Ch14

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Unformatted text preview: ameter and are about 2 to 5 m apart. Typical systems include a UO2 matrix with W fibers, a ZrO2 matrix with W fibers, and a Gd2 O3 matrix with Mo fibers. The composites are grown from a melt produced by induction heating, which allows the center of the ingot to melt while maintaining a solid skin of unmelted but compacted powder on the outside. The unmelted skin serves as a crucible to contain the melt. Since the melt temperature ranges from 2200 to 2800 C, any other type of crucible would contaminate the melt. The oxide matrix can be partially etched away and other elements can be vapor-deposited on the surface. The primary application of such composites is a high–current density electron source for high-power, high-frequency microwave devices. When a sufficiently high electric field is applied to the tips of the exposed fibers, electrons are drawn from the surface of the fiber in a process known as field emission. 14.6.1 Estimation of Nonmechanical Properties of Composites We have learned that several properties of composites can be estimated using the equation Pn c Vi P n i (14.6–1) i where Pc is the estimate for the property of the composite, the Pi values are the properties of components of the composite, the Vi values are the corresponding volume fractions, and the exponent n takes on values of 1 or 1 depending on the direction for which the property is being estimated. This equation can be used to estimate several other properties of composites, including density and, under certain conditions, the electrical and thermal conductivities of the material. In the case of density, the value of n is 1 regardless of the spatial distribution of the components of the composite. For both thermal and electrical conductivities, the equation is only useful if the second phase is in the form of fibers or sheets. For such composites, n has the value of 1 in a direction perpendicular to the fibers (or sheets) and the value 1 in the direction parallel to the fibers (or sheets). | v v 606 iq | e-Text Main Menu | Textbook Table of Contents pg607 [R] G1 7-27060 / IRWIN / Schaffer iq Chapter 14 plm QC2 rps MP Composite Materials 607 SUMMARY ........................................................................................................................................................................... Composites are an important class of materials used in high-performance structural, thermal, and electrical applications. Structural composites often combine the properties of low density, high strength, high elastic modulus, and good fracture and fatigue resistance. They provide an opportunity for tailoring the properties to meet the needs in any given direction or combination of directions. Most composites consist of a reinforcement component in the form of small-diameter fibers, whiskers, particles, or flakes. Aside from particles in concrete, fibers are the most common reinforcing agents. The use of small-diameter fibers made from various pol...
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This note was uploaded on 02/25/2013 for the course PHYS 2202 taught by Professor Sowell during the Spring '10 term at Georgia Institute of Technology.

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