H.O. Pierson. Handbook of carbon, graphite, diamond and fullerenes. Properties, processing and applications. 1993

66

Carbon,

Graphite,

Diamond,

and

Fuilerenes

Melts

Acid salts

(AICI,,

H,BO,,

FeCi,,

PCL,,

ZnCI,)

B

Alkaline

salts

(Ba(OH)*,

LiOH,

KCN,

soda

ash)

B

Metals

(Al, Sb, Babbit,

brass,

Cu, Ga,

Au,

Mg,

Hg,

Ag,

Sn,

Zn)

A

Neutral

salts (KCI, Na.$OJ

A

Oxidizing

salts (sodium

nitrate)

B

Salt

solutions,

neutral

(baking

soda,

KCr(SO,,),,

CuSO,,

Mg(SO,&,

KCL,

sea water,

sewage,

Na2SO.J

A

Solvents

Aliphatic

(butadiene,

butane,

butylene,

cyclohexane,

fuel

oil,

gasoline,

lubric

oil,

propane,

propylene)

A

Aromatic

(benzene,

coal

tar,

creosote,

cumene,

naphtaiene,

petroleum,

styrene,

urethane)

A

Chlorinated,

fluorinated

(Ccl,,

chlorobenzene,

freons,

chloroform,

methyl

chloride,

vinyl

chloride)

A

Oxygenated,

sulfide

(acetaldehyde,

acrolein,

butyl

acetate,

butyl

alcohol,

CS,,

rayon, ether, ethyl

acetate,

futfural,

glycerine,

methanol,

ketones,

sorbitoi,

vinyl

acetate)

A

(A=high,

B=medium,

C=low)

The

oxidation

of

graphite

and

the available

protective

coatings

are

reviewed

in Ch. 9. The

controlled

oxidation

of graphite,

known

as activation,

results

in open

structures

with

extremely

high

surface

area

(see Ch. 5, Sec.

3.0).

Graphite

does

not

react

with hydrogen

at ordinary

temperatures.

It

reacts

in the

1000

- 1500°C

range

to form

methane

(CH,).

The reaction

is

accelerated

in the

presence

of a platinum

catalyst.

With

nickel

catalyst,

the

reaction

begins

at approximately

500”C.[25]

7.4Reaction with Metals

Graphite	reacts with	metals that form carbides	readily such	as the
metal of groups	IV, V and	Vl.[121[251Th ese carbides	are the so-called	hard

Graphite

Structure

and Properties

67

carbides, which include the carbides

of

tungsten,

molybdenum,

titanium,

vanadium

and tantalum,

as well asthe non-metal

carbides ofsilicon

and boron.

Graphite

reacts with

iron to

form

iron carbide,

Fe&,

usually

by

the

direct

solution

of carbon in the

molten iron.

Iron carbide

may also

be formed

from the

reaction

of iron

with

a carbon-containing

gas.

This

reaction

is

known

as case-hardening.

The

reaction

rate of graphite

with

the

precious

metals,

aluminum, and

the Ill-V and II-VI semiconductor

compounds

is low

and graphite

is used

successfully

as

a crucible

to melt

these materials.

Graphite

reacts

readily

with

the

alkali

metals:

potassium,

calcium,

strontium,

and

barium.

The

atoms

of

some of

these

metals,

notably

potassium,

can

readily

penetrate

between

the

basal

planes

of the graphite

crystal

to form intercalated

(or lamellar

compounds)

with useful

properties.

These

compounds

are reviewed

in Ch. 10, Sec. 3.0.

7.5Reaction with Halogens, Acids, and Alkalis

Like

the

alkali

metals,

some halogens,

particularly

fluorine,

form

intercalated

compounds

with

graphite

crystals.

Reaction

usually

starts at

600°C.

However,

graphite

does

not

react with chlorine

at temperatures

below

that

of the

electric

arc.

Oxidizing

acids

attack

graphite

to varying

degree

depending

on the

nature

and

surface

area

of the

material.

The

reaction

with concentrated

nitric

acid

is as

follows:

C + 4HN0,

*

2H,O

+ 4N0,

+ CO,

Depending

on the

reaction

conditions,

other products

may

be formed

such

as graphitic

oxide

&H,O,),

mellitic

acid

(C,(CO,H),)

and

hydrocya-

nit acid (HCN).r)

Another

oxidizing

acid that

attacks

graphite

is boiling

sulfuric

acid.

