Impact of Calendering and Silver Addition to Carbon Nanotube-based

Electrodes Used in Printed Multi-layer Capacitors

Ramea Al-Mubarak

May 19, 2015

1

Outline

• Motivation

• Introduction

• Hypothesis

• Research Goals

• Experimental Methods

• Results and Conclusion

• Recommendations for future studies.

2

Motivation

• Energy harvesting

• Produced electrical energy

storage

• Energy storage device:

minimization of energy loss creating advanced energy storage

devices with large storage capacity and high efficiency.

• Energy storage technologies:

Batteries Super-capacitors

3

Demand for global energy Conventional energy resources Sustainable & clean energy development

Introduction: Capacitors

Devices that physically store electric charge

Energy stored through charge separation (static electricity between two oppositely charged plates)

C α 𝐴

𝑑

Capacitance(C): the ability of a body to store an electrical charge (Measured by Farad)

Traditional Capacitor

4

Some groups printed carbon nanotube sheets*

• The lowest achieved sheet resistance was 78 Ω with 200 printed layers. (0.2 microns).

• Taking the print layers into account, the lowest recorded sheet resistance was 760 Ω with 12 prints.

* Ref: Ryan et al., Inkjet printing of carbon nanotubes . Nanomaterials, 3, 453-468.

5

• Some of the carbon nanotubes might be completely isolated and have no contact with any other carbon nanotubes.

• Electrons are constrained to isolated CNTs, which do not contribute to the conductivity of the printed sheet.

• Some of the CNT may be in contact with other CNT; this contact creates an electron pathway, ultimately resulting in electrical current, which is the reason for conductivity of printed sheets

Electron pathway

Isolated CNT

CNTs Network

6

Hypothesis

* Addition of silver flake ink and silver nanowires into a CNT ink, used for printing the electrode layers of capacitor, will improve the device capacitance: • Lacing and eliminating the isolated tubes of

CNT • Conductivity of silver is very high which will

improve the conductivity of the CNT sheet conductivity

7

Cont. Hypothesis

*Calendering the electrode layers using

different pressure values would smooth the

film surface, eliminate and flatten the

peaks resulting from the protruding CNTs.

8

Before calendering

Electrode

Dielectric

After calendering

Dielectric layer needs to completely cover the electrode to

prevent shorting The smoother the electrode layer is, the thinner the dielectric

layer needed the higher the capacitance of the device

9

Research objectives

• optimize silver percentage and structure (nano-

wires or silver flake) added to CNT ink on the sheet

conductivity.

• Study the influence of calendering the multi-layer

capacitor electrodes.

• Fabricate multi-layer capacitors.

• Formulate carbon ink from a synthetic graphite

powder and conductive carbon black powder.

10

Experimental methods

1- Carbon ink formulation

5 wt% ammonia in DI water (45wt.%)

25.6 wt.% Graphite powder (172.08 𝑚2/𝑔)

Mix 5 min

Adjust PH ≈ 9

Solution resin 23.5%

4.3% Eth

Mix 45 min

1.6% film forming emulsion resin

Slowly mix 15 min

Printed onto PET with a Byrd bar#7

Good film adhesion ,but the sheet resistivity was high, 770 Ω

Ink A

11

Cont. carbon ink formulation

5 wt% ammonia in DI water (13.6 wt.%)

23 wt.% Conductive carbon black powder (S.A=254 𝑚2/𝑔)

Mix medium speed 2 hours

Adjust PH 8-9

Solution resin 37.6%

8.3% Eth

Slowly mix 1 hr

17.6% film forming emulsion resin

Printed onto PET with a Byrd bar#7

High viscose, and it showed very poor adhesion. Films readily peeled off the PET substrate after

curing.

Ink B

12

Ink A + Ink B Ink C

Obtain Ink C with better adhesion and conductivity

properties.

