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SELECTED APPLICATION AREAS. 91 absorption. Highly transparent ferromagnets, new types of indirectly coupled magnetoelectrics, and piezomagnets of enhanced properties are to be expected. At these very fine nanoscales, new work to explore the possibility of super paraelasticity could give rise to families of highly nonlinear elastic materials with potential for tailoring the nonlinearity. Clearly on all other electro-, magneto-, and elasto-optical systems, regular periodicities in the nanoscale internal structures will give rise to interesting new pass or stop bands in their interaction with electromagnetic or acoustic waves, analogous to the Brillouin zone structures in crystalline systems. Again, much work remains to be done to develop the technologies necessary to assemble composites on the regular scaling necessary for these realizations. ULTRASTRUCTURED CERAMICS In conventional ceramic processing the powders employed are often characterized by uncontrolled geometry and chemistry. This results in microstructures (irregularly shaped particles or spheres) and ultrastructures (interphases, secondary phases, and pores between spheres) that produce levels of performance far below the theoretical limit (Figure 26). Figure 26 The impact of ultrastructure processing on ceramic performance.
SELECTED APPLICATION AREAS. 92 The attainment of properties that approach theoretical values in high-temperature structural ceramics by the ultrastructure processing approach is schematically shown in Figure 27. The complete control of raw materials includes the design of molecules and chemistry for powders that, upon densification, provide the compositional stoichiometry necessary for glass-free grain boundaries. The viability of the ultrastructure concept in the chemical processing of mullite (3Al2O3â¢2SiO2) has been demonstrated. Figure 27 Comparison of conventional and sol-gel mullite. In comparison to other high-performance materials such as silicon nitride, Si3N4, mullite is not considered to be a useful high-strength material at low temperatures. Its potential becomes apparent only at elevated temperatures, where Si3N4 matrix ceramics start losing their strength (Figure 27). Mullite-forming gels were made from colloidal beohmite (AlOOH, aluminum monohydroxide) and tetraethoxy-silane (TEOS). Upon sintering at 1200°C, these gels form dense (greater than 99 percent of theoretical density) and translucent mullite. Conventional mullite prepared from mixed powders requires 1600°C. This ability to sinter mullite at temperatures significantly lower than the usual 1600°C sintering temperature represents a very important processing accomplishment. Sintering of the same sol-gel-derived mullite at 1250°C results in infrared- transparent mullite, forming the basis for sol-gel passive infrared optics for the 3 to 5 micron region (Figure 28).
