Selenium Applications

Due to its low price and high reliability selenium is used in converting equipment as well as in photocopying (amorphous), photocells (hexagonal) as well as solar cells and in diode rectifiers, selenide synthesis, fluorescence dyes for television, optical and signal instruments, thermistors and so on. It is also used for green glass decolouration and ruby glass manufacture, for obtaining fine grained structure of ingot steel and in stainless steels - for improving mechanical properties. Selenium is a catalyst in many chemical reactions, also in pharmaceutical industry and many others fields.

The remarkable variation in the electrical conductivity of "metallic" selenium on exposure to light, renders the element of great importance for the construction of certain types of optical apparatus. For this purpose the element is generally used in the form of thin sheets which have been carefully annealed "by heating for some time at 200° C. in order to ensure transformation to the metallic" form. The difficulty in connection with "fatigue" or the slow recovery of normal resistance after exposure to light is usually surmounted by employing a series of cells on a rotating disc or other device whereby each cell is given time to recover while others are in use. The photophone and the optophone have already been mentioned. The gradual development of phototelegraphy, the talking film and television, owes much to the application of selenium, although alternative methods are now available. Faint sources of light, such as the light of stars, may be measured by the use of selenium; the lights of lighthouses and of buoys may be regulated from a distance or by the disappearance of daylight, and the density of smoke or vapours in reaction chambers may be automatically recorded by apparatus employing the element. Explosive charges may be fired from a distance by means of a beam of light and a selenium cell, whilst the interruption of such a beam focussed on to a selenium cell constitutes a serviceable burglar alarm. The amount of selenium at present used for such purposes, however, is very small.

The chief use of selenium and its compounds is in the glass and ceramic industries. The grey crystalline form is generally used. In the former industry the element is employed on an increasing scale for four purposes (a) for decolorising glass - half an ounce of selenium with a little cobalt oxide is added to 1000 lb. of sand; (b) for producing pink- and orange-coloured glassware - the selenium is added to soda-lime or potash-soda glasses under oxidising conditions (the orange colour is produced by a mixture of selenium and uranium); (c) for producing ruby glass - more selenium than in the previous operations is added, together with cadmium sulphide, a distinctive ruby colour of considerable intensity resulting; (d) for producing amber glass, a brilliant glass suitable for high grade table ware - selenium and borax are added to lead glass under oxidising conditions. Instead of the element, sodium or barium selenite is now used by many glass makers in order to avoid loss by volatilisation. Selenium is also used to produce ruby glazes in pottery.

Electric cables treated with a thin layer of metallic selenium are flame-proof to a remarkable degree, thus minimising the risk of fire through short-circuiting of heavy currents. The cotton or rubber used for covering the wire may itself be treated with the selenium and so rendered non-inflammable; flame-proof switchboards, etc., may also be constructed.

Attempts to use selenium either in place of or in conjunction with sulphur in the vulcanisation of rubber do not appear to be completely successful, although it is claimed that rubber vulcanised with sulphur and selenium in the presence of an organic accelerator exhibits enhanced rigidity and- resistance to abrasion. The incorporation of powdered selenium in ordinary rubber for vulcanisation by sulphur is said to provide crystalline selenium nuclei which induce the internal crystallisation of any superfluous sulphur and so prevent the undesirable surface crystallisation (or "bloom"). The effect is attributed to the isomorphous character of selenium and sulphur.

Certain selenium compounds exhibit bactericidal and fungicidal action, and selenious acid may be used as a herbicide against dandelion, Canada thistle and burdock.

Certain selenites, for example of barium and lead, are satisfactory as pigments, yielding paints of a high degree of opacity.

Many toning processes in photography involve the use of selenium compounds. One such process recently recommended consists in treating the print or lantern slide, after developing, fixing and washing, with a solution containing about 1 per cent, of crystalline sodium sulphide and 1 per cent, of sodium selenite or selenious acid; after twenty minutes or so in such a bath intense brown tones are obtained with chlorobromide papers, or brown-violet tones with pure bromide papers.

Selenium is one of the most used heavy atom substitute for sulphur in amino acid methionine for x-ray analysis in sulphur additive, production of herbicides, insecticides, dyes, drugs etc.