handbook of carbon, graphite, diamond and fullerenes

Molded Graphite 95
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.
2.3 Carbonization, Graphitlzatlon, 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
of the part. The temperature is raised slowly to 600°C, at which stage the
binder softens, volatiles are released and the material begins to shrink and
harden. Typical shrinkage is 6%. The parts must be supported by a packing
material to prevent sagging.
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
already hard, and sagging is no longer a problem.
Impregnation. After the carbonization stage, the material has a high
degree of porosity. To further densify it, it is necessary to impregnate it with
coal-tar pitch or a polymer such as phenolic. I mpregnation is usually carried
out in a high-pressure autoclave and the carbonization process is repeated.
In special, limited-use applications, non-carbon impregnating materials
such as silver and lithium fluoride impart specific characteristics, particularly
increased electrical conductivity .PI
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.
Graphitization increases the resistance of the material to thermal
shock and chemical attack. It also increases its thermal and electrical
conductivities.
Puffing. Puffing is an irreversible expansion 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)