- Resistivity Definition: Resistivity is the measure of how much a material opposes the flow of electric current.
- Temperature Impact: The resistivity of most metals increases with temperature, showing a positive temperature coefficient. Semiconductors and insulators, however, see a decrease in resistivity with temperature.
- Alloying Effect: Alloying metals increases their resistivity by adding impurities that disrupt the crystal structure.
- Mechanical Stressing: Stressing a metal increases its resistivity by creating localized strains that block electron movement. Annealing can reverse this effect.
- Factors Affecting Resistivity: Temperature, alloying, mechanical stressing, age hardening, and cold working all significantly impact the resistivity of electrical materials.
Factors affecting the resistivity of electrical materials include lattice vibration, chemical composition and material condition. The main engineering factors discussed here are:
- Temperature
- Alloy composition
- Elastic or plastic strain
- Age hardening
- Cold working and annealing
Temperature
The resistivity of most metals increases with temperature because lattice vibrations scatter conduction electrons more strongly. Over a limited temperature interval, a linear coefficient can approximate the change.
Here,
ρt1 is the material resistivity at temperature t1oC,
and
ρt2 is the material resistivity at temperature t2oC.
α1 is the temperature coefficient of resistance at reference temperature t1oC.
A positive α1 means resistivity increases locally as temperature rises. Use tabulated data or a higher-order model outside the range where the linear approximation is valid.
Many ordinary metals have a positive temperature coefficient near room temperature, but their response is not linear across every temperature. At low temperature, impurity, defect and strain scattering can leave residual resistivity. Only certain materials become superconducting below a material-specific critical temperature; cooling a metal towards absolute zero does not guarantee zero resistivity. In semiconductors, temperature changes both carrier population and mobility, so doping level and operating range determine the net result. The behaviour of insulators is also material- and mechanism-specific rather than one universal negative coefficient.
Alloying
An alloy may be a solid solution, an ordered phase, a multiphase material or a mixture of these structures. Solute atoms and phase boundaries disturb the periodic atomic structure and usually add electron-scattering sites, which often raises resistivity relative to a pure metal. The size of the change depends on element, concentration, phase distribution and heat treatment. Conductivity must therefore be balanced against required strength, corrosion resistance and formability using measured alloy data.
Mechanical Stressing
Elastic strain changes dimensions and electronic structure, producing a reversible resistance response used by strain gauges. Plastic deformation creates dislocations and other defects that often increase residual resistivity. The magnitude and even the direction of the change can depend on temperature, alloy content, prior processing and stress mode. Recovery or recrystallisation during annealing can remove some defects and move resistivity towards the annealed value.
Age Hardening
Age hardening, also called precipitation hardening, changes a supersaturated solid solution into a microstructure containing fine precipitates. Solute atoms leave the matrix and form new phases as ageing proceeds. Both solute concentration and precipitate distribution affect electron scattering, so resistivity can rise or fall during different ageing stages. The trend must be measured for the specific alloy and heat treatment.
Cold Working
Cold working plastically deforms a metal below its recrystallisation temperature. It raises dislocation density and can change texture, phase balance and residual stress. These changes often increase residual resistivity while also raising strength. The effect is not a fixed percentage and may saturate with deformation. Subsequent annealing can recover conductivity by reducing defects or recrystallising the material.
