Composite materials are blends of two or more materials of different physical properties. The individual materials are immiscible with each other and exist as distinct phases. Thus, composite materials are multiphase materials consisting of two or more phases. Different materials are mixed together with the purpose of generating superior materials having properties better than those of the individual materials. Composite materials are a rapidly growing class of materials, with applications in industries such as plastics, automotive, electronic, packaging, aircraft, space, sports, and the biomedical field.
In the design, processing, and applications of composite materials, a thorough understanding of the physical properties is required. It is important to be able to predict the variations of the electromagnetic (electrical conductivity, dielectric constant, and magnetic permeability), mechanical, thermal (thermal conductivity and coefficient of thermal expansion), and mass transport properties of composite materials with the kind, shape, and concentration of filler materials.
The filler material may consist of equiaxed particles ranging anywhere from nanometers to microns in size, discontinuous short fibers or whiskers, small disk- or plate-shaped particles/flakes, or core-and-shell type of complex particles. A number of excellent books are available on composite materials, but for the most part, they are restricted to classification, applications, and manufacturing of composite materials along with the characterization of mechanical properties. The electromagnetic, thermal, and mass transport properties of composite materials have generally received little attention as compared with the mechanical properties even though they are equally important from a practical point of view.
The study of electrical, dielectric, and magnetic properties of composite materials can reveal valuable information regarding the morphology and composition of such systems. For example, the dielectric probes could be used to probe the microstructure and to estimate the filler content of composites, especially when the dielectric constants of the individual materials are significantly different from each other. The electrical properties of composites are important in the design of plastics used in the electronics industry. Pure plastics tend to pick up electrostatic charges, especially under low-humidity conditions.
When earthed, the (charged) plastics discharge and, in the process, damage electronic circuitry and equipment. To overcome the problems associated with electrostatic charge of plastics, electrically conducting filler particles (such as carbon black) are incorporated into the plastic matrix. The incorporation of electrically conducting filler particles into the plastic matrix imparts electrical conductivity to the plastic system, and as a consequence, the buildup of static charge is avoided. The magnetic properties of composite materials are of interest in many industrial applications involving electrical and electronic instruments, electrical power generators and transformers, electric motors, radio, television, telephones, computers, audio and video equipment, etc.
The thermal properties of composite materials are important in many practical applications. For example, knowledge of the coefficient of thermal expansion (CTE) of composites is required in calculating dimensional changes and buildup of internal stresses when composites are subjected to temperature changes. In designing a composite material, it is often necessary to match the CTE of different components. The other very important thermal property of composite materials is their thermal conductivity. In the electronics industry, the packaging material used to encapsulate electronic devices must have a high thermal conductivity in order to dissipate the heat generated by the device as rapidly and effectively as possible.
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