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Amorphous Silicon

Preparation of Amorphous Silicon

Amorphous silicon may be prepared in various ways, nearly all of which consist in reducing silicon compounds with metals.

  1. Silicon fluoride or chloride may be reduced by sodium or potassium, e.g.:

    SiF4 + 4Na = Si + 4NaF.

    Gay Lussac and Thenard passed silicon tetrafluoride gas over heated potassium, and Deville employed silicon tetrachloride vapour and sodium, whilst Hempel and von Haasy passed silicon tetrafluoride on to sodium heated to 400°-500° C. in a cast-iron crucible, and allowed the mass to cool in a current of silicon tetrafluoride, so that sodium silicifluoride, Na2SiF6, was formed, which was easily separated from the silicon by means of water.
  2. A slight modification consists in reducing a silicifluoride with sodium or potassium, thus:

    K2SiF6 + 4K = 6KF + Si.

    This reaction was employed by Berzelius in 1823, who heated the reagents together in an iron tube, and subsequently removed the potassium fluoride by water. Wohler employed sodium silicifluoride and sodium, and heated the mixture, covered with common salt, in a clay crucible.
  3. A further method consists in reducing silica by heating it with a metal. Potassium and sodium were employed, but without great success; powdered magnesium has, however, been found to be a suitable reducing agent, and the reaction may be carried out as follows: Powdered quartz or thoroughly dried precipitated silica is mixed with magnesium powder in the porportions required for the reaction:

    SiO2 + 2Mg = Si + 2MgO,

    and a quarter of the weight of calcined magnesia is added. The mixture is heated in a clay crucible, first at 300°-400° C., further to ensure thorough drying, and then at a red heat. The reaction quickly occurs with evolution of much heat; and, after cooling, the silicon is isolated by treating the mass with hydrochloric acid to dissolve the magnesia, and with hydrofluoric and sulphuric acids to remove the remaining silica; the silicon is then washed with water and dried in a current of hydrogen. If the reaction became too violent there would be a tendency for the reduced silicon to combine with the unchanged magnesium to form silicide according to the reaction:

    SiO2 + 4Mg = SiMg2 + 2MgO;

    thus the silicon formed would be diminished in quantity, and would be impure. The magnesia is added to retard the reaction; and by its means the silicon obtained is of 96-97 per cent, purity.

    As a laboratory experiment silicon may be prepared by heating in a hard glass tube 4 parts by weight of thoroughly dried powdered white sand with 1 part of magnesium powder.
  4. Amorphous silicon is produced when electric sparks are passed through liquid silico-ethane, Si2H6; and thus obtained, possesses the power of reducing cold neutral potassium permanganate solution and boiling copper, gold, and mercuric solutions.

Properties of Amorphous Silicon

Pure, amorphous silicon, prepared from silica by means of magnesium, according to the method of Vigouroux, is a brown, hygroscopic powder with a density of 2.35 at 15° C. and a specific heat of 0.214 at 21° C., which is greater than that of crystallised silicon. Cambi has obtained another variety of amorphous silicon of a reddish-yellow colour and density 2.08, and considers that the different amorphous silicons are not clearly defined allotropic forms, but masses of forms possibly possessing different molecular structures. According to Wilke-Dorfurt, a less reactive, grey form of amorphous silicon exists. The ordinary amorphous silicon can easily be melted in a muffle furnace, and vaporised in the electric furnace, yielding a polyatomic vapour. It readily dissolves in many molten metals; with some, such as magnesium, copper, iron, and nickel, it forms silicides; from others, such as aluminium, it separates in the crystalline state on cooling. It burns with difficulty in air, but brilliantly in oxygen at a red heat, forming silica. The heat of combustion of amorphous silicon was found by von Wartenberg to be 195,000 calories. Previously recorded values are 184,500 calories by Berthelot and 191,000 calories by Mixter. It combines with gaseous fluorine at ordinary temperature, forming the tetrafluoride SiF4; and with chlorine at 450° C., bromine at 500° C. and iodine at a still higher temperature, in each case to form the tetrahalide. Silicon combines with sulphur at 600° C., with nitrogen at 1000° C., and with boron, carbon, titanium, and zirconium in the electric furnace. Gaseous and liquid hydrogen fluoride easily dissolve silicon, but hydrogen chloride, bromide, and iodide have little action on it even at a red heat. Steam reacts slowly with silicon at a red heat, forming silica and hydrogen. Most oxides are reduced by silicon at high temperature, the affinity of this element for oxygen exceeding that of all other elements except the alkali and alkaline earth metals and boron and aluminium.

All aqueous acids, except a mixture of hydrofluoric and nitric acids, are with6ut action on silicon, but fused alkali and aqueous solutions of alkali dissolve it with evolution of hydrogen, thus:

Si + 2KOH + H2O = K2SiO3 + 2H2.

When silicon is fused with sodium carbonate, silicate is formed and carbon monoxide evolved. Fused potassium dichromate, chlorate, and nitrate oxidise silicon, but a mixture of potassium chlorate and fuming nitric acid has no action upon it (cf. carbon).
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