Selenium

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Selenium (Greek selēnē, “moon”), symbol Se, semimetallic element with an atomic number of 34. Selenium is in group 16 (or VIa) of the periodic table.

Selenium was discovered in 1817 by the Swedish chemist Baron Jöns Jakob Berzelius in a sulfuric acid residue. It was so called because it was found in association with tellurium (Latin tellus, “earth”).

PROPERTIES AND OCCURRENCE

Chemically, selenium closely resembles sulfur and is related to tellurium. Like sulfur, it exists in several allotropic (distinctly different) forms: a brick-red powder; a brownish-black, glassy, amorphous mass called vitreous selenium; red monoclinic crystals of specific gravity 4.5; and gray, lustrous crystals called gray selenium. It forms selenious acid and selenic acid, the respective salts of which are called selenites and selenates. Gray selenium melts at 217°C (423°F), boils at about 685°C (about 1265°F), and has a specific gravity of 4.81. The atomic weight of selenium is 78.96.

The element occurs in a few selenide minerals, the most common of which is clausthalite, or lead selenide. It also occurs with free sulfur and in many sulfide ores; it is generally obtained as a by-product in the refining of copper-sulfide ores. The yield from by-product sources, however, is insufficient to supply the rapidly increasing industrial demand for the element.

USES

Gray selenium conducts electricity; it is a better conductor of electricity in light than in darkness, the conductivity varying directly with the intensity of light. It is therefore used in many photoelectric devices. In the form of red selenium or as sodium selenide the element is used to impart a scarlet red color to clear glass, glazes, and enamels. It is also used to a great extent as a decolorizer of glass because it neutralizes the greenish tint produced by iron (ferrous) compounds. Small amounts of selenium are added to vulcanized rubber to increase its resistance to abrasion. Sodium selenate is an insecticide used to combat insects that attack cultivated plants, particularly chrysanthemums and carnations; the insecticide is scattered around the roots and is carried by the sap throughout the plant. Selenium sulfide is used in the treatment of dandruff, acne, eczema, seborrheic dermatitis, and other skin diseases.

Semiconductor

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Semiconductor, solid or liquid material, able to conduct electricity at room temperature more readily than an insulator, but less easily than a metal. Electrical conductivity, which is the ability to conduct electrical current under the application of a voltage, has one of the widest ranges of values of any physical property of matter. Such metals as copper, silver, and aluminum are excellent conductors, but such insulators as diamond and glass are very poor conductors (see Electrical Conductor; Insulation; Metals). At low temperatures, pure semiconductors behave like insulators. Under higher temperatures or light or with the addition of impurities, however, the conductivity of semiconductors can be increased dramatically, reaching levels that may approach those of metals. The physical properties of semiconductors are studied in solid-state physics.

CONDUCTION ELECTRONS AND HOLES

The common semiconductors include chemical elements and compounds such as silicon, germanium; selenium, gallium arsenide, zinc selenide, and lead telluride. The increase in conductivity with temperature, light, or impurities arises from an increase in the number of conduction electrons, which are the carriers of the electrical current (See Electricity; Electron). In a pure, or intrinsic, semiconductor such as silicon, the valence electrons, or outer electrons, of an atom are paired and shared between atoms to make a covalent bond that holds the crystal together (See Chemical Reaction). These valence electrons are not free to carry electrical current. To produce conduction electrons, temperature or light is used to excite the valence electrons out of their bonds, leaving them free to conduct current. Deficiencies, or “holes,” are left behind that contribute to the flow of electricity. (These holes are said to be carriers of positive electricity.) This is the physical origin of the increase in the electrical conductivity of semiconductors with temperature. The energy required to excite the electron and hole is called the energy gap.

See also: Doping

Semiconductor

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Doping

Another method to produce free carriers of electricity is to add impurities to, or to “dope,” the semiconductor. The difference in the number of valence electrons between the doping material, or dopant (either donors or acceptors of electrons), and host gives rise to negative (n-type) or positive (p-type) carriers of electricity. This concept is illustrated in the accompanying diagram of a doped silicon (Si) crystal. Each silicon atom has four valence electrons (represented by dots); two are required to form a covalent bond. In n- type silicon, atoms such as phosphorus (P) with five valence electrons replace some silicon and provide extra negative electrons. In p-type silicon, atoms with three valence electrons such as aluminum (Al) lead to a deficiency of electrons, or to holes, which act as positive electrons. The extra electrons or holes can conduct electricity.

When p-type and n-type semiconductor regions are adjacent to each other, they form a semiconductor diode, and the region of contact is called a p-n junction. (A diode is a two-terminal device that has a high resistance to electric current in one direction but a low resistance in the other direction.) The conductance properties of the p-n junction depend on the direction of the voltage, which can, in turn, be used to control the electrical nature of the device. Series of such junctions are used to make transistors and other semiconductor devices such as solar cells, p-n junction lasers, rectifiers, and many others. See Electronics; Laser; Rectification; Solar Energy; Transistor.

Semiconductor devices have many varied applications in electrical engineering. Recent engineering developments have yielded small semiconductor chips containing hundreds of thousands of transistors. These chips have made possible great miniaturization of electronic devices. More efficient use of such chips has been developed through what is called complementary metal-oxide semiconductor circuitry, or CMOS, which consists of pairs of p- and n-channel transistors controlled by a single circuit. In addition, extremely small devices are being made using the technique of molecular-beam epitaxy.

