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

136 Carbon, Graphite, Diamond, and Fullerenes

5.1 Characteristics

and Properties

Vitreous

carbon

foam

is

produced

in

several

pore

sizes,

usually

described

as number

of pores per inch (ppi). Commercially

available

foams

are respectively

60,100,

and 200

ppi (24,39

and 78 pores

per cm).

These

foams

have low

density,

with

relatively

even

pore

distribution.

Their

properties

are

listed

in Table

6.4.p4)

Vitreous-carbon

foam

is very susceptible

to oxidation due its large

surface

area.

Any

application

involving

an

oxidizing

atmosphere

above

500°C

should

not

be considered.

Table 6.4.

Properties

of Vitreous

Carbon

Foam

Bulk

void

volume,

%

97

Bulk

density,

g/cm3

0.05

Strut

density,

g/cm3

1.49

Strut

resistivity,

1O-‘rohm-cm

50

Crushing

strength,

MPa

(function

of pore

size)

0.07 - 3.4

Surface

area, m*/g

1.62

5.2Applications

Electrodes. Its chemical

inertness, its wide range

of usable

potential

(1.2 to -1 .OVvs.

SCE) and the

hydrodynamic

and structural

advantages

of

its open-pore

foam

structure

make

vitreous

carbon

foam

an

attractive

material for

electrodes

for

lithium-ion

and

other

types

of

batteries, with

many

potential

applications

in electrochemistry.[13)[15)[1s]

High-Temperature

Thermal

Insulation.

A

potential

application

of

vitreous-carbon

foam is high-temperature

thermal

insulation

in vacuum

or

non-oxidizing

atmosphere.

Several

factors

combine

to make this

structure

an excellent

thermal

insulator:

(@he

lowvolumefraction

of the solid phase,

which

limits

conduction;

(b) the

small

cell

size,

which

virtually eliminates

convection

and

reduces

radiation

through

repeated

absorption/reflection

at

the cell walls;

and (c)the

poor conductivity

of the enclosed gas (orvacuum).

An additional

advantage

is its excellent

thermal-shock

resistance

due to its

Vitreous Carbon 137

relatively low modulus compared to the bulk material. Very high thermal gradients can be tolerated.

Adsorption of Hydrocarbons and Other Gases.								In the	activated
form, vitreous-carbon			foam	could	replace	activated-carbon		granules		with-
out the requirement			of a container		(see Ch. 10, Sec. 4.0). Potential				uses are
in emission		control	and recovery.
Other		Applications.		Viireous-carbon		foam	is being	considered		as a
filter for	diesel particulates			and	for the	filtration	of non-carbide-forming
molten	metals.

6.0VITREOUS CARBON SPHERES AND PELLETS

Vitreous

carbon

in the form

of microspheres

or pellets

has

a number

of applications, especially

in the

field

of

catalytic

supports.

6.1

Processing

A

typical

process

for

the

production

of

vitreous-carbon

spheres

is

represented

schematically

in

Fig.

6.9.t”)

The

precursor

is

a partially

polymerized

polymer

such

as

furfuryl

alcohol,

catalyzed

with

p-toluene

sulfonic

acid

and

mixed with

acetone

to obtain

the proper

viscosity

for

atomization.tls)

A pore

former

is added

which

can

be an

organic

material

with

a high boiling

point

or sub-micron

solid

particles

such

as carbon

black.

Atomization

occurs

in

the

thermal

reactor

shown

schematically

in

Fig.

6.10.[“]

The

curing

time

is

very

brief

because

of

the small

size

of

the

particles

(- 45 pm).

The microspheres

are then

heat-treated

from 530 to 1330°C. If required,

they

can be partially oxidized

to create

microand

transitional-pores.

6.2Applications

Catalytic Support.

Vitreous

carbon spheres

are being

considered as

catalyst supports

for

iron

and

other

metals.

The material may offer

some

important

advantages

over other

forms of carbon,

such as lower inorganic

impurities

(which

may poison

the

catalyst)

and

a

more

uniform

pore

structure.

The activation

mechanism

and the properties

and characteristics

of catalytic

materials

are

reviewed

in greater

detail in Ch. 10, Sec.

4.0.

138 Carbon, Graphite, Diamond, and Fullerenes

Other Applications: Other applications include foams, low density fillers for plastics and high-temperature thermal insulation.

