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

Electrical

applications

of molded graphite

are well established

and the

material has

been a standard

for

many

years

due

to

its chemical

inertness,

good

electrical

conductivity

and

ability

to

withstand

the

heat

of

electrical

arcing

with minimum

damage.

The

following

is a list

of current applications:

.Brushes

for electric

motors

.Current-collecting

shoes

for

railroad

conductor

rails

.Heating

elements

for

high-temperature

furnaces

.Electrodes

for

lighting,

electrical-discharge

machining

(EDM)

I and electric-arc

furnaces

.Fuel-cell

components

.Anodes,

cathodes,

and

buss

bars

of

zinc-chloride,

zinc-

bromide,

and other

advanced

batteries

Molded Graphite

117

Fine-grain, high-density,

and

low-porosity

molded

graphite

is exten-

sively

used

for bearings

and seals, particularly

in high-temperature

applica-

tions

(up to 600°C)

where conventional

liquid lubricants

degrade

rapidly. It

has excellent

lubricating

and

frictional

properties

(reviewed

in

Sec.

2.5

above),

it

is

chemically

resistant,

lts

high

thermal

conductivity

helps

dissipate

the

heat

generated

by

rubbing

action

and,

at high temperature

(>5OO”C),

its

compressive

strength

is

superior

to

that

of

most

other

engineering

materials.

However

molded

graphite

cannot

be used to seal

materials

that would

attack

it chemically,

such as aqua

regia,

perchloric

acid,

and oieum

(fuming

sulfuric acid)

(see

Ch. 3, Sec. 7.0).

Sealing

failure

may

also

occur

with

abrasive

materials

such

as chromic

acid, chromic oxide, chrome plating

compounds,

potassium

dichromate,

sodium chromate,

and sodium

dichro-

mate.f*l

The

following

is

a

partial

list

of current

commercial

applications

of

bearings

and

seals:

n

Seal rings for gas-turbine

engines

to

protect

compressor-

shaft

bearing

and engine

oil

system

for

600°C gases,

at

rubbing

speed

of 150

m/s and

10,000

hours service.

•

Seal

rings

in chemical

pumps

for corrosive-fluid

transfer

with

a rubbing

speed

of 15 m/s, operating

up to 100°C for

10,000

hours.

.

Seal rings for water pumps

of industrial

and

truck

diesel

engine, operating

at 120°C and 0.18 N/mm*

(25 psi) for20,OOO

hours.

.

Seals

rings

for

automobile

freon

air

compressor

for

air

conditioning,

operating

up to 1.75 N/mm*

(250 psi) and 93°C.

. Seal

rings for home washing

machine and dishwasher

water

pumps,

operating

at 0.28

N/mm* and to 66°C for 10 years

.

O-rings

for

cylinder

heads

of race-car

engines

Other

mechanical

applications

of molded

graphite

include

glass-lehr

roil bearings,

roller

bearings

for jet-engine

exhaust

nozzles,

metering

seats

for gasoline

pumps,

vane

pumps for

air

compressors,

rupture

disks,

and

many

others.

118 Carbon, Graphite,

Diamond, and Fullerenes

A specialized group

of applications is found in aerospace systems

where the performance of molded graphite

has been

excellent, due to its

high-temperature strength and resistance

to erosion

and thermal-shock

(see Ch. 9) .

Figure 5.16. Graphite reentry nose cone. (Photograph

courtesy Sign Great Lakes

In addition to the seal applications

mentioned

above, molded graphite

has many applications

in areas where

chemical

resistance

is the major

factor. Such applications

are found in chemical reactors, heat exchangers,

steam jets, chemical-vapor

deposition

equipment,

and cathodic-protection

anodes for pipelines,

oil rigs, DC-power

lines, and highway

and building

construction.

Molded Graphite

119

4.7

Nuclear Applications

Molded

graphite

is one of the best

material for nuclear-fission

appiica-

tions

since

it

combines

high

neutron-moderating

efficiency

and

a

low

neutron-absorption

cross

section,

good

mechanical

strength

and

chemical

resistance,

ease

of machinability,

and

relatively

low

cost.[lq

However,

nuclear radiation

affects

the crystal lattice

which

becomes

distorted

by collisions

with fast

neutrons,

other

energetic

particles,

and

displaced

carbon

atoms.

