Ändra sökning
Avgränsa sökresultatet
1 - 11 av 11
RefereraExporteraLänk till träfflistan
Permanent länk
Referera
Referensformat
  • apa
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Annat format
Fler format
Språk
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Annat språk
Fler språk
Utmatningsformat
  • html
  • text
  • asciidoc
  • rtf
Träffar per sida
  • 5
  • 10
  • 20
  • 50
  • 100
  • 250
Sortering
  • Standard (Relevans)
  • Författare A-Ö
  • Författare Ö-A
  • Titel A-Ö
  • Titel Ö-A
  • Publikationstyp A-Ö
  • Publikationstyp Ö-A
  • Äldst först
  • Nyast först
  • Skapad (Äldst först)
  • Skapad (Nyast först)
  • Senast uppdaterad (Äldst först)
  • Senast uppdaterad (Nyast först)
  • Disputationsdatum (tidigaste först)
  • Disputationsdatum (senaste först)
  • Standard (Relevans)
  • Författare A-Ö
  • Författare Ö-A
  • Titel A-Ö
  • Titel Ö-A
  • Publikationstyp A-Ö
  • Publikationstyp Ö-A
  • Äldst först
  • Nyast först
  • Skapad (Äldst först)
  • Skapad (Nyast först)
  • Senast uppdaterad (Äldst först)
  • Senast uppdaterad (Nyast först)
  • Disputationsdatum (tidigaste först)
  • Disputationsdatum (senaste först)
Markera
Maxantalet träffar du kan exportera från sökgränssnittet är 250. Vid större uttag använd dig av utsökningar.
  • 1.
    Ghavanini, Farzan
    et al.
    Department of Microtechnology and Nanoscience, Chalmers University of Technology, 41296 Göteborg, Sweden.
    Jackman, Henrik
    Karlstads universitet, Fakulteten för teknik- och naturvetenskap, Avdelningen för fysik och elektroteknik.
    Lundgren, Per
    Department of Microtechnology and Nanoscience, Chalmers University of Technology, 41296 Göteborg, Sweden.
    Svensson, Krister
    Karlstads universitet, Fakulteten för teknik- och naturvetenskap, Avdelningen för fysik och elektroteknik.
    Enoksson, Peter
    Department of Microtechnology and Nanoscience, Chalmers University of Technology, 41296 Göteborg, Sweden.
    Direct measurement of bending stiffness and estimation of Young’s modulus of vertically aligned carbon nanofibers2013Ingår i: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 113, nr 19Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We have measured the bending stiffness of as-grown vertically aligned carbon nanofibers using atomic force microscopy inside a scanning electron microscope. We show that the assumption of a uniform internal structure is inadequate in describing nanofibers mechanical properties and that a dual phase model is needed. We present a model in which different Young’s moduli are assigned to the inner graphitic core and the outer amorphous carbon shell and show that it provides a better fit to the measurements. We obtain values of 11±8 GPa and 63±14 GPa for the Young’s modulus of the inner core and the outer shell, respectively.

  • 2.
    Ghavanini, Farzan
    et al.
    Chalmers University of Technology.
    Jackman, Henrik
    Karlstads universitet, Fakulteten för teknik- och naturvetenskap, Avdelningen för fysik och elektroteknik.
    Svensson, Krister
    Karlstads universitet, Fakulteten för teknik- och naturvetenskap, Avdelningen för fysik och elektroteknik.
    Lundgren, Per
    Chalmers University of Technology.
    Enoksson, Peter
    Chalmers University of Technology.
    Direct Measurement of the Young’s Modulus of Individual Vertically Aligned Carbon Nanofibers (VACNFs)2011Konferensbidrag (Refereegranskat)
    Abstract [en]

