Electrical Properties of Materials

Starting point for this paragraph is the electrical conduction of metals, a phenomenon with which everyone will be familiar. A simplified crystal structure of metals was already discussed in chapter 3, in the paragraph on Chemical bonds . In a metal, there is question of an electron cloud which moves between the metal ions. When the metal is connected 

In figure 11.4.1 some copper ions can be seen. They vibrate around their lattice positions and the intensity of these vibrations increases as the temperature rises. The vibrations are the reason why the flow of the electrons is inhibited. The electrons continuously collide with the copper ions and that is why we say that the electrons experience resistance. The size of the resistance of the copper wire can be calculated using Ohm s law. 

Both specific resistance and electrical conductivity depend on temperature. Materials can differ considerably as far as electrical conductivity is concerned and this fact is used to subdivide them, as illustrated in table 11.4.2. 

Without going into a complete derivation, let us examine Eq. (6.1) more closely to see where the driving force, electrical current, and electrical conductivity come from. 

The electric field, is a vector quantity, as indicated by the boldfaced type. For the time-being, we will limit the description to an electric field in one direction only and 

When a conductive material is placed within the electric field, current begins to flow, as characterized by the current density, J, of Eq. (6.1). The current density is also a vector quantity, but since our field is in one dimension only, current will similarly flow only in one direction, so that we will use only the scalar quantity from here on, J. The current density is simply the current, /, per unit area in the specimen. A  

We define the quantity in parentheses as the resistance, R, to obtain the more familiar form of Ohm s Law  

Consider a wire, 3 mm in diameter and 2 m long. Use the data in Appendix 8 to answer the following questions. 

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Material Properties Numerical Data System Purdue University Purdue University (CINDAS) evaluated data compiled, correlated, analyzed, and synthesized to generate values for the thermophysical, mechanical, and electrical properties of materials... 

Commonly used materials for cable insulation are poly(vinyl chloride) (PVC) compounds, polyamides, polyethylenes, polypropylenes, polyurethanes, and fluoropolymers. PVC compounds possess high dielectric and mechanical strength, flexibiUty, and resistance to flame, water, and abrasion. Polyethylene and polypropylene are used for high speed appHcations that require a low dielectric constant and low loss tangent. At low temperatures, these materials are stiff but bendable without breaking. They are also resistant to moisture, chemical attack, heat, and abrasion. Table 14 gives the mechanical and electrical properties of materials used for cable insulation. 

L. Solymar and D. Walsh, Lectures on the Electrical Properties of Materials, Oxford University Press, Oxford, 1988. 

The electrical properties of materials are important for many of the higher technology applications. Measurements can be made using AC and/or DC. The electrical properties are dependent on voltage and frequency. Important electrical properties include dielectric loss, loss factor, dielectric constant, conductivity, relaxation time, induced dipole moment, electrical resistance, power loss, dissipation factor, and electrical breakdown. Electrical properties are related to polymer structure. Most organic polymers are nonconductors, but some are conductors. 

Solymar, L. and Walsh, D. 1998. Electrical Properties of Materials. Oxford University Press, New York. Turi, E. 1997. Thermal Characterization of Polymeric Materials, 2nd ed. Academic Press, Orlando, PL. Urban, M. 1996. Attenuated Total Reflectance Spectroscopy of Polymers. Oxford University Press, New York. 

Figure 6.8 One dimensional schematic representation of the energy momentum curve for seven electrons in a conductor with (a) no applied electric field and (b) an applied electric field. Reprinted, by permission, from L. Solymar, and D. Walsh, Lectures on the Electrical Properties of Materials, 5th ed., p. 427. Copyright 1993 by Oxford University Press.

Many of the fundamental relationships and concepts governing the electrical properties of materials have been introduced in the previous section. In this section, we elaborate upon those topics that are more prevalent or technologically relevant in ceramics and glasses than in metals, such as electrical insulation and superconductivity, and introduce some topics that were omitted in Section 6.1.1, such as dielectric properties. 

Before proceeding, it may be useful to summarize some of the myriad of terms and symbols that have been used to this point. Such a summary is presented in Table 6.5. With the exception of a few more terms dealing with superconductivity, we have introduced all the variables we will need to describe the electrical properties of materials. 

ELECTRICAL PROPERTIES OF MATERIALS 593 Table 6.13 Some Electrical Properties of Selected Resins... 

In this section we discuss the electrical properties of materials that can be envisaged as sheets of material with metallic conduction, separated by some distance from each other. We suppose first that there is no disorder. 

The magnetic, optical, and electrical properties of materials often depend on the microstructural details and the morphology of materials. Even if the final state is not a colloid, many products pass through colloidal processing routes prior to the final stage. The availability of methods to produce model particles allows us to study and control the desired properties of the final product. 

In the past two decades, the study of the electrical properties of materials has lead to a considerable increase in the understanding of solid state physics and chemistry. This new understanding has lead to the development of new and unusual electronic materials and devices... 

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