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Elastic Properties of Nanowires - an Atomistic Evaluation

Olsson, Pär LU (2011)
Abstract (Swedish)
Popular Abstract in Swedish

Stora framsteg har gjort det möjligt att, med dagens teknik, inte bara visualisera enskilda atomer, utan även utföra olika typer av operationer och manipulationer på atomskalan med stor precision rent experimentellt. Detta har möjliggjort tillverkning av komplexa nanostrukturer genom montering av komponenter av nanostorlek till större sammansatta system, vilket i sin tur har gett upphov till en rad nya banbrytande praktiska tillämpningar inom vitt skilda områden och discipliner. Nanotråden är en av de viktigaste byggstenarna i nanosystem, och nanotrådars egenskaper skiljer sig i många avseenden från motsvarande egenskaper hos makroskopiska balkar. Detta beror på att en stor andel av de atomer som... (More)
Popular Abstract in Swedish

Stora framsteg har gjort det möjligt att, med dagens teknik, inte bara visualisera enskilda atomer, utan även utföra olika typer av operationer och manipulationer på atomskalan med stor precision rent experimentellt. Detta har möjliggjort tillverkning av komplexa nanostrukturer genom montering av komponenter av nanostorlek till större sammansatta system, vilket i sin tur har gett upphov till en rad nya banbrytande praktiska tillämpningar inom vitt skilda områden och discipliner. Nanotråden är en av de viktigaste byggstenarna i nanosystem, och nanotrådars egenskaper skiljer sig i många avseenden från motsvarande egenskaper hos makroskopiska balkar. Detta beror på att en stor andel av de atomer som utgör nanotråden är ytatomer, och eftersom ytatomer har betydligt färre bindningar till andra atomer än vad bulkatomer har, resulterar det i att de har annan påverkan på strukturens egenskaper jämfört med bulkatomer. Detta medför i slutändan att nanotrådar får alltmer avvikande egenskaper allteftersom dimensionerna minskar. Trots att flera av dessa avvikande egenskaper i många fall är önskade, blir det i vissa fall svårare att förutsäga hur nanotrådarna kommer att bete sig i olika tillämpningar, då beteendet kan avvika markant från hur en makroskopisk struktur förväntas uppföra sig.





Syftet med denna avhandling är att, med hjälp av atomsimuleringar, studera hur de elastiska egenskaperna hos metalliska nanotrådar varierar med storlek och hur väl befintliga makroskopiska kontinuummodeller beskriver de mekaniska egenskaperna. Detta innefattar att undersöka vid vilken storlek befintliga modeller upphör att gälla, samt på vilket sätt och varför resultaten inte stämmer överens med befintliga makro-skopiska kontinuummodeller. Arbetet är av teoretisk karaktär där nanotrådarnas elastiska egenskaper modelleras med hjälp av dynamiska och statiska atomsimuleringar. Simuleringarna är klassiska atomsimuleringar, där växelverkan mellan de olika atomerna sker genom empiriska modellpotentialer. Avhandlingen innehåller fyra artiklar, A-D, som behandlar de elastiska egenskaperna hos metalliska nanotrådar. Olika varianter av belastningsfall simuleras: axiell dragning (artiklar A och B), statisk balkböjning (artikel A), transversella vibrationer (artiklar B och C) samt knäckning (artikel D). I förekommande fall jämförs resultaten med experimentella resultat rapporterade i publicerade vetenskapliga artiklar.



Simuleringarna visar att elasticitetsmodulen hos nanotrådarna varierar med tvär-snittsarean, samt att elasticitetsmodulen både kan öka eller minska när storleken på tvärsnittet minskar. Detta beror på nanotrådens kristallografiska orientering, då olika ytor påverkar de elastiska egenskaperna olika mycket, samt på den olinjära påverkan i olika kristallografiska riktningar. Följden av detta är att böjstyvheten både kan bli högre eller lägre än vad som fås när materialdata för makroskopiska strukturer används. Dessutom visar det sig att, allteftersom nanotrådarna blir mindre och mindre, påverkas böjstyvheten i större utsträckning av det faktum att ytornas elastiska egenskaper skiljer sig från kärnans egenskaper. Detta leder till att tvärsnittet får en heterogen eller kompositliknande karaktär vilket försvårar bestämmandet av böjstyvheten.

The major purpose of this thesis is to study how well the behavior of metallic nanowires can be predicted by elementary macroscopic continuum mechanics. This includes to investigate at what sizes of the cross sectional dimensions and in what way macroscopic continuum mechanical models fail to describe the mechanical response accurately. The presented work is of a theoretical nature, related to experiments reported in the literature, and the mechanical properties and responses are modeled through classical molecular dynamics and molecular statics simulations with empirical potentials to describe the interatomic interactions. (Less)
Abstract
Nanowires constitute one of the fundamental building blocks in assembled nanodevices. Due to the high surface to volume ratio, the fraction of surface atoms is not negligible for nanosized elements. Because of the reduced coordination of surface atoms, the physical properties deviate from those of the bulk, which influences the overall physical properties of nanostructures. Consequently, this provides the structure with physical properties that may deviate significantly from the bulk and it may therefore be inappropriate to employ macroscopic continuum mechanical concepts at this scale, as it could lead to inaccurate predictions of the response.



The major purpose of this thesis is to study how well the behavior of... (More)
Nanowires constitute one of the fundamental building blocks in assembled nanodevices. Due to the high surface to volume ratio, the fraction of surface atoms is not negligible for nanosized elements. Because of the reduced coordination of surface atoms, the physical properties deviate from those of the bulk, which influences the overall physical properties of nanostructures. Consequently, this provides the structure with physical properties that may deviate significantly from the bulk and it may therefore be inappropriate to employ macroscopic continuum mechanical concepts at this scale, as it could lead to inaccurate predictions of the response.