The

simplified

reaction is the

following:

C + 2H,SO,

--,

CO,

+ 2H,O

+ 2S0,

Other

by-products

may

be formed

such as benzoic

acid,

C,H,CO,H,

and

mellitic

acid,

C,(CO,H),.

Hydrofluoric

acid (HF) and the alkali hydroxides generally

do not react

with

graphite.

66 Carbon, Graphite, Diamond, and Fullerenes

REFERENCES

1.“International Committee for Characterization and Terminology of Carbon,” Carbon, 28(5):445-449 (1990)

2.	Bokros, J. C., in ChemisfryandPhysicsofCarbon,	(P. L. Walker, Jr.,
	ed.), Vol. 5, Marcel Dekker Inc., New York (1969)

3.Kohl, W. H ., Handbook of Materials and Techniques for Vacuum

	Devices,	Reinhold		Publishing, New		York (1967)
4.	Palmer, H. B. and Shelef,				M., in ChemistryandPhysicsofCarbon,		(P.
	L. Walker,	Jr.,	ed.),	Vol.	4, Marcel Dekker Inc., New York (1968)
5.	Gustafson,	P.,	Carbon,		24(2)169-176	(1986)

6.Storms, E. K., The Refractory Carbides, Academic Press, New York (1968)

7.	Mantell, C. L., CarbonandGraphiteHandbook,							IntersciencePublishers,
	New York,		(1968)
8.	Perry’s	ChemicalEngineering				Handbook, 6th ed., McGraw-Hill,					New
	York (1984)
9.	Wehr,	M. FL, Richards,			J. A. Jr., and Adair,			T. W. Ill,	Physics		of the
	Atom,	3rd ed., Addison-Wesley				Publishing,		Reading,	MA	(1978)
10.	Chart	of the Atoms,		Sargent-Welch			Scientific	Co., Skokie,		IL (1982)
Il.	Eggers, D. F., Gregory,				N. W., Halsey, G. D., Jr., and				Rabinovitch, B.
	S., Physical		Chemistry,		John	Wiley	& Sons,	New York (1964)
12.	Fitzer,	E., Carbon,		25(2):1633-190			(1987)

13.Graphite, Refractory Material, Bulletin from Le Carbone-Lorraine, Gennevilliers 92231, France

14. Campbell, I. E. and Sherwood, E. M.,	High-Temperature	Materials
and Technology, John Wiley & Sons,	New York (1967)

15.Van Vlack, L. H., Elements of Materials Science and Engineering,

Addison-Wesley Publishing Co., Reading, MA (1980)

16.	Sze,	S. M.,		Semiconductor		Physics		and Technology,	John	Wiley &
	Sons,	New		York (1985)
17.	Fitzer,		E., Carbon, 27(5):621-645				(1989)
18.	Mullendore,			A. M., Sandia		Park	NM,	Private Communication		(1992);
	Nelson,		and	Riley,	Proc.	Phys.	Sot.,	London 57:477	(1945)
19.	Murray,		R. L. andCobb, G. C., Physics, ConceptsandConsequences,
	Prentice		Hall Inc.,		Englewood Cliffs,			NJ (1970)

Graphite

Structure

and Properties

69

20.

Spain,

I. L., in ChemistryandPhysicsofCarbon,

(P. L. Walker

and P.

A. Thrower,

eds.),

Vol. 8, Marcel

Dekker inc., New York

(1973)

21.

Walker,

P. L., Jr.,

Carbon, 28(3-4):261-279

(1990)

22.

Corrosion/Chemica/Compatibi/i~

Tab/es, Bulletin

of the Pure Carbon

Co., St. Marys,

PA

(1984)

23.

Hippo,

E. J., Murdie, N., and Hyjaze,

A.,

Carbon, 27(5):689-695

(1989)

24.

Yavorsky,

I. A. and Maianov,

M. D., Carbon, 7:287-291

(1989)

25.

