Adatom diffusion in high electric fields

Strong electric fields are known to create biased adatom migration on metallic surfaces. We present a Kinetic Monte Carlo model that can simulate adatom migration on a tungsten (W) surface in electric fields. We validate our model by using it to calculate the drift velocity of the adatom at different fields and temperature and comparing the results with experimental data from the literature. We obtain excellent agreement

Modeller för nanostrukturutvecklingen i Fe-legeringar under bestrålning

Neutron irradiation induces structural nano-scale changes in steels that in the long term cause degradation of the mechanical properties of the materials. These processes are important to understand to e.g. ensure the integrity of the steel wall of the reactor pressure vessel during the operational life-time of a nuclear power plant. In this thesis, some of the irradiation defects have been studied by using as a model alloy the iron-carbon (Fe-C) system, as iron and carbon are the basic elements in any steel. The interactions between C and vacancy (V) clusters and between C and self-interstitial atom (SIA) clusters have been studied using Molecular Dynamics simulation techniques. This way C-V clusters, such as C2V and CV2, able to trap large SIA clusters, have been identified and characterized. Using Object Kinetic Monte Carlo (OKMC), an model for the radiation-induced nanostructure evolution in Fe-C has been constructed. The model was validated by reproducing experimental data in terms of V and SIA cluster densities and mean sizes from irradiation experiments at low (340 K) and high operational temperature of light water reactors (560 K), as well as reproducing data from post-irradiation annealing up to 780 K. The new model has allowed a deeper understanding of the effect of carbon on the irradiation defect evolution. It was found that the effect of the immobile C-V complexes can be introduced using generic traps for SIA and vacancy clusters. These generic traps have a binding energy that depends on the size of the trapped cluster, which is supported by previously performed atomistic studies. Different trap regimes need to be used at low and high temperatures to account for the different populations of 1/2 and SIA loops at different temperatures, as observed in previous TEM studies. The traps are found to have an important function as nucleation points that promote the growth of larger clusters. The nanostructure evolution model, which is the main result of this thesis, is fully based on physical considerations and only uses a few parameters for calibration. The model is found to be capable of reproducing the experimental trends both at 340 K, 560 K and for annealing up to 700 K; thereby providing insight into the physical mechanisms of importance to determine the type of nanostructural evolution undergone by Fe alloys during irradiation.Kärnenergi har sedan 1950-talet varit bland de viktigaste sätten att producera elektricitet. År 2010 var 428 kärnkraftverk i bruk i 30 länder och stod för ungefär 16 % av den globala elektricitetsproduktionen. Många av dessa kärnkraftverk byggdes under perioden 1965-1980 och börjar i dag nå slutet av deras planerade livslängd. För att säkerställa att reaktorerna fortsättningsvis är säkra för användning pågår i dag internationell forskning för att förstå degraderingsprocesserna av reaktor-stålen: främst experimentell forskning, men då detta inte alltid är möjligt eller tillräckligt används även datorsimuleringstekniker för att studera materialen från atom- till makro-nivå. Neutronstrålning skapar populationer av nano-defekter i stål, vilka i ett långt perspektiv degraderar materialets mekaniska egenskaper. Det är därför av yttersta vikt att förstå de fysikaliska processerna för hur dessa defektpopulationer och därmed hur materialets nanostruktur utvecklas under långvarig bestrålning. I denna avhandling har använts två datorsimuleringstekniker för att studera dessa strålningsdefekter: Molekyldynamiska simuleringar och Objekt-kinetisk Monte Carlo (OKMC). Med hjälp av OKMC-tekniken har en modell för hur defektpopulationerna - främst vakanskluster och självinterstitiella atomkluster (SIA-kluster) - utvecklas i en järn-kol (Fe-C) legering under bestrålning motsvarande den som stålväggen i ett reaktortryckkärl utsätts för under ca 60 år i ett typiskt kärnkraftverk. Järn och kol är baselementen i alla typer av stål och utgör därför en viktig modell-legering som måste förstås före mera komplexa stålsorter kan studeras. Modellen har påvisat den viktiga roll som små kol-vakans-komplex spelar genom deras förmåga att binda rörliga SIA-kluster, vilket även förstärker SIA-klustrens tillväxt så att stora dislokationsslingor uppstår. Modellen har visat sig framgångsrik i att beskriva nanostrukturutvecklingen i Fe-C under bestrålning vid både 70 °C och 300 °C. Den senare temperaturen är typisk för kommersiella lättvattenreaktorer. Modellen klarar även av att reproducera glödningsexperiment var det redan bestrålade Fe-C-materialet hettas långsamt upp från 60 °C till ca 500 °C. Modellen har ökat förståelsen för de fysikaliska processerna som är viktiga för nanostrukturutvecklingen under bestrålning

