simulations of nanocrystalline Ni reproduce the defect nucleation and propagation results as well as strength values reported in the literature. In addition, HMC with timestep of 8 fs correctly reproduces defect formation and stress-strain response observed in the case of MD simulations with permissible timestep of 2 fs (for the interatomic potential used 2 fs is the highest MD timestep). Simulation time analyses show that by using HMC a saving of the order of 4 can be achieved bringing the atomistic analyses closer to the continuum timescales.
Multiscale Modelling of bcc-Fe Based Alloys for Nuclear Applications [PowerPoint Submission]
Authors: Malerba Lorenzo
Published: June 2019

Understanding the basic mechanisms that determine microstructure changes in neutron irradiated steels is vital for a safe lifetime management of existing nuclear reactors and a safe design of future nuclear options. Low-alloyed ferritic steels containing Cu, Ni, Mn and Si as principal solute atoms are used as structural materials for current reactor vessels. The microstructural evolution under irradiation in alloys is decided by the interplay between defect formation and thermodynamic driving forces, together determining the appearance of phase transformations (precipitation, segregation, …) and favouring or delaying the nucleation and growth of point-defect clusters, their diffusion and their mutual recombination or removal at sinks. A reliable description of the production, evolution and accumulation of radiation damage must therefore start from the atomic level and requires being able to describe multicomponent systems for timescales ranging from few picoseconds to years. This goal demands firstly the fabrication of interatomic potentials for alloys that must be both consistent with the thermodynamic properties of the system and capable of reproducing correctly the characteristic solute-point defect interactions, versus ab initio or experimental data. Secondly the performance of extensive molecular dynamics (MD) simulations, to grasp the main mechanisms of defect production, diffusion, mutual interaction, and interaction with solute atoms and impurities. Thirdly, the development of simulation tools capable of describing the microstructure evolution beyond the timeframe and lengthscale of MD, while reproducing as much as possible the atomic-level origin of the mechanisms governing the evolution of the system, including phase changes.

In this presentation the results of recent efforts made in this direction in the case of Fe-Cu, Fe-Cr and Fe-Ni alloys, as basic model alloys for the description of steels of technological relevance, are highlighted. In particular, advanced techniques to fit interatomic potentials consistent with thermodynamics are proposed and the results of their application to the mentioned alloys are presented. Next, the development of advanced methods, based on the use of artificial intelligence, to improve both the physical reliability and the computational efficiency of kinetic Monte Carlo codes for the study of point-defect clustering and phase changes beyond the scale of MD, is reported. These recent progresses bear the promise of being able, in the near future, of producing reliable tools for the description of the microstructure evolution of realistic model alloys under irradiation.
Ab Initio and Kinetic Rate Theory Modeling of 316SS with Oversized Solute Additions on Radiation-Induced Segregation [PowerPoint Submission]
Authors: Hackett Micah J
Published: June 2019

Deleterious effects of radiation in nuclear reactor systems cause material degradation and the potential for component failure. Radiation damage is fundamentally due to freely migrating point defects produced in collision cascades. A reduction in the freely migrating point defect population should, then, reduce radiation damage and increase component lifetime. The addition of oversized solute atoms such as Zr or Hf to 316SS, a common structural material in reactors, is expected to reduce point defect population through a trapping mechanism that enhances recombination. The mechanism, however, requires a strong binding energy between the oversized solute atom and vacancies in order for the mechanism to significantly reduce the defect population. Experimental measurements of this binding energy are unavailable, but can be determined with atomistic calculations. Ab initio methods are used here to determine binding energies and atomic volumes of either Hf or Zr oversized solutes with vacancies in a face-centered cubic Fe matrix. The binding energies are then used to parameterize a kinetic rate-theory model, which is used here to calculate radiation-induced segregation (RIS). The calculated values of RIS are then compared to experimental measurements to benchmark the calculations and offer insight into the proposed point defect trapping mechanism.
Three-Dimensional Boundary Element-Dislocation Dynamics Modeling of Plastic Flow in Small Volumes [PowerPoint Submission]
Authors: El-Awady Jaffar A. , Takahashi Akiyuki , Ghoniem Nasr M
Published: June 2019

