1.
Phosphorus
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Phosphorus is a chemical element with symbol P and atomic number 15. As an element, phosphorus exists in two major forms—white phosphorus and red phosphorus—but because it is reactive, phosphorus is never found as a free element on Earth. At 0. 099%, phosphorus is the most abundant pnictogen in the Earths crust, with few exceptions, minerals containing phosphorus are in the maximally oxidised state as inorganic phosphate rocks. The glow of phosphorus itself originates from oxidation of the white phosphorus — a process now termed chemiluminescence, together with nitrogen, arsenic, antimony, and bismuth, phosphorus is classified as a pnictogen. Phosphates are a component of DNA, RNA, ATP, and the phospholipids, demonstrating the link between phosphorus and life, elemental phosphorus was first isolated from human urine, and bone ash was an important early phosphate source. Phosphate mines contain fossils, especially marine fossils, because phosphate is present in the deposits of animal remains. Low phosphate levels are an important limit to growth in aquatic systems. The vast majority of compounds produced are consumed as fertilisers. Phosphate is needed to replace the phosphorus that plants remove from the soil, other applications include the role of organophosphorus compounds in detergents, pesticides, and nerve agents. Phosphorus exists as several forms that exhibit different properties. The two most common allotropes are white phosphorus and red phosphorus, from the perspective of applications and chemical literature, the most important form of elemental phosphorus is white phosphorus, often abbreviated as WP. It is a soft and waxy solid consists of tetrahedral P4 molecules. This P4 tetrahedron is also present in liquid and gaseous phosphorus up to the temperature of 800 °C when it starts decomposing to P2 molecules, White phosphorus exists in two crystalline forms, α and β. At room temperature, the α-form is stable, which is common and it has cubic crystal structure and at 195.2 K, it transforms into β-form. These forms differ in terms of the orientations of the constituent P4 tetrahedra. White phosphorus is the least stable, the most reactive, the most volatile, the least dense, White phosphorus gradually changes to red phosphorus. This transformation is accelerated by light and heat, and samples of white phosphorus almost always some red phosphorus. For this reason, white phosphorus that is aged or otherwise impure is also called yellow phosphorus, when exposed to oxygen, white phosphorus glows in the dark with a very faint tinge of green and blue
2.
Monochrome monitor
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A monochrome monitor is a type of CRT computer monitor which was very common in the early days of computing, from the 1960s through the 1980s, before color monitors became popular. They are still used in applications such as computerized cash register systems. Green screen was the name for a monochrome monitor using a green P1 phosphor screen. Abundant in the early-to-mid-1980s, they succeeded Teletype terminals and preceded color CRTs, unlike color monitors, which display text and graphics in multiple colors through the use of alternating-intensity red, green, and blue phosphors, monochrome monitors have only one color of phosphor. All text and graphics are displayed in that color, some monitors have the ability to vary the brightness of individual pixels, thereby creating the illusion of depth and color, exactly like a black-and-white television. But typically only a set of brightness levels was provided to save display memory which was very expensive in the 70s and 80s. Either normal/bright or normal/dim per character as in the VT100 or black/white per pixel in the Macintosh 128K or black, dark gray, light gray, monochrome monitors are commonly available in three colors, if the P1 phosphor is used, the screen is green monochrome. If the P3 phosphor is used, the screen is amber monochrome, if the P4 phosphor is used, the screen is white monochrome, this is the same phosphor as used in early television sets. An amber screen was claimed to give improved ergonomics, specifically by reducing eye strain, well-known examples of early monochrome monitors are the VT100 from Digital Equipment Corporation, released in 1978, and the IBM5151, which accompanied the IBM PC model 5150 upon its 1981 release. This was much higher resolution than the alternative IBM Color Graphics Adapter 320×200 pixel and it could also run most programs written for the CGA cards standard graphics modes. Pixel for pixel, monochrome monitors produce sharper text and images than color CRT monitors, other green screens avoided the heavy afterglow-effects, but at the cost of much more pixelated character images. The 5151, amongst others, had brightness and contrast controls to allow the user to set their own compromise, the ghosting effects of the now-obsolete green screens have become an eye-catching visual shorthand for computer-generated text, frequently in futuristic settings. The opening titles of the first Ghost in the Shell film, green text is also featured in the Swans computer in Lost series. Monochrome monitors are particularly susceptible to burn, because the phosphors used are very high intensity. Another effect of the high-intensity phosphors is a known as ghosting. This has a place in pop culture, as evidenced in movies such as The Matrix. This ghosting effect is deliberate on some monitors, known as long persistence monitors and these use the relatively long decay period of the phosphor glow to reduce flickering
3.
Aperture grille
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An aperture grille is one of two major technologies used to manufacture color cathode ray tube televisions and computer displays, the other is shadow mask. Fine vertical wires behind the front glass of the display screen separate the different colors of phosphors into strips and these wires are positioned such that an electron beam from one of three guns at the rear of the tube is only able to strike phosphors of the appropriate color. That is, the electron gun will strike blue phosphors. The fine wires allow for a finer dot pitch as they can be spaced closer together than the perforations of a shadow mask. During the display of images, a shadow mask warms. Aperture grilles do not exhibit this behavior, when the heat up. Because there are no defined holes, this expansion does not affect the image, and these stabilizing wires provide the easiest way to distinguish aperture grille and shadow mask displays at a glance. The stabilized grille can still vibrate but the sounds need to be loud, additionally, aperture grille displays tend to be vertically flat and are often horizontally flat as well, while shadow mask displays usually have a spherical curvature. The first patented aperture grille televisions were manufactured by Sony in the late 1960s under the Trinitron brand name, subsequent designs, either licensed from Sony or manufactured after the patents expiration, tend to use the -tron suffix, such as Mitsubishis DiamondTron and ViewSonics SonicTron. Cromaclear is an improvement of the aperture grille technology, pioneered by the NEC Corporation, shadow mask Cromaclear Chromatron Trinitron Aperture grille details
4.
Optical phenomenon
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Optical phenomena are any observable events that result from the interaction of light and matter. See also list of topics and optics. A mirage is an example of an optical phenomenon, common optical phenomena are often due to the interaction of light from the sun or moon with the atmosphere, clouds, water, dust, and other particulates. One common example is the rainbow, when light from the sun is reflected and refracted by water droplets, some, such as the green ray, are so rare they are sometimes thought to be mythical. Others, such as Fata Morganas, are commonplace in favored locations, other phenomena are simply interesting aspects of optics, or optical effects. For instance, the colors generated by a prism are often shown in classrooms, optical phenomena include those arising from the optical properties of the atmosphere, the rest of nature, of objects, whether natural or human-made, and of our eyes. Also listed here are unexplained phenomena that could have an optical explanation, there are many phenomena that result from either the particle or the wave nature of light. Some are quite subtle and observable only by precise measurement using scientific instruments, one famous observation is of the bending of light from a star by the Sun observed during a solar eclipse. This demonstrates that space is curved, as the theory of relativity predicts, afterglow Airglow Alexanders band, the dark region between the two bows of a double rainbow. Some consider many of these mysteries to simply be local tourist attractions that are not worthy of thorough investigation. Minnaert, Light and Color in the Outdoors, ISBN 0-387-97935-2 John Naylor Out of the Blue, A 24-hour Skywatchers Guide, CUP,2002, ISBN 0-521-80925-8 Abenteuer im Erdschatten
5.
Luminescence
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Luminescence is emission of light by a substance not resulting from heat, it is thus a form of cold-body radiation. It can be caused by chemical reactions, electrical energy, subatomic motions, or stress on a crystal and this distinguishes luminescence from incandescence, which is light emitted by a substance as a result of heating. Historically, radioactivity was thought of as a form of radio-luminescence, the term luminescence was introduced in 1888 by Eilhard Wiedemann. The dials, hands, scales, and signs of aviation and navigational instruments and this property of these minerals can be used during the process of mineral identification at rock outcrops in the field, or in the laboratory. Fluorophores. org A database of luminescent dyes
6.
Phosphorescence
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Phosphorescence is a type of photoluminescence related to fluorescence. Unlike fluorescence, phosphorescent material does not immediately re-emit the radiation it absorbs, the slower time scales of the re-emission are associated with forbidden energy state transitions in quantum mechanics. As these transitions occur very slowly in certain materials, absorbed radiation is re-emitted at an intensity for up to several hours after the original excitation. Typically, the glow slowly fades out, sometimes within a few minutes or up to a few hours in a dark room, the study of phosphorescent materials led to the discovery of radioactivity in 1896. In simple terms, phosphorescence is a process in which energy absorbed by a substance is released relatively slowly in the form of light and this is in some cases the mechanism used for glow-in-the-dark materials which are charged by exposure to light. Most photoluminescent events, in which a chemical substrate absorbs and then re-emits a photon of light, are fast, light is absorbed and emitted at these fast time scales in cases where the energy of the photons involved matches the available energy states and allowed transitions of the substrate. In the special case of phosphorescence, the photon energy undergoes an unusual intersystem crossing into an energy state of higher spin multiplicity. As a result, the energy can become trapped in the state with only forbidden transitions available to return to the lower energy state. These transitions, although forbidden, will occur in quantum mechanics but are kinetically unfavored. Most phosphorescent compounds are relatively fast emitters, with triplet lifetimes on the order of milliseconds. However, some compounds have triplet lifetimes up to minutes or even hours, if the phosphorescent quantum yield is high, these substances will release significant amounts of light over long time scales, creating so-called glow-in-the-dark materials. S0 + h ν → S1 → T1 → S0 + h ν ′ where S is a singlet, transitions can also occur to higher energy levels, but the first excited state is denoted for simplicity. Some examples of glow-in-the-dark materials do not glow by phosphorescence, for example, glow sticks glow due to a chemiluminescent process which is commonly mistaken for phosphorescence. In chemiluminescence, a state is created via a chemical reaction. The light emission tracks the progress of the underlying chemical reaction. The excited state will then transfer to a dye molecule, also known as a sensitizer or fluorophor, common pigments used in phosphorescent materials include zinc sulfide and strontium aluminate. Use of zinc sulfide for safety related products dates back to the 1930s, however, the development of strontium aluminate, with a luminance approximately 10 times greater than zinc sulfide, has relegated most zinc sulfide based products to the novelty category. Strontium aluminate based pigments are now used in signs, pathway marking
7.
