1. Cosires 2004 Helsinki June 28th – July 2nd 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

2. Cosires 2004 Helsinki June 28th – July 2nd 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

3. Cosires 2004 Helsinki June 28th – July 2nd Introduction: carbon nanotubes (CNTs) Tubular carbon molecules –Strong covalent intra-tube bonding Outstanding axial tensile strength & Young’s modulus –Weak van der Waals inter- tube bonding Ultra-low friction surfaces Ideal candidates for reinforcement fibres in composites –Problem: low shear

4. Cosires 2004 Helsinki June 28th – July 2nd Irradiation and nanotubes Improve load transfer? Vacancies –axial weakening –vacancy-vacancy covalent bond formation [1] Interstitials –inside and between tubes –mobile –dimerization [1] R. Telling et al., Nat. Mat. 2, 333 (2003). Simulation: MD with Brenner C-C interaction with long range van der Waals extension by Stuart et al.

5. Cosires 2004 Helsinki June 28th – July 2nd Caused by vacancies Young’s modulus: Y 0 intact tube Young’s modulus n i defect concentration a Yi coefficients Fitted a Yi values Concentration must be high to reduce Y Irradiation induced structural weakening

6. Cosires 2004 Helsinki June 28th – July 2nd Structural weakening: Tensile strength

7. Cosires 2004 Helsinki June 28th – July 2nd 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 equal at L~200nm –defects

8. Cosires 2004 Helsinki June 28th – July 2nd Force: defect-free nanotubes Short ( 36 Å) commensurate / incommensurate tube being pulled out from a longer one ( 62 Å), at T=0 K At room temperature  ~0.1MPa

9. Cosires 2004 Helsinki June 28th – July 2nd 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).

10. Cosires 2004 Helsinki June 28th – July 2nd Two vacancies Interlayer interstitial Interlayer dimer One vacancy (not covalent) Tubes with defects

11. Cosires 2004 Helsinki June 28th – July 2nd Inter-shell bond strength: results Non-covalent bonding strength 0.1-0.4 nN (vacancy) Covalent bonding strength 4-8 nN (2-vacancy) Defects dominate at (D= 6 nm, L= 500 nm CNT) – –10 -6 Å -2 (~ 1/300 nm, ~ 5x10 17 cm -3 ) for covalent binding – –10 -5 Å -2 (~ 1/10 nm, ~ 2x10 19 cm -3 ) for non-covalent binding Defects in nanotubes are realistic means for load transfer

12. Cosires 2004 Helsinki June 28th – July 2nd 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).

13. Cosires 2004 Helsinki June 28th – July 2nd Bending modulus Bundle = cylindrical macroscopic bar –Bending modulus Simulations: Y,G –Y~vacancies –G~covalent inter-tube bonds –Y B

14. Cosires 2004 Helsinki June 28th – July 2nd Young’s and shear modulus: (5,5)-bundle a Y = 1.2Å Y 0 = 370GPa (uniformly loaded bulk bundle) G = a G n bonds a G = 14.5 N/m

15. Cosires 2004 Helsinki June 28th – July 2nd Bending modulus Analytical approximations[1] –Cross section –Number of secondary cascade atoms N vac (dose, E) N bonds (N vac,interaction area) Y,G as a function of dose and E Y B [1] F. Banhart, Rep. Prog. Phys. 62, 1181 (1999).

16. Cosires 2004 Helsinki June 28th – July 2nd Summary Y only moderately sensive (-3% for 1/50Å density (5,5)) Tensile strength may decrease by 50% –reconstruction improves strength Irradiation induced defects pin CNTs effectively – –10 -6 Å -2 (covalent), 10 -5 Å -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 Y B, 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). [1] [2]