Oxygen-free copper (OFC) or oxygen-free high thermal conductivity (OFHC) copper is a group of wrought high-conductivity

copper Copper is a chemical element with the symbol Cu (from la, cuprum) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pinkis ...

alloys that have been electrolytically refined to reduce the level of

oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as wel ...

to 0.001% or below.

Specification

Oxygen-free copper is typically specified according to the ASTM/ UNS database. The UNS database includes many different compositions of high conductivity electrical copper. Of these, three are widely used and two are considered oxygen-free: * C10100 – also known as ''oxygen-free electronic'' (OFE). This is a 99.99% pure copper with 0.0005% oxygen content. It achieves a minimum 101% IACS conductivity rating. This copper is finished to a final form in a carefully regulated, oxygen-free environment. Silver (Ag) is considered an impurity in the OFE chemical specification. This is also the most expensive of the three grades listed here. * C10200 – also known as ''oxygen-free'' (OF). While OF is considered oxygen-free, its conductivity rating is no better than the more common ETP grade below. It has a 0.001% oxygen content, 99.95% purity and minimum 100% IACS conductivity. For the purposes of purity percentage, silver (Ag) content is counted as copper (Cu). * C11000 – also known as ''electrolytic-tough-pitch'' (ETP). This is the most common copper. It is universal for electrical applications. ETP has a minimum conductivity rating of 100% IACS and is required to be 99.9% pure. It has 0.02% to 0.04% oxygen content (typical). Most ETP sold today meets or exceeds the 101% IACS specification. As with OF copper, silver (Ag) content is counted as copper (Cu) for purity purposes.

Oxygen-free high thermal conductivity

''Oxygen-free high thermal conductivity'' (OFHC) copper is widely used in

cryogenics In physics, cryogenics is the production and behaviour of materials at very low temperatures. The 13th IIR International Congress of Refrigeration (held in Washington DC in 1971) endorsed a universal definition of “cryogenics” and “cr ...

. OFHC is produced by the direct conversion of selected refined

cathode A cathode is the electrode from which a conventional current leaves a polarized electrical device. This definition can be recalled by using the mnemonic ''CCD'' for ''Cathode Current Departs''. A conventional current describes the direction in whi ...

s and castings under carefully controlled conditions to prevent

contamination Contamination is the presence of a constituent, impurity, or some other undesirable element that spoils, corrupts, infects, makes unfit, or makes inferior a material, physical body, natural environment, workplace, etc. Types of contamination W ...

of the pure oxygen-free metal during processing. The method of producing OFHC copper ensures an extra high grade of metal with a copper content of 99.99%. With so small a content of extraneous elements, the inherent properties of elemental copper are brought forth to a high degree. In practice the oxygen content is typically 0.001 to 0.003% with a total maximum impurity level of 0.03%. These characteristics are high

ductility Ductility is a mechanical property commonly described as a material's amenability to drawing (e.g. into wire). In materials science, ductility is defined by the degree to which a material can sustain plastic deformation under tensile stres ...

, high electrical and

thermal conductivity The thermal conductivity of a material is a measure of its ability to conduct heat. It is commonly denoted by k, \lambda, or \kappa. Heat transfer occurs at a lower rate in materials of low thermal conductivity than in materials of high thermal ...

, high impact strength, good

creep Creep, Creeps or CREEP may refer to: People * Creep, a creepy person Politics * Committee for the Re-Election of the President (CRP), mockingly abbreviated as CREEP, an fundraising organization for Richard Nixon's 1972 re-election campaign Art ...

resistance, ease of

welding Welding is a fabrication (metal), fabrication process that joins materials, usually metals or thermoplastics, by using high heat to melt the parts together and allowing them to cool, causing Fusion welding, fusion. Welding is distinct from lower ...

, and low relative volatility under

ultra-high vacuum Ultra-high vacuum (UHV) is the vacuum regime characterised by pressures lower than about . UHV conditions are created by pumping the gas out of a UHV chamber. At these low pressures the mean free path of a gas molecule is greater than approximately ...

Standards

Conductivity is generally specified relative to the 1913

International Annealed Copper Standard The International Annealed Copper Standard (IACS) is a standard established in 1914 by the United States Department of Commerce. It is an empirically derived standard value for the electrical conductivity of commercially available copper. Sometime ...

of S/ m. Advances in the refining process now yield OF and ETP copper that can meet or exceed 101% of this standard. (Ultra-pure copper has a conductivity of S/m, 102.75% IACS.) Note that OF and ETP coppers have identical conductivity requirements.

Oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as wel ...

plays a beneficial role for improving copper conductivity. During the copper

smelting Smelting is a process of applying heat to ore, to extract a base metal. It is a form of extractive metallurgy. It is used to extract many metals from their ores, including silver, iron, copper, and other base metals. Smelting uses heat and a ch ...

process, oxygen is deliberately injected into the melt to scavenge impurities that would otherwise degrade conductivity. There are advanced refining processes such as the Czochralski process than can achieve impurity levels below the C10100 specification by reducing copper grain density. At this time, there are currently no UNS/ASTM classifications for these specialty coppers and the IACS conductivity of these coppers is not readily available.

Industrial applications

For industrial applications, oxygen-free copper is valued more for its chemical purity than its electrical conductivity. OF/OFE-grade copper is used in plasma deposition ( sputtering) processes, including the manufacture of

semiconductors A semiconductor is a material which has an electrical resistivity and conductivity, electrical conductivity value falling between that of a electrical conductor, conductor, such as copper, and an insulator (electricity), insulator, such as glas ...

and superconductor components, as well as in other

devices such as

particle accelerators A particle accelerator is a machine that uses electromagnetic fields to propel charged particles to very high speeds and energies, and to contain them in well-defined beams. Large accelerators are used for fundamental research in particle ...

. In any of these applications, the release of oxygen or other impurities can cause undesirable chemical reactions with other materials in the local environment.

Use in home audio

The high-end speaker wire industry markets oxygen-free copper as having enhanced conductivity or other electrical properties that are supposedly advantageous to

audio signal An audio signal is a representation of sound, typically using either a changing level of electrical voltage for analog signals, or a series of binary numbers for digital signals. Audio signals have frequencies in the audio frequency range of r ...

transmission. In fact, conductivity specifications for common C11000 (ETP) and higher-cost C10200 oxygen-free (OF) coppers are identical; and even the much more expensive C10100 has only a one-percent higher conductivity—insignificant in audio applications. OFC is nevertheless sold for both audio and video signals in audio playback systems and home cinema.

Oxygen-free phosphorus-containing copper

High-electrical-conductivity coppers are distinct from coppers deoxidized by the addition of

phosphorus Phosphorus is a chemical element with the symbol P and atomic number 15. Elemental phosphorus exists in two major forms, white phosphorus and red phosphorus, but because it is highly reactive, phosphorus is never found as a free element on Ear ...

in the smelting process. ''Oxygen-free phosphorus-containing copper'' (CuOFP) is typically used for structural and thermal applications where the copper material will be subject to temperatures high enough to cause

hydrogen embrittlement Hydrogen embrittlement (HE), also known as hydrogen-assisted cracking or hydrogen-induced cracking (HIC), is a reduction in the ductility of a metal due to absorbed hydrogen. Hydrogen atoms are small and can permeate solid metals. Once absorbed ...

or more exactly steam embrittlement. Examples include

brazing Brazing is a metal-joining process in which two or more metal items are joined together by melting and flowing a filler metal into the joint, with the filler metal having a lower melting point than the adjoining metal. Brazing differs from we ...

rods and heat exchanger tubing. Copper alloys containing oxygen as an impurity (in the form of residual oxides present in the metal matrix) can be embrittled if exposed to hot

hydrogen Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic, an ...

. The hydrogen diffuses through the copper and reacts with inclusions of Cu₂O, forming H₂O (

water Water (chemical formula ) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as a ...

), which then forms pressurized water steam bubbles at the

grain boundaries In materials science, a grain boundary is the interface between two grains, or crystallites, in a polycrystalline material. Grain boundaries are two-dimensional crystallographic defect, defects in the crystal structure, and tend to decrease the ...

. This process can cause the grains to be forced away from each other and is known as ''steam embrittlement'' (because

steam Steam is a substance containing water in the gas phase, and sometimes also an aerosol of liquid water droplets, or air. This may occur due to evaporation or due to boiling, where heat is applied until water reaches the enthalpy of vaporization ...

is produced, not because exposure to steam causes the problem). CuOFP has been selected as corrosion-resistant material for the overpack of spent nuclear fuel in the

KBS-3 KBS-3 (an abbreviation of ''kärnbränslesäkerhet'', nuclear fuel safety) is a technology for disposal of high-level radioactive waste developed in Sweden by Svensk Kärnbränslehantering AB (SKB) by appointment from Statens Strålskyddsinstitu ...

concept developed in Sweden and Finland to dispose of

high-level radioactive waste High-level waste (HLW) is a type of nuclear waste created by the reprocessing of spent nuclear fuel. It exists in two main forms: * First and second cycle raffinate and other waste streams created by nuclear reprocessing. * Waste formed by vit ...

in crystalline rock formations.

References

{{reflist Copper