What are Nanotubes?

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Carbon Nanotube Formation Detection of Ni atom
and C2 Gary DeBoer LeTourneau
University Longview, TX NASA Johnson Space
Center Thermal Branch Structures and Mechanics
Division Engineering Directorate Summer, 2000
by Laser Induced Fluorescence
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What areCarbon Nanotubes?
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SEM of Nanotube Bundles
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Why should we care?
Strong light-weight materials
Thermal and electrical properties
Gas (hydrogen) storage
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Whats the Problem?
Nanotubes from Tubes_at_Rice Price 1000/gram
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order in 1/4-gram increments only.
Carbon nanotubes, single-walled Sigma-Aldrich
Package Sizes US 100MG
395.90 500MG
1624.00 Product Comments CarboLex SE-
grade, 12-15 angstrom
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Increase Production
modify current methods or design new methods

• Understand the chemical mechanism
• (particularly the role of the catalyst)

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Nanotube Formation Theories

• Atomic scooter
• Metal clusters (nm diameters)
• Melt (mm sized particles or droplets)

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Laser Induced Fluorescence (LIF)
Detector
Optics
Laser
Sample
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Nanotube diagnostics
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Laser Ablation
target
tube
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Plume Emission Spectrum
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Physical Principles for C2 LIF
Upper electronic state
Fluorescence at 513 nm
Long wavelength filter
Detector
Detector
Absorbance at 473 nm
Intermediate state
Lower electronic state
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C2 LIF
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C2 Rotational Spectra
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Rotational Temperature
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ICCD
LeCroy or Digital Scope
Energy meter
Boxcar Averager
Laser 3 Dye Pump 355 nm
DDG
Laser 4 Dye tunable
Laser 2 IR 1064 nm
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C2 Experiment and Synthetic
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C2 Rot Temperature and Intensity
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C2 Rot Temperature and Position
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Summary of C2 LIF results

• Lifetimes of more than 50 ms
• Rotational temperatures 300-700 K
• Rotational temperature is proportional to
  intensity
• Signal can be seen up to 5 mm from the target
  surface
• Signal propagates at 50 m/s

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Physical Principles for Ni LIF
Upper electronic state
non radiative decay
intermediate state
filter

detector
Fluorescence at 301 nm
Absorbance 224-226 nm
Lower electronic state
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Nickel Transitions in LIF
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ICCD
LeCroy or Digital Scope
Energy meter
Laser 1 Gr 532 nm
Boxcar Averager
DDG 2
Laser 3 Dye Pump 355 nm
DDG 1
60 Hz - 10 Hz
Laser 4 Dye tunable
Laser 2 IR 1064 nm
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Nickel LIF Spectra
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Ni Experiment and Synthetic
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Nickel Temperature
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Nickel Propagation
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Summary of Ni LIF Results

• Lifetime of several milliseconds with a hot
  target, 20 microseconds with a room temperature
  target
• Electronic temperatures from 200 - 1500 K
• Electronic temperature is proportional to signal
  intensity
• Signal can be seen up to 3 mm from the target
• Signal propagates at about 10 m/s

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Co resultsLaser Induced Luminescence(LIL)Lifet
imesCo atom millisecondsCarbon
secondsGeohegan et al.Appl. Phys. Letts.,
2000, 76 (3) p 182
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Other Observations

• Hot emission and cooler LIF is not unique.
  Brinkman, Appl. Phys. B, 1996 64 p. 689
  
  Pobst, IEPC, 1995 95 (28) p. 203
  
  Raiche, Appl. Opt. 1993 32 p. 4629
• Ablation small molecules and atoms. Becker,
  Nanostructured Materials, 1998 10 (5) p. 853
  Song, Applied Surface Science, 1998
  127-129 p 111
  Aguilera, Applied Surface Science, 1998 127-129
  p. 309 Dillon, Advances
  in Laser Ablation of Materials (USA), 1998 p.
  403-408

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Summary of Results

• ablation produces small molecules and atoms
  (lifetimes)
• C2 - hot emission 50 ms C2
  - cooler LIF/LIL 100 ms
• Ni and Co LIF/LIL 3 ms
• Cn LIL 3 s
• C2 propagation 50 m/s
• Ni propagation 10 m/s

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Conclusions

• Inconsistent with the melt theory
• Consistent with atomic catalyst theory
• Could be consistent with small metal cluster
  theory
• Need to know when and where nanotubes are formed

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Future Work

• Analysis of three laser ablation experiments
• Analysis of DC arc spectra
• Further parametric studies
• C2 LIF using two ablation lasers
• Computational modeling for
• nanotube formation mechanisms
• nanotube interactions with other materials

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AcknowledgementsSivaram ArepalliWilliam
HolmesPasha NikolaevCarl ScottBrad FilesSFF
NASA-ASEE