Carbon/Graphite

Properties,

Bulletin

from

The

Stackpole Carbon

Co., St. Marys,

PA

(1987)

Synthetic Carbon and Graphite:

Carbonization and Graphitization

1.0TYPES OF SYNTHETIC CARBON AND GRAPHITE

Chapters

2 and

3 were

a review

of the carbon

atom

and

its bonding

mechanismsand

howthese

atoms combine

to form graphite

crystals.

In this

and the

next

six

chapters,

the focus

will be on how large numbers of these

crystallites are

combined

to

form

synthetic

(and

natural)

carbon

and

graphite

products.

Thevarioustypes

of synthetic

materials

will

be reviewed

including

their

production

processes,

their

properties

and

characteristics,

and their

present and

potential

applications.

In terms

of size,

the

review

proceeds

from

the

size of a single

carbon

atom, to that

of a graphite

crystal,

composed

of thousands

of atoms,

to that

of a graphite

product

such

as an electrode,

which

may weigh

hundreds of

kilograms.

Natural

graphite,

which

is found

in abundance

in many

areas

of the

world, has been

used

since historical

times,

but its applications

always

were

(and still

are)

limited

(see Ch.

10). In the

last century,

the advent

of synthetic

graphite

and

carbon has considerably

increased

the scope

of applications,

although

natural

graphite

still

remains

the

material

of choice

in a few cases.

A large

majority

of

graphite

and

carbon

products

are now synthetic

and

these products

are

continuously

being

improved

and

upgraded.

70

Carbonization

and Graphitization

71

1.l

Synthetic

Graphite

and

Carbon

Products

The carbon terminology was reviewed

in Ch. 3, Sec. 1.I,

and its proper

use

is

important

as

confusion

can easily

prevail

because

of the

many

variations of graphite

and

carbon products.

The

synthetic

graphite

and

carbon

products

can

be divided

into six

major

categories:

1. Molded graphite and carbon (Ch. 5)

2. Viireous (glassy) carbon (Ch. 6)

3. Pyrolytic graphite and carbon (Ch. 7)

4. Carbon fibers (Ch. 8)

5.

Carbon

composites

and

carbon-carbon

(Ch.

9)

6.

Carbon

and

graphite

powders

and

particles

(Ch.

10)

These

divisions

may appear

arbitrary

and overlapping

in some

cases;

for instance,

fibers

and

carbon-carbon

are generally

polymeric

carbons,

although

pyrolytic graphite is often used in their

processing.

These

divisions

however

correspond

to

specific

and

unique

processes,

with

resulting

products that

may have

different

properties.

These

variations

in properties,

as stated

in

Ch.

3

(Sec. 2.1),

stem

from

the

nature

of the

polycrystalline

aggregates,

their

different

crystallite

sizes

and

orientation,

various

degrees

of porosity and

purity,

and

other

characteristics.

1.2

General

Characteristics

of Synthetic

Graphite

and

Carbon

Many

new

graphite-

and carbon-materials

with

improved characteris-

tics

have

been

developed

in the last two decades.

Some

of these

materials

have a strongly

anisotropic

structure

and

properties

that

approach

those of

the

perfect

graphite

crystal.

Others

have a

lesser

degree

of

anisotropy

which is not always

a disadvantage

as, in many

cases,

isotropic

properties

are

a desirable

feature,

as will

be seen

in later

chapters.

A common

characteristic

of graphite

and

carbon

materials,

whatever

their

origin

or processing,

is that

they

are

all derived

from

organic

precur-

sors: molded

graphite

from

petroleum

coke and

coal-tar,

pyrolytic

graphite

from

methane

and other

gaseous

hydrocarbons,

vitreous

carbon

and fibers

from

polymers,

carbon

black from

natural

gas,

charcoal

from

wood,

coal

from

plants,

etc.

These

organic

precursors

must

be carbonized

and,

more

often

than

not,

graphitized,

in order

to form carbon

and graphite

materials.

The

critical

72

Carbon, Graphite,

Diamond,

and Fullerenes

and

complex

phenomena

of carbonization

and

graphitization

are the

two

common

features

of the

production

of all these

synthetic

materials

with

the

notable

exception

of

pyrolytic

graphite,

which

is produced

by the

entirely

different

process

of

vapor

deposition

(reviewed in

Ch. 7).

