Vahur Zadin

Application of the general thermal field model to simulate the behaviour of nanoscale Cu field emitters

Strong field electron emission from a nanoscale tip can cause a temperature rise at the tip apex due to Joule heating. This becomes particularly important when the current value grows rapidly, as in the pre-breakdown (the electrostatic discharge) condition, which may occur near metal surfaces operating under high electric fields. The high temperatures introduce uncertainties in calculations of the current values when using the Fowler–Nordheim equation, since the thermionic component in such conditions cannot be neglected. In this paper, we analyze the field electron emission currents as the function of the applied electric field, given by both the conventional Fowler–Nordheim field emission and the recently developed generalized thermal field emission formalisms. We also compare the results in two limits: discrete (atomistic simulations) and continuum (finite element calculations). The discrepancies of both implementations and their effect on final results are discussed. In both approaches, the electric field, electron emission currents, and Joule heating processes are simulated concurrently and self-consistently. We show that the conventional Fowler–Nordheim equation results in significant underestimation of electron emission currents. We also show that Fowler–Nordheim plots used to estimate the field enhancement factor may lead to significant overestimation of this parameter especially in the range of relatively low electric fields.

Electrodynamics—molecular dynamics simulations of the stability of Cu nanotips under high electric field

The shape memory effect and pseudoelasticity in Cu nanowires is one possible pair of mechanisms that prevents high aspect ratio nanosized field electron emitters to be stable at room temperature and permits their growth under high electric field. By utilizing hybrid electrodynamics molecular dynamics simulations we show that a global electric field of 1 GV/m or more significantly increases the stability and critical temperature of spontaneous reorientation of nanosized Cu field emitters. We also show that in the studied tips the stabilizing effect of an external applied electric field is an order of magnitude greater than the destabilization caused by the field emission current. We detect the critical temperature of spontaneous reorientation using the tool that spots the changes in crystal structure. The method is compatible with techniques that consider the change in potential energy, has a wider range of applicability and allows pinpointing different stages in the reorientation processes.

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.

Molecular dynamics simulations of near-surface Fe precipitates in Cu under high electric fields

High electric fields in particle accelerators cause vacuum breakdowns in the accelerating structures. The breakdowns are thought to be initiated by the modification of material surface geometry under high electric fields. These modifications in the shape of surface protrusions enhance the electric field locally due to the increased surface curvature. Using molecular dynamics, we simulate the behaviour of Cu containing a near-surface Fe precipitate under a high electric field. We find that the presence of a precipitate under the surface can cause the nucleation of dislocations in the material, leading to the appearance of atomic steps on the surface. Steps resulting from several precipitates in close proximity can also form protruding plateaus. Under very high external fields, in some cases, we observed the formation of voids above or below the precipitate, providing additional dislocation nucleation sites.

Application of multiphysics and multiscale simulations to optimize industrial wood drying kilns

Timber industry and export are an important part of Estonian economy, making affordable industrial scale equipment an important investment for small or starting companies. These companies often develop on-site equipment for wood processing and drying, utilizing pre-existing infrastructure to minimize cost and risk. However, under these conditions custom design of the wood drying kilns is often required.In the present study, a finite element simulation based approach is used to simulate and optimize the industrial wood drying process and the design of the custom-made kilns in a multiscale-multiphysics modeling framework. Air flow is calculated by the Navier-Stokes equations or ?-e turbulence model followed by heat transport in the solid and gas phase and moisture dynamics in wood and air. The dense packing of the processed materials is handled by utilizing a porous media approach and homogenization procedure, leading to effective simulations of the moisture and heat balance.Multiphysics-multiscale simulations are successfully adapted to optimize the industrial design of wood drying kilns. The optimization of the kiln design is achieved by estimating the necessary ventilating power and ensuring homogeneous drying of the processed material.

