Radiation Effects in Graphite and Carbon-Based Materials
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graphite consists of two phases: a filler material and a binder phase. The filler material is a petroleum coke made by the delayed coking process, or a coal-tar pitch derived coke. The coke is usually calcined (thermally processed) at ~1300°C prior to being crushed and blended. Typically the binder phase is a coaltar pitch. The binder plasticizes the
RAW PETROLEUM OR PITCH COKE i
' CALCINED AT 1300°C f
CALCINED COKE i
CRUSHED, GROUND AND BLENDED
t
BLENDED PARTICLES
BINDER PITCH
I i
MIXED
i
COOLED
i
EXTRUDED, MOLDED OR ISOSTATICALLY PRESSED
i
GREEN ARTIFACT i
BAKED AT 1000°C
i
r
BAKED ARTIFACT IMPREGNATED TO DENSIFY
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GRAPH ITIZED 2500-2800°C 1
GRAPHITE
Figure 1. The major processing steps in the production of a conventional polygranular graphite.
filler coke particles so that they can be formed. The binder phase is carbonized during the subsequent baking operation (~1000°C). Frequently, engineering graphites undergo an impregnation stage to densify the carbon artifact, followed by rebaking. Useful increases in density and strength are obtained with up to six impregnations, but two or three is more typical. The final stage of the manufacturing process is graphitization (25002800°C), during which, in simplistic terms, carbon atoms in the baked material migrate to form the thermodynamically more stable graphite lattice. Pyrolytic graphite5-8 is deposited at high temperatures, typically 1600-2100°C, onto a carbon or graphite substrate from methane or other hydrocarbon-rich gas mixtures. The deposited carbon has a structure consisting of hexagonally arranged networks of atoms lying parallel to one another in coherent regions. The rate of deposition is temperaturedependent, as is the density of the deposit. Highly oriented pyrolytic graphites are produced by very high-temperature annealing (>3000°C) and/or hot working the deposited carbons—for example, compression-annealed pyrolytic graphite. The latter forms of pyrolytic graphite have near-perfect graphitic structures and as such have been frequently used in fundamental studies of radiation-damage effects in graphite. In its perfect form, the crystal structure of graphite (Figure 2) consists of tightly bonded (covalent) sheets of carbon atoms in a hexagonal lattice network.9 The sheets are weakly bound in an ABAB stacking sequence with a separation of 0.335 nm. The crystals in a manufactured, polygranular graphite are less than perfect, with approximately one layer plane in every six constituting a stacking fault. The graphite crystals have two distinct dimensions, the crystallite size La measured parallel to the basal plane and the dimension Lc measured perpendicular to the basal planes. In a coke-based nuclear graphite, values of La ~ 80 nm and Lc ~ 60 nm are typical." A C/C composite material comprises a carbon or graphite matrix that has been reinforced with carbon or graphite fibers. Multidirectionally reinforced C/C composites are substantially stronger, stiffer, and tougher than conventionally manufactured polygranular graphites and are thus
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