The major purpose of this thesis is to study how well the behavior of metallic nanowires can be predicted by elementary macroscopic continuum mechanics. This includes to investigate at what sizes of the cross sectional dimensions and in what way macroscopic continuum mechanical models fail to describe the mechanical response accurately. The presented work is of a theoretical nature, related to experiments reported in the literature, and the mechanical properties and responses are modeled through classical molecular dynamics and molecular statics simulations with empirical potentials to describe the interatomic interactions.



The thesis begins with a short introduction and discussion of the main topics and numerical methods addressed and used in the appended papers, followed by a brief summary and discussion of the obtained results. The bulk of the thesis consists of four appended papers, A-D. In these, different atomistic simulations are employed to study the elastic properties of single crystal metallic nanowires subjected to different types of loading and dynamic excitation: tensile loading (Papers A and B), transverse loading (Paper A), transverse free vibrations (Papers B and C), and compressive loading leading to buckling (Paper D).



The simulations show that the stiffness varies with size and that Young's modulus may increase or decrease with decreasing size, depending on how the crystal is oriented. The behavior is a consequence of what kind of crystallographic surfaces that are present at the bounding surfaces and the non-linear character of the bulk. Consequently, the flexural rigidity can be either higher or lower than what is estimated using macroscopic material properties, depending on the crystallographic orientation and the surface elastic properties. Moreover, it is shown that the surface influence on the flexural rigidity increases as the cross sectional dimensions decrease. This implies that the inhomogeneous elastic character of the cross section may have to be explicitly taken into account when predicting the flexural rigidity accurately for nanowires of small cross sections. This influence has been found to be particularly important when considering static bending and buckling, and it manifests itself in a lowering of the flexural rigidity, leading to a lowering of critical strain and an increase in deflection. Aside from these features, it is shown that the nanowires deform quite similar to what is expected from elementary beam theory. Moreover, it is found that the elastic behavior converges towards that expected from a bulk structure as the cross sectional dimensions increase. (Less)
Please use this url to cite or link to this publication:
author
supervisor
opponent
  • Professor Pyrz, Ryszard, Aalborg University, Denmark
organization
publishing date
type
Thesis
publication status
published
subject
pages
88 pages
defense location
Lecture hall M:E, M-building, Ole Römers väg 1, Lund University Faculty of Engineering
defense date
2011-06-03 10:15
ISBN
978-91-7473-105-7
language
English
LU publication?
yes
id
2d59837a-fedb-48a7-8057-4788084b680f (old id 1938844)
date added to LUP
2011-05-09 10:16:11
date last changed
2016-09-19 08:45:16
@phdthesis{2d59837a-fedb-48a7-8057-4788084b680f,
  abstract     = {Nanowires constitute one of the fundamental building blocks in assembled nanodevices. Due to the high surface to volume ratio, the fraction of surface atoms is not negligible for nanosized elements. Because of the reduced coordination of surface atoms, the physical properties deviate from those of the bulk, which influences the overall physical properties of nanostructures. Consequently, this provides the structure with physical properties that may deviate significantly from the bulk and it may therefore be inappropriate to employ macroscopic continuum mechanical concepts at this scale, as it could lead to inaccurate predictions of the response.<br/><br>
<br/><br>
The major purpose of this thesis is to study how well the behavior of metallic nanowires can be predicted by elementary macroscopic continuum mechanics. This includes to investigate at what sizes of the cross sectional dimensions and in what way macroscopic continuum mechanical models fail to describe the mechanical response accurately. The presented work is of a theoretical nature, related to experiments reported in the literature, and the mechanical properties and responses are modeled through classical molecular dynamics and molecular statics simulations with empirical potentials to describe the interatomic interactions. <br/><br>
<br/><br>
The thesis begins with a short introduction and discussion of the main topics and numerical methods addressed and used in the appended papers, followed by a brief summary and discussion of the obtained results. The bulk of the thesis consists of four appended papers, A-D. In these, different atomistic simulations are employed to study the elastic properties of single crystal metallic nanowires subjected to different types of loading and dynamic excitation: tensile loading (Papers A and B), transverse loading (Paper A), transverse free vibrations (Papers B and C), and compressive loading leading to buckling (Paper D).<br/><br>
<br/><br>
The simulations show that the stiffness varies with size and that Young's modulus may increase or decrease with decreasing size, depending on how the crystal is oriented. The behavior is a consequence of what kind of crystallographic surfaces that are present at the bounding surfaces and the non-linear character of the bulk. Consequently, the flexural rigidity can be either higher or lower than what is estimated using macroscopic material properties, depending on the crystallographic orientation and the surface elastic properties. Moreover, it is shown that the surface influence on the flexural rigidity increases as the cross sectional dimensions decrease. This implies that the inhomogeneous elastic character of the cross section may have to be explicitly taken into account when predicting the flexural rigidity accurately for nanowires of small cross sections. This influence has been found to be particularly important when considering static bending and buckling, and it manifests itself in a lowering of the flexural rigidity, leading to a lowering of critical strain and an increase in deflection. Aside from these features, it is shown that the nanowires deform quite similar to what is expected from elementary beam theory. Moreover, it is found that the elastic behavior converges towards that expected from a bulk structure as the cross sectional dimensions increase.},
  author       = {Olsson, Pär},
  isbn         = {978-91-7473-105-7},
  language     = {eng},
  pages        = {88},
  school       = {Lund University},
  title        = {Elastic Properties of Nanowires - an Atomistic Evaluation},
  year         = {2011},
}