Publications
207 results found
Chen AP, Heuer A, Finnis M, et al., 2022, The electronic structure of α-Al2O2 grain boundaries containing reactive element segregants, Physical Review Materials, Vol: 6, Pages: 1-9, ISSN: 2475-9953
It has long been known that the addition of small quantities (“doping”) of so-called reactive elements (RE) such as Y, Zr, and Hf to high-temperature Al2O3 scale-forming alloys improves oxidation resistance. The presence of reactive elements at grain boundaries lowers the growth rate of the α-Al2O3 scales, but the cause of the reduced scale growth kinetics is not fully understood. Explanations based on steric effects and explanations based on reducing the grain boundary electronic conductivity have been proposed. We have used density functional theory to study the structural and electronic properties of two Σ7 bicrystal grain boundaries containing Y, Hf, and Zr substitutional defects on Al sites. The presence of RE substitutional defects plays a minimal direct role in reducing the density of electronic states near the valence-band maximum. However, Hf4+ or Zr4+ substitutions at the grain boundary repel the positively charged oxygen vacancy VO2+. As VO2+ contributes a defect state above the valence-band maximum but below the Fermi energy, this indirectly lowers the density of current carrying holes and thus the electronic conductivity of the grain boundary. Replacing Al3+ ions with Hf4+ or Zr4+ ions also makes the grain boundary positively charged, further reducing the hole density.
Chen AP, Foulkes W, Heuer AH, et al., 2022, Diffusion of oxygen in Mg-doped α-Al2O3: the corundum conundrum explained, Physics Review Materials, Vol: 6, Pages: 1-7, ISSN: 2475-9953
It has been a puzzle for over two decades that the enhancement of oxygen diffusion in α-Al_{2}O_{3} ,with respect to the amount of Mg doping, is several orders of magnitude less than expected. The standard model, which envisages that transport is mediated by oxygen vacancies induced to compensate the charge of Mg 2+ ions substituting Al 3+ ions, has not been able to explain this anomaly. Here, we report a detailed study of populations of point defects and defect clusters in Mg-doped α-Al_{2}O_{3}. By taking into account calculated defect formation energies from the literature, the condition of charge neutrality, and the environmental parameters (chemical potentials) under which the anomalous trend in oxygen diffusivities were previously observed, we are able to arrive at an explanation. A non-linear relationship between Mg concentration in the system and key native point defects, which serve as mediators of self-diffusion in α-Al_{2}O_{3_ , is predicted: the concentrations of such defects increase much more slowly in the supersaturation regime than in the pre-saturation regime, matching the anomalous result previously observed in α-Al_{2}O_{3} . We identify the reason for this as buffering by positively charged Mg interstitials and Mg–oxygen vacancy clusters, which compensate the negative charges of Mg substitutional defects (Mg^{1−}Al ). This study answers part of the long-standing question about self-diffusion in alumina, referred to by Heuer and Lagerlöf in 1999 as the Corundum Conundrum.
Heuer AH, Finnis MW, Foulkes WMC, et al., 2022, Comment on “Self-diffusion in high-purity α-Al2O3: Comparison of Ti-doped, Mg-doped and undoped single crystals”, P. Fielitz, S. Ganschow, K. Klemens, and G. Borchardt, J. Eur. Ceram. Soc., 41, (2021), 663-668.", Journal of the European Ceramic Society, Vol: 42, Pages: 1829-1831, ISSN: 0955-2219
We comment on recent observations of O and Al self-diffusion in single-crystal sapphire with variable doping concentrations of Mg and Ti by Fielitz et al [1]. The paper reports a null effect of aliovalent doping on oxygen diffusivity. We posit that the extensive heat treatment involved in their experimental protocol may have caused dopant evaporation near the surface, and therefore the null result in oxygen diffusivity, whereas an effect on Al diffusivity is still discernible due to a greater diffusion depth of Al. We propose that buffering mechanisms are ultimately responsible for modest increases of self-diffusion with respect to dopant concentrations; in the Mg-doped case, DFT calculations suggest that negatively charged Mg interstitial defects are the principal charge compensating defects for positively charged Mg substitutional ions.