See also Computer; Integrated Circuit; Microprocessor.

Electrical Conductor

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Electrical Conductor, any material that offers little resistance to the flow of an electric current. The difference between a conductor and an insulator, which is a poor conductor of electricity or heat, is one of degree rather than kind, because all substances conduct electricity to some extent. A good conductor of electricity, such as silver or copper, may have a conductivity a billion or more times as great as the conductivity of a good insulator, such as glass or mica. A phenomenon known as superconductivity is observed when certain substances are cooled to a point near absolute zero, at which point their conductivity becomes almost infinite. In solid conductors the electric current is carried by the movement of electrons; in solutions and gases, the electric current is carried by ions.

Glass

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Glass, an amorphous substance made primarily of silica fused at high temperatures with borates or phosphates. Glass is also found in nature, as the volcanic material obsidian and as the enigmatic objects known as tektites. It is neither a solid nor a liquid but exists in a vitreous, or glassy, state in which molecular units have disordered arrangement but sufficient cohesion to produce mechanical rigidity. Glass is cooled to a rigid state without the occurrence of crystallization; heat can reconvert glass to a liquid form. Usually transparent, glass can also be translucent or opaque. Color varies with the ingredients of the batch.

The basic ingredient of glass compositions is silica, derived from sand, flint, or quartz.

Silica can be melted at very high temperatures to form fused silica glass. Because this glass has a high melting point and does not shrink or expand greatly with changing temperatures, it is suitable for laboratory apparatus and for such objects subject to heat shock as telescope mirrors. Glass is a poor conductor of both heat and electricity and therefore useful for electrical and thermal insulation. For most glass, silica is combined with other raw materials in various proportions. Alkali fluxes, commonly the carbonates of sodium or potassium, lower the fusion temperature and viscosity of silica. Limestone or dolomite (calcium and magnesium carbonates) act as stabilizers for the batch. Other ingredients such as lead and borax give to glass certain physical properties.

Tektite

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Tektite, any of a class of small, usually dark colored glassy objects found scattered in an S-shaped belt over a large area of the earth's surface. The belt stretches from Australia, through southeastern Asia, eastern Europe, the western coast of Africa, and South America, to Georgia and Texas. Despite their chemical similarity to some igneous and sedimentary rocks found on the earth, tektites do not belong to the rock groups in which they occur. This fact has given rise to various theories about their origin, based mainly on their shape and distribution.

The glassy composition and irregular shapes of tektites strongly suggest an extraterrestrial origin; the molten rounded shapes were probably acquired in flight through the atmosphere at very high speed. The theories proposed include: (1) the tektites represent lunar material thrown off the moon's surface following meteorite collisions; (2) they were formed by a meteorite crashing onto the earth, forcing a great deal of debris into the atmosphere; (3) they are the fragments of an exploding planet or meteorite passing through the atmosphere of the earth.

Absolute Zero

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Absolute Zero, lowest temperature theoretically possible, characterized by complete absence of heat. Absolute zero is approximately -273.16° C (-459.69° F), or zero degree on the Kelvin scale (0 K).

The concept of absolute zero temperature was first deduced from experiments with gases; when a fixed volume of gas is cooled, its pressure decreases with its temperature. Although this experiment cannot be conducted below the liquefaction point of the gas, a plot of the experimental values of pressure versus temperature can be extrapolated to zero pressure. The temperature at which the pressure would be zero is the absolute zero temperature. This experimental concept of a gas-thermometer temperature scale and of absolute zero was subsequently shown to be consistent with the theoretical definitions of absolute zero.

Absolute zero cannot be reached experimentally, although it can be closely approached. Special procedures are needed to reach very low, or cryogenic, temperatures. Liquid helium, which has a normal boiling point of 4.2 K (-268.9° C/-452.0° F), can be produced by cryostats, extremely well-insulated vessels, based on a design by the American mechanical engineer Samuel Collins. If the helium is then evaporated at reduced pressures, temperatures as low as 0.7 K can be obtained. Lower temperatures require the adiabatic (no heat transfer) demagnetization of paramagnetic substances (substances of low magnetizability), such as chrome alum, while they are being surrounded with a liquid helium bath (see Thermodynamics). The method, which was first developed in 1937 by the Canadian-American chemist William Giauque, utilizes a magnetic field that initially aligns the ionic magnets of the material. If the magnetic field is removed, the magnets again assume their random orientation, reducing the thermal energy of the material and thus its temperature. Temperatures as low as 0.002 K have been reached with the demagnetization of paramagnetic salts, and the demagnetization of atomic nuclei has yielded temperatures as low as 0.00001 K.

Temperature measurements at values close to absolute zero also present special problems. Gas thermometers can only be used up to the liquefaction point of helium. At lower temperatures, electric and magnetic measurements must be used to determine the effective temperature.

The concept of absolute zero temperature is also important in theoretical considerations. According to the third law of thermodynamics, the entropy, or state of disorder, of a pure crystal is zero at absolute zero temperature; this is of considerable importance in analyzing chemical reactions and in quantum physics.

See also Chemical Reaction; Quantum Theory; Superconductivity; Superfluidity.