Insights into Energy Absorption Mechanisms in Hierarchical ... · 1.e-004 1.e-003 1.e-002 0.1 1....
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Insights into Energy Absorption Mechanisms in Hierarchical Insights into Energy Absorption Mechanisms in Hierarchical Carbon Carbon Nanotube Nanotube Forests Forests Siddhartha Pathak Siddhartha Pathak 1 1 , , Nisha Nisha Mohan Mohan 1 1 , , Ee Ee J. Lim J. Lim 1 1 , , Parisa Parisa P.S.S. P.S.S. Abadi Abadi 2 2 , , Samuel Graham Samuel Graham 2,3 2,3 , , Baratunde Baratunde A. Cola A. Cola 2,3 2,3 , Julia R. Greer , Julia R. Greer 1 1 1 Materials Science, California Institute of Technology (Caltech), Pasadena, CA, USA, 2 George W. Woodruff School of Mechanical Engineering, 3 School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, GA, USA. Introduction Introduction ● What governs the energy absorption mechanism in VACNTs and their amazing recovery ● How do these properties change under different loading and boundary conditions in VACNTs Compression of VACNT micro Compression of VACNT micro - - pillars pillars Plateau Stress (MPa) x Thermal Conductivity (W/m.K) 1.e-004 1.e-003 1.e-002 0.1 1. 10. 100. 1000. Loss Coefficient 1.e-002 0.1 VACNT foams Al foams Ceramic foams C foam Graphite foam FOAM MATERIALS Glass foam Aerated Concrete Phenolic foams Rigid Polymer foams Polyurethane foams Open Cell Closed Cell Microcellular Structural polymer foam Corks Polyvinylchloride foam Polyethylene foams Flexible Polymer foams -1 0 1 2 3 4 5 6 0 0.2 0.4 0.6 Engineering Stress (MPa) 1 st cycle 2 nd 3 rd 4 th 5 th 2 μm 10 μm 1 st buckle σ buckle Before ε~0.5 ε max Flat punch indenter Si substrate 1 μm 200 nm 100 nm/s After 5 cycles Engineering Strain Densification Elastic Plateau 5 nm 0 2 4 6 8 0 0.2 0.4 0.6 0.8 Engineering Strain Engineering Stress (MPa) 1000 nm/s 100 nm/s 10 nm/s 1000 nm/s 100 nm/s 10 nm/s σ buckle 10 μm 10 nm/s 1000 nm/s After 1 st cycle 1 st cycle 100 nm/s 0 0.05 0.1 0 1 2 3 0 0.05 0.1 0 1 2 3 In-situ cyclic compression tests showing three distinct regimes; elastic, plateau, and densification Tests at slower displacement rates show more pronounced buckling signatures and lesser recovery. Plateau Densification Elastic Max Strain 1 st cycle Unloading Modulus (MPa) 0 0.2 0.4 0.6 0.8 1 0 100 200 300 400 500 600 Cycle number Pre-densification, ɛ ≤ 0.65 Cycle number Post-densification, ɛ ≥ 0.65 1000 nm/s 100 nm/s 10 nm/s 1000 nm/s 100 nm/s 10 nm/s 1000 nm/s 100 nm/s 10 nm/s 1000 nm/s 100 nm/s 10 nm/s 0 100 200 300 400 500 600 1 2 3 4 5 1 2 3 4 5 Unloading Modulus (MPa) ● Changes in unloading modulus at varying maximum strains show a stiffer response at faster rates and ● a 20-30 % drop after 5 load-unload cycles Max Strain 1 st cycle Plateau Densification Elastic % recovery, (ε max - ε i )/ ε max 0 0.2 0.4 0.6 0.8 0 20 40 60 80 100 Cycle number Pre-densification, ɛ ≤ 0.65 Cycle number Post-densification, ɛ ≥ 0.65 1000 nm/s 100 nm/s 10 nm/s 1000 nm/s 100 nm/s 10 nm/s % recovery, (ε max - ε i )/ ε max 1 