Inorganic fibers properties

Wallenberger, F. T., et al., Advanced Inorganic Fibers Processes, Structures, Properties, Applications, Kluwer, 1999. 

Properties. CVD boron fibers have high strength, high modulus, and low density. Their properties are summarized and compared with SiC fibers and other inorganic fibers in Table 19.2 (data supplied by the manufacturers). 

In this book we define inorganic fibers in a general sense as small elongate solid objects composed of any compound or element usually nonbio-logic in origin and often exhibiting distinctive physical, especially mechanical, properties. Inorganic fibers can occur naturally, that is, as mineral fibers or can be produced synthetically. 

Inorganic fibers can also be produced from substances that do not form polymers. Under special conditions, virtually any compound can grow as a fibrous solid. These conditions occasionally exist in nature, but today many inorganic fibers are custom-crafted. A compound synthesized in fibrous form usually possesses additional and desirable mechanical properties over the compound in any other form. 

Asbestos is a fibrous inorganic material. It is mined and exploited because of its unique chemical and physical properties, in part the result of its distinctive fibrous form. The hazards, as we understand them, are also attributed to this fibrous character, but asbestos represents only a fraction of the many inorganic fibers now in use. Furthermore, although it is a readily recognizable form, fiber has no precise scientific or technical definition. Thus, to address the health effects of asbestos, federal government (OSHA—Oc-... 

The focus here on self-organizing polymers, electrical and optical properties, and biosynthesis is not even close to comprehensive in the enumeration of avenues and opportunities in new polymeric materials. Degradable plastics that do not remain in our environment forever [24] and polymer precursors to ceramics [19] and inorganic fibers are two of many more areas in which new polymeric materials will provide new challenges to engineers in production and processing. 

Inorganic fibers Felt Pure felt To 2200°F Excellent heat resistance, poor mechanical properties. Resilient, compressible and strong, but not impermeable. Resists medium-strength mineral acids and dilute mineral solutions if not intermittently dried. Resists oils, greases, waxes, most solvents. Damaged by alkalies. 

In extrusion, in addition to the nature and the properties of the materials used to make the moldable mixture, the additives used, the pH, the water content, and the force used in extrusion are also of importance with respect to the properties of the monolith products [31]. The additives applied in extrusion are, e.g., celluloses, CaCl2, ethylene, glycols, diethylene glycols, alcohols, wax, paraffin, acids [13,16,24,30], and heat-resistant inorganic fibers [24]. Besides water, other solvents can also be used, such as ketones, alcohols, and ethers [15,16]. The use of additives may lead to improved properties of the monoliths, such as the production of microcracks that enhance the resistance to thermal shock [4,12], better porosity and adsorbability [15,16], and enhanced mechanical strength or a low thermal expansion [24]. 

By far the most important application fields for inorganic fibers are the insulation and reinforcing sectors. Fibers are also used as fillers and as filter materials. As with other materials, functional properties such as electrical, optical or magnetic properties are becoming increasingly important for fibers, in addition to mechanical and electrical properties. 

Inorganic fibers can currently be produced from a wide range of element combinations and further fiber-types are in development (see Section 5.2.7), so that a classification according to chemical composition, as favored by preparative chemists, is not reasonable. Other possible classification criteria are e.g. the production process, the source of the fibers (natural or synthetic), their degree of order (amorphous or crystalline), their thermal stability (27 - 2227°C) or physical properties (tensile strength, elasticity modulus). The boundaries between the individual fiber types are, however, often fluid. 

Asbestos, the first inorganic fiber material used, is currently still exclusively produced from natural mineral deposits. It is formed by the hydrothermal conversion of basic and ultrabasic volcanic rock (olivine and pyroxene) to serpentine upon which the actual asbestos formation takes place leading to two asbestos sorts with different structures serpentine asbestos and amphibole asbestos. Asbestos can be produced synthetically by several hours heating of a polysilicic acid/metal oxide mixture (e.g. Mg, Fe, Co, Ni) in water at 300 to 350°C and 90 to 160 bar. The properties of four important asbestos types are summarized in Table 5.2-2. 

In some applications, it is useful to incorporate nonplastic substances into a plastic object, to reduce its cost or improve its performance in some way. Fillers are typically used to lower the cost of the plastic, and generally consist of minerals of some kind. Reinforcements are often more expensive, per unit mass or volume, than the plastics, but provide improvement in properties such as strength and/or rigidity. They usually consist of either organic or inorganic fibers. Use of fillers and reinforcements is less common in packaging applications than in uses such as automotive components or housewares, but is sometimes significant. In addition, these additives are more commonly used with thermoset polymers than with thermoplastics. 

Nowadays, in place of asbestos, fibrous reinforcements are used that include glass fiber, steel fiber, aramid fiber, potassium titanate fiber, etc. Since these fibrous reinforcements have their own specific properties, in practice, a mixture of them is used. Potassium titanate fiber is a hard inorganic fiber. It can improve the strength, the heat resistance and the wear resistance of the fiietion material. In addition, it can enhance the friction coefficient of the friction material through its abrasive property. 

Ideally, pigment should provide only color. They are expected to be heat- and light-stable and not have any effect on fiber properties. However interactions do occur. Some are predictable—such as the interaction between some inorganic sulfides with nickel stabilizers. Most interactions are unpredictable and are found only via an Edisonian approach, that is, produce the fiber and test it. 

In the scientific context of this book, generic and specific compositions are more meaningful than trade names. Therefore, compositional descriptions have been used throughout the book to characterize a given fiber, notably in discussions which relate structures to properties. Trade names have been used only when absolutely necessary. No attempt has been made to suppress the diversity of trivial names as they appear in the literature. A fiber by any name is still a fiber. In summary, the book introduces a unified view of advanced inorganic fibers to the aspiring materials science student and attempts to foster cross-fertilization among the experts in the field. 

In summary however, vapor phase processes offer a more effective route to advanced inorganic fibers than solid or liquid phase processes because they facilitate greater control over diameter, length, aspect ratio, and properties of the resulting whiskers, microfibers, and nanotube structures. 

F. T. Wallenberger, Inorganic fibers and microfabricated parts by laser assisted chemical vapour deposition (LCVD) structures and properties, Cer. International, 23,119-126 (1997). 

With a few exceptions, metal matrix composites (MMCs) are still in a development stage although it is well known that the presence of inorganic fibers in a metal matrix has a major effect on the properties of the metal. The reinforcement and manufacturing costs are inevitably too high for the potential markets, such as that of automotive engines [29-33]. 

Advanced inorganic fibers processes-structures-properties-applications/ contributors, Frederick T.Wallenberger... [et al.] editor, Frederick T. Wallenberger. p. cm. — (Materials technology series)... 

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