When you consider the usability of any metal, conductivity should be taken into account. Conductivity really consists of two properties, electrical and thermal (heat). Although copper has long been valued for its excellent heat and electrical conductivity properties, other factors, such as strength, corrosion resistance, and malleability should also be considered. Solutions to issues raised by those considerations can affect conductivity.
Keytometals.com states that electrolytic tough pitch (ETP) copper, the preferred material for current-carrying members, has a conductivity rating of 101 percent IACS (International Annealed Copper Standard) in soft temper with 220 MPa (megapascals, a unit of stress) tensile strength, and 97 percent in spring rolled temper at 345 to 380 MPa tensile strength. Under the standard, conductivity is expressed as a percentage of the standard. One hundred percent IACS represents a conductivity of 58 megasiemens per meter (MS/m), equivalent to a resistivity of 1/58 ohm per meter for a wire one square millimeter in cross section. ETP is 99.9 percent copper and 0.04 percent oxygen. A megapascal equals 10 bars or approximately 145.038 pounds per square inch.
Conductive heat transfer is calculated using Fourier's Law, namely q = k A dT / s,
where q equals heat transferred per unit time (watts or British Thermal Units [BTU] per hour), A equals heat transfer area (meters squared or feet squared), k equals thermal conductivity of the material (watts per meter times degrees Kelvin or watts per meter times degrees centigrade, or Btu per [hour times degrees Fahrenheit times feet squared divided by feet]), dT equals temperature difference across the material (in Kelvin or degrees centigrade or degrees Fahrenheit), and s equals material thickness (meters or feet). So, copper at 25, 125, and 225 degrees centigrade has a thermal conductivity of 401,000, 400,000, and 398,000 watts per meter Kelvin, respectively.
Conductivity and Other Considerations
Only silver is a better electrical conductor than copper. Corrosion resistance, machinability, fatigue characteristics, malleability, formability, and strength are other factors considered in metal applications. Copper has been preferred over silver for electrical applications because of the relative expense of silver and the speed at which silver corrodes. To improve characteristics other than conductivity, copper is alloyed with other metals. Dilution of copper from its pure state causes a rapid decline in its conductive power. Brasses and bronzes, which are 60 to 80 percent copper, have conductivity ratings of 25 to 50 percent IACS.
The use of copper and the extent or selection of material to be alloyed are subject to significant performance considerations, where strength and non-maleability are trade-offs against conductivity. A design consideration becomes whether the decreased conductivity is acceptable.
With increasing emphasis on the efficient use of power, the power industry has been looking for improved conductive materials for normal environments while the search for superconductors continues.
- Photo Credit copper cable image by Witold Krasowski from Fotolia.com
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