    Vertically aligned carbon nanofibers (VACNFs) are synthesized in a plasma-enhanced chemical vapor deposition process (PECVD) in which the position, diameter, length, and alignment of individual nanofibers can be controlled accurately. This has provided an unprecedented opportunity to realize a new bottom-up-engineered material with excellent mechanical and electrical properties which could exploit the third dimension at a reasonable cost. VACNFs have been already employed in a number of applications including electron emitters, gene delivery arrays, and nanoelectromechanical systems. However, no direct measurement of the Young’s modulus of VACNFs has been reported yet. Qi et al. have used nanoindentation method to measure the collective response of a forest of VACNFs with a distribution in length and diameter of the constituent nanofibers. Kaul et al., have reported in situ uniaxial compression tests on individual VACNFs but they have not provided enough information to evaluate the accuracy of their measurements. Indirect estimation of the VACNFs Young’s modulus has also been reported by Eriksson et al. from measurements of the resonance frequency of a nanofiber deposited on top of an excitation electrode. Here, we report on direct measurements of VACNFs Young’s modulus using a piezoresistive atomic force microscope (AFM) cantilever implemented inside a scanning electron microscope (SEM). The VACNFs were grown from Ni catalyst seeds, patterned using electron-beam lithography on top of a stoichiometric TiN underlayer. The VACNFs were grown in a commercially available PECVD chamber (AIXTRON BlackMagic™). The nanofibers were approached from the side and pushed at the tip (resembling a cantilever beam) and force-deflection curves were obtained. By calibrating the AFM sensor the bending stiffness of the nanofiber could be determined. The Young’s modulus was then estimated by taking the nanofibers dimensions into account. The sub-nano Newton force precision provided by the AFM force-sensor together with the fact the individual VACNFs could be observed in the SEM simultaneously during the measurements, has enabled us to measure the nanofibers Young’s modulus with a high precision. Preliminary measurements indicate that VACNFs posses a Young’s modulus between 40 to 100 GPa which is comparable to CVD grown carbon nanotubes of similar diameter

  • 3.
    Jackman, Henrik
    Karlstads universitet, Fakulteten för hälsa, natur- och teknikvetenskap (from 2013), Institutionen för ingenjörsvetenskap och fysik.
    Mechanical behaviour of carbon nanostructures2014Doktorsavhandling, sammanläggning (Övrigt vetenskapligt)
    Abstract [en]

    Abstract

    Carbon nanotubes (CNTs) have extraordinary mechanical and electrical properties. Together with their small dimensions and low density, they are attractive candidates for building blocks in future nanoelectromechanical systems and for many other applications. The extraordinary properties are however only attained by perfectly crystalline CNTs and quickly deteriorate when defects are introduced to the structure. The growth technique affects the crystallinity where in general CNTs grown by arc-discharge are close to perfectly crystalline, while CVD-grown CNTs have large defect densities. Mechanical deformation also affects these properties, even without introducing defects. When CNTs are bent they behave similarly to drinking straws, i.e. they buckle or ripple and their bending stiffness drops abruptly.

    In this thesis, the mechanical behaviour of individual CNTs and vertically aligned carbon nanofibers (VACNFs) has been studied by performing force measurements inside electron microscopes. Cantilevered CNTs, and VACNFs, were bent using a force sensor, yielding force-deflection curves while their structure was imaged simultaneously.

    We have found that CNTs grown by arc-discharge have a high enough crystallinity to possess a Young’s modulus close to the ideal value of 1 TPa. CVD-grown CNTs possess a Young’s modulus that is about one order of magnitude smaller, due to their large defect density. The VACNFs are yet another order of magnitude softer as a result of their cup-stacked internal structure.  We also found that a high defect density will increase the critical strain for the rippling onset and the relative post-rippling stiffness. Multi-walled CNTs with a small inner diameter are less prone to ripple and have a larger relative post-rippling stiffness. Our findings show large variations in the onset of rippling and the bending stiffness before and after rippling. These variations open up possibilities of tailoring the mechanical properties for specific applications.

    Ladda ner fulltext (pdf)
    fulltext
  • 4.
    Jackman, Henrik
    Karlstads universitet, Fakulteten för teknik- och naturvetenskap, Avdelningen för fysik och elektroteknik.
    Mechanical properties of carbon nanotubes and nanofibers2012Licentiatavhandling, sammanläggning (Övrigt vetenskapligt)
    Abstract [en]

    Carbon nanotubes (CNTs) have extraordinary electrical and mechanical properties, and many potential applications have been proposed, ranging from nanoscale devices to reinforcement of macroscopic structures. However, due to their small sizes, characterization of their mechanical properties and deformation behaviours are major challenges. Theoretical modelling of deformation behaviours has shown that multi-walled carbon nanotubes (MWCNTs) can develop ripples in the walls on the contracted side when bent above a critical curvature. The rippling is reversible and accompanied by a reduction in the bending stiffness of the tubes. This behaviour will have implications for future nanoelectromechanical systems (NEMS). Although rippling has been thoroughly modelled there has been a lack of experimental data thus far. In this study, force measurements have been performed on individual MWCNTs and vertically aligned carbon nanofibers (VACNFs). This was accomplished by using a custom-made atomic force microscope (AFM) inside a scanning electron microscope (SEM). The measurements were done by bending free-standing MWCNTs/VACNFs with the AFM sensor in a cantilever-to-cantilever fashion, providing force-displacement curves. From such curves and the MWCNT/VACNF dimensions, measured from SEM-images, the critical strain for the very onset of rippling and the Young’s modulus, E, could be obtained. To enable accurate estimations of the nanotube diameter, we have developed a model of the SEM-image formation, such that intrinsic diameters can be retrieved. We have found an increase in the critical strain for smaller diameter tubes, a behaviour that compares well with previous theoretical modelling. VACNFs behaved very differently, as they did not display any rippling and had low bending stiffnesses due to inter-wall shear. We believe that our findings will have implications for the design of future NEMS devices that employ MWCNTs and VACNFs.

