Are nano-composites worth the effort?

The mechanics of small objects:
Selected experiments from various landscapes
H. Daniel Wagner
Department of Materials and Interfaces
Weizmann Institute of Science
GDR Mecano
7-8 April 2011, Poitiers, France
Outline
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Nano-objects and macro-problems
Single NT mechanics: Nano-acrobatical exercises
Current experiments with polyhedral inorganic fullerene
particles
From nano-objects to nano-composites
The hard question: Are nano-composites worth the effort?
Conclusions
Nano-objects and macro-problems
 70s – 90s: microfiber mechanical tests (complex but became standard)
 1991: nano-era (NT, NF, NW, Npillars…)
 C NT, WS2 NT, MoS NT, ES polymer fibers, but also virus membranes (nm),
basilar membrane within the cochlea of the inner ear (mm), nanoplatelets and
nanocomposite biomaterials in general…
 Major testing challenges (handling, gripping, testing, nN forces, nm extensions…)
 AFM, Hi-res microscopies, nano-indentation / compression / tension / bending / …
 Some interesting questions (and contradictions)
Iijima 1991
Inspiring effect
It was conjectured and demonstrated that other layered compounds
are likely to form fullerene-type
(including nanocylindrical) structures:
BN, BC3, WS2, MoS2
Tenne 1992
Tenne et al, Nature, 1992
GRAPHENE
Novoselov & Geim 2004
Single NT mechanics: Nano-acrobatical exercises
Compression (from TEM
observations + modeling)
Tensile strength tests
Interfacial adhesion tests
Lourie, Cox, Wagner Phys. Rev. Lett. 1998
Lourie, Wagner J. Mater. Res. 1998
~100-150 GPa
crit = ENT
+ (2K/)(L/mr)2
(mr/L)2
Confirmed by molecular
dynamics
Srivastava, Phys Rev Lett 1999
200
BUCKLING STRESS (GPa)
COMPRESSIVE STRENGTH
OF SINGLE MWNT
BUCKLING OF
THICK-WALLED NANOTUBES
(h/r = 0.72)
150
strong
interface
LOWER BOUND
100
weak
interface
50
0
0
5
10
15
20
25
NORMALIZED LENGTH, L/r
Lourie, Cox, Wagner Phys. Rev. Lett. 1998
Lourie, Wagner J. Mater. Res. 1998
30
Attaching CNTs to AFM tip - ESEM
Barber, Cohen, Kenig, Wagner, Compos. Sci. Technol. 2004
Young’s modulus from single MW WS2 nanotube - buckling
OPTION 1
Euler‟s analysis:
OPTION 2
Elastica analysis:
F
 
E  FEuler L2 /  2 I
E=170 GPa
F
y
2 EI
cos   cos  
F
E ~ 150 GPa
Kaplan-Ashiri et al, PNAS 2006
Barber, Cohen, Kenig, Wagner, Compos. Sci. Technol. 2004
Full stress-strain signature of a single MW WS2 NT :
E ~ 145 GPa
Kaplan-Ashiri et al, PNAS 2006
(
HOW ABOUT CNT STRENGTH?
Specifically:
What is the role of individual defects (vacancies…) in an almost
perfect nano structure ? Are stochastic models still valid ?
(and continuum mechanics in general ?)
1. Gao et al., PNAS 2003
Below  30 nm (approx length scale), the material becomes insensitive to
preexisting flaws: The strength of a perfect mineral platelet is maintained
despite defects. The failure criterion is not governed by the Griffith criterion
( ~ c-0.5).
Gao in, Griffith out ?
2. What happens if a finite # of atoms are extracted from the CNT structure?
(i) Sammalkorpi et al (Phys Rev B 2004): simulations show that NT strength
decreases by 15% with a single missing atom
(ii) Mielke et al (Chem Phys Lett 2004): 26% drop of the tensile strength
relative to theoretical strength with only a single missing atom !
MM calculations and quantum mechanics in,
Griffith and Gao out?
3. To deal with nano-defects (and size effects), can one still rely on
traditional stochastic strength models?
Weibull‟s statistical model (1950s) -
 ~ V -1/b
(not too many CNT data sets around to verify this…)
THE WEIBULL MODEL PROVIDES
EXCELLENT FIT IN ALL CASES !
  x  
F ( x)  1  exp    
    
