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Silicon Dioxide

Silicon dioxide

The chemical compound silicon dioxide, also known as silica, is the oxide of silicon, chemical formula SiO₂. It is found in nature in several forms, including quartz and opal. In fact, silica has 17 crystalline forms (see [http://www.minsocam.org/MSA/collectors_corner/arc/silicanom.htm Nomenclature of Silica]). Also, many forms of life include silica structures, including microorganisms such as diatoms, plants such as horsetail, and animals such as hexactinellid sponges. It is manufactured in several forms including glass (in colorless high purity form called fused silica), synthetic amorphous silica and silica gel (used e.g. as desiccants in brand new clothes and leather goods). Silica, with alumina, is a crucial ingredient in clay and allows for the development of a interlocking crystal matrix after firing in earthenware, stoneware and porcelain ceramic processes. Silica is a major ingredient of Portland cement. The ceramic re-entry heat protection tiles mounted on the bottom side of the Space Shuttles are made mostly of silica, as are the firebricks used in steel processing. The most common constituent of sand in inland continental settings and non-tropical coastal settings is silica, usually in the form of quartz because the considerable hardness of this mineral resists erosion. However, the composition of sand varies according to local rock sources and conditions. Inhaling crystalline silica dust can lead to silicosis. Variants found in high-pressure impacts are coesite and stishovite. Silica is also used as a food additive, primarily as a flow agent in powdered foods, or to absorb water. See the ingredients list for [http://www.bk.com/Food/Nutrition/ingredients.aspx Burger King]. The chemical stability of silicon dioxide and its electrical insulation properties are a major reason why silicon is the dominant material for semiconductor devices. It is used to separate the active regions of devices and to form insulating surfaces.

Chemistry

Silicon dioxide can be formed when silicon is exposed to oxygen (or air) at extremely high temperatures. This can occasionally happen naturally in fires, or in lightning strikes onto sand. Silicon dioxide is attacked by strong acids particularly hydrofluoric acid (HF). HF is used to remove or pattern silicon in the semiconductor industry.

Reference

- R. K. Iler, The Chemistry of Silica (ISBN 047102404X)

External links

- (Tridymite)
- (Quartz)
- (Cristobalite)
- [http://www.cdc.gov/niosh/npg/npgd0552.html NIOSH Pocket Guide to Chemical Hazards] (amorphous)
- [http://www.cdc.gov/niosh/npg/npgd0553.html NIOSH Pocket Guide to Chemical Hazards] (crystalline, as respirable dust) Category:Silicon compounds Category:Oxides Category:Ceramics ja:二酸化ケイ素

Chemical compound

A chemical compound is a chemical substance formed from two or more elements, with a fixed ratio determining the composition. For example, dihydrogen monoxide (water, ₂) is a compound composed of two hydrogen atoms for every oxygen atom. In general, this fixed ratio must be fixed due to some sort of physical property, rather than an arbitrary man-made selection. This is why materials such as brass, the superconductor YBCO, the semiconductor aluminium gallium arsenide, or chocolate are considered mixtures or alloys rather than compounds. A defining characteristic of a compound is that it has a chemical formula. Formulas describe the ratio of atoms in a substance, and the number of atoms in a single molecule of the substance (thus the formula for ethene is ₂₄ rather than ₂). The formula does not indicate that a compound is composed of molecules; for example, sodium chloride (table salt, ) is an ionic compound. Compounds may have a number of possible phases. Most compounds can exist as solids. Molecular compounds may also exist as liquids or gases. All compounds will decompose to smaller compounds or individual atoms if heated to a certain temperature (called the decomposition temperature). Every chemical compound that has been described in the literature carries a unique numerical identifier, its CAS number.

Types of compounds

- Acids
- Bases
- Ionic compounds
- Salts
- Oxides
- Organic compounds

Silicon

Silicon (Latin: silicium) is the chemical element in the periodic table that has the symbol Si and atomic number 14. A tetravalent metalloid, silicon is less reactive than its chemical analog carbon. It is the second most abundant element in the Earth's crust, making up 25.7% of it by weight. It occurs in clay, feldspar, granite, quartz and sand, mainly in the form of silicon dioxide (also known as silica) and silicates (compounds containing silicon, oxygen and metals). Silicon is the principal component of glass, cement, ceramics, most semiconductor devices, and silicones, the latter a plastic substance often confused with silicon. Silicon is widely used in semiconductors because the semiconductor Germanium has a problem with reverse leakage current flow, and because its native oxide forms better semiconductor/dielectric interfaces than almost all other material combinations.

Notable characteristics

In its crystalline form, silicon has a dark gray color and a metallic luster. Even though it is a relatively inert element, silicon still reacts with halogens and dilute alkalis, but most acids (except for a combination of nitric acid and hydrofluoric acid) do not affect it. Elemental silicon transmits more than 95% of all wavelengths of infrared light. Pure silicon crystals are rarely found in nature, as natural silicon is usually found as silica (SiO₂). Pure silicon crystals can be found as inclusions in gold, or in volcanic exhalations. Pure silicon has a negative temperature co-efficient of resistance, since the number of free charge carriers increases with temperature.

Applications

Silicon is a very useful element that is vital to many human industries. Silicon dioxide in the form of sand and clay is an important ingredient of concrete and brick and is also used to produce Portland cement. Silicon is a very important element for plant and animal life. Diatoms extract silica from water to build their protective cell walls. Other uses:
- Pottery/Enamel - It is a refractory material used in high-temperature material production and its silicates are used in making enamels and pottery.
- Steel - Silicon is an important constituent of some steels.
- Glass - Silica from sand is a principal component of glass. Glass can be made into a great variety of shapes and with a many different physical properties. Silica is used as a base material to make window glass, containers, and insulators, and many other useful objects.
- Abrasives - Silicon carbide is one of the most important abrasives.
- Semiconductor - Ultrapure silicon can be doped with other elements to adjust its electrical response by controlling the number and charge (positive or negative) of current carriers. Such control is necessary for transistors, solar cells, semiconductor detectors and other semiconductor devices which are used in electronics and other high-tech applications.
- Photonics - Silicon can be used as a continuous wave raman laser to produce coherent light with a wavelength of 1,698 nm.
- Medical materials - Silicones are flexible compounds containing silicon-oxygen and silicon-carbon bonds; they are widely used in applications such as artificial breast implants and contact lenses.
- LCDs and solar cells - Hydrogenated amorphous silicon has shown promise in the production of low-cost, large-area electronics in applications such as LCDs. It has also shown promise for large-area, low-cost solar cells.
- Construction - Silica is a major ingredient in bricks because of its low chemical activity.

History

Silicon (Latin silex, silicis meaning flint) was first identified by Antoine Lavoisier in 1787, and was later mistaken by Humphry Davy, in 1800, for a compound. In 1811 Gay Lussac and Thénard probably prepared impure amorphous silicon through the heating of potassium with silicon tetrafluoride. In 1824 Berzelius prepared amorphous silicon using approximately the same method of Lussac. Berzelius also purified the product by repeatedly washing it. Because silicon is an important element in semiconductor and high-tech devices, the high-tech region of Silicon Valley, California, is named after this element.

Occurrence

Silicon is a principal component of aerolites which are a class of meteoroids and also of tektites which is a natural form of glass. Measured by weight, silicon makes up 25.7% of the earth's crust and after oxygen is also the second most abundant element. Elemental silicon is not found in nature. It occurs most often as oxides and as silicates. Sand, amethyst, agate, quartz, rock crystal, flint, jasper, and opal are some of the forms in which the oxide appears. Granite, asbestos, feldspar, clay, hornblende, and mica are a few of the many silicate minerals.

