'Oxygen Free Copper'
The term 'oxygen free copper' can be a little confusing, partly because it is inaccurate, at best it will be 'oxygen reduced', and partly because of different standards. Even copper manufacturers sometimes give what look like conflicting accounts of the properties and names of various grades of copper, so there is some difficulty in comparing different samples. A summary is given here primarily based on British standards.
The most basic type of copper is 'electrolytic tough pitch' which is subdivided into two grades:
British Standard (BS) C101, ISO and CEN standard Cu-ETP.
BS C100, ISO and CEN standard Cu-ETP1.
The C100 version is of higher purity and is made from Grade A cathode copper. Typical purity of 99.95% is specified for ETP copper, with oxygen impurity at an optimum level of 200 ppm. The oxygen is added intentionally and has beneficial effect on conductivity, as described in the main text. This level of oxygen makes welding a problem because hydrogen then combines with the oxygen to produce water vapour, which causes fractures. An oxygen level under 3 to 4 ppm is needed for reliable welding, and for this purpose reduced oxygen copper was developed.
Deoxidised copper is produced by several different methods, one being to use a deoxidising agent such as phosphorus:
BS C106, ISO-CEN Cu-DHP has low enough oxygen for welding, and is commonly used for tubes, extrusions etc. The fairly high phosphorus content however reduces conductivity typically 15% below that of ETP. The copper purity is about the same as ETP, but phosphorus is at about 200ppm.
A better grade, Cu-DLP has reduced phosphorus content at 50ppm, again has similar copper purity to ETP, and only about 2% lower conductivity.
Other methods of oxygen removal include carbon reduction, which avoids adding impurities.
Oxygen Free copper is produced in several grades:
BS C103, Cu-OF and C110, Cu-OFE are melted and cast in an oxygen free atmosphere to give very low oxygen level, typically under 2ppm. For use in vacuum seals, e.g in glass vacuum tubes, the C110 grade needs to be oxidised on its surface at high temperature to gives good adhesion, but even small amounts of phosphorus can reduce the adhesion, so the level is kept below 3ppm.
Cu-OF has conductivity almost identical to ETP, but Cu-OFE is about 1.5% better. OFHC (a registered trade-mark) appears to be similar to Cu-OFE. HC means 'high conductivity' while OFE means 'oxygen free electronic grade'. Copper purity is typically 99.997% for both.
There are also various alloys of copper used in electronics applications, and these include Copper-silver, copper-chromium etc. which have better mechanical properties. Typically only 1 or 2% of the other metal is added to the alloy.
Copper of extremely high purity, or with large crystal size are also available, sometimes at a high price, but one example of twin speaker cable is available with a specification of 99.999% OFC, for as little as 36p (UK) per metre. (about 50 cents US). As far as I am aware there is no reliable and repeatable evidence from either measurement or listening tests that expensive audio cables can improve sound quality, but it is well known and easily demonstrated that some unusual speaker cable construction techniques can add unnecessary parallel capacitance which can cause instability in some amplifiers.