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Thermal conductivity of graphene oxide / polyvinyl alcohol composites Sunnam KIM1, Jumpei SHIMAZU1, Tsuyoshi FUKAMINATO, Tomonari OGATA3, Seiji KURIHARA1,2 (1Graduate School of Science and Technology Kumamoto University, 2-39-1 Kurokami, Chuo-ku, Kumamoto, 860-8555, Japan 2Kumamoto Institute for Photo-electro Organics, 3Inovate Collaboration Organization, Kumamoto University)

Introduction

Accumulation of heat

Efficient heat

dissipating materials

lightweight, easy processing,

high resistance to corrosive

environments, low cost

Small and wearable smart devices with downscaled circuits

problem

Damage of device with short life expectancy

Polymer

Thermal conductivity (W/mK)

1.E+00 1.E+01 1.E+02 1.E+03 1.E+04

In modern ubiquitous society,

(iWatch-Apple)

Polymer

composites

Low thermal conductivity

wood glass steel copper Al

diamond Gold

High Low

filler

Mechanical strength Thermal conductivity Low density Low CTE

coefficient of thermal expansion

advantages

Expensive Hard manufacturing process

Compatibility

Disadvantages

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Introduction

Polyvinyl alcohol (PVA)

Graphene oxide

graphene • A lot of oxygen groups

• Amphiphilic properties

• Solubility/compatibility

• Chemical modification

Various functionality

Electrical, thermal, mechanical

Superior properties

Oxidation / Exfoliation

graphite

Graphene oxide (GO)

(process)

• Low cost

• Mass production

Water soluble semi-crystalline polymer

Interaction between GO and PVA

For GO/ PVA composites

Area (m2)

Thickness (m)

Thermal energy (W)

Temperature difference T=T1-T2

T１(K) Thermal conductivity (W・K-1・m-1)

T２(K)

Heat transfer mechanism

For high thermal conductivity of GO/PVA composites

Polymer composite

In this work,

Dielectric materials : by phonon* Metals : by electron

e-

+

e-

+ + +

*Phonon : crystal lattice vibrations

Ex) diamond

• high crystallinity • Molecular interaction between GO and PVA

Water soluble and Semi-crystalline polymer * PVA : * GO: Graphene derivative ; Super thermal conductivity of graphene

• Preparation of GO/PVA composite • Investigation of thermal conductivity

Introduction

𝜆 =C𝑝𝑣𝑙

3

Cp is the specific heat capacity per unit volume; v is the average phonon velocity; and l is the phonon mean free path.

Debye equation Phonon velocity wave speed: elastic property/density

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For thermal conductivity of GO/PVA composites,

Parameters

• Crystallinity • Degree of polymerization

• GO content • Oxidation degree of GO

• Interaction between GO and PVA • Silica packing

PVA

GO

I. PVA matrix

II. GO filler

III. Composite structure

PVA-GO

In this work,

• Degree of saponification and polymerization

i. Effect of crystallinity of PVA on thermal conductivity

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h

Result FT-IR測定結果による各複合体のGOとPVAの水素結合

GO：5wt%

Hydrogen bonding of GO/PVA (depending on degree of polymerization)

0 500 1000 nm 0

500

1000 nm

0 30 180 nm

1.0nm

Changes in O–H stretching vibration band at 3300 cm-1

and C–O band at 1084 cm-1

FTIR spectra : Hydrogen bonding of hydroxyl groups & crystallinity

Fig. AFM image of GO

composites Degree of

polymerization Degree of

saponification GO content

GO/PVA300 300 98.0 ~ 100% Completely Hydrolyzed

5wt% to PVA GO/PVA500 500

GO/PVA2000 2000

Interaction between GO and PVA (depending on degree of polymerization)

OH (cm-1) Change

amount

(cm-1) 0% 5%

GO/PVA300 3278.0 3272.6 -5.4

GO/PVA500 3272.7 3271.3 -1.4

GO/PVA2000 3271.6 3271.7 0.1

OH peaks of GO/PVA300, GO/PVA500, GO/PVA2000

3266

3268

3270

3272

3274

3276

3278

3280

GO/PVA 300 GO/PVA 500 GO/PVA 2000

OH

ban

d (c

m-1

)

GP/PVA composites

GO 0%

GO 5%

Strong hydrogen bonding between PVA chains High entanglement with long chains

Interaction between GO and PVA

PVA-GO

PVA

① Less interaction between GO and PVA ② Aggregation of GO

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Crystallinity of GO/PVA300, GO/PVA500, GO/PVA2000 from IR results

