In the case of metallic materials is the influence of structure and temperature given Mathiessen rule that the resistivity can be divided into two components:
ρ = ρS + ρT
where ρS is a component depends only on the structure and composition is related to the metal and its processing technology and does not depend on temperature. On the other hand component ρT is a part dependent only on temperature. The alloy has a structural component usually more influential than the pure metals, alloys, because most shows less dependence on temperature change in resistance than pure metals.
With increasing temperature in the metal increased the amplitude of thermal motion of atoms, reduces mean time between collisions of electrons with thermal lattice vibrations, and thus leads to the growth of resistivity with temperature. Since the range of measured temperatures, that R = ρ.L/S, where L (the length of the conductor) and S (cross-sectional area) are constant, the increase in electrical resistance can be expressed using a simplified formula:
R = R0 . [1 + α.(t – t0)]
where R is the resistance at temperature t and R0 is the resistance at temperature t0. We can then express from relation (1) formula to obtain the temperature coefficient of electrical resistance of α:
|α =||R – R0|
|R0 . (t – t0)|
|t||[°C]||– temperature at the end of the measurement|
|t0||[°C]||– temperature at the beginning of the measurement|
|R||[Ω]||– resistance at temperature t|
|R0||[Ω]||– resistance at temperature t0|
Temperature coefficient of resistivity α observed at t0 = 20 °C is presented in Tables.
The semiconductor materials, like metal used for the temperature dependence of resistance on temperature. Unlike metals, but the principle is the conductivity of different semiconductors. At absolute zero temperature, all electrons are tightly bound to their cores and the material can not conduct current. Electrons must be some power to "jumped" over the so-called band gap into the conduction band and can participate in the current leadership. With increasing temperature will therefore increase the concentration of charge carriers and the electrical resistance of the material will decrease. While this phenomenon by trying to suppress conventional semiconductor devices, thermistors at him trying to contrast the composition of appropriate technology and highlight.
NTC thermistors, as representatives of the semiconductor components are negative temperature coefficient of resistance and preparation of semiconducting materials, usually based on oxides of nickel, manganese, cobalt, iron and titanium. For the temperature dependence of resistance of NTC thermistor is used in engineering practice the expression:
|R = R0·e||[–β·(||1||–||1||)]|
|T||[K]||– thermodynamic temperature measurement at the end|
|T0||[K]||– thermodynamic temperature at the beginning of the measurement|
|R||[Ω]||– resistance of the thermodynamic temperature T|
|R0||[Ω]||– resistance of the thermodynamic temperature T0|
The coefficient β is related to the activation energy ΔE of the charge carriers, which is significantly affected by the composition and preparation technology semiconductor thermistor. To calculate the coefficient β we use the measured thermistor resistance values:
The relationship between the activation energy ΔE and the calculated coefficient β can be expressed with sufficient accuracy as:
ΔE = β.k
(Boltzmann constant k = 1,38.10–23 J·K–1)