	Furfuryl	Polymerization	p-Toluene
			Sulfonic
	Alcohol
			Acid
		I

v

1Particle Generation1

I	Sieving	I

$

Spheres

Figure 6.9. Processing flow-chart for vitreous carbon spheres.[17]

Vitreous Carbon 139

Aerosol Generator
Dispersion Air --J/L	Polymer	Inlet
2-	Dilution Air

Fi’ter\eIl--cEF:g

9!?=r N2

Exhaust

Collector

Figure 6.10. Schematic of vitreous-carbon

production apparatus.[17]

140 Carbon, Graphite, Diamond, and Fullerenes

REFERENCES

1.Dubgen, R., Glassy Carbon - A Material for Use in Analytical

Chemistry, Publication of Sigri, D-8901 Meitingen, Germany (1985)

2.Jenkins, G. M. and Kawamura, K., Polymeric Carbons, Cambridge

Univ. Press, Cambridge, UK (1976)

3.	Fitzer, E., Schaefer, W., and Yamada,			S., Carbon,		7:643-648	(1969)
4.	Inagaki,	M., et al, Carbon, 27(2):253-257			(1989)
5.	Inakagi,	M. et al, Carbon. 29(8):1239-l		243 (1991)
6.	Fitzer, E. and Schaefer, W.,		Carbon,	8:353-364 (1970)
7.	Cowlard,	F. and Lewis J., J.	of Mat. Sci.		2507-512	(1967)
8.	Doremus, R., G/ass Science,		John Wiley		& Sons,	New York	(1973)

9.Jenkins, G. M. and Kawamura, K., Nature, 231:175-176 (May 21, 1971)

10.Lewis, J. C., Redfern, B., and Cowlard, F. C., So/id-State/Electronics, 6:251-254, Pergammon Press (1963)

11.	Lausevic, 2. and Jenkins, G. M., Carbon, 24(5):651-652 (1986)

12.Van der Linden, W. E., and Dieker, J. W., Analytica Chimica Acta,

119:1-24 (1980)

13. Wang, J., Hectrochimica Acta, 26(12):1721-1726 (1981)

14. Reticulated vitreous Carbon, Brochure form ERG, Oakland, CA

94608 (1976)

15.Sherman, A. J., Tuffias, R. H., and Kaplan, R. B., Ceramic Bull.,

70(6):1025-1029 (1991)

16.	Sherman,	A. J. and Pierson, H. O., Ultrastructures		for	Co/d Cathode
	Emitters,	Final Report (ULT/TR-89-6762),	Air Force Electronic Systems
	Division, Hanscom AFB, MA (April 1989).
17.	Levendis,	Y. and Flagan, R., Carbon,	27(2):265-283		(1989)
18.	Moreno-Castilla, C., et al, Carbon, 18:271-276 (1980)

Pyrolytic Graphite

1.0GENERAL CONSIDERATIONS

The

production

of molded

graphite

and vitreous

carbon,

described

in

the previous

two chapters,

relies on the

carbonization

(pyrolysis)

of a solid,

inorganic

substance

such

as coal-tar pitch,

petroleum

fractions

or polymers.

This

chapter

is a review

of another

type

of carbon

material,

produced

by a

fundamentally

different

process

that

is

based

on

a

gaseous

precursor

instead

of a

solid

or

liquid.

The

process

is

known

as

chemical

vapor

deposition

(CVD)

and

the

product

as

pyrolytic

carbon

or

graphite,

some-

times

referred

to as pyrocarbon

or pyrographite.

To simplify,

in this

chapter

the material will be referred

to as pyrolytic

graphite,

regardless

of the degree

of graphitization.

Pyrolytic

graphite

is different

from

another

standpoint:

although

pro-

duced in

bulk

form,

its

main

use

is in the

form

af coatings,

deposited

on

substratessuch

as molded

graphite, carbon

fibers,

or poiouscarbon-carbon

structures.

As such,

it is part

of a composite

structure

and is not as readily

identifiable

as

other

forms of

carbon.

It

is similar

in

this

respect

to CVD

diamond

and

diamond-like

carbon

(DLC)

described

in Chs.

13 and

14.

Pyrolytic

graphite

is the

only

graphitic

material

that can

be

produced

effectively

as

a coating.

