Alumina Fibers microstructure

Figure 6.5 (a) Microstructure of an alumina fiber. TEM. (b) Microstructure of an (alumina+zirconia) fiber. SEM. The white particles are zirconia. 

Fig. 3 Left Preceramic paper microstructure (1 - alumina fiber, 2 - cellulose (pulp) fiber) right Stress-strain behaviour of paper with various cellulose/alumina-fiber ratio.

Adding inorganic components to alumina-based precursors also has a strong affect on crystallization behavior and the fiber microstructure. For instance, small grain size can be achieved by stabilizing transition alumina by adding Si02... 

Sabol, S.M. 1994. Development of textured and tabular grain microstructure in alumina fibers via seeded sol-gel processing. Ph.D. diss., Pennsylvania State University. 

Sabol, S.M., and R.E. Tressler. 1990. Microstructural tailoring of polycrystalline alumina fibers. HiTemp Review 15 1-11. ... 

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). 

The microstructural features of transition alumina fibers virhich can be formed by calcining dry spun alumina-silica gel fibers are shown in Table III. The first phase that crystallizes from the amorphous gel is r -alumina. This phase is a spinel with vacancies distributed in the octahedral sites [10]. The calcination temperatures corresponding to each transition alumina can be estimated from Figure 2b. 

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]. 

Wilson (1990) has provided some details of the microstructural evolution in this fiber. A fine-grained o-AljOj fiber is obtained by seeding the high temperature a-alumina with a very fine hydrous colloidal iron oxide. The fine iron oxide improves the nucleation rate of a-Al Oj, with the result that a high density, ultra-fine, homogeneous o-Al203 fiber is obtained. The rationale for seeding with iron oxide as follows. Basic salts of aluminiun decompose into transition aluminum oxide spinels such as above 400°C. These transition cubic spinels... 

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