2
1
CVD MW C NTs:  = 109 GPa,  = 1.7
LN(-LN(1-F))
0
-1
AD MW C NTs:  = 31 GPa,  = 2.4
-2
MW WS2 NTs:  = 13.3 GPa,  = 7.7
-3
CVD C MWNT
AD C MWNT
WS2 MWNT
-4
1
2
3
4
LN(Strength, GPa)
5
6
Weibull in, Gao et al. ‘out’ ?
Barber, Kaplan-Ashiri, Cohen, Tenne, Wagner, Compos. Sci. Technol. 2005
Barber, Andrews, Schadler, Wagner, Appl. Phys. Lett. 2005
4. N. M. Pugno and R. S. Ruoff (Phil Mag 2004; J Appl Phys 2006):
1
1
 2





 f n   c 1   1  n 2
 2a 
 = crack tip radius
a = crack length
n = # missing atoms
c = ideal strength
(i) Available carbon-nanotube (CNT) tensile strength data do not obey the “classical”
Weibull statistical model.
(ii) Role of isolated defects (such as missing atoms) is critical ! Strength is „quantized‟
– Quantum Fracture Mechanics (QFM)
QFM in, Weibull out ?
5. C. Klein (J Appl Phys 2007): CNT fracture-strength data are consistent
with Weibull‟s model ! No evidence for a lower crack length limit (thus no
evidence for insensitivity to pre-existing flaws)!
Weibull in, QFM out ? Gao et al. out ?
)
Current experiments with polyhedral inorganic fullerene
particles (in progress)
Inorganic layered materials can form hollow
multilayered polyhedral nanoparticles (4 to 300 nm).
Excellent tribological and wear resisting properties.
Measuring and evaluating the stiffness of individual
nanoparticle is non-trivial.
In-situ technique for stiffness measurements of
individual WS2 nanoparticle 80 nm or larger using
high resolution scanning electron microscope
(HRSEM).
Elucidation of the compression failure strength and
the elastic behavior under uniaxial compression.
Tevet et al., Nanotechnology (2010)

•
A transmission electron microscope (TEM- Philips model CM120) equipped
with energy dispersive X-ray analyzer (EDS- EDAX model Phoenix) used
for the analysis of the powders.
Uniaxial compression experiment performed in a high resolution scanning
electron microscope (HRSEM- LEO model Supra, 7426) equipped with a
Kleindiek nanomanipulator.
Tevet et al., Nanotechnology (2010)
Tevet et al., Nanotechnology (2010)
From nano-objects to nano-composites
ES nanofibers and yarns
Capillary
tip
Polymer
solution
Fiber
formation
A solid fiber is generated as the
electrified jet is continuously
stretched due to the electrostatic
repulsions between the surface
charges and the evaporation of
solvent
First patent: 1934
Fiber mat
Renewed interest: 1990s
Advantages of electrospinning
1. Conventional fiber spinning limited to D > 2 mm
2. Production of continuous polymer nanofibers (unlike submicron whiskers,
nanorods, nanotubes, nanowires)
3. Low cost, relatively easy
PMMA / CNT
NANOTUBES HAVE NO CHOICE BUT TO ALIGN !
Tensile test in ESEM
RT tensile testing (inverted microscope)
PMMA / p-CNT (1.5%) electrospun fiber
This is somewhat counterintuitive, what is going on?
1. Where on the stress-strain
curve does necking start ?
 2. TEM observation of necking and final fracture
Sui & Wagner, Nano Lett. (2009)
PMMA/p-MWNTs
PMMA/p-SWNTs
STABLE NECKING, EXTENSIVE PULL-OUT
Sui & Wagner, Nano Lett. (2009)
EXTENSIVE (STABLE) NECKING, NO PULL-OUT ?
Sui & Wagner, Nano Lett. (2009)
emax (%)
σmax (Mpa)
E (Mpa)
Pnecking (%)
Toughness
Gc (MJ/m3)
PMMA
38.20
84.43
960.09
25.6
25.39
PMMA/pMWNTs
79.70
155.74
2153.83
133.6
107.36
PMMA/pSWNTs
84.55
85.71
902.03
111.5
65.97
What happens when functionalizing the CNT surface?
p-MWCNTs
10 nm 100 nm
COOH-MWCNTs
10 nm
100 nm
ID/IG=1.09f-MWCNTs
ID/IG=0.96
10 nm
D band – structural defects
100 nm
G band – tangential shearing mode of C
Covalent functionalization produces defects in the graphene structure, leading to
mechanical weakening of the nanotube and, therefore, of the nanocomposite.
Sui et al., Appl Phys Lett (2009)
ONE-STEP FABRICATION OF
YARNS / MICROCABLES MADE OF NANOFIBERS
Liu, Tasis, Prato, Wagner, Advanced Materials, 2007
Liu, Eder, Burgert, Tasis, Prato, Wagner, Applied Physics Letters, 2007
MICROCABLES
Liu, Tasis, Prato, Wagner, Advanced Materials, 2007
Liu, Eder, Burgert, Tasis, Prato, Wagner, Applied Physics Letters, 2007
MICROCABLE
TOUGHNESS
Sui et al. (Submitted, 2010)
Sui et al. (Quasi-submitted, 2011)
The hard question: Are nano-composites worth the effort?
FIRST SPECIFIC NANO-EFFECT WITH POTENTIAL BENEFITS:
Young’s modulus of fibers unexpectedly high at nano-scale
Size effect (diameter) on Young’s modulus
PAMPS
Polypyrrole
A true nano-effect; Cutoff diameter varies
ES PMMA
Possible explanations for the increase in E with decreasing D
1. Added stiffness originates from the production of additional surface area
(Nysten et al, PRL 2000; PRB 2004, and others):
Esf  g/D3
2. Added stiffness originates from the presence of ‘confined supermolecular
structures in amorphous regions’ of the polymer, of size D = 2Lcorr (Arinstein &
Zussmann, Nature Nano 2007)
Other models exist, none fully satisfactory
The hard question: Are nano-composites worth the effort?
SECOND SPECIFIC NANO-EFFECT WITH POTENTIAL BENEFIT:
Traditional composites: Fiber functionalization increases stress
transfer, thus increases composite strength, but decreases
toughness (‘brittle fracture’)
Nano-composites: Tube functionalization increases stress transfer,
and increases toughness
Standard fracture toughness of CNT/epoxy nanocomposites

