3. Contact Materials

In electrical power engineering, metals are primarily used as conductors. The specific electrical resistance ρ or the electrical conductivity κ of metals depend on the freely moving electrons in the lattice (represented by ρ₀). The electrons are scattered, and thus the specific electrical resistivity ρ is increased, by temperature-induced lattice vibrations (ρ_T) and lattice disturbances, such as defects, dislocations, grain boundaries, or foreign atoms (ρ_G) (Eq. (3.1)) [1]. The movement of the electrons transports energy and thus also heat. This means that metallic conductors with a low specific electrical resistance ρ also have a high thermal conductivity λ. This indirect proportionality is described by the Wiedemann-Franz-Lorenz law, taking into account the thermodynamic temperature T and Lorenz number L (Eq. (3.2)). In the temperature range in which electrical power engineering systems are operated, it is the case that the movement of electrons dominates energy transport. At very low temperatures near absolute zero, lattice vibration is the dominant mechanism of energy transport that must be taken into account in thermal conductivity [1]. The mechanical properties of pure metals are altered by cold forming or alloying. This increases the number of lattice disturbances and thus the specific electrical resistance due to greater scattering of electrons. This allows the mechanical and electrical properties to be set according to specific applications. As a result, the properties of the contact materials must not change impermissibly during a long operating period and at maximum operating temperature, as this can strongly negatively affect the contact and long-term behavior of current-carrying connections. The behavior of conductor and coating materials is therefore of fundamental importance and will be examined in more detail below (Fig. 3.1).