Detailed accounts of the manufacture of polygranular synthetic graphite may be found elsewhere.2,4,7 Figure 2 summarizes the major processing steps in

the manufacture of synthetic graphite. Synthetic graphite consists of two phases: a filler material and a binder phase. The predominant filler materials are petroleum cokes made by the delayed coking process or coal-tar pitch-derived cokes. The structure, shape, and size distribution of the filler particles are major variables in the manufacturing process. Thus, the properties are greatly influenced by coke morphol­ogy. For example, the needle coke used in arc furnace electrode graphite imparts low electrical resistivity and low coefficient of thermal expansion (CTE), resulting in anisotropic graphite with high thermal shock resistance and high electrical conductivity, which is ideally suited for the application. Such needle-coke materials would, however, be wholly unsuited for nuclear graphite applications, where a premium is placed upon isotropic behavior (see Chapter 4.10, Radiation Effects in Graphite). The coke is usually calcined (thermally processed) at 1300 °C prior to being crushed and blended.

The calcined filler, once it has been crushed, milled, and sized, is mixed with the binder (typically a coal-tar pitch) in heated mixers, along with certain additives to improve processing (e. g., extrusion oils). The formulations (i. e., the amounts of specific ingre­dients to make a specified grade) are carefully fol­lowed to ensure that the desired properties are attained in the final products. The warm mix is trans­ferred to the mix cylinder of an extrusion press, and

the mix is extruded to the desired diameter and length. Alternately, the green mix may be molded into the desired form using large steel molds on a vertical press. Vibrational molding and isostatic pressing may also be used to form the green body. The green body is air — or water-cooled and then baked to completely pyrolyze the binder.

Baking is considered the most important step in the manufacture of carbon and graphite. The pitch binder softens upon heating and goes through a liquid phase before irreversibly converting into a solid car­bon. Consequently, the green articles can distort or slump in baking if they are not properly packed in the furnace. If the furnace-heating rate is too rapid, the volatile gases evolved during pyrolysis cannot easily diffuse out of the green body, and it may crack. If a sufficiently high temperature is not achieved, the baked carbon will not attain the desired density and physical properties. Finally, if the baked artifact is cooled too rapidly after baking, thermal gradients may cause the carbon blocks to crack. For all of these reasons, utmost care is taken over the baking process.

Bake furnaces are usually directly heated (electric elements or gas burning) and are of the pit design. The furnaces may be in the form of a ring so that the waste heat from one furnace may be used to preheat the adjacent furnace. The basic operational steps include (1) loading, (2) preheating (on waste gas), (3) gas heating (on fire), (4) cooling (on air), and (5) unloading. Typical cycle times are of the order of hundreds of hours (Figure 3). The green bodies are stacked into the furnace and the interstices filled with pack materials (coke and/or sand). Thermocouples are placed at set locations within the furnace to allow

direct monitoring and control of the furnace temper­ature. More modern furnaces may be of the car — bottom type, in which the green bodies are packed into saggers (steel containers) with ‘pack’ filling the space between the green body and the saggers. The saggers are loaded onto an insulated rail car and rolled into a furnace. The rail car is essentially the bottom of the furnace. Thermocouples are placed within the furnace to allow direct monitoring and control of the baking temperature.

The furnaces are unpacked when the product has cooled to a sufficiently low temperature to prevent damage. Following unloading, the baked carbons are cleaned, inspected, and certain physical properties determined. The carbon products are inspected, usually on a sampling basis, and their dimensions, bulk density, and specific resistivity are determined. Measurement of the specific electrical resistivity is of special significance since the electrical resistivity correlates with the maximum temperature attained during baking. Minimum values of bulk density and maximum values of electrical resistivity are specified for each grade of carbon/graphite that is manufactured.

Certain baked carbon products (those to be fur­ther processed to produce synthetic graphite) will be densified by impregnation with a petroleum pitch, followed by rebaking to pyrolyze the impregnant pitch. Depending upon the desired final density, pro­ducts may be reimpregnated several times. Useful increases in density and strength are obtained with up to six impregnations, but two or three are more common. The final step in the production of graphite is a thermal treatment that involves heating the carbons to temperatures in excess of 2500 °C. Graphitization is achieved in an Acheson furnace in which heating occurs by passing an electric current throughout the baked products and the coke pack that surrounds them. The entire furnace is covered with sand to exclude air during operation. Longitu­dinal graphitization is increasingly used in the indus­try today. In this process, the baked forms are laid end to end and covered with sand to exclude air. The current is carried in the product itself rather than through the furnace coke pack. During the pro­cess of graphitization (2500-3000 °C), in simplistic terms, carbon atoms in the baked material migrate to form the thermodynamically more stable graphite lattice.

Certain graphite require high chemical purity. This is achieved by selecting very pure cokes, utiliz­ing a high graphitization temperature (>2800 °C), or by including a halogen purification stage in the manufacture of the cokes or graphite, either during graphitization or as a postprocessing step. Graphite manufacture is a lengthy process, typically 6-9 months in duration.