We present a self-consistent formulation of 3-D Parametric Dislocation Dynamics (PDD) with the Boundary Element method (BEM) to describe dislocation motion, and hence microscopic plastic flow in finite volumes. We develop quantitative measures of the accuracy and convergence of the method by considering a comparison with known analytical solutions. It is shown that the method displays absolute convergence with increasing the number of quadrature points on the dislocation loop and the surface mesh density. For example, at a distance of one lattice parameter from the surface, the relative error is less than 5% for a surface mesh with an element size of 1000 × 2000 lattice parameters, and 64 quadrature points. The Eshelby twist in a finite length cylinder containing a coaxial screw dislocation is also used to benchmark the method. Simulation results of single slip behavior in cylindrical microcrystals is presented, and the general features are compared to single-crystal compression experiments. The method is utilized to study size scaling aspects of plastic flow in small volumes and assess the role of the dislocation starvation mechanism.
Cooling Mechanism and Structural Change of Local Regions With a Different Cooling Rate of Excimer Laser Annealed Si
Authors: Lee Byoung-Min , Seong Baek Seok , Kim Hak Rho , Munetoh Shinji , Motooka Teruaki
Published: June 2019

To investigate the cooling mechanism and the local structural changes of excimer laser-annealed silicon (Si), molecular dynamics (MD) simulations were performed. Heat flow of molten Si showed a strong dependency of the local region during a natural cooling. An amorphous-to-liquid transition near an interface in the temperature range of 1600 K ∼ 1800 K was expected with the results of the local diffusion coefficients calculated by integrating the velocity autocorrelation functions. It was confirmed that the structure of the interface region affected the cooling rate of the overall system. The structural properties at the various local regions after a cooling were assessed in terms of the configurational properties including the coordination and bond-angle distributions. A spontaneous nucleation of Si near a interface was observed during a natural cooling.
Growth study of nanocrystalline Ni and Ni3Al using molecular dynamics
Authors: Zhang Z. Y. , Meng X. K. , Wang J. , Huang Hanchen , Liu X.-Y.
Published: June 2019

The growth of nanocrystalline materials has received much attention recently due to its importance in nanocrystalline thin film properties and fabrications. Research studies of this subject have so far focused on the face-centered-cubic metals. Two mechanisms, grain-rotation induced grain coalescence and curvature-driven grain-boundary migration are considered as the dominant mechanisms in the nanograin growth. In this work, we use molecular dynamics method to simulate the growth of Ni and Ni3Al alloy. We find the above mechanisms can describe the growth behavior well. A detailed comparison of the nanograin growth between the two systems is discussed in terms of grain rotation and grain sliding. We also study the temperature effect and the size effect in the nanograin growth. The tendency of twinning in the nanograin growth is discussed.
Simulations of Multi-atom Vacancies in Diamond
Authors: Laszlo Istvan , Kertesz Miklos , Gogotsi Yury
Published: June 2019

Multi-atom vacancies and pores in diamond-structured carbon phases play an important role in carbon sieves and carbon based storage. The size and shape of pores have a profound effect on the energetics of adsorptive storage. We are modeling large numbers of vacancy configurations with a combination of ab initio density functional theory (DFT), tight binding DFT and simpler methods based on Brenner-potentials and modified Brenner potentials. The more accurate calculations serve as the basis of the parametrization which is then used in lattice Monte Carlo simulations on more complex vacancies. The results will be put in the context of SiC-derived porous carbon materials with the purpose to explore basic questions of energetics in porous carbons.
Multiscale Modeling of Growth and Structure of Silicon Nanoparticles in an Oxide Matrix
Authors: Yu Decai , Lee Sangheon , Hwang Gyeong S
Published: June 2019

