Alumina fibers properties

Particle or discontinuously reinforced MMCs have become important because they are inexpensive compared to continuous fiber-reinforced composites and they have relatively isotropic properties compared to the fiber-reinforced composites. Figures la and b show typical microstmctures of continuous alumina fiber/Mg and siUcon carbide particle/Al composites, respectively. 

The Nextel series of fibers produced by the 3M Company consists of a variety of aluminosilicate fibers. These contain mainly AljOj+SiOj and some B2O3. The compositions and properties of Nextel 312, Nextel 440, Nextel 550, Nextel 610, and Nextel 720 fibers are given in Table 6.3. Nextel 610 is, of course, the a-alumina fiber described above. The sol-gel manufacturing process used by 3M Co. has metal alkoxides as the starting materials. Metal alkoxides are M(OR)j -type compounds where M is the metal and n is the metal valence, and R is an organic compound. Selection of an appropriate organic group is very important. It should provide sufficient stability and volatility to the alkoxide so M-OR... 

The properties of alumina listed in Sects. 4-10 show the unusual performance of pure alumina, leading to the variety of applications given in Table 1. Practical aluminas with impurities and defects have somewhat degraded properties, but often are superior to many other materials, and have a variety of specialized applications such as refractories, electronic components, and catalyst substrates. In [1] there are articles discussing the future of alumina. There will continue to be incremental improvements in processing methods and properties, leading to expansion of present applications. What really new areas of application of alumina are likely These predictions are speculative, but the most promising new applications of alumina will probably be in electronic circuits, optical components, and biomaterials. Alumina fibers for composites and optics are attractive if they can be made pure, defect-free, and cheap. Because of its excellent properties other unsuspected applications of alumina will undoubtedly be developed. 

NANOCRYSTALLINE ALUMINA FIBERS STRUCTURE, PROPERTIES, APPLICATION... 

Features Crosslinkable with external crosslinking agents can be compded. to high solids with clays, whiting, and alumina trihydrate fillers imparts good tens, str. good adhesion to wide variety of fibers Properties 0.20 p particle size dens. 8.40 Ib/gal vise. 300 cps pH 8.7 anionic 53% solids S/B ratio 58/42... 

The alumina based fibers discussed in section 3.2 possess a range of compositions. They can be short, as with the S affil fibers or continuous, as with the others described. Their properties at room temperature depend on the ct-alumina content and at high temperature, the presence of any second phase (31). The Saffil fiber contains a few percent of silica with the remainder of the composition being alumina in one of its transition phases or as a mixture of transition phases and a-alumina. Table 7 shows the changes in processing of fibers of this type (32).The properties of alumina based fibers areshown in Table 8. Figure 3 shows the tensile curves of a pure a-alumina fiber, the Fiber FP, which had a grain size of 0.5 p,m (23). 

TABLE 9. Tensile properties of directionally solidified YAG-alumina fibers as a function of temperature. 

For a further improvement of mechanical properties MDF materials may be reinforced with fibers. Carbon and alumina fibers gave the best results (Park, 1998). The optimum fiber amount was 10-15% by volume. 

Table IV summarizes the effect of various oxides on the resulting fiber properties [30-31]. The addition of increasing amounts of alumina, beryllia, yttria or nitride has already been discussed in Chapter 4.1.2. These modifiers tend to increase the modulus or stiffness of a glass fiber. In addition, alumina and baria tend to increase the density, alumina and strontia tend to increase the refractive index, and zinc oxide and zirconia tend to increase the alkali resistance of a fiber.

F. T. Wallenberger, N. E. Weston and S. D. Brown, Melt processed calda-alumina fibers optical and structural properties, in Growth of materials for infrared detectors, R. E. Longshore J. Baars, Eds., Proceedings of the SPIE, Society of Photo-Optical Instrumentation Engineers, Bellington, WA, Volume 1484,116-124 (1991). 

Table III. Property change during the processing of alumina fibers containing 4% silica.

J. Dinwoodie, Saffil alumina fiber, manufacture, properties and application in high temperature furnaces, Canadian Ceramics Quarterly, 23-29, Feb. 1996. 

Lipowitz,J.,andRabe,J.A. (1997). Structure and properties of Sylramic silicon carbide fiber—a polycrystalline, stoichiometric fi-SiC composition. Ceram Eng Sci. Proc 18 145-157. Dhingra, A. K. (1980). Alumina fiber FP. Phil Trans R Soc. A294 411-417. 

Continuous alumina fibers are available from several suppliers. Chemical compositions and properties of the various fibers are significantly different. 

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