0

20

40

60

80

100

120

140

160

180

47.8 wt.% Ink A 67.5 wt.% Ink A 74 wt.% Ink A

Sh

ee

t R

es

isti

vit

y Ω

52.2 wt.% Ink B 32.5 wt.% Ink B 26 wt.% Ink B

13

Inks specifications

Ink A Ink B Ink C

Carbon wt.% 25.6% 23.0% Theoretical 25.10%

Exp. 23.62%

- Graphite 20.80%

- Conductive carbon black 4.30%

Solution resin wt.%

23.5% 37.6% 26.10%

Emulsion resin wt.%

1.59% 17.58% 4.58%

Sheet resistivity Ω 770 - 104.2

32.50% of Ink B to Ink A sheet resistivity 86.4% Surface area of conductive carbon black≈ 32.3% higher than the surface area of graphite powder

14

2- Blending Silver with Commercial CNT Ink and

Formulated Graphite Ink

Silver Flake (D: 2 microns, t: 50-200 nm) 75 wt% silver flake, AR=40-10

Silver nanowires (D: 90 nm, L: 25 μm) 1.25 wt% silver nanowires, AR= 277.8

CNT (D=0.93 nm, AR=1600, Specific surface area= 790 𝑚2

𝑔

Carbon black: D=30 nm), Graphite (layers of carbon atoms in hexagonal ring)

15

Ink blends preparation: (Manually mixed)

Graphite ink + Silver flake ink

Silver wt.%: 5, 10, 15, 20, 25, 30, and 35 wt.%

CNT ink + silver nanowires

Silver wt.%: 2, 5, 8, 10, 20, and 30 wt%

CNT ink + Silver flake

Silver wt.%: 5, 10, 20, 35, 50, 75, and 90 wt.%

For instance , a blend of 10 wt.% of silver nanowire in CNT was prepared by adding 7.56 ml silver nanowire to

8.5 g CNT ink

16

DOE for films drown down printed by blends of silver with CNT or graphite ink.

Draw-down printed onto PET by Byrd bar. Cured in an oven at 120 ºC for 10 min. Silver flake ink + carbon samples sintered using a NovaCentrix

PulseForge 1200 unit at 370 V, 1400 μs, and 1.5 OLF at 20

FPM (Sheet resistivity 28% less than samples cured in the oven alone)

17

Sheet resistivity measured

(Four-Point Collinear Probe)

Average thickness and roughness measured

(Bruker GT-K interferometer microscope)

Bulk resistivity and conductivity calculated.

Bruker ContourGT-K - Optical Microscope bruker.com

18

3- Multi-Layer Capacitor Fabrication

Phase I: Printing the electrode layers and screen selecting

DOE for screen printing the electrodes layer

Screen 1:Mesh count 325SS and 0.0011" wire diameter

Screen 2: mesh count 105SS and 0.003" wire diameter

19

Phase II: Printing Complete Multi-Layer Capacitors

Stack of 3 layers was printed.

A dielectric layer (ELECTRODAG PF-

455B) was printed on top of the electrode

layer.

Cured using a Fusion UV drier equipped

(passed through the UV drier 2 times)

Third layer was printed on top of the

dielectric.

The capacitance of was measured using

an E4980A Precision LCR Meter (Agilent

Technology).

Complete multilayer printed capacitor (Three layer structure)

20

4- Calendering

Nip pressures (10, 25, 30 psi). 1 pass through the

calender.

Roughness, thickness, and sheet resistivity .

Compared to the values without

calendering . Study the influence of

calendering on sheet conductivity and its surface topography.

21

Results and Discussion

1- Effect of Silver Addition to Commercial CNT Ink

Silver Nanowires

Blends Incompatible and clearly unprintable and unstable at wt.% beyond 10 wt.% silver nanowires

a- CNT blended with 5% silver nanowires ink

b- CNT blended with 20% silver nanowires ink

Compatibility of CNT with silver nanowires.

Silver nanowires will spontaneously aggregate when added to aqueous solutions with a low amount of ions.

