The doctoral dissertations of the former Helsinki University of Technology (TKK) and Aalto University Schools of Technology (CHEM, ELEC, ENG, SCI) published in electronic format are available in the electronic publications archive of Aalto University - Aaltodoc.
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Molecular Dynamics Simulations of Strained and Defective Carbon Nanotubes

Maria Sammalkorpi

Dissertation for the degree of Doctor of Science in Technology to be presented with due permission of the Department of Electrical and Communications Engineering for public examination and debate in Auditorium T2 at Helsinki University of Technology (Espoo, Finland) on the 17th of December, 2004, at 12 o'clock noon.

Overview in PDF format (ISBN 951-22-7379-9)   [1170 KB]
Dissertation is also available in print (ISBN 951-22-7378-0)

Abstract

Carbon nanotubes are tubular molecules of pure carbon with typical diameters of 1 nm – 100 nm and lengths from 100 nm up to several cm. The nanotubes have outstanding electronical and mechanical properties which has resulted in remarkable scientific interest and in proporsals of various applications. For example, their ability to be either metals or semiconductors enables the usage of nanotubes as components of electronic devices, while excellent mechanical characteristics motivate the use of nanotubes as reinforcement agents in composite structures and in nanoelectromechanical devices.

This thesis aims to contribute to the understanding of the mechanical properties of carbon nanotubes and it contains two parts. The first part concentrates on initially defect-free but strained nanotubes and on the deformations and defects induced by the strain. The employed methods are empirical and tight binding molecular dynamics simulations. As results the criteria for uniform and discontinuous buckling deformations are reported. In addition, defect formation and strain relaxation are discussed and the stability of various strained and deformed structures is assessed.

The second part of the thesis evaluates defects as a means to improve the bulk mechanical properties of a nanotube sample. Defects, and irradiation as a method of inducing them, are proposed to improve mechanical load transfer between a nanotube and its surroundings. These proposals are verified by analytics and molecular dynamics simulations based on classical empirical potential. The load transfer between nanotubes is found to improve significantly in the presence of defects. This concept is extended to bundles of nanotubes where the improved tube-tube load transfer is predicted to increase shear and stiffen the bundle at moderate irradiation doses. The load transfer has great significance for reinforcement of polymer composites in which the nanotube bundles may act as reinforcement fibers. Furthermore, the mechanical degradation of individual tubes as a result of the defects is also assessed. Point defects have little effect on the axial stiffness of an individual tube but the tensile strength may decrease to a fraction of the strength for a perfect tube. Although individual tubes deteriorate in strength because of the defects, the results indicate that the overall mechanical properties of a nanotube sample can be significantly improved by imperfections in the structure of the tubes.

This thesis consists of an overview and of the following 6 publications:

  1. Huhtala M., Kuronen A. and Kaski K., 2002. Carbon nanotube structures: molecular dynamics simulation at realistic limit. Computer Physics Communications 146, number 1, pages 30-37. © 2002 Elsevier Science. By permission.
  2. Huhtala M., Kuronen A. and Kaski K., 2002. Computational studies of carbon nanotube structures. Computer Physics Communications 147, numbers 1-2, pages 91-96. © 2002 Elsevier Science. By permission.
  3. Huhtala M., Kuronen A. and Kaski K., 2002. Carbon nanotubes under bending strain. In: Bernier P., Ajayan P., Iwasa Y. and Nikolaev P. (editors), Making Functional Materials with Nanotubes. Materials Research Society Symposium Proceedings, volume 706, pages 289-294. © 2002 Materials Research Society. By permission.
  4. Huhtala M., Krasheninnikov A. V., Aittoniemi J., Stuart S. J., Nordlund K. and Kaski K., 2004. Improved mechanical load transfer between shells of multiwalled carbon nanotubes. Physical Review B 70, 045404. © 2004 American Physical Society. By permission.
  5. Sammalkorpi M., Krasheninnikov A. V., Kuronen A., Nordlund K. and Kaski K., Mechanical properties of carbon nanotubes with vacancies and related defects. Physical Review B, accepted for publication. © 2004 American Physical Society. By permission.
  6. Sammalkorpi M., Krasheninnikov A. V., Kuronen A., Nordlund K. and Kaski K., Irradiation-induced stiffening of carbon nanotube bundles. Nuclear Instruments and Methods in Physics Research B, accepted for publication. © 2004 Elsevier Science. By permission.

Keywords: point defects, irradiation, nanocomposites, load transfer, molecular dynamics, nanotube

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© 2004 Helsinki University of Technology

Last update 2011-05-26