Transition alumina fibers

Transition alumina fibers are obtained by the sol-gel route, as long as the firing temperature is... 

If a shrinkage resistant transition alumina fiber is desired, sintering of the porous fiber should take place in a temperature range that is actually quite narrow for pure alumina. Thus, most commercially available transition alumina fibers are stabilized with oxide additives such as silica. The addition of silica (Figure 2b) considerably enlarges the apparent stability domain of transition aluminas, the formation of a-alumina being shifted to almost 1250°C for only 5 wt.% Si02 [34]. 

The preparation of continuous transition alumina fibers requires the use of liquid precursors with a higher viscosity, e.g. 30- 100 Pa.s. The control of the viscosity of the spinning dope is achieved through (1) the use of high molecular weight polymers and (2) proper vaporization of the solvent in a vacuum [37]. After heat treatment, the fibers are composed of one or several transition aluminas, depending on the sintering temperature. 

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. 

As shown in Figure 5 of Chapter 4, the modulus increases as the alumina content of the fibers increases. For a given composition, the stiffness of alumina-silica fibers produced by the sol-gel route depends on the residual porosity and hence the processing conditions. For example, the modulus of alumina fibers with a 96 AbOrSiOz (wt.%) composition increases from 110 GPa for the highly porous r -transition alumina fiber, to 280-300 GPa for the dense 5-AI2O3 fiber prepared at higher temperature. 

Thermal Insulation is by far the most Important present application of oxide fibers. Transition alumina fibers, e.g., eta-alumina fibers, are produced at Intermediate firing temperatures and are used as supports for catalysts and Insulation tiles such as those used for the space shuttle orbiter [1-2]. Carbon fiber felts are used as internal thermal insulation for vacuum furnaces at extremely high temperatures. Activated carbon fibers, which are obtained by partial oxidation of selected carbon fibers, have extremely small pores and very high specific surface areas, ranging from 500 to 3000 mVg. They are of great interest in ultrafiltration as membranes for the treatment of used waters and liquids [3-5]. 

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... 

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). 

Colloidal sols can be prepared from aluminum chloride (or nitrate) by dissolving AlCb (or AI(N03)3-9H20) and aluminum pellets in water under reflux, followed by filtration [27]. The hydrolysis/polycondensation reaction is continued until the viscosity of the precursor is suitable for dry spinning. The green fibers are dried, prefired to 800°C and sintered at 1300°C in air. Crystallization of the gel occurs above 700°C. First transition aluminas are formed and then converted to a-alumina [27],... 

Boria-free fibers consist of nanocrystalline ri- or y-transition alumina and are porous when fired at 1000°C. They are fully sintered and crystallized as mullite at 1400°C. B20rmodified fibers are poorly crystallized as transition alumina when fired at 1000°C, display a low specific surface area and transform completely to mullite at 1400°C (density 3.00 glcni). Boria seems to enhance sintering at low temperatures before mullitization, and the boria-modified fibers exhibit a lower mullite grain size. 

The precursor of a similar fiber was prepared by polycondensation of an organoaluminum compound, such as monoisopropoxydiethyl aluminum, dissolved in ethylether [60]. Some isopropoxy groups were presumably replaced by a phenoxy group, such as ethyl 0-hydroxybenzoate, in order to improve the spinnability of the final dope. The polyaluminoxane was dissolved in benzene, the ether was distilled off and ethyl silicate was added. After concentration, the dope was dry spun and the green fibers were aged in a humid atmosphere and calcined. The fibers (Table II) had a glassy appearance and were composed of a nanocrystalline Al-Si spinel phase (or r /y-transition alumina) in an amorphous silica based matrix [33] [53]. After mullitization that starts at 1150 C and is complete after 2 min at 1400 C, the fibers were composed of mullite and corundum [33]. 

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]. 

In the following sections some examples are given of the ways in which these principles have been utilized. The first example is the use of these techniques for the low temperature preparation of oxide ceramics such as silica. This process can also be used to produce alumina, titanium oxide, or other metal oxides. The second example describes the conversion of organic polymers to carbon fiber, a process that was probably the inspiration for the later development of routes to a range of non-oxide ceramics. Following this are brief reviews of processes that lead to the formation of silicon carbide, silicon nitride, boron nitride, and aluminum nitride, plus an introduction to the synthesis of other ceramics such as phosphorus nitride, nitrogen-phosphorus-boron materials, and an example of a transition metal-containing ceramic material. 

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... 

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. 

The nano alumina is end-attached and the fibers project out about 0.2 to 0.3 pm into the flow stream. This results in an open space, free for fluid to flow imimpeded through the 2 pm average pore size, allowing moderate to high flowrate at low pressme drop. Computations show that there is a local electropositive field that projects out up to about 1 pm beyond the nano alumina (2) that attracts nano size particles (e.g.-viras) that pass close by, increasing the capture cross section. The filter media s thickness is about 0.8 mm thick, resulting in approximately 400 pores that a particle must transit before exiting as... 

The history of ceramics is as old as civilization, and our use of ceramics is a measure of the technological progress of a civilization. Ceramics have important effects on human history and human civilization. Earlier transitional ceramics, several thousand years ago, were made by clay minerals such as kaolinite. Modem ceramics are classified as advanced and fine ceramics. Both include three distinct material categories oxides such as alumina and zirconia, nonoxides such as carbide, boride, nitride, and silicide, as well as composite materials such as particulate reinforced and fiber reinforced combinations of oxides and nonoxides. These advanced ceramics, made by modem chemical compounds, can be used in the fields of mechanics, metallurgy, chemistry, medicine, optical, thermal, magnetic, electrical and electronics industries, because of the suitable chemical and physical properties. In particular, photoelectron and microelectronics devices, which are the basis of the modern information era, are fabricated by diferent kinds of optical and electronic ceramics. In other words, optical and electronic ceramics are the base materials of the modern information era. 

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