Silicon

I

Introduction

Silicon, symbol Si, semimetallic element that is the second most common element on Earth, after oxygen. The atomic number of silicon is 14. Silicon is in group 14 (or IVa) of the periodic table. It was first isolated from its compounds in 1823 by the Swedish chemist Baron Jöns Jakob Berzelius.

II

Properties and Occurrence

Silicon is prepared as a brown amorphous powder or as grey-black crystals. It is obtained by heating silica, or silicon dioxide (SiO₂), with a reducing agent, such as carbon or magnesium, in an electric furnace. Crystalline silicon has a hardness of 7, compared to 5 to 7 for glass. Silicon melts at about 1410° C (about 2570° F), boils at about 2355° C (about 4271° F), and has a relative density of 2.33. The atomic weight of silicon is 28.086.

Silicon is not attacked by nitric, hydrochloric, or suphuric acids, but it dissolves in hydrofluoric acid, forming the gas, silicon tetrafluoride, SiF ₄ (see Fluorine). It dissolves in sodium hydroxide, forming sodium silicate and hydrogen gas. At ordinary temperatures silicon is impervious to air, but at high temperatures it reacts with oxygen, forming a layer of silica that does not react further. At high temperatures it also reacts with nitrogen and chlorine to form silicon nitride and silicon chloride, respectively.

Silicon constitutes about 28 per cent of the Earth's crust. It does not occur in the free, elemental state, but is found in the form of silicon dioxide and in the form of complex silicates. Silicon-containing minerals constitute nearly 40 per cent of all common minerals, including more than 90 per cent of igneous-rock-forming minerals. The mineral quartz, varieties of quartz (such as cornelian, chrysoprase, onyx, flint, and jasper), and the minerals cristobalite and tridymite are the naturally occurring crystal forms of silica. Silicon dioxide is the principal constituent of sand. The silicates (such as the complex aluminium, calcium, and magnesium silicates) are the chief constituents of clays, soils, and rocks in the form of feldspars, amphiboles, pyroxenes, micas, and zeolites, and of semi-precious stones, such as olivine, garnet, zircon, topaz, and tourmaline.

III

Uses

Silicon is used in the steel industry as a constituent of silicon-steel alloys. In steelmaking, molten steel is deoxidized by the addition of small amounts of silicon; ordinary steel contains less than 0.03 per cent of silicon. Silicon steel, which contains from 2.5 to 4 per cent silicon, is used in making the cores of electrical transformers because the alloy exhibits low hysteresis (see Magnetism). A steel alloy, known as duriron, containing about 15 per cent silicon, is hard, brittle, and resistant to corrosion; duriron is used in industrial equipment that comes in contact with corrosive chemicals. Silicon is also used as an alloy in copper, brass, and bronze.

Silicon is a semiconductor, in which the resistivity to the flow of electricity at room temperature is in the range between that of metals and that of insulators. The conductivity of silicon can be controlled by adding small amounts of impurities, called dopants. The ability to control the electrical properties of silicon, and its abundance in nature, have made possible the development and widespread application of transistors and integrated circuits—the silicon chip—used in the electronics industry. In addition, although silicon is usually an inefficient emitter of light, research has shown that in the future it may be possible to create lasers using densely packed silicon nanocrystals. Electrical connections could then be replaced by silicon optical connections, which transmit signals much faster.

Silica and silicates are used in the manufacture of glass, glazes, enamels, cement, and porcelain, and have important individual applications. Fused silica, a glass made by melting quartz or hydrolysing silicon tetrachloride, is characterized by a low coefficient of expansion and high resistance to most other chemicals. Silica gel is a colourless, porous, amorphous substance; it is prepared by removing part of the water from a gelatinous precipitate of silicic acid, SiO₂·H ₂O, which is formed by adding hydrochloric acid to a solution of sodium silicate. Silica gel absorbs water and other substances and is used as a drying and decolorizing agent.

Sodium silicate, Na₂SiO₃, an important synthetic silicate, is a colourless, water-soluble, amorphous solid that melts at 1088° C (1990° F). It is prepared by reacting silica (sand) and sodium carbonate at a high temperature or by heating sand with concentrated sodium hydroxide under pressure. The aqueous solution of sodium silicate, called water glass, is used for preserving eggs; as a substitute for glue in making boxes and other containers; as a binder in artificial gemstones; as a fireproofing agent; and as a binder and filler in soaps and cleansers. Another important silicon compound is the silicon-carbon compound carborundum, which is used as an abrasive.

Silicon monoxide, SiO, is used as a coating to protect other materials, the outer surface oxidizing to the dioxide SiO₂. Such layers are applied also as components of interference filters.