carb nano.pdf

7/25/2019 carb nano.pdf

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Carbon Nanotubes Mechanical Properties J. Gil Sevillano - TECNUN

Structure and mechanicalproperties of Carbon nanotubes

J. Gil Sevillano

TECNUN2003


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Many people considers C nanotubes

(discovered in 1991) as the fiber and cable

material of the future:

Very high stiffness

Very high strength

High aspect ratio

Low density


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Recent press quotation

Not (only) Science Fiction: An Elevator to Space!

With advances toward ultrastrong fibers, the concept of building an

elevator 60,000 miles high to carry cargo into space is moving from the

realm of science fiction to the fringes of reality (?)


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"Technically it's feasible," said Robert Cassanova, director

of the NASA Institute for Advanced Concepts. "There'snothing wrong with the physics."

The key to the concept's feasibility lies in the material that

will be used to construct the ribbon between the Earth and

outer space:

NANOTUBES

Nanotubes are essentially sheets of graphite -- a lattice of carbon --

seamlessly rolled into long tubes that are mere

nanometers in diameter. These are 100 times as strong as steel,

but much lighter.

NIAC has given more than 500,000 to Seattle-based

HighLift Systems to develop the concept

under the leadershipof the company's chief technology officer, Bradley Edwards.

Not science-fiction at all!


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Carbon nanotubes

Elastic modulus and strength of C multi-walled C

nanotubes (MWCNT) measured by direct tension in a

TEM (Demczyk et al., 2002):

E (TPa) f (GPa) fF (N) Tube diameter (nm)

0.91 ( 0.18) 150 ( 45) 18 12.5

Other experimental values in the literature range from 0.1 to1.8 TPa and 10 to 150 GPa for, respectively the CNT Young

modulus and strength.


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Theoretical estimations for graphene sheets Young modulus and

fracture strength are, respectively, 1.03 TPa and 140 to 177 GPa

(Demczyk et al., 2002). A thickness of 0.34 nm of the sheet has been

assumed for calculating the stress.

First-principles calculations for SWCNT (single-walled C nanotubes,

Zhou et al., 2001) yield E = 0.76 TPa and = 6.25 GPa.

Molecular dynamics have yielded 3.62 TPa and 9.6 GPa for the same

properties (Yao et al., 2001).

More recent results of MD yield 1.24 to 1.35 TPa for the Young

modulus (Jin and Yuan, 2003) for SWCNT or 1.05 TPa (Li and Chou,2003) for MWCNT.


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Bundles of CNT

Oriented CNT bundles can be considered Van der Waals solids,

with very weak transverse properties, highly anisotropic.

Up to now, most realizations of CNT cables yield very

modest results: Vigolo et al. (2000) have measured 9 to

15 GPa for the apparent Young modulus and about 150

MPa for the apparent tensile strength of bundles oforiented 1.4 nm diameter SWCNT (fibres up to 100 m

diameter, 1.3 to 1.5 g/cm3 density).

However, Baughman (2000) has quoteda tensile stregth

of 36 GPa with a 6% elongation for small diameter CNT

bundles.


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What are CNT?

Tubules of closed graphene sheets

[A very good recent review: Ruoff et al., C. R. Physique, in press, 2003]


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3-D surfaces based on graphene sheets

may be imagined and some of them,

CNT among others, are possible


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Graphene is in-plane anisotropic

The orientation on the sheet planeis defined by the chiral vector


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Chiral vector


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SWCNT of different chiral vector


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MWCNT


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Atomic resolution

STM image of

nanotubes

Nanotube

ends


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CTN are fabricated by different

methods


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Direct-current electric arc discharge method

Laser ablation method


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Laser ablation method


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NT towers or carpets grown by cathalisis


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Mechanical testing of SWCNT: direct tensile testing


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Elastic modulus measured by vibration


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TENSILE TESTS

WARNING: stress calculated assuming an

equivalent thickness of 0.34 nm for each layer ofloaded CNT!

SWCNT (Yu et al., 2000a)

E (mean): 1002 GPa

MWCNT (Yu et al., 2000b)

E: 270 to 950 MPa


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An example of calculation of elastic constants of CNT

[with an engineer-mind method]

(Li and Chou, 2003)


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SWCNT as a frame structure

Covalent bonds are

treated as connecting

beam elements between C

atoms, resisting stretching,

bending and torsion.

Bean dimensions and force

constants are fitted to the

molecular force field

constants (linear behaviourassumed)

Analysis of tensile deformation

Id. of shear


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Interlayer forces in MWCNT

Non-directional Van der Waalsforces are simulated with a

Lennard-Jones potential

transmitted by rods connected

by rotatable end joints (onlytensile or compressive forces

are transmited)

(Strongly non-linear response)


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1

2

3

1

2 2


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3


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Another calculation:

2

3

SWCNT (Natsuki et al. 2003)


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As mentioned at the beginning, not only rigidity

but CNT strength is exceptional as well!


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An optimistic view?

The strength depends on orientation


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Mechanical behaviour is a function of

chiral vector of SWCNT

Stone-Wales defect formation leads to

(soft) plastic behaviour or to brittle

fracture depending on chirality

Sometimesthe results

reported are

rather poor


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Other questions to be considered for the

design of and with CNT

E.g.: flattening, buckling

Fl tt i f V d l f


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Flattening from Van der waals forces


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TEM

FEM calculation

Buckling on bending, Pantano et al., 2004


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Torsional buckling


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The importance, due to the high expected strength,

the known high stiffnessin tensile load, and the load

density, of CNT materials means that theirmechanical properties deserve and will surely

receive scrutiny for decades to come

(R. S. Ruoff, D. Quiang and W. K. Liu, 2003)

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