Effect of exfoliation on the electrical resistivity of intercalated graphite

Synthetic

Metals, 12 (1985)

533-538

EFFECT OF EXFOLIATION

533

ON THE ELECTRICAL

RESISTIVITY -__

OF INTERCALATED

GRAPHITE

D. D. L. CHUNG and LAN W. WONG Department of Metallurgical Engineering and Materials University, Pittsburgh, PA 15213 (U.S.A.)

Science,

Carnegie-Mellon

ABSTRACT Exfoliation of graphite-bromine was found to increase the a-axis electrical resistivity (p ) by a factor of 230 and decrease the c-axis electrical resistivity (p 1 by a factor of 10, so that the anisotropy in the electrical resistivity (p /p ) decreases by a factor of 5 X 104. During heating of graphite-bromine, p, first ‘in&eased slightly and then decreased abruptly upon exfoliation. During subsequent cooling, ,Q increased abruptly upon collapse of the exfoliated structure. The c-axis electrical res?stance did not vary much with temperature upon exfoliation or collapse.

INTRODUCTION The has

low

electrical

been

of

resistivity

scientific

temperature,

intercalated

times

the

the

along heating

heating

is

concern

to

therefore

the

the

Miyauchi (based

back

to

In

the

pyrolytic axis

[43

the

about value

This

a-axis

al.

of

artificial

past

cooling.

the

subject

and

0319-6779/85/$3.30

this

that

the

200°C.

Upon

was work,

heating,

such

have

electrical

was

to

a-axis at

The

as

dozens

to

be

a

hundreds

reversible;

effect

critical

[33).

of

when

graphite

electrical

of

When the

reversible

intercalated

an

addition,

electrical 3OOO’C)

‘the

and is

conductor.

of It

is

that

the

resistivity was

of

increased

a-axis

electrical

decrease

started

graphite-bromine by

22-25%

resistivity at

about

upon

decreased 320°C

during

exfoliation.

investigated

resistivity In

(HOPG).

resistivity

can

of

graphite

cooling,

attributed we

by

directions

past

mechanically

expansion

resistivity

in-plane

paper.

reported

before

the heating

constrained

irreversible.

intercalated

heat-treated

c-axis

graphite

electrical

of

is

the

electrical

graphite

effect

present

the

along Upon

expanding

is

manner,

expansion

on

application

et

on

heating

the

exfoliation

[1,21,

expansion

a controlled

excessive,

irreversible

the

graphite interest.

exfoliates

(unless

in

intercalated

technological

graphite

c-axis

occurs

of

and

of the

the

effect

of

irreversible

graphite-bromine effect

of

reversible

based

exfoliation on

highly

exfoliation

on

oriented on

the

c-

studied.

0 ElsevierSequoia/Printedin The Netherlands

534 EXPERIMENTAL

Sample preparation Highly Carbide

oriented Corporation,

vapor

at

room

temperature

to

The

measuring

the

of

outer was

To

the

and

28

sample

Also and

was

is

Graphite

of

the

Foam

c-axis

to

in

bromine

air

at

room

furnace

heating.

a

almost

c-axis,

top

on

each

on each

surface

contacts

were

24

on

voltage

12

A

For of

contact

as

V,

the

sample

two

on

each

served made

as a on

probes,

DC

the

while

variable

power

to

current

along

the

measured

and

recorded

the

to

0.006

by

the

c-axis)

c-axis)

The

c-axis,

the

by

using

by

of

the The

was

inside

The

probe

were

voltage

0.136

across

temperature chart

small

(LVDT).

which sample.

the

in.

a

measured

transducer

of

(perpendicular

sample

and

air was

thermocouple

surface

was

in

the

measured

were

expansion

differential

top

sample

heated

The

resistivity,

c-axis

(perpendicular

variable

the

electrical

the

was

sample.

surface

c-axis

along

a chromel-alumel

the

heated

made

Silver

sample.

a

served

the

the

the

sample;

four

A

method.

to

were

contact

probes.

linear

of

four-probe wires

the

contacts

the

by

of

thickness

of

expansion

touched

dimensions

the

and

recorders,

X the the

while

cooled.

results exfoliation

intercalation HOPG

[51,

exfoilated

Intercalation on

exposure

desorb

Union

96 bromine. by

contacts

exfoliation the

the to

repeatedly

included

Grafoil

ZYX

by

probes.

surrounded

simultaneously

effects

resistivity

provided

2 by to

contact

resistivity,

graphite-bromine

Effect of irreversible The

out

using

other

inner

current

measured

it

the

were

Experimental

as

touched

initial

a-axis

and

which

was

g. The

along

sample

carried

c-axis) the

two

current

connected

that

The

expansion the

was

such

in.

