Pierson Hugh O. Handbook of carbon, graphite, diamond and fullerene. New Jersey / carbon

186

Carbon,

Graphite,

Diamond,

and

Fullerenes

3.4

Structure

of Mesophase-Pitch

Carbon

Fibers

Cross

Section.

The

cross-section

structure of

mesophase-pitch

carbon

fibers

is one

of the four

types

shown in Fig. 8.12

and is determined

by

the

spinning method,

the

temperature

of

stabilization, and the

partial

pressure of

oxygen. t2)t19) The

formation

of a skin-core

structure

or skin

effect

is often

observed.

This

structure is similar to that

of the PAN-based

carbon

fiber

shown

in Fig. 8.8

above.

Radial

Onion-Skin

Onion-Skin

Onion-Skin

and Center-

and Center

Radial

Random

Interlayer

Spacing

and

Crystallite

Size.

The

increase

of

the

apparent

crystallite

size

of

mesophase-pitch

fibers

as

a function

of heat-

treatment

is shown

in Fig. 8.7,

and

the decrease

of the

interlayer

spacing

(c spacing)

in Fig.8.9.p)

Th e spacing

of the

heat-treated

fiber

is relatively

close to that of the ideal graphite

crystal

(-

0.340

nm

vs.

0.3355

nm),

indicating

a decreased

turbostratic

stacking

of the

basal

planes

and

a well-

ordered

structure.

in this

respect,

the

difference

between

these

fibers and

the

PAN-based

fibers is pronounced.

The

large

crystaiiites

of the

heat-treated

pitch-based

fibers,

which

is

structurally

close to the

perfect

graphite

crystal

and

well

aligned

along

the

fiber

axis,

offer

few

scattering

sites

for

phonons.

This

means

that

these

fibers

have

a high

thermal

conductivity

along

the

fiber

axis

since,

as

mentioned

in Ch. 3, Sec. 4.3, the transfer

of heat

in a graphite

crystal occurs

mostly

by

lattice

vibration

(see

Sec.

6.6

below).

Carbon

Fibers

187

However,

this

high

degree of crystallinity

also

results

in low shear

and

compressive

strengths.

In addition,

these carbon fibers tend to have flaws

such as pits, scratches,

striations,

and flutes.

These flaws

are detrimental

to tensile

properties

but do not essentially

affect

the

modulus and

the

thermal

conductivity.t16)

Rayon-based

fibers were the first carbon

fibers

produced

commer-

cially. They were

developed

in the

1960’s

specifically

for the reinforcement

of ablative

componentsfor rockets

and missiles.

However, they are difficult

to process

into high-strength,

high-modulus

fibers and have been

replaced

in most structural

applications

by

PAN or pitch-based

fibers.

A

number of rayon fibers are available, but

the most

suitable is the

highly

polymerized viscose

rayon.

The molecular

structure

is asfollows:tll)

OH

OH

CH-0

CH-0

-CH’

\

\

I

CH-0-CH’

CH-0

\

\

CH-CH

CH-dH

As can be seen, this structure

has many heteroatoms

(0 and H) which

must

be

removed.

Moreover, many carbon atoms are

lost

due

to the

formation

of volatile

carbon oxides

during pyrolysis. As a result

the

carbon

yield

is low (~30%)

and shrinkage

is high.

The rayon

precursor

is first heated to 400°C at the

relatively slow

rate

of 10°C per hour.

During

this step, the fiber is stabilized;

H,O is formed

from

188

Carbon,

Graphite,

Diamond,

and

Fullerenes

the hydroxyl

groups

in the

molecule

and

the

fiber

depolymerizes

with

the

evolution

of CO and

CO,

and the formation

of volatile,

tar-like

compounds.

This

depolymerization

makes

it impossible

to stretch the fiber

at this stage.

The

heating

rate can be increased

and the carbon

yield

maximized

by

heating

the

fiber

in

a

reactive

atmosphere

such

as

air

or chlorine,

or

impregnating

it with

flame

retardants

and

carbonization

promoters.t11l

Carbonization

is

the

next

step

and

is carried

out

in

an

inert

atmosphere

at a temperature

range

of

1000

- 1500°C.

At this

stage,

the

fiber

has

a low

modulus

(35 GPa,

5 Msi),

a low tensile

strength

and

a low

density

(1.3

g/cm3).

The

degree

of preferred

orientation

can

be considerably

increased

by

stretching

the carbonized

fiber

at very

high

temperature

(-

2700

- 28OO”C),

resulting

in a high-strength

and

high-modulus fiber.

However,

this

requires

complicated

and

expensive

equipment,

the

process

is costly,

and the

yield

of unbroken

fiber

is low.

For these

reasons,

stretched

fibers

are no longer

produced

since

they

cannot

compete

with

lower-cost, PAN-based fibers. As

mentioned

in Sec. 2.2 above,

PAN

is stretched

prior to carbonization,

which

is a considerably

cheaper

and

more

reliable

than

stretching

at very

high

temperature.