Production

Silicon is commercially prepared by the heating of high-purity silica in an electric arc furnace using carbon electrodes. At temperatures over 1900 °C, the carbon reduces the silica to silicon according to the chemical equation :SiO₂ + C → Si + CO₂ Liquid silicon collects in the bottom of the furnace, and is then drained and cooled. The silicon produced via this process is called metallurgical grade silicon and is at least 99% pure. Using this method, silicon carbide, SiC, can form. However, provided the amount of SiO₂ is kept high, silicon carbide may be eliminated, as explained by this equation: :2SiC + SiO₂ → 3Si + 2CO In 2000, metallurgical grade silicon cost about $ 0.56 per pound ($1.23/kg).[http://minerals.usgs.gov/minerals/pubs/commodity/silicon/760301.pdf]. $

Purification

The use of silicon in semiconductor devices demands a much greater purity than afforded by metallurgical grade silicon. Historically, a number of methods have been used to produce high-purity silicon.

Physical methods

Early silicon purification techniques were based on the fact that if silicon is melted and re-solidified, the last parts of the mass to solidify contain most of the impurities. The earliest method of silicon purification, first described in 1919 and used on a limited basis to make radar components during World War II, involved crushing metallurgical grade silicon and then partially dissolving the silicon powder in an acid. When crushed, the silicon cracked so that the weaker impurity-rich regions were on the outside of the resulting grains of silicon. As a result, the impurity-rich silicon was the first to be dissolved when treated with acid, leaving behind a more pure product. In zone melting, the first silicon purification method to be widely used industrially, rods of metallurgical grade silicon are heated to melt at one end. Then, the heater is slowly moved down the length of the rod, keeping a small length of the rod molten as the silicon cools and resolidifies behind it. Since most impurities tend to remain in the molten region rather than resolidify, when the process is complete, most of the impurities in the rod will have been moved into the end that was the last to be melted. This end is then cut off and discarded, and the process repeated if a still higher purity was desired.

Chemical methods

Today, silicon is instead purified by converting it to a silicon compound that can be more easily purified than silicon itself, and then converting that silicon compound back into pure silicon. Trichlorosilane is the silicon compound most commonly used as the intermediate, although silicon tetrachloride and silane are also used. When these gases are blown over silicon at high temperature, they decompose to high-purity silicon. In the Siemens process, high-purity silicon rods are exposed to trichlorosilane at 1150 °C. The trichlorosilane gas decomposes and deposits additional silicon onto the rods, enlarging them according to chemical reactions like :2 HSiCl₃ → Si + 2 HCl + SiCl₄ Silicon produced from this and similar processes is called polycrystalline silicon. Polycrystalline silicon typically has impurity levels of 1 part per billion or less. At one time, DuPont produced ultrapure silicon by reacting silicon tetrachloride with high-purity zinc vapors at 950 °C, producing silicon according to the chemical equation :SiCl₄ + 2 Zn → Si + 2 ZnCl₂ However, this technique was plagued with practical problems (such as the zinc chloride byproduct solidifying and clogging lines) and was eventually abandoned in favor of the Siemens process.

Crystallization

The majority of silicon crystals grown for device production are produced by the Czochralski process, since it is the cheapest method available. However, silicon single-crystals grown by the Czochralski method contain impurities since the crucible which contains the melt dissolves. For certain electronic devices, particularly those required for high power applications, silicon grown by the Czochralski method is not pure enough. For these applications, float-zone silicon (FZ-Si) can be used instead.

Isotopes

Silicon has nine isotopes, with mass numbers from 25-33. Si-28 (the most abundant isotope, at 92.23%), Si-29 (4.67%), and Si-30 (3.1%) are stable; Si-32 is a radioactive isotope produced by argon decay. Its half-life, has been determined to be approximately 132 years, and it decays by beta emission to P-32 (which has a 14.28 day half-life [http://www.sciencegateway.org/isotope/phosp32.html]) and then to S-32.

Precautions

A serious lung disease known as silicosis often occurred in miners, stonecutters, and others who were engaged in work where siliceous dust was inhaled in great quantities.

Silicon is not silicone

Casual speakers often make the mistake of interchanging the words silicon and silicone; they are not the same. The first, of course, is the element. The second is a class of chemical compounds (in particular, inorganic polymers) that contain the element silicon, the most notable members of the class being silicone rubbers and silicone gels.

Silicon-based life

Since Silicon is analogous to Carbon, some scientists have proposed the possibility of Silicon-based life. This concept is especially popular in science-fiction.

Compounds

For Silicates see category Silicate. See also Silane (SiH₄), silicic acid (H₄SiO₄), Silicon carbide (SiC), Silicon dioxide (SiO₂), Silicon tetrachloride (SiCl₄), Silicon tetrafluoride (SiF₄), Trichlorosilane (HSiCl₃)

References

- [http://periodic.lanl.gov/elements/14.html Los Alamos National Laboratory – Silicon]

External links

- [http://www.webelements.com/webelements/elements/text/Si/key.html WebElements.com – Silicon]
- [http://mineral.galleries.com/minerals/elements/silicon/silicon.htm Mineral.Galleries.com – Silicon]
- [http://www.processpecialties.com/siliconp.htm Silicon wafer processing information by Process Specialties Inc] Category:Metalloids Category:Semiconductor materials ko:규소 ja:ケイ素 th:ซิลิคอน

Oxygen

Oxygen is a chemical element in the periodic table. It has the symbol O and atomic number 8. The element is very common, found not only on Earth but throughout the universe, usually covalently bonded with other elements. Unbound oxygen (usually called molecular oxygen, O₂, a diatomic molecule) first appeared on Earth during the Paleoproterozoic era (between 2500 million years ago and 1600 million years ago) and as a product of the metabolic action of early anaerobes (archaea and bacteria). The presence of free oxygen drove most of the organisms then living to extinction. The atmospheric abundance of free oxygen in later geological epochs and up to the present has been largely driven by photosynthetic organisms, roughly three quarters by phytoplankton and algae in the oceans and one quarter from terrestrial plants.

Characteristics

At standard temperature and pressure, oxygen is mostly found as a gas consisting of a diatomic molecule with the chemical formula O₂. O₂ has two energetic forms:
- The low-energy predominant single-bonded diradical triplet oxygen. This native diradical quality of oxygen contributes to its destructive chemical nature. This form is stabilized by the degeneracy effect.
- The high-energy double-bonded molecule singlet oxygen. Oxygen is a major component of air, produced by plants during photosynthesis, and is necessary for aerobic respiration in animals. The word oxygen derives from two words in Greek, οξυς (oxys) (acid, sharp) and γεινομαι (geinomai) (engender). The name "oxygen" was chosen because, at the time it was discovered in the late 18th century, it was believed that all acids contained oxygen. The definition of acid has since been revised to not require oxygen in the molecular structure. Liquid O₂ and solid O₂ have a light blue color and both are highly paramagnetic. Liquid O₂ is usually obtained by the fractional distillation of liquid air. Liquid and solid O₃ (ozone) have a deeper color of blue. A recently discovered allotrope of oxygen, tetraoxygen (O₄), is a deep red solid that is created by pressurizing O₂ to the order of 20 GPa. Its properties are being studied for use in rocket fuels and similar applications, as it is a much more powerful oxidizer than either O₂ or O₃.

Applications

Liquid oxygen finds use as an oxidizer in rocket propulsion. Oxygen is essential to respiration, so oxygen supplementation has found use in medicine (as oxygen therapy). People who climb mountains or fly in airplanes sometimes have supplemental oxygen supplies (as air). Oxygen is used in welding (such as the oxyacetylene torch), and in the making of steel and methanol. Oxygen presents two absorption bands centered in the wavelengths 687 and 760 nanometers. Some scientists have proposed to use the measurement of the radiance coming from vegetation canopies in those oxygen bands to characterize plant health status from a satellite platform. This is because in those bands, it is possible to discriminate the vegetation's reflectance from the vegetation's fluorescence, which is much weaker. The measurement presents several technical difficulties due to the low signal to noise ratio and due to the vegetation's architecture, but it has been proposed as possibility to monitor the carbon cycle from satellite, thus in a global scale. Oxygen, as a mild euphoric, has a history of recreational use that extends into modern times. Oxygen bars can be seen at parties to this day. In the 19th century, oxygen was often mixed with nitrous oxide to promote an analgesic effect; indeed, such a mixture (Entonox) is commonly used in medicine today.