40 wt.

electrical

reversible

the

which

temperature

0.100

the of

coil,

sample

weighed

to

while

probe

probe

the

resistivity

sample the

kindly

stage

about

copper

the

while

the

served

effect

heater

a

to

allowed

by

the

to

probe,

sample;

contacts

simultaneously

using

two

measuring

the

electrical

resistance

resistivity,

connected

study

c-axis

attach

(perpendicular

For

probe.

supply

was

was

was

measured

to

as a current

surface two

of

graphite

was

used

c-axis

served

voltage

it

concentration

intercalated

was

surfaces

surface

bromine

Subsequently

resistivity

paint

opposite

with

which

technique

electrical

conducting

same

the

(HOPG),

intercalated

an intercalate of

Experimental

was

graphite

temperature.

Exfoliation

the

pyrolytic

are

and

shown

in the

table

which

are

HOPG;

is like greatly resistivity.

in Table are

the

is

except

decreased After

the

for

values

available

exfoliated

the

on

a-axis

and

ZYX

graphite,

c-axis

electrical

1. published

commercially

Grafoil

Grafoil

exfoliation

a less a-axis

intercalation,

for from

natural severe electrical exfoliation

Union crystals

Graphite

Carbide rolled

Foam

Corporation. into

a

sheet;

compression. resistivity increased

but the

had

little

a-axis

effect

electrical

5.35

Electrical

Table I.

Rdsistivities

(in R.cm) at Rbom Temperature.

, Pristine

This work

Intercalated(Br)HOPG Exfoliated(Br)HOPG

From Ref. 5

ZYX graphite Graphite Foam Grafoil

resistivity so

HOPG

by

that

the

104.

a factor

of

anisotropy

After

230

in the

1.6 X IO-4 6.8 X 10-l 2.4 X 10-6 6.3 X 10-l 5.4 X 10-4 3.0 X 10e2

4.4 X lo3 2.6 X 105 5.5 X lo1

8 X 1O-4 I 6X1O-35.5X1O-2 1.1 x 10-3 1.1 x 10-l

I 9 1.0 x lo2

and

decreased

electrical

the

exfoliation,

%lP,

PC

pa

Material

the

resistivity

a-axis

resistivity

c-axis

resistivity

by

(p,/p,)

decreased

by

is

even

higher

a factor

than

of

20,

a factor

of

that

pristine

of

5 X

graphite. Our Our is

value

value in

p,

of

p,

of

exfoliated

for

turn

probably

lower due

the

graphite graphite

that

of

cracking

exfoliation variation

shows

expansion

at

heating.

sample

Figure

cooling,

along

resistivity

also

at

the

at

during 130°C,

cycles,

irreversible

decrease more

Although by

the

a factor

appreciable

of

the

heating

expansion

after

going

reversible

than

c-axis 20

dependence

electrical in Cycle on

of

ZYX

Graphite that

electrical

graphite

Foam

rolling

[51,

[51. which

increases

p,,

(pc)

the

resistivity

thickness No. the

began

of

c-axis.

Cycle

whereas

of

the

along

1.

The

the

c-axis

expansion

increasing

c-axis

temperature at

heating. while

the

change

was

with

upon

the

c-axis

in

that

that

implies

thickness/initial

cooling

to

c-axis)

resistivity

began

at

and

at

14O’C.

16O’C

while

with began

the

during collapse

14O’C.

decreasing

increased both

at

than this

to

resistivity

variation

c-axis

began

appreciably

In

the

110°C

resistivity

also

shows

lower

the

heating,

the

comparable

[53;

of

in

and

during

began

2

expansion

heating

14O’C

During

the

(change

during

decreasing

The

the

is

perpendicular

Figure

1

is

Grafoil

Effect of reversible

fractional

and

exfoliated

than

to

temperature

of

for

during

11O’C

during

During collapse before was

No.

2, the

and while

also

large

Cycle

cooling the

resistivity

at

the

In

fractional

in Cycle expansion began

13O’C.

However,

the

1.

In

Cycle

by

a factor

NO. No.

No.

2.

began

increasing

both

cycles,

the

resistivity No.

2,

showed the

an

resistivity

1.

changed c-axis

and

decrease.

reversible.

resistivity

heating

the

began

in Cycle

resistivity

heating,

cooling,

the

through that

electrical

electrical

of

resistance

40 did

in

Cycle

not

show

NO. any

temperature.