The

low-modulus

rayon-based

fibers

are the only ones now produced

in the form

of carbon

cloth

or felt

(Thornel

WCA, VCL, VCK, and VCX from

Amoco

Performance

Products).

Primary

uses

are

in carbon-carbon

com-

posites and high-temperature

insulation.

5.0

CARBON

FIBERS

FROM VAPOR-PHASE

(CVD)

REACTION

The

direct

growth

of

carbon

fibers

from the vapor-phase

has

been

investigated

for

a number

of

years

and

the

potential

for

producing

a

economically

viable

material

with

properties

matching

those

of

existing

PAN

or pitch-based

fibers appears good.f20)-f231

Vapor-phase

fibers

are

produced

by the catalytic

decomposition

of a

hydrocarbon

such as

methane

or

benzene.

The

seed

catalysts

are

iron

particles

or iron metallo-organics

such

as ferrocene,

(C,H,),Fe.

Growth

occurs

in the

temperature

range

of 1000

- 1150°C.

Thefibersstill

have

a large

spread

in theirtensile

strength

(3000 - 8000

MPa).

However,

the higher values

compare

favorably

with

those

of high-

strength

PAN-based

fibers

(see

following

section).

Carbon

Fibers

189

Vapor-phase

fibers

are only

produced

in short

lengths

at the

present

time.

Maximum

reported

length

is 50 mm with diameters

from 0.5

- 2 pm.

Such

short-length

fibers

would be suitable

for the random

reinforcement of

composites and

in the production

of carbon-carbon

(see

Ch. 9).

Carbon fibers

are produced

as a multifilament

bundle known

as a tow.

The number of filaments

pertow

is500,

1,000,

3,000,

6,OOOor 12,000. The

smaller

tow

sizes

are usually

reserved

for weaving

and braiding

while

the

larger ones

are for unidirectional

tape winding.

Both

are used primarily

in

aerospace

applications.

Still

largertows,

with filament

count up to 320,000,

are also

produced

but

mainly

for less-demanding

applications

such

as

sporting

goods (see Ch.

9).t8)

Carbon

fibers

are difficult to test due to theiranisotropic

structure,

their

brittleness,

the variation

in their

diameter,

and the

need

to mold them

in an

epoxy

matrix to be

able

to measure

some properties.

Furthermore,

the

strength

is dependent

to some degree

on the length

and diameter

of the test

specimen

and on the testing techniques.

As a rule, the longerthe

specimen

and

the

larger

the

fiber

diameter,

the

lower

the results,

as

shown in Fig.

8.13.t’)

This

is due

to the

greater

chance

of having

structural

defects

in the

larger specimens.

These

factors

must

always

be considered

when comparing

properties

from

various

groups

of fibers and

the

data shown

in the

following

sections

is to

be viewed

with

this

in mind.t24)

PAN-based carbon

fibers are heat-treated

to various

degrees

of

structural

re-ordering.

This determines the

final

strength

and

modulus

of

elasticity.

The

fibers

are commonly divided

into the three

following classes

based on the

value of the

modulus:

n

Standard-modulus

fibers

m Intermediate-modulus

fibers

(also

known

as Type II)

= High-modulus

fibers

(also known

as Type

I)

Their

properties are

summarized

in Table 8.6.

I

I

I

I

I

I

2

8000

AS-4

(Hercules)

B

5”

6000

F

5

4000

Q)

=

2

2000

C

0

2

4

6

8

10

12

Fiber

Diameter,pm

6000

1

I

I

I

I 1

g

5000

-

T 800

(Torayca)

\-

g

4000

-

-

‘4

F

3000

-

T 300 (Torayca)

2

f

2000

-

2

1000

-

fi

01

I

I

I

1

0

5

10

20

30

Gauge

Length,

nm

Standard

(Type11)

(Type1)

Intermediate

High

Modulus

Modulus

Modulus

Modulus,

GPA

205

- 235

275-

310

345

- 550

Msi

30

- 34

40 - 45

50 - 80

Tensile

strength, MPa

3450

- 4650

4350

- 6900

1860

- 4140

ksi

500

- 675

630 - 1000

270

- 600

Tensile

strain, %

1.4-

1.6

1.6

- 2.2

0.81

- 0.9

Density,

g/cm3

1.76

- 1.79

1.76

- 1.79

1.87

Properties

of

selected commercially

available

fibers are

shown in

Table

8.7.

The

data

are obtained

from

suppliers’

technical

brochures.

As a rule, higher-modulus fibers

have lowertensilestrength

and tensile

strain

(elongation).

The

compressive-failure

strain

is

dependent

on

the

modulus;

it increases

with

decreasing

modulus.t25]

The

failure

occurs

by

kinking

or microbuckling.