Commercial CNT ink contained 0.5-15 wt.% water,

22

% by which Silver Nanowires addition decreases sheet and

bulk resistivity in comparasion to the control samples, CNT

80

82

84

86

88

90

92

94

96

98

100

2 5 8 10

%b

y w

hic

h s

he

et

an

d b

ulk

r

es

isti

vit

y d

ec

re

ase

s

Silver Nanowires wt.%

% of SheetResistivitychange

% of BulkResistivitychange

2 wt.% silver nanowires : Decreases sheet resistivity by ≈ 88%, and the bulk resistivity by ≈ 98%.

23

0.00

20.00

40.00

60.00

80.00

100.00

120.00

140.00

160.00

0 2 4 6 8 10 12

Sh

ee

t R

es

isti

vit

y (

Ω/□

)

Silver nanowires wt.% in ink

The effect of silver nanowire addition to CNT ink on sheet resistivity

Taking into consideration the cost and conductivity improvement optimum % silver nanowires 3 wt.%.

- High electrical conductivity of silver

- High aspect ratio (= 277.78)

connecting the isolated CNT and forming

long-range connectivity in the random

systems, according to percolation theory

24

Silver flake

0.00

20.00

40.00

60.00

80.00

100.00

120.00

140.00

160.00

0 10 20 30 40 50 60 70 80 90 100

Sh

ee

t R

es

isti

vit

y (Ω

/□)

%Novacentrix silver flake in ink

In over the range of 0-10 wt.%. Max. value of 154.1 Ω at 20% silver (0.328% higher) % by which the resistivity is improved is almost the

same beyond 50 wt.%. Optimum 35 wt.% silver flake (87.29% lower)

-5

15

35

55

75

95

5 10 20 35 50 75 90

% b

y w

hic

h s

he

et

re

sis

tiv

ity

de

cr

ea

se

s

Silver flake wt.%

25

0.000000

0.005000

0.010000

0.015000

0.020000

0 10 20 30 40 50 60 70 80 90 100

Bu

lk R

es

isti

vit

y (

Ω.c

m)

-650

-550

-450

-350

-250

-150

-50

50

150

5 10 20 35 50 75 90

% b

y w

hic

h b

ulk

re

sis

tiv

ity

d

ec

re

as

es

or

in

cr

ea

se

s

Silver flake wt.%

Max. 0.0156 Ω.cm(10 wt.%).

Improvement started at about 35 wt.% (22.8% lower)

26

• Bulk resistivity (98% lower):

75 wt.% silver flake ink = achieved by adding 2 wt.% silver nanowires.

• Cost and bulk resistivity results :

adding silver flake is not beneficial.

silver flake is micro-sized which might separate and insulate the CNTs instead of connecting them.

• At 35% resistivity starts to decrease:

might have nothing to do with percolation or connecting the CNTs

Silver flake might begin to dominate the resistivity response and the CNT began to act like an impurity

The improvement might be due high conductivity of silver.

27

2- Effect of Silver Flake Addition on Fabricated

Graphite Ink

0

50

100

150

200

250

300

350

400

0 5 10 15 20 25 30 35

Sh

ee

t r

es

isti

vit

y,

Ω/□

Silver flak wt% in ink

-5

5

15

25

35

45

55

65

75

5 10 15 20 25 30 35% b

y w

hic

h s

he

et

re

sis

tiv

ity

de

cr

ea

se

s

Silver flake wt.%

Compatible at all % No aggregation was

observed. 5 wt.% silver flake

reduced the sheet resistivity to 50.69%, compared to the 100% graphite ink.

28

0

0.1

0.2

0.3

0.4

0.5

0.6

0 5 10 15 20 25 30 35

Bu

lk r

es

isti

vit

y,

Ω.c

m

Silver flake wt.% in ink

0

10

20

30

40

50

60

70

80

5 10 15 20 25 30 35

% b

y w

hic

h b

ulk

re

sis

tiv

ity

de

cr

es

es

Silver flake wt.%

Bulk resistivity decreased with silver flake addition.

20-25 wt% increases

29

Multi-Layer Capacitor Fabrication

1- Screen selecting

Screen printed ink film thickness and roughness

Screen 1: Thickness 35 nm, roughness is 15.4 nm. Screen 2 : Thickness 155 nm, roughness was 32.7 nm.

