Structural and tribological properties of zero- and one-dimensional nanocrystals
Date
2012-05-28
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Abstract
Doktoritöö käsitleb tänapäeva füüsika ja materjaliteaduse üht aktuaalset uurimisvaldkonda – süsteemi omaduste sõltuvust süsteemi suurusest, mis on eriti oluline nanoskaalas süsteemide juures, kus pindala/ruumala suhe on oluliselt suurem kui makroskoopilistes süsteemides. Väga paljud nanostruktuuride omadused sõltuvad otseselt struktuuri suurusest ja arusaadavalt on triboloogilised omadused (s.t. hõõrdumisega seotud omadused) üheks suurusest sõltuvate omaduste klassiks.
Töö eesmärgiks oli käsitleda teatavate nanoosakeste ja nanotraatide struktuurilisi ja triboloogilisi omadusi, luua nende kirjeldamiseks teoreetilised mudelid ning töötada välja mudelitel baseeruvad eksperimentaalsed meetodid. Eriline tähelepanu oli suunatud pentagonaalsete nanokristallide uurimisele.
Pentagonaalsed nanostruktuurid pakuvad huvi eeskätt seetõttu, et maksomaailmas sellise sümmeetriaga kristalle üldjuhul ei leidu, mistõttu on alust nanokristallide puhul oodata nanoefektide eriti suureulatuslikku esiletõusu. Töö tulemusena töötati välja mitmed mudelid, kuidas antud sümmeetriast tingitud „üleliigne“ energia jaguneb nanokristalliidis ja milliseid erinevaid kujusid võib energia jagunemine põhjustada. Praktilisest sisukohast on eriliselt huvipakkuvad nö kaksikkiht-struktuurid ja teatavad „nõel“struktuurid, kus struktuursete pingete tõttu kasvab osakese pinnale nõelakujuline teravik.
Töö teises osas käsitleti nanoosakeste triboloogiliste omaduste modelleerimist ja mudelite eksperimentaalset kontrolli. Mudelid aitavad ennustada, millistel tingimustel nanoosakesed liikuvate pindade vahel veerevad või libisevad, kui palju erineb nanoosakeste „kleepumine“ pinna külge makroskoopiliste pindade vastavatest näitajatest ning millises ulatuses nanoosakeste mehaanilised omadused erinevad makroskoopilistest materjalidest.
Koostöö käesoleva ja mitme teise projekti raames tehtavate uurimistööde vahel on võimaldanud TÜ Füüsika Instituudis välja töötada unikaalne nanomanipulatsiooni eksperimentaalne kompleks, mille abil on võimalik väga täpselt mõõta nanoosakeste mehaanilisi omadusi ja visualiseerida nende reaktsiooni mitmesugustele mõjutustele.
From the point of view of modern physics, size of the physical system under the investigation is crucially important and in many cases essentially determines the properties of the system. For a nanoscale material, when surface/volume ratio is much higher compared to a bulk material. Structure and related properties are strongly affected by the presence of large surface area. Tribology, i.e. frictional properties have interfacial nature and the tremendously differ at the nanoscale. The goal of this dissertation is consider some of structural and tribological properties of nanoparticles, nanorods and nanowires, propose corresponding theoretical models and demonstrate several experimental methods based upon those. In particular, structural properties of pentagonal nanocrystals concerned with the presence of intrinsic stresses and their stress relaxation are investigated and described in the first part of the thesis. Tribological properties of nanoparticles and nanowires on a flat surface are studied and described in the second part. Pentagonal nanocrystals, such as nanoparticles and nanorods possess internal mechanical stresses that may result in various structural transformations in them. The author had proposed a theoretical model of stress relaxation in pentagonal nanocrystals, such as nanoparticles and nanorods, related to formation of shell layer with crystal lattice mismatch. The shell layer of that kind can diminish intrinsic energy of the nanocrystals, and therefore stabilize it. Furthermore, a theoretical model of growth of metallic whiskers in a material with internal stresses was proposed. Extruding of nanoscale whiskers can be another way of stress relaxation in pentagonal nanocrystals. The author had developed and implemented a theoretical model for numerical simulations of a nanoparticle manipulated by external forces on a flat surface. In one case, nanoparticle is affected by a concentrated force that may original from a sharp tip. In the other case, the nanoparticle is clutched between 2 surfaces, when the top surface is vertically loaded and pulled with a certain velocity. For the both cases rolling and sliding motion of the nanoparticle were distinguished depending on the parameters. An experimental setup for real-time manipulation of nanoscale objects inside a scanning electron microscope had been elaborated in the University of Tartu. The author had developed the software for controlling and measurements in the nanomanipulation setup. Then, experimental methods for measurements of friction of nanoparticles and nanowires were proposed and demonstrated. The author had elaborated theoretical models for interpretation of the experimental data and measurement static and kinetic friction between the nanowire and flat surface.
From the point of view of modern physics, size of the physical system under the investigation is crucially important and in many cases essentially determines the properties of the system. For a nanoscale material, when surface/volume ratio is much higher compared to a bulk material. Structure and related properties are strongly affected by the presence of large surface area. Tribology, i.e. frictional properties have interfacial nature and the tremendously differ at the nanoscale. The goal of this dissertation is consider some of structural and tribological properties of nanoparticles, nanorods and nanowires, propose corresponding theoretical models and demonstrate several experimental methods based upon those. In particular, structural properties of pentagonal nanocrystals concerned with the presence of intrinsic stresses and their stress relaxation are investigated and described in the first part of the thesis. Tribological properties of nanoparticles and nanowires on a flat surface are studied and described in the second part. Pentagonal nanocrystals, such as nanoparticles and nanorods possess internal mechanical stresses that may result in various structural transformations in them. The author had proposed a theoretical model of stress relaxation in pentagonal nanocrystals, such as nanoparticles and nanorods, related to formation of shell layer with crystal lattice mismatch. The shell layer of that kind can diminish intrinsic energy of the nanocrystals, and therefore stabilize it. Furthermore, a theoretical model of growth of metallic whiskers in a material with internal stresses was proposed. Extruding of nanoscale whiskers can be another way of stress relaxation in pentagonal nanocrystals. The author had developed and implemented a theoretical model for numerical simulations of a nanoparticle manipulated by external forces on a flat surface. In one case, nanoparticle is affected by a concentrated force that may original from a sharp tip. In the other case, the nanoparticle is clutched between 2 surfaces, when the top surface is vertically loaded and pulled with a certain velocity. For the both cases rolling and sliding motion of the nanoparticle were distinguished depending on the parameters. An experimental setup for real-time manipulation of nanoscale objects inside a scanning electron microscope had been elaborated in the University of Tartu. The author had developed the software for controlling and measurements in the nanomanipulation setup. Then, experimental methods for measurements of friction of nanoparticles and nanowires were proposed and demonstrated. The author had elaborated theoretical models for interpretation of the experimental data and measurement static and kinetic friction between the nanowire and flat surface.
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Keywords
nanostruktuursed materjalid, mehaanilised omadused, nanokristallid, hõõrdumine, nanostructured materials, mechanical properties, nanocrystals, frictions (physics)