Fluorescence
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Fluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation. It is a form of luminescence, in most cases, the emitted light has a longer wavelength, and therefore lower energy, than the absorbed radiation. Fluorescent materials cease to glow immediately when the radiation source stops, unlike phosphorescence, Fluorescence also occurs frequently in nature in some minerals and in various biological states in many branches of the animal kingdom. An early observation of fluorescence was described in 1560 by Bernardino de Sahagún and it was derived from the wood of two tree species, Pterocarpus indicus and Eysenhardtia polystachya. The chemical compound responsible for this fluorescence is matlaline, which is the product of one of the flavonoids found in this wood. The name was derived from the fluorite, some examples of which contain traces of divalent europium. In a key experiment he used a prism to isolate ultraviolet radiation from sunlight, the specific frequencies of exciting and emitted light are dependent on the particular system. S0 is called the state of the fluorophore, and S1 is its first excited singlet state. A molecule in S1 can relax by various competing pathways and it can undergo non-radiative relaxation in which the excitation energy is dissipated as heat to the solvent. Excited organic molecules can also relax via conversion to a triplet state, relaxation from S1 can also occur through interaction with a second molecule through fluorescence quenching. Molecular oxygen is an extremely efficient quencher of fluorescence just because of its unusual triplet ground state, in most cases, the emitted light has a longer wavelength, and therefore lower energy, than the absorbed radiation, this phenomenon is known as the Stokes shift. The emitted radiation may also be of the wavelength as the absorbed radiation. Molecules that are excited through light absorption or via a different process can transfer energy to a second sensitized molecule, the fluorescence quantum yield gives the efficiency of the fluorescence process. It is defined as the ratio of the number of photons emitted to the number of photons absorbed, Φ = Number of photons emitted Number of photons absorbed The maximum fluorescence quantum yield is 1.0, each photon absorbed results in a photon emitted. Compounds with quantum yields of 0.10 are still considered quite fluorescent, thus, if the rate of any pathway changes, both the excited state lifetime and the fluorescence quantum yield will be affected. Fluorescence quantum yields are measured by comparison to a standard, the quinine salt quinine sulfate in a sulfuric acid solution is a common fluorescence standard. The fluorescence lifetime refers to the time the molecule stays in its excited state before emitting a photon. This is an instance of exponential decay, various radiative and non-radiative processes can de-populate the excited state
8.
Cathode ray tube
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The cathode ray tube is a vacuum tube that contains one or more electron guns and a phosphorescent screen, and is used to display images. It modulates, accelerates, and deflects electron beam onto the screen to create the images, the images may represent electrical waveforms, pictures, radar targets, or others. CRTs have also used as memory devices, in which case the visible light emitted from the fluorescent material is not intended to have significant meaning to a visual observer. In television sets and computer monitors, the front area of the tube is scanned repetitively and systematically in a fixed pattern called a raster. An image is produced by controlling the intensity of each of the three beams, one for each additive primary color with a video signal as a reference. A CRT is constructed from an envelope which is large, deep, fairly heavy. The interior of a CRT is evacuated to approximately 0.01 Pa to 133 nPa. evacuation being necessary to facilitate the flight of electrons from the gun to the tubes face. That it is evacuated makes handling an intact CRT potentially dangerous due to the risk of breaking the tube and causing a violent implosion that can hurl shards of glass at great velocity. As a matter of safety, the face is made of thick lead glass so as to be highly shatter-resistant and to block most X-ray emissions. Flat panel displays can also be made in large sizes, whereas 38 to 40 was about the largest size of a CRT television, flat panels are available in 60. Cathode rays were discovered by Johann Hittorf in 1869 in primitive Crookes tubes and he observed that some unknown rays were emitted from the cathode which could cast shadows on the glowing wall of the tube, indicating the rays were traveling in straight lines. In 1890, Arthur Schuster demonstrated cathode rays could be deflected by electric fields, the earliest version of the CRT was known as the Braun tube, invented by the German physicist Ferdinand Braun in 1897. It was a diode, a modification of the Crookes tube with a phosphor-coated screen. In 1907, Russian scientist Boris Rosing used a CRT in the end of an experimental video signal to form a picture. He managed to display simple geometric shapes onto the screen, which marked the first time that CRT technology was used for what is now known as television. The first cathode ray tube to use a hot cathode was developed by John B. Johnson and Harry Weiner Weinhart of Western Electric and it was named by inventor Vladimir K. Zworykin in 1929. RCA was granted a trademark for the term in 1932, it released the term to the public domain in 1950. The first commercially made electronic television sets with cathode ray tubes were manufactured by Telefunken in Germany in 1934, in oscilloscope CRTs, electrostatic deflection is used, rather than the magnetic deflection commonly used with television and other large CRTs
9.
Fluorescent light
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A fluorescent lamp or a fluorescent tube is a low pressure mercury-vapor gas-discharge lamp that uses fluorescence to produce visible light. An electric current in the gas excites mercury vapor which produces ultraviolet light that then causes a phosphor coating on the inside of the lamp to glow. A fluorescent lamp converts electrical energy into useful light much more efficiently than incandescent lamps, the typical luminous efficacy of fluorescent lighting systems is 50–100 lumens per watt, several times the efficacy of incandescent bulbs with comparable light output. Compact fluorescent lamps are now available in the same sizes as incandescents and are used as an energy-saving alternative in homes. Because they contain mercury, many fluorescent lamps are classified as hazardous waste, the United States Environmental Protection Agency recommends that fluorescent lamps be segregated from general waste for recycling or safe disposal, and some jurisdictions require recycling of them. Fluorescence of certain rocks and other substances had been observed for hundreds of years before its nature was understood, by the middle of the 19th century, experimenters had observed a radiant glow emanating from partially evacuated glass vessels through which an electric current passed. The explanation relied on the nature of electricity and light phenomena as developed by the British scientists Michael Faraday in the 1840s and James Clerk Maxwell in the 1860s. Little more was done with this phenomenon until 1856 when German glassblower Heinrich Geissler created a vacuum pump that evacuated a glass tube to an extent not previously possible. Geissler invented the first gas-discharge lamp, the Geissler tube, consisting of an evacuated glass tube with a metal electrode at either end. When a high voltage was applied between the electrodes, the inside of the lit up with a glow discharge. By putting different chemicals inside, the tubes could be made to produce a variety of colors, more important, however, was its contribution to scientific research. One of the first scientists to experiment with a Geissler tube was Julius Plücker who systematically described in 1858 the luminescent effects that occurred in a Geissler tube and he also made the important observation that the glow in the tube shifted position when in proximity to an electromagnetic field. Alexandre Edmond Becquerel observed in 1859 that certain substances gave off light when they were placed in a Geissler tube and he went on to apply thin coatings of luminescent materials to the surfaces of these tubes. Fluorescence occurred, but the tubes were very inefficient and had an operating life. Inquiries that began with the Geissler tube continued as even better vacuums were produced, the most famous was the evacuated tube used for scientific research by William Crookes. That tube was evacuated by the highly effective mercury vacuum pump created by Hermann Sprengel, Research conducted by Crookes and others ultimately led to the discovery of the electron in 1897 by J. J. Thomson and X-rays in 1895 by Wilhelm Roentgen. While Becquerel was interested primarily in conducting research into fluorescence. Although Edison lost interest in fluorescent lighting, one of his employees was able to create a gas-based lamp that achieved a measure of commercial success
10.
Light-emitting diode
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A light-emitting diode is a two-lead semiconductor light source. It is a p–n junction diode, which emits light when activated, when a suitable voltage is applied to the leads, electrons are able to recombine with electron holes within the device, releasing energy in the form of photons. This effect is called electroluminescence, and the color of the light is determined by the band gap of the semiconductor. LEDs are typically small and integrated optical components may be used to shape the radiation pattern, appearing as practical electronic components in 1962, the earliest LEDs emitted low-intensity infrared light. Infrared LEDs are still used as transmitting elements in remote-control circuits. The first visible-light LEDs were also of low intensity and limited to red, modern LEDs are available across the visible, ultraviolet, and infrared wavelengths, with very high brightness. Early LEDs were often used as indicator lamps for electronic devices and they were soon packaged into numeric readouts in the form of seven-segment displays and were commonly seen in digital clocks. Recent developments in LEDs permit them to be used in environmental, LEDs have allowed new displays and sensors to be developed, while their high switching rates are also used in advanced communications technology. LEDs have many advantages over incandescent light sources including lower energy consumption, longer lifetime, improved physical robustness, smaller size, and faster switching. Light-emitting diodes are now used in applications as diverse as aviation lighting, automotive headlamps, advertising, general lighting, traffic signals, camera flashes, as of 2017, LED lights home room lighting are as cheap or cheaper than compact fluorescent lamp sources of comparable output. They are also more energy efficient and, arguably, have fewer environmental concerns linked to their disposal. Electroluminescence as a phenomenon was discovered in 1907 by the British experimenter H. J. Round of Marconi Labs, using a crystal of silicon carbide, russian inventor Oleg Losev reported creation of the first LED in 1927. His research was distributed in Soviet, German and British scientific journals, rubin Braunstein of the Radio Corporation of America reported on infrared emission from gallium arsenide and other semiconductor alloys in 1955. Braunstein observed infrared emission generated by simple diode structures using gallium antimonide, GaAs, indium phosphide, in 1957, Braunstein further demonstrated that the rudimentary devices could be used for non-radio communication across a short distance. As noted by Kroemer Braunstein …had set up a simple optical communications link, the emitted light was detected by a PbS diode some distance away. This signal was fed into an amplifier and played back by a loudspeaker. Intercepting the beam stopped the music and we had a great deal of fun playing with this setup. This setup presaged the use of LEDs for optical communication applications, by October 1961, they had demonstrated efficient light emission and signal coupling between a GaAs p-n junction light emitter and an electrically-isolated semiconductor photodetector
11.
