1,721,069 research outputs found
Adsorption properties of hydrogen on (10,0) single-walled carbon nanotube through density functional theory
No full textThe density functional theory (DFT) has been used to simultaneously investigate physi-/chemi- sorption properties of hydrogen on the (10,0) single-walled carbon nanotube (SWCNT) walls. Physisorption of H-2 outside the CNT with a vertical orientation to the tube axis above the center of a hexagon surface is the most stable state of physisorption and its binding energy is very weak, -0.792 kcal/mol. In the chemisorp ion of two hydrogen atoms the most stable state is above two adjacent carbon atoms of a hexagon with a C-H bond length of 1.10 Angstrom and one C-H bond energy of -45.761 kcal/mol. Based on these results, we have also investigated the transition state and the reaction pathway from physisorption to chernisorption of hydrogen on the CNT. The energy barrier of the reaction from physisorption to chemisorption is about 78.837 kcal/mol and the reaction is not spontaneous at 0 K. Through the calculations of the Gibbs free energy change from physisorption to chemisorption with temperatures, we learned that it is not easy for the reaction to occur, which is a major obstacle for the practical use of the CNT as a hydrogen storage medium. (C) 2004 Elsevier Ltd. All rights reserved.This research was supported by a grant (code #:
03K1501-02210) from ‘Center for Nanostructured
Materials Technology’ under ‘21st Century Frontier
R&D Programs’ of Ministry of Science and Technology,
Korea
Study on cap closure mechanism of single-walled carbon nanotubes by molecular dynamics
No full textThe closing mechanism of zigzag single-walled carbon nanotubes (SWCNT) was investigated using the molecular dynamics (MD) simulation at the experimental arcdischarge temperature of 3000 K. The (10,0) SWCNT with a diameter of 0.78 nm showed a dome-shape tip which evolved into a saddle-shaped cap that was caused by double heptagon-octagon pairs. In the case of (18,0) SWCNT with a diameter of 1.404 nm, a zipper-like closing mechanism was observed and the flat cap was obtained.This research was performed by the financial support of
‘Center for Nanostructured Materials Technology’ under
‘21st Century Frontier R&D Programs’ of the Ministry of
Science and Technology, Korea
Deposition of fluorinated amorphous carbon thin films with low dielectric constant and thermal stability
No full text
MODIFIED MEMBRANE FINITE-ELEMENT FORMULATION CONSIDERING BENDING EFFECTS IN SHEET-METAL FORMING ANALYSIS
No full textA modified membrane finite element formulation is derived to take the bending effect into account for sheet metal forming analysis. The algorithm developed is applied to a cylindrical and a square cup drawing problem to confirm its validity. The results show that the bending effect is appreciable in a class of deep drawing problems. It is also noted that the present algorithm enhances the convergence of a solution procedure and prevents numerical buckling
Numerical modeling of the multi-stage sheet pair hydroforming process
No full textThe need for reduction of the weight is an important issue in the sheet metal forming industry. The hydroforming process has become an effective manufacturing process, because it can be adapted to the manufacturing of complex structural components into a single body with high structural stiffness. Tube hydroforming has been successfully developed in industry such as in the manufacturing of the components of automotive vehicles. There still, however, remains the constraint of the geometrical shape that can be fabricated by tube hydroforming. In order to improve such a constraint in product shape of the tube hydroforming process and in the formability of conventional sheet metal forming, the hydroforming process of sheet metal has been studied. The sheet pair hydroforming process has been reported to have several important advantages compared with the conventional deep drawing process. Uniform deformation over the whole region is a main advantage in the sheet hydroforming process. However, the deformation of the sheet metal can be concentrated on a small region in the case of rectangular cup with a steep slope in the wall. In this paper, the multi-stage hydroforming process of a sheet pair is proposed in order to increase the formability of a structural parts having a shape like that of an oil pan. For the strict comparison of the proposed process, the blank holding force is controlled by a hydraulic valve to a constant value during deformation. The deformed shape and strain distribution of the manufactured parts with the proposed process are compared with the results of simulation. (C) 2004 Elsevier B.V. All rights reserved
A tension split Hopkinson bar for investigating the dynamic behavior of sheet metals
No full textThe dynamic response of sheet metals at high strain rate is investigated with a tensile split Hopkinson bar test using plate type specimens. The tension split Hopkinson bar inevitably causes some errors in the strain at grips with the plate type specimens, since the grip and specimens disturb the one-dimensional wave propagation in bars. To validate the experiment, the level of error induced from the grips is estimated by comparing the waves acquired from experiments with the Pochhammer-Chree solution. The optimum geometry of the specimen is determined to minimize the loading equilibrium error. High strain rate tensile tests are then performed with auto-body sheet metals in order to construct their appropriate constitutive models for use in crash-worthiness evaluation
Nanomechanical behavior of beta-SiC nanowire in tension: Molecular dynamics simulations
No full textThe molecular dynamics (MD) simulation employing a Tersoff potential was performed to examine the nanomechanical behavior of the beta-SiC nanowire in tension. The elongation was much larger than that of the bulk beta-SiC. We observed non-homogeneous deformation, and the fracture behavior was found to depend on size, orientation and temperature of the specimen. The Young's modulus calculated in this study generally decreased with temperatures and increased with the radius, namely, the diameter of the beta-SiC nanowire as long as the length scale remained the same. The initial orientation was found to have a more serious effect on the Young's modulus than size and temperature. The [1 1 1] Young's modulus is much higher than that of the [001] orientation. The fracture of the beta-SiC nanowire in the [001] orientation showed two different modes, which is brittle at 100 K and ductile at 300 and 500 K. The ductile fracture was accompanied by formation of an atomic chain. In the [1 1 1] orientation, it was always fractured in the ductile mode and thus an atomic chain was formed before rupture.This research was performed with the financial support of the Center for Nanostructured Materials Technology under the 21st Century Frontier R&D Program of Ministry of Science and Technology, Korea
High coverage of hydrogen on a (10,0) single-walled boron nitride nanotube
No full textThe binding energy of hydrogen atoms to a (10,0) single-walled boron nitride nanotube (SWBNNT) is calculated at 25%, 50%, 75%, and 100% coverage using the density functional theory. The average binding energy is highest at 50% coverage when the H atoms are adsorbed on the adjacent B and N atoms along the tube axis and the value is -53.93 kcal/mol, which is similar to half of the H - H binding energy. Also, the band gap (-4.29 eV) of the pristine (10,0) SWBNNT is decreased up to -2.01 eV for the H-adsorbed BNNT with 50% coverage.This research was supported by Grant No. 04K1501-
02210 from the Center for Nanostructured Materials Technology
under the 21st Century Frontier R&D Programs of
the Ministry of Science and Technology, Korea
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