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

Molded Graphite

107

The

thermal

expansion

of molded

graphite

is an

important

property

since

many

applications

involve

high

temperature,

and

expansion

values

must

be known

accurately.

Thermal Shock. The low modulus,

high thermal

conductivity,

and

low

thermal

expansion

combine

to give molded graphite

excellent

thermal-shock

resistance.

It is difficult to

rupture

the

material by thermal

shock alone.

3.7

Electrical

Resistivity

The

electrical

resistivity

of the graphite

crystal

was

reviewed

in Ch. 3,

Sec.

5.

The

crystal

has a high

degree

of electrical

anisotropy

with

resistivity

that is low

in the

ab directions

and

high in the

c direction.

Molded

graphites

(even

the

isostatically

molded

material

where

the

crystallites

are essentially

random)

have higher

resistivity

than the single-crystal

in the

ab directions,

but can

still

be considered

electrical

conductors.

Table

5.9

lists typical

electrical-resistivity

values

of

several

molded

carbons

and graphites

and

selected

low-resistivity

metals.

Table 5.9.

Electrical

Resistivity

of Molded

Graphite

and

Selected

Metals

Electrical

Resistivity

Material

at 25°C j&J-m

Molded

carbon

-

50

Electrographite

(from

petroleum

coke)

7.6

Electrographite

(from

lampblack)

30.5

Pyrolytic

graphite

(ab

direction)

2.5

- 5.0

Aluminum

0.026

Copper

0.017

Tungsten

0.056

Silver

0.016

The electrical resistivity of

molded

graphite, like that

of the graphite

crystal and metals,

increases

with temperature,

above

approximately

4OO”C, and the material has the

positive

temperature coefficient of resis-

tance which is typical

of metals.

Below

4OO”C, the

coefficient is usually

slightly

negative. At high temperature, graphite becomes a better conduc-

torthan

the most-conductive refractory metal. The effect of temperature on

5

I

= 0.5 -

1

I

I

1

i

I

I

I

I

-273

0

500

1000

1500

2000

2500

Temperature,

“C

amount of energy. The emissivity of a material depends

on its structure and

on its surface

conditions.

Molded

graphite, being black, has high emissivity,

which is an impor-

tant advantage in high-temperature applications. Table

5.10 lists the total

Material

Total Emissivity

Graphite

(petroleum-coke base)

0.70

- 0.90

Graphite

(lampblack base)

0.85

- 0.95

Molded

Carbon

0.60

- 0.80

Lampblack

0.90

- 0.99

Silver

0.04

Nickel, oxidized

0.87

Tungsten, polished

0.15

4.0

APPLICATIONS

AND

MARKET

OF MOLDED

GRAPHITE

4.1

General Considerations

The

technology

of molded

graphite is versatile,

improvements

in the

manufacturing

techniques

are continuously

made

and the scope

of applica-

tions

is gradually

expanding.

This

expansion

is the

direct

result

of asizeable

research

effort

carried out by

many

workers

in universities,

government

laboratories,

and

industry.

In the

last

twenty

years

or

so,

several

major applications

of molded

graphite

have developed

into an important,

complex,

and diverse

industrial

market.

Because

of this

diversity,

the

classification

of these

applications is

a critical

factor

if a proper perspective

of the

industry

is to

be

obtained.

The

classification

by product

function

is the

primary

one

used

in this

book.

It is

used

in this

chapter

for

molded

graphite

and

in the

following

chapters

for the

other

main

types

of

carbon

and

graphite

materials,

i.e.,

polymeric,

pyrolytic,

fibers, carbon-carbon

and

powders.

it is based on the

following

major

functions:

electrical,

structural

and

mechanical,

chemical

and

refractory,

and

nuclear.

These

functions

corresponds

roughly

to the

various

segments

of industry

and

end-use

market,

such

as electronic

and

semiconductor,

metal

processing,

chemicals,

automobile,

and

aerospace.

The

world

market

for

molded

graphite

is large

as

shown

in

Table

5.11 ?I

($

millions)

Steel Production

Electrodes

2250

Aluminum

Production

Anodes

940

Cathodes

250

Specialty

Graphites

300

Total

3740

Electrodes for the Production

of Electric-Arc Steel. As shown

in

Table

5.11, the production

of electrodesfor

steel

and aluminum

processing

is the

largest

application

of molded graphite,

in terms of both tonnage

and

dollars.

As

mentioned

in

Sec.

1.1,

electrodes

are one of the original

applications

and

have

been

manufactured

with

essentially

the

same

process

for almost

a century.

The

largest

use

of these

electrodes

is

in

the

production

of

steel

in

electric-arc furnaces, mostly

for the reclamation

of ferrous

scrap.t’)

These

electrodes

must

have

good

electrical

conductivity, good refractory

proper-

ties,

and low cost. They gradually

erode in use and at times break

altogether

and

must

be replaced

at regular

intervals.

The

worldwide

production

of steel is slowly

increasing,

but the

produc-

tion

of electric-arc

furnace

steel

is increasing

at a more rapid

rate.

This rate

reached

28%

of total

production

in 1990

as shown

in Fig. 5.11.

The

major

producers

by area

are Europe,

North

America,

and

Japan.

Molded Graphite

111

.

.

.

.

1

I

I

I

1

8

7Total

7

6

Steel

Production

5

4

Electrical

Steel

3

2

1981

1982

1984

1986

1988

1990

Year

Figure 5.11. World

production

of steel.[‘]

The

gradual

increase

in electric-arc

steel

production

does

not

neces-

sarily translate

into a gradual increase

in the tonnage

of graphite

electrodes.