2 3 4 5 1 2 3 4 5 40 50 60 70 80 90 100 Cycle number Pre-densification, ɛ ≤ 0.65 Cycle number Post-densification, ɛ ≥ 0.65 % rel. recovery,( rec i / rec i-1 ) 1000 nm/s 100 nm/s 10 nm/s 1000 nm/s 100 nm/s 10 nm/s 1000 nm/s 100 nm/s 10 nm/s 1 2 3 4 5 1 2 3 4 5 40 50 60 70 80 90 100 ● VACNTs show higher recovery at faster rates ● and in the pre- densification regime. Their recoverability decreases progressively beyond densification Time (sec) Displacement (μm) 0 1000 2000 3000 0 3 6 9 12 15 18 10 μm 2 μm 2 μm Cycle number % recovery, (ε max - ε i )/ ε max 130s hold 1500s hold 1000 nm/s 100 nm/s 10 nm/s 1000 nm/s with 1 2 3 4 5 40 50 60 70 80 90 100 No hold time ● When VACNT micro-pillars were allowed similar amounts of time for the reconfiguration to occur, they show similar % recovery. ● Thus it is the time spent by the VACNTs under high strains, rather than the loading history, that determines the permanence of their deformation. Loss coefficient, η Cycle number Cycle number 1000 nm/s 100 nm/s 10 nm/s 1000 nm/s 100 nm/s 10 nm/s 1000 nm/s 100 nm/s 10 nm/s 1000 nm/s 100 nm/s 10 nm/s Area ∆U Area U 1 2 U U i 1 2 3 4 5 1 2 3 4 5 0 0.05 0.1 0.15 Pre-densification, ɛ ≤ 0.65 Post-densification, ɛ ≥ 0.65 ● VACNTs show relatively high values of loss coefficient (energy dissipaption) under compression – comparable to polymeric foams. ● Similar to the trends of modulus and %recovery, the loss coefficient also increases at faster rates. Compression vs. Indentation Compression vs. Indentation Acknowledgments Acknowledgments ● Supported by the Institute for Collaborative Biotechnologies through contract W911NF-09-D-0001 from the U.S. Army Research Office. ● SP gratefully acknowledges support from the W.M. Keck Institute for Space Studies Postdoctoral Fellowship program for this work. References References (1) S Pathak, Ee J. Lim, Parisa P.S.S. Abadi, S Graham, Baratunde A. Cola, Julia R. Greer. Higher Recovery and Better Energy Dissipation at Faster Strain Rates in Carbon Nanotube Bundles: an in-situ Study. ACS Nano, 2012 DOI: 10.1021/nn300376j (2) S Pathak, N Mohan, P PSS Abadi, S Graham, B A Cola, JR Greer. Compressive Response of Vertically Aligned Carbon Nanotube Films via In-Situ Flat Punch Indentations, Advanced Functional Materials, 2012 – submitted (3) Abadi PPS; Hutchens S; Taphouse JH; Greer JR; Cola BA; Graham,S, The Effect of Morphology on the Micro-Compression Response of Carbon Nanotube Forests. Carbon 2012- submitted. Flat punch Indentations on VACNT film Flat punch Indentations on VACNT film -1 0 1 2 3 4 5 6 0 0.2 0.4 0.6 Engineering Stress, MPa Indentation Stress, MPa Engineering Strain Indentation Strain Compression Indentation 1000 nm/s 176.9±11 MPa 173.7±2 MPa Modulus @ ε=0.5 0.05±0.01 0.005±0.001 Loss Coefficient @ ε=0.5 95.7±2.8 % 4.3±0.3 % Recovery @ ε=0.5 2.69±0.1 MPa (buckling) 1.75±0.3 MPa (shear) Instability stress Compression Indentation 1000 nm/s ● VACNTs show distinct modes of deformation in different boundary conditions – shear under