arXiv subjects

Alejandro Strachan

Publications and source records attributed to Alejandro Strachan.

At least 55 records · Page 3Linked to original sources

arXiv2022

Systematic Builder for All-Atom Simulations of Plastically Bonded Explosives

The shock to detonation transition in heterogeneous plastically bonded explosives is dominated by energy localization into hotspots that arise from the interaction of the shockwave with microstructural features and defects. The complex polycrystalline structure of these materials leads to a network of hotspot that can coalesce into deflagration and detonation waves. Significant progress has been made on the formation and potency of hotspots using atomistic simulations, but most of the work has focused on ideal and isolated defects. Hence, developed a method, denoted PBXGen, to build realistic PBX microstructures for all-atom simulations. PBXGen is generally applicable, and we demonstrate it with two systems: an RDX-polystyrene PBX with a 3D microstructure and a TATB-polystyrene with columnar grains. The resulting structure exhibit key features of PBXs, albeit at smaller scales, and are validated against experimental mechanical and shock properties.

cond-mat.mtrl-sci↗

arXiv2021

Active learning and molecular dynamics simulations to find high melting temperature alloys

Active learning (AL) can drastically accelerate materials discovery; its power has been shown in various classes of materials and target properties. Prior efforts have used machine learning models for the optimal selection of physical experiments or physics-based simulations. However, the latter efforts have been mostly limited to the use of electronic structure calculations and properties that can be obtained at the unit cell level and with negligible noise. We couple AL with molecular dynamics simulations to identify multiple principal component alloys (MPCAs) with high melting temperatures. Building on cloud computing services through nanoHUB, we present a fully autonomous workflow for the efficient exploration of the high dimensional compositional space of MPCAs. We characterize how uncertainties arising from the stochastic nature of the simulations and the acquisition functions used to select simulations affect the convergence of the approach. Interestingly, we find that relatively short simulations with significant uncertainties can be used to efficiently find the desired alloys as the random forest models used for AL average out fluctuations.

cond-mat.mtrl-sci↗

arXiv2021

Sim2Ls: FAIR simulation workflows and data

Just like the scientific data they generate, simulation workflows for research should be findable, accessible, interoperable, and reusable (FAIR). However, while significant progress has been made towards FAIR data, the majority of science and engineering workflows used in research remain poorly documented and often unavailable, involving ad hoc scripts and manual steps, hindering reproducibility and stifling progress. We introduce Sim2Ls (pronounced simtools) and the Sim2L Python library that allow developers to create and share end-to-end computational workflows with well-defined and verified inputs and outputs. The Sim2L library makes Sim2Ls, their requirements, and their services discoverable, verifies inputs and outputs, and automatically stores results in a globally-accessible simulation cache and results database. This simulation ecosystem is available in nanoHUB, an open platform that also provides publication services for Sim2Ls, a computational environment for developers and users, and the hardware to execute runs and store results at no cost. We exemplify the use of Sim2Ls using two applications and discuss best practices towards FAIR simulation workflows and associated data.

cs.DL↗

arXiv2021

The Potential Energy Hotspot: Effects from Impact Velocity, Defect Geometry, and Crystallographic Orientation

In energetic materials, the localization of energy into "hotspots" when a shock wave interacts with the material's microstructure is known to dictate the initiation of chemical reactions and detonation. Recent results have shown that, following the shock-induced collapse of pores with circular cross-sections, more energy is localized as internal potential energy (PE) than can be inferred from the kinetic energy (KE) distribution. This leads to a complex thermo-mechanical state that is typically overlooked. The mechanisms associated with pore collapse and hotspot formation and the resulting energy localization are known to be highly dependent on material properties, especially its ability to deform plastically and alleviate strain energy, as well as the size and shape of the pore. Therefore, we use molecular dynamics simulations to characterize shock-induced pore collapse and the subsequent formation of hotspots in TATB, a highly anisotropic molecular crystal, for various defect shapes, shock strengths and crystallographic orientations. We find that the the localization of energy as PE is consistently higher and its extent larger than as localized as KE. A detailed analysis of the MD trajectories reveal the underlying molecular process that govern the effect of orientation and pore shape on the resulting hotspots. We find that the regions of highest PE for a given KE relate to not the impact front of the collapse, but the areas of maximum plastic deformation, while KE is maximized at the point of impact. A comparison with previous results in HMX reveal less energy localization in TATB which could be a contributing factor to its insensitivity.

