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

Natural Graphite

227

Natural

graphite

is

classified

into

three general

types:

flake

(also

known

as plumbago),

crystalline

(vein), and amorphous,

varying

in physical

properties, appearance, chemical composition, and

impurities.

These

differences stem

from the type

of precursor

material

(oil,

coal,

or

other

carbonaceous

deposits)

and

the

natural

process

by

which

graphite

was

formed.

Table

10.2 summarizes

the characteristics

of these

three

types.t2)t3]

Table

10.1.

Characteristics

and

Properties

of the

Three

Types

of Natural

Graphite

Type

Property

Flake

Crystalline

Amorphous

Composition

Carbon,

%

90

96

81

Sulfur,

%

0.10

0.70

0.10

Density,

g/cm3

2.29

2.26

2.31

Degree

of

graphitization,

%

99.9

100

28

d-spacing

(002),

nm

0.3355

0.3354

0.3361

Resistivity,

S2acm

0.031

0.029

0.091

Morphology

Plate

Plate

Granular

Needle

Flake

graphite

is found

disseminated

in

metamorphosed

silica-rich

quartzites,

gneisses,

and

marbles. Crystalline

(vein)

graphite

occurs

in

transverse,

igneous,

or metamorphic

rocks,

where

it was

formed

by the

transformation

of

oil

precursors.

Amorphous

graphite

is the result

of the

metamorphosis

of coal

exposed

to high

pressure.

Graphite

is also

present

in the universe

as evidenced

by

near-perfect

crystals

frequently

found

in

meteorites.

Besides the

U.S.,

the major producers

of

graphite

are Brazil

(flake),

Canada

(flake), China

(flake

and

amorphous),

Korea

(amorphous),

Mada-

gascar

(flake),

Mexico

(amorphous),

Sri

Lanka

(flake),

and

the

former

Soviet

Union

(flake

and

amorphous).

The

total

world

production

was

estimated

at over

650,000

tons in 1991

at an average

price

of $1400/tori

for

flake and

$165/tori

for

amorphous.t*j

After mining, flake and crystalline

graphites

are milled,

usually

with

ball

mills,

although

ring

rollers

and

jet

mills

are

also

used.

A high

percentage

of the graphite-flake

production

goes

into

the

fabrication

of refractory

crucibles

for

the

metal

industry.

High-quality

materials

are

required

such as No.

1 flake which

is >90%

carbon,

and

20 -

90 mesh.

Other

major

applications

arefound

in friction

products,

lubricating

oils

and greases,

dry-film

lubricants,

batteries, conductive

coatings,

electri-

cal

brushes,

carbon

additives,

paints,

pencil

manufacturing,

and others.t4j

Carbon-derived

powders and

particles

comprise a family

of synthetic

materials,

known under

the generic

term of carbon black,

made

by burning

hydrocarbons

in insufficient

air. Carbon-black

particles

are

aggregates

of

graphite

microcrystals,

each

only

a few unit

cells in size

and

so small

that

they are

generally

not

detectable

by diffraction

techniques.

The physical

properties

of these

materials

are essentially

determined

by the

nature

and

extent of their

surface

areas.

Natural Graphite

229

The

term

carbon

black is often narrowed

to

designate

the materials

produced

by two basic

methods

known

as channel and thermal

processes.tl)

Other carbon

blacks are

lampblack

and

acetylene

black

(see

below).

Channel

Process.

In the channel

process,

thousands

of small flames

of natural

gas

impinge

on a cool metallic

surface

which

can

be a channel,

a roller,

or a rotating’disk.

The

carbon

black

forms

on the

cool

surface

and

is then

exposed

to high

temperature

in air to oxidize

the

surface

of each

particle.

These

particles

are in the

form

of small

spheroids.

The

channel

carbon

black

has the

smallest

particle

size (- 10 nm), the

highest

surface

area, and

the

highest

volatile

content

of all carbon

blacks.

Thermal

Process.

In the thermal

process,

the carbon black is formed

by the

thermal

decomposition

of

natural

gas

in

the

absence

of

air

in a

preheated

firebrick-lined

chamber.

The process

produces

a coarser grade

than the channel

process

with

particle

size

up to 500

nm and

lower

surface

area.

Composition

and

Properties.

Table

10.2 lists

the

composition

and

typical

properties

of carbon

black.

Table

10.2.

Composition

and

Properties

of Carbon

Black

Composition:

Hydrogen

0.5 - 1.0 %

Nitrogen

0.02 - 0.09

%

Oxygen

2.5 - 7.0 %

Sulfur

O.Ol-

0.03

%

co2

0.1 - 1.5 %

co

0.2

- 4.0

%

Carbon

balance

Surface

area:

25 - 150

m2/g

Particle

size:

lo-500nm

Oil

absorption:

0.5

- 1.5 cm3/g

230 Carbon, Graphite,

Diamond,

and

Fullerenes

Applications.