indentation, and buckling in compression. During indentation the VACNT film reaches its critical shear stress before it can buckle. ● Shearing under indentation results in (<5%) recovery, compared to compression (> 95%). ● VACNT recovery in compression is strain rate dependent - higher recovery at faster rates ● Thus utility of VACNTs in protective applications with high energy dissipation requirements depends on the applied strain rate as well as on the loading/boundary conditions. Hierarchical morphology of VACNTs: ●continuous film 260X magnification ●nominally vertical alignment of CNTs at 30,000X ●intertwined network at 240,000X ●individual multiwalled CNTs (TEM) VACNT growth VACNT growth 4 2 0 d p corr eng 0 h u corr eng eng unload eng E * p corr - corrected load u corr - corrected displacement h 0 - initial height of VACNT pillar d 0 - initial dia of VACNT pillar In-situ on-edge flat punch indentation ●first buckle formed close to the bottom substrate ●followed by a large displacement burst ~20μm ●caused by a vertical shear in the VACNT film ●a series of tangled CNT ‘micro-rollers’ act as effective lubricants during the process -5 0 5 10 15 20 25 -0.4 0.0 0.4 0.9 1.3 1.8 2.2 -5 0 5 10 15 20 25 0 10 20 30 40 50 60 70 -0.4 0.0 0.4 0.9 1.3 1.8 2.2 -5 5 15 25 -0.4 0.1 0.6 1.1 1.6 2.1 64 66 68 0.533 0.553 0.573 1 st cycle 1 st 10 nm/s 2 nd 3 45 B ur s t Load, mN Displacement, μm Indentation Stress, MPa Indentation Strain Load, mN Displacement, μm Indentation Stress, MPa Indentation Strain ins 50 μm 1000 nm/s 100 nm/s 10 nm/s Unloaded after burst Unloaded from h≈65μm 0 0.1 0.2 0.3 0.4 0.5 0.6 0 0.1 0.2 0.3 0.4 0.5 0.6 ins 20 μm 19 0 10 20 30 40 50 60 70 Unloading Modulus, MPa Cycle number 1000 nm/s 100 nm/s 10 nm/s 1000 nm/s 100 nm/s 10 nm/s 0 40 80 120 160 200 1 2 3 4 5 % recovery, (h max -h i )/ h max Displacement before unload, μm 1000 nm/s 100 nm/s 10 nm/s 1000 nm/s 100 nm/s 10 nm/s 0 10 20 0 25 50 75 0 10 20 0 25 50 75 0 20 40 60 80 100 0 25 50 75 Cycle number Loss coefficient, η 1000 nm/s 100 nm/s 10 nm/s 1000 nm/s 100 nm/s 10 nm/s 1 2 3 4 5 0 0.002 0.004 0.006 0.008 0.01 Ex-situ in-bulk indentation ●three distinct regions: (i) a short elastic regime (ii) sudden displacement burst (instability), (iii) sloped plateau region. ●modulus values comparable to compression ●however, the VACNTs in indentation show very poor recovery and loss coefficient after the burst 0 10 20 30 40 50 60 0 1 2 3 4 5 0 0.1 0.2 0.3 0.4 0.5 0.6 Load, mN Displacement, μm Indentation Stress, MPa Indentation Strain Film thickness 25 μm Film thickness 141.5 μm 120 μm dia flat punch indenter 100nm/s 0 10 20 30 40 50 60 70 Plateau Substrate effect Effect of substrate on buckle formation Buckle formation in the VACNT film is strongly influenced by the constraints imposed by the stiff substrate. Thus, indentations on a thin 25 μm VACNT film result in additional localized folds to continuously form along the VACNT film-height, and the indenter starts probing the folded/collapsed densified