1 Material Science Prof. Satish V. Kailas Associate Professor Dept. of Mechanical Engineering, Indian Institute of Science, Bangalore India Chapter 14. Electrical properties Engineering materials are important in everyday life because of their versatile structural properties. Other than these properties, they do play an important role because of their physical properties. Prime physical properties of materials include: electrical properties; thermal properties; magnetic properties; and optical properties. The electrical behaviors of engineering materials are diverse, and so are their uses in electrical applications. Few examples of electrical applications: copper and aluminium wires used in power transmission; amorphous silicon in solar cells; LEDs; gallium-arsenide chips used in modern day computers, cell phones, digital assistants; etc. This chapter shall describe electrical properties and behavior of engineering materials in electrical applications. First, basic electrical property, electrical conductivity, will be described followed by different types of electrical conduction including semi- and superconductivities. Some peculiar phenomena like ferro-electricity and peizo-electricity are explained in addition to electrical insulation Electrical conduction Electrical conductivity of a material is defined in terms of ease with which a material transmits an electrical current. Electrical current (I) is flow of electrons, and driving force for the flow of electrons is called voltage (V). Ohm s law relates these parameters as follows: V α I V = IR...(1) where R is the materials resistance to flow of electrons through it. V, I, and R respectively have units as volts, amperes, and ohms (Ω).
2 Electrical resistance of a material is influenced by its geometric configuration; hence a new parameter called electrical resistivity (ρ) is defined such as it is independent of the geometry. ρ = RA l...(2) where A cross-sectional area perpendicular to the direction of the current, and l the distance between points between which the voltage is applied. Units for ρ are ohm-meters (Ω-m). Reciprocal of the electrical resistivity, known as electrical conductivity (σ), is used to express the electrical behavior of a material, which is indicative of the ease with which a material allows of flow of electrons. 1 σ = = ρ l RA....(3) Electrical conductivity has the following units: (Ω-m) -1 or mho/meter. Resistivity or conductivity does not depend on the dimensions of the material. These properties are microstructure-sensitive instead, like many other intrinsic properties, for example yield strength. Thus either of it (usually conductivity) allows us to compare different materials. Solid engineering materials exhibit very wide range of electrical conductivity (about 27 orders of variation). Hence the materials for electrical applications are classified according to their electrical conductivity as: conductors, semiconductors and insulators/dielectrics. Metals are conductors having conductivities in range of 10 7 (Ω-m) -1, while semiconductors have conductivities in range from 10-6 to 10 4 (Ω-m) -1, and materials with conductivity lower than (Ω-m) -1 are termed as insulators. In components designed to conduct electrical energy, minimizing losses is important to conserve energy as well as minimize heating. The electric power (P) lost when a current flows through a material is given by: P = VI = I 2 R..(4) Electrical losses are also known as Joule heating losses. Second form of Ohm s law can be obtained by combining the equations (3) and (4) to give: I A V = σ J = σe (5) l where J current density (A/cm 2 ), E electric field strength (V/cm).
3 Current density can be defined in different terms as J = n. q. v..(6) where n number of charge carriers (1/cm 3 ), q charge on each carrier (1.6x10-19 C), v - average drift velocity (cm/s), which occurs as a result of an applied electric or magnetic field. The term v /E is called mobility, μ (cm 2 /V.s) of the carriers, thus σ = n.q.μ.(7) From the above equation, the following can be concluded: electrical cond