Rare-earth
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Scandium and yttrium are considered rare earth elements because they tend to occur in the same ore deposits as the lanthanides and exhibit similar chemical properties. They are not especially rare, but they tend to occur together in nature and are difficult to separate from one another, however, because of their geochemical properties, rare earth elements are typically dispersed and not often found concentrated as rare earth minerals in economically exploitable ore deposits. The first such mineral discovered was gadolinite, a composed of cerium, yttrium, iron, silicon. This mineral was extracted from a mine in the village of Ytterby in Sweden, a table listing the seventeen rare earth elements, their atomic number and symbol, the etymology of their names, and their main usages is provided here. The distinction between the groups is more to do with atomic volume and geological behavior, Rare earth elements became known to the world with the discovery of the black mineral Ytterbite by Lieutenant Carl Axel Arrhenius in 1787, at a quarry in the village of Ytterby, Sweden. Arrheniuss ytterbite reached Johan Gadolin, a Royal Academy of Turku professor, anders Gustav Ekeberg isolated beryllium from the gadolinite but failed to recognize other elements that the ore contained. After this discovery in 1794 a mineral from Bastnäs near Riddarhyttan, Sweden, in 1803 they obtained a white oxide and called it ceria. Martin Heinrich Klaproth independently discovered the same oxide and called it ochroia, in 1839 Carl Gustav Mosander, an assistant of Berzelius, separated ceria by heating the nitrate and dissolving the product in nitric acid. He called the oxide of the soluble salt lanthana and it took him three more years to separate the lanthana further into didymia and pure lanthana. Didymia, although not further separable by Mosanders techniques, was a mixture of oxides, in 1842 Mosander also separated the yttria into three oxides, pure yttria, terbia and erbia. The earth giving pink salts he called terbium, the one that yielded yellow peroxide he called erbium, so in 1842 the number of known rare earth elements had reached six, yttrium, cerium, lanthanum, didymium, erbium and terbium. This confusion led to false claims of new elements, such as the mosandrium of J. Lawrence Smith. There were no further discoveries for 30 years, and the element didymium was listed in the table of elements with a molecular mass of 138. In 1879 Delafontaine used the new process of optical-flame spectroscopy. Also in 1879, the new element samarium was isolated by Paul Émile Lecoq de Boisbaudran from the mineral samarskite, the samaria earth was further separated by Lecoq de Boisbaudran in 1886 and a similar result was obtained by Jean Charles Galissard de Marignac by direct isolation from samarskite. They named the element gadolinium after Johan Gadolin, and its oxide was named gadolinia, the fractional crystallization of the oxides then yielded europium in 1901. In 1839 the third source for rare earths became available and this is a mineral similar to gadolinite, uranotantalum. This mineral from Miass in the southern Ural Mountains was documented by Gustave Rose, the exact number of rare earth elements that existed was highly unclear, and a maximum number of 25 was estimated
12.
World War II
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World War II, also known as the Second World War, was a global war that lasted from 1939 to 1945, although related conflicts began earlier. It involved the vast majority of the worlds countries—including all of the great powers—eventually forming two opposing alliances, the Allies and the Axis. It was the most widespread war in history, and directly involved more than 100 million people from over 30 countries. Marked by mass deaths of civilians, including the Holocaust and the bombing of industrial and population centres. These made World War II the deadliest conflict in human history, from late 1939 to early 1941, in a series of campaigns and treaties, Germany conquered or controlled much of continental Europe, and formed the Axis alliance with Italy and Japan. Under the Molotov–Ribbentrop Pact of August 1939, Germany and the Soviet Union partitioned and annexed territories of their European neighbours, Poland, Finland, Romania and the Baltic states. In December 1941, Japan attacked the United States and European colonies in the Pacific Ocean, and quickly conquered much of the Western Pacific. The Axis advance halted in 1942 when Japan lost the critical Battle of Midway, near Hawaii, in 1944, the Western Allies invaded German-occupied France, while the Soviet Union regained all of its territorial losses and invaded Germany and its allies. During 1944 and 1945 the Japanese suffered major reverses in mainland Asia in South Central China and Burma, while the Allies crippled the Japanese Navy, thus ended the war in Asia, cementing the total victory of the Allies. World War II altered the political alignment and social structure of the world, the United Nations was established to foster international co-operation and prevent future conflicts. The victorious great powers—the United States, the Soviet Union, China, the United Kingdom, the Soviet Union and the United States emerged as rival superpowers, setting the stage for the Cold War, which lasted for the next 46 years. Meanwhile, the influence of European great powers waned, while the decolonisation of Asia, most countries whose industries had been damaged moved towards economic recovery. Political integration, especially in Europe, emerged as an effort to end pre-war enmities, the start of the war in Europe is generally held to be 1 September 1939, beginning with the German invasion of Poland, Britain and France declared war on Germany two days later. The dates for the beginning of war in the Pacific include the start of the Second Sino-Japanese War on 7 July 1937, or even the Japanese invasion of Manchuria on 19 September 1931. Others follow the British historian A. J. P. Taylor, who held that the Sino-Japanese War and war in Europe and its colonies occurred simultaneously and this article uses the conventional dating. Other starting dates sometimes used for World War II include the Italian invasion of Abyssinia on 3 October 1935. The British historian Antony Beevor views the beginning of World War II as the Battles of Khalkhin Gol fought between Japan and the forces of Mongolia and the Soviet Union from May to September 1939, the exact date of the wars end is also not universally agreed upon. It was generally accepted at the time that the war ended with the armistice of 14 August 1945, rather than the formal surrender of Japan
13.
Chemiluminescence
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Chemiluminescence is the emission of light, as the result of a chemical reaction. There may also be limited emission of heat, the decay of this excited state to a lower energy level causes light emission. In theory, one photon of light should be given off for each molecule of reactant and this is equivalent to Avogadros number of photons per mole of reactant. In actual practice, non-enzymatic reactions seldom exceed 1% QC, quantum efficiency, in a chemical reaction, reactants collide to form a transition state, the enthalpic maximum in a reaction coordinate diagram, which proceeds to the product. Normally, reactants form products of chemical energy. The difference in energy between reactants and products, represented as Δ H r x n, is turned into heat, since vibrational energy is generally much greater than the thermal agitation, it rapidly disperses in the solvent through molecular rotation. This is how exothermic reactions make their solutions hotter, in a chemiluminescent reaction, the direct product of the reaction is an excited electronic state. Chemiluminescence differs from fluorescence or phosphorescence in that the excited state is the product of a chemical reaction rather than of the absorption of a photon. It is the antithesis of a reaction, in which light is used to drive an endothermic chemical reaction. Here, light is generated from an exothermic reaction. A standard example of chemiluminescence in the setting is the luminol test. Here, blood is indicated by luminescence upon contact with iron in hemoglobin, when chemiluminescence takes place in living organisms, the phenomenon is called bioluminescence. A light stick emits light by chemiluminescence, luminol in an alkaline solution with hydrogen peroxide in the presence of iron or copper, or an auxiliary oxidant, produces chemiluminescence. This is a reaction of phosphorus vapor, above the solid, with oxygen producing the excited states 2. Another gas phase reaction is the basis of nitric oxide detection in commercial analytic instruments applied to environmental air-quality testing, ozone is combined with nitric oxide to form nitrogen dioxide in an activated state. NO+O3 → NO2+ O2 The activated NO2 luminesces broadband visible to infrared light as it reverts to an energy state. A photomultiplier and associated electronics counts the photons that are proportional to the amount of NO present, the ozone reaction produces a photon count proportional to NO that is proportional to NO2 before it was converted to NO. In the case of a sample that contains both NO and NO2, the above reaction yields the amount of NO and NO2 combined in the air sample
14.
Incandescence
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Incandescence is the emission of electromagnetic radiation from a hot body as a result of its temperature. The term derives from the Latin verb incandescere, to glow white, incandescence is a special case of thermal radiation. Incandescence usually refers specifically to visible light, while thermal radiation refers also to infrared or any electromagnetic radiation. For a detailed discussion of the intensity and spectrum of incandescence, see the article and this limit is called the Draper point. The incandescence does not vanish below that temperature, but it is too weak in the spectrum to be perceivable. At higher temperatures, the substance becomes brighter and its changes from red towards white. Incandescence is exploited in incandescent light bulbs, in which a filament is heated to a temperature at which a fraction of the falls in the visible spectrum. The majority of the radiation however, is emitted in the part of the spectrum. If the filament could be hotter, efficiency would increase, however. More efficient light sources, such as fluorescent lamps and LEDs, sunlight is the incandescence of the white hot surface of the sun. The word incandescent is also used figuratively to describe a person who is so angry that they are imagined to glow or burn red hot or white hot, red heat List of light sources Figurative use, Rangers 1 Unirea-Urziceni 4 etc
15.
Dislocation
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In materials science, a dislocation is a crystallographic defect or irregularity within a crystal structure. The presence of dislocations strongly influences many of the properties of materials, some types of dislocations can be visualized as being caused by the termination of a plane of atoms in the middle of a crystal. In such a case, the planes are not straight. The two primary types of dislocations are edge dislocations and screw dislocations, mixed dislocations are intermediate between these. Mathematically, dislocations are a type of defect, sometimes called a soliton. Dislocations behave as stable particles, they can move around, two dislocations of opposite orientation can cancel when brought together, but a single dislocation typically cannot disappear on its own. Two main types of dislocations exist, edge and screw, dislocations found in real materials are typically mixed, meaning that they have characteristics of both. A crystalline material consists of an array of atoms, arranged into lattice planes. One approach is to begin by considering a 3D representation of a crystal lattice. The viewer may then start to simplify the representation by visualising planes of atoms instead of the atoms themselves, an edge dislocation is a defect where an extra half-plane of atoms is introduced mid way through the crystal, distorting nearby planes of atoms. When enough force is applied from one side of the crystal structure, a simple schematic diagram of such atomic planes can be used to illustrate lattice defects such as dislocations. In an edge dislocation, the Burgers vector is perpendicular to the line direction, the stresses caused by an edge dislocation are complex due to its inherent asymmetry. A screw dislocation is much harder to visualize, imagine cutting a crystal along a plane and slipping one half across the other by a lattice vector, the halves fitting back together without leaving a defect. This is similar to the Riemann surface of the complex logarithm, if the cut only goes part way through the crystal, and then slipped, the boundary of the cut is a screw dislocation. It comprises a structure in which a path is traced around the linear defect by the atomic planes in the crystal lattice. Perhaps the closest analogy is a spiral-sliced ham, in pure screw dislocations, the Burgers vector is parallel to the line direction. Despite the difficulty in visualization, the caused by a screw dislocation are less complex than those of an edge dislocation. This equation suggests a long cylinder of stress radiating outward from the cylinder, please note, this simple model results in an infinite value for the core of the dislocation at r=0 and so it is only valid for stresses outside of the core of the dislocation
16.