History

Oxygen was first discovered by Michał Sędziwój, Polish alchemist and philosopher in late 16th century. Sędziwój assumed the existence of oxygen by warming nitre (saltpeter). He thought of the gas given off as "the elixir of life". Oxygen was again discovered by the Swedish pharmacist Carl Wilhelm Scheele sometime before 1773, but the discovery was not published until after the independent discovery by Joseph Priestley on August 1, 1774, who called the gas dephlogisticated air (see phlogiston theory). Priestley published his discoveries in 1775 and Scheele in 1777; consequently Priestley is usually given the credit. It was named by Antoine Laurent Lavoisier after Priestley's publication in 1775.

Occurrence

Oxygen is the second most common component of the earth's atmosphere (20.947% by volume).

Compounds

Due to its electronegativity, oxygen forms chemical bonds with almost all other elements (which is the origin of the original definition of oxidation). The only elements to escape the possibility of oxidation are a few of the noble gases. The most famous of these oxides is dihydrogen monoxide, or water (H₂O). Other well known examples include compounds of carbon and oxygen, such as carbon dioxide (CO₂), alcohols (R-OH), aldehydes, (R-CHO), and carboxylic acids (R-COOH). Oxygenated radicals such as chlorates (ClO₃⁻), perchlorates (ClO₄⁻), chromates (CrO₄²⁻), dichromates (Cr₂O₇²⁻), permanganates (MnO₄⁻), and nitrates (NO₃⁻) are strong oxidizing agents in and of themselves. Many metals such as iron bond with oxygen atoms, iron (III) oxide (Fe₂O₃). Ozone (O₃) is formed by electrostatic discharge in the presence of molecular oxygen. A double oxygen molecule (O₂)₂ is known and is found as a minor component of liquid oxygen. Epoxides are ethers in which the oxygen atom is part of a ring of three atoms.

Isotopes

Oxygen has fifteen known isotopes with atomic masses ranging from 12 to 26. Three of them are stable and twelve are radioactive. The radioisotopes all have half lives of less than three minutes. The stable isotopes have mass numbers of 16, 17 and 18, of which oxygen-16 is the most common (over 99%).

Precautions

Oxygen can be toxic at elevated partial pressures (i.e. high relative concentrations). This is important in some forms of scuba diving, such as with a rebreather. Certain derivatives of oxygen, such as ozone (O₃), singlet oxygen, hydrogen peroxide, hydroxyl radicals and superoxide, are also highly toxic. The body has developed mechanisms to protect against these toxic species. For instance, the naturally-occurring glutathione can act as an antioxidant, as can bilirubin which is normally a breakdown product of hemoglobin. Highly concentrated sources of oxygen promote rapid combustion and therefore are fire and explosion hazards in the presence of fuels. This is true as well of compounds of oxygen such as chlorates, perchlorates, dichromates, etc. Compounds with a high oxidative potential can often cause chemical burns. The fire that killed the Apollo 1 crew on a test launchpad spread so rapidly because the pure oxygen atmosphere was at normal atmospheric pressure instead of the one third pressure that would be used during an actual launch. (See partial pressure.) Oxygen derivatives are prone to form free radicals, especially in metabolic processes. Because they can cause severe damage to cells and their DNA, they are thought to be related to cancer and aging.

References

- [http://periodic.lanl.gov/elements/8.html Los Alamos National Laboratory – Oxygen]
- [http://physics.nist.gov/cgi-bin/AtData/main_asd Nist atomic spectra database]
- [http://chartofthenuclides.com/default.html Nuclides and Isotopes Fourteenth Edition]: Chart of the Nuclides, General Electric Company, 1989

External links

- [http://www.priestleysociety.net Priestley Society, Dedicated to Joseph Priestley the man who discovered oxygen]
- [http://www.best-home-remedies.com/minerals/oxygen.htm Oxygen - Benefits, Deficiency Symptoms And Food Sources]
- [http://www.josephpriestley.info Joseph Priestley Information Website, about the man who discovered oxygen]
- [http://periodic.lanl.gov/elements/8.html Los Alamos National Laboratory – Oxygen]
- [http://www.webelements.com/webelements/elements/text/O/index.html WebElements.com – Oxygen]
- [http://education.jlab.org/itselemental/ele008.html It's Elemental – Oxygen]
- [http://members.tripod.com/tjaartdb0/html/oxygen_toxicity.html Oxygen Toxicity]
- [http://www.uigi.com/oxygen.html Oxygen (O2) Properties, Uses, Applications]
- [http://www.compchemwiki.org/index.php?title=Oxygen Computational Chemistry Wiki]
- [http://koti.mbnet.fi/antitz/dime/en Tests with liquid oxygen :-)] Category:Nonmetals Category:Chalcogens als:Sauerstoff ko:산소 ms:Oksigen ja:酸素 simple:Oxygen th:ออกซิเจน

Quartz

Quartz is the most abundant mineral in the Earth's continental crust. It has a hexagonal crystal structure made of trigonal crystallized silica (silicon dioxide, SiO₂), with a hardness of 7 on the Mohs scale. Density is 2.65 g/cm³. The typical shape is a six-sided prism that ends in six-sided pyramids, although these are often twinned, distorted, or so massive that only part of the shape is apparent from a mined specimen. Additionally a bed is a common form, particularly for varieties such as amethyst, where the crystals grow up from a matrix and thus only one termination pyramid is present. A quartz geode consists of a hollow rock (usually with an approximately spherical shape) with a core lined with a bed of crystals.

Varieties

Quartz is one of the world's most common crustal minerals and goes by a bewildering array of different names. The most important distinction between types of quartz is that of macrocrystalline (individual crystals visible to the unaided eye) and the microcrystalline or cryptocrystalline varieties (aggregates of crystals visible only under high magnification). Chalcedony is a generic term for cryptocrystalline quartz. The cryptocrystalline varieties are either translucent or mostly opaque, while the transparent varieties tend to be macrocrystalline. Although many of the varietal names historically arose from the colour of the mineral, current scientific naming schemes refer primarily to the microstructure of the mineral. Colour is a secondary identifier for the cryptocrystalline minerals, although it is a primary identifier for the macrocrystalline varieties. This does not always hold true. Not all varieties of quartz are naturally occurring. Prasiolite, an olive coloured material, is produced by heat treatment. Although citrine occurs naturally, the majority is the result of heat-treated amethyst. Carnelian is widely heat-treated to deepen its colour. Carnelian] Because natural quartz is so often twinned, much quartz used in industry is synthesized. Large, flawless and untwinned crystals are produced in an autoclave via the hydrothermal process: emeralds are also synthesized in this fashion. Quartz occurs in hydrothermal veins and pegmatites. Well-formed crystals may reach several metres in length and weigh hundreds of kilograms. Erosion of pegmatites may reveal expansive pockets of crystals, known as "cathedrals." Quartz is a common constituent of granite, sandstone, limestone, and many other igneous, sedimentary, and metamorphic rocks. Some quartz crystal structures are piezoelectric and are used as oscillators in electronic devices such as quartz clocks and radios. Lechatelierite is an amorphous silica glass SiO₂ which is formed by lightning strikes in quartz sand.