region. The stiff Si substrate in the shorter sample also causes a steeper rise in the load-displacement response in the plateau region. ind eff ind * E 2 a p A p corr corr ind a S A S E eff 2 2 i i s s eff E E E 2 2 1 1 1 p corr - corrected load u corr - corrected displacement a - indenter radius E eff - effective modulus S - unloading stiffness E s - sample modulus E i - indenter modulus - Poisson’s ratio sample - Poisson’s ratio indenter s i a u corr ind 2 VACNTs (vertically aligned carbon nanotubes) – ● unique hierarchical structure resulting in ● mechanical properties that combine the best of polymeric (high recoverability) and metallic foams (higher strengths even at large ~60-80% strains). ● Photolithography - Photoresist application - UV Exposure - developing ● E-beam evaporation - catalyst Ti (30nm) /Al (10 nm)/Fe (3 nm) ● Photoresist removal ● CVD CNT growth - Pressure: 750 mbar - Temp: 750 C - Carbon source gas: Acetylene ● Multiwall CNT, dia 8.8±2nm, density ~80mg/cm 3 10 μm VACNT pillars 10 μm VACNT pillars Load, mN Indentation Strain (a) 0 2 4 6 8 10 12 14 0 10 20 30 0 0.5 1 1.5 2 2.5 Load, mN Displacement, μm Indentation Stress, MPa Indentation Strain 0 10 20 30 (b) (c) (d) Indent on edge Indent on bulk (a) 100 nm/s 0 0.1 0.2 0.3 0 2 4 6 8 10 12 14 0 10 20 30 0 0.5 1 1.5 2 2.5 Load, mN Displacement, μm Indentation Stress, MPa Indentation Strain 0 10 20 30 (b) (c) (d) Indent on edge Indent on bulk (a) 100 nm/s 0 0.1 0.2 0.3 CNT ‘micro-roller’
Transcript of Insights into Energy Absorption Mechanisms in Hierarchical ... · 1.e-004 1.e-003 1.e-002 0.1 1....
Insights into Energy Absorption Mechanisms in Hierarchical Insights into Energy Absorption Mechanisms in Hierarchical Carbon Carbon NanotubeNanotube ForestsForests
Siddhartha Pathak Siddhartha Pathak 11, , NishaNisha Mohan Mohan 11, , EeEe J. Lim J. Lim 11, , ParisaParisa P.S.S. P.S.S. AbadiAbadi 22, , Samuel Graham Samuel Graham 2,32,3, , BaratundeBaratunde A. Cola A. Cola 2,32,3, Julia R. Greer , Julia R. Greer 11
1 Materials Science, California Institute of Technology (Caltech), Pasadena, CA, USA, 2 George W. Woodruff School of Mechanical Engineering, 3 School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, GA, USA.
IntroductionIntroduction
● What governs the energy absorption mechanism in VACNTs and their amazing recovery ● How do these properties change under different loading and boundary conditions in VACNTs
Compression of VACNT microCompression of VACNT micro--pillarspillars
Plateau Stress (MPa) x Thermal Conductivity (W/m.K)1.e-004 1.e-003 1.e-002 0.1 1. 10. 100. 1000.
Loss
Coe
ffici
ent
1.e-002
0.1
VACNT foams
Al foams
Ceramic foams
C foamGraphite foam
FOAM MATERIALS
Glass foam
Aerated Concrete
Phenolic foams
Rigid Polymer foams
Polyurethane foams
Open Cell
Closed Cell
Microcellular Structural polymer foam
Corks
Polyvinylchloride foam
Polyethylene foams
Flexible Polymer foams
Plateau Stress (MPa) x Thermal Conductivity (W/m.K)1.e-004 1.e-003 1.e-002 0.1 1. 10. 100. 1000.
