1 / 19

Effect of Defects in the Mechanical Properties of Carbon Nanotubes

Effect of Defects in the Mechanical Properties of Carbon Nanotubes. PHY 472 / Lehigh University Instructor: Prof. Slava V. Rotkin By: Paul A. Belony, Jr. Common Defects observed in CNT’s. Main types of defects 3 main groups Point defects Topological defects Hybridization defects

priscilla-mays
Download Presentation

Effect of Defects in the Mechanical Properties of Carbon Nanotubes

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Effect of Defects in the Mechanical Properties of Carbon Nanotubes PHY 472 / Lehigh University Instructor: Prof. Slava V. Rotkin By: Paul A. Belony, Jr.

  2. Common Defects observed in CNT’s • Main types of defects • 3 main groups • Point defects • Topological defects • Hybridization defects • Subgroups • Vacancies (PD) • Metastable atoms • Pentagons (TD) • Heptagons (TD) • Stone–Wales (SW or 5–7–7–5) (TD) • Fictionalization (HD) (sp3 bonds) • Discontinuities of walls • Distortion in the packing configuration of CNT bundles • Etc…

  3. Stone-Wales (5-7-7-5) Defects • Stone-Wales defect is: • Composed of Two Pentagon-Heptagon pairs • Formed by rotating a sp2 bond by 900 • In other words, formed when bond rotation in a graphitic network transforms four hexagons into two pentagons and two heptagons, which is accompanied by elongation of the tube structure along the axis connecting the pentagons, and shrinking along the perpendicular direction

  4. Main Mechanical Properties affected by Defects • Stiffness • Ultimate Strength • Ultimate Strain • Flexibility • Buckling and robustness under high pressure • CNT’s are among the most robust materials • High Elastic Modulus (order of 1 TPa) • High Strength (range up to 100’s of GPa)

  5. Main effects on Stiffness • Changes in stiffness observed. • Stiffness decrease with topological defects and increase with fictionalization • Defect generation and growth observed during plastic deformation and fracture of nanotubes Deepak Srivastava etal. Qiang Lu and Baidurya Battacharya

  6. Force-Displacement curves are used in order to show the response of the SWNTs under loading • Reduced units: • 1 time ru = 0.03526 ps • 1 force ru = 1.602 nN • 1 displacement ru = 1 Ǻ SWNTs with more defects are likely to break at smaller strains and have less strength as well. Force–displacement curves of nanotubes with various average numbers of defects. • the Young modulus can be calculated as the initial slope of the force–displacement curve; • The ultimate strength is calculated at the maximum force point, σU = (Fmax)/A, where F is the maximum axial force, A is the cross section area • the ultimate strain, which corresponds to the ultimate strength, is calculated as εU = (ΔLU)/L, where L is the original tube length.

  7. i, j  indices in Cartesian coordinate system Α, β  atomic indices Ωtot  total volume Lα,β fraction of the length of αβ bond inside the averaging volume Types of stresses • virial stress • atomic (BDT) stress • Lutsko stress • Yip stress

  8. The applied force is computed by summing the interatomic forces for the atoms along the end of the nanotube where the displacement is prescribed. • The stress is calculated from the cross-sectional area S = πdh (h is the chosen interlayer separation of graphite) Belytschko et al. Force-Deflection curve for a model of zigzag NT (Normalized to Stress vs. Strain) Crack formation in a [40, 40] armchair NT with SW defect (evolution from left to right 12.5 – 12.8 ps) Evolution of cracks in the NT Bond-breaking spreads sideways after the initially weakened bond failed The crack grows in the direction of maximum shear

  9. Elastic modulus before defect • Defect free (9,0) nanotube with periodic boundary conditions • Strains applied using conjugate gradients energy minimization • - All stress and strain measures yield a Young’s modulus value of 1.002TPa

  10. 5-7-7-5 Defect on CNTs • The graph shows the graph for Lutsko stress profile for (9,0) zigzag NT with (5-7-7-5) defect • The defected region facilitates Stress amplification • When applied strains increase, this amplification reduces • A different situation is observed for (n,n) armchair NT; there is a decrease in stress at the defect region

  11. N. Chandra, S. Namilae and C. Shet • Contour plots of the longitudinal strain ε33 strain and stress σ33 near the defected region drawn at different applied strain levels. • Strain contours at an applied strain of 1%. b) Stress contours at an applied strain of 1%. • c) Strain contours at an applied strain of 3%. d) Stress contours at an applied strain of 3%. • e) Strain contours at an applied strain of 5%. f) Stress contours at an applied strain of 5%. • g) Strain contours at an applied strain of 8%. h) Stress contours at an applied strain of 8%.

  12. NT possess residual forces at zero strain (even when defect free) At about 1 TPa there’s a reduction of stiffness away from the defect-free straight line

  13. Measuring Mechanical Properties L. Forro etal. • Use of Atomic Force Microscopy (AFM) • For individual CNTs • 3D representation of the adhesion of a SWNT to an alumina ultra-filtration (tube is clamped allowing mechanical testing) • How AFM works in schematic way (a load F is applied to the suspending portion of the NT with length L. So the max deflection d is topologically measured

  14. Fracture process of a (6, 6) SWNT with three SW defects: • crack initiation and propagation (A–I); • (b) corresponding force time history.

  15. Diameter dependence The slope of the graphs seem to be very close to each other for different curvature of the NTs So, stiffness values of various tubes of same SW defect but different diameters do not change significantly Stiffness in the range of 0.61TPa to 0.63TPa for different (n,0) tubes Curvature is not significantly affected by Mechanical properties of SW defect

  16. Charality Dependence There’s a significant change in the measured stiffness when the charality varies

  17. Defects can occur in the form of atomic vacancies. • High levels of such defects can lower the tensile strength by up to 85%. • Due to the almost one-dimensional structure of CNTs, the tensile strength of the tube is dependent on the weakest segment of it in a similar manner to a chain, where a defect in a single link will greatly diminish the strength of the entire chain.

  18. Summary • Mechanical behavior of defects such as 5-7-7-5 defect is examined • A considerable decrease in stiffness at 5-7-7-5 defect location in different nanotubes is observed • Changes in diameter don’t affect the decrease in stiffness significantly • Changes in chirality have different effect on stiffness

  19. 7-5-5-7 STM images and corresponding atomic positions for a C2 dimer absorbed into different nanotubes: (a) and (b) show a (10,10) tube; (c) and (d) a (17,0) tube. All under a 10% strain (tip at 10.5 eV.) (10,10) NT under 10% strain Heterojunction

More Related