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50 100 150 200 250 300 350 400 450 500 550 Temperature, °C
Figure 1. Variation of apparent viscosity with temperature of various pitch fractions. Reprinted with permission. Materials Technology Distinguished lecture 1986, "The Future of Carbon-Carbon Composites" (Professor Erich Fitier).
weight fractions (sparging) as shown in figure 2. In general, however, as the temperature of the pitch is increased, the rate of mesophase formation and viscosity increase. Mesophase spheres similar to those shown in figure 3 begin to appear at temperatures greater than 400°C, growing until they coalesce to become a continuous phase. At this stage, a preferred orientation of large numbers of these liquid crystals imparts a directionality or anisotropy to the properties of the resultant coke (ref. 28). Maintaining the temperature for longer times results in the viscosity increasing rapidly until the pitch becomes a brittle, predominantly crystalline solid (coke). Pitch-based matrices, when graphitized, are dense (~1.9 g/cm ).
Carbonization of isotropic coal tar or petroleum pitch produces a 50- to 60percent coke yield under atmospheric conditions. However, if the carbonization is performed very slowly or under high pressure (29000 psi (200 MPa)), the coke yield can be increased to 70 to 80 percent (ref. 5). If the 100-percent mesophase pitch, first produced from an initially isotropic pitch, is stabilized (oxidized) before carbonization, an even greater yield, up to 92 percent, can be achieved (ref. 27).
Figure 2. Percentage mesophase (anisotropy) versus reaction time at 425°C under static (ODAx+j and stirring conditions <•).
The microstructure of carbonized cokes is influenced by the temperature and pressure history used in the processing. Previous studies have shown that lowpressure carbonization, 1000 psi (6.9 MPa), of coal tar pitch produces a needlelike (flow) coke texture, in which the mesophase is deformed through bubble percolation. However, under higher pressures (9860 psi (68 MPa)), the coke has a mosaic anisotropic texture, and gas formation is suppressed. Under high pressures the mesophase forms at lower temperatures. At even higher pressures, 29000 psi (200 MPa), coalescence of the mesophase does not occur (ref. 36).
High-pressure impregnation/carbonization forces thermoplastic pitch into very small fissures and cracks generated during previous carbonization/graphitization steps. Using pressures up to 30000 psi (207 MPa) reduces the temperature associated with the thermal degradation and improves the carbon yield by reducing the loss of volatiles (foaming). The effect of pressure increases the viscosity of any liquid. This effect produces a less compliant thermoplastic, more matrix cracks, and a smaller and more reactive graphitic structure. Conversely, the retention of the volatiles tends to decrease the viscosity, producing a more compliant matrix.
McAllister (ref. 4) has shown that high-pressure impregnation/carbonization of three-dimensional fiber preforms with coal tar pitch increases the yield and density of the final CC composite. Following six cycles of pitch impregnation/carbonization, Figure 3. Photomicrograph of mesophase spheres in isotropic pitch.
a CC composite was obtained with a density of ~ 1.9 g/cm3. These data compare with the data for a density of 1.6 g/cm3 following eight impregnations at atmospheric pressure. Under atmospheric conditions, the maximum achievable density is ~1.8 g/cm3.
Thermoset Resin Matrices
Phenolics and epoxy resins are two types of commonly used thermosetting resins. Both these resins are cured prior to carbonization; since they are thermosets, they will not flow from the fibrous preforms during first carbonization.
Resin matrix composites are fabricated from preimpregnated carbon fiber layers (prepreg) of woven fiber cloth. This type of material is preferred when fabricating complex shapes. The materials technology involves the impregnation of one layer of carbon fibers (or carbon fabric) with a resin. This prepreg is partially cured (B-staged) to a fixed degree of tackiness and can be used immediately or refrigerated for 6 to 12 months. Prepregs are cut and combined according to the need and hot-pressed or cured in an autoclave to produce a rigid solid. They are then pyrolyzed and subsequently densified by repeated infiltration/carbonization cycles. This process is described in detail by Curry et al. (ref. 37). Although some new resin precursors have been developed that have higher carbon content (>80 percent), the carbon char yield in commercially available resins can be as low as 50 percent or as high as 70 percent, depending on the resin and processing conditions. See table 3 (ref. 38).
Table 3. Precursory Organic Matrices and Their Char Yields (ref. 38*)
*Reprinted by permission of the Society for the Advancement of Material and Process Engineering. Schmidt, D. L.: Carbon-Carbon Composites. SAMPEJ., May-June 1972, pp. 9-19.
In a typical process, multiple layers of prepreg fabric (fiber and resin) are laid up on a mold before being pressed at a given temperature (150°C to 300°C) under a given pressure of 100 psi to 500 psi (0.69 MPa to 3.45 MPa) for up to 10 hours. The exact processing conditions are dependent on the time/viscosity properties of the resin used in the prepreg. A typical hot-pressing cycle is shown in figure 4. During hot pressing, the resin softening allows the air pockets to be squeezed from between the laminates. The pressure is usually applied continuously during cool down (5 to 10 hours).
Figure 4. Typical hot-pressing cycle for processing phenolic matrix composites.
Composites described in the previous paragraph are carbonized at atmospheric pressure under an inert gas flow. Carbonization to ~800°C is usually carried out slowly (~ 10°/hour) in order to prevent the rapid evolution of volatiles such as H2O and CH4 and to prevent consequent delamination between the woven layers. The carbonization cycle may take many hours or sometimes days to complete.
During carbonization the resin matrix is converted to carbon, the porosity increases typically from ~3 to ~25 percent, and the density decreases from 1.5 g/cm3 to 1.3 g/cm3. The carbonized composite is repeatedly heat-treated to higher temperatures (graphitized) and reinfiltrated to complete the densification process.
Densification cycles can be carried out using either resin, CVI, or pitch feedstocks. If a resin impregnant is chosen, one with low room temperature viscosity is used. Furfural alcohol and phenolic dissolved in isopropanol are