A-Alumina fibers

Researchers at du Pont (Dhingra, 1980) made a continuous a-alumina fiber by spinning a viscous solution. Du Pont Co. does not make these fibers any more. Nevertheless, we give a brief description of this process of making alumina fiber because it represents an important technique. The basic fabrication procedure involves three steps ... 

An important polycrystalline a-alumina fiber has been developed by 3M Co. This a-alumina fiber, trade name Nextel 610, is made via a sol-gel route. The sol-gel process of making fibers involves the following steps common to all sol-gel processing ... 

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 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). 

FIGURE 3. Tensile behavior of apure a-alumina fiber, Fiber FP, as a ftinction of temperature. 

FIGURE 5. Thepure a-alumina fiber. Fiber FP, crept from 1000°C and the creep rate increased with temperature. (Reprinted, from reference 23, with kind permission ofKluwer Academic / Plenum Publishers). 

Figure 7 compares the creep behavior of the three nearly pure a-alumina fibers. It can be seen that Fiber FP crept at the lowest rate at each temperature. The smaller grains in the Nextel 610 fibers allowed them to creep faster whilst the porosity and residual stresses in the Almax fibers meant that they crept the fastest (23). 

FIGURE 8. A comparison of the creep rate of the Nextel 650 and PRD-166 fibers, both of which are composed of a-alumina and zirconia, the Nextel 720 which is composed of a-alumina and mullite and the pure a-alumina Fiber FP. (Reprinted, from reference 12, "Mechanical and Microstructural Characterization of Nextel 650 Alumina-Zirconia Fibres , with kind permission of Elsevier). 

A comparison of creep rates of single crystal a-alumina fibers and directionally solidified YAG-alumina fibers, at 1400°C, reveals that the former shows no creep at a rate... 

Alumina based fibers are subject to creep, even at low temperatures, This phenomenon is exacerbated by the fine grain size of the fibers [20] [54] [67] [70] [73] [79-80]. At 1200°C and with an applied stress of 70 MPa, the strain rate for the fine-grained Nextel 610 a-alumina fiber is higher than that of Fiber FP with a coarser microstructure. Both strain rates are about one order of magnitude higher than that for a bulk alumina ceramic with a grain size of 1.2 jm. The aeep of Nextel 610 is already measurable at a temperature as low as 900 C under an applied stress of 200-500 MPa [70]. 

In this equation, A is a constant, Q is the apparent activation energy, R is the gas constant, G is the shear modulus, and p and n are the inverse grain size and stress exponents, respectively. The values of n and Q, measured for the fibers considered here, are listed in Table VI. The high values of n suggest that creep deformation is rate controlled by an interface reaction-limited grain boundary diffusional process [78], Mullite fibers are more resistant to creep than a-alumina fibers. For example, the strain rate of the Nextel 480 mullite fiber at 1200 C is almost two orders of magnitude lower than that of the Nextel 610 a-alumina fiber [55]. 

The creep of alumina based fibers can be reduced by adding a second phase. For example, the steady state creep rate of a PRD-166 a-alumina/t-ZrOa (Y) fiber is about one order of magnitude lower than that of the single phase a-alumina fiber (Fiber FP) [78]. This effect is thought to be related, at least at low temperatures (T<1100°C), to the fact that the dispersed zirconia particles at grain boundaries limit the mobility of intergranular dislocations [67],... 

Both carbon fiber/lead and alumina/lead composites have been utilized for the fabrication of electrodes in lead add batteries with a significant weight reduction. For example, reinforcing lead unidirectionally with 25 vol.% of a-alumina fibers increases the matrix stiffness by a factor of 20 and reduces the electrical conductivity only slightly. Due to the stiffening effect of the fibers, pure lead can be used instead of a complex alloy, thus reducing the cost of the matrix and improving its corrosion resistance [49]. 

Saitow, Y, Iwanaga, K., Itou, S., Fukumoto, T. and Utsunomiya, T. (1992) Preparation of continuous high purity a-alumina fiber. In Proceedings of the 37th International SAMPE Symposium, March 9-12, pp. 

Oxide fibers include glass fibers, mullite fibers, zirconia fibers and alumina fibers. Of these, a-alumina-based fibers have been used intensively for ceramic matrix composites. Fiber FP, manufactured by Du Pont in 1979, was the first wholly a-alumina fiber produced [34]. At present, Almax (Mitsui Mining Material Co. Ltd., Japan) and Nextel 610 (3M Co., USA) are commercially available a-alumina fibers. Almax contains 99.5% alumina and has an elastic modulus of 330 GPa, and Nextel 610 has a tensile strength of 2.4 GPa and an elastic modulus of 380 GPa [35]. 

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