SiC composites are a family of materials of varied constituents and architectures. Up to the point of writing this chapter, nuclear-grade SiC composites (those specifically developed for application in fast neutron environments and exhibiting neutron irradi­ation damage resistance) are more precisely defined as continuous fiber-reinforced ceramic composites. The history of development for these materials has been reviewed in a number of publications.29,77-79 The primary constituents of these nuclear-grade com­posites are the continuous SiC fiber, a fiber/matrix interphase material that can be SiC or pyrolytic graph­ite or a combination of the two, and a matrix of SiC infiltrated into the woven fiber preform. The most common matrix material is derived from chemical vapor infiltration (CVI), and is essentially identical in structure, properties, and irradiation response to the CVD SiC discussed in previous sections. While there has been little direct study on the effects of irradiation on the material properties of the SiC interphase, it can be assumed that it would also behave in a similar manner to the SiC matrix. However, the effect of neutron irradiation on pyrolytic graphite interphase (if used) will be substantially different from that on both matrix and fiber. While the effect of irradiation on the underlying properties of graphite interphase has not been well studied, it can be assumed that the interphase will behave in a similar manner to nuclear graphite (discussed in Chapter 4.05, Radiation Dam­age of Reactor Pressure Vessel Steels).

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Figure 21 Example of braided nuclear-grade SiC/SiC composite. Fiber: Hi-Nicalon™ Type-S; Interphase: Multilayer SiC with pyrolytic carbon; Matrix: CVI SiC deposited through an isothermal process. Reproduced from Nozawa, T.; Lara-Curzio, E.; Katoh, Y.; Shinavski, R. J. Tensile properties of advanced SiC/SiC composites for nuclear control rod applications. Wiley: 2007; pp 223-234.

An example of an SiC/SiC composite that has been developed for high-temperature gas-cooled reactor control rod applications is shown in Figure 21. The basic textile weaving of the composite is evident on inspection of Figure 21(a). In this case, a ±55° weave is depicted. For the polished section of Figure 21(b), large voids, which are an unavoidable characteristic of chemical vapor infiltrated materials and also the primary reason why it is difficult to produce gas — impermeable SiC/SiC composite, are clearly observed. In Figure 21(c), the complicated structure of the interphase is seen. In this case, alternating layers of SiC and pyrolytic graphite have been applied. The pyrolytic graphite layer between the SiC layers is quite thin (tens of nanometer), with a relatively thick graphite layer in contact with the fiber itself.

From the earliest study of SiC/SiC composites under irradiation, it was clear that the fiber was the key to performance. As with the impure forms of SiC monolithic ceramics (cf. Figure 17), the impure and oxygen-rich early grades of SiC fiber (trade name Nicalon™) were quite unstable under neutron irra­diation.12,80,81 Researchers were able to directly link an irradiation-induced shrinkage of the SiC-based
fibers with debonding of the fiber-matrix interface that severely compromised the ability to load the high-strength fibers.80 Composite mechanical prop­erties such as strength suffered appreciably.

With continued evolution of the fiber systems to increasingly pure, stoichiometric materials, the irradiation stability improved significantly. Presently, there are two commercial fiber systems used in nuclear-grade composites, both of which have rela­tively low impurity contents and are approaching a 1:1 stoichiometry. Specifically, the ~11 micron Hi-Nicalon™ Type-S fiber has the nominal chemis­try of SiC105, 0.2%-O, while the ^7.5 and ~10 pm Tyranno™ SA-3 fibers have the nominal chemistry of SiC1.07, 0.5% Al. Study has revealed that these ‘near stoichiometric’ fibers exhibit irradiation — induced swelling similar to that of CVD,82 thus avoiding the debonding phenomenon mentioned in the previous paragraph. For this reason, composites fabricated from these materials are superior under irradiation to their predecessors. Consistent with the discussion of properties of irradiated monolithic SiC, the following discussion will be limited to the more pure, near stoichiometric fiber materials.

