Irradiationinduced stiffening of carbon nanotube bundles Maria Sammalkorpi
Irradiation-induced stiffening of carbon nanotube bundles Maria Sammalkorpi (née Huhtala)1, Arkady Krasheninnikov 2, Antti Kuronen 1, Jussi Aittoniemi 1, Kai Nordlund 2, Steve Stuart 3, Kimmo Kaski 1 1 Laboratory of Computational Engineering, Helsinki University of Technology 2 Accelerator Laboratory, University of Helsinki 3 Department of Chemistry, Clemson University Cosires 2004 Helsinki June 28 th – July 2 nd
Outline Introduction Irradiation as a means of tailoring carbon nanotube (CNT) and nanotube bundle properties – Irradiation effects in nanotubes – Load transfer in multi-walled nanotubes – Nanotube bundle stiffening Summary Cosires 2004 Helsinki June 28 th – July 2 nd
Introduction: carbon nanotubes (CNTs) Tubular carbon molecules – Strong covalent intra-tube bonding Outstanding axial tensile strength & Young’s modulus – Weak van der Waals intertube bonding Ultra-low friction surfaces Ideal candidates for reinforcement fibres in composites – Problem: low shear Cosires 2004 Helsinki June 28 th – July 2 nd
Irradiation and nanotubes Improve load transfer? Vacancies – axial weakening – vacancy-vacancy covalent bond formation [1] Simulation: MD with Brenner C-C interaction with long range van der Waals extension by Stuart et al. Interstitials – inside and between tubes – mobile – dimerization [1] R. Telling et al. , Nat. Mat. 2, 333 (2003). Cosires 2004 Helsinki June 28 th – July 2 nd
Irradiation induced structural weakening Caused by vacancies Young’s modulus: Y 0 intact tube Young’s modulus ni defect concentration a. Yi coefficients Fitted a. Yi values Concentration must be high to reduce Y Cosires 2004 Helsinki June 28 th – July 2 nd
Structural weakening: Tensile strength Cosires 2004 Helsinki June 28 th – July 2 nd
Load transfer Can irradiation defects improve the load transfer? Setup: What is the minimum force required to pull out the inner tube? Contributions – capillary force – tube-tube shear – defects equal at L~200 nm Cosires 2004 Helsinki June 28 th – July 2 nd
Force: defect-free nanotubes Short (36Å) commensurate / incommensurate tube being pulled out from a longer one (62Å), at T=0 K At room temperature t~0. 1 MPa Cosires 2004 Helsinki June 28 th – July 2 nd
Tubes with defects Single vacancies reconstruct to protrude a dangling bond 0. 5 -0. 7Å out of plane [1] Two vacancies in adjanced planes of graphite can form a covalent bond [2] Interstitials are mobile in room temperature – Model: metastable covalent configurations [1] El-Barbary et al. , PRB 68, 144107 (2003). [2] R. Telling et al. , Nature Materials 2, 333 (2003). Cosires 2004 Helsinki June 28 th – July 2 nd
Tubes with defects One vacancy Two (not covalent) vacancies Interlayer interstitial dimer Cosires 2004 Helsinki June 28 th – July 2 nd
Inter-shell bond strength: results Non-covalent bonding strength 0. 1 -0. 4 n. N (vacancy) Covalent bonding strength 4 -8 n. N (2 -vacancy) Defects dominate at (D=6 nm, L=500 nm CNT) – 10 -6Å-2 (~1/300 nm, ~5 x 1017 cm-3) for covalent binding – 10 -5Å-2 (~1/10 nm, ~2 x 1019 cm-3) for non-covalent binding Defects in nanotubes are realistic means for load transfer Cosires 2004 Helsinki June 28 th – July 2 nd
Nanotube bundle stiffening Ref. [1]: electron irradiation of bundles of single-walled carbon nanotubes (SWNTs) result in – (1) dramatic increase of the bundle bending modulus – (2) followed by a decrease at higher doses What is the reason for this nonlinear behavior? [1] Kís et al. , Nature Materials 3, 153 (2004). Cosires 2004 Helsinki June 28 th – July 2 nd
Bending modulus Bundle = cylindrical macroscopic bar – Bending modulus Simulations: Y, G – Y~vacancies – G~covalent inter-tube bonds – YB Cosires 2004 Helsinki June 28 th – July 2 nd
Young’s and shear modulus: (5, 5)-bundle a. Y = 1. 2Å Y 0 = 370 GPa (uniformly loaded bulk bundle) G = a. G nbonds a. G = 14. 5 N/m Cosires 2004 Helsinki June 28 th – July 2 nd
Bending modulus Analytical approximations[1] – Cross section – Number of secondary cascade atoms Nvac(dose, E) Nbonds(Nvac, interaction area) Y, G as a function of dose and E YB [1] F. Banhart, Rep. Prog. Phys. 62, 1181 (1999). Cosires 2004 Helsinki June 28 th – July 2 nd
Summary Y only moderately sensive (-3% for 1/50Å density (5, 5)) Tensile strength may decrease by 50% [1] – reconstruction improves strength Irradiation induced defects pin CNTs effectively – 10 -6Å-2 (covalent), 10 -5Å-2 (non-covalent) Preliminary results for bundle bending modulus – Increase and followed by decrease due to interplay between 1) Increased shear via inter-tube bonds 2) Degrade of strength due to vacancies – Overestimate of max YB, Y 0 too large? Irradiation good tool to improve nanotube mechanical properties for usage as reinforcement agents [1] M. Sammalkorpi et al, submitted. , [2] M. Huhtala et al, PRB 70, ? ? (2004). Cosires 2004 Helsinki June 28 th – July 2 nd [2]
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