Oxide fibers properties

Oxide fibers are manufactured by thermal or chemical processes into a loose wool mat, which can then be fabricated into a flexible blanket combined with binders and formed into boards, felts, and rigid shapes or fabricated into ropes, textiles and papers. The excellent thermal properties of these products make them invaluable for high temperature industrial appHcations. 

Nonoxide fibers, such as carbides, nitrides, and carbons, are produced by high temperature chemical processes that often result in fiber lengths shorter than those of oxide fibers. Mechanical properties such as high elastic modulus and tensile strength of these materials make them excellent as reinforcements for plastics, glass, metals, and ceramics. Because these products oxidize at high temperatures, they are primarily suited for use in vacuum or inert atmospheres, but may also be used for relatively short exposures in oxidizing atmospheres above 1000°C. 

The most important properties of refractory fibers are thermal conductivity, resistance to thermal and physical degradation at high temperatures, tensile strength, and elastic modulus. Thermal conductivity is affected by the material s bulk density, its fiber diameter, the amount of unfiberized material in the product, and the mean temperature of the insulation. Products fabricated from fine fibers with few unfiberized additions have the lowest thermal conductivities at high temperatures. A plot of thermal conductivity versus mean temperature for three oxide fibers having equal bulk densities is shown in Figure 2. 

Oxide fibers such as silica and alumina, which have oxidation stability and insulating properties, are used for heat insulators, such as the TPS (Fig. 3) on the upper part of the Space Shuttle. On the surface of the TPS, a coating of sil-... 

Under these conditions, many types of continuous oxide fiber were developed. The physical properties of these oxide fibers are shown in Table 2 [11]. Methods for preparation of these oxide fibers include spinning of a sol, a solution, or slurry, usually containing fugitive organics as part of a precursor. 

Table 2 Physical properties of various oxide fibers...

The need to develop fibers with better microstructural stability at elevated temperatures and ability to retain their properties between 1000-2000°C. The requirements of fiber properties for strong and tough ceramic composites have been discussed by DiCarlo.83 A small diameter, stoichiometric SiC fiber fabricated by either CVD or polymer pyrolysis, and a microstructur-ally stable, creep-resistant oxide fiber appear to be the most promising reinforcements. 

Crystalline oxide fibers represent an important dass of ceramic fibers mainly because of their superior oxidation resistance, being oxides. We describe the processing, structure, and properties of oxide fibers, mainly alumina and some alumina+silica-type fibers. 

Table 6.4 Composition and properties of some non-Nextel oxide fibers. 

Several oxide fibers on the basis of Zr02, Pb(Zr,Ti)03 (PZT), Y3AI5O12 (YAG) and YBa2Cu30x, which exhibit e.g. catalytic, magnetic, dielectric or superconducting properties, are currently under development or in evaluation for special applications. 

Fiber strength cannot be maintained if the oxidation process goes too far. It is therefore important to find a balance between fiber properties and the ability of the fibef s surface to interact with the surrounding materials. Moderate oxidation generally gives the best performance. 

Zirconia stabilized by yttria (3 mol. %) was used as initial nanopowder. It was produced from the salt containing hydro-cellulose fibers using the thermal treatment. During the treatment the solvent was removed and oxide fibers composed from nanograins of PSZ were formed. The fibers were grinded in fine dispersed powder from which half-finished products were formed. Some properties of the powder are presented in Table 1. 

Oxide fibers have been commercially available since the 1970s. Control of the microstructure through careful processing is essential to obtain the desired properties, which for ceramic fibers for sfrucfural applicafions are... 

Cost Nonoxide fibers cost thousands of dollars per kilogram. Oxide fibers, even those that have been commercially available for years, sell for hundreds of dollars per kilogram. The main reason is that production volumes are small. Most hber-reinforced CMCs utilize a layer between the fiber and matrix to optimize mechanical properties. The methods used for depositing this layer tend to be expensive and difficult to scale up for production. 

Spun non-oxide fibers (i.e., those made by the first three processes above) are produced in tows consisting of hundreds of filaments with diameters of 10 to 20 pm. Typical ranges of properties are given in Table ES-1. All of these fibers are based on SiC except for amorphous Si-B-N-C, a promising new fiber. 

Commercial polycrystalline oxide fibers are produced by spinning and hydrolyzing precursors. First a fiber precursor solution is filtered and concentrated to remove excess solvent, forming a viscous spin dope. Then, continuous filaments are extruded by spinning. The filaments are pyrolyzed to remove volatile components and then heat treated above 800X (1,472°F) to crystallize and sinter the fiber. Polycrystalline oxide fibers are produced in tows of200 to 1,000 fibers with diameters of 10 to 16 pm (0.39 to 0.63 mils). Typical ranges of properties are listed in Table ES-1. 

Recommendations 2, 3, and 4 (listed in order of decreasing priority) are related to performance, which is also considered to be a high priority. Recommendation 2 is the most important in this category. The oxidation resistance of oxide fibers is attractive, but poor creep resistance is a significant limitation. Thus, Recommendation 3 addresses the need to improve this property. Recommendation 4 (regarding non-oxide fibers) is last in this category because the committee concluded that resources directed toward property improvement in fiber coatings and oxide fibers was more important. The committee is satisfied that the preliminary properties reported for Si-B-N-C amorphous fibers are sufficiently attractive to stimulate the research needed to verify them. 

The CMC market is divided into two classes, oxide and non-oxide materials. Oxide composites consist of oxide fibers (e.g., alumina [AI2O3]), interfacial coatings, and matrices. If any one of these three components consists of a non-oxide material (e.g., silicon carbide [SiC]), the composite is classified as a non-oxide composite. These classes have different properties, different levels of development, and different potential applications. 

Composite behavior has also been studied in oxide systems (e.g., oxide fiber-reinforced porous oxide matrix composites with no interfacial coatings). Oxide composites have the attractive features of oxidation resistance, alkali corrosion resistance, low dielectric constants, and potentially low cost. Because of these properties, oxide CMCs could be attractive for hot gas filters, exhaust components of aircraft engines, chemical processing equipment, and long-life, lower temperature components. 

The committee made two assumptions to formulate the interrelationships between application requirements and fiber properties. First, the combination of tensile strength, creep resistance, and rupture resistance exhibited by available SiC fibers is approaching theoretical limits. Second, enhancing the creep strength of oxide fibers by several hundred degrees centigrade may be possible although this has not been demonstrated. 

In the Tyranno family of non-oxide fibers, Ti was originally incorporated into the fiber during processing to create a very fine P-SiC grain size. In the Tyranno ZM fiber and its derivatives, zirconium (Zr) is included to improve its high-temperature properties and resistance to NaCl corrosion. 

TEMPERATURE AND TIME DEPENDENCE OF PROPERTIES OF NON-OXIDE FIBERS... 

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