A first principles-based multiscale model is developed to examine mechanisms underlying Si nanocrystal formation in Si-rich SiO2. Using the multiscale approach, we have found that the embedded nanocrystal formation is mainly driven by suboxide penalty arising from incomplete O coordination, with a minor contribution of strain, and it is primarily controlled by O diffusion rather than excess Si diffusion and agglomeration. The overall behavior of Si cluster growth from our Monte Carlo simulations based on these fundamental findings agrees well with experiments.
New Ferromagnetic Nitrides, CaN and SrN, and their “Recipe”
Authors: Geshi Masaaki , Kusakabe Koichi , Nagara Hitose , Suzuki Naoshi
Published: June 2019

We have designed new materials, CaN and SrN in a rock-salt structure (RS). They are ferromagnetic nitrides. Their ferromagnetic states are quite stable. Our calculations for the formation energy and first-principles MD simulations suggested that RS-CaN is at least metastable and stable in normal condition. Based on our results we have proposed the synthesis process of RS-CaN. Our proposed process is (1) heat up α-Ca3N2 until it is transformed into β-Ca3N2, (2) compress it above 50 GPa until 2Ca3N2 →2CaN+Ca reaction occurs, (3) cool down them to room temperature and (4) decompress them into an ambient pressure. We consider that this high pressure and high temperature synthesis is one of the hopeful method to synthesize new materials.
Phase Separation and Solidification of Fluid Phosphorus
Authors: Katayama Yoshinori , Inamura Yasuhiro , Saitoh Hiroyuki , Utsumi Wataru
Published: June 2019

Fluid phosphorus exhibits a macroscopic phase separation during a pressure-induced first-order phase transition between low-density fluid and high-density fluid. Solidification process of the phase-separated fluid sample was monitored by x-ray radiography and the obtained solid sample was investigated by optical and scanning electron microscopy. A solid mixture of black parts and red parts was obtained from the phase-separated sample. Three different morphologies were observed in the red part of the sample.
High Pressure Structural Phase Transition and Elastic Properties of MgX (X = S, Se, Te) Semiconducting Compounds
Authors: Varshney Dinesh , Kaurav Netram , Choudhary Kamal Kumar , Kinge R. , Singh R. K.
Published: June 2019

Pressure-induced structural aspects of NaCl-type (B1) to CsCl-type (B2) structure in MgX [X = S, Se, Te] semiconductors are presented. An effective interionic interaction potential (EIOP) with long range Coulomb, van der Waals (vdW) interaction and the short-range repulsive interaction upto second-neighbor ions within the Hafemeister and Flygare approach is developed. Particular attention is devoted to evaluate the vdW coefficients following the variational method, as both the cation and the anion are polarizable. Our result on vast volume discontinuity in pressure volume phase diagram identifies the structural phase transition from B1 to B2 structure. The estimated value of the phase transition pressure (Pt) is consistent with results previously published. The variations of elastic constants with pressure follow a systematic trend identical to that observed in others compounds of NaCl type structure family.
Synchrotron Moessbauer Spectroscopy and Resistivity Studies of Iron Oxide Under High Pressure
Authors: Struzhkin Viktor V. , Eremets Mikhail I. , Eremets Ivan M. , Lin Jung-Fu , Sturhahn Wolfgang , Zhao Jiyong , Hu Michael Y
Published: June 2019

The strong electron correlations play a crucial role in the formation of a variety of electronic and magnetic properties of the transition metal oxides. In strongly correlated electronic materials many theoretical predictions exist on pressure-induced insulator-metal transitions, which are followed by a collapse of localized magnetic moments and by structural phase transitions [1]. The high-pressure studies provide additional degree of freedom to control the structural, electronic, optical, and magnetic properties of transition metal oxides. With the development of the high-pressure diamond-anvil-cell technique the experimental studies of such transitions are now possible with the advanced synchrotron techniques. In our studies, the iron monooxide Fe0.94O was studied under high pressures up to 200 GPa in diamond anvil cells. The single crystals enriched with Fe57 isotopes have been prepared for nuclear resonance measurements. The results of synchrotron Mössbauer spectroscopy (nuclear forward scattering -NFS), and electro-resistivity measurements suggest a complicated scenario of magnetic interactions governed by band-broadening effects.
Magnetic Collapse and Insulator-Metal Transitions in Some 3D Metal Oxides Under High Pressures
Authors: Lyubutin Igor S. , Gavriliuk Alexander G. , Struzhkin Viktor
Published: June 2019