cond-mat.mtrl-sci↗

arXiv2021

Hybrid Polymer-Garnet Materials for All-Solid-State Energy Storage Devices

Hybrid electrolyte materials comprising polymer-ionic salt matrixes embedded with garnet particles constitute a promising class of materials for the realization of all-solid-state batteries. In addition to providing solutions to the safety issues inherent to current liquid electrolytes, hybrid polymer electrolytes offer advantages over other solid-state electrolytes. This is because their functional properties such as ionic conductivity, electrochemical stability, mechanical and thermal properties can be tailored to a particular application by independently optimizing the properties of the constituent materials. Thereby, providing a rational approach to solving bottlenecks currently preventing solid-state electrolytes from practical implementation into battery devices. This review starts with a survey of solid-state electrolytes, focusing on their materials and ion transport limitations. Next, we summarize the current understanding of transport mechanisms in composite polymer electrolytes (CPEs) with the purpose of identifying materials solutions for further improving their properties. The overall goal of the review is to foster heightened research interest in these hybrid structures to rapidly advance development of future all-solid-state battery devices.

physics.app-ph↗

arXiv2020

A Hotspots Better Half: Non-Equilibrium Intra-Molecular Strain in Shock Physics

Shockwave interactions with material microstructure localizes energy into hotspots, which act as nucleation sites for complex processes such as phase transformations and chemical reactions. To date, hotspots have been described via their temperature fields. Nonreactive, all-atom molecular dynamics simulations of shock-induced pore collapse in a molecular crystal show that more energy is localized as potential energy (PE) than can be inferred from the temperature field and that PE localization persists through thermal diffusion. The origin of the PE hotspot is traced to large intra-molecular strains, storing energy in modes readily available for chemical decomposition.

cond-mat.mtrl-sci↗

arXiv2020

Parsimonious neural networks learn interpretable physical laws

Machine learning is playing an increasing role in the physical sciences and significant progress has been made towards embedding domain knowledge into models. Less explored is its use to discover interpretable physical laws from data. We propose parsimonious neural networks (PNNs) that combine neural networks with evolutionary optimization to find models that balance accuracy with parsimony. The power and versatility of the approach is demonstrated by developing models for classical mechanics and to predict the melting temperature of materials from fundamental properties. In the first example, the resulting PNNs are easily interpretable as Newton's second law, expressed as a non-trivial time integrator that exhibits time-reversibility and conserves energy, where the parsimony is critical to extract underlying symmetries from the data. In the second case, the PNNs not only find the celebrated Lindemann melting law, but also new relationships that outperform it in the pareto sense of parsimony vs. accuracy.

cs.LG↗

arXiv2020

Tuning martensitic transformations via coherent second phases in nanolaminates using free energy landscape engineering

We explore the possibilities and limitations of using a coherent second phase to engineer the thermo-mechanical properties of a martensitic alloy by modifying the underlying free energy landscape that controls the transformation. We use molecular dynamics simulations of a model atomistic system where the properties of a coherent, nanoscale second phase can be varied systematically. With a base martensitic material that undergoes a temperature-induced transformation from a cubic austenite to a monoclinic martensite, the simulations show a significant ability to engineer the transformation temperatures, from a ~50% reduction to a ~200% increase, with 50 at. % of the cubic second phase. We establish correlations between the properties of the second phase and the transformation characteristics and microstructure, via the free energy landscape of the two-phase systems. Coherency stresses have a strong influence on the martensitic variants observed and can even cause the non-martensitic second phase to undergo a transformation. Reducing the stiffness of second phase increases the transformation strain and modifies the martensitic microstructure, increasing the volume fraction of the transformed material. This increase in transformation strain is accompanied by a significant increase in the Af and thermal hysteresis, while the Ms remains unaltered. Our findings on the tunability of martensitic transformations can be used for informed searches of second phases to achieve desired material properties, such as achieving room temperature, lightweight shape memory alloys.

cond-mat.mtrl-sci↗

arXiv2020

The thermal conductivity of "pastas" in neutron star matter

This investigation explores the phononic thermal conductivity of nuclear star matter as it undergoes the "topological" transition to the "pasta" regime, and further down to the solid-liquid phase transition. The study was carried out using molecular dynamics simulations with nuclear potentials embedded in an effective (i.e. Thomas-Fermi) Coulomb potential. The thermal conductivity experiences a dramatic change within a narrow temperature interval around T=1 MeV. This change accomplishes the "pasta" breakdown during a heating process. The thermal conductivity by flipping protons' or neutrons' velocity further shows a decoupling for asymmetric nuclear star matter.

nucl-th↗

arXiv2020

Tunability of martensitic behavior through coherent nanoprecipitates and other nanostructures

