Electric properties

Resistivity

The German physicist Georg Simon Ohm discovered the basic law of electric conduction, which is now called Ohm’s law. His law relates the voltage (V, measured in volts), the current (I, in amperes), and the resistance (R, in ohms) according to the formula V = RI. A current I through a solid induces a voltage V; the resistance R is the constant of proportionality. The value of R is an important factor in the design of electrical circuits. It is determined by the shape of the resistor: a long narrow object has more resistance than a short wide one of the same material. For solids, the important parameter is the resistivity ρ, which is given in units of ohm-metres. It is the resistivity per volume unit and is independent of shape. The relationship between R and ρ is R = ρL/A, where A is the area of the resistor and L is the length. These dimensions are measured in the direction of the current: L is the length of the current path, and A is the cross-sectional area. The resistance of a copper bar depends on its shape, but at a given temperature every piece of pure copper has the same resistivity. Thus the resistivity is a fundamental parameter of a material and is investigated by scientists. Resistivities of solids span a wide range of values. Certain metals have zero resistivity at low temperatures; they are called superconductors. At the other extreme, very good insulators such as sulfur and polystyrene have resistivities larger than one quadrillion ohm-metres. At room temperature, the metal with the lowest value of resistivity is silver, with ρ = 1.6 × 10⁻⁸ ohm-metre; the second best conductor is copper, with ρ = 1.7 × 10⁻⁸ ohm-metre. Copper, rather than silver, is used in household wires because of the high cost of silver.

Conduction through ion hopping

Electrical conductivity σ is the inverse of resistivity and is measured in units of ohm-metre⁻¹. Electrical current is produced by the motion of charges. In crystals, electrical current is due to the motion of both ions and electrons. Ions move by hopping occasionally from site to site; all solids can conduct electricity in this manner. When the voltage is zero, there is no net current because the ions hop randomly in all directions. The imposition of a small voltage causes the ions to slightly favour one direction of motion, which leads to a net flow of charge in that direction; this constitutes an electrical current. The electricity conducted by this process is quite small and is usually negligible compared with that carried by the electrons. When an ion hops, it must migrate to a vacant site, which could be either an interstitial or a vacancy. Ionic conductivity can occur because hopping ions cause vacancies to move through the solid. An ion hops to the vacant site, thereby filling the vacancy, while creating a new one at the ion’s former site. Repeating this process causes the position of the vacancy to migrate through the crystal. The motion of the vacancy arises from the motion of ions, which carry charge and contribute to electrical conductivity.

Ion hops are induced by thermal fluctuations. Most of the ions move within their lattice site, vibrating around this point. Temperature is defined as the average energy of this vibrational motion; the more the ions move, the higher the temperature. An individual ion at times moves slowly and at times vibrates quite rapidly but usually has an energy near the average value. Each ion shares its vibrational energy with its neighbouring ions. An ion typically has some neighbours with small vibrations and others with large ones. The average energy shared with the neighbours is close to the average energy of all the atoms. As a random process, however, it occasionally happens that all neighbours of an ion may have large vibrations, in which case the ion will acquire an unusually high energy. This energy may be high enough to cause it to leave its site and hop to a neighbouring site. A thermal fluctuation is the rare process in which the energy at a local site may be much higher or lower than the average energy in the crystal. Probability theory shows that the higher the temperature, the more frequent are these thermal fluctuations. Ions therefore hop more often at high temperature.

A few solids conduct electricity better by ion motion than by electron motion. These unusual materials are technologically important in making batteries. All batteries have two electrodes separated by an electrolyte, which is a material that conducts ions better than electrons. An example of a crystal electrolyte is β-alumina, which readily conducts monovalent cations such as silver (Ag⁺) and sodium (Na⁺). Among all ions, silver has the largest value of ionic conductivity in many different electronic insulators. The copper ion (Cu⁺) forms the same type of chemical bonds as does the silver ion, but the copper ion, because of its smaller radius, does not migrate as well within an electrolyte. Silver ions fit perfectly into the interstitial sites of the crystal lattices of several electrolytes, while the smaller copper ions permit the neighbouring ions to collapse around them, inhibiting further hopping. There are a few good conductors of the inexpensive copper ion that can be used as solid electrolytes in batteries. Silver is too costly and heavy to use in large-volume batteries such as those found in automobiles, but it is used in the smaller batteries that power devices such as hearing aids.

Conduction electrons

Electrons carry the basic unit of charge e, equal to 1.6022 × 10⁻¹⁹ coulomb. They have a small mass and move rapidly. Most electrons in solids are bound to the atoms in local orbits, but a small fraction of the electrons are available to move easily through the entire crystal. These so-called conduction electrons carry the electrical current. Solids with many conduction electrons are metals, while those with a few are semimetals or semiconductors. In insulators, nearly all the electrons are bound, and very few electrons are capable of carrying current. A typical metal has one or more conduction electrons in each atomic unit cell, a semiconductor may have only one conduction electron for each thousand unit cells, and an insulator may have one conduction electron per one million or one trillion unit cells.

The bonding properties of the individual atoms of a solid determine the behaviour of the bulk solid. The electrical properties of a solid can usually be predicted from the valence and bonding preferences of its atoms. In the argon atom, for example, all atomic shells are filled with electrons. The electrons of solid argon remain in the atomic shells; none are conduction electrons, and the electrical resistivity is therefore high. Solid argon, like all the rare gas solids, is a good insulator. A few conduction electrons are contributed by impurities, and so the conductivity, though small, is not zero. These conduction electrons move quite readily through the solid. The term mobility is used to describe how well a conduction electron moves through the solid in response to a voltage. Conductivity is the product of mobility, the electrical charge e, and the number N of conduction electrons per unit volume: σ = Neμ, where σ is the conductivity and μ is the mobility. The mobility of the rare gas solids is high, but their conductivity is nonetheless low because there is a small number of conduction electrons.

Electrical insulators

Like the rare gas solids, most ionic solids are electrical insulators. In sodium chloride, for example, each sodium atom donates its single valence electron to a chlorine atom, thus forming a solid composed of Na⁺ and Cl⁻ ions. All electrons are in filled shells at low temperature, and in a perfect crystal there are no conduction electrons. Sodium chloride is thus an insulator with a very high resistivity. Some conduction electrons are provided by impurities or thermal excitations. At high temperatures large ion vibrations from thermal fluctuations may knock an electron out of a filled shell, upon which it becomes a conduction electron and contributes to the conductivity. The number of conduction electrons created by thermal excitations is small for most insulators. Although defects can be responsible for producing conduction electrons, they can also destroy the conducting ability of electrons by trapping them. The defects have local orbitals that provide a lower energy state for the electron than the one occupied in the conduction state. A conduction electron becomes bound at the defect, ceasing to contribute to the conductivity. This process is very efficient in insulators, so the few conduction electrons provided by impurities and thermal fluctuations are usually trapped at other defects. By definition, an insulator is a solid that does not provide a stable environment for conduction electrons.