Реферат: Aluminium

Magnetic ordering paramagnetic[3]

Electrical resistivity (20 °C) 28.2 nΩ·m

Thermal conductivity (300 K) 237 W·m−1·K−1

Thermal expansion (25 °C) 23.1 µm·m−1·K−1

Speed of sound (thin rod) (r.t.) (rolled) 5,000 m·s−1

Young's modulus 70 GPa

Shear modulus 26 GPa

Bulk modulus 76 GPa

Poisson ratio 0.35

Mohs hardness 2.75

Vickers hardness 167 MPa

Brinell hardness 245 MPa

CAS registry number 7429-90-5

2. Characteristics

Aluminium is a soft, durable, lightweight, malleable metal with appearance ranging from silvery to dull grey, depending on the surface roughness. Aluminium is nonmagnetic and nonsparking. It is also insoluble in alcohol, though it can be soluble in water in certain forms. The yield strength of pure aluminium is 7–11 MPa, while aluminium alloys have yield strengths ranging from 200 MPa to 600 MPa.[5] Aluminium has about one-third the density and stiffness of steel. It is ductile, and easily machined, cast, drawn and extruded.

Corrosion resistance can be excellent due to a thin surface layer of aluminium oxide that forms when the metal is exposed to air, effectively preventing further oxidation. The strongest aluminium alloys are less corrosion resistant due to galvanic reactions with alloyed copper.[5] This corrosion resistance is also often greatly reduced when many aqueous salts are present however, particularly in the presence of dissimilar metals.

Aluminium atoms are arranged in a face-centred cubic (fcc) structure. Aluminium has a stacking-fault energy of approximately 200 mJ/m².[6]

Aluminium is one of the few metals that retain full silvery reflectance in finely powdered form, making it an important component of silver paints. Aluminium mirror finish has the highest reflectance of any metal in the 200–400 nm (UV) and the 3000–10000 nm (far IR) regions, while in the 400–700 nm visible range it is slightly outdone by tin and silver and in the 700–3000 (near IR) by silver, gold, and copper.[7]

Aluminium is a good thermal and electrical conductor, by weight better than copper. Aluminium is capable of being a superconductor, with a superconducting critical temperature of 1.2 kelvin and a critical magnetic field of about 100 gauss.[8]

3. Isotopes

Aluminium has nine isotopes, whose mass numbers range from 23 to 30. Only 27Al (stable isotope) and 26Al (radioactive isotope, t1/2 = 7.2 × 105 y) occur naturally; however, 27Al has a natural abundance of 99.9+ %. 26Al is produced from argon in the atmosphere by spallation caused by cosmic-ray protons. Aluminium isotopes have found practical application in dating marine sediments, manganese nodules, glacial ice, quartz in rock exposures, and meteorites. The ratio of 26Al to 10Be has been used to study the role of transport, deposition, sediment storage, burial times, and erosion on 105 to 106 year time scales.[9] Cosmogenic 26Al was first applied in studies of the Moon and meteorites. Meteoroid fragments, after departure from their parent bodies, are exposed to intense cosmic-ray bombardment during their travel through space, causing substantial 26Al production. After falling to Earth, atmospheric shielding protects the meteorite fragments from further 26Al production, and its decay can then be used to determine the meteorite's terrestrial age. Meteorite research has also shown that 26Al was relatively abundant at the time of formation of our planetary system. Most meteorite scientists believe that the energy released by the decay of 26Al was responsible for the melting and differentiation of some asteroids after their formation 4.55 billion years ago.[10]

4. Natural occurrence

In the Earth's crust, aluminium is the most abundant (8.3% by weight) metallic element and the third most abundant of all elements (after oxygen and silicon).[11] Because of its strong affinity to oxygen, however, it is almost never found in the elemental state; instead it is found in oxides or silicates. Feldspars, the most common group of minerals in the Earth's crust, are aluminosilicates. Native aluminium metal can be found as a minor phase in low oxygen fugacity environments, such as the interiors of certain volcanoes.[12] It also occurs in the minerals beryl, cryolite, garnet, spinel and turquoise.[11] Impurities in Al2O3, such as chromium or cobalt yield the gemstones ruby and sapphire, respectively. Pure Al2O3, known as Corundum, is one of the hardest materials known.[11]

Although aluminium is an extremely common and widespread element, the common aluminium minerals are not economic sources of the metal. Almost all metallic aluminium is produced from the ore bauxite (AlOx(OH)3-2x). Bauxite occurs as a weathering product of low iron and silica bedrock in tropical climatic conditions.[13] Large deposits of bauxite occur in Australia, Brazil, Guinea and Jamaica but the primary mining areas for the ore are in Ghana, Indonesia, Jamaica, Russia and Surinam.[14] Smelting of the ore mainly occurs in Australia, Brazil, Canada, Norway, Russia and the United States. Because smelting is an energy-intensive process, regions with excess natural gas supplies (such as the United Arab Emirates) are becoming aluminium refiners.

5. Production and refinement

Although aluminium is the most abundant metallic element in the Earth's crust, it is rare in its free form, occurring in oxygen-deficient environments such as volcanic mud, and it was once considered a precious metal more valuable than gold. Napoleon III, emperor of France, is reputed to have given a banquet where the most honoured guests were given aluminium utensils, while the other guests had to make do with gold.[15][16] The Washington Monument was completed, with the 100 ounce (2.8 kg) aluminium capstone being put in place on December 6, 1884, in an elaborate dedication ceremony. It was the largest single piece of aluminium cast at the time. At that time, aluminium was as expensive as silver.[17] Aluminium has been produced in commercial quantities for just over 100 years.

Aluminium is a strongly reactive metal that forms a high-energy chemical bond with oxygen. Compared to most other metals, it is difficult to extract from ore, such as bauxite, due to the energy required to reduce aluminium oxide (Al2O3). For example, direct reduction with carbon, as is used to produce iron, is not chemically possible, since aluminium is a stronger reducing agent than carbon. However there is an indirect carbothermic reduction possible by using carbon and Al2O3 which forms an intermediate Al4C3 and this can further yield aluminum metal at a temperature of 1900-2000°C. This process is still under development. This process costs less energy and yields less CO2 than the Hall-Héroult process.[18] Aluminium oxide has a melting point of about 2,000 °C. Therefore, it must be extracted by electrolysis. In this process, the aluminium oxide is dissolved in molten cryolite and then reduced to the pure metal. The operational temperature of the reduction cells is around 950 to 980 °C. Cryolite is found as a mineral in Greenland, but in industrial use it has been replaced by a synthetic substance. Cryolite is a chemical compound of aluminium, sodium, and calcium fluorides: (Na3AlF6). The aluminium oxide (a white powder) is obtained by refining bauxite in the Bayer process of Karl Bayer. (Previously, the Deville process was the predominant refining technology.)

The electrolytic process replaced the Wöhler process, which involved the reduction of anhydrous aluminium chloride with potassium. Both of the electrodes used in the electrolysis of aluminium oxide are carbon. Once the refined alumina is dissolved in the electrolyte, its ions are free to move around. The reaction at the cathode is:

Al3+ + 3 e− → Al