In electronic theory, negatively charged electrons orbit around the nucleus (positive charge) in fixed orbits. Materials in which electrons are easily attracted by an external electric field and fly off their orbits to become free electrons are called conductors. Metals can conduct electricity because, under the influence of an external electric field, they generate a large number of free electrons. These free electrons move from low potential to high potential, forming an electron flow, or electric current.
Metals are composed of atoms arranged in a regular lattice, a crystal structure known as a lattice. When free electrons are accelerated and directed by an electric field, they constantly collide with atoms and with each other, slowing their movement and dissipating energy. Consequently, conductive materials present a certain resistance to the flow of electric current. This resistance is called resistance, representing the conductor's resistance to the flow of current.
Resistivity is the resistance of a conductor made of a specific material that has a standard unit of length and a standard unit of cross-sectional area, at a given temperature.
This definition can be better understood through a formula:
R=ρ·L/A
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R is the resistance of the conductor, with the unit in ohms (Ω).
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ρ (rho) is the resistivity, with the unit in ohm-meters (Ω·m).
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L is the length of the conductor, with the unit in meters (m).
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A is the cross-sectional area of the conductor, with the unit in square meters (m2).
Resistivity and conductivity are inversely related.
The greater the conductivity, the smaller the resistance of the material and the better the conductivity.
In eddy current testing technology, conductivity does not require absolute value but relative value. The most commonly used is the International Annealed Copper Standard (IACS), which is a non-International Unit unit that represents metal conductivity.
In order to facilitate the distinction between materials, the International Electrotechnical Commission (IEC) stipulated in 1913 that the industrial high-purity copper calibration in an annealed state with a resistivity of 1 m 1 m long and 1 mm21 mm2 cross-sectional area at 20°C temperature was 1.7241×10−8Ω⋅m1.7241×10−8Ω⋅m, the conductivity of 100%IACS100%IACS, and the resistivity of other metals or alloys ρXρX and conductivity σxσx are the ratio of the conductivity of industrial high-purity copper in this annealed state under 20°C as the conductivity of the metal or alloy, which is expressed as a percentage, that is, %IACS or PIACS (P is a percentage)
Temperature
Resistivity, conductivity and temperature coefficient of resistance of typical metals and alloys
Alloy composition
For solid solution alloy materials (impacts are uniformly distributed in metal substrates), if the arrangement of alloy atoms is irregular, that is, disordered solid solution, its resistivity generally increases with the increase of alloy components. However, if the alloy atoms are arranged in a certain proportion into a very regular crystal lattice, that is, an ordered solid solution, its resistivity will have a minimum value as the alloy components change.
Materials of different alloy components have different conductivity, which is not only the basis of the material sorting method in eddy current detection technology, but also one of the important factors that must be considered in eddy current detection that affects the impedance of the detection coil.
Impurity content
Impurities in metals can cause metal lattice distortion, affecting the arrangement of atoms in the material, causing electron scattering, and increasing resistivity.
Stress
The internal stress present in the metal can cause the metal lattice to deform, increasing the chance of electron collision, thereby increasing the resistivity. For example, within the elastic range, the stress of unidirectional tensile or torsion will increase the resistivity of the metal, while under the action of unidirectional compressive stress, the resistivity will be reduced for most metals, or the internal stress of the metal after cold processing and heat treatment will also reduce the conductivity.
Normal deformation achieved by hot and cold processing
The result of normal deformation is that the atomic arrangement structure is deformed and the chances of electron collision increase. The greater the degree of deformation, the greater the resistivity increases. However, for cold-processed metals, after long-term high-temperature heating such as annealing and eliminating lattice deformation, the resistivity can be reduced to a low value close to the original value.
Heat treatment process
The conductivity of the same material will vary under different heat treatment states. Single crystal metals or fully annealed high purity metals tend to have high conductivity, while alloys have lower conductivity. Annealing of metals such as aluminum, silver, copper, iron after cold processing will reduce the resistivity. The resistance of the material usually decreases with the increase of the annealing temperature, but when the annealing temperature is higher than the recrystallization temperature, the resistance will increase instead.
In addition, different types of materials (insulators, conductors, semiconductors) also have different conductivity.
To have deeper understanding of eddy current testing, you will need to know Magnetic permeability as well.
Check our post for Part two.
KEYWORDS:
How temperature affects metal conductivity, Effect of impurities on resistivity, Metal lattice defects and conductivity, Physics fundamentals of eddy current testing, Conductivity vs. resistivity