In the systems with strong electron correlations, many theories predict the high-pressure-induced dielectric-metal transition, which is followed by collapse of localized magnetic moment and structural phase transition. In this report, summary results of many last experiments on the influence of high pressure on the magnetic and crystal structure as well on the electronic and transport properties of 3d metal oxides is presented. Along with X-ray diffraction, optical absorption, Raman scattering and electroresistivity measurements, several synchrotron radiation techniques have also been applied to perform the high-pressure experiments with compound iron oxides having different crystal structures.
Composition, structure and diversity of coastal vegetation in the northeastern of Cozumel, Mexico
Authors: Collantes-Chávez-Costa Alejandro , Alanis-Rodríguez Eduardo , Yam-Uicab Oscar , López-Contreras Cruz , Sarmiento-Muñoz Tania , Tapia-Muñoz José Luis
Published: June 2019

Background: Cozumel island, Mexico has one of the best preserved and threatened coastal vegetation in the Mexican Caribbean. Increasing the ecological knowledge about these communities can help to establish conservation priorities.
Question: How are the structure and composition of well-preserved coastal vegetation communities in the Island of Cozumel?
Study site and dates: The study was conducted in the northeastern coast of the island of Cozumel, at the Municipality of Cozumel, Quintana Roo, Mexico, from July 2014 to November 2014.
Methods: Five interception lines of 80 m were randomly established along environmental gradient, perpendicular to the coastal line, in each three different environments: rocky beaches, sandy beaches, and coastal dunes. Species richness, diversity, and importance value index of the species in the community were computed, and floristic composition recorded.
Results: We recorded 23 families, 35 genera and 37 species. The most representative families are Poaceae with four species, Rubiaceae and Asteraceae with three species each; Sideroxylon and Euphorbia were the richest genera, with two species each. The three coastal environments presented no statistical differences in its relative density, coverage and diversity, but differed in species composition and richness (higher in rocky beaches and coastal dunes).
Conclusions: Two dissimilar coastal plant communities can be recognized into the halophilous vegetation type, which are established in sandy and rocky environments respectively. The floristic composition and the species association of the rocky beaches reveal the relationship between the Island of Cozumel coastal vegetation and plant communities in other Caribbean islands.
Editorial 75 aniversario 97(2) 2019
Authors: Meave Jorge A.
Published: June 2019

Editorial
Green certificate trading
Authors: Brick Kerri , Visser Martine
Published: June 2019

Policies to promote renewable electricity are increasingly seen as a way to reduce the negative environmental impacts associated with electricity consumption and meet growing electricity demand. This paper reviews the international experience with one such policy, namely, renewable energy certificates, and considers important design aspects of a national green certificate system. Within a South African context, a green certificate system would provide a mechanism with which to verify compliance with any future renewable energy obligations, and would encourage renewable electricity generation in the current monopoly environment. In terms of a national green certificate framework, international experience has shown that renewable energy certificates must be both accredited and standardized, with enforcement of penalties for non-compliance with renewable energy quotas. Above all, a long-term and stable policy environment is crucial for developing renewable energy markets.
The challenges and potential options to meet the peak electricity demand in Mauritius
Authors: Elahee Khalil
Published: June 2019