History

The name "quartz" comes from the German "Quarz", which is of Slavic origin. Quartz is the most common material identified as the mystical substance maban in Australian Aboriginal mythology. Roman naturalist Pliny the Elder believed quartz to be permanently frozen ice. He supported this idea by saying that quartz is found near glaciers in the Alps and that large quartz crystals were fashioned into spheres to cool the hands. He also knew of the ability of quartz to split light into a spectrum. Nicolas Steno's study of quartz paved the way for modern crystallography. He discovered that no matter how distorted a quartz crystal, the long prism faces always made a perfect 60 degree angle.

Piezoelectricity

Quartz is also a type of piezoelectric crystal that creates electricity through a process called piezoelectricity when mechanical stress is put upon it. One of the earliest uses for a quartz crystal was a phonograph pickup. Today, one of the most ubiquitous piezoelectric uses of quartz is as a crystal oscillator -- in fact these oscillators are often simply called "quartzes".

References

- Hurlbut, Cornelius S.; Klein, Cornelis, 1985, Manual of Mineralogy, 20th ed., ISBN 0471805807
- [http://mineral.galleries.com/minerals/silicate/quartz.htm Quartz group - Mineral Galleries]
- [http://mineral.galleries.com/minerals/silicate/quartz/quartz.htm Quartz - Mineral Galleries]
- [http://www.minerals.net/mineral/silicate/tecto/quartz/quartz.htm Quartz - Mineral.net]
- [http://rockhoundingar.com/quartz.html Arkansas quartz, Rockhounding Arkansas]
- [http://www.minsocam.org/MSA/collectors_corner/arc/silicanom.htm Gilbert Hart Nomenclature of Silica, American Mineralogist, Volume 12, pages 383-395, 1927]
- [http://www.mindat.org/min-3337.html Mindat.org] Category:Minerals Category:Quartz varieties ko:석영 ja:石英

Opal

The mineraloid opal is amorphous SiO₂·nH₂O; hydrated silicon dioxide, the water content sometimes being as high as 20%. Opal ranges from colorless through white, milky blue, gray, red, yellow, green, brown and black. Often many of these colors can be seen at once, caused by interference and diffraction of light passing through minute, regularly arranged apertures within the microstructure of opal, known as Bragg's lattice. These apertures are filled with secondary silica and form thin lamellae inside the opal during solidification. The term opalescence is commonly and erroneously used to describe this unique and beautiful phenomenon, which is correctly termed play of color. Contrarily, opalescence is correctly applied to the milky, turbid appearance of common or potch opal. Potch does not show a play of color. The veins of opal displaying the play of color are often quite thin, and this has given rise to [http://www.opalsdownunder.com.au/articles/cutting.htm unusual methods] of preparing the stone as a gem. An opal doublet is a thin layer of colorful material, backed by a black mineral, such as ironstone, basalt or obsidian. The darker backing emphasizes the play of color, and results in a more attractive display than a lighter potch. Given the texture of opals, they can be quite difficult to polish to a reasonable lustre. The triplet cut backs the colored material with a dark backing, and then has a cap of clear quartz (rock crystal) on top, which takes a high polish, and acts as a protective layer for the comparatively delicate opal.

Common opal

Besides the gemstone varieties that show a play of color, there are other kinds of common opal such as the milk opal, milky bluish to greenish; resin opal, honey-yellow with a resinous lustre; wood opal, caused by the replacement of the organic material in wood with opal; menilite brown or grey; hyalite, a colorless glass-clear opal sometimes called Muller's Glass; geyserite, (siliceous sinter) deposited around hot springs or geysers; and diatomite or diatomaceous earth, the accumulations of diatom shells or tests. Opal is a mineraloid gel which is deposited at relatively low temperature and may occur in the fissures of almost any kind of rock, being most commonly found with limonite, sandstone, rhyolite, and basalt. The word opal comes from the Sanskrit upala, the Greek opallios, and the Latin opalus, meaning "precious stone." Opal is one of the mineraloids that can form or replace fossils. The resulting fossils, though not of any extra scientific interest, appeal to collectors. fossil

Sources of opal

About 95% of the world's opal comes from Australia, in particular the town of Coober Pedy in South Australia, is a major source. Common, water, jelly, and fire opal are found mostly in Mexico and Mesoamerica. Another Australian town, Lightning Ridge in New South Wales, is the main source of black opal, opal containing a predominantly dark background (dark-gray to blue-black displaying the play of color). Boulder opal has a main source in Quilpie, Queensland. The main source of opal in the United States is Spencer, Idaho. The opal is the official gemstone of South Australia. Opal is the official birthstone of the month of October.

Synthetic opal

As well as occurring naturally, opals of all varieties have been synthesized experimentally and commercially. The resulting material is distinguishable from natural opal by its regularity; under magnification, the patches of colour are seen to be arranged in a "lizard skin" or "chicken wire" pattern. Synthetics are further distinguished from naturals by the former's lack of fluorescence under UV light. Synthetics are also generally lower in density and are often highly porous; some may even stick to the tongue. Two notable producers of synthetic opal are the companies Kyocera and Inamori of Japan. Most so-called synthetics, however, are more correctly termed imitations, as they contain substances not found in natural opal (e.g., plastic stabilizers). The Gilson opals often seen in vintage jewellery are actually an imitation consisting of laminated glass with bits of foil interspersed.

References and external links

- Hurlbut, Cornelius S.; Klein, Cornelis, 1985, Manual of Mineralogy, 20th ed., ISBN 0471805807
- [http://mineral.galleries.com/minerals/mineralo/opal/opal.htm Mineral Galleries - Opal]
- [http://www.mindat.org/min-3004.html Mindat.org - Opal]
- [http://www.minerals.net/mineral/silicate/tecto/quartz/opal.htm Minerals.net - Opal]
- [http://ist-socrates.berkeley.edu/~eps2/wisc/Lect16b.html Berkeley - Great photos]
- [http://www.theimage.com/gemstone/opal/opal.html Gem gallery - Opal] Category:Quartz varieties Category:Gemstones ja:オパール

Horsetail

- Subgenus Equisetum Equisetum arvense - Field or Common Horsetail
Equisetum bogotense - Andean Horsetail
Equisetum diffusum - Himalayan Horsetail
Equisetum fluviatile - Water Horsetail
Equisetum palustre - Marsh Horsetail
Equisetum pratense - Shade Horsetail
Equisetum sylvaticum - Wood Horsetail
Equisetum telmateia - Great Horsetail
- Subgenus Hippochaete Equisetum giganteum - Giant Horsetail
Equisetum myriochaetum - Mexican Giant Horsetail
Equisetum hyemale - Rough Horsetail
Equisetum laevigatum - Smooth Horsetail
Equisetum ramosissimum - Branched Horsetail
Equisetum scirpoides - Dwarf Horsetail
Equisetum variegatum - Variegated Horsetail The horsetails are vascular plants, comprising 15 species of plants in the genus Equisetum. This genus is the only one in the family Equisetaceae, which in turn is the only family in the order Equisetales and the class Equisetopsida. This class is often placed as the sole member of the Division Equisetophyta (also called Arthrophyta in older works), though some recent molecular analyses place the genus within Pteridophyta, related to Marattiales. The molecular data, however, are somewhat ambiguous as of yet. Other classes and orders of Equisetophyta are known from the fossil record, where they were important members of the world flora during the Carboniferous period. Carboniferous The name horsetail arose because it was thought that the stalk resembled a horse's tail; the name Equisetum is from the Latin equus, "horse", and seta, "bristle". Other names, rarely used, include candock (applied to branching species only), and scouring-rush (applied to the unbranched or sparsely branched species). The name scouring-rush refers to its rush-like appearance and because the stems are coated with abrasive silica that led them to be used for scouring cooking pots in the past. The genus is near-cosmopolitan, being absent only from Australasia and Antarctica. They are perennial plants, either herbaceous, dying back in winter (most temperate species) or evergreen (some tropical species, and the temperate Equisetum hyemale). They mostly grow 0.2-1.5 m tall, though E. telmateia can exceptionally reach 2.5 m, and the tropical American species E. giganteum 5 m, and E. myriochaetum 8 m. In these plants the leaves are greatly reduced, being represented only by whorls of small, translucent scales. The stems are green and photosynthetic, also distinctive in being hollow, jointed, and ridged (with (3-) 6-40 ridges). There may or may not be whorls of branches at the nodes; when present, these branches are identical to the main stem except smaller. The spores are borne in a cone-like structures (strobilus, pl. strobili) at the tip of some of the stems. In many species they are unbranched, and in some (e.g. E. arvense) they are non-photosynthetic, produced early in spring separately from photosynthetic sterile stems. In some other species (e.g. E. palustre) they are very similar to sterile stems, photosynthetic and with whorls of branches. spore Horsetails are mostly homosporous, though in E. arvense, smaller spores give rise to male prothalli. The spores have four elaters that act as moisture-sensitive springs, ejecting the spores through a weak spot of the sporangia. Many plants in this genus prefer sandy soils, though some are aquatic and others adapted to wet clay soils. One horsetail, E. arvense, can be a nuisance weed because it readily regrows after being pulled out. The stalks arise from rhizomes that are deep underground and almost impossible to dig out. It is also unaffected by many herbicides designed to kill seed plants. The foliage is poisonous to grazing animals if eaten in large quantities. The horsetails were a much larger and more diverse group in the distant past before seed plants became dominant across the Earth. Some species were large trees reaching to 30 m tall. The genus Calamites (Family Calamitaceae) is abundant in coal deposits from the Carboniferous period. ---- The superficially similar flowering plant, Mare's tail (Hippuris vulgaris), unrelated to the genus Equisetum, is occasionally misidentified and misnamed as a horsetail.