Crystal defect
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Crystalline solids exhibit a periodic crystal structure. The positions of atoms or molecules occur on repeating fixed distances, however, the arrangement of atoms or molecules in most crystalline materials is not perfect. The regular patterns are interrupted by crystallographic defects, point defects are defects that occur only at or around a single lattice point. They are not extended in space in any dimension, strict limits for how small a point defect is are generally not defined explicitly. However, these defects typically involve at most a few extra or missing atoms, larger defects in an ordered structure are usually considered dislocation loops. For historical reasons, many point defects, especially in ionic crystals, are called centers, for example a vacancy in many ionic solids is called a luminescence center and these dislocations permit ionic transport through crystals leading to electrochemical reactions. These are frequently specified using Kröger–Vink Notation, vacancy defects are lattice sites which would be occupied in a perfect crystal, but are vacant. If a neighboring atom moves to occupy the vacant site, the moves in the opposite direction to the site which used to be occupied by the moving atom. The stability of the crystal structure guarantees that the neighboring atoms will not simply collapse around the vacancy. In some materials, neighboring atoms actually move away from a vacancy, a vacancy is sometimes called a Schottky defect. Interstitial defects are atoms that occupy a site in the structure at which there is usually not an atom. They are generally high energy configurations, small atoms in some crystals can occupy interstices without high energy, such as hydrogen in palladium. A nearby pair of a vacancy and an interstitial is often called a Frenkel defect or Frenkel pair and this is caused when an ion moves into an interstitial site and creates a vacancy. Due to fundamental limitations of material purification methods, materials are never 100% pure, in the case of an impurity, the atom is often incorporated at a regular atomic site in the crystal structure. This is neither a vacant site nor is the atom on an interstitial site, the atom is not supposed to be anywhere in the crystal, and is thus an impurity. In some cases where the radius of the atom is substantially smaller than that of the atom it is replacing. These types of defects are often referred to as off-center ions. There are two different types of defects, Isovalent substitution and aliovalent substitution
17.
Electronic band structure
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In solid-state physics, the electronic band structure of a solid describes the range of energies that an electron within the solid may have and ranges of energy that it may not have. Band theory derives these bands and band gaps by examining the allowed quantum mechanical wave functions for an electron in a large, the electrons of a single, isolated atom occupy atomic orbitals each of which has a discrete energy level. When two atoms together to form into a molecule, their atomic orbitals overlap. The Pauli exclusion principle dictates that no two electrons can have the quantum numbers in a molecule. Similarly if a large number N of identical atoms come together to form a solid, such as a crystal lattice, the atoms atomic orbitals overlap. Since the number of atoms in a piece of solid is a very large number the number of orbitals is very large. The energy of adjacent levels is so close together that they can be considered as a continuum and this formation of bands is mostly a feature of the outermost electrons in the atom, which are the ones responsible for chemical bonding and electrical conductivity. The inner electron orbitals do not overlap to a significant degree, Band gaps are essentially leftover ranges of energy not covered by any band, a result of the finite widths of the energy bands. The bands have different widths, with the widths depending upon the degree of overlap in the atomic orbitals from which they arise, two adjacent bands may simply not be wide enough to fully cover the range of energy. For example, the associated with core orbitals are extremely narrow due to the small overlap between adjacent atoms. As a result, there tend to be large band gaps between the core bands, higher bands involve comparatively larger orbitals with more overlap, becoming progressively wider at higher energies so that there are no band gaps at higher energies. Band theory is only an approximation to the state of a solid. These are the necessary for band theory to be valid, Infinite-size system, For the bands to be continuous. Since a macroscopic piece of material contains on the order of 1022 atoms, with modifications, the concept of band structure can also be extended to systems which are only large along some dimensions, such as two-dimensional electron systems. Homogeneous system, Band structure is a property of a material. Practically, this means that the makeup of the material must be uniform throughout the piece. Non-interactivity, The band structure describes single electron states, the existence of these states assumes that the electrons travel in a static potential without dynamically interacting with lattice vibrations, other electrons, photons, etc. Not only are there local small-scale disruptions, but also local charge imbalances and these charge imbalances have electrostatic effects that extend deeply into semiconductors, insulators, and the vacuum
18.
Crystal
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A crystal or crystalline solid is a solid material whose constituents are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. In addition, macroscopic single crystals are usually identifiable by their geometrical shape, the scientific study of crystals and crystal formation is known as crystallography. The process of crystal formation via mechanisms of crystal growth is called crystallization or solidification, the word crystal derives from the Ancient Greek word κρύσταλλος, meaning both ice and rock crystal, from κρύος, icy cold, frost. Examples of large crystals include snowflakes, diamonds, and table salt, most inorganic solids are not crystals but polycrystals, i. e. many microscopic crystals fused together into a single solid. Examples of polycrystals include most metals, rocks, ceramics, a third category of solids is amorphous solids, where the atoms have no periodic structure whatsoever. Examples of amorphous solids include glass, wax, and many plastics, Crystals are often used in pseudoscientific practices such as crystal therapy, and, along with gemstones, are sometimes associated with spellwork in Wiccan beliefs and related religious movements. The scientific definition of a crystal is based on the arrangement of atoms inside it. A crystal is a solid where the form a periodic arrangement. For example, when liquid water starts freezing, the change begins with small ice crystals that grow until they fuse. Most macroscopic inorganic solids are polycrystalline, including almost all metals, ceramics, ice, rocks, solids that are neither crystalline nor polycrystalline, such as glass, are called amorphous solids, also called glassy, vitreous, or noncrystalline. These have no periodic order, even microscopically, there are distinct differences between crystalline solids and amorphous solids, most notably, the process of forming a glass does not release the latent heat of fusion, but forming a crystal does. A crystal structure is characterized by its cell, a small imaginary box containing one or more atoms in a specific spatial arrangement. The unit cells are stacked in three-dimensional space to form the crystal, the symmetry of a crystal is constrained by the requirement that the unit cells stack perfectly with no gaps. There are 219 possible crystal symmetries, called space groups. These are grouped into 7 crystal systems, such as cubic crystal system or hexagonal crystal system, Crystals are commonly recognized by their shape, consisting of flat faces with sharp angles. Euhedral crystals are those with obvious, well-formed flat faces, anhedral crystals do not, usually because the crystal is one grain in a polycrystalline solid. The flat faces of a crystal are oriented in a specific way relative to the underlying atomic arrangement of the crystal. This occurs because some surface orientations are more stable than others, as a crystal grows, new atoms attach easily to the rougher and less stable parts of the surface, but less easily to the flat, stable surfaces
19.
Valence band
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In solid-state physics, the valence band and conduction band are the bands closest to the Fermi level and thus determine the electrical conductivity of the solid. On a graph of the band structure of a material. This distinction is meaningless in metals as the highest band is partially filled, in semiconductors and insulators the two bands are separated by a band gap, while in semimetals the bands overlap. A band gap is a range in a solid where no electron states can exist due to the quantization of energy. Electrical conductivity of non-metals is determined by the susceptibility of electrons to excitation from the band to the conduction band. In solids, the ability of electrons to act as charge carriers depends on the availability of vacant electronic states and this allows the electrons to increase their energy when an electric field is applied. This condition is satisfied in the conduction band, since the valence band is full in non-metals. As such, the conductivity of a solid depends on its capability to flow electrons from the valence to the conduction band. Hence, in the case of a semimetal with an overlap region, if there is a small band gap, then the flow of electrons from valence to conduction band is possible only if an external energy is supplied, these groups with small Eg are called semiconductors. If the Eg is sufficiently high, then the flow of electrons from valence to conduction band becomes negligible under normal conditions, there is some conductivity in semiconductors, however. This is due to thermal excitation—some of the electrons get enough energy to jump the gap in one go. Once they are in the band, they can conduct electricity. The hole is an empty state that allows electrons in the band some degree of freedom. Electrical conduction for more information about conduction in solids, and another description of band structure, electronic band structure Fermi sea Semiconductor for a full explanation of the band structure of materials
20.
Conduction band
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In solid-state physics, the valence band and conduction band are the bands closest to the Fermi level and thus determine the electrical conductivity of the solid. On a graph of the band structure of a material. This distinction is meaningless in metals as the highest band is partially filled, in semiconductors and insulators the two bands are separated by a band gap, while in semimetals the bands overlap. A band gap is a range in a solid where no electron states can exist due to the quantization of energy. Electrical conductivity of non-metals is determined by the susceptibility of electrons to excitation from the band to the conduction band. In solids, the ability of electrons to act as charge carriers depends on the availability of vacant electronic states and this allows the electrons to increase their energy when an electric field is applied. This condition is satisfied in the conduction band, since the valence band is full in non-metals. As such, the conductivity of a solid depends on its capability to flow electrons from the valence to the conduction band. Hence, in the case of a semimetal with an overlap region, if there is a small band gap, then the flow of electrons from valence to conduction band is possible only if an external energy is supplied, these groups with small Eg are called semiconductors. If the Eg is sufficiently high, then the flow of electrons from valence to conduction band becomes negligible under normal conditions, there is some conductivity in semiconductors, however. This is due to thermal excitation—some of the electrons get enough energy to jump the gap in one go. Once they are in the band, they can conduct electricity. The hole is an empty state that allows electrons in the band some degree of freedom. Electrical conduction for more information about conduction in solids, and another description of band structure, electronic band structure Fermi sea Semiconductor for a full explanation of the band structure of materials
21.