The

coating

can

be

made

sufficiently

thick

that,

after

removing

the substrate,

a free-standing

object

remains.

Pyrolytic

graphite

is a key

element

in the

technology

of carbon

and

is

used

extensively

in the

coating

of specialty

molded

graphites

and

in the

processing

of carbon-carbon

components.

141

142

Carbon,

Graphite,

Diamond,

and

Fullerenes

1.l

Historical

Perspective

The

CVD

of carbon materials

is not new.

As mentioned

in the pioneer

work of Powell,

Oxley, and

Blocher,tlj

its first

practical

use

was developed

in

the

1880’s

in

the

production

of

incandescent

lamps

to

improve

the

strength

of filaments

by carbon deposition

and

a patent was

issued

over a

hundred

years

ago,

covering

the

basis

of the

CVD

of carbon!*]

The

CVD

process

developed

slowly

in the next

fifty

years,

and

was

limited

mostly

to pyro

and extraction

metallurgy,

and

little workwas

done on

graphite

deposition.

It is only

since the end

of World

War II that

the

CVD of graphite

began

to expand

rapidly

as researchers

realized

the potential

of this

technique

for

the

formation

of coatings

and

free-standing

shapes.

The

importance

and

impact

of pyrolytic

graphite

have

been

growing

ever

since.

1.2The Chemical Vapor Deposition Process

CVD is nowawell-established				processthat		hasreached major production
status in areas		such	as semiconductors and cutting toots.				It is a vapor-phase
process	which	relies	on the chemical		reaction	of a vapor	near or on a heated
surface	to form a solid deposit and gaseous by-products.						The process is very
suitable	to the	deposition of carbon,			as reviewed be~ow.tq

1.3Pyrolytic Graphite as a Coating

Although,

as mentioned

above,

pyrolytic

graphite

is used

by itself

as

free-standing

structures

such

as crucibles

or rocket

nozzles

(see

Sec. 4.0),

its major

use

is

in

the

form

of

coatings

on

substrates

such

as

molded

graphite,

carbon

foam,

carbon fibers,

metals,

and

ceramics.

Composite

Nature

of

Coatings.

The

surfaces

of

many

materials

exposed

to the

environment

are prone

to the effects

of abrasion,

corrosion,

radiation,

electrical

or

magnetic

fields,

and

other conditions.

These

surfaces

must

have

the

ability

to withstand

these

environmental

conditions

and/or provide

certain

desirable

properties

such

as

reflectivity,

semi-

conductivity,

high

thermal

conductivity,

or erosion

resistance.

To obtain

these

desirable

surface

properties,

a coating

is deposited

on

the

bulk

material

to form

a composite

in which

bulk

and

surface

properties

may

be very

different.t4)

	Pyrolytic	Graphite	143
Table 7.1 summarizes the surface	properties that	may be obtained	or
modified by the use of pyrolytic graphite	coatings.

Table 7.1. Material Properties	Affected by Pyrolytic Graphite Coatings
Electrical	Resistivity
Optical	Reflectivity
Mechanical	Wear
	Friction
	Hardness
	Adhesion
	Toughness
Porosity	Surface area
	Pore	size
	Pore	volume
Chemical	Diffusion
	Corrosion
	Oxidation

2.0THE CVD OF PYROLYTIC GRAPHITE

The CVD

of pyrolytic

graphite

is theoretically

simple

and

is based on

the thermal

decomposition

(pyrolysis)

of a hydrocarbon

gas.

The

actual

mechanism

of

decomposition

however

is

complex

and not

completely

understood.t5j

This

may

be due in part

to the fact

that most of the studies

on the subject

of hydrocarbon

decomposition

are focused

on the improve-

ment

of fuel

efficiency

and the prevention

of carbon

formation

(e.g.,

soot),

rather

than

the

deposition

of a coating.

Although

many

studies

of the

CVD

of graphite

have

been

carried out,

a better understanding

of the pyrolysis

reactions,

a more

accurate

predic-

tion of the results,

and more

complete

experimental,

thermodynamic,

and

kinetic

investigations

are

still

needed.

144 Carbon, Graphite, Diamond, and Fullerenes

2.1

Thermodynamics

and Kinetics Analyses

The

CVD of pyrolytic

graphite

can be optimized

by experimentation.

