arXiv subjects

Wahyu Setyawan

Publications and source records attributed to Wahyu Setyawan.

14 recordsLinked to original sources

arXiv2017

The AFLOW Fleet for Materials Discovery

The traditional paradigm for materials discovery has been recently expanded to incorporate substantial data driven research. With the intent to accelerate the development and the deployment of new technologies, the AFLOW Fleet for computational materials design automates high-throughput first principles calculations, and provides tools for data verification and dissemination for a broad community of users. AFLOW incorporates different computational modules to robustly determine thermodynamic stability, electronic band structures, vibrational dispersions, thermo-mechanical properties and more. The AFLOW data repository is publicly accessible online at aflow.org, with more than 1.7 million materials entries and a panoply of queryable computed properties. Tools to programmatically search and process the data, as well as to perform online machine learning predictions, are also available.

cond-mat.mtrl-sci↗

arXiv2016

Object Kinetic Monte Carlo Simulations of Radiation Damage in Bulk Tungsten Part-II: With a PKA Spectrum Corresponding to 14-MeV Neutrons

Object kinetic Monte Carlo was employed to study the effect of dose rate on the evolution of vacancy microstructure in polycrystalline tungsten under neutron bombardment. The evolution was followed up to 1.0 displacement per atom (dpa) with point defects generated in accordance with a primary knock-on atom (PKA) spectrum corresponding to 14-MeV neutrons. The present study includes the effect of grain size (2.0 and 4.0 $μ$m) but excludes the impact of transmutation or pre-existing defects beyond grain boundary sinks. Vacancy cluster density increases with dose rate, while the density of vacancies decreases. Consequently, the average vacancy cluster size and the fraction of vacancies in visible clusters decrease with increasing dose rate. The density of vacancies and vacancy clusters decrease with grain size such that the average size of the clusters remains similar. However, the average size is larger for larger grains at dose rates < 4.5 x 10-7 dpa/s. The trend of vacancy accumulation as a function of dose, dose rate, and grain size is similar to that obtained with the High Flux Isotope Reactor (HFIR) PKA spectrum. However, the amount of vacancy accumulation and the vacancy microstructure are quite different. Compared to the HFIR case, we find that even though the dose rates are 2.5 times higher, the density of vacancies and the average vacancy cluster sizes are lower. In addition, a void lattice forms only for the lowest two dose rates (4.5 x 10-8 and 4.5 x 10-9 dpa/s). In contrast, a void lattice formed at all dose rates studied using the HFIR PKA spectrum. We discuss in detail the factors that lead to these different microstructures.

cond-mat.mtrl-sci↗

arXiv2014

Cascade morphology transition in bcc metals

Energetic atom collisions in solids induce shockwaves with complex morphologies. In this paper, we establish the existence of a morphological transition in such cascades. The order parameter of the morphology is defined as the exponent, $b$, in the defect production curve as a function of cascade energy ($N_F \sim E_{MD}^b$). Response of different bcc metals can be compared in a consistent energy domain when the energy is normalized by the transition energy, $μ$, between the high- and the low-energy regime. Using Cr, Fe, Mo and W data, an empirical formula of $μ$ as a function of displacement threshold energy, $E_d$, is presented for bcc metals.

cond-mat.mtrl-sci↗

arXiv2014

Displacement cascades and defects annealing in tungsten, Part I: defect database from molecular dynamics simulations

Molecular dynamics simulations have been used to generate a comprehensive database of surviving defects due to displacement cascades in bulk tungsten. Twenty one data points of primary knock-on atom (PKA) energies ranging from 100 eV (sub-threshold energy) to 100 keV ($\sim$780$\times E_d$, where $E_d$ is the average displacement threshold energy) have been completed at 300 K, 1025 K and 2050 K. Within this range of PKA energies, two regimes of power-law energy-dependence of the defect production are observed. A distinct power-law exponent characterizes the number of Frenkel pairs produced within each regime. The two regimes intersect at a transition energy which occurs at approximately 250$\times E_d$. The transition energy also marks the onset of the formation of large self-interstitial atom (SIAs) clusters (size 14 or more). The observed defect clustering behavior is asymmetric, with SIA clustering increasing with temperature, while the vacancy clustering decreases. This asymmetry increases with temperature such that at 2050 K ($\sim 0.5 T_m$) practically no large vacancy clusters are formed, meanwhile large SIA clusters appear in all simulations. The implication of such asymmetry on the long-term defect survival and damage accumulation is discussed. In addition, rare $<$100$>${110} SIA loops are observed.

