H.O. Pierson. Handbook of carbon, graphite, diamond and fullerenes. Properties, processing and applications. 1993
.pdf86 Carbon, Graphite, Diamond, and Fuiierenes
REFERENCES
1.Graphite, A Refracto~Material, Technical Brochure, Carbone Lorraine, Genneviiiiers, France (1990)
2.Jenkins, G. M. and Kawamura, K., Polymeric Carbons, Cambridge Univ. Press, Cambridge UK (1976)
3. |
inagaki, M., et al, Carbon, 27(2):253-257 (1989) |
4. |
Fitzer, E., Carbon, 25(2):163-l 90 (1987) |
5.Ayache, J., Oberlin, A. and inagaki, M., Carbon, 28(2&3):353-362 (1990)
6. Walker, P. L., Carbon, 24(4):379-386 (1986)
7.Kochiing, K. H., McEnaney, B., Muiier, S. and Fitzer, E., Carbon, 23(5):601-603 (1985)
8. Akezuma, M. et al, Carbon, 25(4):517-522 (1987)
9.Honda, H., Carbon, 26(2):139-l 36 (1988)
10.Mochida, I., Shimimzu, K., and Korai, Y., Carbon, 28(2&3):31 l-319 (1990)
11. |
Walker, P. L., Jr., Carbon, 28(2&3):261-279 (1990) |
12. |
Eser, S. and Jenkins, R. G., Carbon, 27(6):877-887 (1989) |
13.Lim, Y. S. and Lee, B. I., Effect of Aromatic Hydrocarbon Addition on Mesophase Formation, Fiber-Tex 7990, (J. D. Buckley, ed.), NASA Conf. Pubi. 3128 (1991)
14. |
Eggers, D. F., Jr., Gregory, N. W., |
Halsey, J. D., Jr. and Rabinovitch, |
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|
B. S., Physical Chemistry, John Wiley & Sons, New York |
(1964) |
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15. |
Manteii, C. L., CarbonandGraphiteHandbook, IntersciencePublishers, |
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New York (1968) |
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|
16. |
Kawamura, K. and Bragg, R. H., |
Carbon, 24(3):301-309 |
(1986) |
17.Murty, H. N., Biederman, D. L. and Heir&, E. A., Carbon, 7:667-681 (1969)
18.Mochida, I., Ohtsubo, Ft., and Takeshita, K., Carbon, 18(2&3):25-30 (1990)
19. |
Cowlard, F. and Lewis. J., J. of Mat. Science 2:507-512 (1967) |
20.Sonobe, N., Kyotani, T. and Tomita, A., Carbon, 29(1):61-67 (1991)
21.Sonobe, N., Kyotani, T., and Tomita, A., Carbon, 28(4):483-488 (1990)
5
Molded Graphite: Processing,
Properties, and Applications
1.O |
GENERAL |
CONSIDERATIONS |
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Molded |
graphite |
can |
be defined |
as a synthetic |
(or |
artificial) |
graphitic |
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product |
manufactured |
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by a compaction |
process |
from |
a mixture of carbon |
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filler and |
organic |
binder |
which |
is subsequently |
carbonized |
and |
graphitized. |
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Parts |
of considerable |
size, weighing |
several hundred |
kilograms, |
such |
as the |
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electrodes |
shown in |
Fig. 5.1, |
are |
manufactured |
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in large |
quantities!‘] |
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The basic process was invented |
by E. G. Acheson, |
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who |
produced |
the |
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first molded |
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graphite |
in 1896. |
The |
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original |
applications |
of molded |
graphite |
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were |
electrodes |
for |
electric-arc |
furnaces |
and |
movie |
projectors. |
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Many |
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improvements |
have |
been made since then and the applications |
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have |
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increased |
considerably |
in scope. |
Molded |
graphite |
is found in almost |
every |
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corner |
of the |
industrial |
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world |
and forms |
the base |
of the |
traditional |
graphite |
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industry. |
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It is often difficult |
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to obtain details |
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of |
a specific |
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process, |
particularly if |
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such |
details |
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are not protected |
by a patent |
or cannot |
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be revealed |
by suitable |
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analyses. |
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Most |
graphite |
producers |
claim |
that |
such secrecy |
is necessary |
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because |
of the high cost of developing |
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new grades |
of molded |
graphite, |
and |
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the need for the new |
product |
to remain |
ahead |
of competition |
long |
enough |
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for the |
producer |
to recover |
his expenses |
and realize |
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a profit!*) |
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Fortunately, |
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a great |
deal of information |
on the basic |
materials |
and processes |
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is disclosed |
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in the |
open |
literature. |
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87
88 Carbon, Graphite, Diamond, and Fullerenes
Figure |
5.1. Graphite electrode. |
(Photograph |
courtesy of Carbon/Graphite |
Group |
Inc., |
St. Marys, PA.) |
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2.0 PROCESSING |
OF MOLDED |
GRAPHITES |
2.1Raw Materials (Precursors)
Raw |
Materials |
Selection. |
The |
selection |
of |
the appropriate |
raw |
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(precursor) |