The

carbon

source (hydrocarbon

gas), the method

of activating

the decom-

position reaction (thermal, plasma, laser, etc.),

and the

deposition

variables

(temperature,

pressure,

gas flow,

etc.)

can be changed

until

a satisfactory

deposit

is

achieved.

However,

this

empirical

approach

may

be

too

cumbersome

and, for

more

accurate

results,

it should

be combined

with a

theoretical

analysis.

Such

an

analysis

is

a valuable

step

which,

if properly

carried

out,

predicts

what

will

happen

to the

reaction, what

the

resulting

composition

of

the

deposit

will

be (i.e.,

stoichiometry),

what

type

of

carbon

structure

to

expect,

and

what

the

reaction

mechanism

(i.e., the

path

of the reaction

as

it forms

the

deposit)

is

likely

to

be.

The

analysis

generally

includes

two

steps:

1.

The

calculation

of

the

change

in

the

free

energy

of

formation

for

a

given

temperature

range;

this

is

a

preliminary,

relatively

simple

step

which

provides

information

on the

feasibility

of the

reaction.

2.

The

minimization

of the free

energy

of formation

which

is

a more complete

analysis

carried out

with

a computer

program.

2.2AG Calculations and Reaction Feasibility

Thermodynamics

of CVD Carbon. The CVD of carbon

(as all CVD

reactions)

is

governed

by

two ‘factors:

(a)

thermodynamics,

that is

the

driving

force

which

indicates

the

direction

the

reaction

is going

to proceed

(if at

all),

and

(b) kinetics, which

defines

the

transport

process

and

determines

the

rate-control

mechanism,

i.e.,

how

fast

it is going.

Chemical

thermodynamics

is

concerned with

the

interrelation

of

various

forms

of

energy

and the transfer

of

energy

from

one chemical

system

to another

in accordance

with

the

first

and

second

laws

of thermo-

dynamics.

In the

case

of

CVD,

this

transfer

occurs

when the gaseous

compounds,

introduced

in the deposition

chamber,

react to form the carbon

deposit

(and

by-products

gases).

Pyrolytic Graphite

145

AG Calculation:

The first

step

is to

ensure

that

the

desired

CVD

reaction

will

take

place

in a given

temperature

range.

This

will

happen

if the

thermodynamics

is favorable, that

is, if the transfer

of energy

(i.e.,

thefree-

energy

change

of the reaction,

known

asAG,)

is negative.

To calculate

AG,,

it is necessary

to know

the thermodynamic

properties

of each

component,

specifically

their

free-energy

of

formation

(also

known

as Gibbs

free

energy),

AGr.

The values of AG,

of the reactants

and

products

for

each

temperature

can

be obtained

from

thermodynamic-data

tables such

as the

JANAF

Thermochemical

Tables

and

others.t6jm

It

should

be noted that

the

negative

free-energy

change

is

a

valid

criterionforthefeasibility

of areaction

only ifthe reaction

aswritten

contains

the major species

that

exist

at equilibrium.

2.3Minimization of Gibbs Free Energy

Experimentation

shows

that

the best,

fully

dense,

and

homogeneous

carbon

deposits

are

produced

at an optimum

negative

value

of AG.

For

smaller negative values, the reaction rate

is

very

low

and,

for

higher

negative values, vapor-phase

precipitation

and

the

formation

of soot

can

occur.

Such

factors

are not

revealed

in the

simple

free-energy

change

calculation.

A more

complete

analysis

is often

necessary.

A method

of analysis

is the

minimization

of the

Gibbs

free energy,

a

calculation

based

on

the

rule

of

thermodynamics

which

states

that

a

system

will

be in equilibrium

when

the

Gibbs

free

energy

is at a minimum.

The

objective

then

is the

minimization

of the total free

energy

of the system

and

the

calculation

of

equilibria

at

constant

temperature

and

volume

or

constant

pressure.

It is a

complicated

and

lengthy

operation

but, fortu-

nately,

computer

programs

are

now

available

that

simplify

the

task

considerably.f8)t9)

These

programs

provide

the

following

information:

•

The

composition

and

amount

of deposited

material

that

is theoretically

possible

at agiven

temperature,

pressure,

and

concentration

of input

gases

. The

existence

of gaseous

species

and

their

equilibrium

partial

pressures

.

The

possibility

of multiple

reactions

with

the

inclusion

of

the

substrate

as a possible

reactant