Loss
Coe
ffici
ent
1.e-002
0.1
VACNT foams
Al foams
Ceramic foams
C foamGraphite foam
FOAM MATERIALS
Glass foam
Aerated Concrete
Phenolic foams
Rigid Polymer foams
Polyurethane foams
Open Cell
Closed Cell
Microcellular Structural polymer foam
Corks
Polyvinylchloride foam
Polyethylene foams
Flexible Polymer foams
-1
0
1
2
3
4
5
6
0 0.2 0.4 0.6
Engi
neer
ing
Stre
ss (M
Pa)
1st cycle
2nd
3rd4th 5th
2 µm
10 µm
1st buckle
σbuckle
Before
ε~0.5 εmax
Flat punch indenter
Si substrate
1 µm
200 nm
100 nm/s
After 5 cycles
Engineering Strain
Densification
ElasticPlateau
5 nm
0
2
4
6
8
0 0.2 0.4 0.6 0.8Engineering Strain
Engi
neer
ing
Stre
ss (M
Pa)
1000 nm/s100 nm/s10 nm/s
1000 nm/s100 nm/s10 nm/s
σbuckle
10 µm
10 nm/s
1000 nm/s
After 1st cycle1st cycle
100 nm/s
0 0.05 0.10
1
2
3
0 0.05 0.10
1
2
3
In-situ cyclic compression tests showing three distinct regimes; elastic, plateau, and densification
Tests at slower displacement rates show more pronounced buckling signatures and lesser recovery.
PlateauDensificationElastic
Max Strain
1st cycle
Unl
oadi
ng M
odul
us (M
Pa)
0 0.2 0.4 0.6 0.8 10
100
200
300
400
500
600
Cycle number
Pre-densification, ɛ ≤ 0.65
Cycle number
Post-densification, ɛ ≥ 0.651000 nm/s100 nm/s10 nm/s
1000 nm/s100 nm/s10 nm/s
1000 nm/s100 nm/s10 nm/s
1000 nm/s100 nm/s10 nm/s
0
100
200
300
400
500
600
1 2 3 4 5 1 2 3 4 5
Unl
oadi
ng M
odul
us (M
Pa)
● Changes in unloading modulus at varying maximum strains show a stiffer response at faster rates and● a 20-30 % drop after 5 load-unload cycles
Max Strain
1st cycle
Plateau
DensificationElastic
% re
cove
ry, (ε m
ax-ε
i)/ ε
max
0 0.2 0.4 0.6 0.80
20
40
60
80
100
Cycle number
Pre-densification, ɛ ≤ 0.65
Cycle number
Post-densification, ɛ ≥ 0.65
1000 nm/s100 nm/s10 nm/s
1000 nm/s100 nm/s10 nm/s%
reco
very
, (ε m
ax-ε
i)/ ε
max
1 2 3 4 5 1 2 3 4 540
50
60
70
80
90
100
Cycle number
Pre-densification, ɛ ≤ 0.65
Cycle number
Post-densification, ɛ ≥ 0.65
% re
l. re
cove
ry,(
rec i/
rec i-
1)
1000 nm/s100 nm/s10 nm/s
1000 nm/s100 nm/s10 nm/s
1000 nm/s100 nm/s10 nm/s
1 2 3 4 51 2 3 4 540
50
60
70
80
90
100● VACNTs show higher recovery at faster rates ● and in the pre-densification regime. Their recoverability decreases progressively beyond densification
Time (sec)
Dis
plac
emen
t (µm
)
0 1000 2000 30000
3
6
9
12
15
18
10 µm
2 µm
2 µm
Cycle number
% re
cove
ry, (ε m
ax-ε
i)/ ε
max
130s hold1500s hold
1000 nm/s100 nm/s10 nm/s
1000 nm/s with
1 2 3 4 540
50
60
70
80
90
100
No hold time
● When VACNT micro-pillars were allowed similar amounts of time for the reconfiguration to occur, they show similar % recovery.● Thus it is the time spent by the VACNTs under high strains, rather than the loading history, that determines the permanence of their deformation.
Loss
coe
ffici
ent, η
Cycle number Cycle number
1000 nm/s100 nm/s10 nm/s
1000 nm/s100 nm/s10 nm/s
1000 nm/s100 nm/s10 nm/s
1000 nm/s100 nm/s10 nm/s
Area∆U
Area U
12 UUi
1 2 3 4 51 2 3 4 50
0.05
0.1
0.15
Pre-densification, ɛ ≤ 0.65 Post-densification, ɛ ≥ 0.65
● VACNTs show relatively high values of loss coefficient (energy dissipaption) under compression – comparable to polymeric foams. ● Similar to the trends of modulus and %recovery, the loss coefficient also increases at faster rates.