Poulou A, Mellan TA, Finnis MW, 2021, Stability of Zr-Al-C and Ti-Al-C MAX phases: A theoretical study, PHYSICAL REVIEW MATERIALS, Vol: 5, ISSN: 2475-9953
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- Citations: 4
Fernandez AD, Charchar P, Cherstvy AG, et al., 2020, The diffusion of doxorubicin drug molecules in silica nanoslits is non-Gaussian, intermittent and anticorrelated, PHYSICAL CHEMISTRY CHEMICAL PHYSICS, Vol: 22, Pages: 27955-27965, ISSN: 1463-9076
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- Citations: 38
Finnis MW, Csanyi G, Daff T, et al., 2020, A grand canonical approach for modelling hydrogen trapping at vacancies in alpha-Fe, Physical Review Materials, Vol: 4, ISSN: 2475-9953
Vacancies in iron are hydrogen traps, important in the understanding of hydrogen embrittlement of steel. We present a grand canonical approach to computing the trap occupancy as a function of both temperature and hydrogen concentration from practically zero to supersaturation. Our method couples a purpose-made machine-learned H-Fe potential, which enables rapid sampling with near-density-functional-theory accuracy, with a statistical mechanical calculation of the trap occupancy using the technique of nested sampling. In contrast to the conventional assumption (based on Oriani theory) that at industrially relevant hydrogen concentrations and ambient conditions vacancy traps are are fully occupied, we find that vacancy traps are less than fully occupied under these conditions, necessitating a reevaluation of how we think about “mobile hydrogen” in iron and steel.
Korbmacher D, Glensk A, Duff A, et al., 2019, Ab initio based method to study structural phase transitions in dynamically unstable crystals, with new insights on the beta to omega transformation in titanium, Physical Review B, Vol: 100, Pages: 1-11, ISSN: 2469-9950
We present an approach that enables an efficient and accurate study of dynamically unstable crystals over the full temperature range. The approach is based on an interatomic potential fitted to ab initio molecular dynamics energies for both the high- and low-temperature stable phases. We verify by comparison to explicit ab initio simulations that such a bespoke potential, for which we use here the functional form of the embedded atom method, provides accurate transformation temperatures and atomistic features of the transformation. The accuracy of the potential makes it an ideal tool to study the important impact of finite size and finite time effects. We apply our approach to the dynamically unstable β (bcc) titanium phase and study in detail the transformation to the low-temperature stable hexagonal ω phase. We find a large set of previously unreported linear-chain disordered (LCD) structures made up of three types of [111]β linear-chain defects that exhibit randomly disordered arrangements in the (111)β plane.
Mellan TA, Duff A, Grabowski B, et al., 2019, Fast anharmonic free energy method with an application to vacancies in ZrC, Physical Review B: Condensed Matter and Materials Physics, Vol: 100, Pages: 024303-1-024303-11, ISSN: 1098-0121
We propose an approach to calculate the anharmonic part of the volumetric-strain and temperature-dependent free energy of a crystal. The method strikes an effective balance between accuracy and computational efficiency, showing a ×10 speedup on comparable free energy approaches at the level of density functional theory, with average errors less than 1 meV/atom. As a demonstration we make predictions on the thermodynamics of substoichiometric ZrCx, including vacancy concentration and heat capacity.
Kumar A, Barda H, Klinger L, et al., 2018, Anomalous diffusion along metal/ceramic interfaces, Nature Communications, Vol: 9, Pages: 1-8, ISSN: 2041-1723
Interface diffusion along a metal/ceramic interface present in numerous energy and electronic devices can critically affect their performance and stability. Hole formation in a polycrystalline Ni film on an α-Al2O3 substrate coupled with a continuum diffusion analysis demonstrates that Ni diffusion along the Ni/α-Al2O3 interface is surprisingly fast. Ab initio calculations demonstrate that both Ni vacancy formation and migration energies at the coherent Ni/α-Al2O3 interface are much smaller than in bulk Ni, suggesting that the activation energy for diffusion along coherent Ni/α-Al2O3 interfaces is comparable to that along (incoherent/high angle) grain boundaries. Based on these results, we develop a simple model for diffusion along metal/ceramic interfaces, apply it to a wide range of metal/ceramic systems and validate it with several ab initio calculations. These results suggest that fast metal diffusion along metal/ceramic interfaces should be common, but is not universal.
