Electrical conductivity primarily refers to the capability of physical bodies to conduct electric current and is secondarily a quantitative measure of this property. Conductivity is attributed to free-charge carriers. The conductive property of a substance is characterized by its conductivity, σ, defined according to Ohm’s Law by the expression j = σE. Here, j is the electric current density and E is the strength of the electric field. The magnitude ρ = 1/σ is called resistivity.
In metals, free electrons are the charge carriers. Among all metals, silver has the least resistivity (ρ = 1.6 × 10−8 Ohm/m at 300 K). Special high resistance alloys (used, e.g., for making electric heaters in electric furnaces) have ρ values up to 1.5 × 10−6 Ohm/m at 300 K. The dependence of ρ on temperature, T, has the form ρT/ρT0 = 1 + α(T − T0) ; here ρT, ρT0 are the resistivity at T, T0 , respectively, and α is the temperature resistance factor. For most metals, α = (3 ÷ 5) × 10−3 K−1 ; for high resistance alloys, α is substantially lower (e.g., for constantan, α = 10−5 K−1 ).
At temperatures close to absolute zero, the conductivity of some metals abruptly falls to zero. This phenomenon is called low-temperature (classical) superconductivity and is the converse of high-temperature superconductivity, which arises at significantly higher temperatures (80 K and higher) in nonmetals of special compositions.
The conductivity of semiconductors is attributed to electron transition in a conduction band (electron conductivity in n-type semiconductors) or in a valance band (hole conductivity in p-type semiconductors). The intrinsic conductivity of semiconductors is attributed to the movement in opposite directions of the same quantities of electrons and holes. Practically the most significant is conductivity due to donor-type or acceptor-type additive, which provides high electron or hole conductivity, correspondingly. Semiconductor resistivity very strongly decreases with temperature: ρT/ρT0 = exp(A/t), where A is a coefficient depending on a semiconductor’s properties.
The conductivity of electrolytes (such as solutions of acids, alkalis and salts in water and other dissolvents along with molten salts) is attributed to positive and negative ions. The resistivity of electrolytes decreases with temperature (in contrast to metals): ρT/ρT0 = 1 − α(T − T0) . For most electrolytes, the values of α and ρT0 depend on solution concentration.
The conductivity of gases and plasma is attributed to free electrons. The input of ions to conductivity is usually small. The two types of gas conductivity are: unself-maintained, which occurs due to gas ionization by external factors (such as X-radiation); and self-maintained, when ionization is due only to internal processes in a gas or plasma. Self-maintained conductivity of low-temperature (less then 10,000 K) plasma initially rises very strongly with temperature, then this rise significantly decelerates. To increase low-temperature plasma conductivity, alkali metals or their compounds are seeded into it.