Molecular dynamics simulations show that coherent precipitates can significantly affect the properties of martensitic transformations in Ni$_{63}$Al$_{37}$ alloys. The precipitates, consisting of non-martensitic Ni$_{50}$Al$_{50}$, modify the free energy landscape that governs the phase transformation and result in a significant reduction of the thermal hysteresis, at comparably minor expense of transformation strain, and modification of transformation temperatures. Importantly, this paper shows that free energy landscape engineering is possible with nanostructures potentially accessible through standard metallurgical processing routes. The atomistic-level nucleation and transformation mechanisms within the nanoprecipitate systems are explored and compared with epitaxial nanolaminates and nanowires. The simulations reveal three distinct regimes of transformation mechanisms and martensitic nanostructure as a function of volume fraction of the non-martensitic phase. Free energy landscape engineering is generally applicable and could contribute to the design of new shape memory alloys with novel properties, such as light weight alloys that operate at room temperature.

cond-mat.mtrl-sci↗

arXiv2020

Functional uncertainty quantification for isobaric molecular dynamics simulations and defect formation energies

Functional uncertainty quantification (FunUQ) was recently proposed to quantify uncertainties in models and simulations that originate from input functions, as opposed to parameters. This paper extends FunUQ to quantify uncertainties originating from interatomic potentials in isothermal-isobaric molecular dynamics (MD) simulations and to the calculation of defect formation energies. We derive and verify a computationally inexpensive expression to compute functional derivatives in MD based on perturbation theory. We show that this functional derivative of the quantities of interest (average internal energy, volume, and defect energies in our case) with respect to the interatomic potential can be used to predict those quantities for a different interatomic potential, without re-running the simulation. The codes and scripts to perform FunUQ in MD are freely available for download. In addition, to facilitate reproducibility and to enable use of best practices for the approach, we created Jupyter notebooks to perform FunUQ analysis on MD simulations and made them available for online simulation in nanoHUB. The tool uses cloud computing resources and users can view, edit, and run end-to-end workflows from a standard web-browser without the need to need to download or install any software.

physics.comp-ph↗

arXiv2020

Online simulation powered learning modules for materials science

Simulation tools are playing an increasingly important role in materials science and engineering and beyond their well established importance in research and development, these tools have a significant pedagogical potential. We describe a set of online simulation tools and learning modules designed to help students explore important concepts in materials science where hands-on activities with high-fidelity simulations can provide insight not easily acquired otherwise. The online tools, which involve density functional theory and molecular dynamics simulations, have been designed with non-expert end-users in mind and only a few clicks are required to perform most simulations, yet they are powered by research-grade codes and expert users can access advanced options. All tools and modules are available for online simulation in nanoHUB.org and access is open and free of charge. Importantly, instructors and students do not need to download or install any software. The learning modules cover a range of topics from electronic structure of crystals and doping, plastic deformation in metals, and physical properties of polymers. These modules have been used in several core undergraduate courses at Purdue's School of Materials Engineering, they are self contained, and are easy to incorporate into existing classes.

cond-mat.mtrl-sci↗

arXiv2020

Error correction in multi-fidelity molecular dynamics simulations using functional uncertainty quantification

We use functional, Fréchet, derivatives to quantify how thermodynamic outputs of a molecular dynamics (MD) simulation depend on the potential used to compute atomic interactions. Our approach quantifies the sensitivity of the quantities of interest with respect to the input functions as opposed to its parameters as is done in typical uncertainty quantification methods. We show that the functional sensitivity of the average potential energy and pressure in isothermal, isochoric MD simulations using Lennard-Jones two-body interactions can be used to accurately predict those properties for other interatomic potentials (with different functional forms) without re-running the simulations. This is demonstrated under three different thermodynamic conditions, namely a crystal at room temperature, a liquid at ambient pressure, and a high pressure liquid. The method provides accurate predictions as long as the change in potential can be reasonably described to first order and does not significantly affect the region in phase space explored by the simulation. The functional uncertainty quantification approach can be used to estimate the uncertainties associated with constitutive models used in the simulation and to correct predictions if a more accurate representation becomes available.

physics.comp-ph↗

arXiv2020

Uncharacteristic second order martensitic transformation in metals via epitaxial stress fields

While most phase transformations, e.g. ferroelectric or ferromagnetic, can be first or second order depending on external applied fields, martensitic transformations in metallic alloys are nearly universally first order. We demonstrate that epitaxial stress originating from the incorporation of a tailored second phase can modify the free energy landscape that governs the phase transition and change its order from first to second. High-fidelity molecular dynamics simulations show a remarkable change in the character of the martensitic transformation in Ni-Al alloys near the critical point. We observe the continuous evolution of the transformation order parameter and scaling with power-law exponents comparable to those in other ferroic transitions exhibiting critical behavior. Our theoretical work provides a foundation to recent experimental and computational results on martensites near critical points.

cond-mat.mtrl-sci↗

arXiv2019

Ferroelectricity in free standing perovskite-based nanodots: A density functional theory study