Atomistic behavior of metal surfaces under high electric fields

Combining classical electrodynamics and density functional theory (DFT) calculations, we develop a general and rigorous theoretical framework that describes the energetics of metal surfaces under high electric fields. We show that the behavior of a surface atom in the presence of an electric field can be described by the polarization characteristics of the permanent and field-induced charges in its vicinity. We use DFT calculations for the case of a W adatom on a W{110} surface to confirm the predictions of our theory and quantify its system-specific parameters. Our quantitative predictions for the diffusion of W-on-W{110} under field are in good agreement with experimental measurements. This work is a crucial step towards developing atomistic computational models of such systems for long-term simulations.Peer reviewe

Simulation of the nanostructure evolution under irradiation in Fe-C alloys

Peer reviewe

The nanostructure evolution in Fe–C systems under irradiation at 560 K

Peer reviewe

Migration barriers for surface diffusion on a rigid lattice : Challenges and solutions

Abstract Atomistic rigid lattice Kinetic Monte Carlo is an efficient method for simulating nano-objects and surfaces at timescales much longer than those accessible by molecular dynamics. A laborious part of constructing any Kinetic Monte Carlo model is, however, to calculate all migration barriers that are needed to give the probabilities for any atom jump event to occur in the simulations. One of the common methods of barrier calculations is Nudged Elastic Band. The number of barriers needed to fully describe simulated systems is typically between hundreds of thousands and millions. Calculations of such a large number of barriers of various processes is far from trivial. In this paper, we will discuss the challenges arising during barriers calculations on a surface and present a systematic and reliable tethering force approach to construct a rigid lattice barrier parameterization of face-centred and body-centred cubic metal lattices. We have produced several different barrier sets for Cu and for Fe that can be used for KMC simulations of processes on arbitrarily rough surfaces. The sets are published as Data in Brief articles and available for the use.Peer reviewe

Application of artificial neural networks for rigid lattice kinetic Monte Carlo studies of Cu surface diffusion

Kinetic Monte Carlo (KMC) is a powerful method for simulation of diffusion processes in various systems. The accuracy of the method, however, relies on the extent of details used for the parameterization of the model. Migration barriers are often used to describe diffusion on atomic scale, but the full set of these barriers may become easily unmanageable in materials with increased chemical complexity or a large number of defects. This work is a feasibility study for applying a machine learning approach for Cu surface diffusion. We train an artificial neural network on a subset of the large set of 2(26) barriers needed to correctly describe the surface diffusion in Cu. Our KMC simulations using the obtained barrier predictor show sufficient accuracy in modelling processes on the low-index surfaces and display the correct thermodynamical stability of these surfaces.Peer reviewe

Suitability of Paper-Based Substrates for Printed Electronics

Flexible plastic substrates are widely used in printed electronics; however, they cause major climate impacts and pose sustainability challenges. In recent years, paper-based electronics has been studied to increase the recyclability and sustainability of printed electronics. The aim of this paper is to analyze the printability and performance of metal conductor layers on different paper-based substrates using both flexography and screen printing and to compare the achieved performance with that of plastic foils. In addition, the re-pulpability potential of the used paper-based substrates is evaluated. As compared to the common polyethylene terephthalate (PET) substrate, the layer conductivity on paper-based substrates was found to be improved with both the printing methods without having a large influence on the detail rendering. This means that a certain surface roughness and porosity is needed for the improved ink transfer and optimum ink behavior on the surface of the substrate. In the case of uncoated paper-based substrates, the conductivity and print quality decreased by preventing the formation of the proper and intimate ink-substrate contact during the ink transfer. Finally, the re-pulpability trials together with layer quality analysis detected very good, coated substrate candidates for paper-based printed electronics competing with or even outperforming the print quality on the reference PET foil