Mellan TA, Duff AI, Finnis MW, 2018, Spontaneous Frenkel pair formation in zirconium carbide, Physical Review B, Vol: 98, ISSN: 2469-9950
With density functional theory we have performed molecular dynamics simulations of ZrC which displayed spontaneous Frenkel pair formation at a temperature of 3200 K, some 500 K below the melting point. To understand this behaviour, rarely seen in equilibrium simulations, we quenched and examined a set of lattices containing a Frenkel pair. Five metastable structures were found, and their formation energies and electronic properties were studied. Their thermal generation was found to be facilitated by a reduction of between 0.7 and 1.5 eV in formation energy due to thermal expansion of the lattice. With input from a quasi-harmonic description of the defect free energy of formation, an ideal solution model was used to estimate lower bounds on their concentration as a function of temperature and stoichiometry. At 3000 K (0.81 of the melting temperature) their concentration was estimated to be 1.2% per mole in a stoichiometric crystal, and 0.3% per mole in a crystal with 10% per mole of constitutional vacancies. Their contribution to heat capacity, thermal expansion and bulk modulus was estimated.
Surendralal S, Todorova M, Finnis MW, et al., 2018, First-principles approach to model electrochemical reactions: understanding the fundamental mechanisms behind Mg corrosion, Physical Review Letters, Vol: 120, ISSN: 0031-9007
Combining concepts of semiconductor physics and corrosion science, we develop a novel approach that allows us to perform ab initio calculations under controlled potentiostat conditions for electrochemical systems. The proposed approach can be straightforwardly applied in standard density functional theory codes. To demonstrate the performance and the opportunities opened by this approach, we study the chemical reactions that take place during initial corrosion at the water-Mg interface under anodic polarization. Based on this insight, we derive an atomistic model that explains the origin of the anodic hydrogen evolution.
Hadian R, Grabowski B, Finnis MW, et al., 2018, Migration mechanisms of a faceted grain boundary, PHYSICAL REVIEW MATERIALS, Vol: 2, ISSN: 2475-9953
We report molecular dynamics simulations and their analysis for a mixed tilt and twist grain boundary vicinal to the Σ7 symmetric tilt boundary of the type {123} in aluminum. When minimized in energy at 0K, a grain boundary of this type exhibits nanofacets that contain kinks. We observe that at higher temperatures of migration simulations, given extended annealing times, it is energetically favorable for these nanofacets to coalesce into a large terrace-facet structure. Therefore, we initiate the simulations from such a structure and study as a function of applied driving force and temperature how the boundary migrates. We find the migration of a faceted boundary can be described in terms of the flow of steps. The migration is dominated at lower driving force by the collective motion of the steps incorporated in the facet, and at higher driving forces by the step detachment from the terrace-facet junction and propagation of steps across the terraces. The velocity of steps on terraces is faster than their velocity when incorporated in the facet, and very much faster than the velocity of the facet profile itself, which is almost stationary. A simple kinetic Monte Carlo model matches the broad kinematic features revealed by the molecular dynamics. Since the mechanisms seem likely to be very general on kinked grain-boundary planes, the step-flow description is a promising approach to more quantitative modeling of general grain boundaries.
Tautschnig MP, Harrison NM, Finnis MW, 2017, A model for time-dependent grain boundary diffusion of ions and electrons through a film or scale, with an application to alumina, Acta Materialia, Vol: 132, Pages: 503-516, ISSN: 1359-6454
A model for ionic and electronic grain boundary transport through thin films, scales or membranes with columnar grain structure is introduced. The grain structure is idealized as a lattice of identical hexagonal cells – a honeycomb pattern. Reactions with the environment constitute the boundary conditions and drive the transport between the surfaces. Time-dependent simulations solving the Poisson equation self-consistently with the Nernst-Planck flux equations for the mobile species are performed. In the resulting Poisson-Nernst-Planck system of equations, the electrostatic potential is obtained from the Poisson equation in its integral form by summation. The model is used to interpret alumina membrane oxygen permeation experiments, in which different oxygen gas pressures are applied at opposite membrane surfaces and the resulting flux of oxygen molecules through the membrane is measured. Simulation results involving four mobile species, charged aluminum and oxygen vacancies, electrons, and holes, provide a complete description of the measurements and insight into the microscopic processes underpinning the oxygen permeation of the membrane. Most notably, the hypothesized transition between p-type and n-type ionic conductivity of the alumina grain boundaries as a function of the applied oxygen gas pressure is observed in the simulations. The range of validity of a simple analytic model for the oxygen permeation rate, similar to the Wagner theory of metal oxidation, is quantified by comparison to the numeric simulations. The three-dimensional model we develop here is readily adaptable to problems such as transport in a solid state electrode, or corrosion scale growth.