We use density functional theory to investigate the possibility of polar and multiferroic states in free-standing, perovskite-based nanodots at their limit of miniaturization: single unit cell with termination to allow centrosymmetry. We consider both A-O and B-O2 terminations for three families of nanodots: i) A=Ba with B=Ti, Zr, and Hf; ii) A=Ca and Sr with B=Ti; and iii) A = Na and K with B=Nb. We find all A-O terminated dots to be non-polar and to exhibit cubic symmetry, regardless of the presence of ferroelectricity in the bulk. These dots all carry a net magnetic moment, except Ca8TiO6. On the other hand, all B-O2 terminated nanodots considered in this study relax to a non-cubic ground state. Rather surprisingly, a subset of these structures (BaTi8O12, BaHf8O12, BaZr8O12, SrTi8O12, CaTi8O12 and KNb8O12) exhibit polar ground states. We propose a new structural parameter, the cluster tolerance factor (CTF), to determine whether a particular combination of A and B will yield a polar ground state in the nanodot geometry, analogous to the Goldschmidt factor in bulk ferroelectrics. We also find that in all polar B-O2 terminated nanodots are also magnetic. The multiferroic state originates from separation between spin density in peripheral B atoms and polarity primarily caused by the off-center central A atom. Our findings stress that surface termination plays a crucial role in determining whether ferroelectricity is completely suppressed in perovskite-based materials at their limit of miniaturization.

cond-mat.mtrl-sci↗

arXiv2018

The use of strain to tailor electronic thermoelectric transport properties: A first principles study of 2H-phase CuAlO2

Using first principles calculations, the use of strain to adjust electronic transport and the resultant thermoelectric (TE) properties is discussed using 2H phase CuAlO2 as a test case. Transparent oxide materials, such as CuAlO2, a p-type transparent conducting oxide (TCO), have recently been studied for high temperature thermoelectric power generators and coolers for waste heat. Given TCO materials with relative ease of fabrication, low cost of materials, and non-toxicity, the ability to tailor them to specific temperature ranges, power needs, and size requirements, through the use of strain opens an interesting avenue. We find that strain can have a significant effect on these properties, in some cases detrimental and in others beneficial, including the potential for n-type power factors larger than the highest p-type case. The physical reasons for this behavior are explained in the terms of the thermoelectric transport distribution and the Landauer distribution of modes.

cond-mat.mtrl-sci↗

arXiv2018

Enhanced Electron Transport in Thin Copper Films via Atomic-Layer Materials Capping

Using first-principles calculations based on density functional theory and non-equilibrium Green's functions, we characterized the effect of surface termination on the electronic transport properties of nanoscale Cu slabs. With ideal, clean (111) surfaces and oxidized ones as baselines we explore the effect of capping the slabs with graphene, hexagonal boron nitrate, molybdenum disulfide and stanene. Surface oxide suppresses balistic conductance by a factor of 10 compared to the ideal surface. Capping the ideal copper surface with graphene slightly increase conductance but MoS$_2$ and stanene have the opposite effect due to stronger interactions at the interface. Interestingly, we find that capping atomistically roughed copper surfaces with graphene or MoS$_2$ decreases the resistance per unit length by 20 and 13%, respectively, due to reduced scattering. The results presented in this work suggest that two-dimensional materials can be used as an ultra-thin liner in metallic interconnect technology without increasing the interconnect line resistivity significantly.

cond-mat.mtrl-sci↗

arXiv2017

Novel doping alternatives for single-layer transition metal dichalcogenides

Successful doping of single-layer transition metal dichalcogenides (TMDs) remains a formidable barrier to their incorporation into a range of technologies. We use density functional theory to study doping of molybdenum and tungsten dichalcogenides with a large fraction of the periodic table. An automated analysis of the energetics, atomic and electronic structure of thousands of calculations results in insightful trends across the periodic table and points out promising dopants to be pursued experimentally. Beyond previously studied cases, our predictions suggest promising substitutional dopants that result in p-type transport and reveal interesting physics behind the substitution of the metal site. Doping with early transition metals (TMs) leads to tensile strain and a significant reduction in the bandgap. The bandgap increases and strain is reduced as the d-states are filled into the mid TMs; these trends reverse are we move into the late TMs. Additionally, the Fermi energy increases monotonously as the d-shell is filled from the early to mid TMs and we observe few to no gap states indicating the possibility of both p- (early TMs) and n- (mid TMs) type doping. Quite surprisingly, the simulations indicate the possibility of interstitial doping of TMDs; the energetics reveal that a significant number of dopants, increasing in number from molybdenum disulfide to diselenide and to ditelluride, favor the interstitial sites over adsorbed ones. Furthermore, calculations of the activation energy associated with capturing the dopants into the interstitial site indicate that the process is kinetically possible. This suggets that interstitial impurities in TMDs are more common than thought to date and we propose a series of potential interstitial dopants for TMDs relevant for application in nanoelectronics based on a detailed analysis of the predicted electronic structures.

cond-mat.mtrl-sci↗