In fact the

opposite

is

happening

and

the

total

consumption

of graphite

electrodes

is decreasing

(Fig. 5.12)

due to a pronounced

decrease

in the

consumption of

graphite

per

ton

of

electric

steel

produced.

In 1975,

an

average

of 7.5

kg of molded

graphite

was

required

to produce

one

ton

of

electric-arc

steel.

In 1990, this

amount had dropped

to 5 kg. The downward

trend is continuing.

This

reduction

is the result

of

two

factors:

(a) improvements

of the

properties

of the

graphite

materials,

particularly

a considerable

reduction

of

the thermal

expansion

and resulting

increase in thermal-shock

resistance,

and (b) improvements

in arc-furnace

operating

techniques.

I.

I ..I..

.

* I

1975

1980

1985

1990

Year

Electrodes

for

Aluminum

Production.

Aluminum

is

processed

electrolytically

and

the

production

of

the

necessary

electrodes

is

the

second-largest

application

of molded

graphite

(see Table

5.11

above)!‘]

The anodes

are similar

to those

used

in electric-arc

steel production

and are

also

manufactured

from

petroleum-coke

filler

and

coal-tar

pitch.

The

aluminum

collects

at

the

cathodes

which

are

large

blocks

lining

the

electrolytic

cell.

These

cathodes

were

originally

made

of baked

carbon

based on anthracite

coal

but, in recent

years,

have

been

upgraded

and

are

now

made

of molded

graphite

from

petroleum

coke.

The world

production

of aluminum

was

estimated

at 14.2 x 1O6metric

tons

in 1990.

The consumption

of molded-graphite

anode

is approximately

400 kg per metric ton of aluminum

and

the

total

estimated

consumption

worldwide

is 5.68 x 1O6 metric

tons.

Melting,

Smelting,

and

Casting

of

Metals.

Molded

graphite

has

numerous applications

in the processing

of ferrous and non-ferrous

metals

and alloys such

as

copper,

copper-nickel,

brass,

bronze,

zinc,

aluminum

alloys, nickel

and its alloys, precious

metals,

and grey and ductile

irons.t31t151

The

wide variety

of these

applications

is shown

in the

following

partial

list:

Molded

Graphite

113

.Moldsfor

centrifugal

casting of brass and bronze bushings

and sleeves

.Molds

for pressure casting

of steel slab and railroad-car

wheels

.Dies for continuous

casting

and

extrusion

of aluminum

and other non-ferrous

metals

.Extrusion

guides and run-out

tables

.Hot-pressing

molds

and plungers

.Pumps

for molten aluminum

and zinc

.Brazing

fixtures

.Furnace

linings

.Sintering

boats and trays

The factors

to consider in selecting

a suitable

grade

of molded

grahite

for

metal

processing

are: high thermal

conductivity

to reduce

the thermal

stresses;

high hardness to reduce

abrasive

wear and improve

the

lifetime

of

the

die; and

high

density,

fine

grain,

and low

porosity

to minimize

chemical

attack

by molten metals

and

by dissolved

atomic

oxygen (the

latter

especially

found

in nickel

and

its alloys) .Fig.

5.13 shows

typical

molded-graphite

casting dies.

114

Carbon,

Graphite,

Diamond,

and Fullerenes

4.3

Semiconductor

and

Related

Applications

In 1947,

Bardeen,

Brattain, and Shockley

of Bell

Telephone

Labora-

tories

demonstrated

the transistor

function with

alloyed

germanium

and this

date

is generally

recognized

as the

start

of the solid-state

semiconductor

industry.

The

era

of

integrated

circuits

(IC’s)

was

inaugurated

in

1959,

when,

for the

first

time, several

components

were

placed

on a single

chip

at Texas

Instruments.

These

developments

resulted

in a drastic

price

reduction

in all aspects

of solid-state

circuitry

and the cost

per unit of information

(bit)

has

dropped

by an estimated

three

orders

of magnitude

in the last twenty years. This

cost

reduction

been

accompanied

by a similar

decrease

in size,

and today

circuit

integration

has

reached

the

point where

more

than

a million

components

can be

put

on a single

chip.

This

progress

is largely

due to the development

of glass

and ceramic

fabrication

techniques, single-crystal production

processes,

and

thin-film

technologies

such

as evaporation,

sputtering

and

chemical vapor

deposi-

tion (CVD). These advances

were

made

possible

in part

by the

availability

of high-purity

molded graphite

and itsextensive

use as molds,

crucibles

and

other

components

as shown

by the following

examples.

Molded

Graphite

for

Crystal

Pulling.

Single

crystals

of silicon,

germanium

and

WVand

//-V/semiconductors

are usually

produced

by the

ribbon

or the

Czochralski

crystal-pulling

techniques.

The

latter

is shown

schematically

in Fig. 5.14.

The

process

makes extensive

use

of molded

graphite,

as shown

on the figure.

The crucible

holding

the

molten

material

is made of high-purii

graphite

lined

with quartz,

and so are the support

and

the heater.

In some

cases, the

crucible is coated with

pyrolytic

boron

nitride

deposited

by chemical

vapor

deposition

(CVD).

Other

Molded

Graphite

Applications

in Semiconductor

Process-

ing.

The

following

is a partial

list

of current applications

of molded

graphite

in semiconductor

processing:t16)

n

Boats

and assemblies

for

liquid-phase

epitaxy

n

Crucibles

for molecular-beam

epitaxy

.

Susceptors

for metallo-organic

CVD

.

Wafer trays for plasma-enhanced

CVD

n

Shields,

electrodes,

and

ion sources

for ion

implantation

.

Electrodes

for

plasma

etching