186 research outputs found
Large-scale molecular dynamics simulations of cluster impact and erosion processes on a diamond surface
Yasutaka Yamaguchi and JĂŒrgen Gspann. Phys. Rev. B 66, 155408, 2002. Copyright 2002 by the American Physical Society
Supported magnetic nanoclusters: Softlanding of Pd clusters on a MgO surface
Low-energy deposition of neutral Pd_N clusters (N=2-7 and 13) on a MgO(001)
surface F-center (FC) was studied by spin-density-functional molecular dynamics
simulations. The incident clusters are steered by an attractive "funnel"
created by the FC, resulting in adsorption of the cluster, with one of its
atoms bonded atop of the FC. The deposited Pd_2-Pd_6 clusters retain their
gas-phase structures, while for N>6 surface-commensurate isomers are
energetically more favorable. Adsorbed clusters with N > 3 are found to remain
magnetic at the surface.Comment: 5 pages, 2 figs, Phys.Rev.Lett., accepte
Quantifying alignment in carbon nanotube yarns and similar two-dimensional anisotropic systems
Abstract: The uniaxial orientational order in a macromolecular system is usually specified using the Hermans factor which is equivalent to the second moment of the system's orientation distribution function (ODF) expanded in terms of Legendre polynomials. In this work, we show that for aligned materials that are twoâdimensional (2D) or have a measurable 2D intensity distribution, such as carbon nanotube (CNT) textiles, the Hermans factor is not appropriate. The ODF must be expanded in terms of Chebyshev polynomials and therefore, its second moment is a better measure of orientation in 2D. We also demonstrate that both orientation parameters (Hermans in three dimensional (3D) and Chebyshev in 2D) depend not only on the respective fullâwidthâatâhalfâmaximum of the peaks in the ODF but also on the shape of the fitted functions. Most importantly, we demonstrate a method to rapidly estimate the Chebyshev orientation parameter from a sample's 2D Fourier power spectrum, using an analysis program written in Python which is available for open access. As validation examples, we use digital photographs of dry spaghetti as well as scanning electron microscopy images of directâspun carbon nanotube fibers, proving the technique's applicability to a wide variety of fibers and images
Ein neues Konzept fĂŒr die Anwendung von einwandigen Kohlenstoffnanoröhren fĂŒr die pH-Sensorik
Einwandige Kohlenstoffnanoröhren (SWCNTs) gelten aufgrund ihrer hohen LadungstrĂ€germobilitĂ€t, des hohen OberflĂ€che-Volumen-VerhĂ€ltnisses und der Tatsache, dass all ihre Atome mit der Umgebung wechselwirken, als hoch interessant fĂŒr die Sensorik. Hier wird ein neues Konzept fĂŒr die Messung des pH-Wertes, der als wichtigster Parameter der FlĂŒssiganalytik gilt, auf Basis von SWCNTs vorgestellt und anhand der Charakterisierung der hergestellten Sensoren die FunktionsfĂ€higkeit nachgewiesen
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Mechanical properties of carbon nanotube fibres: St Venant's principle at the limit and the role of imperfections
Carbon nanotube (CNT) fibres, especially if perfect in terms of purity and alignment, are of extreme anisotropy. With their high axial strength but ready slippage between the CNTs, there is utmost difficulty in transferring the force applied uniformly. Finite element analysis is used to predict the stress distribution in CNT fibres loaded by grips attached to their surface, along with the resulting tensile stress-strain curves. This study demonstrates that in accordance with St Venantâs principle very considerable length-to-diameter ratios (~ 103) are required before the stress becomes uniform across the fibre, even at low strains. It is proposed that lack of perfect orientation and presence of carbonaceous material between bundles greatly enhances the stress transfer, thus increasing the load it can carry before failing by shear. It is suggested that a very high strength batch of fibres previously observed experimentally had an unusually high concentration of internal particles, meaning that the pressure exerted by the grips would assist stress transfer between the layers. We conclude, that the strength of CNT fibres depends on the specific testing geometries and that imperfections, whether by virtue of less-than-perfect orientation or of embedded impurities, are actually major positive contributors to the observed strength.The authors are grateful to USN ONR GLOBAL for the provision of funding under award number N62909-14-1-N200. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the Office of Naval Research.This is the accepted manuscript. The final version is available at http://www.sciencedirect.com/science/article/pii/S0008622315004728
Quantifying alignment in carbon nanotube yarns and similar twoâdimensional anisotropic systems
The uniaxial orientational order in a macromolecular system is usually specified using the Hermans factor which is equivalent to the second moment of the system\u27s orientation distribution function (ODF) expanded in terms of Legendre polynomials. In this work, we show that for aligned materials that are twoâdimensional (2D) or have a measurable 2D intensity distribution, such as carbon nanotube (CNT) textiles, the Hermans factor is not appropriate. The ODF must be expanded in terms of Chebyshev polynomials and therefore, its second moment is a better measure of orientation in 2D. We also demonstrate that both orientation parameters (Hermans in three dimensional (3D) and Chebyshev in 2D) depend not only on the respective fullâwidthâatâhalfâmaximum of the peaks in the ODF but also on the shape of the fitted functions. Most importantly, we demonstrate a method to rapidly estimate the Chebyshev orientation parameter from a sample\u27s 2D Fourier power spectrum, using an analysis program written in Python which is available for open access. As validation examples, we use digital photographs of dry spaghetti as well as scanning electron microscopy images of directâspun carbon nanotube fibers, proving the technique\u27s applicability to a wide variety of fibers and images
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