The good electrical and thermal conductivity are important properties of copper. However, the great importance of this material for technology only results from the combination of the various good properties, which – also in combination with other metals – have been used and further developed for years. In addition, copper has excellent corrosion resistance and is 100 percent recyclable without any loss of quality.
Properties at a glance
|Relative atomic mass||63,546|
|Melting point||1083 °C|
|Boiling temperature||2595 °C|
|Electrical conductivity at 20 °C||57 m/(Ω*mm2)|
|Thermal conductivity at 20 °C||394 W/m*K|
|Temperature coefficient of electrical conductivity||0,0039/K|
|Thermal expansion||17*10-6/K (von 25 bis 300 °C)|
|Speziﬁc heat||0,39 J/g*K (20 bis 400 °C)|
|Heat of fusion||214 J/g|
|Crystal structure||Cubic face-centred|
Properties in detail
Copper is the only metal that has a salmon red colour. Besides gold, it is the only coloured metallic element.
In the periodic system of elements, copper is in the first subgroup with silver and gold. The structure of pure copper consists of a homogeneous phase and crystallises in a face-centred cubic lattice. Pure copper is a salmon-coloured ductile metal with a melting point of 1083 °C and a boiling point of 2595 °C. In its chemical compounds, copper practically always occurs in monovalent and divalent form.
Individual physical properties of copper depend strongly on the degree of purity or on the impurities. On the other hand, it is possible to change certain physical properties in wide ranges by alloying the metals with each other, by hot and/or cold forming or by heat treatment. In this way, many characteristic values of copper materials can be adapted to the respective application.
Copper is one of the specifically heavy utility metals. In the solid state, the density of pure copper at 20 °C is 8.93 g/cm3.
As a result of the (failed) attempt to make a clear technical-physical distinction between “light” and “heavy” metals, the term “heavy metals” has been used for about 40 years in public and often also in so-called specialist literature for “heavy” metals with simultaneously toxic properties. The distinction between a physical quantity (density = mass/volume = “heaviness”) and a biological property (toxicity) was thus lost. Metals such as copper and zinc, on the other hand, which are also physically “heavy” and biologically also essential for life, are subsumed in such lists on an equal footing with elements that are actually critical to health, such as cadmium or mercury. Physico-technically, no uniform definition could be found for a sharp distinction between “heavy” and “light” metals, a fact that the renowned chemist and biologist John Diffus of the International Union of Pure and Applied Chemistry (IUPAC) has been drawing attention to for decades: According to this, there is a tendency, which is not supported by the facts, that all so-called ‘heavy metals’ and their compounds have highly toxic or ecotoxic properties. This has no basis in chemical or toxicological data. According to IUPAC, there are almost 40 different definitions of the term in the literature. Diffus is clearly in favour of a new classification based on the chemical properties of the metals.
Nowadays, we tend to speak of base metals or technology metals.
Depending on the type and proportion of alloying elements added, the density of the resulting copper alloys changes. With the lighter elements silicon or aluminium, the density decreases, while it is hardly changed by nickel, and to a lesser extent by tin or zinc.
A change in temperature causes the materials to expand or shrink; their volume changes analogously. The change in length is calculated with the help of the linear coefficient of thermal expansion. The spatial coefficient of expansion is about three times as large as the linear coefficient of expansion. It should be noted that the coefficient of expansion is temperature dependent. The linear coefficient of expansion of pure copper is reduced by the addition of nickel, hardly affected by aluminium and increased by zinc and tin.
The most important physical property of pure copper is its high thermal and electrical conductivity. After silver, copper has the highest electrical conductivity of all metals. It is common to state the electrical conductivity of metals in % I.A.C.S (International Annealed Copper Standard) or in MS/m. 100 % I.A.C.S. corresponds to 58.00 MS/m (= 58.00 m/W mm2). The electrical conductivity of pure copper is already strongly reduced by small impurities. The specific electrical resistance depends on the temperature. The influence is stronger with unalloyed copper than with copper alloys. The conductivity of copper materials is reduced by cold forming. Heat-treatable copper alloys, on the other hand, have a higher conductivity in the quenched and tempered state than in the solution-annealed state. The thermal conductivity of pure copper decreases slightly with increasing temperature, whereas it increases with temperature in copper alloys.