P
KI 
2a
W
B W
 
 
 
  
 0.886  4.64 a  13.32 a 2  14.72 a 3  5.6 a

W
W
W
W

3/ 2

1 a
W




4



P – maximum load
cm
B – specimen thickness
W – specimen characteristic length
a - crack length before failure
Fracture Toughness
1.5
Carbon-based epoxy
composites
1.4
1.3
KIc (MPa*m1/2)
1.2
1.1
1.0
0.9
0.8
Pure
epoxy
Carbon
Black
NH2-MWCNT
(good dispersion)
pristine
NH2-MWCNT
MWCNT
(poor
(good
dispersion)
dispersion)
pristine
COOH-MWCNT
MWCNT (poor
Carbon dispersion)
nanofibers
0.7
0.6
0.0
Lachman et al. Composites A (2010)
44
44
Pullout mechanism at the nano-scale:
G pullout 
L2po  i
d
In traditional (micro)fiber-based composites, lc << Lfiber and Lpo  lc
So, using the Kelly-Tyson force balance:
G pullout 
1
i
In nanotube-based composites, lc ≥ LCNT
and so Lpo  LCNT
G pullout   i
In CNT-based composites, enhanced adhesion leads to increased toughness !
(specifically nano-scale effect)
Lachman et al. Composites A (2010)
45
(Shear-lag model – Greszczuk)
Polymer-CNT interfacial adhesion
160
■ pristine nanotube pullout
● modified nanotube pullout
140
Similar approach: CNT wetting
by various liquids and polymers
Barber, Cohen, Wagner, Phys. Rev. Lett. 2004
Barber, Cohen, Wagner, Nano Lett. 2004
Barber, Cohen, Wagner, Phys. Rev. B 2005
IFSS, average (MPa)
120
100
80
60
40
20
0
-20
0
500
1000
1500
2000
2500
Embedded length, L (nm)
Barber et al. Adv Mater (2005)
Barber et al. Phys Rev Lett (2004)
The hard question: Are nano-composites worth the effort?
HOW ABOUT A MULTISCALE (NANO-MICRO, HYBRID)
COMPOSITE?
Qian et al.,
J Mater Chem (2010)
Bekyarova et al, Langmuir (2007)
Kepple et al, Carbon (2008)
30% improvement in ILSS
50% improvement in GIc
Qian et al, J Mater Chem (2010)
Improvement in 
Lachman et al., Quasi-submitted (2011)
The hard question: Are nano-composites worth the effort?
INSPIRATIONAL GUIDING PRINCIPLES FROM BIOLOGICAL
NANO-COMPOSITES
(i) Optimized particle dispersion and packing through self-assembly
(Vf ~ 90-95%)
(ii) Optimized interfacial adhesion
(iii) Very high toughness
Wagner, News & Views, Nature Nanotechnology Dec 2007
Weiner, Addadi, Wagner, Mater. Sci.Eng. C 2000
Weiner & Wagner, Ann. Rev. Mater. Sci. 1998
Last 50 years:
micron fiber-based composites revolution
LM 61.5 m long blades
(17.7 tons)
Next 50 years:
nano objects-based composites revolution ?
MAY 2010
Acknowledgments
Israel Science Foundation
NES Magnet (IMIC)
US-Israel BSF
NOESIS (EU)
Partners & colleagues
XiaoMeng Sui, Noa Lachman, Ofer Tevet
Sidney Cohen, Reshef Tenne
Philippe Poulin (CRPP Pessac, France)
Pulickel Ajayan (Rice)
Brian Wardle (MIT)
P. Fratzl (Max-Planck Inst., Golm)