Structure and dispersion of carbon nanotubes Janis M. Brown Air Force Research Laboratory, MLBCO,...
-
Upload
meagan-walton -
Category
Documents
-
view
215 -
download
3
Transcript of Structure and dispersion of carbon nanotubes Janis M. Brown Air Force Research Laboratory, MLBCO,...
Structure and dispersion of carbon nanotubesJanis M. Brown
Air Force Research Laboratory, MLBCO, WPAFB, OH 45433-7750
David P. AndersonUniversity of Dayton Research Institute,
300 College Park, Dayton, OH 45469-0168
Jian Zhao, Kumar Chokalingham, Max Belfor and Dale W. Schaefer,Department of Chemical and Materials Engineering, University of Cincinnati, Cincinnati, OH 45221-0012
Jan IlavskyAdvanced Photon Source,
Argonne National Laboratory, Argonne, IL 60439
AbstractSmall-angle light scattering and ultra small-angle X-ray scattering are used to assess the morphology of single-walled (SWNTs) and carbon nanofibers (CNFs). For CNFs, a power-law scattered-intensity profile with a slope of –1.08 is consistent with the rod-like morphology. For SWNTs, however, scattering profiles characteristic of rod-like morphology are not observed on any length-scale from 1 nm to 50 m. Rather, disordered objects are found that we identify as a network of carbon “ropes” enmeshed with polyelectrolyte dispersants. The effectiveness of polyelectrolyte dispersants is assessed using small-angle light scattering in conjunction with exposure to ultrasound. In the presence of an anionic polyelectrolyte, sonication can assist dispersion of both SWNTs and CNFs. In the presence of a cationic agent, however, sonication can induce aggregation. SWNTs respond differently to ultrasound depending on whether residual synthesis catalyst is present. Four dispersants are studied, of which sodium polystyrene sulfonate is the most effective and polyallylamine hydrochloride is the least effective.
d
I
q = 2/d = (4/) sin(/2) = 1/length
d = /2 sin (/2)
PDLIGHT
Q(Å-1).00001 .001 10
SAXS
.1
Real Space
Reciprocal Space
Length
Stiffness
Diameter
Surfaceroughness
BondLength
-1
-4
USAXS
I M
R q-1
Bragg Scattering and Length Scale
M ~V ~R3
M ~V ~R2
M ~V ~R1
M ~V ~R1.66 2.5
Mass Fractal Dimension = DSurface Fractal Dimension = Ds
I q 2
I q 2
I q 1
I q 4
I ~ M ~ RD ~ q D
Real Space Reciprocal Space
I qDs 2D
I ~ M R ~ Q-1
M~RD
The Concept of “Dimension”
GA = E (a/R)3+C
Aggregate Size
Grubber
Sti
ffn
ess
G = a (E/Grubber)1/3+C
= a (10000)1/4
= 10 a 1500 Å
Silica
SWNTS must be dispersed to impart superior performance.
Identify the backbone
Find the “chemical length,” L
Model as a rod with same chemical lengthAnd thickness = primary particle diameter
GA = E(a/R)3+C ~ R-4L ~ Rc
Bulk Modulus of Silica/Carbon
Stiffness of Aggregate
a
L
Witten, Rubenstein, Colby Model
Reinforcement by Disordered Fillers Show a Limiting Length Scale
SWNT 1-3 nm
Conventional Intermediate Modulus Fiber6000 nm (6 microns)
ASI CNFs100-200 nm
MWNT20-30 nm
• Differences in mechanical properties and processing are expected as one scales down several orders of magnitude
• CNFs are a good intermediate size fiber to address scaling issues
Carbon “Fibers” - Scaling Down Several Orders of Magnitude
1.2 µ
ASI Carbon Nanofibers (CNF)
0.5 µ
Ropes of single walled tubes (SWNTs)
10-3
10-2
10-1
100
101
102
103
104
105
106
107
108
x-ra
y C
ross
Sec
tio
n (
cm-1
)
10-5 10-4 10-3 10-2 10-1
q(Å-1)
-1.06
-2.80
-3.27
210 Å
4570 Å
Jian PULS Soln USAX Soln
Light +USAXS+SAXS
Hollow Tube
0.5 µm
210 Å Wall
ASI Carbon Nanofibers
1000 Å
3000 Å
Reinforcing element is a “polymer,” not a rod.
SWNTNetwork
RopeNetwork
10-2
10-1
100
101
102
103
104
105
106
107
108
109
X-R
ay C
ross S
ecti
on
(cm
-1)
10-5 10-4 10-3 10-2 10-1
q (Å)-1
28µ
0.22µ
722 Å
-4
-2.1
-2.2
Rope Diameter
RopeMesh Size
SWNT Gel Mesh Size
Dispersed SWNTs are Not Rod-like at Any Length Scale
NH2 NH2
NH3
NH3
NH3
+
NH3
NH3 NH3
+ +
+
+
H20
pH >8
sonicate
Polyelectrolyte “Surfactant”
Scattering
SWNT
Polyelectrolyte Dispersion
0.001
0.01
0.1
1
10
100
1000
In
ten
sity
/con
c.(m
l/g)
10-6
10-5
10-4
10-3
q(Å-1)
ARSWNT in solvents
no sonication
-2
'0973_ARSWNT_PAAHCL_1_0m' '0968_ARSWNT_PMAA_1_0m' '0955_ARSWNT_PSSO31_0m_D'
PSSO3
PAAHCl
I ~ M
Light scattering measures “dispersibility”
0.1
1
10
100
1000
Int
ensi
ty/c
onc.