0.4

0.45

0.5

0.55

0.6

0.65

GO/PVA 300 GO/PVA 500 GO/PVA 2000

Cry

stal

linit

y fr

om

FTI

R

GP/PVA composites

GO 0%

GO 5%

② Crystallinity decreased with high degree of polymerization, more amorphous ③ For GO/PVA2000, Crystallinity decreased with GO addition GO is no more nucleation for crystallization

① For GO/PVA300, Crystallinity increased by GO addition GO is for nucleation of crystallization

Crystallinity of GO/PVA (depending on degree of polymerization)

A1146 : absorbance at 1146 cm-1 A1084: absorbance at 1084 cm-1

A1146

A1084 × 100 ＝

Crystallinity （%）

Imai K, Bulletin of Nagaoka University

of Technology 1979; 1: 67-69

Defect generation for GO/PVA2000 by unbalanced stress on the area between aggregates of GO, indicating less interaction between GO and PVA

FE-SEM images of GO/PVA (depending on degree of polymerization)

PVA2000

GO

GO

GO

PVA300

PVA GO/PVA

For PVA300, PVA2000, and GO/PVA300,

No defect

(a)

(b)

(c)

(d)

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%

Thermal conductivity of GO/PVA (depending on GO content)

For GO/PVA500 and GO/PVA2000, no improvement of thermal conductivity by GO addition ; due to lower crystallinity and less interaction between GO and PVA

(LFA results without GO)

1. High crystallinity 2. Strong interaction

between GO and PVA 3. Good distribution of GO

in the composite

Thermal conductivity of GO/PVA300 was improved

by GO addition.

• GO content to PVA300 : 0 ~ 30wt%

ii. Effect of GO content on thermal conductivity

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Before heat treatment

2θ

19.5°

Inte

nsi

ty (

a.u

.)

Inte

nsi

ty (

a.u

.)

GO 0

GO 5

GO 10

GO 20

GO 30

*

*

*

After heat treatment

2θ

19.5°

GO 0

GO 5

GO 10

GO 20

GO 30

* * *

*

*

For GO5 : No 002 peak of GO For GO10, GO20 and GO30 : shift to lower angle

002 peak of GO

0.8 nm

Crystallinity increased by heat

treatment

Sharpness

XRD results of GO/PVA300 composites

GO PVA

Crystallinity decreased

by GO addition

5.34˚

5.89˚

5.26˚

2θ = 11.5˚

1.6 nm

2θ = 5.34˚

aggregates

Aggregation of GO

0.4

0.45

0.5

0.55

0.6

0.65

0.7

0.75

0.8

0 5 10 15 20 25 30

Before heat treat

After heat treat

PVA 500

PVA 500 heat treat

A1

14

6/A

10

84

GO content wt%

0

10

20

30

40

50

0 5 10 15 20 25 30

Before heat treat

After heat treat

Cry

sta

llinity %

GO content wt%

Heat treatment

Crystallinity increase

GO addition

Aggregates

Crystallinity decrease

Crystallinity by FTIR Crystallinity by XRD

Crystallinity of GO/PVA300 (depending on GO content)

where A1146 is the absorbance at 1146 cm-1 A1084 is the absorbance at 1084 cm-1

A1146

A1084 × 100 ＝ Crystallinity （%）

Ic = the area of the polymer crystalline phase Ia = the area of the amorphous-phase

Ic

Ic + Ia × 100 ＝ Crystallinity （%）

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0.1

0.2

0.3

0.4

0.5

0.6

0 5 10 15 20 25 30

Before heat treat

After heat treat

Th

erm

al condu

ctivi

ty (

W/m

K)

GO content %

• For only PVA without GO, Thermal conductivity increased by the heat treatment as crystallinity increased • For GO5, thermal conductivity increased by GO addition

Results

• More contents of GO(10~30wt%), No further improvement of thermal conductivity Due to decrease of crystallinity and aggregation of GO

PVA 500 Before heat treatment

Thermal conductivity of GO/PVA300 (depending on GO content)

PVA300

After heat treatment

Next plan : Effect of GO content under 5wt%

OH peaks of GO/PVA300 composites

Amorphous

Crystal

O-H

5%

1%

PVA300

3%

Wavenumber/cm-1

Abs

Lower wavenumber

Stronger hydrogen bond

by GO addition

Shift to lower wavenumber for OH

stretching vibration band for GO/PVA300 :

bond lengthening

OH bands of GO/PVA300 (depending on GO content; ~5 wt% of GO)

GO content : ~ 5wt%

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Crystallinity of GO/PVA300 from IR results