Paxton AT, Sutton AP, Finnis MW, 2017, The challenges of hydrogen and metals, Journal: Philosophical Transactions A: Mathematical, Physical and Engineering Sciences, Vol: 375, ISSN: 1471-2962
The Royal Society Scientific Discussion Meeting ‘The challenges of hydrogen and metals’ was held in Carlton House Terrace, London, UK, on 16–18 January 2017. This is the introductory article to the discussion meeting issue which includes contributed papers and seven discussion papers. Here, we introduce the motivation to hold the Meeting and give a brief overview of the contents. We conclude with acknowledgements.This article is part of the themed issue ‘The challenges of hydrogen and metals’.
Faber KT, Asefa T, Backhaus-Ricoult M, et al., 2017, The role of ceramic and glass science research in meeting societal challenges: Report from an NSF-sponsored workshop., Journal of the American Ceramic Society, Vol: 100, Pages: 1777-1803, ISSN: 0002-7820
Under the sponsorship of the U.S. National Science Foundation, a workshop on emerging research opportunities in ceramic and glass science was held in September 2016. Reported here are proceedings of the workshop. The report details eight challenges identified through workshop discussions: Ceramic processing: Programmable design and assembly; The defect genome: Understanding, characterizing, and predicting defects across time and length scales; Functionalizing defects for unprecedented properties; Ceramic flatlands: Defining structure‐property relations in free‐standing, supported, and confined two‐dimensional ceramics; Ceramics in the extreme: Discovery and design strategies; Ceramics in the extreme: Behavior of multimaterial systems; Understanding and exploiting glasses and melts under extreme conditions; and Rational design of functional glasses guided by predictive modeling. It is anticipated that these challenges, once met, will promote basic understanding and ultimately enable advancements within multiple sectors, including energy, environment, manufacturing, security, and health care.
Paxton AT, Sutton AP, Finnis MW, 2017, The challenges of hydrogen and metals, The Challenges of Hydrogen and Metals, Publisher: Royal Society, The, ISSN: 1471-2962
Cheah WL, McComb DW, Finnis MW, 2017, Structure and ionic diffusivity in an yttria-stabilised zirconia/strontium titanate multilayer., Acta Materialia, Vol: 129, Pages: 388-397, ISSN: 1359-6454
Enhanced ionic conductivity observed in a heteroepitaxial multilayer of yttria-stabilised zirconia and (YSZ) and strontium titanate (STO) has variously been attributed to lattice dilation or a disordered oxygen sublattice, leading to high interfacial mobility of anions, as compared to those of the constituent bulk oxides. We seek to understand the mechanism of ionic motion in such heterostructures by first simulating the atomic structure assuming coherent interfaces. After investigating possible low-energy interface structures using a genetic algorithm, we perform molecular dynamics simulations on these structures to examine the anionic diffusivity in the system. We find that the extreme biaxial tensile strain in the YSZ layer, as imposed between layers of STO, induces phases that differ from fluorite. The lowest energy structure is an unknown phase, which we refer to as quasi-cubic and whose cation sublattice resembles an extension of the perovskite; this structure does not lead to enhanced ionic conductivity, in contradiction to some reports in the literature.
Finnis MW, 2017, Structure and ionic diffusivity in an yttria-stabilised zirconia/strontium titanate multilayer, Acta Materialia
Heuer AH, Azar MZ, Guhl H, et al., 2016, The band structure of polycrystalline Al2O3 and its influence on transport phenomena, Journal of the American Ceramic Society, Vol: 99, Pages: 733-747, ISSN: 1551-2916
Guhl H, Lee HS, Tangney P, et al., 2015, Structural and electronic properties of sigma7 grain boundaries in alpha-Al2O3, Acta Materialia, Vol: 99, Pages: 16-28, ISSN: 1359-6454
Applying simulated annealing with a classical potential followed by screening of low-energy structures with density functional theory, we examined the atomic and electronic structures of the View the MathML source and View the MathML source symmetric tilt grain boundaries in α-Al2O3. The lowest energy View the MathML source boundary exhibits a pronounced pattern of alternating columns of exclusively four- or fivefold coordinated Al atoms, with a grain boundary energy of 1.84 Jm−2. For the View the MathML source boundary, numerous structures were found with energy just below 2.11 Jm−2. Furthermore, by analysing the full set of candidate structures generated by simulated annealing for the two grain boundaries, we find that the number of fivefold coordinated Al atoms tends to increase with grain boundary energy, which we can also correlate with the behaviour of the electronic density of states. On the other hand, we find no systematic trend with energy that might be expected for other quantities, notably the excess volume of the interface. We compare simulated high-resolution transmission electron microscope (HRTEM) images of the lowest energy calculated structures with experimental images. The disparate structural and electronic features of these two boundaries suggest reasons for their very different oxygen diffusion coefficients that have been observed experimentally.