Compression vs. IndentationCompression vs. Indentation
AcknowledgmentsAcknowledgments● Supported by the Institute for Collaborative Biotechnologies through contract W911NF-09-D-0001 from the U.S. Army Research Office.● SP gratefully acknowledges support from the W.M. Keck Institute for Space Studies Postdoctoral Fellowship program for this work.
ReferencesReferences(1) S Pathak, Ee J. Lim, Parisa P.S.S. Abadi, S Graham, Baratunde A. Cola, Julia R. Greer. Higher Recovery and Better Energy Dissipation at Faster Strain Rates in Carbon Nanotube Bundles: an in-situ Study. ACS Nano, 2012 DOI: 10.1021/nn300376j (2) S Pathak, N Mohan, P PSS Abadi, S Graham, B A Cola, JR Greer. Compressive Response of Vertically Aligned Carbon Nanotube Films via In-Situ Flat Punch Indentations, Advanced Functional Materials, 2012 – submitted(3) Abadi PPS; Hutchens S; Taphouse JH; Greer JR; Cola BA; Graham,S, The Effect of Morphology on the Micro-Compression Response of Carbon Nanotube Forests. Carbon 2012- submitted.
Flat punch Indentations on VACNT filmFlat punch Indentations on VACNT film
-1
0
1
2
3
4
5
6
0 0.2 0.4 0.6
Engi
neer
ing
Stre
ss, M
PaIn
dent
atio
n St
ress
, MPa
Engineering StrainIndentation Strain
Compression
Indentation
1000 nm/s
176.9±11 MPa173.7±2 MPaModulus
@ ε=0.5
0.05±0.010.005±0.001Loss Coefficient @ ε=0.5
95.7±2.8 %4.3±0.3 %Recovery @ ε=0.5
2.69±0.1 MPa(buckling)
1.75±0.3 MPa(shear)Instability stress
CompressionIndentation1000 nm/s
● VACNTs show distinct modes of deformation in different boundary conditions – shear under indentation, and buckling in compression. During indentation the VACNT film reaches its critical shear stress before it can buckle.● Shearing under indentation results in (<5%) recovery, compared to compression (> 95%).● VACNT recovery in compression is strain rate dependent -higher recovery at faster rates● Thus utility of VACNTs in protective applications with high energy dissipation requirements depends on the applied strain rate as well as on the loading/boundary conditions.Hierarchical morphology of VACNTs:
●continuous film 260X magnification●nominally vertical alignment of CNTsat 30,000X ●intertwined network at 240,000X●individual multiwalled CNTs (TEM)
VACNT growthVACNT growth
420d
pcorreng
0hucorr
eng
engunloadeng E *
pcorr - corrected loaducorr - corrected displacementh0 - initial height of VACNT pillard0 - initial dia of VACNT pillar
In-situ on-edge flat punch indentation●first buckle formed close to the bottom substrate●followed by a large displacement burst ~20µm●caused by a vertical shear in the VACNT film●a series of tangled CNT ‘micro-rollers’ act as effective lubricants during the process
-5
0
5
10
15
20
25
-0.4
0.0
0.4
0.9
1.3
1.8
2.2
-5
0
5
10
15
20
25
0 10 20 30 40 50 60 70-0.4
0.0
0.4
0.9
1.3
1.8
2.2
-5
5
15
25
-0.4
0.1
0.6
1.1
1.6
2.1
64 66 68
0.533 0.553 0.573
1st cycle
1st
10 nm/s
2nd
3 4 5
Burst
Load
, mN
Displacement, µm
Indentation Stress, MPa
Indentation Strain
Load
, mN
Displacement, µm
Indentation Stress, MPa
Indentation Strain
ins
50 µm1000 nm/s100 nm/s10 nm/s
Unloaded after burst
Unloaded from h≈65µm
0 0.1 0.2 0.3 0.4 0.5 0.6
0 0.1 0.2 0.3 0.4 0.5 0.6
ins
20 µ
m19
0 10 20 30 40 50 60 70
Unl
oadi
ng M
odul
us, M
Pa
Cycle number
1000 nm/s100 nm/s10 nm/s
1000 nm/s100 nm/s10 nm/s
0
40
80
120
160
200
1 2 3 4 5
% re
cove
ry, (
h max
-hi)/
hm
ax
Displacement before unload, µm
1000 nm/s100 nm/s10 nm/s
1000 nm/s100 nm/s10 nm/s
0
10
20
0 25 50 750
10
20
0 25 50 75
0
20
40
60
80
100
0 25 50 75
Cycle number
Loss
coe
ffici
ent, η
1000 nm/s100 nm/s10 nm/s
1000 nm/s100 nm/s10 nm/s
1 2 3 4 50
0.002
0.004
0.006
0.008
0.01
Ex-situ in-bulk indentation●three distinct regions: (i) a short elastic regime (ii) sudden displacement burst (instability), (iii) sloped plateau region.
