Physical properties of Aluminium
|Aluminium is a tin-white metal which may be highly polished. The commercial metal is extremely sonorous, malleable, and ductile. It is best worked at 100° to 150°; at 600° it is easily broken, and at a slightly higher temperature it may be powdered in a mortar. Aluminium has a crystalline structure when slowly cooled. Apparently the crystals are regular octahedra. After being worked, the metal is devoid of structure. Cast aluminium is about as hard as silver, and becomes harder when hammered. |
The density of aluminium varies with the treatment to which it has been subjected. The density of the cast metal (containing 0.36 per cent. impurities) is 2.703 at 18° C.; that of the worked metal may be less than 2.703, but it increases on annealing and may reach the value 2.7085 at 18°. Mallet gives the density of pure aluminium as 2.583 at 4° C. The mean coefficient of expansion between 0° and 100° is 2.432×10-5 for hard-drawn metal and 2.454×10-5 for the annealed metal.
The value of Young's modulus for aluminium is about 6.91×103 (in kilos, per sq. mm.) at 20°, the rigidity being about 2.7×103 (in kilos, per sq. mm.); both these values diminish considerably with rise of temperature. The compressibility is 1.49×10-6 per atmosphere. The tensile strength is affected by the form, method of casting, and subsequent treatment, the ultimate strength being (in tons per sq. inch) 7 in castings, 11 in sheet aluminium, and from 13 to 29 in aluminium wire.
The thermal conductivity of aluminium is 0.3435 at 0° and 0.3619 at 100° (absolute units), according to the measurements of Lorenz, while Jaeger and Disselhorst give 0.4804 and 0.4923 as the corresponding figures for aluminium containing 0.4 per cent, of copper and 0.5 per cent, of iron. Aluminium is therefore a slightly better heat conductor than zinc. The electrical conductivity is 35.6×104 reciprocal ohms per cm. cube at 0°, according to Sturm; while the conductivity at 0° and temperature-coefficient of the resistance (between 0° and 100°) are given by Broniewski as 40.1×104 and 4.25×10-3 respectively for annealed aluminium, and 38.5×104 and 4.10×10-3 for the chilled metal.
Aluminium melts at 658 ± 1°, boils at 1800°, and expands at 4.8 per cent, on fusion. Owing to the formation of a protective film of oxide, it is possible to heat a piece of aluminium wire above the melting-point without destroying its shape. Being a metal of low atomic weight, aluminium has a high specific heat, which, moreover, has a large temperature-coefficient. According to E. H. Griffiths and E. Griffiths, the specific heat is 0.2096 at 0° and 0.2252 at 100°. Intermediate values are given by the expression s = 0.20957(l + 9.161×10-4t – 1.7×10-6t2), and hence the mean specific heat is 0.2180 between 0° and 100°, and 0.2196 between 20° and 100°. For the latter value, Schmitz found 0.2191. Bontscheff has experimented over a wide temperature interval and gives the following values for the specific heat: -
The results may be expressed by the formula: -
s = 0.20890 + 1.6187t×-4 - 2.9425t2×-7 + 46183t3×-10.
At -193.9° the specific heat of aluminium is only 0.086, and at -253.9 it has fallen to 0.0024; at these extremely low temperatures the specific heat is proportional to the cube of the absolute temperature. The atomic heat of aluminium at ordinary temperatures is rather lower than might be anticipated, since, adopting the mean specific heat between 20° and 100°, the atomic heat is only 5.95. Griffiths and E. Griffiths give the following as the most probable values of the atomic heat of aluminium at various temperatures: -
The latent heat of fusion of aluminium is 70 to 80 cals. per gram. Aluminium is paramagnetic, the magnetic susceptibility at ordinary temperature baing + 1.8×-6 c.g.s. electromagnetic units per unit volume. The atomic refraction of aluminium in its compounds is 9.5 (for the Hα line; Gladstone and Dale's formula).
The electrode potential of aluminium is not known with any accuracy, the value εH = -1.03 to 1.28 volts being uncertain and probably numerically too great. In the potential series aluminium probably occupies the following position: -
Alkali and alk. earth metals, Mg, Mn, Al, Zn, Cr, Cd, Fe, etc.
When aluminium is used as anode in passing a current through an aqueous solution, and the voltage does not exceed some 25 volts, the current that passes quickly falls to almost zero. This seems to be due to the formation of a layer of aluminium hydroxide on the anode. The critical voltage, above which an appreciable current can be made to flow, varies with the temperature and the nature of the electrolyte. This property of aluminium is utilised at times for obtaining a direct from an alternating current.
The arc and spark spectra of aluminium are fairly simple. The most intense lines in these spectra, i.e. the " hauptlinien," are (Exner and Haschek) the following: -
arc: 2568.08, 2575.20, 2652.56, 2660.50, 3082.30, 3092.89, 3944.20, 3961.71
spark: 2816.41, 3082.30, 3092.89, 3944.22, 3961.74, 4529.70, 5696.71.