materials |
is |
the |
first and |
critical step |
in |
the manufacturing |
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process. |
It determines |
to a great degree, |
the properties |
and the cost of the |
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final product. |
The characteristics |
of these raw materials |
such as the particle |
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size and |
ash |
content |
of cokes, |
the |
degree of carbonization |
of pitch, |
the |
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particle |
structure |
of lampblack, |
and the |
impurities |
and particle size |
of |
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natural graphite |
must |
be taken |
into account. |
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To use high-grade, expensive raw materials to produce an undemanding |
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product, |
such |
as a grounding |
anode for |
electrolytic |
protection, would |
be |
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wasteful |
and economically |
unsound |
since |
these |
electrodes |
do not require |
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optimum |
properties and cost |
is the |
overriding factor. |
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On the other |
hand, |
nuclear applications demand a graphite with the lowest -possible impurities and the highest-possible mechanical properties. This requires the selection of premium-grade precursor materials with c9st somewhat secondary .
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Molded |
Graphite |
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89 |
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Raw |
materials |
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can |
be |
divided |
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into |
four |
generic |
categories: |
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fillers, |
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binders, |
impregnants, |
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and |
additives.t’)-t4) |
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Fillers. |
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The |
filler |
is |
usually |
selected |
from |
carbon |
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materials |
that |
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graphitize |
readily. |
As |
mentioned |
in |
Ch. 4, |
such |
materials |
are generally |
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cokes, |
also |
known |
in industry |
as “softfillers”. They |
graphitize |
rapidly |
above |
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2700°C |
(the |
graphitization |
process |
is described |
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in Sec. |
2.4 |
below). |
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Other |
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major fillers |
are synthetic |
graphite |
from recycled |
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electrodes, |
natural |
graph- |
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ite, and |
carbon |
black |
(see Ch. |
10). |
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Petroleum |
coke |
is the filler of choice |
in most applications. |
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it is a porous |
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by-product |
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of the |
petroleum |
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industry |
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and an almost-pure |
solid |
carbon |
at |
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room |
temperature. |
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it |
is |
produced |
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by |
destructive |
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distillation |
without |
the |
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addition |
of hydrogen, |
either |
by a continuous |
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process |
(fluid |
coking) or, more |
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commonly, |
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by a batch |
process |
(delayed |
coking). |
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The |
batch |
process |
consists |
of |
heating |
high-boiling |
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petroleum |
feed- |
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stocks |
under pressure |
to approximately |
430°C |
usually |
for |
several |
days.t5j |
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This |
promotes |
the |
growth |
of |
mesophase-liquid |
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polycylic |
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crystals. |
The |
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material |
is then calcined |
up to 12OO”C, to remove |
almost |
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all |
the |
residual |
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hydrogen, |
and |
finally |
ground |
and |
sized. |
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By varying |
the |
source |
of oil and the |
process |
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parameters, |
it is possible |
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to obtain various |
grades |
of petroleum-coke |
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filler |
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with different |
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properties. |
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The industry |
commonly |
uses |
three grades: |
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Needle |
coke, |
a |
premium |
grade |
with |
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distinctive |
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needle- |
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shape |
particles, |
produced |
by delayed |