cond-mat.mtrl-sci↗

arXiv2014

Displacement cascades and defects annealing in tungsten, Part II: Object kinetic Monte Carlo Simulation of Tungsten Cascade Aging

We describe the results of object kinetic Monte Carlo (OKMC) simulations of the annealing of primary cascade damage in bulk tungsten using a comprehensive database of cascades obtained from molecular dynamics [1] as a function of primary knock-on atom (PKA) energy and direction, and temperatures of 300, 1025 and 2050 K. An increase in SIA clustering but decrease in vacancy clustering with temperature combined with disparate mobilities of SIAs versus vacancies causes an interesting temperature effect on cascade annealing, which is quite different from what one would expect. The annealing efficiency (ratio of number of defects after and before annealing) exhibits an inverse U-shape curve as a function of temperature. In addition, we will also describe the capabilities of our newly developed OKMC code; KSOME (kinetic simulations of microstructure evolution) used to carryout these simulations

cond-mat.mtrl-sci↗

arXiv2013

Asymmetry of radiation damage properties in Al-Ti nanolayers

Molecular dynamics (MD) simulations were employed with empirical potentials to study the effects of multilayer interfaces and interface spacing in Al-Ti nanolayers. Several model interfaces derived from stacking of close-packed layers or face-centered cubic \{100\} layers were investigated. The simulations reveal significant and important asymmetries in defect production with $\sim$60% of vacancies created in Al layers compared to Ti layers within the Al-Ti multilayer system. The asymmetry in the creation of interstitials is even more pronounced. The asymmetries cause an imbalance in the ratio of vacancies and interstitials in films of dissimilar materials leading to $>$90% of the surviving interstitials located in the Al layers. While in the close-packed nanolayers the interstitials migrate to the atomic layers adjacent to the interface of the Al layers, in the \{100\} nanolayers the interstitials migrate to the center of the Al layers and away from the interfaces. The degree of asymmetry and defect ratio imbalance increases as the layer spacing decreases in the multilayer films. Underlying physical processes are discussed including the interfacial strain fields and the individual elemental layer stopping power in nanolayered systems. In addition, experimental work was performed on low-dose (10$^{16}$ atoms/cm$^2$) helium (He) irradiation on Al/Ti nanolayers (5 nm per film), resulting in He bubble formation $\sim$1 nm in diameter in the Ti film near the interface. The correlation between the preferential flux of displaced atoms from Ti films to Al films during the defect production that is revealed in the simulations and the morphology and location of He bubbles from the experiments is discussed.

cond-mat.mtrl-sci↗

arXiv2013

AFLOW: An automatic framework for high-throughput materials discovery

Recent advances in computational materials science present novel opportunities for structure discovery and optimization, including uncovering of unsuspected compounds and metastable structures, electronic structure, surface, and nano-particle properties. The practical realization of these opportunities requires systematic generation and classification of the relevant computational data by high-throughput methods. In this paper we present Aflow (Automatic Flow), a software framework for high-throughput calculation of crystal structure properties of alloys, intermetallics and inorganic compounds. The Aflow software is available for the scientific community on the website of the materials research consortium, aflowlib.org. Its geometric and electronic structure analysis and manipulation tools are additionally available for online operation at the same website. The combination of automatic methods and user online interfaces provide a powerful tool for efficient quantum computational materials discovery and characterization.

cond-mat.mtrl-sci↗

arXiv2010

High-throughput electronic band structure calculations: challenges and tools

The article is devoted to the discussion of the high-throughput approach to band structures calculations. We present scientific and computational challenges as well as solutions relying on the developed framework (Automatic Flow, AFLOW/ACONVASP). The key factors of the method are the standardization and the robustness of the procedures. Two scenarios are relevant: 1) independent users generating databases in their own computational systems (off-line approach) and 2) teamed users sharing computational information based on a common ground (on-line approach). Both cases are integrated in the framework: for off-line approaches, the standardization is automatic and fully integrated for the 14 Bravais lattices, the primitive and conventional unit cells, and the coordinates of the high symmetry k-path in the Brillouin zones. For on-line tasks, the framework offers an expandable web interface where the user can prepare and set up calculations following the proposed standard. Few examples of band structures are included. LSDA+U parameters (U, J) are also presented for Nd, Sm, and Eu.

cond-mat.mtrl-sci↗

arXiv2009

Comparative Study of Nonproportionality and Electronic Band Structures Features in Scintillator Materials

The origin of nonproportionality in scintillator materials has been a long standing problem for more than four decades. In this manuscript, we show that, with the help of first principle modeling, the parameterization of the nonproportionality for several systems, with respect to their band structure curvature suggests a correlation between carrier effective mass and energy response. We attribute this correlation to the case where free electrons and holes are the major energy carriers. Excitonic scintillators do not show such a definitive trend. This model suggests a potential high-throughput approach for discovering novel proportional scintillators in the former class of materials.