Duff AI, Davey T, Korbmacher D, et al., 2015, Improved method of calculating ab initio high-temperature thermodynamic properties with application to ZrC, Physical review B: Condensed matter and materials physics, Vol: 91, ISSN: 1098-0121
Thermodynamic properties of ZrC are calculated up to the melting point (Tmelt≈3700K), using density functional theory (DFT) to obtain the fully anharmonic vibrational contribution, and including electronic excitations. A significant improvement is found in comparison to results calculated within the quasiharmonic approximation. The calculated thermal expansion is in better agreement with experiment and the heat capacity reproduces rather closely a CALPHAD estimate. The calculations are presented as an application of a development of the upsampled thermodynamic integration using Langevin dynamics (UP-TILD) approach. This development, referred to here as two-stage upsampled thermodynamic integration using Langevin dynamics (TU-TILD), is the inclusion of tailored interatomic potentials to characterize an intermediate reference state of anharmonic vibrations on a two-stage path of thermodynamic integration between the original DFT quasiharmonic free energy and the fully anharmonic DFT free energy. This approach greatly accelerates the convergence of the calculation, giving a factor of improvement in efficiency of ∼50 in the present case compared to the original UP-TILD approach, and it can be applied to a wide range of materials.
Duff AI, Finnis MW, Maugis P, et al., 2015, MEAMfit: A reference-free modified embedded atom method (RF-MEAM) energy and force-fitting code, Computer Physics Communications, Vol: 196, Pages: 439-445, ISSN: 0010-4655
Ab initio modeling of materials has become routine in recent years, largely due to the success of density functional theory (DFT). However, for many processes in materials, realism is achieved only when millions of atoms are considered. Currently, such large scale simulations are beyond ab initio capabilities so that one has to resort to effective interatomic potentials that well represent ab initio data on smaller scales. Two of the more widely used types of interatomic potentials are embedded atom method (EAM) and modified embedded atom method (MEAM) potentials. Here we present a code that can use ab initio generated energies and forces to obtain representative EAM and reference-free MEAM type effective interatomic potentials. We illustrate the use of this code with ab initio computed thermal excitations in ZrC.
Korbmacher D, Glensk A, Grabowski B, et al., 2015, Ab initio description of the Ti BCC to ω transition at finite temperatures, Pages: 755-756
Cecchin D, de la Rica R, Bain RES, et al., 2014, Plasmonic ELISA for the detection of gp120 at ultralow concentrations with the naked eye, NANOSCALE, Vol: 6, Pages: 9559-9562, ISSN: 2040-3364
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- Citations: 38
Rogal J, Divinski SV, Finnis MW, et al., 2014, Perspectives on point defect thermodynamics, PHYSICA STATUS SOLIDI B-BASIC SOLID STATE PHYSICS, Vol: 251, Pages: 97-129, ISSN: 0370-1972
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- Citations: 51
Gryaznov D, Finnis MW, Evarestov RA, et al., 2014, Oxygen vacancy formation energies in Sr-doped complex perovskites: ab initio thermodynamic study, Solid State Ionics, Vol: 254, Pages: 11-16, ISSN: 0167-2738
Sarsam J, Finnis MW, Tangney P, 2013, Atomistic force field for alumina fit to density functional theory, JOURNAL OF CHEMICAL PHYSICS, Vol: 139, ISSN: 0021-9606
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- Citations: 10
Horton RM, Haslam AJ, Galindo A, et al., 2013, New methods for calculating the free energy of charged defects in solid electrolytes, JOURNAL OF PHYSICS-CONDENSED MATTER, Vol: 25, ISSN: 0953-8984
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- Citations: 2
Heuer AH, Nakagawa T, Azar MZ, et al., 2013, On the Growth of Al_2 O_3 Scales, Acta Materialia, Vol: 61, Pages: 6670-6683
Mukhopadhyay S, Finnis MW, Harrison NM, 2013, Electronic structures and phonon free energies of LaCoO3 using hybrid-exchange density functional theory, PHYSICAL REVIEW B, Vol: 87, ISSN: 1098-0121
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- Citations: 31
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