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 ...
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 ...
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 wh ...
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
...
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 str ...
, high
electrical
Electricity is the set of physical phenomena associated with the presence and motion of matter that has a property of electric charge. Electricity is related to magnetism, both being part of the phenomenon of electromagnetism, as described ...
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
In materials science and metallurgy, toughness is the ability of a material to absorb energy and plastically deform without fracturing.creep resistance, ease of
welding
Welding is a 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 is distinct from lower temperature techniques such as bra ...
, and low
relative volatility Relative volatility is a measure comparing the vapor pressures of the components in a liquid mixture of chemicals. This quantity is widely used in designing large industrial distillation processes. In effect, it indicates the ease or difficulty of u ...
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 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 ...
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 ...
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
In physics, sputtering is a phenomenon in which microscopic particles of a solid material are ejected from its surface, after the material is itself bombarded by energetic particles of a plasma or gas. It occurs naturally in outer space, and ca ...
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 ...
devices such as particle accelerators. 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
Speaker wire is used to make the electrical connection between loudspeakers and audio amplifiers. Modern speaker wire consists of two or more electrical conductors individually insulated by plastic (such as PVC, PE or Teflon) or, less commonly ...
industry markets oxygen-free copper as having enhanced conductivity or other electrical properties that are supposedly advantageous to audio signal 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
Home cinema, also called home theaters or theater rooms, are home entertainment audio-visual systems that seek to reproduce a movie theater experience and mood using consumer electronics-grade video and audio equipment that is set up in a room ...
.
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 ...
welding
Welding is a 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 is distinct from lower temperature techniques such as bra ...
/
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
A heat exchanger is a system used to transfer heat between a source and a working fluid. Heat exchangers are used in both cooling and heating processes. The fluids may be separated by a solid wall to prevent mixing or they may be in direct conta ...
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 ...
. The hydrogen diffuses through the copper and reacts with inclusions of Cu2O, forming H2O (
water
Water (chemical formula ) is an Inorganic compound, inorganic, transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living ...
), 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 defects in the crystal structure, and tend to decrease the electrical and thermal ...
. This process can cause the grains to be forced away from each other and is known as ''steam embrittlement'' (because steam 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
Spent nuclear fuel, occasionally called used nuclear fuel, is nuclear fuel that has been irradiated in a nuclear reactor (usually at a nuclear power plant). It is no longer useful in sustaining a nuclear reaction in an ordinary thermal reactor and ...
in the KBS-3 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.
See also
*
Copper wire and cable
Copper has been used in electrical wiring since the invention of the electromagnet and the telegraph in the 1820s. The invention of the telephone in 1876 created further demand for copper wire as an electrical conductor.
Copper is the electri ...