300

% Crystallinity of GO/PVA300 from XRD results

Crystallinity by FTIR

A1146 is the absorbance at 1146 cm-1 A1084 is the absorbance at 1084 cm-1

A1146

A1084 × 100 ＝ Crystallinity（%）

Crystallinity by XRD

Result : Crystallinity increased by GO addition

Crystallinity of GO/PVA300 (depending on GO content)

Ic = the area of the crystalline phase Ia = the area of the amorphous-phase

Ic

Ic + Ia × 100 ＝ Crystallinity （%）

Increase of interaction between GO and PVA

PVA GO PVA

PVA GO

GO GO

< 5wt% > 5wt%

Aggregation of GO Free oxygen groups Intercalated H2O

TG of GO/PVA300 (depending on GO content)

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Condu

ctiv

ity /

W/m･

K

GO/PVA300

%

Thermal conductivity of GO/PVA300

Thermal conductivity of GO/PVA300 (depending on GO content)

Because of increase of interaction between GO and PVA Increase of crystallinity

Because of aggregation of GO, excessive oxygen groups of GO. intercalated H2O

Dependence of thermal conductivity on GO content : best for 4wt%

Oxidation degree of GO : phonon scattering by free oxygen groups Interaction between GO and PVA : hydrogen bonding

• Chemical reduction of GO

iii. Effect of oxidation degree of GO on thermal conductivity

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XPS elemental analysis results of GO, rGO10, rGO30 and rGO60

Peak separation results of C1s about GO, rGO10 , rGO30, rGO60 from XPS results

GO rGO10

rGO30 rGO60

Reduced GO by chemical reduction with hydrazine

1ml of N2H4・H2O / 20mg of GO

Reaction time at 90°C

10 min (rGO10)

30 min (rGO30)

60 min (rGO60)

C-O

C-C C-O

Oxidation degree

high

low

GO rGO10 rGO30 rGO60

0min

GO rGO10 rGO30 rGO60

1h

GO rGO10 rGO30 rGO60

1Day

15min

GO rGO10 rGO30 rGO60

Photograph of water dispersion at 0min, 15min, 1h, 1day after 2h sonication about

GO, rGO10, rGO30, rGO60 (each GO concentration : 0.1 wt%)

Dispersion

GO>rGO10>rGO30>rGO60

Reduced GO by chemical reduction with hydrazine

good bad dispersion

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PVA300 GO/PVA300

rGO10/PVA300 rGO30/PVA300 rGO60/PVA300

rGO60/PVA300 > rGO30/PVA300 > rGO10/PVA300 > GO/PVA300

PVAに対する分散性

GO>rGO10>rGO30>rGO60

Observation of optical microscope of rGO/PVA300 (aggregates)

GO aggregates: poor dispersion of GO in PVA

wt%

Thermal conductivity depending on oxidation degree of GO

rGO10 : 0.72W/mK rGO30 : 0.68 W/mK rGO60 : 0.59 W/mK

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• Silica packing • PVA2000 (for mechanical strength)

iv. Effect of composite structure on thermal conductivity

Silica

GO

厚み方向

SEM images (front view (left), sectional view (right)) of GO/PVA2000, GO/(Silica/PVA2000=5/95), GO/(Silica/PVA2000=25/75),

GO/(Silica/PVA2000=50/50) (each GO contents : 5%)

Silica, GO, and PVA were well

dispersed.

Silica

GO

厚み方向

Top view

FESEM images of GO/silica/PVA2000 (depending on silica content)

Cross section Top view Cross section

Silica 0wt%

Silica 5wt%

Silica 25wt%

Silica 50wt%

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Result Silica/PVA2000のLFA

%

Thermal conductivity of silica

: 1.38 W/m･K

No effect of silica addition on thermal

conductivity

LFA results of Silica/PVA2000

LFA results of Silica/PVA2000

Result LFA測定によるGO/Silica/PVA2000の熱伝導率

GO/(Silica/PVA2000=25/75)

GO/(Silica/PVA2000=50/50)

Thermal conductivity

increased with GO addition

Silica 50wt%

Silica 0wt%

Silica

GO

厚み方向

Thermal conductivity of GO/silica/PVA2000 (depending on GO content)

Silica packing

Thermal-percolation structure

Thermal conductivity of silica : 1.38 W/m･K

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For high thermal conductivity of GO/PVA composites,

• Higher crystallinity • Lower degree of polymerization

• GO content under 5 wt% • Thermal conductivity increase with reduced GO but the problem of poor distribution

• Interaction between GO and PVA via hydrogen bonding • thermal-percolation structure was achieved, by silica packing

I. PVA matrix

II. GO filler

III. Composite structure

Conclusion

PVA GO GO

Silica

GO

厚み方向

Heat transfer

GO

GO

GO

PVA