Using DL_POLY to understand radiation damage effects and soft matter (glasses, liquids, supercritical fluids) Kostya Trachenko Queen Mary University of London Collaborators: Martin Dove, Volker Heine Vadim Brazhkin William Smith, Ilian Todorov, Michael Seaton Chenxing Yang, Dima Bolmatov, Eva Zarkadoula, Ling Wang, Aaron Diver Support from MCC Consortium and Prof R Catlow Starting from 2017, DL_POLY is taught in Queen Mary in third year. Twice the number of students we expected (40-50). I started 20 years ago, working on glasses with Martin and Volker. DL_POLY 2 (now “Classic”) Two-level systems PRL 1998 Glasses under pressure Explains how glasses relax under pressure ~ln(t) Glasses under pressure We vs two PRLs: a story about 1-order phase transition in SiO2 glass: experiment vs theory PRL 2004 Radiation damage effects Future of nuclear industry is linked to safe encapsulation of nuclear waste 1. Radiation damage in materials to be used to encapsulate highly radioactive nuclear waste: ceramic materials 2. Radiation damage in nuclear fuels in current and future nuclear reactors: metals 3. Radiation damage in future fusion reactors: metals Crystalline ceramics as waste forms . Safe and more durable alternative to glasses: CaTiO3, CaZrTi2O7, BaAl2Ti6O16 and TiO2 - geochemically stable minerals which have immobilised uranium and thorium for billions of years. Can incorporate high-level radioactive waste (Pu, Sr, Ba, Cs) . Pyrochlore (Gd, Sm, Hf-based)-rich titanate phases for Pu encapsulation: high waste load plus criticality control (neutron absorbants) . Other potential “waste forms”: ZrSiO4, ZrO2, Gd2Zr2O7 and so on What are the effects of radiation damage? How do they affect performance of waste forms? Effect of radiation-induced amorphization on diffusion What are long-term (millions of years) effects of irradiation on the performance of waste forms? Case study: zircon ZrSiO4 minerals are ~1 billion years old, completely amorphous yet intact Absorbs large ions like Pu on Zr site Can we learn from Nature? What are the effects of radiation damage? Processes are too fast and collision cascades are too small for experiments -> MD simulations Challenge of large energy and large size: Recoil energies: 100-500 keV Require very large MD boxes Not achievable previously Molecular dynamics simulation of radiation damage: challenge of large energy and large size Details: 1. Empirical potentials; short-range ZBL potential at short <~1 Å distances 2. Perfectly scalable MD code based on domain decomposition strategy (DL_POLY 3 and 4) 3. Parallel computers (Cambridge HPC, HPCx, HECToR, Archer) 4. Adapted MD code to handle out-of-equilibrium conditions (variable time step, boundary scaling) High-energy U recoils in ZrSiO4 50 keV U recoil and their overlap Density variations appeared inside cascades and during their overlap Density variations later confirmed in diffuse X-ray scattering The value of percolation threshold in the continuum percolation is pc=0.3 (approx.) (non-trivial because it is continuous percolation) This gives channels of increased diffusion and explains percolation-type increases of transport (Geisler et al, Trachenko et al, 2003, 2004) Amorphizability A material is amorphizable if it is able to form a covalent network Resistant vs amorphizable materials Some materials are easily amorphized (silicates and titanates) Others are extremely resistant to amorphization (e.g., ZrO2, Gd2Zr2O7) What is the nature of the process of resistance to amorphization by radiation damage? The same question is relevant in other areas: GaN and ZnO are considerably more resistant than GaAs, GaSb, Si etc Understanding resistance to amorphization to radiation damage Resistance is governed by activation barriers for damage recovery and the nature of the bond (but may or may not correlate with ratio of ionic “radii”, topology, coordinations, defect disorder energetics etc) Look at the process in detail Rutile TiO2 Two types of damage relaxation: 1. Elastic reversible relaxation of crystalline lattice around the swollen cascade – large peak in Ndef 2. Irreversible topological damage 1-2 ps 50-100 ps Resistance to amorphization 50 keV U recoil Poor recovery Intermediate recovery Good and perfect recovery SiO2 GeO2 TiO2 MgO Al2O3 t=1 ps t=5, 50 ps MD simulations reproduce experimental behaviour of resistance to amorphization. Damage increases with the stiffness of O-O interaction Trachenko, Todorov, Dove, Artacho and W. Smith, PRB 2006 Why do empirical potentials reproduce activation barriers that govern damage recovery? Resistance to amorphization . Classical MD simulations reproduce experimental behaviour of resistance to amorphization . MD simulations can be used to predict highly resistant materials where resistance to