External links

- [http://www.btinternet.com/~pigott/equisetum/ UK National Collection] - includes a taxonomic list of all known species and hybrids
- [http://members.eunet.at/m.matus/ The Wonderful World of Equisetum]
- [http://www.fiu.edu/~chusb001/giant_equisetum.html Giant horsetails]
- [http://www.floridata.com/ref/e/equi_hye.cfm Equisetum hyemale] Category:Equisetophyta Category: cryptogams ja:トクサ ja:スギナ

Hexactinellid

Hexactinellid sponges are sponges with a skeleton made of four- and/or six-pointed silaceous spicules, often referred to as 'glass sponges'. Hexactinellids are relatively uncommon nowadays and are mostly found at substantial depths. There are big differences between hexactinellids and other sponges. They are often cup-shaped animals with sturdy internal skeletons made up of fused spicules. Much of their body tissues are syncitia, extensive regions of multinucleate cytoplasm. They are fairly common relative to Demosponges as fossils, but this is thought to be at least in part because their spicules are sturdier than spongin and fossilize better. Unlike other sponges, they do not possess the ability to contract. One ability they do possess is a unique system for rapidly conducting electrical impulses across their bodies, making it possible for them to respond quickly to external stimuli. Hexactinellids like the "Venus Flower Basket" have a tuft of fibres that extends outward like an inverted crown at the base of their skeleton. These fibres are between 50 and 175 mm long and about the thickness of a human hair. They work as fibre optics that are surprisingly similar to the fibres used in modern telecoms networks and could even be more handy than the man-made versions. The biological fibres of the sponge conduct light beautifully when they are illuminated, and use the same optical principles that modern engineers use to design industrial fibre optics. Despite not having the ultra-high transparency needed for telecoms networks, they do have other advantages; unlike commercial fibre, it is possible to tie them in tight knots without them cracking or breaking. Another advantages is the fact that these biological fibres are produced by chemical deposition at the temperature of seawater. For the moment, human fibre optics can only be produced with a high-temperature furnace and expensive equipment. The earliest known hexactinellids are from the earliest Cambrian or late Neoproterozoic. Category:Porifera

Glass

For eyeglasses, see glasses. For the drinking vessel, see glass (drinkware), Glass (disambiguation). The materials definition of a glass is a uniform amorphous solid material, usually produced when a suitably viscous molten material cools very rapidly to below its glass transition temperature, thereby not giving enough time for a regular crystal lattice to form. A simple example is when table sugar is melted and cooled rapidly by dumping the liquid sugar onto a cold surface. The resulting solid is amorphous, not crystalline like the sugar was originally, which can be seen in its conchoidal fracture. The word glass comes from Latin glacies (ice) and corresponds to German Glas, M.E. glas, A.S. glaes. Germanic tribes used the word glaes to describe amber, recorded by Roman historians as glaesum. Anglo-Saxons used the word glaer for amber. The remainder of this article will be concerned with a specific type of glass—the silica-based glasses in common use as a building, container or decorative material. ---- In its pure form, glass is a transparent, relatively strong, hard-wearing, essentially inert, and biologically inactive material which can be formed with very smooth and impervious surfaces. These desirable properties lead to a great many uses of glass. Glass is, however, brittle and will break into sharp shards. These properties can be modified, or even changed entirely, with the addition of other compounds or heat treatment. Common glass is about 70% amorphous silicon dioxide (Si O₂), which is the same chemical compound found in quartz, or in its polycrystalline form, sand.

Properties and Uses

sand.]] One of the most obvious characteristics of ordinary glass is that it is transparent to visible light (not all glassy materials are). The transparency is due to an absence of electronic transition states in the range of visible light, and to the fact that such glass is homogeneous on all length scales greater than about a wavelength of visible light (inhomogeneities cause light to be scattered, breaking up any coherent image transmission). Ordinary glass does not allow light at a wavelength of lower than 400 nm, also known as ultraviolet light or UV, to pass. This is due to the addition of compounds such as soda ash (sodium carbonate). Pure SiO₂ glass (also called fused quartz) does not absorb UV light and is used for applications that require transparency in this region, although it is more expensive. This type of glass can be made so pure that hundreds of kilometres of glass are transparent at infrared wavelengths in fibre optic cables. Individual fibers are given an equally transparent cladding of SiO₂/O₂ glass, which has only slightly different optical properties (the germanium contributing to a lower index of refraction). Undersea cables have sections doped with erbium, which amplify transmitted signals by laser emission from within the glass itself. Amorphous SiO₂ is also used as a dielectric material in integrated circuits, due to the smooth and electrically neutral interface it forms with silicon. Glasses used for making optical devices are commonly categorized using a six-digit glass code, or alternatively a letter-number code from the Schott Glass catalog. For example, BK7 is a low-dispersion borosilicate crown glass, and SF10 is a high-dispersion dense flint glass. The glasses are arranged by composition, refractive index, and Abbe number. Glass is sometimes created naturally from volcanic magma. This glass is called obsidian, and is usually black with impurities. Obsidian is a raw material for flint knappers, who have used it to make extremely sharp knives since the stone age. Obsidian collection is prohibited by law in some places (including the United States), but the same toolmaking techniques can be applied to industrially-made glass.

Glass Ingredients

Pure silica (SiO₂) has a melting point of about 2000 °C (3600 °F), and while it can be made into glass for special applications (see fused quartz), two other substances are always added to common glass to simplify processing. One is soda (sodium carbonate Na₂CO₃), or potash, the equivalent potassium compound, which lowers the melting point to about 1000 °C (1800 °F). However, the soda makes the glass water-soluble, which is obviously undesirable, so lime (calcium oxide, CaO) is the third component, added to restore insolubility. Hence the use of the term soda-lime glass. Soda-lime glasses account for about 90% of manufactured glass. Most common glass has other ingredients added to change its properties. Lead glass, such as lead crystal or flint glass, is more 'brilliant' because the increased refractive index causes noticeably more 'sparkles', while boron may be added to change the thermal and electrical properties, as in Pyrex. Adding barium also increases the refractive index. Thorium oxide gives glass a high refractive index and low dispersion, and was formerly used in producing high-quality lenses, but due to its radioactivity has been replaced by lanthanum oxide in modern glasses. Large amounts of iron are used in glass that absorbs infrared energy, such as heat absorbing filters for movie projectors, while cerium(IV) oxide can be used for glass that absorbs UV wavelengths (biologically damaging ionizing radiation).

