Copper (Cu) has been known for more than 10,000 years, with its discovery dating back as far as the Neolithic period. In addition to gold and silver, it was one of the first materials people worked with and processed. There are a lot of regions where it became so widespread and widely used between 5000BC and 3000BC that this period has been labeled “Copper Age” there. (People used it to make tools, weapons, jewelry, goods, works of art and a lot more.) Triggered by the discovery and growing use of electricity, it gained additional importance in the 19th century due to its being the second best metallic conductor after silver, which it outprices because it can be found in much higher quantities.
Today, two different types of copper are used for electrical conductors:
- Copper containing oxygen (CU-ETP1)
- Oxygen-free copper (CU-OF1) for special applications
Basic properties of copper
- Precious metal
- Chemical element, pure (as opposed to alloys like bronze, brass or steel)
- Chemical Symbol „Cu“
- Density 8,92 g/cm3 (20 °C)
- Melting point 1357,77 K (1084,62 °C)
- Electrical conductivity 58,58 m/Ohm mm² = 101 % IACS (International Annealed Copper Standard)
The relative electrical conductivity of copper has been set as being 100%, with the following values derived from this for other metals:
silver = 106%, gold = 72%, iron = 17%
- Very high thermal conductivity (400 W/(m*K))
- The good thermal conductivity is in line with the good electrical conductivity
- Strength in recrystallized condition < 200 N/mm²
Elongation > 15 % (dependent on diameter)
- High ductility
- Resistant to corrosion from most environmental impacts
Exceptions: oxidizing acids, hydrous ammonia and halogenated gases, hydrogen sulfide, seawater
- Good solderability
- Anti-bacterial agent
The markets that wires and strands - made either from bare or plated copper - target can be categorized into two major areas:
- Further processing into cables, extrusion
- Further processing, without being extruded, into flat ribbons, braids etc.
The type of copper largely applied in both fields is CU-ETP1.
- Material code: CW003A
- Description: CU-ETP1 as per DIN EN1977
- Predominantly electrolytic copper with a copper content of at least 99.9 %, and an oxygen content of less than 400 ppm
- Manufacturing methods: Contirod® or Southwire® (casting wheel)
The molten copper is poured on a casting wheel (Southwire) or a conveyor belt (Contirod) thus taking on the form of an endless strand. While still maintaining its melting heat, it is fed through a multi-stage hot-rolling mill where it is reshaped into continuous cast wire rod, which is the starting product for the production of cables, strands and ultra-fine wires.
There is, however, one drawback to using CU-ETP: while the hot copper is being poured on the casting wheel or the conveyor belt, it is exposed to the ambient air. As a consequence, the copper absorbs small quantities of oxygen from it. This poses no problem for a great deal of applications, but in particular areas even this minuscule amount of oxygen absorbed provokes the contraction of the so-called “hydrogen disease”.
The hydrogen disease, also known as embrittlement, is a phenomenon caused by a chemical reaction. When heated up beyond 500 °C, the metal grid of the copper is impregnated by nuclear hydrogen (H2) and reacts with nuclear oxygen atoms (O) to form water (H2O):
Cu2O + H2 → 2 Cu + H2O (vapor)
The oxygen atoms are located at the grain boundaries in the form of a thin network pattern. The water vapor generated breaks up the structure at the grain boundaries and brings about hollow spaces, which causes the entire structure to be impaired.
Evidence of oxygen in deoxidized and highly conductive copper wires with a maximum diameter of 12 mm (0.5 in) can be provided by means of the so-called „embrittlement test“ according to DIN EN ISO 2626. A sample is heated up in a hydrogenous atmosphere. If the metal contains oxygen, the above-mentioned structure-impairing reaction kicks in. Two methods can be employed to verify the embrittlement: the Flex Life Test (also known as back-and-forth bending) and the microscopic examination.
The bending scope during the Flex life test is 180°, with the following chain of cause and effect taking place: the material contains oxygen → a reaction with hydrogen occurs → the structure is impaired → the wire simply disintegrates.
A more sophisticated approach can be taken by executing a microscopic examination. Here, a specific cut is being scrutinized under the microscope at a magnification factor of 200. Gas pores and cracked spots as typical signs for embrittlement are easy to recognize here:
Micrograph of CU-ETP
- Material code: CW007A
- Description: CU-OF1 nach DIN EN1977
- Predominantly electrolytic copper with a copper content of at least 99,95 %, and an oxygen content of less than 10 ppm
- Strong hydrogen resistance as per ISO 2626 (Copper-Hydrogen embrittlement test) required
- Manufacturing methods: Dipforming and Upcasting
Dipforming: a so-called „mother rod“ with a purified and scraped surface is run through molten copper. The latter settles down on the mother rod, prompting the diameter of the wire to increase significantly. Using a hot-rolling mill it will be calibrated to its final diameter afterwards.
Upcasting: having reached its final sub-assembly diameter, the wire is hoisted through a cooled ingot mold and wound up without further treatment.
Fields of application for CU-OF1
Because of its high electrical conductivity, copper is mainly employed in electrical engineering. This field includes a special segment that is characterized by the hydrogen disease playing an important role:
When carbon brushes (slide contacts in electric motors) are produced, composite wires (e.g. special high-flexible copper braids) have to be sintered (or caked) with the particles of carbon present. In order to accomplish this, the green carbon compact is annealed in pure hydrogen atmosphere. If oxygen-carrying copper (CU-ETP) was used here, the carbon brush connector would invariably brittle on the spot owing to its being exposed to an extreme vibrational and motional strain.
All wires and strands manufactured for this application have to be absolutely free of oxygen by all means. From start to finish the product undergoes consistent quality control, which reliably prevents mistaken materials from being used. LEONI’s quality level surpasses that of DIN EN ISO 2626.
Periodic table of elements
”Hydrogen embrittlement test“ as per DIN EN ISO