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A solid with conventional crystalline properties but exhibiting a point-group symmetry inconsistent with translational periodicity. Like crystals, quasicrystals display discrete diffraction patterns, crystallize into polyhedral forms, and have long-range orientational order, all of which indicate that their structure is not random. But the unusual symmetry and the finding that the discrete diffraction pattern does not fall on a reciprocal periodic lattice suggest a solid that is quasiperiodic. Their discovery in 1982 contradicted a long-held belief that all crystals would be periodic arrangements of atoms or molecules.
Industry:Science
A solid-state device which has a capacitance that varies with the voltage applied across it. The name varactor is a contraction of the words variable and reactor. Typically the device consists of a reverse-biased <i>pn</i> junction that has been doped to maximize the change in capacitive reactance for a given change in the applied bias voltage. The device has two primary applications: frequency-tuning of radio-frequency circuits including frequency-modulation (FM) transmitters and solid-state receivers, and nonlinear frequency conversion in parametric oscillators and amplifiers.
Industry:Science
A solid-state heat engine which employs the electron gas as a working fluid. It directly converts heat energy into electrical energy using the Seebeck effect. This phenomenon can be demonstrated using a thermocouple which comprises two legs (thermoelements) of dissimilar conducting materials joined at one end to form a junction. If this junction is maintained at a temperature which differs from ambient, a voltage is generated across the open ends of the thermoelements. When the circuit is completed with a load, a current flows in the circuit and power is generated. In practice the thermocouples are fabricated generally from <i>n</i>- and <i>p</i>-type semiconductors, and several hundred are connected electrically in series to form a module which is the active component of a thermoelectric generator. Provided a temperature difference is maintained across the device, it will generate electrical power. Heat is provided from a variety of sources depending on the application, and they include burning fossil fuels in terrestrial and military applications, decaying long-life isotopes in medical and deep-space applications, and waste heat. The performance of the thermoelectric generator, in terms of efficiency, output power, and economic viability, depends upon its temperature regime of operation; the materials used in the module construction; its electrical, thermal, and geometrical design; and the generator load. The power output spectrum of thermoelectric generators spans 14 orders of magnitude and ranges from nanowatt generators fabricated using integrated circuit technology to the nuclear reactor–powered 100-kW SP-100 generator intended to provide electrical power to orbiting space stations.
Industry:Science
A solution is at the saturation point when dissolved solute in it crystallizes from it at the same rate at which it dissolves. Under prescribed experimental conditions of temperature and pressure, a solution can contain at saturation only one fixed amount of dissolved solute. However, it is possible to prepare relatively stable solutions which contain a quantity of a dissolved solute greater than that of the saturation value provided solute phase is absent. Such solutions are said to be supersaturated. They can be prepared by changing the experimental conditions of a system so that greater solubility is obtained, perhaps by heating the solution, and then carefully returning the system to or near its original state. The addition of solute phase will immediately relieve supersaturation. Solutions in which there is no spontaneous formation of solute phase for extended periods of time are said to be metastable. There is no sharp line of demarcation between an unstable and a metastable solution. In fact, the latter is poorly defined and much influenced by many factors such as mechanical shock and the presence of minute quantities of foreign materials. The process whereby initial aggregates within a supersaturated solution develop spontaneously into particles of new stable phase is known as nucleation. The greater the degree of supersaturation, the greater will be the number of nuclei formed. This should be avoided in gravimetric analysis because of the formation of many small crystals that tend to coprecipitate excess amounts of foreign ions by virtue of their great surface area.
Industry:Science
A time interval equal to <math> \sqrt\{360\} </math> of the time required for one complete cycle of alternating current. Mechanical rotation is often measured in degrees, 360° constituting one complete revolution. In describing alternating voltages and currents, the time for one complete cycle is considered to be equivalent to 360 electrical degrees (360°) or 2π electrical radians. For example, if the frequency <i>f</i> is 60 cycles per second (60 Hz), 360° corresponds to 1/√<span style="border-top:1px solid black;">60</span> second and 1 electrical degree to 1/√<span style="border-top:1px solid black;">21,600</span> second.
Industry:Science
A solution of a partial differential equation for the case of a point source of unit strength within the region under examination. The Green's function is an important mathematical tool that has application in many areas of theoretical physics including mechanics, electromagnetism, acoustics, solid-state physics, thermal physics, and the theory of elementary particles.
The underlying physics in each of these areas is generally described by some linear partial differential equation which relates the physical variable of interest (electrostatic potential or pressure amplitude in a sound wave, for example) to a source function. For present purposes the source may be regarded as an independent entity, although in some applications (for example, particle physics) this view masks an inherent nonlinearity. The source may be physically located within the region of interest, it may be simulated by certain boundary conditions on the surface of that region, or it may consist of both possibilities. A Green's function is a solution to the relevant partial differential equation for the particular case of a point source of unit strength in the interior of the region and some designated boundary condition on the surface of the region. Solutions to the partial differential equation for a general source function and appropriate boundary condition can then be written in terms of certain volume and surface integrals of the Green's function.
Industry:Science
A solution of two or more liquids, the composition of which does not change upon distillation. The composition of the liquid phase at the boiling point is identical to that of the vapor in equilibrium with it, and such mixtures or azeotropes form constant-boiling solutions. The exact composition of the azeotrope changes if the boiling point is altered by a change in the external pressure.
A solution of two components which form an azeotrope may be separated by distillation into one pure component and the azeotrope, but not into two pure components. Standard solutions are often prepared by distillation of aqueous solutions until the azeotropic composition is reached. At 1 atm (760 mmHg or approximately 100 kilopascals) pressure, hydrogen chloride and water form an azeotrope containing 20.24% by weight of HCl.
Industry:Science
A solution to the (imaginary-time) nonlinear field equation which arises in Yang-Mills field theory—a modern nonlinear generalization of Maxwell electromagnetic theory that is believed to give a fundamental description of elementary particles and forces. The instanton solution carries information about quantum tunneling. (Initially, the instanton was called a pseudoparticle.)
Industry:Science
A sonic boom is the loud impulsive sound produced by any object moving faster than the local sound speed. Human-made sonic booms have existed for a long time, due to cracking whips or artillery shells, but they were not recognized as such before World War I, when booms interfered with Ernest Esclangon's attempts to localize German guns by acoustical goniometry (direction finding).
Then sonic booms quickly returned to obscurity until the birth of the “supersonic age” in 1947 when Charles Yeager broke the sound barrier on the aircraft Bell X-1. This event was followed by the design of supersonic fighters and the launch in the 1960s of civil supersonic projects (the B2707 in the United States, the Soviet TU144, and the British-French Concorde), at a time of cold war competition when the future of air travel was confidently forecast as supersonic. But high costs, increasing environmental concerns, surging oil prices, and the political turmoil of the 1970s contradicted optimistic anticipations. The Concorde alone achieved the dream of supersonic travel, but only for wealthy customers flying transatlantic. Indeed, the Federal Aviation Administration banned civil supersonic flight over U.S. territory in 1973 as a protection against undesirable booms (at that time, the Concorde's). This ban was also adopted by other countries.
Industry:Science
A sorting operation in which mixtures of particles of mixed sizes, and often of different specific gravities, are separated into fractions by the action of a stream of fluid. Water is ordinarily used as the sorting fluid, but other liquids or air or other gases may be used.
Industry:Science