These

two

phenomena,

carbonization

and graphitization,

are the topics

of this

chapter.

2.0

THE CARBONIZATION

(PYROLYSIS)

PROCESS

The carbonization

process, also known

as pyrolysis,

can be defined

as

the

step

in which

the

organic

precursor

is transformed

into

a material

that

is essentially

all

carbon.

The mechanism

of

carbonization

is reviewed

below in general

terms.

Additional

information

on

the

carbonization

of

specific

materials

is given

in subsequent

chapters.

2.1Principle of Carbonization

Carbonization

Cycle.

Carbonization

is basically

a heating

cycle.

The

precursor

is heated

slowly

in a reducing

or inert environment,

over

a range

of temperature

that

varies

with

the

nature of the

particular

precursor

and

may extend

to 1300°C.

The

organic

material

is decomposed

into a carbon

residue and volatile compounds

diffuse

out to the atmosphere.

The process

is complex

and

several

reactions

may take place

at the same

time

such as

dehydrogenation,

condensation

and

isomerization.

The

carbon

content of the

residue

is a function

of the

nature

of the

precursor

and the

pyrolysis

temperature.

It usually

exceeds

90 weight

% at

900°C and

99 weight

% at 1300°C.

The

diffusion

of the volatile

compounds

to the atmosphere

is a critical

step and

must

occur

slowly

to

avoid

disruption

and

rupture

of the

carbon

network.

As a result,

carbonization

is usually

a slow

process.

Its duration

may vary

considerably,

depending

on the

composition

of the

end-product,

the type of precursor,

the thickness

of the material,

and other factors. Some

carbonization

cycles,

such

as

those

used

in

the

production

of

large

electrodes

or some

carbon-carbon

parts,

last

several

weeks.

Others

are

considerably

shorter,

such

as the

carbonization

cycle

to

produce

carbon

fibers, since

these

fibers

have

a small

cross-section

and the

diffusion

path

is short.

The specifics

of each

cycle

will

be

reviewed

in more

detail

in the

following

chapters.

Carbonization

and

Graphitization

73

Characteristics

of

Carbonized

Materials.

After

carbonization,

the

residual

material

is essentially

all

carbon.

However,

its structure

has

little

graphitic

order

and

consists

of

an

aggregate

of small

crystallites,

each

formed

of a few

graphite

layer

planes

with

some degree

of parallelism

and

usually

with

many

imperfections.

These crystallites

are generally

randomly

oriented

as described

in Ch. 3, Sec.

2.0

and

shown

in Fig. 3.4.

The

carbonized

material

is often

called

“amorphous”

or “baked car-

bon”.

It

is

without

long-range

crystalline

order

and

the

deviation

of

the

interatomic

distances

of the carbon

atoms

(from the perfect

graphite

crystal)

is greater

than

5% in both the basal

plane

(ab

directions)

and

between

planes

(c direction),

as

determined

by x-ray

diffraction.

Amorphous

carbon

is

hard,

abrasion

resistant,

brittle,

and

has

low

thermal-

and electrical-conductivities.

In afew

cases,

these

characteristics

are

desirable

and

amorphous

carbon

is

found

in

applications

such

as

contacts,

pantographs,

current

collectors

and brushes for operation

on flush

mica

commutators,

as well

as

in special

types

of carbon-carb0n.t’)

In most

instances

however,

amorphous

carbon

is only the intermedi-

ate

stage

in the

manufacture

of synthetic

graphite

products.

2.2

Precursor

Materials

and

Their

Carbon

Yield

The

carbon

yield

is

defined

as

the

ratio

of

the

weight

of

the

carbon

residue

after

carbonization

to

the

weight

of

the

material

prior to

carbonization.

Typical

carbon

yields of common

and potential

precursor

materials

are

shown

in Table

4.1 .t*JtsJThese

yields

are

not

fixed

but

depend

to

a great

degree

on

the

heating

rate,

the

composition

of

the

atmosphere,

the

pressure,

and other

factors

(see

below).

The

nature

of the

carbon

yield,

given

in

the

last

column,

i.e.,

coke

or char,

is

reviewed

in

the

following

section

on

graphitization.