●modulus values comparable to compression●however, the VACNTs in indentation show very poor recovery and loss coefficient after the burst
0
10
20
30
40
50
60
0
1
2
3
4
50 0.1 0.2 0.3 0.4 0.5 0.6
Load
, mN
Displacement, µm
Indentation Stress, MPa
Indentation Strain
Film thickness 25 µm
Film thickness 141.5 µm
120 µm dia flat punch indenter
100nm/s0 10 20 30 40 50 60 70
Plateau
Substrate effect
Effect of substrate on buckle formationBuckle formation in the VACNT film is strongly influenced by the constraints imposed by the stiff substrate. Thus, indentations on a thin 25 µm VACNT film result in additional localized folds to continuously form along the VACNT film-height, and the indenter starts probing the folded/collapsed densified region. The stiff Si substrate in the shorter sample also causes a steeper rise in the load-displacement response in the plateau region.
indeffind *E
2ap
Ap corrcorr
ind
aS
ASEeff 22
i
i
s
s
eff EEE
22 111
pcorr - corrected loaducorr - corrected displacementa - indenter radiusEeff - effective modulusS - unloading stiffnessEs - sample modulusEi - indenter modulus
- Poisson’s ratio sample- Poisson’s ratio indenter
si
aucorr
ind 2
VACNTs (vertically aligned carbon nanotubes) –● unique hierarchical structure resulting in● mechanical properties that combine the best of polymeric (high recoverability) and metallic foams (higher strengths even at large ~60-80% strains).
● Photolithography - Photoresist application- UV Exposure- developing
● E-beam evaporation- catalyst Ti (30nm) /Al (10 nm)/Fe (3 nm)
● Photoresist removal● CVD CNT growth - Pressure: 750 mbar
- Temp: 750 C- Carbon source gas: Acetylene
● Multiwall CNT, dia 8.8±2nm, density ~80mg/cm3
10 µm
VACNT pillars 10 µm
VACNT pillars
0
2
4
6
8
10
12
14
0 10 20 300
0.5
1
1.5
2
2.5
Load
, mN
Displacement, µm
Indentation Stress, MPa
Indentation Strain
0 10 20 30
(b)
(c)
(d)
Indent on edge
Indent on bulk
(a)
100 nm/s0 0.1 0.2 0.3
0
2
4
6
8
10
12
14
0 10 20 300
0.5
1
1.5
2
2.5
Load
, mN
Displacement, µm
Indentation Stress, MPa
Indentation Strain
0 10 20 30
(b)
(c)
(d)
Indent on edge
Indent on bulk
(a)
100 nm/s0 0.1 0.2 0.3
0
2
4
6
8
10
12
14
0 10 20 300
0.5
1
1.5
2
2.5
Load
, mN
Displacement, µm
Indentation Stress, MPa
Indentation Strain
0 10 20 30
(b)
(c)
(d)
Indent on edge
Indent on bulk
(a)
100 nm/s0 0.1 0.2 0.3
CNT ‘micro-roller’