Exciton
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An exciton is a bound state of an electron and an electron hole which are attracted to each other by the electrostatic Coulomb force. It is an electrically neutral quasiparticle that exists in insulators, semiconductors, the exciton is regarded as an elementary excitation of condensed matter that can transport energy without transporting net electric charge. An exciton can form when a photon is absorbed by a semiconductor and this excites an electron from the valence band into the conduction band. In turn, this leaves behind a positively charged electron hole, the electron in the conduction band is then effectively attracted to this localized hole by the repulsive Coulomb forces from large numbers of electrons surrounding the hole and excited electron. This attraction provides an energy balance. Consequently, the exciton has slightly less energy than the unbound electron, the wavefunction of the bound state is said to be hydrogenic, an exotic atom state akin to that of a hydrogen atom. However, the energy is much smaller and the particles size much larger than a hydrogen atom. This is because of both the screening of the Coulomb force by other electrons in the semiconductor, and the effective masses of the excited electron. The electron and hole may have parallel or anti-parallel spins. The spins are coupled by the interaction, giving rise to exciton fine structure. In periodic lattices, the properties of an exciton show momentum dependence, the concept of excitons was first proposed by Yakov Frenkel in 1931, when he described the excitation of atoms in a lattice of insulators. He proposed that this state would be able to travel in a particle-like fashion through the lattice without the net transfer of charge. Excitons may be treated in two limiting cases, depending on the properties of the material in question, molecular excitons may even be entirely located on the same molecule, as in fullerenes. This Frenkel exciton, named after Yakov Frenkel, has a binding energy on the order of 0.1 to 1 eV. Frenkel excitons are typically found in alkali halide crystals and in molecular crystals composed of aromatic molecules. In semiconductors, the constant is generally large. Consequently, electric field screening tends to reduce the Coulomb interaction between electrons and holes, the result is a Wannier exciton, which has a radius larger than the lattice spacing. Small effective mass of electrons that is typical of semiconductors also favors large exciton radii, as a result, the effect of the lattice potential can be incorporated into the effective masses of the electron and hole
22.
Energy gap
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In solid-state physics, a band gap, also called an energy gap or bandgap, is an energy range in a solid where no electron states can exist. It is closely related to the HOMO/LUMO gap in chemistry, therefore, the band gap is a major factor determining the electrical conductivity of a solid. Every solid has its own characteristic energy-band structure and this variation in band structure is responsible for the wide range of electrical characteristics observed in various materials. In semiconductors and insulators, electrons are confined to a number of bands of energy, the term band gap refers to the energy difference between the top of the valence band and the bottom of the conduction band. Electrons are able to jump from one band to another, however, in order for an electron to jump from a valence band to a conduction band, it requires a specific minimum amount of energy for the transition. The required energy differs with different materials, electrons can gain enough energy to jump to the conduction band by absorbing either a phonon or a photon. In contrast, a material with a band gap is an insulator. In conductors, the valence and conduction bands may overlap, so they may not have a band gap, the conductivity of intrinsic semiconductors is strongly dependent on the band gap. Band-gap engineering is the process of controlling or altering the band gap of a material by controlling the composition of certain semiconductor alloys, such as GaAlAs, InGaAs and it is also possible to construct layered materials with alternating compositions by techniques like molecular-beam epitaxy. These methods are exploited in the design of heterojunction bipolar transistors, laser diodes, the distinction between semiconductors and insulators is a matter of convention. One approach is to think of semiconductors as a type of insulator with a band gap. Insulators with a band gap, usually greater than 4 eV, are not considered semiconductors. Electron mobility also plays a role in determining a materials informal classification, the band-gap energy of semiconductors tends to decrease with increasing temperature. When temperature increases, the amplitude of atomic vibrations increase, leading to larger interatomic spacing, the interaction between the lattice phonons and the free electrons and holes will also affect the band gap to a smaller extent. The relationship between band gap energy and temperature can be described by Varshnis empirical expression, E g = E g − α T2 T + β, in a regular semiconductor crystal, the band gap is fixed owing to continuous energy states. In a quantum dot crystal, the gap is size dependent. It is also known as quantum confinement effect, Band gaps also depend on pressure. Band gaps can be direct or indirect, depending on the electronic band structure
23.
Electron hole
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In physics, chemistry, and electronic engineering, an electron hole is the lack of an electron at a position where one could exist in an atom or atomic lattice. Holes in a crystal lattice can move through the lattice as electrons can. They play an important role in the operation of devices such as transistors, diodes. However they are not actually particles, but rather quasiparticles, they are different from the positron, if an electron is excited into a higher state it leaves a hole in its old state. This meaning is used in Auger electron spectroscopy, in computational chemistry, in crystals, electronic band structure calculations lead to an effective mass for the electrons, which typically is negative at the top of a band. The negative mass is a concept, and in these situations a more familiar picture is found by considering a positive charge with a positive mass. In solid-state physics, a hole is the absence of an electron from a full valence band. A hole is essentially a way to conceptualize the interactions of the electrons within a full system. In some ways, the behavior of a hole within a crystal lattice is comparable to that of the bubble in a full bottle of water. Hole conduction in a band can be explained by the following analogy. Imagine a row of people seated in an auditorium, where there are no spare chairs, someone in the middle of the row wants to leave, so he jumps over the back of the seat into an empty row, and walks out. The empty row is analogous to the band, and the person walking out is analogous to a free electron. Now imagine someone else comes along and wants to sit down, the empty row has a poor view, so he does not want to sit there. Instead, a person in the crowded row moves into the empty seat the first person left behind, the empty seat moves one spot closer to the edge and the person waiting to sit down. The next person follows, and the next, et cetera, one could say that the empty seat moves towards the edge of the row. Once the empty seat reaches the edge, the new person can sit down, in the process everyone in the row has moved along. If those people were charged, this movement would constitute conduction. If the seats themselves were positively charged, then only the vacant seat would be positive and this is a very simple model of how hole conduction works
24.
Forbidden gap
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In solid-state physics, a band gap, also called an energy gap or bandgap, is an energy range in a solid where no electron states can exist. It is closely related to the HOMO/LUMO gap in chemistry, therefore, the band gap is a major factor determining the electrical conductivity of a solid. Every solid has its own characteristic energy-band structure and this variation in band structure is responsible for the wide range of electrical characteristics observed in various materials. In semiconductors and insulators, electrons are confined to a number of bands of energy, the term band gap refers to the energy difference between the top of the valence band and the bottom of the conduction band. Electrons are able to jump from one band to another, however, in order for an electron to jump from a valence band to a conduction band, it requires a specific minimum amount of energy for the transition. The required energy differs with different materials, electrons can gain enough energy to jump to the conduction band by absorbing either a phonon or a photon. In contrast, a material with a band gap is an insulator. In conductors, the valence and conduction bands may overlap, so they may not have a band gap, the conductivity of intrinsic semiconductors is strongly dependent on the band gap. Band-gap engineering is the process of controlling or altering the band gap of a material by controlling the composition of certain semiconductor alloys, such as GaAlAs, InGaAs and it is also possible to construct layered materials with alternating compositions by techniques like molecular-beam epitaxy. These methods are exploited in the design of heterojunction bipolar transistors, laser diodes, the distinction between semiconductors and insulators is a matter of convention. One approach is to think of semiconductors as a type of insulator with a band gap. Insulators with a band gap, usually greater than 4 eV, are not considered semiconductors. Electron mobility also plays a role in determining a materials informal classification, the band-gap energy of semiconductors tends to decrease with increasing temperature. When temperature increases, the amplitude of atomic vibrations increase, leading to larger interatomic spacing, the interaction between the lattice phonons and the free electrons and holes will also affect the band gap to a smaller extent. The relationship between band gap energy and temperature can be described by Varshnis empirical expression, E g = E g − α T2 T + β, in a regular semiconductor crystal, the band gap is fixed owing to continuous energy states. In a quantum dot crystal, the gap is size dependent. It is also known as quantum confinement effect, Band gaps also depend on pressure. Band gaps can be direct or indirect, depending on the electronic band structure
25.
Crystal lattice
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In crystallography, crystal structure is a description of the ordered arrangement of atoms, ions or molecules in a crystalline material. Ordered structures occur from the nature of the constituent particles to form symmetric patterns that repeat along the principal directions of three-dimensional space in matter. The smallest group of particles in the material that constitutes the pattern is the unit cell of the structure. The unit cell completely defines the symmetry and structure of the crystal lattice. The repeating patterns are said to be located at the points of the Bravais lattice, the lengths of the principal axes, or edges, of the unit cell and the angles between them are the lattice constants, also called lattice parameters. The symmetry properties of the crystal are described by the concept of space groups, all possible symmetric arrangements of particles in three-dimensional space may be described by the 230 space groups. The crystal structure and symmetry play a role in determining many physical properties, such as cleavage, electronic band structure. The crystal structure of a material can be described in terms of its unit cell, the unit cell is a box containing one or more atoms arranged in three dimensions. The unit cells stacked in three-dimensional space describe the arrangement of atoms of the crystal. Commonly, atomic positions are represented in terms of fractional coordinates, the atom positions within the unit cell can be calculated through application of symmetry operations to the asymmetric unit. The asymmetric unit refers to the smallest possible occupation of space within the unit cell and this does not, however imply that the entirety of the asymmetric unit must lie within the boundaries of the unit cell. Symmetric transformations of atom positions are calculated from the group of the crystal structure. Vectors and planes in a lattice are described by the three-value Miller index notation. It uses the indices ℓ, m, and n as directional parameters, which are separated by 90°, by definition, the syntax denotes a plane that intercepts the three points a1/ℓ, a2/m, and a3/n, or some multiple thereof. That is, the Miller indices are proportional to the inverses of the intercepts of the plane with the unit cell, if one or more of the indices is zero, it means that the planes do not intersect that axis. A plane containing a coordinate axis is translated so that it no longer contains that axis before its Miller indices are determined, the Miller indices for a plane are integers with no common factors. Negative indices are indicated with horizontal bars, as in, in an orthogonal coordinate system for a cubic cell, the Miller indices of a plane are the Cartesian components of a vector normal to the plane. Likewise, the planes are geometric planes linking nodes
26.