Colors

UV Metals and metal oxides are added to glass during its manufacture to change its color. Manganese can be added in small amounts to remove the green tint lent by iron, or in higher concentrations to give glass an amethyst color. Like manganese, selenium can be used in small concentrations to decolorize glass, or in higher concentrations to impart a reddish color. Small concentrations of cobalt (0.025 to 0.1%) yield blue glass. Tin oxide with antimony and arsenic oxides produce an opaque white glass, first used in Venice to produce an imitation porcelain. 2 to 3% of copper oxide produces a turquoise color. Pure metallic copper produces a very dark red, opaque glass, which is sometimes used as a substitute for gold in the production of ruby-colored glass. Nickel, depending on the concentration, produces blue, or violet, or even black glass. Adding titanium produces yellowish-brown glass. Metallic gold, in very small concentrations (around 0.001%), produces a rich ruby-colored glass, while lower concentrations produces a less intense red, often marketed as "cranberry". Uranium (0.1 to 2%) can be added to give glass a fluorescent yellow or green color. Uranium glass is typically not radioactive enough to be dangerous, but if ground into a powder, such as by polishing with sandpaper, and inhaled, it can be carcinogenic. Silver compounds (notably silver nitrate) can produce a range of colors from orange-red to yellow. The way the glass is heated and cooled can significantly affect the colors produced by these compounds. The chemistry involved is complex and not well understood. New colored glasses are frequently discovered.

History of glass

Naturally occurring glass, such as obsidian, has been used since the stone age. The first documented instructions for glass making is in Egypt around 1500 BC, when glass was used as a glaze for pottery and other items. In the first century BC the technique of blowing glass was developed and what had once been an extremely rare and valuable item became much more common. During the Roman Empire many forms of glass were created, usually for vases and bottles. Glass was made from sand, plant ash and lime. The earliest use of glass was as a colored, opaque, or transparent glaze applied to ceramics before they were fired. Small pieces of colored glass were considered valuable and often rivaled precious gems as jewelry items. As time passed, it was discovered (most likely by a potter) that if glass is heated until it becomes semi-liquid, it can be shaped and left to cool in a new, solid, independently standing shape. In the first century BC, somewhere at the eastern end of the mediterranean, a new invention caused a true revolution in the glass industry. This was the discovery of glassblowing, both free-blowing and mold-blowing. The color of "natural glass" is green to bluish green. This color is caused by the varying amounts of naturally occurring iron impurities in the sand. Common glass today usually has a slight green or blue tint, arising from these same impurities. Glassmakers learned to make colored glass by adding metallic compounds and mineral oxides to produce brilliant hues of red, green, and blue - the colors of gemstones. When gemcutters learned to cut glass, they found clear glass was an excellent refractor of light, the popularity of cut clear glass soared, that of colored glass diminished. Glass objects from the 7th and 8th centuries have been found on the island of Torcello near Venice. These form an important link between Roman times and the later importance of that city in the production of the material. About 1000 AD, an important technical breakthrough was made in Northern Europe when soda glass was replaced by glass made from a much more readily available material: potash obtained from wood ashes. From this point on, northern glass differed significantly from that made in the Mediterranean area, where soda remained in common use. The 11th century saw the emergence, in Germany, of new ways of making sheet glass by blowing spheres, swinging these out to form cylinders, cutting these while still hot, and then flattening the sheets. This technique was perfected in 13th century Venice. Until the 12th century, stained glass (i.e., glass with some coloring impurities, usually metals) was not widely used. The centre for glass making from the 14th century was Venice, which developed many new techniques and became the center of a lucrative export trade in dinner ware, mirrors, and other luxury items. Eventually some of the Venetian glass workers moved to other areas of northern Europe and glass making spread with them. The Crown glass process was used up to the mid-1800s. In this process, the glassblower would spin around 9 lb (4 kg) of molten glass at the end of a rod until it flattened into a disk approximately 5 ft (1.5 m) in diameter. The disk would then be cut into panes. Venetian glass was highly prized between the 10th and 14th centuries as they managed to keep the process secret. Around 1688, a process for casting glass was developed, which led to its becoming a much more commonly used material. The invention of the glass pressing machine in 1827 allowed the mass production of inexpensive glass articles. The Cylinder method was invented by William J. Blenko in the early 1900s. Art is sometimes etched into glass via acid or other caustic substance (causing the image to be eaten into the glass). Traditionally this was done by a trained artisan after the glass was blown or cast. In the 1920s a new mold-etch process was invented, in which art was etched directly into the mold, so that each cast piece emerged from the mold with the image already on the surface of the glass. This reduced manufacturing costs and, combined with a wider use of colored glass, led to cheap popular glassware in the 1930s, which later became known as Depression glass.

Glass tools

Since glass is strong and unreactive, it is a very useful material. Many household objects are made of glass. Drinking glasses, bowls, and bottles are often made of glass, as are light bulbs, mirrors, the picture tubes of computer monitors and televisions, and windows. In laboratories doing research in chemistry, biology, physics and many other fields, flasks, test tubes, lenses and other laboratory equipment are often made of glass. For these applications, borosilicate glass (such as Pyrex) is usually used for its strength and low coefficient of thermal expansion, which gives greater resistance to thermal shock and allows for greater accuracy in laboratory measurements when heating and cooling experiments. For the most demanding applications, quartz glass is used, although it is very difficult to work. Most such glass is mass-produced using various industrial processes, but most large laboratories need so much custom glassware that they keep a glassblower on staff. Volcanic glasses, such as obsidian, have long been used to make stone tools, and flint knapping techniques can easily be adapted to mass-produced glass.

Glass art

Even with the availability of common glassware, hand blown or lampworked glassware remains popular for its artistry. Some artists in glass include Lino Tagliapietra, Sidney Waugh, Rene Lalique, Dale Chihuly, and Louis Comfort Tiffany, who were responsible for extraordinary glass objects. The term "crystal glass", derived from rock crystal, has come to denote high-grade colorless glass, often containing lead, and is sometimes applied to any fine hand-blown glass. There are many techniques for creating fine glass art; each is suitable for certain kinds of object and unsuitable for others. Someone who works with hot glass is called a glassblower or lampworker, and these techniques are how most fine glassware is created. Glass can also be cut with a diamond saw, and polished to give gleaming facets. Objects made out of glass include vessels (bowls, vases, and other containers), paperweights, marbles, beads, smoking pipes, bongs, and sculptures. Colored glass is often used, and sometimes the glass is painted, although many glassblowers consider this crude. A significant exception is the collection of pieces by the Blaschkas. The Harvard Museum of Natural History has a collection of extremely detailed models of flowers made of painted glass. These were lampworked by Leopold Blaschka and his son Rudolph, who never revealed the method he used to make them. The Blaschka Glass Flowers stand as an inspiration to glassblowers today. See [http://www.hmnh.harvard.edu/exhibitions/glassflowers.html the Harvard Museum of Natural History's page on the exhibit] for further information. Stained glass is an art form with a long history; many churches have beautiful stained-glass windows.