DISCUSSION The (Table

resistivity 1) are

changes

consistent

with

for one

reversible another.

(Figs.

1

and

2)

and

irreversible

exfoliation

1

536 I

I

Cycle No. I

/

r

-A-

TEMPERATURE

-A-,

(“C)

Fig. 1. Variation of the c-axis electrical resistivity (p ) [circles] and fractional expansion (change in thickness/initial thCickness along the caxis) [triangles] with temperature during heating (full lines and solid symbols) and cooling (dashed lines and open symbols) in Cycle No. 1.

The the

increase

bending

much

more

to

decrease

previously

of

No.

2.

graphite-bromine

the

exfoliation

the

increase

Miyauchi

makes

in Cycle

et

used

is

by

This

of

et

easier by

1, 11O’C

to

230)

is

difference

Miyauchi

observed No.

attributed

(a factor al.

exfoliation

temperature

(160°C

al.,

and

Miyauchi

in Cycle

is as a more

et No.

al.

2) is

reason.

in the

c-axis

electrical

the

graphite

layers.

for

to

material

microstructure onset

of by

graphite

observed

due

magnitude

1.2) reported

expansion we

reported

temperature

Cycle

that

this

of

planar

high

to

of

quality

resistivity

The

layers.

and The

electrical

(a factor

poorer

1I.

bending

The

for

[

attributed

the

than

the

a-axis

graphite that

compared

The

been

the

well-oriented

(200°C)

to

the

than

to

extensive

also

of

greater

attributed

in

the

resistivity X-ray

due

to

diffraction

exfoliation evidence

is also

attributed

for

the

bending

higher

for

Cycle

had

[ 1 I. the

These based

onset

of

exfoliation

temperatures on HOPG

are [2].

expansion

consistent

with

was

those

previously

No.

reported

1

537

6

-0

-0. 5

-0 4

z c:

-0. 3-

QY

-0. 2

-0. I

I0 TEMPERATURE Pig. 2. Variation

of

the

c-axis

(“C) electrical

resistivity

(p

)

[circles]

and

the

fractional expansion (change in thickness/initial thicknesl along the caxis) [triangles] with temperature during heating (full lines and solid symbols) and cooling (dashed lines and open symbols) in Cycle No. 2.

In

Cycle

heating

at

of

1,

a lower

resistivity scale

No.

the

intercalate.

electrical

temperature

is sensitive

expansion.

c-axis

to

Such The

than

the

small-scale

small-scale

buckling

of

resistivity start

of

structural structural

these

started the

large

expansion.

changes changes

intercalate

its

which

include

pockets

decrease

This occur

the

occurs

is

upon

because

before

the

the large-

formation

of

during

large-scale

the

pockets

expansion. The

increase

decrease

is

intercalate The Cycle bending

the

attributed

c-axis

to

the

electrical formation

resistivity of

upon

microcracks,

heating

which

later

before

the

evolve

large

into

the

pockets. resistivity

No.

change

2 was

of

Although heating,

in

this

the

much

graphite

the effect

observed more layers

c-axis cannot

that

electrical be

in Cycle

reversible.

used

No. This

occurred

in Cycle

resistivity for

1 was is

electrical

not

attributed No.

changed

reversible, to

the

whereas slight

that

in

irreversible

1. abruptly

switching.

This

and

reversibly

is because

the

upon c-axis

538 electrical

resistivity

resistance

did

resistivity the

as well

change

activated

decreased

not

of

vary

as resistance

a-axis

electrical

while

much

the

with

c-axis

thickness

temperature.

increased

quite

with

temperature

resistance

increased,

On abruptly

the with

may

be

so

other

that

hand,

temperature exploited

the the

[4], for

c-axis a-axis so that

thermally

switching.

ACKNOWLEDGMENT Technical

assistance

by

John

Knoblich

of

Carnegie-Mellon

appreciated.

REFERENCES 1

S. H. Anderson

and

D. D. L. Chung,

Synth.

Met.

2

S. H. Anderson

and

D. D. L. Chung,

Carbon

22,

3

D. D. L. Chung

4

K. Miyauchi,

Y. Takahaski

5

C. Uher

L. M. Sander,

and

and Lan W.

Wong, and

to

Rev.

253

(1983).

(1984).

be published.

T. Mukaibo,

Phys.

8, 343

B 27,

Carbon 1326

9, 807 (1983).

(1971).

University

is

greatly