Scintillator
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A scintillator is a material that exhibits scintillation — the property of luminescence, when excited by ionizing radiation. Luminescent materials, when struck by a particle, absorb its energy. A scintillation detector or scintillation counter is obtained when a scintillator is coupled to a light sensor such as a photomultiplier tube, photodiode. PMTs absorb the light emitted by the scintillator and re-emit it in the form of electrons via the photoelectric effect, the subsequent multiplication of those electrons results in an electrical pulse which can then be analyzed and yield meaningful information about the particle that originally struck the scintillator. Vacuum photodiodes are similar but do not amplify the signal while silicon photodiodes, on the other hand, the first device which used a scintillator was built in 1903 by Sir William Crookes and used a ZnS screen. The scintillations produced by the screen were visible to the eye if viewed by a microscope in a darkened room. The technique led to a number of important discoveries but was obviously tedious, scintillators gained additional attention in 1944, when Curran and Baker replaced the naked eye measurement with the newly developed PMT. This was the birth of the scintillation detector. Scintillators are used by the American government as Homeland Security radiation detectors, scintillators can also be used in particle detectors, new energy resource exploration, X-ray security, nuclear cameras, computed tomography and gas exploration. Other applications of scintillators include CT scanners and gamma cameras in medical diagnostics, scintillators generate light in fluorescent tubes, to convert the ultra-violet of the discharge into visible light. Scintillation detectors are used in the petroleum industry as detectors for Gamma Ray logs. There are many desired properties of scintillators, such as density, fast operation speed, low cost, radiation hardness. High density reduces the size of showers for high-energy γ-quanta. The range of Compton scattered photons for lower energy γ-rays is also decreased via high density materials and this results in high segmentation of the detector and leads to better spatial resolution. Usually high density materials have heavy ions in the lattice, significantly increasing the photo-fraction, the increased photo-fraction is important for some applications such as positron emission tomography. High stopping power for electromagnetic component of the ionizing radiation needs greater photo-fraction, high operating speed is needed for good resolution of spectra. Precision of time measurement with a detector is proportional to √τsc. Short decay times are important for the measurement of time intervals, high density and fast response time can allow detection of rare events in particle physics
27.
Near ultraviolet
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Ultraviolet is an electromagnetic radiation with a wavelength from 10 nm to 400 nm, shorter than that of visible light but longer than X-rays. UV radiation constitutes about 10% of the light output of the Sun. It is also produced by electric arcs and specialized lights, such as lamps, tanning lamps. Consequently, the effects of UV are greater than simple heating effects. Suntan, freckling and sunburn are familiar effects of over-exposure, along with risk of skin cancer. Living things on dry land would be damaged by ultraviolet radiation from the Sun if most of it were not filtered out by the Earths atmosphere. More-energetic, shorter-wavelength extreme UV below 121 nm ionizes air so strongly that it is absorbed before it reaches the ground, Ultraviolet is also responsible for the formation of bone-strengthening vitamin D in most land vertebrates, including humans. The UV spectrum thus has both beneficial and harmful to human health. Ultraviolet rays are invisible to most humans, the lens in a human eye ordinarily filters out UVB frequencies or higher, and humans lack color receptor adaptations for ultraviolet rays. Under some conditions, children and young adults can see ultraviolet down to wavelengths of about 310 nm, near-UV radiation is visible to some insects, mammals, and birds. Small birds have a fourth color receptor for ultraviolet rays, this gives birds true UV vision, reindeer use near-UV radiation to see polar bears, who are poorly visible in regular light because they blend in with the snow. UV also allows mammals to see urine trails, which is helpful for animals to find food in the wild. The males and females of some species look identical to the human eye. Ultraviolet means beyond violet, violet being the color of the highest frequencies of visible light, Ultraviolet has a higher frequency than violet light. He called them oxidizing rays to emphasize chemical reactivity and to them from heat rays. The terms chemical and heat rays were eventually dropped in favour of ultraviolet and infrared radiation, in 1878 the effect of short-wavelength light on sterilizing bacteria was discovered. By 1903 it was known the most effective wavelengths were around 250 nm, in 1960, the effect of ultraviolet radiation on DNA was established. The discovery of the ultraviolet radiation below 200 nm, named vacuum ultraviolet because it is absorbed by air, was made in 1893 by the German physicist Victor Schumann
28.
Photomultiplier
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Elements of photomultiplier technology, when integrated differently, are the basis of night vision devices. Research that analyzes light scattering, such as the study of polymers in solution use a laser. Photomultipliers are typically constructed with an evacuated glass housing, containing a photocathode, several dynodes, incident photons strike the photocathode material, which is usually a thin vapor-deposited conducting layer on the inside of the entry window of the device. Electrons are ejected from the surface as a consequence of the photoelectric effect and these electrons are directed by the focusing electrode toward the electron multiplier, where electrons are multiplied by the process of secondary emission. The electron multiplier consists of a number of electrodes called dynodes, each dynode is held at a more positive potential, by ≈100 Volts, than the preceding one. A primary electron leaves the photocathode with the energy of the photon, or about 3 eV for blue photons. A small group of primary electrons is created by the arrival of a group of initial photons, the primary electrons move toward the first dynode because they are accelerated by the electric field. They each arrive with ≈100 eV kinetic energy imparted by the potential difference, upon striking the first dynode, more low energy electrons are emitted, and these electrons are in turn accelerated toward the second dynode. The geometry of the chain is such that a cascade occurs with an exponentially-increasing number of electrons being produced at each stage. This last stage is called the anode, the necessary distribution of voltage along the series of dynodes is created by a voltage divider chain, as illustrated in the Figure. In the example, the photocathode is held at a high voltage of order 1000V. The capacitors across the final few dynodes act as reservoirs of charge to help maintain the voltage on the dynodes while electron avalanches propagate through the tube. Many variations of design are used in practice, the design shown is merely illustrative, the side-on design is used, for instance, in the type 931, the first mass-produced PMT. Besides the different photocathode materials, performance is affected by the transmission of the window material that the light passes through. A large number of models are available having various combinations of these. Either of the manuals mentioned will provide the information needed to choose a design for a particular application. The invention of the photomultiplier is predicated upon two prior achievements, the discoveries of the photoelectric effect and of secondary emission. The first demonstration of the effect was carried out in 1887 by Heinrich Hertz using ultraviolet light
29.
Metastable state
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In physics, metastability denotes the phenomenon when a dynamical system spends an extended time in a configuration other than the systems state of least energy. During a metastable state of finite lifetime, all state-describing parameters reach, a conceptual analogy may be drawn with a ball resting in a hollow on a slope. Small perturbations will result in the ball settling back into its current hollow, isomerisation is another common example of metastability. Higher energy isomers are long lived as they are prevented from rearranging to their ground state by barriers in the potential energy. The metastability concept originated in the physics of first-order phase transitions and it then acquired new meaning in the study of aggregated subatomic particles or in molecules, macromolecules or clusters of atoms and molecules. Later, it was borrowed for the study of decision-making and information transmission systems, many complex natural and man-made systems can demonstrate metastability. Metastability is common in physics and chemistry – from an atom to statistical ensembles of molecules at molecular levels or as a whole, the abundance of states is more prevalent as the systems grow larger and/or if the forces of their mutual interaction are spatially less uniform or more diverse. In these systems, the equivalent of thermal fluctuations in molecular systems is the noise that affects signal propagation. Non-equilibrium thermodynamics is a branch of physics that studies the dynamics of statistical ensembles of molecules via unstable states, being stuck in a thermodynamic trough without being at the lowest energy state is known as having kinetic stability or being kinetically persistent. The particular motion or kinetics of the atoms involved has resulted in getting stuck, metastable states of matter range from melting solids, boiling liquids and sublimating solids to supercooled liquids or superheated liquid-gas mixtures. Extremely pure, supercooled water stays liquid below 0 °C and remains so until applied vibrations or condensing seed doping initiates crystallization centers and this is a common situation for the droplets of atmospheric clouds. Metastable phases are common in condensed matter, for example, diamond is a metastable form of carbon at standard temperature and pressure. It can be converted to graphite, but only after overcoming an activation energy – an intervening hill, martensite is a metastable phase used to control the hardness of most steel. The bonds between the blocks of polymers such as DNA, RNA and proteins are also metastable. Metastable polymorphs of silica are commonly observed, in some cases, such as in the allotropes of solid boron, acquiring a sample of the stable phase is difficult. Generally speaking, emulsions/colloidal systems and glasses are metastable, sandpiles are one system which can exhibit metastability if a steep slope or tunnel is present. Sand grains form a pile due to friction and it is possible for an entire large sand pile to reach a point where it is stable, but the addition of a single grain causes large parts of it to collapse. The avalanche is a problem with large piles of snow
30.
Valence (chemistry)
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In chemistry, the valence or valency of an element is a measure of its combining power with other atoms when it forms chemical compounds or molecules. The concept of valence was developed in the half of the 19th century and was successful in explaining the molecular structure of inorganic and organic compounds. The combining power or affinity of an atom of an element was determined by the number of atoms that it combined with. In methane, carbon has a valence of 4, in ammonia, nitrogen has a valence of 3, in water, oxygen has a valence of 2, and in hydrogen chloride, chlorine has a valence of 1. Chlorine, as it has a valence of one, can be substituted for hydrogen, so phosphorus has a valence of 5 in phosphorus pentachloride, PCl5. Valence diagrams of a compound represent the connectivity of the elements, examples are, Valence only describes connectivity, it does not describe the geometry of molecular compounds, or what are now known to be ionic compounds or giant covalent structures. A line between atoms does not represent a pair of electrons as it does in Lewis diagrams and this definition differs from the IUPAC definition as an element can be said to have more than one valence. It is in manner, according to Frankland, that their affinities are best satisfied. Following these examples and postulates, Frankland declares how obvious it is that This “combining power” was afterwards called quantivalence or valency. In 1857 August Kekulé proposed fixed valences for many elements, such as 4 for carbon, and used them to propose structural formulas for many organic molecules, which are still accepted today. Most 19th-century chemists defined the valence of an element as the number of its bonds without distinguishing different types of valence or of bond. For main-group elements, in 1904 Richard Abegg considered positive and negative valences, in 1916, Gilbert N. Lewis explained valence and chemical bonding in terms of a tendency of atoms to achieve a stable octet of 8 valence-shell electrons. According to Lewis, covalent bonding leads to octets by the sharing of electrons, the term covalence is attributed to Irving Langmuir, who stated in 1919 that the number of pairs of electrons which any given atom shares with the adjacent atoms is called the covalence of that atom. The prefix co- means together, so that a co-valent bond means that the share a valence. Pauling also considered hypervalent molecules, in which main-group elements have apparent valences greater than the maximal of 4 allowed by the octet rule. Similar calculations on transition-metal molecules show that the role of p orbitals is minor, so that one s, for elements in the main groups of the periodic table, the valence can vary between 1 and 7. Many elements have a common valence related to their position in the periodic table, the Latin/Greek prefixes uni-/mono-, bi-/di-, ter-/tri-, quadri-/tetra- and quinque-/penta- are used to describe ions in the charge states 1,2,3,4,5 respectively. Polyvalence or multivalence refers to species that are not restricted to a number of valence bonds
31.