Effect

of Pressure

on

Carbon

Yield and Structure: The nature

and

the

length

of the

carbonization

cycle are important

factors

in controlling

the

carbon

yield.

For instance,

the

effect

of gas pressure

can

be considerable.

Figure

4.1 shows

this

effect

on the yield

of three

grades

of coal-tar

pitch,

with

various

softening

points. t4t In this

particular

case,

high pressure

more than

doubles

the

yield.

Pressure

can

also

modify

the

structure

of the

resulting

carbon

and

change

its

graphitization

characteristics.t5)

74 Carbon, Graphite, Diamond, and Fullerenes

Table 4.1. Typical			Carbon		Yield of Various	Precursor Materials
					Average		carbon	Type of
Precursor						yield	(%)	carbon*
Aromatic hvdrocarbons
Coal-tar	pitches					40 - 60		Coke
Petroleum		fractions				50 - 60		Coke
Naphtalene,		C,,H,						Coke
Anthracene,		C,,H,,						Coke
Acenaphtalene,			C,,H,					Coke
Phenantrene,			C,,H,,					Char
Biphenyl,	C,,H,,							Char
Polvmers
Polyvinyl	chloride,			(CH,CHCI),			42	Coke
Polyimide		(Kapton),		(C,,H,,O,N,),			60	Coke
Polyvinylidene			chloride,		(CH,CCI,),		25	Char
Polyfurfural		alcohol,		(C,O,H,),		50 - 56		Char
Phenolics,		(C,,O,H,o),				52 - 62		Char
Polyacrylonitrile			(PAN),		(CH,CHCN),	46 - 50		Char
Cellulose,		(C120,,H,,),					20	Char

* Coke is a graphitizable carbon, char is non-graphitizable (see Sec. 3.0 below).

60 - I I I

Softening Point 67 “C

Softening Point 77 “C

Softening Point 126 “C -

- Heating Rate, 10 Wmin

		I	I	I
	00	10	100	1000
		Gas Pressure, Bar
Figure	4.1. Effect of gas	pressure on weight	change	during pyrolysis of various
pitches	at 600°C.141

				Carbonization		and Graphitization					75
2.3	Carbonization Mechanism of Aromatic					Hydrocarbons
As shown in Table				4.1, the graphite	precursors can be divided					into two
major	classes:		(a) aromatic hydrocarbons and				(b) polymers,		each		with
different		carbonization		characteristics.
	Structure of Aromatic Hydrocarbons. The term							hydrocarbon			refers
to an organic compound that contains only carbon							and hydrogen.		Aromatics
are hydrocarbons			characterized by the		presence		of at least one			benzene
ring.	Aromatics		have	a graphite-like	structure		and	graphite		is	often
considered as the parent of all these compounds.							The structure		of benzene
is shown		below:
				H
				I

H\,/C\,/H

I II

The

structures

of the

aromatics listed

in Table

4.1 are

shown

in Figs.

4.2 (coke

formers)

and 4.3 (char formers).

Some

of the

most

important

aromatics

are

the following:t6]

. Anthracene

is a linear,

planar

molecule

with three

benzene

rings.

In an autoclave

at approximately

45O”C, it begins

to

lose

the

hydrogen

atoms

in the

9,lO

positions.

Free

radicals

are formed

and

condensed

into

gradually

larger,

planar

molecules

and

eventually

coke

is formed.

.

Phenanthrene

is a branched,

planar

isomerof

anthracene.

Itcarbonizestoacharinamannersimilartoanthracenebut

with

a lower

yield.

•

Biphenyl

has

two

benzene

rings

connected

by

a single

carbon-carbon

bond.

It

is

non-planar

with

free

rotation

around

this

bond.

It carbonizes

to a char.

Mesophase. The general

carbonization

mechanism

of polyaromatic

hydrocarbons

is relatively

simple,

at least in theory,

since it proceeds

by the

rupturing

of the carbon-hydrogen

bonds and the removal

of the hydrogen.[*j

Some

of

these

hydrocarbons

first

go

through

an

intermediate

liquid

or

plastic

stage which

occurs

at temperatures

above

approximately

400°C.

This stage is the

so-called

“mesophase”,

in which

the

material shows

the