Architectural glass

Float (annealed) glass

90% of the world's flat glass is produced by the float glass process invented in the 1950s by Sir Alastair Pilkington of Pilkington Glass, in which molten glass is poured onto one end of a molten tin bath. The glass floats on the tin, and levels out as it spreads along the bath, giving a smooth face to both sides. The glass cools and slowly solidifies as it travels over the molten tin and leaves the tin bath in a continuous ribbon. The glass is annealed by cooling in a temperatured controlled oven called a "lehr". The finished product has near-perfect parallel surfaces. A very small amount of the tin is imbedded in the glass on the side it touched. The tin side is easier to make into a mirror. This "feature" quickened the switch from plate to float glass. Glass is produced in standard metric thicknesses of 2, 3, 4, 5, 6, 8, 10, 12, 15, 19 and 22 mm. Molten glass floating on tin in a nitrogen/hydrogen atmosphere will spread out to a thickness of about 6mm and stop due to surface tension. Thinner glass is made by streching the glass while it floats on the tin and cools. Similarly thicker glass is pushed back and not permitted to expand as it cools on the tin. Annealed glass is considered a hazard in architectural applications as it breaks in large, jagged shards that can cause serious injury. Building codes across the world restrict the use of annealed glass in areas where there is a high risk of breakage and injury, for example in bathrooms, in door panels, fire exits and at low heights in schools.

Sheet glass

Before Pilkington's invention, flat glass panels were generally made as plate glass or sheet glass. Sheet glass (sometimes called window glass or drawn glass) was made by dipping a leader into a vat of molten glass then pulling that leader straight up while a film of glass hardened just out of the vat. This film or ribbon was pulled up continiously held by tractors on both edges while it cooled. After 12 meters or so it was cut off the vertical ribbon and tipped down to be further cut. This glass is clear but has thickness variations due to small temperature changes just out of the vat as it was hardening. These variations cause lines of slight distortions. You may still see this glass in older houses. Float glass replaced this process.

Plate glass

The plate glass process starts with extruded or rolled glass that is rather rough. The rough panes are ground flat then pollished clear. This is a fairly expensive process. Before the float process, mirrors were plate glass as sheet glass had distortions that would be too objectionable if made into mirrors.

Cylinder glass

school Glass is blown into a cylindrical iron mold. The ends are cut off and a cut is made down the side of the cylinder. The cut cylinder is then placed in an oven where the cylinder bends flat into a glass sheet. Before the introduction of the Pilkington method this was a popular method for glass manufacture. William J. Blenko used this method in the early 1900s to make stained glass. These imperfect panes have led to the misconception that glass is actually a high-viscosity liquid at room temperature, which is not the case. (See below.)

Insulated glazing

Main article: insulated glazing. Insulated glazing, or double glazing is a piece of glazing consisting of two or more layers of glazing separated by a spacer along the edge and sealed to create a dead air space between the layers.
Toughened glass
dead air space with tempered glass]] Toughened glass (or tempered glass) a type of safety glass that has increased strength and a tendency to shatter in small, square pieces when broken. It is typically used in unframed assemblies such as frameless doors and in structurally loaded applications. Using toughened glass could pose a security risk in some situations due to the tendency the glass has to shatter utterly upon edge impact. Toughened glass is typically assumed to be six times the strength of annealed glass. This is because any surface flaws tend to be pressed closed by the retained compressive forces, while the core layer remains relatively free of the defects which could cause a crack to begin. However, this strength comes with a penalty. Due to the balanced stresses in the glass, any damage to the glass edges will result in the glass shattering into thumbnail sized pieces. Because of this, the glass must be cut to size before toughening and cannot be re-worked once toughened. Also, ironically, the toughened glass surface is not as hard as annealed glass and is more susceptible to scratching. Toughened glass is made from annealed glass via a thermal tempering process. The glass is cut to the required size and any required processing (such as polishing the edges or drilling holes in the glass) is carried out before the toughening process starts. Toughened glass can also be made by a chemical process where typically some of the sodium ions at the surface are replaced with potassium ions. This was used on some fighter aircraft canopies. The glass is placed onto a roller table, taking it through a furnace which heats it to above its annealing point of 600 °C. The glass is then rapidly cooled with forced draughts of air. This rapidly cools the glass surface below its annealing point, causing it to harden and contract, while the inner portion of the glass remains free to flow for a short time. The final contraction of the inner layer induces compressive stresses in the surface of the glass balanced by tensile stresses in the body of the glass. The pattern of cooling can be revealed by observing the glass with polarized light. Though the underlying mechanism was not known at the time, the effects of "tempering" glass have been known for centuries. In the 1640s, Prince Rupert of Bavaria (1619–1682), who was grandson of James I of England, and nephew of Charles I, brought the discovery of what are now known as "Prince Rupert's Drops" to the attention of the King. These are remarkable teardrop shaped bits of glass which are produced by allowing a molten drop of glass to fall into a bucket of water, thereby rapidly cooling it. The very rapid cooling produces tremendously high tensile stress in the glass giving it unusual qualities such as the ability to withstand a blow from a hammer on the bulbous end without breaking. However, even the smallest scratch on the "tail" of the drop will allow the large amount of potential energy contained in the internal stresses of the glass to be released, causing it to explosively shatter so thoroughly that it is converted to a fine powder. The drops were often used as a practical joke, as the King would tell a subject to hold the bulb end in the palm of their hand while he broke the tip, producing a small explosion in the surprised person's hand. A video of the technique can be seen here [http://www.museumofglass.org/VHS/video/rupert_drop_demo_bb.html].
Laminated glass
Prince Rupert's Drop Laminated glass is a type of safety glass that holds together when shattered. In the event of breakage, it is held in place by an interlayer of PVB between its two or more layers of glass. The interlayer keeps the layers of glass bonded even when broken, and its high strength prevents the glass from breaking up into large sharp pieces. This produces a characteristic "spiderweb" cracking pattern when the impact is not enough to completely pierce the glass. Laminated glass is normally used when there is a possibility of human impact or where the glass could fall if shattered. Shopfront glazing and windshields are typically laminated glasses. The PVB interlayer also gives the glass a much higher sound insulation rating, due to the damping effect, and also blocks 99% of transmitted UV light. Using toughened glass on windshields would be a problem when a small stone hits the windshield at speed, if it were toughened and the stone hit with enough force the whole windshield would shatter into the small squares making visiblilty difficult and likely the wind would blow the small squares into the driver and passengers. Laminated glass was invented in 1903 by the French chemist Edouard Benedictus, inspired by a laboratory accident. A glass flask had become coated with the plastic cellulose nitrate and when dropped shattered but did not break into pieces. Benedictus fabricated a glass-plastic composite to reduce injuries in car accidents. However, it was not immediately adopted by automobile manufacturers, and the first widespread use of laminated glass was in the eyepieces of gas masks during World War I. Today, laminated glass is produced by bonding two or more layers of ordinary annealed glass together with a plastic interlayer, usually polyvinyl butyral (PVB). The PVB is sandwiched by the glass which is passed through rollers to expel any air pockets and form the initial bond then heated to around 70 °C in a pressuredized oil bath. The tint at the top of some car windshields is in the PVB. A typical laminated makeup would be 3 mm glass / 0.38 mm interlayer / 3 mm glass. This gives a final product that would be referred to as 6.38 laminated glass. Multiple laminates and thicker glass increases the strength. Bulletproof glass panels, made up of thick glass often toughened and several interlayers often thicker than that in windshields, can be as thick as 50 mm. A similar glass is often used in airliners on the front windows, often three sheets of 6mm toughened with thick PVB between them.
Low-emissivity glass
Metallic-based coatings applied to one or more surfaces of insulated glass can greatly decrease the glass unit's ability to transfer thermal energy, resulting in more efficient windows.
Self-cleaning glass
A recent innovation is so-called self-cleaning glass, aimed at building, automotive and other technical applications. A 50 nanometre coating of titanium dioxide on the outer surface of glass introduces two mechanisms which lead to the self-cleaning property. The first is a photocatalytic effect, in which ultra-violet rays catalyse the breakdown of organic compounds on the window surface; the second is a hydrophilic effect in which water is attracted to the surface of the glass, forming a thin sheet which washes away the broken-down organic compounds.
Evacuated glazing
Another recent innovation for insulated glazing is evacuated glass, which as yet is produced commercially only in Japan. The extreme thinness of evacuated glazing offers many new architectural possibilities, particularly in building conservation and historicist architecture, where evacuated glazing can replace traditional (much less energy-efficient) single glazing. An evacuated glazing unit is made by sealing the edges of two glass sheets, typically by using a solder glass, and evacuating the space inside with a vacuum pump. The evacuated space between the two sheets can be very shallow and yet be a good insulator, yielding insulative window glass with nominal thicknesses as low as 6mm overall. The reasons for this low thickness are deceptively complex, but the potential insulation is good essentially because there can be no convection or gasesous conduction in a vacuum. Unfortunately, evacuated glazing does have some disadvantages; its manufacture is complicated and difficult. For example, a necessary stage in the manufacture of evacuated glazing is "offgassing"; that is, heating it liberates any gasses adsorbed on the inner surfaces, which could otherwise later escape and destroy the vacuum. This heating process currently means that evacuated glazing cannot be toughened or heat-strengthened. If an evacuated safety glass is required, the glass must be laminated. The high temperatures necessary for offgassing also tend to destroy the highly effective "soft" low-emissivity coatings that are often applied to one or both of the internal surfaces (i.e. the ones facing the air gap) of other forms of modern insulative glazing, in order to prevent loss of heat through infra-red radiation. Slightly less effective "hard" coatings are still suitable for evacuated glazing, however. Furthermore, because of the atmospheric pressure present on the outside of an evacuated glazing unit, its two glass sheets must somehow be held apart in order to prevent them flexing together and touching each other, which would defeat the object of evacuating the unit. The task of holding the panes apart is performed by a grid of spacers, which typically consist of small stainless steel discs that are placed around 20mm apart. The spacers are small enough that they are visible only at very close distances, typically up to 1m. However, the fact that the spacers will conduct some heat often leads in cold weather to the formation of temporary, grid-shaped patterns on the surface of an evacuated window, consisting either of small circles of interior condensation centered around the spacers, where the glass is slightly colder then average, or, when there is dew outside, small circles on the exterior face of the glass, in which the dew is absent because the spacers make the glass near them slightly warmer. The conduction of heat between the panes, caused by the spacers, tends to limit evacuated glazing's overall insulative effectiveness. Nevertheless, evacuated glazing is still as insulative as much thicker conventional double glazing and tends to be stronger, since the two constituent glass sheets are pressed together by the atmosphere, and hence react practically as one thick sheet to bending forces. Evacuated glazing also offers very good sound insulation in comparison with other popular types of window glazing.
Glass as a liquid
One common belief is that glass is a super-cooled liquid of practically infinite viscosity when at room temperature. Supporting evidence for this position is that old windows are often thicker at the bottom than at the top. It is then assumed that the glass was once uniform, but has flowed to its new shape. One possible source of this belief is that when panes of glass were commonly made by glassblowers, the technique that was used was to spin molten glass so as to create a round, mostly flat and even plate (the Crown glass process, described above). This plate was then cut to fit a window. The pieces were not, however, absolutely flat; the edges of the disk would be thicker because of centripetal forces. When actually installed in a window frame, the glass would be placed thicker side down for the sake of stability. Also, the sparkle is greater and the visual effect stronger when the thicker side is down. There is anecdotal evidence that occasionally such glass has been found thinner side down, as would be caused by carelessness at the time of installation. Writing in the American Journal of Physics, physicist Edgar D. Zanotto states "...the predicted relaxation time for GeO₂ at room temperature is 10³² years. Hence, the relaxation period (characteristic flow time) of cathedral glasses would be even longer" (Am. J. Phys, 66(5):392-5, May 1998). In layperson's terms, he wrote that glass at room temperature is very strongly on the solid side of the spectrum from solids to liquids.
Evidence against glass flow