As

a result,

the

properties

are

altered

to

some

extent. Strength and hardness generally

increase

and dimensional

changes

become

noticeable

particularly

at high

temperature, as

shown

in

Fig.

5.17.W3lWl

I

-

Perpeidlcularto

-

Extrusion

Direction

---

Parallel to

Extrusion

Direction

0

5

10

15

20

25

Equivalent Fission Fluence (1021cmB2)

Figure 5.17.

Length changes

of typical

nuclear graphite

as a function

of fission

fluence.[16~[1gl

3.

Specialty

Graphite, Technical

Brochure,

Great Lakes

Carbon

Corp.,

St. Marys, PA (1990)

4.

Schroth,

P. and Gazda,

I. W., in Electric FurnaceSteelmaking

(C. R.

Taylor,

Ed.),

71-79, The Iron

and Steel

Sot.

(1985)

5.

Eser, S. and

Jenkins,

R. G.,

Carbon, 27(6):877-887

(1989)

6.

Akezuma,

M. et al, Carbon, 25(4):517-522

(1987)

7.

Sato,Y,

Kitano, T., Inagaki, M., andsakai,

M., Carbon,28(1):143-148

(1990)

8.

Charette,

A. et al, Carbon, 29(7):1015-l

024 (1991)

10.

High-Purity

Graphite Products forthe Semiconductorlndustry, Bulletin

from The

Carbon/Graphite

Group Inc.,

St. Marys, PA 15857

11.

A Machinist’s Guide for Graphite, Bulletin

from the Graphite Materials

Div., Carbone of America,

St. Marys, PA 15857

13.

Carbon/Graphite Properties,

Bulletin

from the Graphite Materials

Div., Carbone of America,

St. Marys,

PA 15857

14.

Carbon Products, Bulletin

from

The Carbon/Graphite Group Inc., St.

Marys, PA 15857

Casting Industry, Bulletin from the

Graphite Materials

Div., Carbone

of America, St. Marys,

PA 15857

(1988)

16.

Products for the Semiconductorlndusfry, Bulletin from

Ringsdorff, D-

5300 Bonn-2, Germany

(1988)

124

Carbon,

Graphite, Diamond,

and

Fullerenes

Phenolics.

Phenolics, typically

(C,sO,H,J,, are a class of polymers

which

are obtained

by a condensation

reaction, usually between phenol and

formaldehyde,

with

elimination of water.

The monomer has the following

chemical structure:

When

heated

to

about

250°C extensive cross-linking

occurs

and

a

hard, rigid,

and insoluble

polymer

is formed.

Polyimide.

The

polymer

polyimide,

(C,,H,,O,N,),,

in the form

of a

film (known then as Kapton

or Novax),

is unusual

in

the

sense

that

it

carbonizes

to

form

a char which

becomes

a well-oriented

graphite

after

heat-treatment

to 3000°C

(see

Ch. 4, Sec.

3.5).t4)t5j

Polyacrylonitrile.

The

polymer polyacrylonitrile

(PAN)

is used

mostly

for the

production

of carbon

fibers.

Cellulose.

Cellulose

is the major constituent of wood

and other

plants.

Cotton

and

rayon

are almost

pure

cellulose

and the

latter

is a precursor

for

carbon

fibers.

The

pyrolysis

of cellulose,

like

that

of

PAN,

is reviewed

in

Ch. 8.

Others.

Other

polymers

are

being

investigated

as potential

precur-

sors for glassy

carbon,

such

as polyvinylidene

chloride

(CH,CCI,),,

polyvi-

nyl alcohol

(CH,CHOH),

polyphenylene

oxide

and

aromatic

epoxy.

The

latter two compounds

have a high

carbon

yield.

Molding.

The

precursor

polymer

is

often

combined with

other

materials

such

as solvents

to obtain the

desired molding

and carbonization

characteristics.

Some

of these compositions

and processes

are considered

proprietary

by the

manufacturers.

The

precursor

compound

is heated

and

molded

to the

desired

shape

by standard plastic

processing,

i.e., injection

molding,

extrusion,

or

com-

pression molding.

Since

carbonization

is accompanied

by a large

shrink-