coking from selected |
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feedstocks |
with low concentration |
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of insolubles. |
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It is used |
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in applications |
requiring |
high thermal-shock |
resistance |
and |
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low electrical |
resistivity. |
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. Anode |
coke |
for less |
demanding |
applications. |
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Isotropic |
coke |
in |
applications |
where |
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isotropic |
properties |
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and a fine-grained |
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structure |
are |
required. |
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Binders. |
The |
most |
common |
binder |
is coal-tar |
pitch |
which |
is a hard, |
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brittle |
and |
glassy |
material, |
described |
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in Ch. 4, Sec. 2.3. |
It is a by-product |
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of metallurgical-coke |
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production |
and |
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is obtained |
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by the |
distillation |
or heat |
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treatment |
of coal-tar. |
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From |
35 to 60 |
kg of pitch |
are produced |
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from |
every |
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metric ton |
of coal. |
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The |
composition |
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of coal-tar pitch |
is complex |
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and may |
vary consider- |
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ably |
since |
it depends |
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on the |
degree |
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of |
refinement |
of the |
available |
coke- |
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oven |
tars. |
Two |
factors |
can |
noticeably |
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influence |
the |
quality |
and |
graphitiza- |
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tion characteristics |
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of the |
pitch: |
(a) its softening |
point |
and (b) the |
content |
of |
90 |
Carbon, Graphite, Diamond, and Fullerenes |
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insoluble |
complexes |
of quinoline |
(C,H,N), |
This |
content may |
vary widely |
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from one |
pitch |
to another!jl |
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Other binders |
such as petroleum |
pitch |
and |
thermosetting |
resins |
are |
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used |
for |
specialty |
applications. |
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2.2 |
Production |
Process |
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A |
typical |
production-process |
flow diagram |
for molded |
graphite |
is |
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shown |
in Fig. |
5.2.[‘]p1 |
The production |
steps |
are |
as follows. |
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Weighing
Scales
Graphitization
[ Machining ]
$
1 Inspection ]
Figure 5.2. Production-process flow diagram of molded graphite.[11[2]
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Molded Graphite |
91 |
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Milling |
and Sizing. |
Filler |
and |
binder |
are ground |
or milled |
to the |
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particle-size |
requirement |
which |
may |
vary |
from 1 pm |
(flour) to 1.25 |
cm. A |
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batch |
usually |
consists |
of more than one size. |
This allows better control of |
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the packing |
characteristics |
and |
optimizes |
the |
density |
of the final |
product. |
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Table |
5.1 |
lists the grain |
size of various |
grades |
of molded |
graphites |
and its |
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effect |
on |
properties.t3j |
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Table 5.1. |
Particle |
Sizes |
and |
Characteristics |
of Graphite |
Grades |
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Grade |
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Grain |
Size |
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Properties |
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Medium |
grain |
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Up to |
1.25 cm |
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Low |
density |
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Low |
thermal |
expansion |
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Low |
strength |
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High |
permeability |
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Fine |
grain |
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0.05 |
to 0.15 |
cm |
Medium |
density |
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Medium |
thermal |
expansion |
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Medium |
strength |
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Medium |
permeability |
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Micrograin |
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4 |
pm |
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to 75 pm |
High |
density |
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High |
thermal |
expansion |
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High |
strength |
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Low |