This paper reviews the current challenges facing Mauritius in terms of meeting peak electricity demand. As a fast-developing island-economy with a very high population density, this is a crucial issue. The more so that it imports 80% of its energy requirements in terms of fossil fuels, relies significantly on tourism and needs to protect its fragile ecosystems. The nature of the peak electricity demand and its evolution is firstly analysed. Reference is made to past scenarios for electricity supply, the obstacles to their implementation and their relevance in terms of sustainability. The forecasts underpinning the latter scenarios are found to be over-estimated. Demand-Side Management projects are discussed and their potential to promote an alternative scenario based on revised forecasts are discussed. Hence a new Maurice Ile Durable      (Mauritius Sustainable Island, MID) scenario is proposed in view of stabilising the peak demand, reducing the rate of increase of total electricity demand and making the capacity margin positive. The newly-devised scenario is not only more sustainable but also addresses several political and socio-economic issues to bring holistic win-win solutions. Institutional and regulatory reforms as well as a relevant Business Framework are also important in order to meet the challenges of MID. The new scenario relies only on existing technology with an excellent track-record and provides the transition to a more sustainable future.
The technical pre-feasibility to use briquettes made from wood and agricultural waste for gasification in a downdraft gasifier for electricity generation
Authors: Malatji Pholoso , Mamphweli Ntshengedzeni Sampson , Meincken Martina
Published: June 2019

Biomass can be converted to energy through various thermochemical and biological processes. Gasification is one of the thermochemical processes that has recently gained popularity, because it achieves higher conversion efficiencies than, for example, incinerators, boilers or furnaces. Fixed-bed downdraft gasifiers are preferred for electricity generation, because they produce very little tar, but on the other hand, they are limited with regard to biomass properties, such as particle size, bulk density and moisture content. Biomass material with a heterogeneous size is usually processed into pellets or briquettes, which have to be mechanically strong enough to be handled. Cohesive strength is provided by residual moisture and lignin present in most biomass. However, the briquetting process becomes more complicated if one wants to add agricultural waste products that do not necessarily contain lignin as binders. The aim of this work was to process wood chips, grape skins and chicken litter into briquettes that are mechanically stable and have a sufficiently high energy content, as well as adequate bulk density for gasification. The performance of these briquettes in a downdraft gasifier was simulated with a program developed for wood, which was modified to optimise the briquette yield. The results showed a gasification performance comparable to solid pine wood, implying that the blended briquettes could be used as fuel for a downdraft biomass gasifier. Unfortunately, the briquettes proved too instable to experimentally verify the performance in a gasifier. This paper describes the properties of the briquettes as well as the gasification simulation results.
Reliability worth assessment of electricity consumers: a South African case study
Authors: Dzobo Oliver , Gaunt Trevor , Herman Ronald
Published: June 2019

This paper discusses the results obtained from a customer survey conducted in Cape Town, South Africa, using in-person interviews with approximately 275 sample business customers. The survey included customer interruption cost estimation questions based on the direct costing method. The results obtained show that customer interruption cost for business customers varies with duration and time of occurrence of power interruptions. The variation was shown to be dependent on customer segment. Furthermore, it revealed that business customers can be grouped in terms of the investment they make to mitigate the impact of power interruptions on their activities, such as the use of backup power supplies.
Comparing performance of MLP and RBF neural network models for predicting South Africa’s energy consumption
Authors: Oludolapo Olanrewaju A. , Adisa Jimoh A. , Pule Kholopane A.
Published: June 2019

In view of the close association between energy and economic growth, South Africa’s aspirations for higher growth, more energy is required; formulating a long-term economic development plan and implementing an energy strategy for a country /industry necessitates establishing the correct relationship between energy and the economy. As insufficient energy or a lack thereof is reported to be a major cause of social and economic poverty, it is very important to select a model to forecast the consumption of energy reasonably accurately. This study presents techniques based on the development of multilayer perceptron (MLP) and radial basis function (RBF) of artificial neural network (ANN) models, for calculating the energy consumption of South Africa’s industrial sector between 1993 and 2000. The approach examines the energy consumption in relation to the gross domestic product. The results indicate a strong agreement between model predictions and observed values, since the mean absolute percentage error is below 5%. When performance indices are compared, the RBF-based model is a more accurate predictor than the MLP model.