This

tendency

is shown

in both

PAN-

and

pitch-

based

fibers. The mean-failure

strain for PAN-based

fibers

is 2.11 - 2.66%.

It is much

lower

for

pitch-based

fibers: 0.24

- 0.98%;

this may be the result

of the

greater

suceptibility

of

these fibers

to defects and

handling.

The

testing

procedure

is described

in

Ref. 25.

Carbon

fibers based on isotropic pitch have low strength

and

modulus

with

tensile

strength

averaging

870 - 970 MPa and modulus

of 40 - 55 GPa

(Data from Carboflex,

Ashland

Oil Co., Ashland, KY, and Kureha

Chemicals,

Japan).

Mesophase

pitch-based

carbon

fibers

generally have

the

highest

stiffness

of all carbon

fibers

with

modulus

of elasticity

up to

965 GPa (140

Msi),

considerably

higher

than

PAN-based

fibers.

The tensile

strength

however

is much

lower, averaging only

half.

As mentioned

in the

previous

section,

the

compressive-failure

strain

is

low.

Tensile

strength

Modulus

Product

Mpa

ksi

GPa

Msi

Standard

Modulus

AS-4

(1)

3930

570

248

36

Celion G30-500 (2)

Thornel T-300 (3)

3650

530

230

33

Torayca T-300 (4)

3525

512

230

33

Grafil 33-650 (5)

4480

650

Intermediate

Modulus

(Tvpe

II)

IM-6

(1)

4340

630

275

40

IM-7

(1)

5030

730

275

40

IM-8

(1)

5860

850

275

40

IM-9

(1)

6890

1000

275

40

Celion

G40

- 600

(2)

4140

600

Celion

G40

- 700

(2)

4820

700

Celion

G40

- 800

(2)

5510

800

Thornel T-650/42 (3)

4820

700

298

42

Hitex 46-8 (6)

5760

825

296

43

Hiah Modulus (Tvpe

I)

UHM

(1)

4140

600

448

65

Celion GY-70 (2)

1860

270

520

75

Torayca

M60

(4)

2410

350

550

80

(1) Product

of Hercules, Magma,

UT

(2) Product

of BASF,

Germany, and Charlotte, NC

(3) Product

of Amoco

Performance

Products,

Atlanta,

GA

(4) Product

of Toray,

Japan

(5) Product

of Grafil,

Sacramento,

CA

(6) Product

of BP Chemicals,

UK,

and Santa

Ana, CA

Carbon Fibers 193

Table 8.8. Summary

of Physical Properties of Mesophase Pitch-Based

Carbon Fibers

Tensile

modulus,

GPa

380 - 827

Msi

55-

120

Tensile

strength,

MPa

1900

- 2370

ksi

274

- 350

Tensile

strain, %

0.25

- 0.5

Density,

g/cm3

2.0

- 2.18

800

1200

1600

2OW

2400

2800

3200

Heat-Treatment Temperature, “C

800

I

I

I

I

1

m

8 .

600

-

s

:

400

-

S

ifiii

200

-

s

I

I

I

I

I

O-

800

1200

1600

2060

2400

2800

3200

194

Carbon,

Graphite,

Diamond,

and Fullerenes

6.5

Properties

of Rayon-Based

Carbon

Fibers

The properties

of stretched-graphitized

rayon-based

carbon fibers are

shown

in Table

8.9.

The

data

is to

be considered

for

its

historical

value,

since

the material

is no longer

produced

commercially

(data from

Union

Carbide

Corp.)

Table

8.9. Summary

of Physical

Properties

of Rayon-Based

Carbon

Fibers

Tensile

modulus,

GPa

173-520

Msi

25

- 75

Tensile

strength,

MPa

1200

- 2650

ksi

180-

385

Density, g/cm3

1.40

- 1.80

Thermal

Conductivity.

As

mentioned

above, the

fibers

with

the

highest

degree

of

orientation

such as

the

pitch-based

fibers

have

the

highest

thermal

conductivity.

As

shown

in

Table

8.10,

their

conductivity

along

the

axis

is higher

than

even

the

best

metal

conductor.

PAN-based

fibers,

on

the other

hand,

have

much

lower

conductivity

because

of their

more

pronounced

isotropic

structure.

Thermal

Expansion.

The

thermal

expansion

of

carbon

fibers,

measured

along

the axis,

is extremely

low

and

similar

to

that

of

pyrolytic

graphite in the

abdirection,

i.e.,

slightly

negative

at room temperature

and

slowly

increasing

with

increasing

temperature

(see Ch. 7, Fig.

7.11).

The

thermal

coefficient

of expansion

(CTE)

at room

temperature

is as follows

(data

from

Amoco

Performance

Products).

PAN-based

fibers:

-0.6 to -1 .l

m/m.K

x 1Oe6

Pitch-based

fibers:

-1.3

to -1.45 m/m-K

x 1Oe6