permeability |
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Mixing. |
Filler |
and binder |
are |
weighed |
in the |
proper |
proportion |
and |
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blended |
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with large |
mixers |
into a homogeneous |
mix where |
each filler particle |
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is coated |
with |
the |
binder. |
Blending |
is usually |
carried |
out |
at 160 |
- 170°C |
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although |
temperatures |
may |
reach |
as high as 315°C |
on |
occasion. |
When |
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mixing |
at lower |
temperatures |
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(belowthe |
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melting point of the |
binder), |
volatile |
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solvents |
such |
as |
acetone |
or |
alcohol |
are often |
added to |
promote |
binder |
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dispersion. |
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The |
final |
properties |
of the molded |
product |
are |
controlled |
to |
a great |
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degree |
by |
the |
characteristics |
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of the |
filler-binder |
paste |
such |
as: |
(a) |
the |
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temperature |
dependence |
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of |
the |
viscosity, |
(b) the |
general |
rheological |
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behavior, |
and |
(c) the |
hydrodynamic |
interaction |
between |
filler |
particles.pj |
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Forming Techniques. |
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Three |
major techniques |
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are used |
to form |
the |
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graphite |
mix: |
extrusion, |
compression |
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(uniaxial |
loading), |
and |
isostatic |
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pressing. |
They are |
shown |
graphically |
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in Fig. 5.3. |
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92 Carbon, Graphite, Diamond, and Fullerenes
Extrusion
Compression Molding
lsostatic Molding
Graphite
- Paste
Fluid
Figure 5.3. Forming techniques |
for molded graphites. |
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The |
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selection |
of a given |
technique |
has |
a great |
influence |
on the |
final |
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properties |
of the |
molded |
product |
as shown |
in Table |
5.2. |
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Extrusion. |
Extrusion |
is a major |
technique |
which |
is favored |
for |
the |
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production |
of |
parts having |
a constant |
cross-section, |
such as |
electrodes. |
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The |
mix |
is cooled |
to just |
above the |
softening |
point |
(approximately |
125”C), |
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then |
extruded |
through steel |
dies, |
cut to length and |
rapidly |
cooled to solidify |
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the |
pitch |
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before distortion |
occurs. |
The resulting |
shape |
is known |
in |
the |
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industry |
as a “green shape”. |
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Molded Graphite |
93 |
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Table 5.2. Characteristics |
of Forming Techniques |
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Technique |
Characteristics |
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Extrusion |
Anisotropic |
properties |
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Non-uniformity |
of cross-section |
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Presence |
of flow lines and laminations |
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Limited |
to |
parts |
of constant |
cross-section |
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Production |
of large parts possible |
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Low cost |
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Compression |
Non uniformity |
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Edge effect |
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Presence |
of flow lines and |
laminations |
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Medium |
cost |
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lsostatic |
Isotropic |
properties |
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Uniformity |
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No flow lines or laminations |
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High cost |
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Extrusion |
pressures |
are |
on the |
order of |
7 MPa (100 psi). |
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Some |
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alignment |
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of the |
|
coke-filler |
particles |
takes |
place |
which |
imparts |
anisotropy |
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to the properties |
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of the finished |
product. |
This |
anisotropy |
can be controlled |
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to some |
extent |
by |
changing |
the |
mix |
formulation |
|
and |
the |
extrusion |
geom- |
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etry.t4) The |
center |
of the |
extruded |
material |
is usually |
of lower quality |
than |
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the |
material |
near |
the outside |
edge and defects such as |
flow |
lines |