amorphization operates on the time scale of picoseconds. Successfully predicted resistance of series of pyrochlores. Slower recovery processes are possible. Even then, predictions work if the barriers are sufficiently high (cascade size in MD simulations on ps time scale=in billion-year-old samples from NMR experiments) Radiation damage in iron Simulate radiation damage in fusion reactors: ~1 Mev Fe recoil atoms from 14 MeV neutrons Also relevant for nuclear fuels: fission products (many MeVs) These energies were not studied before, yet are important to simulate Require up to 1 billion atoms in MD box DL_POLY development work supported by the EPSRC grant, in collaboration with Ilian Todorov and Michael Seaton Radiation damage-specific developments: - Identification of radiation damage and defects - Electronic energy loss mechanism (friction term and two-temperature models) General developments related to very large system sizes: -Calculation of properties on the fly (one configuration of 250 mln atoms is about 100 Gb, ~10,000-frame history file is ~10-100 Tb in size) - Change the paradigm of how to run&analyze MD results – MD of the future - Relevant for many interested in phenomena operating on microscale: shock, fracture, initiation of micro-cracks, micro-structural changes, interfacial effects, macromolecules, biological systems and so on Current work: finish the development work Calculation on the fly of: - elastic constants - velocity correlation functions (thermal conductivity, density of states etc) - New algorithms of defect identification Write it up and include in the next release. Our new postdoc is happy to lead. 0.2 - 0.5 MeV Fe recoils in iron These energies have not been simulated before • Many-body (embedded atom) potential optimized against several defect energies in alpha-iron • 100-500 million atoms, system size ~100-200 nm • 24,000-65,000 parallel HeCTOR processors • Fearless PhD student 1. What does the collision cascade actually look like? First visualization of high-energy cascades 2. Novel insights into the cascade structure: continuous damage morphology vs sub-cascade branching Iron movies Can defects induced by shock waves be permanent? 500 keV Fe recoils in iron anharmonicity displaced atoms defect atoms Look at defects locally: Novel large (~100 atoms) clusters of interstitials and vacancies. Quantify and discuss their stability. IoP Selected Highlight and News Story in JPCM 2013 Current work: interfaces in Y2O3-ZrO2 0% Y 5% Y 8% Y 10% Y 11% Y 12% Y 15% Y 20% Y Cubic phase Orthorhombic (not monoclinic) phase Liquids gases: solids: fluidity density viscosity duality of liquids bulk modulus … heat capacity No theory Trachenko and Brazhkin, Reports on Progress in Physics 2016 Energy and heat capacity of matter E =3NT/2 Gases, solids gas Esolid=3NT Liquids Eliquid=? A Granato JNCS 2002 Landau school: “liquids have no small parameter” Landau&Lifshitz, Statistical Physics: “The absence of smallness of particle vibrations… Interactions in a liquid are both strong and system-specific Liquid energy can not be calculated in general form 3 N E NT U (r)g(r)dV 2 2 Allen and Tildesley “Computer simulation of liquids” Frenkel reduction τ - time between particle jumps t<τ : solid t>τ : liquid ω>ωF=1/τ : liquid supports solid-like collective modes. Frequency gap. ω<ωF=1/τ : hydrodynamic modes Prediction: liquid supports two transverse modes with ω>ωF and one longitudinal mode (propagating in two different regimes) Experiments: liquid Hg • Van Der Waals, hard-spheres models of liquids etc: ideal-gas cv=3kB/2 • Can liquid cv be understood on the basis of collective modes as in solids? Supercritical fluids Food industry, e.g. Purification and extraction decaffination (non-toxic) of unwanted solvents, CO2 of coffee and tea, CO2 supercritical fluids: green and effective Supercritical H2 in Jupiter, Saturn, exoplanets, brown dwarfs plants to treat toxic and hazardous wastes: “supercritical H2O oxidation” as alternative to Extraction and removing of incineration and landfills actinides in nuclear waste, CO2 New proposal: the Frenkel line in the supercritical region Textbooks: no difference Our proposal: the new Frenkel between a gas and a liquid line separates two distinct states above the critical point. 2012-2013: Physics Today 2012, PRL 2013, Nature Comm. 2013 Recently reviewed in Rep. Prog. Phys. 2016 Thermodynamic crossover at the Frenkel line Shear modes at ω>1/τ Frenkel line corresponds to τ≈τD Important change is the inability to support rigidity (solid-like shear modes) at ANY frequency: rigidity is lost completely the supercritical system crosses over from “rigid” liquid to “non-rigid” gas- like fluid What kind of shear modes can propagate in liquids and supercritical fluids? ds P 1 dP 1 1 d P dt G