A primary

use

of carbon

black is as a filler

in rubber to

improve

the strength,

stiffness,

hardness,

and

wear-

and

heat-resistance.

Carbon-black

fillers

are

found

in practically

every

rubber

product.

Carbon

black is also

the

base

of most

black

typographical

inks.

This

is a large

market

since

an average

of 16 kg of ink, containing

11 - 13% by

weight

of

carbon

black,

is required for

every

ton

of news-print.

Other

applications

are

found

in

paints, enamels,

lacquers,

molded

carbon and

graphite,

and

many

others.

Lampblack

is one of the

oldest

forms

of carbon, first

produced

by the

ancient

Chinese

who

made

it by collecting

the soot

from

a burning

oil lamp.

It was

used

then

(and

now)

as a black

pigment

for

inks

and

paints.t’]

Production.

Lampblack

is produced in

multiple

furnaces

where

a

preheated

feed of coal tar and petroleum

oil is burned in a controlled

airflow.

The particles

are calcined

in the flue

gas to remove

excess

residual

oil and

aromatic

compounds

and,

still

entrained

by the flue gas,

are

carried

in a

large chamber.

The

sudden

expansion

of the gases

causes

the

particles

to

settle

by

gravity.

The feed-to-air

ratio in this

confined

partial combustion

is the

critical

processing

factor that controls

the

characteristics

and

properties

of

the

resulting

lampblack

as shown

in Table

10.3.

Table

10.3.

Characteristics

of Lampblack

as a Function

of Processing

Oil-to-air ratio

Low

High

Air

velocity

High

Low

Yield

High

Low

Color

Strength

Low

High

Density

High

Low

Blue

Undertone

Stronn

Weak

Particle Size.

The

particle

size

of lampblack

averages

100 - 200

nm

and, to a great

degree,

controls

the

color

and

oil adsorption

as shown

in

Table

10.4.

Natural Graphite

231

Table 10.4. Effect of Particle Size

on Properties

of Lamblack

Particle

Size

Large

Small

Color

Blue

Black

Color Intensity

Low

High

Oil Adsorption

Low

High

Properties.

Lampblack

is a soft, flocculent,

amorphous

material with

an apparent

density

that

can vary

by a factor of twenty,

depending on the

amount

of occluded

gases and the shape

of the

particle. The occluded

gases

include hydrogen,

oxygen,

nitrogen,

and

often

sulfur

compounds

originating

from

the

feed

coal

tar.

Composition.

A typical

composition

of lampblack

(dried

at 105°C) is

the following:

Acetylene

black is obtained by thethermal

decomposition of acetylene

(C,H,).

The

gas

is

piped

into

a

retort

preheated

at

800°C

where it

decomposes

in the absence

of air. The

reaction

is strongly

exothermic

and

does

not

require

additional

heat.t’]

Composition

and

Properties.

Acetylene

black

is

similar

to

high-

grade

lampblack

but with

greater

purity,

higher

liquid-absorption

capacity,

and higher electrical

conductivity.

Table

10.5

lists its composition

and

typical

properties.

Applications.

The

principle

applications

of acetylene

black

are in the

production

of

dry

cells

and as

a

filler

in

rubber and

plastic

materials,

particularly

if electrical

conductivity

is required.

Composition:

Polymerization products

<l

%

Carbon

99 %

Ash

0.03

- 0.04

%

Moisture

0.05

- 0.06

%

Density:

2.05

g/cm3

Apparent

density:

0.02

g/cm3

Surface

area:

65

m*/g

Particle

size:

3-l 30 nm

Undertone:

blue

Compounds of graphite

are formed

when

foreign species

such

as

atoms,

ions,

or molecules

are

inserted

between

the

layers of the

graphite

lattice.

These

compounds

can be divided

into

two

general

classes

with

clearly

different

characteristics:

(a) the

covalent

compounds,

and (b)

the

intercalation

compounds.f5)

As the

name

implies,

the covalent

graphite

compounds

have

covalent

two-electron

bonds

between

the

foreign

and

the carbon

atoms.

These

bonds

disrupt

the

R bonds

between

layers

(see

Ch. 3, Sec. 1.2) and the

delocalized

n electrons are no longer free to move, thereby causing

a drastic

reduction

in the electrical

conductivity

of the

material.

This

is accompanied

by a loss of planarization

of the

layers

which

are

converted

to a puckered

structure,

somewhat

similarto

that

of diamond

(see Ch. 11, Fig.

11.3).

The

interlayer

spacing

increases

considerably

as

a result

of this

insertion

and

may reach 0.7 nm (vs.

0.3353

nm

for

graphite).

Two covalent

graphite

compounds

are known,

namely

graphite

oxide

and graphite

fluoride.t6)

Graphite

Oxide.

The

nominal

composition

of graphite

oxide is C,O

with oxygen

atoms

forming

C-O-C

bridges

in the

meta-position.

Hydrogen

is also

present

in varying

amounts

depending

on synthesis

conditions.

The