Thermal runaway of metal nano-tips during intense electron emission

When an electron emitting tip is subjected to very high electric fields, plasma forms even under ultra high vacuum conditions. This phenomenon, known as vacuum arc, causes catastrophic surface modifications and constitutes a major limiting factor not only for modern electron sources, but also for many large-scale applications such as particle accelerators, fusion reactors etc. Although vacuum arcs have been studied thoroughly, the physical mechanisms that lead from intense electron emission to plasma ignition are still unclear. In this article, we give insights to the atomic scale processes taking place in metal nanotips under intense field emission conditions. We use multi-scale atomistic simulations that concurrently include field-induced forces, electron emission with finite-size and space-charge effects, Nottingham and Joule heating. We find that when a sufficiently high electric field is applied to the tip, the emission-generated heat partially melts it and the field-induced force elongates and sharpens it. This initiates a positive feedback thermal runaway process, which eventually causes evaporation of large fractions of the tip. The reported mechanism can explain the origin of neutral atoms necessary to initiate plasma, a missing key process required to explain the ignition of a vacuum arc. Our simulations provide a quantitative description of in the conditions leading to runaway, which shall be valuable for both field emission applications and vacuum arc studies.

Atomistic modeling of metal surfaces under high electric fields: Direct coupling of electric fields to the atomistic simulations

We propose a novel tool to perform electrodynamics-molecular dynamics and electrodynamics-kinetic Monte Carlo simulations. The tool generates finite elements in- and outside the atomistic domain, uses them to solve a system of linear differential equations and offers the interface to output the results into atomistic simulations. The tool shows high tolerance against crystallographic orientation in the material and robustness against dynamic atomistic processes there.

Influence of Breathing on Foucault Cardiogram Origination

A Foucault cardiogram (i.e. magnetic induction cardiogram / eddy currents cardiogram) is determined by a volume integral over the space surrounding the inductor. It depends on the time-variable conductivity distribution in the thorax. Localization of the region-specific contributions into the integral signal can be computed. Thus, the location of the regions where the Foucault cardiogram is originated can be mapped. Using this instrument, the influence of breathing on the origination of the signal has been studied.

Au nanowire junction breakup through surface atom diffusion

Metallic nanowires are known to break into shorter fragments due to the Rayleigh instability mechanism. This process is strongly accelerated at elevated temperatures and can completely hinder the functioning of nanowire-based devices like e.g. transparent conductive and flexible coatings. At the same time, arranged gold nanodots have important applications in electrochemical sensors. In this paper we perform a series of annealing experiments of gold and silver nanowires and nanowire junctions at fixed temperatures 473, 673, 873 and 973 K (200 °C, 400 °C, 600 °C and 700 °C) during a time period of 10 min. We show that nanowires are especially prone to fragmentation around junctions and crossing points even at comparatively low temperatures. The fragmentation process is highly temperature dependent and the junction region breaks up at a lower temperature than a single nanowire. We develop a gold parametrization for kinetic Monte Carlo simulations and demonstrate the surface diffusion origin of the nanowire junction fragmentation. We show that nanowire fragmentation starts at the junctions with high reliability and propose that aligning nanowires in a regular grid could be used as a technique for fabricating arrays of nanodots.

Laser-induced asymmetric faceting and growth of a nano-protrusion on a tungsten tip

Irradiation of a sharp tungsten tip by a femtosecond laser and exposed to a strong DC electric field led to reproducible surface modifications. By a combination of field emission microscopy and scanning electron microscopy, we observed asymmetric surface faceting with sub-ten nanometer high steps. The presence of faceted features mainly on the laser-exposed side implies that the surface modification was driven by a laser-induced transient temperature rise on a scale of a couple of picoseconds in the tungsten tip apex. Moreover, we identified the formation of a nano-tip a few nanometers high located at one of the corners of a faceted plateau. The results of simulations emulating the experimental conditions are consistent with the experimental observations. The presented technique would be a new method to fabricate a nano-tip especially for generating coherent electron pulses. The features may also help to explain the origin of enhanced field emission, which leads to vacuum arcs, in high electric field devi...