Soft copper has a tensile strength of about 200 MPa, a yield strength of 40-80 MPa and an elongation at break of over 40 %. If cold working takes place, the tensile strength increases to at least 350 MPa and the yield strength to at least 320 MPa, but the elongation at break then drops to values below 5 %. Pure copper has no hot-brittle range and can be formed well even in the warm state. At the same time, copper does not become brittle even at low temperatures. Since copper also has considerable fatigue strength characteristics, it is also suitable as a material for oscillating stresses without the fear of brittle fractures. The mechanical properties of the different pure copper grades practically do not differ from each other. Only the creep behaviour can be positively influenced by small additions of alloys such as silver. With alloys, strength values up to approx. 700 MPa, in special cases even up to 1500 MPa, can be achieved, but then the conductivity can decrease considerably.
In its chemical compounds, copper is monovalent and divalent, in some exceptional cases – e.g. in K3[CuF6] – also trivalent, in one case – namely in Cs2(CuF6) – even tetravalent. In aqueous solutions, copper is the only useful metal that shows a normal potential that is nobler than that of hydrogen. Against acids, the corrosion behaviour of copper depends not only on their type and concentration, but also on the amount of oxygen or an oxidising agent present. Copper is resistant in non-oxidising acids that do not contain dissolved oxygen. Alkaline aqueous solutions of the hydroxides and carbonates of the alkaline earth and alkali metals – with the exception of NH3 – have little effect on copper.
In the atmosphere – also in sea air – copper is very resistant. Its surface is initially covered with a dark brown to almost black protective layer, which over time usually turns into the green patina familiar from old copper roofs. Patina is a mixture of basic copper salts (sulphate, carbonate, near the sea also chloride), whose quantity ratio is determined by the concentration of the corresponding basic substances in the air. Humid ammonia and hydrogen sulphide vapours have an unfavourable effect. Copper also has good resistance to drinking and industrial water (cold and hot water). It is therefore an excellent material for water pipes. The resistance is linked to the formation of an even protective layer.
Copper is not only a naturally occurring element found in various forms and concentrations in the earth’s crust, oceans, lakes and rivers, but also a trace element essential for life. The life of flora and fauna has developed within the framework of this natural presence of copper. Therefore, most organisms have an intrinsic mechanism for its use. For the human organism, copper has the significance of an essential trace element, i.e. humans need copper in order to survive. Usually, the daily requirement of an adult of approx. 2 milligrams is reached by the intake of a balanced diet with a rich portion of cereals, meat, root vegetables, legumes, nuts or even chocolate. Since copper is particularly important for the metabolism, a copper deficiency can lead to serious health problems.
Copper is the only utility metal that is nobler than hydrogen in the normal voltage series. The good corrosion resistance of copper materials is based on their ability to form stable covering layers that protect the material from further corrosion attack (a visible example of this are the green church roofs). The addition of alloying elements has a positive effect on the formation of the covering layer.
In the atmosphere, these compact and protective layers consist of oxides and hardly soluble basic salts. In solutions, the corrosion behaviour is primarily influenced by the presence of oxygen or other oxidising agents. Depending on the environmental parameters, the medium can be permanently corrosive or lead to the formation of a protective layer. However, an attack of the copper can only take place if the attacking agent contains oxygen or oxidising agents or has an oxidising effect itself, and in weakly attacking agents (e.g. water containing oxygen), the protective layers addressed inhibit or prevent the progression of corrosion.
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Copper is a significant functional metal and an important element of technical and socio-economic progress.More