    Ladda ner fulltext (pdf)
    KUS_2012_18
  • 5.
    Jackman, Henrik
    et al.
    Karlstads universitet, Fakulteten för teknik- och naturvetenskap, Avdelningen för fysik och elektroteknik.
    Krakhmalev, Pavel
    Karlstads universitet, Fakulteten för teknik- och naturvetenskap, Avdelningen för maskin- och materialteknik.
    Svensson, Krister
    Karlstads universitet, Fakulteten för teknik- och naturvetenskap, Avdelningen för fysik och elektroteknik.
    Bending modulus of freestanding carbon nanotubes2010Konferensbidrag (Övrigt vetenskapligt)
  • 6.
    Jackman, Henrik
    et al.
    Karlstads universitet, Fakulteten för teknik- och naturvetenskap, Avdelningen för fysik och elektroteknik.
    Krakhmalev, Pavel
    Karlstads universitet, Fakulteten för teknik- och naturvetenskap, Avdelningen för maskin- och materialteknik.
    Svensson, Krister
    Karlstads universitet, Fakulteten för teknik- och naturvetenskap, Avdelningen för fysik och elektroteknik.
    Direct Measurements Of Bending Stiffness And Rippling Phenomena In Free-Standing Carbon Nanotubes2011Konferensbidrag (Refereegranskat)
  • 7.
    Jackman, Henrik
    et al.
    Karlstads universitet, Fakulteten för teknik- och naturvetenskap, Avdelningen för fysik och elektroteknik.
    Krakhmalev, Pavel
    Karlstads universitet, Fakulteten för teknik- och naturvetenskap, Avdelningen för maskin- och materialteknik.
    Svensson, Krister
    Karlstads universitet, Fakulteten för teknik- och naturvetenskap, Avdelningen för fysik och elektroteknik.
    High resolution SEM imaging of carbon nanotubes: deconvolution and retrieval of intrinsic nanotube dimensions2012Konferensbidrag (Refereegranskat)
    Abstract [en]

    Characterizing physical properties of individual nanotubes is crucial for their implementation in nano electromechanical systems (NEMS). This requires measurements on suspended or free-standing structures together with accurate determination of the nanotubes dimensions. In situ methods are often used where physical measurements are performed inside electron microscopes [1-3]. Transmission electron microscopy (TEM) has the advantage of high resolution, providing accurate determination of both dimensions and the internal structure. The space inside a TEM is however rather restricted, leaving limited room for additional probes [4]. Scanning electron microscopy (SEM) on the other hand, has a large specimen chamber which facilitates the addition of probes, but the image resolution is lower, making the evaluation of material properties less accurate or even impossible for very thin nanotubes [1]. One way to solve this is to first measure the physical properties inside an SEM, and then determine the diameter using a TEM afterwards [1]. This approach requires transfer of the nanotube from the SEM to a suitable TEM sample holder, and analysis of the same sample-location in both instruments. It would thereby be advantageous to obtain accurate structural information directly inside the SEM [2]. We have studied the mechanisms involved in SEM image formation of small multiwalled nanotubes, 2-5 nm in diameter. The electron-probe shape in an SEM broadens the sample details, and the image can be seen as a convolution of the secondary electron yield at each sample position and the probe shape. By comparing SEM and TEM images, we found that the probe intensity profile was best described by a linear combination of Gaussian and Lorentzian distributions. Using the obtained probe shape, the SEM images could then be deconvoluted to reveal more details, including the inner diameter in some cases. We also show how the outer diameter can be obtained by differentiating image profiles, a method that does not require any detailed knowledge regarding the probe shape and is reliable down to dimensions comparable to the electron-probe size. This significantly improves the capabilities of in-situ SEM experiments by enabling accurate characterizations of nanofibres inside SEM instruments, without the need for subsequent TEM imaging

