Crystals

C O N S I D E R:
Properties: Crystals differ in physical properties, in: hardness, cleavage, optical properties, heat conductivity, and electrical conductivity. [Harris, William H., and Judith S. Levey, eds. The New Columbia Encyclopedia. New York and London: Columbia University Press, 1975.]

New Columbia Encyclopedia: Crystal. A solid body bounded by natural plane faces that are the external expression of a regular internal arrangement of constituent atoms, molecules, or ions. The particles in a crystal occupy positions with definite geometrical relationships to each other. The positions form a kind of scaffolding, called a crystalline lattice; the atomic occupancies of lattice positions re determined by the chemical composition of the substance. The formation of a crystal by a substance passing from a gas or liquid to a solid state, or by going out of solution (by precipitation or evaporation), is called crystallization. A crystalline substance is uniquely defined by the combination of its chemistry and the structural arrangement of its atoms. In all crystals of any specific substance the angles between corresponding faces are constant (Steno's Law, or the First Law of Crystallography). Crystalline substances are grouped, according to the type of symmetry they display, into 32 classes. These in turn are grouped into seven systems on the basis of the relationships of their axes, i.e., imaginary straight lines passing through the ideal centers of the crystals. Crystals may be symmetrical with relation to planes, axes, and centers of symmetry. Planes of symmetry divide crystals into equal parts (mirror images) that correspond point for point, angle for angle, and face for face. Axes of symmetry are imaginary lines about which the crystal may be considered to rotate, assuming, after passing through a rotation of 60 degrees, 90 degrees, 120 degrees, or 180 degrees, the identical position in space that it originally had. Centers of symmetry are points from which imaginary straight lines may be drawn to intersect identical pints equidistant from the center on opposite sides. The crystalline systems are cubic, or isometric (three equal axes, intersecting at right angles); hexagonal (three equal axes, intersecting at 60 degree angles in horizontal plane, and a fourth, longer of shorter, axes, perpendicular to the lane of the other three); tetragonal (two equal, horizontal axes at right angles and one axis longer or shorter than the other two and perpendicular to their plane); orthorhombic (three unequal axes intersecting at right angles); monoclinic (three unequal axes, two intersecting at right angles and the third at an oblique angle to the plane of the other two); trigonal, or rhombohedral (three equal axes intersecting at oblique angles); and triclinic (three unequal axes intersecting at oblique angles). In all systems in which the axes are unequal there is a definite axial ratio for each crystal substance. Crystals differ in physical properties, i.e., in

hardness

cleavage

optical properties

heat conductivity

electrical conductivity.

These properties are important since they sometimes determine the use to which the crystals are put in industry. For example, crystalline substances that have special electrical properties are much used in communications equipment. These include quartz and Rochelle salt, which supply voltage upon the application of mechanical force (see PIEZOELECTRIC EFFECT), and germanium, silicon, galena, and silicon carbide, which carry current unequally in different crystallographic directions (semiconductor rectifier). F. C. Phillips, An Introduction to Crystallography (1970); J. D. Dana, Manual of Mineralogy (18th ed., rev. by C. S. Huribut, Jr., 1971)

[Harris, William H., and Judith S. Levey, eds. The New Columbia Encyclopedia. New York and London: Columbia University Press, 1975.]

Crystals, Magical Servants of the Space Age, by Kenneth F. Weaver, Assistant Editor, National Geographic Magazine [ca. 1975].
What is a crystal? From the rock of earth's crust to the lacy snowflake, most solid matter is crystalline. This means that its atoms, for reasons not entirely understood by scientists, arrange themselves in orderly three-dimensional patterns called lattices. Alum and table salt . . . . reflect this inner order in their outer shape. Most crystalline matter, however, is a conglomeration of small crystals, and its external appearance belies the inner order. Because of their atomic structure, some crystals exert strange forces on light and electricity or possess great strength--properties that man finds increasingly useful . . . . [pg. 280]

The miraculous "IC"
Take a tiny chip of synthetic crystal, cram it with scores of minute electronic devices, and you have the integrated circuit, or IC, a little giant that has revolutionized electronics technology. Successor to the bulky vacuum tube and the transistor, the IC goes through an ingenious manufacturing process . . . . The end product is an engineer's delight: rugged, long-lasting, low in cost, fantastically speedy. Magnified 12 times . . . . the IC with its gold leads and parallel prongs for soldering also reveals unsuspected beauty.

Slicing off a waver 12/1000 of an inch thick, IC technicians polish it even thinner. Silicon provides a good material for the circuits because its electrical properties can be precisely altered by adding controlled amounts of impurities, called dopants. A wafer will eventually carry a grid of hundreds of IC's . . . . [pg. 286]

Mighty midget, a chip that houses 44 electronic components slips through the eye of an ordinary sewing needle. Such IC compactness pays off not only in space saved but in reduced time required for electricity to pass from component to component--a vital factor in computers that must do hundreds of millions of computations in a second . . . . pg. 291]

. . . . Using new techniques to crowd components closer and closer, one firm says it can already put nearly a million [ICs] on a two-inch slice--although it has not yet solved the problem of making all of them usable . . . . [p. 294]

. . . . When these supercircuits and "computers on a slice" come out of a laboratory some years hence, the stuff of dreams will become reality. The computer, adaptable to an infinite number of tasks, will become remarkably cheap by today's standards . . . . [pg. 294]

. . . . The marvels we have looked at so far all involve the special properties of semiconductor crystals--chiefly silicon.