30

0.0

100.0

200.0

300.0

400.0

500.0

600.0

700.0

800.0

900.0

1000.0

Sheet Resistivity

Ω

Screen 1, mesh count 325SS

Screen 2, mesh count 105SS

Screen 1: Sheet resistivity is ≈ 766 Ω. Screen 2: Sheet resistivity 148.4 Ω

Sheet resistivity for films printed using screen 1 and 2

31

2- Influence of Calendering on Sheet Thickness,

Resistivity, and surface topography

No influence on electrodes thickness, roughness, and sheet resistivity.

No effect on high CNT peaks.

Eliminated the short peaks.

Before calendering After calendering

32

3- Printed capacitors capacitance

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

CNT CNT CNT+Silver flake

ca

pa

cit

an

ce

, n

F

CNTS14, t=0.03microns

CNTS22, t=0.16micron

CNTNS35S22, t=0.8microns

Capacitors fabricated using a 35 wt. % silver flake and 65 wt. % CNT blend for printing the electrode layers enabled an 8.6 % increase in capacitance over CNT

electrode-based capacitors

Optimum point 3 wt.% provides

sheet resistivity ≈ 5 Ω. 72.4% higher

than the sheet resistivity of 35%

silver blind

33

Ink/Blend Sheet resistivity, Ω

Thickness % sheet resistivity

change

100% CNT 153.60 (+/- 0.874) 0.14 (+/- 0.034) -

100% Graphite 327.84 (+/- 28.784) 13.46 (+/- 4.839) -

2 wt.% silver nanowires with CNT

18.13 (+/- 1.282) 0.02 (+/- 0.003) -88.2 (+/-0.767)

3 wt.% silver nanowires with CNT

5 - -96.74

35% silver flake with CNT

19.52 (+/- 2.727) 0.89 (+/- 0.600) -87.3 (+/-1.717)

5 wt.% silver flake in graphite

161.66 (+/- 13.847) 23.54 (+/- 3.889) 50.69 (+/- 0.731)

CNT screen printed 148.44 (+/- 8.580) 0.155 (+/- 0.037) -

35 wt.% silver flake in CNT screen printed

1.78 (+/- 0.569) 1.54 (+/- 0.460) -

SWCNT (Literature) 78 0.2 -

34

Conclusion • Ink was formulated from graphite powder (327.8 Ω, 0.432

Ω.cm).

• Commercial CNT ink (153.6 Ω,0.002 Ω.cm)

• To compare the two inks:

CNT has a sheet resistivity about 53% higher than formulated ink.

bulk resistivity 99.5% higher than formulated ink.

CNT is more conductive, but it is more expensive.

• By adding 5 wt.% silver flake to Ink C, sheet resistivity was

reduced 50.7% and its bulk resistivity was decreased 12%.

35

• By adding 5 wt.% silver flake to CNT ink, sheet

resistivity was reduced 7.5% and bulk resistivity

increased by 632.6%.

• Adding silver flake to synthetic graphite ink is

efficient. But adding it to CNT ink not effeciant.

• The optimum silver % in the CNT ink was found to

be 3 wt.% silver nanowires.

• Results showed that calendering did not improve the

conductivity of the CNT printed sheets.

36

Recommendations for future studies

• Mixing nano-sized silver particles with the CNT ink

instead of using larger micro-sized silver flake particles.

• Fabricating an ink from scratch using CNT and silver

nanowire powders to avoid any aggregation and

incompatibility issues.

• Finally, print a stacked capacitor with more than three

layers to increase overall capacitance.

37

Acknowledgements

• Dr. Margaret Joyce, professor, supervisor.

• Dr. Paul D. Fleming III, professor, committee member.

• Dr. Thomas Joyce, professor, committee member.

• Colleagues: Bhushan Hiralal Pati, Veronika Husovska,

James Atkinson, Michael Joyce, Ali Eshkeiti.

38