  • 8.
    Jackman, Henrik
    et al.
    Karlstads universitet, Fakulteten för teknik- och naturvetenskap, Avdelningen för fysik och elektroteknik.
    Krakhmalev, Pavel
    Karlstads universitet, Fakulteten för teknik- och naturvetenskap, Avdelningen för maskin- och materialteknik.
    Svensson, Krister
    Karlstads universitet, Fakulteten för teknik- och naturvetenskap, Avdelningen för fysik och elektroteknik.
    Image formation mechanisms in scanning electron microscopy of carbon nanotubes,and retrieval of their intrinsic dimensions.2013Ingår i: Ultramicroscopy, ISSN 0304-3991, Vol. 124, s. 35-39Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We present a detailed analysis of the image formation mechanisms that are involved in the imaging of carbon nanotubes with scanning electron microscopy (SEM). We show how SEM images can be modelled by accounting for surface enhancement effects together with the absorption coefficient for secondary electrons, and the electron-probe shape. Images can then be deconvoluted, enabling retrieval of the intrinsic nanotube dimensions. Accurate estimates of their dimensions can thereby be obtained even for structures that are comparable to the electron-probe size (on the order of 2 nm). We also present a simple and robust model for obtaining the outer diameter of nanotubes without any detailed knowledge about the electron-probe shape.

    Ladda ner fulltext (pdf)
    preprint_jackman
  • 9.
    Jackman, Henrik
    et al.
    Karlstads universitet, Fakulteten för teknik- och naturvetenskap, Avdelningen för fysik och elektroteknik.
    Krakhmalev, Pavel
    Karlstads universitet, Fakulteten för teknik- och naturvetenskap, Avdelningen för maskin- och materialteknik.
    Svensson, Krister
    Karlstads universitet, Fakulteten för teknik- och naturvetenskap, Avdelningen för fysik och elektroteknik.
    Large variations in the onset of rippling in concentric nanotubes.2014Ingår i: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 104, artikel-id 021910Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We present a detailed experimental study of the onset of rippling in highly crystalline carbon nanotubes. Modeling has shown that there should be a material constant, called the critical length, describing the dependence of the critical strain on the nanotube outer radius. Surprisingly, we have found very large variations, by a factor of three, in the critical length. We attribute this to a supporting effect from the inner walls in multiwalled concentric nanotubes. We provide an analytical expression for the maximum deflection prior to rippling, which is an important design consideration in nanoelectromechanical systems utilizing nanotubes.

    Ladda ner fulltext (pdf)
    APL 2014
  • 10.
    Jackman, Henrik
    et al.
    Karlstads universitet, Fakulteten för teknik- och naturvetenskap, Avdelningen för fysik och elektroteknik.
    Krakhmalev, Pavel
    Karlstads universitet, Fakulteten för teknik- och naturvetenskap.
    Svensson, Krister
    Karlstads universitet, Fakulteten för teknik- och naturvetenskap, Avdelningen för fysik och elektroteknik.
    Measurements of the critical strain for rippling in carbon nanotubes2011Ingår i: Applied Physics Letters, ISSN 0003-6951, Vol. 98, nr 18, s. 3 pages-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We report measurements of the bending stiffness in free standing carbon nanotubes, using atomic force microscopy inside a scanning electron microscope. Two regimes with different bending stiffness were observed, indicative of a rippling deformation at high curvatures. The observed critical strains for rippling were in the order of a few percent and comparable to previous modeling predictions. We have also found indications that the presence of defects can give a higher critical strain value and a concomitant reduction in Youngs modulus.

    Ladda ner fulltext (pdf)
    fulltext
  • 11.
    Jackman, Henrik
    et al.
    Karlstads universitet, Fakulteten för hälsa, natur- och teknikvetenskap (from 2013), Institutionen för ingenjörsvetenskap och fysik.
    Krakhmalev, Pavel
    Karlstads universitet, Fakulteten för hälsa, natur- och teknikvetenskap (from 2013), Institutionen för ingenjörsvetenskap och fysik.
    Svensson, Krister
    Karlstads universitet, Fakulteten för hälsa, natur- och teknikvetenskap (from 2013), Institutionen för ingenjörsvetenskap och fysik.
    Mechanical behavior of carbon nanotubes in the rippled and buckled phase2015Ingår i: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 117, nr 8, s. 084318-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We have studied the mechanical behavior of multi-walled carbon nanotubes for bending strains beyond the onset for rippling and buckling. We found a characteristic drop in the bending stiffness at the rippling and buckling onset and the relative retained stiffness was dependent on the nanotube dimensions and crystallinity. Thin tubes are more prone to buckle, where some lose all of their bending stiffness, while thicker tubes are more prone to ripple and on average retain about 20\% of their bending stiffness. In defect rich tubes the bending stiffness is very low prior to rippling but these tubes retain up to 70\% of their initial bending stiffness.

    Ladda ner fulltext (pdf)
    fulltext
1 - 11 av 11
RefereraExporteraLänk till träfflistan
Permanent länk
Referera
Referensformat
  • apa
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Annat format
Fler format
Språk
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Annat språk
Fler språk
Utmatningsformat
  • html
  • text
  • asciidoc
  • rtf