Many other kinds of crystals offer their own unique capabilities. Ruby crystals, for example, can be made into lasers, which amplify light and produce enormously bright and powerful beams [See "The Laser's Bright Magic," by Thomas Meloy, NGS, Dec. 1966]

Some crystals convert light into electricity. Crystalline selenium does this in a photographer's light meter. The amount of light hitting the selenium determines the strength of current it creates, and thus controls how far the pointer moves across the dial.

Many spacecraft wear thousands of thin purple-blue rectangles of silicon. These are solar cells; they provide the spacecraft with electricity converted from sunlight at rates as high as 10 watts per square foot of surface.

Piezoelectric crystals, such as quartz or Rochelle salt, give off electricity when twisted or pressed, or vibrate rapidly when an alternating current is applied. [The name comes from a Greek root meaning to press or squeeze and is pronounced pee-AY-zoh.]

At the Bell Telephone Laboratories in Murray Hill, NJ, I saw a simple but revealing demonstration of how such crystals work. In an exhibit case, a wafer of quartz is connected to an electric meter. A mechanical hammer periodically strikes the crystal. At each blow, the needle on the meter jumps, showing that a current is passing through.

Piezoelectric crystals are used by the millions for all sorts of useful tasks. In many phonograph pickups they respond to the vibrations of the stylus in the record grooves by giving off a weak current, whose variations are amplified and sent on to the loud-speaker. They work similarly in microphones. In submarine sonar systems, they detect sounds. And in radio stations thin quartz wafers vibrating at carefully determined rates control the broadcast frequencies.

In 1963 an IBM physicist, J.B. Gunn, discovered that a speck of gallium arsenide, a man-made crystal, gives off microwave radiations when a current passes through it. These vibrations pulse at the rate of one to ten billion a second, overlapping the radar range.

. . . . Industry has been quick to see the possibilities of "Gunn devices" made from these crystals Within a few years we will probably see on the market radar sets light enough and cheap enough for blind people to carry, or for everyday use in automobiles and small boats.

In General Electric's Space Sciences Laboratories near Philadelphia, Dr. Willard H. Sutton showed me the finest crystals I have ever seen. They were sapphire "whiskers," about half an inch long, but so fine that a billion would weigh only a pound. In a mass they looked like cotton candy, but they were amazingly stiff to the touch . . . .

Delicate as they may seem, fine whiskers such as these, or thin filaments of such substances as boron, can be mixed with metals or plastics to produce incredibly tough, heat-resistant, new Space Age materials. Some, lighter than aluminum yet stiffer and stronger than steel, theoretically make possible sky-scrapers five times as high as the Empire State Building, or suspension bridges twice as long as today's longest.

Military men wax enthusiastic about the advantages for nose cones, rocket nozzles, and parts of supersonic jet planes. And before long your dentist may fill your cavities with crystal-reinforced compounds.

Of all the improbable mysteries I ran across in preparing this article, the strangest concerned the "liquid crystals." Fred Davis showed me in the Westinghouse Research Laboratories at Pittsburgh. Derivatives of cholesterol, these organic materials flow like viscous liquids, yet have the inner structural orderliness of solid crystals. They respond with high sensitivity to temperature changes.

Mr. Davis spread liquid crystals on a thin sheet of black plastic. Colorless at first, the liquid quickly took on color as he warmed it with an electric light bulb. Reds, greens, and blues chased each other across the surface in fleeting patterns, faster than the neon lights of Las Vegas. Cooler areas showed red, hotter areas blue . . . .

Doctors are experimenting with such liquid crystals as a possible aid in cancer diagnosis. Most cancers show up as hot spots because more blood circulates in the tumor area. Benign tumors are usually cooler . . . . [p. 295-95]

["Crystals, Magical Servants of the Space Age," by Kenneth F. Weaver, Assistant Editor, National Geographic Magazine [ca. 1975].

R  E  F  E  R  E  N  C  E  S 

2 Crystal adj [14c] 1: consisting of or resembling crystal: Clear, Lucid 2: relating to or using a crystal [a __ radio receiver]

Crystalline adj [ME cristallin, fr. MF & L; MF, fr. L crystallinus, fr. Gk krystallinos, fr. krystallos] [15c] 1: resemblng crystal: as a: strikingly clear or sparkling [__ air] [a __ lake] b: Clear-cut 2: made of crystal: composed of crystals 3: constituting or relating to a crystal

Crystalline lens n [1794]: the lens of the eye in vertebrates

Crystallite n [G kristallit, fr. Gk krystalos] [1805] 1A: a minute mineral form [as in glassy volcanic rocks] that marks the beginning of crystallization b: a single grain in a polycrystalline substance 2: Micelle

Crystallize vt [1598] 1: to cause to form crystals or assume crystalline form 2: to cause to take a definite form [tried to __ his toughts] 3: to coat with crystals esp. of sugar [__ grapes] vi: to become crystallized

Crystal pleat n [1976]: any of a series of narrow sharply pressed pleats all turned in one direction