The effect of neutron irradiation on the Weibull mean strength of individual ‘near stoichiometric’ fibers is given in Figure 22.83’84 Within inherent sta­tistical scatter, no change in strength is observed for either the Hi-Nicalon™ Type-S or the Tyranno™ SA-3 bare fibers. The numbers inset to the figure indicate the irradiation temperature of the SiC fibers, with no apparent function of irradiation temperature on strength observed. From the same study, the effect of irradiation on composite properties is also observed. Figure 2 3 67 gives the proportional limit stress for which the load departs from elastic behavior and the ultimate tensile strength. As with the fiber data, and the data for monolithic CVD SiC (Figure 18), the composite strength does not exhibit any statistically meaningful change. Supporting studies14,82,83,85-87 on the strength in tension or bending of neutron-irradiated stoichiometric fiber composites support the fact that at least up to ^40 dpa, composite strength is not significantly affected by irradiation. A recent study88 on the fracture toughness of irradiated and unirradiated Hi-Nicalon™ Type-S composites also reports no appreciable change. However, a minor dif­ference in the fracture surface (length of fiber pull out) and a trend in the fiber-matrix interphase properties are reported,89 suggesting that mechanical property evolution may occur at higher doses.

In the unirradiated state, the thermal conductivity of SiC composites is dependent on variables includ­ing the fibers and matrix constituents, processing, and the level of porosity. For the nuclear composite considered here, there is considerable thermal conduc­tivity anisotropy and temperature dependence typical of all ceramics. This is demonstrated in Figure 24, which gives the measured and calculated thermal conductivity for the two nuclear-grade SiC compo­sites.90 Presented are Hi-Nicalon™ Type-S fiber and Tyranno™ SA fiber composites, each matrix infiltrated through CVI.58 Architectures included balanced (1:1:1 for x:y:z) and unbalanced (1:1:4) 3D forms and 2D laminates (SW: satin weave, PW: plain weave.) In each case, a pyrolytic graphite interphase was applied. The conductivity for all materials is presented in the through thickness direc­tion (perpendicular to the plate and the fabric for the 2D composite.) This typically represents the low — conductivity direction.

As evident from Figure 24 and the supporting analysis by Katoh,90 the fiber makes a significant con­tribution to the thermal conductivity of these highly stoichiometric fiber composites, and this conductivity is a fairly strong function of temperature. However, the absolute conductivity is only a fraction of that for the highest thermal conductivity CVD SiC (cf. Figure 7.)

Figure 23 Effect of neutron dose on tensile proportional limit and ultimate tensile stresses for composites. Data labels indicate the nominal irradiation temperature in °C for Hi-Nicalon™ Type-S (upright) and Tyranno™ SA-3 (oblique) composites. Reproduced from Katoh, Y.; Snead, L. L.; Nozawa, T.; Kondo, S.; Busby, J. T. J. Nucl. Mater. 2010, 403, 48-61.

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Figure 24 Thermal conductivity of representative nuclear-grade SiC/SiC composite in unirradiated condition. Reproduced from Katoh, Y.; Nozawa, T.; Snead, L. L.; Hinoki, T.; Kohyama, A. Fus. Eng. Des. 2006, 81, 937-944.

As with the CVD SiC discussed in section 4.07.3, silicon carbide composite also undergoes significant degradation in thermal conductivity because of neutron irradiation. The data is somewhat limited;

however, Figure 25 gives the ambient through­thickness thermal conductivity for a plain weave Hi-Nicalon™ Type-S, multilayer SiC interphase, and CVI SiC matrix composite. It is noted that, in

Effect of neutron irradiation on the through-thickness thermal conductivity of Hi-Nicalon™ Type-S, CVI matrix

Figure 26 Comparison of the thermal defect resistance for neutron irradiated CVD SiC and Hi-Nicalon™ Type-S, CVI matrix composite.

comparison to the conductivity shown in Figure 24 (second from lowest curve), the ambient through­thickness thermal conductivity for the material of Figure 25 is relatively low (10.2 ± 2.2Wm~1K~1). This is mostly ascribed to the large porosity for that composite. Nevertheless, the figure clearly shows a significant, irradiation temperature-dependent reduc­tion in thermal conductivity as a function ofirradiation dose. The fact that this is temperature dependent suggests that the degradation is due to the produc­tion of stable point defects and clusters, as discussed in Section 4.07.3, although this may not be the sole factor determining the degradation. Figure 26 provides the accumulated thermal defect resistance at the lowest and highest irradiation temperature for the composite materials of Figure 25, compared with high-conductivity CVD SiC. It is interesting to note that the thermal defect resistance for the composite, while accumulating in the same manner as that of the CVD SiC, is about an order of magnitude greater than that of CVD SiC at a given dose (at least prior to saturation.) This greater accumulation of thermal defect resistance has been recently observed by Katoh67 The reason for this is unclear, although it is plausible that, in addition to defect production, propa­gation of internal interfaces (e.g., cracks) in the com­posite is occurring under irradiation. It is also possible that the defects population responsible for phonon scattering for the composite material is stabilized at a higher level than that of the highly pure CVD SiC.90