Composite Materials

Engineering materials constitute the foundation of technology, whether the technology pertains to structural, electronic, thermal, electrochemical, environmental, biomedical or other applications. The history of human civilization evolved from the Stone Age to the Bronze Age, the Iron Age, the Steel Age and to the Space Age (simultaneously the Electronic Age). Each age is marked by the advent of certain materials. The Iron Age brought tools and utensils. The Steel Age brought rails and the Industrial Revolution. The Space Age was brought about by structural materials (e.g., composite materials) that are both strong and lightweight. The Electronic Age was brought about by semiconductors. Materials include metals, polymers, ceramics, semiconductors and composite materials.

The transfer of heat by conduction is involved in the use of a heat sink to dissipate heat from an electronic package, the heating of an object on a hot plate, the operation of a heat exchanger, the melting of ice on an airport runway by resistance heating, the heating of a cooking pan on an electric range, and in numerous industrial processes that involve heating or cooling. Effective transfer of heat by conduction requires materials (such as a heat sink material) of high thermal conductivity. In addition, it requires a good thermal contact between the two surfaces (such as the surface of a heat sink and the surface of a printed circuit board) across which heat transfer occurs. Without good thermal contacts, the use of expensive thermal conducting materials for the components is not cost effective. The attainment of a good thermal contact requires a thermal interface material, such as a thermal grease, which must be thin (small in thickness) between the mating surfaces, must conform to the topography of the mating surface and preferably should have a high thermal conductivity as well. This chapter addresses materials for thermal conduction, including materials of high thermal conductivity and thermal interface materials. In addition, this chapter addresses materials for thermal insulation and heat retention, which are important in buildings for the purpose of energy saving. Thermoelectric applications can be considered a subset of thermal applications, but they are addressed in Chapter 6 rather than this chapter.

Composite materials are traditionally designed for use as structural materials. With the rapid growth of the electronics industry, composite materials are finding more and more electronic applications. Owing to the vast difference in property requirements between structural composites and electronic composites, the design criteria for these two groups of composites are different. While structural composites emphasize high strength and high modulus, electronic composites emphasize high thermal conductivity, low thermal expansion, low dielectric constant, high/low electrical conductivity and/or electromagnetic interference (EMI) shielding effectiveness, depending on the particular electronic application. Low density is desirable for both aerospace structures and aerospace electronics. Structural composites emphasize processability into large parts, such as panels, whereas electronic composites emphasize processability into small parts, such as stand-alone films and coatings. Owing to the small size of the parts, material costs tend to be less of a concern for electronic composites than structural composites. For example, electronic composites can use expensive fillers, such as silver particles, which serve to provide high electrical conductivity.

This chapter covers composite materials for electromagnetic applications, including structural and non-structural composite materials. Among the structural composite materials, both polymer-matrix and cement-matrix composites are addressed.

Thermoelectric phenomena involve the transfer of energy between electric power and thermal gradients. They are widely used for cooling and heating, including air conditioning, refrigeration, thermal management and the generation of electrical power from waste heat.

Electrical insulators are also known as dielectrics. Most ionic solids and molecular solids are insulators because of the negligible concentration of conduction electrons or holes.

Light is electromagnetic radiation in the visible region, which includes light of colors violet, blue, green, yellow, orange and red, in order of increasing wavelength from 0.4 to 0.7 pm. The visible region only constitutes a small part of the electromagnetic spectrum, which includes γ-rays, X-rays, ultraviolet, visible, infrared, microwave and radio (TV) radiation, in order of increasing wavelength from 10⁻¹⁴ to 10⁴ m, as shown in Figure 8.1.

An electron is associated with a spin, which results in a magnetic moment. The magnetic moment of an electron is 9.27 × 10⁻²⁴ A.m², or 1 Bohr magneton (β). A filled orbital contains two electrons of opposite spin, so the net magnetic moment of a filled orbital is zero.

Electrochemical behavior pertains to chemical processes brought about by the movement of charged species (ions and electrons) under the influence of an electric field. It occurs when there are ions that can move in a medium (liquid or solid) due to an electric field (i.e., a voltage gradient). The positive ions (cations) move toward the negative end of the voltage gradient, while the negative ions (anions) move toward the positive end of the voltage gradient (Figure 10.1). The medium containing the movable ions is called the electrolyte The electrical conductors in contact with the electrolyte for the purpose of applying the electric field are called electrodes. The electrode at the positive end of the voltage gradient is called the positive electrode or anode The electrode at the negative end of the voltage gradient is called the negative electrode or cathode. The electrodes must be sufficiently inert so that they do not react with the electrolyte. In order to apply the electric field, the electrodes are connected to a DC power supply (or a battery) such that the cathode is connected to the negative end of the power supply (so that the cathode becomes negative) and the anode is connected to the positive end of the battery (so that the anode becomes positive). In this way, electrons flow in the electrical leads (the outer circuit) from the anode to the cathode, while ions flow in the electrolyte. The electron flow in the outer circuit and the ion flow in the electrolyte constitute a loop of charge flow. Note that the anions flow from cathode to anode in the electrolyte, while the electrons flow from anode to cathode in the outer circuit and both anions and electrons are negatively charged.

Vibrations are undesirable for structures, owing to the need for structural stability, position control, durability (particularly durability against fatigue), performance and noise reduction. Vibrations are of concern to large structures such as aircraft, as well as small structures such as electronics.

Smart structures are important because of their relevance to hazard mitigation, structural vibration control, structural health monitoring, transportation engineering and thermal control. Research on smart structures has emphasized the incorporation of various devices in a structure for providing sensing, energy dissipation, actuation, control or other functions. Research on smart composites has emphasized the incorporation of a smart material in a matrix material for enhancing smartness or durability. Research on smart materials has emphasized the study of materials (e.g., piezoelectric materials) used for making the devices. However, relatively little attention has been given to the development of structural materials (e.g., concrete and composites) that are inherently able to provide some of the smart functions, so that the need for embedded or attached devices is reduced or eliminated, thereby lowering cost, enhancing durability, increasing the smart volume and minimizing mechanical property degradation (which usually occurs in the case of embedded devices).