Graphite structure is largely dependent upon the manufacturing process. Graphites are classified according to their ‘grain’ size8 from coarse-grained (containing grains in the starting mix that are gener­ally >4 mm) to microfine-grained (containing grains in the starting mix that are generally <2 pm). The forming process will tend to align the grains to impart ‘texture’ to the green body. The extrusion process will align the grains with their long axis parallel to the forming axis, whereas molding and vibrational molding will tend to align the long axis of the particles in the plane perpendicular to the forming axis. Thus, molded graphite has two perpendicular with-grain (WG) orientations and one against-grain (AG) orientation, whereas extruded graphite has one WG orientation (parallel to the billets long axis) and two AG orientations. Isostatically pressed graphite does not exhibit a preferred orientation. Examples of various graphite microstructures are present in Figures 4-10. The graphite grades shown in Figures 4-10 have all either been used in nuclear applications or been candidates for nuclear reactor use.9 Grade AGOT (Figure 4) was used as the moderator in the earliest nuclear reactors in the United States. Pile grade A (PGA) graphite (Figure 5) was used as the moderator in the early air­cooled reactors and Magnox reactors in the United Kingdom.9 Grade NBG-18 is a candidate for the next generation of high-temperature reactors. Grade IG-110 (Figures 7 and 8) is the moderator material

in the high-temperature test reactor in Japan, and grade 2020 graphite (Figures 9 and 10) was a candidate for the core support structure of the modular high — temperature gas-cooled reactor in the United States.

A comparison of Figures 4 and 7 indicates the range of nuclear graphite textures. Figure 4 shows the structure of AGOT graphite, an extruded medium-grained, needle-coke graphite (maximum filler size ~-0.7 5 mm) and Figure 7 shows the struc­ture of IG-110 graphite, an isostatically pressed, fine-grained graphite (maximum filler size ~10 pm). Similarly, grade 2020 (Figures 9 and 10) is also a fine-grained, isostatically pressed graphite. The UK graphite PGA is extruded needle-coke graphite with a relatively coarse texture (Figure 5). The large individual needle-coke filler particles (named needle coke because of their acicular structure) can clearly be distinguished in this graphite. Another dominant feature of graphite texture can easily be distinguished in Figure 5, namely porosity. Graphite single crys­tal density is 2.26 gcm~3, while the bulk density is 1.75 gcm~ . The difference can be attributed to the porosity that is distributed throughout the graph­ite structure.10 About half the total porosity is open to the surface, while the remainder is closed. In the case of PGX graphite, large pores in the structure result in relatively low strength. The formation of pores and cracks in the graphite during manufacture adds to the texture arising from grain orientation and causes anisotropy in the graphite physical properties. Three classes of porosity may be identified in syn­thetic graphite:

• Those formed by incomplete filling of voids in the green body by the impregnant pitch; the voids originally form during mixing and forming.

• Gas entrapment pores formed from binder phase pyrolysis gases during the baking stage of manufacture.

• Thermal cracks formed by the anisotropic shrink­age of the crystals in the filler coke and binder.

Isotropic behavior is a very desirable property in nuclear graphite (see Chapter 4.10, Radiation Effects in Graphite) and is achieved in modern nuclear graphite through the use of cokes11 with an isotropic structure in the initial formulation.

Coke isotropy results in large measure from the optical domain structure of the calcined coke. The optical domain size is a measure of the extended — preferred orientation of the crystallographic basal planes. Essentially, the optical domain size and struc­ture (domains are the isochromatic regions in the coke and binder revealed when the structure is viewed at high magnification on an optical micro­scope under polarized light) controls the isotropy of the filler coke. Anisotropic ‘needle’ cokes have relatively large extended optical domains, whereas ‘isotropic’ cokes exhibit smaller, randomly orientated domains. The domain structure of a coke is devel­oped during delayed coking through pitch pyrolysis chemistry (mesophase formation) and coking trans­port phenomena. At the atomic scale, orientation of the crystallographic structure is characterized using X-ray diffraction analysis. The crystal spacing within the graphitized artifact may be determined (the dimensions a and c in Figure 1). Moreover, the extent to which the basal planes are parallel to one another, or crystal coherence length (la), and the mean height over which the layers are stacked in a coherent fashion (4) may be defined. These two para­meters, 4 and 4 (the crystal coherence lengths), define the perfection of the crystal (contained within the graphitized coke and binder) and the degree of graphitization.

An important feature of artificial graphite struc­ture, which has a controlling influence upon the material properties, is that the structural feature dimensions span several orders of magnitude. The crystal lattice parameters are fractions of a nanometer (a = 0.246 nm, c = 0.67 nm). The crystallite ‘coherent domains’ or extent of three-dimensional order, la and 4, are typically tens of nanometers (4 = stack height = 15-60 nm and la = stack width = 25-60 nm). The thermal microcracks between planes are typically the size of crystallites. Within the graphite, the opti­cal domain (extended orientation of crystallites) may typically range from 5 to 200 pm and largely controls the isotropy of synthetic graphite. As discussed ear­lier, graphite grain size (usually refers to largest filler particles) is a manufacturing variable and is typically in the range 1 pm to 5 mm. Finally, the pore size, depending upon the category and location (pores could be within filler or binder phases) is commensu­rate with grain size. The largest pores (excluding ther­mal cracks between the crystal layers) are typically 10 pm to a few millimeters.