The most persistent lines in the spark spectrum of aluminium, i.e. the "ultimate" lines, which should be looked for when seeking the traces of aluminium, are (Exner and Haschek's wave-lengths) 3961.74,* 3944.22,* 3092.82, 3082.30, 2816.41, those asterisked being the most sensitive.
The arc spectrum of aluminium contains a number of very characteristic bands due to aluminium oxide, which disappear when the arc is surrounded by an atmosphere of hydrogen.
Aluminium always contains a little occluded gas, which may be extracted by fusing the metal in vacuo.
Aluminium readily combines with the halogens. A compact piece of the metal is only superficially oxidised when heated in air or oxygen, but in thin foil it burns brilliantly when heated in oxygen. Aluminium powder begins to oxidise rapidly at 400°, and readily burns if strongly heated at one point, aluminium oxide and a little nitride being produced. Aluminium rapidly oxidises if the surface is amalgamated with mercury, an arborescent growth of alumina quickly forming all over the metal. At high temperatures finely divided aluminium readily unites with sulphur, selenium, phosphorus, and arsenic; it also combines with antimony, but with more difficulty. Aluminium combines directly with nitrogen, producing a nitride; it also combines with carbon, silicon, and boron.
Owing to the extremely large heat of formation of aluminium oxide, aluminium is able to reduce many oxides, with the evolution of much heat, Thus, aluminium powder burns readily when heated in the oxides of sulphur, nitrogen, and carbon. The reduction of a solid oxide is best accomplished by mixing the powdered oxide with an equivalent of granulated aluminium and starting the reaction by putting a little barium peroxide and magnesium powder mixture on the top and lighting it with a match or a piece of burning magnesium ribbon. In this way the oxides of iron, manganese, chromium, etc., can be easily reduced to metal, so much heat being generated that both the metal and the alumina produced are melted in the reaction. These reductions are generally known as " thermit" reactions (vide infra). In a few cases the reduction of an oxide by aluminium is an endothermic change and can only be brought about by supplying the necessary heat, e.g. the reduction of calcium and magnesium oxides. Thermit reactions may be readily applied to the preparation of phosphides, arsenides, silicides, and borides by simultaneously reducing two oxides.
At ordinary temperatures aluminium is unattacked by air-free water, but ordinary water slowly acts upon it. The rate of corrosion is greatly increased by the presence of impurities, particularly by traces of sodium, copper, or iron, and is also augmented by rise of temperature. Aluminium powder slowly decomposes water at 100°, and the powder, when ignited in air and plunged into steam, continues to burn, with the evolution of hydrogen. Aluminium amalgam decomposes water readily, aluminium hydroxide and hydrogen being produced. To effect this reaction it is only necessary to amalgamate superficially the surface of aluminium foil by immersing it in aqueous mercuric chloride, and tfeen wash the foil with cold water. The aluminium-mercury couple thus formed, owing to the ease with which it decomposes water, constitutes a valuable reducing agent.
Aluminium is rapidly corroded by dilute hydrogen peroxide, aluminium hydroxide being formed.
Aluminium dissolves readily in hydrochloric acid, dilute or concentrated, hydrogen and aluminium chloride being produced. The action of dilute sulphuric acid is very slow; the concentrated acid attacks the metal with the evolution of sulphur dioxide. Phosphoric acid, dilute or concentrated, readily attacks aluminium, hydrogen being evolved. In comparison with its action on other metals, the action of nitric acid on aluminium is extremely slow. With 5 to 20 per cent, acid at 25° to 30° the main reaction is as follows: -
Al + 4HNO3 = Al(NO3)3 + 2H2O + NO.
With a large excess of acid, nitrogen appears among the gaseous products. A little ammonium nitrate is produced. Nitric acid of density 1.15 dissolves aluminium faster than the 1.45 acid, and the rate of solution of the metal increases with the fineness of division, thick foil dissolving very slowly, but coarse turnings much more rapidly. Organic acids attack aluminium only very slowly, but if the protecting layer of hydrogen that forms on the metal is removed (e.g. by operating in a vacuum) the rate of solution is greatly accelerated.
Alkali hydroxides in aqueous solution rapidly dissolve aluminium, alkali aluminates and hydrogen being produced. With ammonium hydroxide, the products are aluminium hydroxide and hydrogen. Aluminium also dissolves in aqueous alkali carbonates, carbon dioxide and hydrogen being evolved. Aqueous solutions of salts, e.g. sodium chloride, slowly attack aluminium, but only in the presence of oxygen; the addition of a small quantity of a weak organic acid hastens the corrosion.
|When two aluminium rods are immersed in water, their ends being separated by only 0.1 mm., and a condensed spark discharge passed between them by means of a powerful induction coil, a colloidal solution of aluminium is obtained.|