Oxidation
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Redox is a chemical reaction in which the oxidation states of atoms are changed. Any such reaction involves both a process and a complementary oxidation process, two key concepts involved with electron transfer processes. Redox reactions include all chemical reactions in which atoms have their oxidation state changed, in general, the chemical species from which the electron is stripped is said to have been oxidized, while the chemical species to which the electron is added is said to have been reduced. It can be explained in terms, Oxidation is the loss of electrons or an increase in oxidation state by a molecule, atom. Reduction is the gain of electrons or a decrease in state by a molecule, atom. As an example, during the combustion of wood, oxygen from the air is reduced, the reaction can occur relatively slowly, as in the case of rust, or more quickly, as in the case of fire. Redox is a portmanteau of reduction and oxidation, the word oxidation originally implied reaction with oxygen to form an oxide, since dioxygen was historically the first recognized oxidizing agent. Later, the term was expanded to encompass oxygen-like substances that accomplished parallel chemical reactions, ultimately, the meaning was generalized to include all processes involving loss of electrons. The word reduction originally referred to the loss in weight upon heating a metallic ore such as an oxide to extract the metal. In other words, ore was reduced to metal, antoine Lavoisier showed that this loss of weight was due to the loss of oxygen as a gas. Later, scientists realized that the atom gains electrons in this process. The meaning of reduction then became generalized to all processes involving gain of electrons. Even though reduction seems counter-intuitive when speaking of the gain of electrons, it help to think of reduction as the loss of oxygen. Since electrons are charged, it is also helpful to think of this as reduction in electrical charge. The electrochemist John Bockris has used the words electronation and deelectronation to describe reduction and oxidation processes respectively when they occur at electrodes and these words are analogous to protonation and deprotonation, but they have not been widely adopted by chemists. The term hydrogenation could be used instead of reduction, since hydrogen is the agent in a large number of reactions. But, unlike oxidation, which has been generalized beyond its root element, the word redox was first used in 1928. The processes of oxidation and reduction occur simultaneously and cannot happen independently of one another, the oxidation alone and the reduction alone are each called a half-reaction, because two half-reactions always occur together to form a whole reaction
32.
Plasma display
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A plasma display panel is a type of flat panel display common to large TV displays 30 inches or larger. They are called plasma displays because they use small cells containing electrically charged ionized gases, Plasma displays are bright, have a wide color gamut, and can be produced in fairly large sizes—up to 3.8 metres diagonally. They had a very low-luminance dark-room black level compared with the grey of the unilluminated parts of an LCD screen at least in the early history of the competing technologies. LED-backlit LCD televisions have been developed to reduce this distinction, the display panel itself is about 6 cm thick, generally allowing the devices total thickness to be less than 10 cm. The plasma that illuminates the screen can reach a temperature of at least 1200 °C, typical power consumption is 400 watts for a 127 cm screen. Most screens are set to shop mode by default, which draws at least twice the power of a setting of less extreme brightness. The lifetime of the latest generation of displays is estimated at 100,000 hours of actual display time. This is the time over which maximum picture brightness degrades to half the original value. Plasma screens are made out of glass. This may causes glare from reflected objects in the viewing area, currently, plasma panels cannot be economically manufactured in screen sizes smaller than 82 centimetres. Although a few companies have been able to make plasma enhanced-definition televisions this small, with the trend toward large-screen television technology, the 32 inch screen size is rapidly disappearing. Competing display technologies include cathode ray tube, organic light-emitting diode, AMLCD, Digital Light Processing DLP, SED-tv, LED display, field emission display, and quantum dot display. Capable of producing deeper blacks allowing for superior contrast ratio Wider viewing angles than those of LCD, images do not suffer from degradation at less than straight ahead angles like LCDs. LCDs using IPS technology have the widest angles, but they do not equal the range of plasma primarily due to IPS glow, LCD panel backlights nearly always produce uneven brightness levels, although this is not always noticeable. High-end computer monitors have technologies to try to compensate for the uniformity problem, unaffected by clouding from the polishing process. Some LCD panel types, like IPS, require a process that can introduce a haze usually referred to as clouding. Less expensive for the buyer per square inch than LCD, particularly when equivalent performance is considered, earlier generation displays were more susceptible to screen burn-in and image retention. Recent models have a pixel orbiter that moves the entire picture slower than is noticeable to the human eye, earlier generation displays had phosphors that lost luminosity over time, resulting in gradual decline of absolute image brightness. Newer models have advertised lifespans exceeding 100000 hours, far longer than older CRTs Uses more electrical power, on average, older CCFL backlights for LCD panels used quite a bit more power, and older plasma TVs used quite a bit more power than recent models
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Diffusion
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Diffusion is the net movement of molecules or atoms from a region of high concentration to a region of low concentration. This is also referred to as the movement of a substance down a concentration gradient, a gradient is the change in the value of a quantity with the change in another variable. The word diffusion derives from the Latin word, diffundere, which means to spread out, a distinguishing feature of diffusion is that it results in mixing or mass transport, without requiring bulk motion. Thus, diffusion should not be confused with convection, or advection, an example of a situation in which bulk flow and diffusion can be differentiated is the mechanism by which oxygen enters the body during external respiration. The lungs are located in the cavity, which expands as the first step in external respiration. This expansion leads to an increase in volume of the alveoli in the lungs and this creates a pressure gradient between the air outside the body and the alveoli. The air moves down the gradient through the airways of the lungs and into the alveoli until the pressure of the air. The air arriving in the alveoli has a concentration of oxygen than the “stale” air in the alveoli. The increase in oxygen concentration creates a concentration gradient for oxygen between the air in the alveoli and the blood in the capillaries that surround the alveoli, oxygen then moves by diffusion, down the concentration gradient, into the blood. The other consequence of the air arriving in alveoli is that the concentration of carbon dioxide in the alveoli decreases and this creates a concentration gradient for carbon dioxide to diffuse from the blood into the alveoli. The pumping action of the heart then transports the blood around the body, as the left ventricle of the heart contracts, the volume decreases, which increases the pressure in the ventricle. This creates a gradient between the heart and the capillaries, and blood moves through blood vessels by bulk flow. The concept of diffusion is widely used in, physics, chemistry, biology, sociology, economics, however, in each case, the object that is undergoing diffusion is “spreading out” from a point or location at which there is a higher concentration of that object. In the phenomenological approach, diffusion is the movement of a substance from a region of high concentration to a region of low concentration without bulk motion, according to Ficks laws, the diffusion flux is proportional to the negative gradient of concentrations. It goes from regions of higher concentration to regions of lower concentration, some time later, various generalizations of Ficks laws were developed in the frame of thermodynamics and non-equilibrium thermodynamics. From the atomistic point of view, diffusion is considered as a result of the walk of the diffusing particles. In molecular diffusion, the molecules are self-propelled by thermal energy. Random walk of small particles in suspension in a fluid was discovered in 1827 by Robert Brown, the theory of the Brownian motion and the atomistic backgrounds of diffusion were developed by Albert Einstein
34.
Aluminium phosphate
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Aluminium phosphate is a chemical compound. The anhydrous form is found in nature as the mineral berlinite, many synthetic forms of anhydrous aluminium phosphate are known. They have framework structures similar to zeolites and some are used as catalysts or molecular sieves, the dihydrate, AlPO4·2H2O is found as the minerals variscite and meta-variscite. A synthetic hydrated form, AlPO4·1. 5H2O is also known, commercially, an aluminium phosphate gel is available. AlPO4 is isoelectronic with Si2O4, silicon dioxide, berlinite looks like quartz and has a structure that is similar to quartz with silicon replaced by Al and P. The AlO4 and PO4 tetrahedra alternate, like quartz, AlPO4 exhibits chirality and piezoelectric properties. Crystalline AlPO4 when heated, converts to tridymite and cristobalite forms, the structure can be regarded as an assembly of phosphate anions, aluminium cations and water. There are many of aluminium phosphate molecular sieves, generically known as ALPOs, the first ones were reported in 1982. They all share the chemical composition of AlPO4 and have framework structures with microporous cavities. The frameworks are made up of alternating AlO4 and PO4 tetrahedra, the denser cavity-less crystalline AlPO4 mineral, berlinite, shares the same alternating AlO4 and PO4 tetrahedra. These organic molecules act as templates to direct the growth of the porous framework, in medicine an aluminium phosphate sol is used as adsorbent for toxoid and in the dried form, which contains hydrated AlPO4, as an antacid. Industrially the Al-PO4-H2O system is used as the basis of many adhesives, binders and cements
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Hydrogen
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Hydrogen is a chemical element with chemical symbol H and atomic number 1. With a standard weight of circa 1.008, hydrogen is the lightest element on the periodic table. Its monatomic form is the most abundant chemical substance in the Universe, non-remnant stars are mainly composed of hydrogen in the plasma state. The most common isotope of hydrogen, termed protium, has one proton, the universal emergence of atomic hydrogen first occurred during the recombination epoch. At standard temperature and pressure, hydrogen is a colorless, odorless, tasteless, non-toxic, nonmetallic, since hydrogen readily forms covalent compounds with most nonmetallic elements, most of the hydrogen on Earth exists in molecular forms such as water or organic compounds. Hydrogen plays an important role in acid–base reactions because most acid-base reactions involve the exchange of protons between soluble molecules. In ionic compounds, hydrogen can take the form of a charge when it is known as a hydride. The hydrogen cation is written as though composed of a bare proton, Hydrogen gas was first artificially produced in the early 16th century by the reaction of acids on metals. Industrial production is mainly from steam reforming natural gas, and less often from more energy-intensive methods such as the electrolysis of water. Most hydrogen is used near the site of its production, the two largest uses being fossil fuel processing and ammonia production, mostly for the fertilizer market, Hydrogen is a concern in metallurgy as it can embrittle many metals, complicating the design of pipelines and storage tanks. Hydrogen gas is flammable and will burn in air at a very wide range of concentrations between 4% and 75% by volume. The enthalpy of combustion is −286 kJ/mol,2 H2 + O2 →2 H2O +572 kJ Hydrogen gas forms explosive mixtures with air in concentrations from 4–74%, the explosive reactions may be triggered by spark, heat, or sunlight. The hydrogen autoignition temperature, the temperature of spontaneous ignition in air, is 500 °C, the detection of a burning hydrogen leak may require a flame detector, such leaks can be very dangerous. Hydrogen flames in other conditions are blue, resembling blue natural gas flames, the destruction of the Hindenburg airship was a notorious example of hydrogen combustion and the cause is still debated. The visible orange flames in that incident were the result of a mixture of hydrogen to oxygen combined with carbon compounds from the airship skin. H2 reacts with every oxidizing element, the ground state energy level of the electron in a hydrogen atom is −13.6 eV, which is equivalent to an ultraviolet photon of roughly 91 nm wavelength. The energy levels of hydrogen can be calculated fairly accurately using the Bohr model of the atom, however, the atomic electron and proton are held together by electromagnetic force, while planets and celestial objects are held by gravity. The most complicated treatments allow for the effects of special relativity
36.
