Metals used as conductive materials should possess high conductivity, sufficient mechanical strength, resistance to oxidation and corrosion, ease of processing and welding, and resource availability should also be considered. Ms. Can will share with you today the resistivity properties of commonly used conductive materials.
1 Performance requirements of conductive materials
Copper, aluminum, and their alloys are the most commonly used conductive metals. In certain special applications, precious metals and other metals are also used. Copper was the earliest widely used conductive material, possessing excellent electrical conductivity and mechanical properties. Aluminum's electrical conductivity is approximately 62% that of copper, and its density is 33% that of copper. Aluminum's specific strength (strength to density ratio) is about 30% higher than copper's. For the same electrical resistance, aluminum's cross-sectional area is 168% that of copper, while aluminum's weight is only 54% of copper's.
The development of modern science and technology demands that certain conductive materials not only possess excellent conductivity but also high strength, hardness, and comprehensive properties such as heat resistance, corrosion resistance, and wear resistance. Therefore, conductive copper alloys, aluminum alloys, and composite metal conductors for various applications have developed rapidly.
Conductive alloys have lower electrical conductivity than their counterparts in pure metals, but significantly improved strength and heat resistance.
Composite conductors made from two or more different metals through certain composite processes have many unique advantages: high strength, high hardness, high elasticity, good wear resistance, heat resistance, corrosion resistance and thermal conductivity, special magnetic properties and coefficient of thermal expansion, etc.
2. Resistance of conductive metals
Resistance is an important indicator of a metal's conductivity and a key parameter in the design and selection of conductive metals. The resistance of a metal is often expressed as resistivity, as shown in equation (1).
ρ=RS/L……………………(1)
In formula (1):
R—Resistance of a conductor, measured in Ω;
S—Conductor cross-section, unit mm2;
L – Length of the conductor, in meters (m).
In alternating current (AC) conditions, an alternating magnetic field is generated in the conductor, and the current is not uniformly distributed across the entire conductor cross-section. The current density increases closer to the surface; this is known as the skin effect. To reduce the influence of the skin effect, the diameter of the single wire can be reduced, and stranded or hollow wire structures can be used. (Further details about frequency are needed for a more accurate translation.)
When the surface is high, it can be plated with highly conductive metallic silver or other materials.
3. Main factors affecting resistance
●Temperature
The resistance of a metal increases with increasing temperature. Generally, the resistance has a linear relationship with temperature, as shown in equation (2):
R=R0[1+α(T-T0)]……………………(2)
In formula (2):
R—resistance at temperature T;
R0—Resistance at temperature T0;
α — Temperature coefficient of resistance.
● Alloying elements and impurities
Alloying elements and impurities can cause lattice distortion in metals, leading to increased electrical resistance. The extent of this effect can be categorized into three types:
(1) Effect on the base metal. It is related to the content of the element or impurity and the difference in valence electrons between it and the base metal.
(2) The greater the difference between the atomic radius of the element or impurity and the base metal, the greater the increase in lattice distortion and resistance.
(3) When an element or impurity forms a solid solution in the base metal, the resistance increases significantly; when a two-phase mixture is formed, the resistance changes as a straight line.
●Cold deformation
The increase in electrical resistance of pure metals due to cold deformation is generally no more than 4%, while that of alloys is slightly higher.
●Heat treatment
Cold deformation of metals increases strength and hardness, but decreases electrical conductivity and ductility. Annealing can restore these properties. However, if the annealing temperature is too high or the heating time is too long, the mechanical properties of the metal deteriorate, but the change in electrical resistance is not significant.
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