Oxide Fiber Composites 3315 Alumina Mullite Mullite Precursor-derived particles alumina Mullite 1 μm (a) Schematics of the matrix topologies produced by mullite/alumina particle mixtures(top) and mullite particles bonded by precursor-derived a (bottom).(b)Compositional maps produced by energy-dispersive spectroscopy of TEM foils. The top image shows a particle mixture of 80% and 20% alumina(without precursor addition), whereas the bottom one is of a mullite powder compact bonded by 15% precursor-derived during the same aging cycle. Some shrinkage occurs for higher alumina content(> 30%0), but its evolution remains extremely luggish in relation to that for pure alumina powder(inset in (a) Mullite/Alumina Mixture Fig. 14(a)), by about four orders of magnitude. The results con- 90M/10A firm that the mullite network is effective in inhibiting matrix 100M densification, even for relatively large amounts of the sinterable Despite the absence of shrinkage, both Young,s modulus E nd the toughness f increase appreciably with aging time, by a a88 80M/20A 098 factor of 3-4(Fig. 14(b): a consequence of surface-diffusion- ontrolled sintering at the particle junctions. The implications 9.96Fo5 for crack deflection are addressed in a subsequent section. Alumin 70M30A Yet further enhancements in matrix properties have been C achieved through the design of all-mullite matrices. 43 The con- 0.94 cept uses two particle populations with vastly dissimilar sizes (e.g, I and 0.1 um) and exploits the differences in their sinterin kinetics. When present in the appropriate proportion, the small- 100 er particles can be readily sintered to the larger particles without (b)Mullite compromising the stability of the main network. In principle, a similar structure could be achieved using mullite precursor so- Modulus, E lutions. However, the temperatures required for mullitization are well beyond those that the present fibers can withstand An additional processing enhancement involves use of a time- delayed setting agent(e.g, AIN)in the slurry. The agent (a) Effects of aging at 1200.C on the porosity s with compositions ranging from M)to 60% mullite and 40% alumina(60M/ property changes of pure mullite. Sin diffusion nism with diffusivity =4x10-30m'Is.The 0.1 02 ferred junction toughness is Tj=3 J/'m": only slightly higher than the ace energy contribution(2y=2 J/m-) ging time, t (h)during the same aging cycle. Some shrinkage occurs for higher alumina content (  30%), but its evolution remains extremely sluggish in relation to that for pure alumina powder (inset in Fig. 14(a)), by about four orders of magnitude. The results con- firm that the mullite network is effective in inhibiting matrix densification, even for relatively large amounts of the sinterable phase. Despite the absence of shrinkage, both Young’s modulus E and the toughness G increase appreciably with aging time, by a factor of 3–4 (Fig. 14(b)): a consequence of surface-diffusion￾controlled sintering at the particle junctions.42 The implications for crack deflection are addressed in a subsequent section. Yet further enhancements in matrix properties have been achieved through the design of all-mullite matrices.43 The con￾cept uses two particle populations with vastly dissimilar sizes (e.g., 1 and 0.1 mm) and exploits the differences in their sintering kinetics. When present in the appropriate proportion, the small￾er particles can be readily sintered to the larger particles without compromising the stability of the main network. In principle, a similar structure could be achieved using mullite precursor so￾lutions. However, the temperatures required for mullitization are well beyond those that the present fibers can withstand without degradation. An additional processing enhancement involves use of a time￾delayed setting agent (e.g., AlN) in the slurry.43 The agent Fig. 13. (a) Schematics of the matrix topologies produced by mullite/alumina particle mixtures (top) and mullite particles bonded by precursor-derived alumina (bottom). (b) Compositional maps produced by energy-dispersive spectroscopy of TEM foils. The top image shows a particle mixture of 80% mullite and 20% alumina (without precursor addition), whereas the bottom one is of a mullite powder compact bonded by 15% precursor-derived alumina. (Adapted from Fujita et al. 40). Fig. 14. (a) Effects of aging at 12001C on the porosity of mixed mullite– alumina compacts with compositions ranging from 100% mullite (de￾noted 100M) to 60% mullite and 40% alumina (60M/40A). (b) Corre￾sponding property changes of pure mullite. Sintering occurs by a surface diffusion mechanism with diffusivity dSDS 5 4 1030 m3 /s. The in￾ferred junction toughness is Gj3 J/m2 : only slightly higher than the surface energy contribution (2g2 J/m2 ). November 2006 Oxide Fiber Composites 3315