Argon
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Argon is a chemical element with symbol Ar and atomic number 18. It is in group 18 of the table and is a noble gas. Argon is the third-most abundant gas in the Earths atmosphere, at 0. 934% and it is more than twice as abundant as water vapor,23 times as abundant as carbon dioxide, and more than 500 times as abundant as neon. Argon is the most abundant noble gas in Earths crust, comprising 0. 00015% of the crust, nearly all of the argon in Earths atmosphere is radiogenic argon-40, derived from the decay of potassium-40 in the Earths crust. In the universe, argon-36 is by far the most common argon isotope, the name argon is derived from the Greek word ἀργόν, neuter singular form of ἀργός meaning lazy or inactive, as a reference to the fact that the element undergoes almost no chemical reactions. The complete octet in the atomic shell makes argon stable. Its triple point temperature of 83.8058 K is a fixed point in the International Temperature Scale of 1990. Argon is produced industrially by the distillation of liquid air. Argon is also used in incandescent, fluorescent lighting, and other gas-discharge tubes, Argon makes a distinctive blue-green gas laser. Argon is also used in fluorescent glow starters, Argon has approximately the same solubility in water as oxygen and is 2.5 times more soluble in water than nitrogen. Argon is colorless, odorless, nonflammable and nontoxic as a solid, liquid or gas, Argon is chemically inert under most conditions and forms no confirmed stable compounds at room temperature. Although argon is a gas, it can form some compounds under extreme conditions. Argon fluorohydride, a compound of argon with fluorine and hydrogen that is stable below 17 K, has been demonstrated. Although the neutral ground-state chemical compounds of argon are presently limited to HArF, ions, such as ArH+, and excited-state complexes, such as ArF, have been demonstrated. Theoretical calculation predicts several more argon compounds that should be stable but have not yet been synthesized, Argon was suspected to be a component of air by Henry Cavendish in 1785. Argon was first isolated from air in 1894 by Lord Rayleigh and Sir William Ramsay at University College London by removing oxygen, carbon dioxide, water and they had determined that nitrogen produced from chemical compounds was 0. 5% lighter than nitrogen from the atmosphere. The difference was slight, but it was important enough to attract their attention for many months and they concluded that there was another gas in the air mixed in with the nitrogen. Argon was also encountered in 1882 through independent research of H. F. Newall, each observed new lines in the color spectrum of air that did not match known elements
37.
Carrier generation and recombination
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In the solid-state physics of semiconductors, carrier generation and recombination are processes by which mobile charge carriers are created and eliminated. Carrier generation and recombination processes are fundamental to the operation of many semiconductor devices, such as photodiodes, LEDs. They are also critical to a analysis of p-n junction devices such as bipolar junction transistors. Like other solids, semiconductor materials have a band structure determined by the crystal properties of the material. The actual energy distribution among the electrons is described by the Fermi level, at absolute zero temperature, all of the electrons have energy below the Fermi level, but at non-zero temperatures the energy levels are filled following a Boltzmann distribution. In semiconductors the Fermi level lies in the middle of a band or band gap between two allowed bands called the valence band and the conduction band. The valence band, immediately below the band, is normally very nearly completely occupied. The conduction band, above the Fermi level, is nearly completely empty. Because the valence band is so full, its electrons are not mobile. However, if an electron in the valence band acquires enough energy to reach the conduction band, furthermore, it will also leave behind an electron hole that can flow as current exactly like a physical charged particle. In a material at thermal equilibrium generation and recombination are balanced, the equilibrium carrier density that results from the balance of these interactions is predicted by thermodynamics. The resulting probability of occupation of states in each energy band is given by Fermi–Dirac statistics. Recombination and generation are happening in semiconductors, both optically and thermally, and their rates are in balance at equilibrium. The product of the electron and hole densities is a constant at equilibrium, when there is a surplus of carriers, the rate of recombination becomes greater than the rate of generation, driving the system back towards equilibrium. Likewise, when there is a deficit of carriers, the rate becomes greater than the recombination rate. As the electron moves from one band to another, the energy. The following models are used to describe generation and recombination, depending on which particles are involved in the process, the localized state can absorb differences in momentum between the carriers, and so this process is the dominant generation and recombination process in silicon and other indirect bandgap materials. It can also dominate in direct bandgap materials under conditions of very low carrier densities, the energy is exchanged in the form of lattice vibration, a phonon exchanging thermal energy with the material
38.
Reactive oxygen species
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Reactive oxygen species are chemically reactive chemical species containing oxygen. Examples include peroxides, superoxide, hydroxyl radical, and singlet oxygen, in a biological context, ROS are formed as a natural byproduct of the normal metabolism of oxygen and have important roles in cell signaling and homeostasis. However, during times of stress, ROS levels can increase dramatically. This may result in significant damage to cell structures, cumulatively, this is known as oxidative stress. ROS are also generated by sources such as ionizing radiation. Ionizing radiation can generate damaging intermediates through the interaction with water, since water comprises 55–60% of the human body, the probability of radiolysis is quite high under the presence of ionizing radiation. In the process, water loses an electron and becomes highly reactive, then through a three-step chain reaction, water is sequentially converted to hydroxyl radical, hydrogen peroxide, superoxide radical and ultimately oxygen. The hydroxyl radical is extremely reactive and immediately removes electrons from any molecule in its path, turning that molecule into a free radical, mitochondria convert energy for the cell into a usable form, adenosine triphosphate. The process in which ATP is produced, called oxidative phosphorylation, the last destination for an electron along this chain is an oxygen molecule. Superoxide is not particularly reactive by itself, but can inactivate specific enzymes or initiate lipid peroxidation in its protonated form, the pKa of hydroperoxyl is 4.8. Thus, at physiological pH, the majority will exist as superoxide anion, if too much damage is present in mitochondria, a cell undergoes apoptosis or programmed cell death. This cytochrome C binds to Apaf-1, or apoptotic protease activating factor-1, using energy from the ATPs in the mitochondrion, the Apaf-1 and cytochrome C bind together to form apoptosomes. The apoptosomes bind to and activate caspase-9, another free-floating protein, the caspase-9 then cleaves the proteins of the mitochondrial membrane, causing it to break down and start a chain reaction of protein denaturation and eventually phagocytosis of the cell. Superoxide dismutases are a class of enzymes catalyze the dismutation of superoxide into oxygen and hydrogen peroxide. As such, they are an important antioxidant defense in all cells exposed to oxygen. In mammals and most chordates, three forms of superoxide dismutase are present, sOD1 is located primarily in the cytoplasm, SOD2 in the mitochondria and SOD3 is extracellular. The first is a dimer, while the others are tetramers, sOD1 and SOD3 contain copper and zinc ions, while SOD2 has a manganese ion in its reactive centre. The genes are located on chromosomes 21,6, and 4, respectively
39.
Carbon
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Carbon is a chemical element with symbol C and atomic number 6. It is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds, three isotopes occur naturally, 12C and 13C being stable, while 14C is a radioactive isotope, decaying with a half-life of about 5,730 years. Carbon is one of the few elements known since antiquity, Carbon is the 15th most abundant element in the Earths crust, and the fourth most abundant element in the universe by mass after hydrogen, helium, and oxygen. It is the second most abundant element in the body by mass after oxygen. The atoms of carbon can bond together in different ways, termed allotropes of carbon, the best known are graphite, diamond, and amorphous carbon. The physical properties of carbon vary widely with the allotropic form, for example, graphite is opaque and black while diamond is highly transparent. Graphite is soft enough to form a streak on paper, while diamond is the hardest naturally occurring material known, graphite is a good electrical conductor while diamond has a low electrical conductivity. Under normal conditions, diamond, carbon nanotubes, and graphene have the highest thermal conductivities of all known materials, all carbon allotropes are solids under normal conditions, with graphite being the most thermodynamically stable form. They are chemically resistant and require high temperature to react even with oxygen, the most common oxidation state of carbon in inorganic compounds is +4, while +2 is found in carbon monoxide and transition metal carbonyl complexes. The largest sources of carbon are limestones, dolomites and carbon dioxide, but significant quantities occur in organic deposits of coal, peat, oil. For this reason, carbon has often referred to as the king of the elements. The allotropes of carbon graphite, one of the softest known substances, and diamond. It bonds readily with other small atoms including other carbon atoms, Carbon is known to form almost ten million different compounds, a large majority of all chemical compounds. Carbon also has the highest sublimation point of all elements, although thermodynamically prone to oxidation, carbon resists oxidation more effectively than elements such as iron and copper that are weaker reducing agents at room temperature. Carbon is the element, with a ground-state electron configuration of 1s22s22p2. Its first four ionisation energies,1086.5,2352.6,4620.5 and 6222.7 kJ/mol, are higher than those of the heavier group 14 elements. Carbons covalent radii are normally taken as 77.2 pm,66.7 pm and 60.3 pm, although these may vary depending on coordination number, in general, covalent radius decreases with lower coordination number and higher bond order. Carbon compounds form the basis of all life on Earth
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