Multiwalled Carbon Nanotubes: Synthesis and Applications

1
Multiwalled Carbon Nanotubes Synthesis and
Applications

• Rodney Andrews
• University of Kentucky
• Center for Applied Energy Research
• NSF-MRSEC Advanced Carbon Materials Center

2
Carbon Fiber Diameters
3
Nanotube Applications

• Nano-electronics
• Metallic and semi-conducting varieties
• Molecular circuits
• Electromagnetic interference shielding
• Stealth composites and coatings

4
Nanotube Applications

• Energy Storage
• Hydrogen storage
• Super-capacitors and batteries
• Field emission devices
• Flat panel displays

5
High Strength to Weight Materials
6
Enabling Technology

• Need for viable synthesis technology
• Large scale
• Low cost
• High purity
• Controlled properties
• Lack of entanglement
• Easily separable
• Currently this does not exist

7
Commercial Processes

• SWNT
• MER Corp. 250 g/day (US)
• Carbolex 50 g/day (US)
• Tubes_at_Rice (US)
• Nanoledge (France)
• MWNT
• MER Corp. 2 kg/day (US)
• Hyperion Catalysis 80 kg/day (US)

8
Commercial Processes

• CVD MWNT
• Showa Denko 40,000 kg/yr (Japan)
• Nikkiso (Japan)
• Fullerene International Technologies
• MER-Mitsubishi joint venture
• Based on MERs arc technology

9
CVD Synthesis of MWNT

• You meant to coke the catalyst?

10
MWNT Process Developed at CAER

• Vapor Growth (CVD) Process
• quartz tube furnace
• quartz plate substrates
• Ar/H2 atmosphere
• liquid feed
• xylene as hydrocarbon source
• ferrocene catalyst
• controlled injection rate
• Product
• high purity (gt95 MWNT produced)
• 60-65 carbon conversion to MWNT
• aligned mats normal to growth surface

R. Andrews et al., Chem Phys Lets, 303 (1999)
467-474.
11
SEM of High Purity MWNT Arrays
12
HRTEM of a Single MWNT
13
Reactor System
14
Parameters Studied
15
Temperature of Reaction Zone
16
Carbon Partial Pressure
17
Catalyst Loading

• Production rate increases with FeC ratio
• Diameter distribution widens

18
Distribution of MWNT Diameters
Diameter function of temperature, partial
pressure and time.
19
Outer diameter relates to particle size.
Core diameter is constant (3-10 nm, average of
6nm)
20
Growth Mechanism

• Form of carbon depends on metal particle size
• graphite
• graphite whiskers (VGCF)
• nanofibers
• MWNT
• SWNT
• As metal becomes smaller
• curvature is eventually favored (MWNT)
• ultimately, SWNT is only stable form

21
Lawn Growth Mechanism

• Growth on substrates and walls
• metal particle seeds growth
• extrusion up from surface
• tip growth following a particle

22
Nanotube Composites

• You want how many grams of nanotubes?

23
Difficulties in Use of Nanotubes

• Purity is an issue working in this field
• B. McEnaney, 18 Oct, 2000
• Supply problem
• Nanotube form effects dispersability
• Entanglement (birds nest)
• Physical linkages

24
Literature Review

• Recent search yielded 100 articles on nanotube
  composite materials
• Topics covered include
• Nanotube-polymer composites
• Metal-matrix nanotube composites
• Nanotube-glass composites
• Nanotube-carbon composites

25
MWNT Materials

• CVD synthesis
• xylene / ferrocene
• low temperature, 725 oC
• high purity, gt 95

26
Pyrograf III

• Nanofibers
• Applied Sciences, Inc.
• entangled
• some pyrolytic carbon

• commercially available
• lower cost, 65/lb

27
Solution Processing
28
Dispersion to single MWNT level

• Ultrasonic mixing
• Individual dispersion

• Surfactant assisted

29
Tensile strength and modulus
30
Experimental and Theoretical Moduli
31
Alignment in shear field
32
Melt Processing Shear Mixing
33
Shear Mixing of MWNT into Polymers

• Haake Polylab Shear Mixer
• 50 gram charges
• 0 - 25 wt fiber
• Matrices
• HIPS
• PP
• ABS
• Pitch
• Mixing Energy

34
Dispersion of MWNT in PP

• Determination
• Optical microscopy
• SEM and TEM
• 0 - 10 rating

35
Mixing Energy Increases with Loading
36
Mixing Energy for Dispersion
MWNT in HIPS
37
Melt Processing

• Thin Films

38
Surface resistivity
39
Conductive Plastics

• Current technology
• carbon blacks
• 10-15 loadings
• loss of mechanical properties
• MWNT Composites
• 0.1 - 1 wt loadings
• low percolation threshold
• tunable

40
Melt Processing

• Polymer and Pitch Fibers

41
Fiber Formation
1 mm die, L/D 20
Composite Filament
Rotating wind-up drum
42
Polymer Fibers with Aligned MWNT
43
MWNT/PP Fibers
44
Carbon Fiber with 1wt MWNT
45
Carbon fiber with 2wt MWNT
46
MWNT-Pitch Fibers

• Western KY Coal Extract Pitch
• Strength 373 MPa
• Modulus 28.7 GPa
• WKy 1 wt MWNT
• Strength 506 MPa
• Modulus 34 GPa
• Aromaticity of pitch seems to aid dispersion

47
Failure of NT-Composite Materials
48
NT-Composite Failure(Dickey)
Sword-in-sheath fracture
MWNTs align normal to the crack direction as
they bridge the crack
a. nanotube fracture in crack wake b. debonding
at nanotube-PS interface c. cavities from
nanotube pullout
49
Failure modes of MWNT Composites

• Bridging during crack formation
• Good adhesion
• telescopic failure
• Poor adhesion
• MWNT pull-out

50
Telescopic failure

• sword-in-sheath
• Good MWNT-matrix adhesion
• Outer shells fail at some defect
• Inner shells slide out intact

51
MWNT Reinforcement

• Tensile properties
• Large enhancement in modulus
• Little effect on tensile strength
• Similar to what has been seen in VGCF composites
  (ref. Tibbetts)
• Compressive properties
• Similar to tensile
• Tensile/compressive ? 1 with increasing
  structural perfection (ref. Wagner)

52
Benzyne Functionalization of MWNT(Meier, Andrews)

• Benzyne addition
• on sidewall of MWNT
• Composite polystyrene films
• Improved dispersion
• Improved matrix-nanotube adhesion
• Results
• Good dispersion
• Reduction in film brittleness
• Improved flexibility over blank films and
  unfunctionalized MWNT composites
• Increased flexural strength!!

53
Acknowledgements