Altex fiber

The Nextel 720 contains the same alumina to silica ratio as in the Altex fiber, that is around 85% wt AI2O3 and 15 % wt Si02- The fiber has a circular cross section and a diameter of 12 im. The sol-gel route and higher processing temperatures have induced the growth of alumina rich mullite and alpha alumina. Unlike other alumina-silica fibers the Nextel... 

The combination of silica with alumina can retain transitional forms of alumina, as in the Altex fiber but the combination in the Nextel 480 fiber gives a mullite structure whereas the combination in the Nextel 720 fiber gives a mullite structure in which a-alumina grains are embedded. All three fibers, however lose strength above 1100°C, as shown in Figure 9. The fibers show very different creep behavior, as can be seen from Figure 10, with the Nextel 720 fiber showing the lowest creep rate of all oxide fibers. 

FIGURE 9. Temperature dependence of tensile strength of the Nextel 720, Nextel480 and Altex fibers all contain alumina and silica in various forms. (Reprinted, from reference 22, with kind permission of Marcel Dekker Inc.). 

Fibers with a large excess of alumina with respect to the nominal mullite composition exhibit similar features. When fired below the mullitization temperature, they consist of a mixture of transition alumina and amorphous silica with (pre-crystallized Nextel 720 fiber) or without (Altex fiber) corundum. Conversely, after mullitization in the 1100-1200"C temperature range (Figure 2a), the crystalline phases present in the fibers are mullite and corundum [18] [33]. 

Fig. 7. Effects of fiber type (1300and Altex) and test environment (RT-air and 77K-LNj) on the relation between mode I interlaminar fiacture touglmess and increment of crack length in fiber/Epoxyll laminates.

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