cond-mat.mtrl-sci↗

arXiv2008

Structures and topological transitions of hydrocarbon films on quasicrystalline surfaces

Lubricants can affect quasicrystalline coatings surfaces by modifying commensurability of the interfaces. We report results of the first computer simulation studies of physically adsorbed hydrocarbons on a quasicrystalline surface: methane, propane, and benzene on decagonal Al-Ni-Co. The grand canonical Monte Carlo method is employed, using novel Embedded Atom Method potentials generated from it ab initio calculations, and standard hydrocarbon interactions. The resulting adsorption isotherms and calculated structures show the films' evolution from submonolayer to condensation. We discover the presence and absence of the 5- to 6-fold topological transition, for benzene and methane, respectively, in agreement with a previsouly formulated phenomenological rule based on adsorbate-substrate size mismatch.

cond-mat.mtrl-sci↗

arXiv2008

Influence of Mo on the Fe:Mo:C nano-catalyst thermodynamics for single-walled carbon nanotube growth

We explore the role of Mo in Fe:Mo nanocatalyst thermodynamics for low-temperature chemical vapor deposition growth of single walled carbon nanotubes (SWCNTs). By using the size-pressure approximation and ab initio modeling, we prove that for both Fe-rich (~80% Fe or more) and Mo-rich (~50% Mo or more) Fe:Mo clusters, the presence of carbon in the cluster causes nucleation of Mo2C. This enhances the activity of the particle since it releases Fe, which is initially bound in a stable Fe:Mo phase, so that it can catalyze SWCNT growth. Furthermore, the presence of small concentrations of Mo reduce the lower size limit of low-temperature steady-state growth from ~0.58nm for pure Fe particles to ~0.52nm. Our ab initio-thermodynamic modeling explains experimental results and establishes a new direction to search for better catalysts.

cond-mat.mtrl-sci↗

arXiv2006

Theoretical study of the thermal behavior of free and alumina-supported Fe-C nanoparticles

The thermal behavior of free and alumina-supported iron-carbon nanoparticles is investigated via molecular dynamics simulations, in which the effect of the substrate is treated with a simple Morse potential fitted to ab initio data. We observe that the presence of the substrate raises the melting temperature of medium and large $Fe_{1-x}C_x$ nanoparticles ($x$ = 0-0.16, $N$ = 80-1000, non- magic numbers) by 40-60 K; it also plays an important role in defining the ground state of smaller Fe nanoparticles ($N$ = 50-80). The main focus of our study is the investigation of Fe-C phase diagrams as a function of the nanoparticle size. We find that as the cluster size decreases in the 1.1-1.6-nm-diameter range the eutectic point shifts significantly not only toward lower temperatures, as expected from the Gibbs-Thomson law, but also toward lower concentrations of C. The strong dependence of the maximum C solubility on the Fe-C cluster size may have important implications for the catalytic growth of carbon nanotubes by chemical vapor deposition.

cond-mat.mtrl-sci↗

arXiv2006

Xe films on a decagonal Al-Ni-Co quasicrystal surface

The grand canonical Monte Carlo method is employed to study the adsorption of Xe on a quasicrystalline Al-Ni-Co surface. The calculation uses a semiempirical gas-surface interaction, based on conventional combining rules and the usual Lennard-Jones Xe-Xe interaction. The resulting adsorption isotherms and calculated structures are consistent with the results of LEED experimental data. In this paper we focus on five features not discussed earlier (Phys. Rev. Lett. 95, 136104 (2005)): the range of the average density of the adsorbate, the order of the transition, the orientational degeneracy of the ground state, the isosteric heat of adsorption of the system, and the effect of the vertical cell dimension.

cond-mat.soft↗

arXiv2005

Evolution of topological order in Xe films on a quasicrystal surface

We report results of the first computer simulation studies of a physically adsorbed gas on a quasicrystalline surface, Xe on decagonal Al-Ni-Co. The grand canonical Monte Carlo method is employed, using a semi-empirical gas-surface interaction, based on conventional combining rules, and the usual Lennard-Jones Xe-Xe interaction. The resulting adsorption isotherms and calculated structures are consistent with the results of LEED experimental data. The evolution of the bulk film begins in the second layer, while the low coverage behavior is epitaxial. This transition from 5-fold to 6-fold ordering is temperature dependent, occurring earlier (at lower coverage) for the higher temperatures.

cond-mat.mtrl-sci↗