and |
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laminations |
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are |
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difficult |
to |
avoid. |
On |
the |
plus |
side, |
it is |
the lowest-cost |
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technique |
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which |
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is |
satisfactory |
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for most large parts, such as furnace |
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electrodes. |
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It represents |
the largest |
tonnage |
of molded |
graphite. |
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Compression |
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(Unlaxlal) |
Molding. |
The |
mix |
in compression |
molding |
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is usually |
a fine |
powder |
(flour) |
as opposed |
to the |
coarser |
material |
used in |
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extrusion. |
|
Tungsten |
carbide |
dies |
are frequently |
used with |
pressures |
on the |
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order of 28 to 280 MPa (4000 |
to 40,000 |
psi). |
Complex |
|
shapes |
can |
be |
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produced |
|
by this |
process (Fig. |
5.4).[*) |
However, |
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die-wall |
|
friction |
and |
die |
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edge effect may cause non-uniformity |
in the |
density and other properties |
of |
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the |
finished |
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product. |
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lsostatic molding. In isostatic molding, pressure is applied from all directions through a rubber membrane in a liquid-filled chamber, resulting
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Molded Graphite |
95 |
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in a material |
with great |
uniformity, |
isotropic |
properties, |
and generally |
with |
|
few |
defects. |
However, |
the molding |
process |
is expensive |
and cost is higher |
|
than |
extrusion |
or compression molding. |
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|
2.3Carbonization, Graphitization, and Machining
Carbonization. |
Carbonizing |
(also known as baking) the green shape |
|
is the next step (see Ch. 4, Sec. 2). |
Carbonization |
takes place in a furnace |
|
in an inert or reducing |
atmosphere. |
The process |
may last from a few days |
to several weeks |
depending |
|
on the constituents, |
|
and the size |
and geometry |
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of the |
part. |
The |
temperature |
|
is raised |
slowly |
to 600°C |
at which |
stage |
the |
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binder |
softens, |
volatiles |
are |
released |
and the |
material |
begins |
to shrink |
and |
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harden. |
Typical |
shrinkage |
is 6%. The |
parts must |
be supported |
by a packing |
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material |
to |
prevent |
|
sagging. |
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The temperature |
is then |
|
raised |
to 760 |
to |
|
980°C |
(or |
up to |
1200°C |
in |
|||||||||||||||||||||
special |
|
cases). |
This |
can |
be |
done faster than the first temperature |
|
step, |
|||||||||||||||||||||||||
since |
|
most |
of |
the |
volatiles |
|
have |
by |
now |
been |
removed, |
the |
material |
is |
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already |
hard, |
and |
sagging |
is no longer |
a problem. |
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Impregnation. |
|
|
After |
the |
|
carbonization |
|
stage, |
the |
material |
has |
|
a high |
|||||||||||||||||||
degree |
of porosity. |
|
To further |
densify |
it, it is necessary |
to impregnate |
|
it with |
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coal-tar |
pitch or a polymer such |
as phenolic. |
Impregnation |
is usually |
carried |
||||||||||||||||||||||||||||
out in a high-pressure |
|
autoclave |
and the carbonization |
process |
is repeated. |
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In special, |
limited-use |
applications, |
non-carbon |
|
impregnating |
|
materials |
||||||||||||||||||||||||||
such |
as |
silver |
and |
lithium |
fluoride |
impart specific |
characteristics, |
particu- |
|||||||||||||||||||||||||
larly |
increased |
electrical |
conductivity.t6) |
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Graphitization. |
|
During |
|
graphitization, |
the |
parts |
are |
heated |
up to |
|||||||||||||||||||||||
3000°C |
(see Ch. 4, |
Sec. |
3). |
|
The |
temperature |
|
cycle |
is |
shorter |
than |
the |
|||||||||||||||||||||
carbonization |
|
cycle |
and |
varies |
depending |
on the |
size |
of the |
parts, |
|
lasting |
||||||||||||||||||||||
from |
as |
short |
as |
a few |
hours |
to |
as |
long |
as |
three |
weeks. |
|
It |
is |
usually |
||||||||||||||||||
performed |
in |
a |
resistance |
|
furnace |
(the original |
Acheson |
|
cycle) |
or |
in |
a |
|||||||||||||||||||||
medium-frequency |
|
|
induction |
|
furnace. |
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||||||||||
|
Graphitization |
|
increases |
|
the |
resistance |
|
of |
the |
material |
to thermal |
||||||||||||||||||||||
shock |
and |
chemical |
|
attack. |
|
|
It also |
increases |
|
its |
thermal |
|
and |
electrical |
|||||||||||||||||||
conductivities. |
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|
Puffing. |
Puffing |
is an irreversibleexpansion |
|
|
of molded |
graphite |
|
which |
||||||||||||||||||||||||
occurs |
during |
graphitization |
|
when |
volatile |
species, |
such |
as sulfur from |
the |
||||||||||||||||||||||||
coke, |
are released. |
|
|
Puffing |
is detrimental |
as |
it causes |
cracks |
and |
other |
|||||||||||||||||||||||
structural |
defects. |
|
It can be eliminated |
(or at least |
considerably |
reduced) |