Atomistic Study of Surface Effects in Metals
Date
2018
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Abstract
Materjalide kõige väiksemad koostisosad mõjutavad kõige suuremaid esemeid. Euroopa Tuumauuringute keskuses, mis on kuulsaks saanud Higgsi bosoni avastamisega, plaanitakse ehitada uus osakestekiirendi. Praeguse Suure Hadronite Põrguti kõrvale peaks valmima Kompakte Lineaarpõrguti, mille pikkuseks pakutakse välja 50 kilomeetrit. Selle abil saab palju täpsemalt uurida Higgsi bosonit, et paremini aru saada maailma ehitusest.
Uus kiirendi tähendab ka uut tehnoloogiat, nimelt osakeste kiirendamiseks kasutatakse ülikõrget elektrivälja. Eksperimentides on aga selgunud, et kiirendi ise ei pea sellisele elektriväljale vastu – tema vasest korpuses tekkivad elektrilised läbilöögid, teiste sõnadega välk. Selle ennetamiseks on vaja paremini tundma õppida vase käitumist kõrge elektrivälja keskkonnas, ja seda kõige väiksemal, atomistlikul, tasandil. Väikese, ainult mõnest aatomist koosneva kõrgendiku tekkimine materjali pinnale võib võimenduda, kuni tulemuseks on massiline aatomite aurustumine ja elektriline läbilöök, mis seiskab kogu suure kiirendi töö.
Elektriline läbilöök on liiga kiire ja liiga ekstreemne sündmus, et seda saaks jälgida mikroskoobi abil. Seetõttu võetakse appi arvutisimulatsioonid, kus aatomite liikumist saab vaadata kuitahes täpselt. Doktoritöös on simuleeritud, kuidas materjali pind muudab oma kuju kõrge elektrivälja mõjul ja kuidas ta oma kuju pärast välja kadumist taastab.
Selgub, et materjali struktuuri defektid pinna all – dislokatsioonid – aktiveeruvad piisavalt, et liikuda pinnale ja tekitada seal kõrgendikke, mille kohta on teada, et nad soodustavad läbilöögi tekkimist.
Kuju taastumine töötab teistsuguse mehhanismi, pinnaenergia vähendamise teel, mida uuriti nanotraatide näitel. Nanotraadid lagunevad piisavalt pika aja jooksul väikesteks tilkadeks, nagu kraanist voolav veejuga. Tilkadeks lagunemine on väga hästi ennustatav ja saadud tulemusi saab laiendada nii kiirendi läbilöögi probleemile kui ka nanotehnoloogiale tervikuna.
The smallest building blocks of materials affect the behavior of the largest objects. The European Organization for Nuclear Research, famous for its discovery of the Higgs boson, is planning the construction of a new particle accelerator. Next to the current Large Hadron Collider, the Compact Linear Collider should be constructed, with a planned length of 50 kilometers. It will allow a much more precise study of the Higgs boson to better understand the underpinnings of the Universe. The new collider means new technology, namely extremely high electric fields are used to accelerate the particles. Experiments have shown, however, that the accelerator structure itself is unable to bear such a high electric field. Electrical breakdowns, in other words lightning, are observed inside the copper structure. To prevent this, the behavior of copper under high field conditions needs to be studied, specifically at the smallest atomistic scale. A small protrusion consisting of only a few atoms can grow until massive atom evaporation leads to a breakdown and interrupts the work of the whole machine. Electrical breakdown is too fast and too extreme to study in a microscope. Therefore, computer simulations are employed, where the motion of atoms can be observed with high precision. In this doctoral work, the modification of material surface and its subsequent relaxation are simulated. It turns out that the structural defects under the material surface – dislocations – activate sufficiently to move to the surface and create plateaus, which are known to promote breakdowns. Relaxation works with a different mechanism – the reduction of surface energy – which was studied using nanowires. Given sufficient time, nanowires break up into small droplets, like a water stream from a tap. This breakup is very predictable, and the results can be extended both to the accelerator breakdown problem, and nanotechnology in general.
The smallest building blocks of materials affect the behavior of the largest objects. The European Organization for Nuclear Research, famous for its discovery of the Higgs boson, is planning the construction of a new particle accelerator. Next to the current Large Hadron Collider, the Compact Linear Collider should be constructed, with a planned length of 50 kilometers. It will allow a much more precise study of the Higgs boson to better understand the underpinnings of the Universe. The new collider means new technology, namely extremely high electric fields are used to accelerate the particles. Experiments have shown, however, that the accelerator structure itself is unable to bear such a high electric field. Electrical breakdowns, in other words lightning, are observed inside the copper structure. To prevent this, the behavior of copper under high field conditions needs to be studied, specifically at the smallest atomistic scale. A small protrusion consisting of only a few atoms can grow until massive atom evaporation leads to a breakdown and interrupts the work of the whole machine. Electrical breakdown is too fast and too extreme to study in a microscope. Therefore, computer simulations are employed, where the motion of atoms can be observed with high precision. In this doctoral work, the modification of material surface and its subsequent relaxation are simulated. It turns out that the structural defects under the material surface – dislocations – activate sufficiently to move to the surface and create plateaus, which are known to promote breakdowns. Relaxation works with a different mechanism – the reduction of surface energy – which was studied using nanowires. Given sufficient time, nanowires break up into small droplets, like a water stream from a tap. This breakup is very predictable, and the results can be extended both to the accelerator breakdown problem, and nanotechnology in general.