(ml/
g)
10-6 10-5 10-4 10-3
q(Å-1)
0.1% ARSWNT in 600ul PAAHCL
-2
-1
RTC0973_ARSWNT_PAAHCL_1_0m RTC0974_ARSWNT_PAAHCL_1_2m RTC0975_ARSWNT_PAAHCL_1_4m RTC0976_ARSWNT_PAAHCL_1_8m RTC0977_ARSWNT_PAAHCL_1_12m RTC0978_ARSWNT_PAAHCL_1_16m
Agglomeration20 µm 60 µm
0.001
0.01
0.1
1
10
100
In
ten
sity
/con
c.(m
l/g)
10-6
10-5
10-4
10-3
q(Å-1)
0.1% ARSWNT in 1200ul PMAA
-2
RTC0968_ARSWNT_PMAA_1_0m RTC0969_ARSWNT_PMAA_1_2m RTC0970_ARSWNT_PMAA_1_4m RTC0971_ARSWNT_PMAA_1_6m RTC0972_ARSWNT_PMAA_1_8m
Breakup83 µm 24 µm
0.001
0.01
0.1
1
In
ten
sity
/con
c.(m
l/g)
10-6
10-5
10-4
10-3
q(Å-1)
0.1% ARSWNT in 600ul PSSO3
-2
RTC0955_ARSWNT_PSSO31_0m_D RTC0956_ARSWNT_PSSO31_2m_D
No effect13 µm
PSSO3
Good Dispersion
PMAAIntermediate
PAAHClPoor Dispersion
SonicationStudies
0.1
1
10
100
1000
In
ten
sity
/con
c.(m
l/g)
10-6
10-5
10-4
10-3
q(Å-1)
0.1% ARSWNT in 600ul PAAHCL
-2
-1
RTC0973_ARSWNT_PAAHCL_1_0m RTC0974_ARSWNT_PAAHCL_1_2m RTC0975_ARSWNT_PAAHCL_1_4m RTC0976_ARSWNT_PAAHCL_1_8m RTC0977_ARSWNT_PAAHCL_1_12m RTC0978_ARSWNT_PAAHCL_1_16m
Coil
Rod
Potential Separation Technology
Rod-like Remnant in Sonicated “Poor” Dispersions
0.01
0.1
1
10
100
In
ten
sity
/con
c.(m
l/g)
10-6
10-5
10-4
10-3
q(Å-1)
0.1%fiber in solvents
2min sonication
'0986_PR21_PAAHCL_1_2m' '0965_PR21_PMMA_2m' '0967_PR21_PSSO3_2mb'
Same Trends as SWNTS
-1
Nanofibers (0.1% Fiber) in 3 Dispersants
Conclusions
• The utility of small-angle X-ray and light scattering to measure the dispersion of both SWNTs and CNTs in water suspension was demonstrated. • Even well dispersed both forms of carbon exist in an aggregated state.
• The SWNT aggregates are fractal structures - seem to be the analogue
of the network of ropes seen by electron microscopy of dried samples.• The ropes in suspension are swollen compared to the dried
counterparts.• No evidence of a persistence length of order of the diameter of an
isolated SWNT.• CNTs are also aggregated, but the morphology is side-by-side
association of a limited number of tubes.– Morphology remains rod-like.– Porod’s law is observed on length-scales smaller than the radius
of a single tube - the surface is presumed smooth and the interface sharp.
– Both CNTs and SWNTs respond to dispersion aids in a similar fashion.
– Anionic polyelectrolytes are the best dispersants.– There is evidence for phase separation of the dispersant around
the tube clusters.– Suspensions respond differently to ultrasound.
In good dispersants ultrasound has minimal effect. In poor solvents it induces aggregation.
• Residual catalyst has an effect on sonication.– Clean SWNT suspensions (catalyst removed) have little response
to ultrasound regardless of the dispersion aid.– For As Rec’d SWNTs in the poorest dispersant precipitation
observed after 10 min of sonication.
AcknowledgementsThis research was funded by the United States Air Force Research Laboratory, partially
through contract F33615-97-D5405 and contract F33615-00-D-5006.
The UNICAT facility at the Advanced Photon Source (APS) is supported by the University of Illinois at Urbana-Champaign, Materials Research Laboratory (U.S. Department of Energy,
the State of Illinois-IBHE-HECA, and the National Science Foundation), the Oak Ridge National Laboratory (U.S. DOE under contract with UT-Battelle LLC), the National Institute of
Standards and Technology (U.S. Department of Commerce) and UOP LLC. The APS is supported by the U.S. Department of Energy, Basic Energy Sciences, Office of Science
under contract No. W-31-109-ENG-38.