Oxygen-free copper or Oxygen-free high thermal conductivity copper is a group
of wrought high conductivity copper alloys that have been electrolytically
refined to reduce the level of oxygen to .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. 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 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.
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 content is counted as copper. C11000 - also known as
Electrolytic-Tough-Pitch. 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. Most ETP
sold today will meet or exceed the 101% IACS specification. As with OF copper,
silver content is counted as copper for purity purposes.
= Oxygen-free high thermal conductivity Oxygen-free high thermal conductivity
copper is widely used in cryogenics. OFHC is produced by the direct
conversion of selected refined cathodes and castings under carefully controlled
conditions to prevent contamination of the pure oxygen-free metal during
processing. The method of producing OFHC copper ensures 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. These characteristics are high ductility, high electrical and thermal
conductivity, high impact strength, good creep resistance, ease of welding, and
low relative volatility under high vacuum.
Standards Conductivity is generally specified
relative to the 1913 International Annealed Copper Standard of 58 MS/m.
Advances in the refining process now yield OF and ETP copper that can meet or
exceed 101% of this standard. Note that OF and ETP coppers have identical
conductivity requirements. Oxygen plays a beneficial role for
improving copper conductivity. During the copper smelting 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 be used to reduce impurity levels to
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 processes, including the
manufacture of semiconductors and superconductor components, as well as in
high vacuum 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 industry
markets oxygen-free copper as having enhanced conductivity or other
electrical properties that are supposedly advantageous to audio signal
transmission. However, conductivity specifications for common C11000
Electrolytic-Tough-Pitch and higher-cost C10200 Oxygen-Free coppers are
identical. Much more expensive C10100, a highly refined copper with silver
impurities removed and oxygen reduced to 0.0005%, 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 in the smelting
process. Oxygen-free phosphorus-containing copper is
typically used for structural and thermal applications where the copper
material will be subject to temperatures high enough to cause hydrogen
embrittlement or more exactly steam embrittlement. Examples include
welding/brazing rods and heat exchanger tubing.
Indeed, copper alloys which contain oxygen as an impurity can be embrittled
if exposed to hot hydrogen. The hydrogen diffuses through the copper and reacts
with inclusions of Cu2O, forming H2O, which then forms pressurized water steam
bubbles at the grain boundaries. This process can cause the grains to be
forced away from each other, and is known as steam embrittlement.
CuOFP has been selected as corrosion resistant material for the overpack of
spent nuclear fuel in the KBS-3 concept developed in Sweden and Finland to
dispose high-level radioactive waste in crystalline rock formations.
See also Copper wire and cable
References