- If medieval glass has flowed perceptibly, then ancient Roman and Egyptian objects should have flowed proportionately more—but this is not observed.
- If glass flows at a rate that allows changes to be seen with the naked eye after centuries, then changes in optical telescope mirrors should be observable (by interferometry) in a matter of days—but this also is not observed. Similarly, it should not be possible to see Newton's rings between decade-old fragments of window glass—but this can in fact be quite easily done.
- Likewise, precision optical lenses and mirrors used in microscopes and telescopes should gradually deform and lose focus. This is also not observed.
See also

- Aluminum Oxynitride
- Art glass
- Beveled glass
- Bulletproof glass
- Fiberglass
- Magnifying glass
- Prince Rupert's Drops
- Pyrex
- Stained glass
- Glass-reinforced plastic
References

- "Do Cathedral Glasses Flow?" Am. J. Phys., 66 (May 1998), pp 392–396
- Noel C. Stokes; The Glass and Glazing Handbook; Standards Australia; SAA HB125-1998
- [http://www.oxbowbooks.com/bookinfo.cfm/ID/36444/MID/11027 Glass Beads from Anglo-Saxon Graves: A Study on the Provenance and Chronology of Glass Beads from Anglo-Saxon Graves, Based on Visual Examination] by Birte Brugmann
External links

- [http://www.cmog.org/ Corning Museum of Glass], especially Research, Teach, and Learn section.
- [http://math.ucr.edu/home/baez/physics/General/Glass/glass.html Is glass liquid or solid?] by Philip Gibbs on the spr USENET physics FAQ
- [http://dwb.unl.edu/Teacher/NSF/C01/C01Links/www.ualberta.ca/~bderksen/windowpane.html Antique windowpanes and the flow of supercooled liquids]
- [http://dwb.unl.edu/Teacher/NSF/C01/C01Links/www.ualberta.ca/~bderksen/florin.html article on the non-flowness of glass]
- [http://tafkac.org/science/glass.flow/ Page devoted to the AFU glass flow controversy, with links to citations]
- [http://www.glassnotes.com/WindowPanes.html Page stating that glass does not flow]
- [http://1st.glassman.com/articles/glasscolouring.html Substances used in the Making of Colored Glass]
- [http://www.activglass.com/index_eng.htm Self-cleaning glass] - Product information from Pilkington.
- [http://www.straightdope.com/classics/a1_120.html The Straight Dope article on glass], article discusses why glass is a liquid treated as a solid
- [http://www.waste-management-information.org.uk Recycling Glass - Waste Management Issues]
- Category:Glass art ms:Kaca ja:ガラス simple:Glass th:กระจก

Fused silica

Fused quartz is a man-made material manufactured principally from sand. It is non-crystalline, and in a high purity state is a useful material for high performance fluid and gas delivery. Its mechanical and thermal properties are superior to that of glass due to its purity [or rather, its lack of impurities]. For these reasons, it finds use in situations such as semiconductor fabrication and laboratory equipment. The name quartz comes from the mineral of the same basic chemical composition. The natural material is crystalline.

Chemistry

Fused quartz is composed of silicon dioxide, which is also called silica (Si O₂). This material was originally called fumed silica because the high purity manufacturing process involves chemical gassification of silicon, oxidation of this gas to silicon dioxide, and thermal fusion of the resulting dust. From this final melting process it also gets the name fused silica.

Applications

Specially prepared fused silica is also the key starting material used to make optical fiber for telecommunications. Because of its strength and high melting point (compared to ordinary glass), fused silica is used as the envelope of halogen lamps, which must operate at a high envelope temperature to achieve their combination of high brightness and long life. The combination of strength, thermal stability, and UV transparency makes it an excellent substrate for projection masks for photolithography. Fused quartz has nearly ideal properties for fabricating first surface mirrors such as those used in telescopes. The material behaves in a predictable way and allows the optician to put a very smooth polish onto the surface and produce the desired figure with fewer testing iterations.

Physical Properties

The extremely low coefficient of thermal expansion accounts for its remarkable ability to undergo large, rapid temperature changes without cracking. Fused silica is transparent to ultraviolet light (down to around 170 nm where it has a transmittance of about 50% and a transmittance of only a few percent at 160 nm for an optic with thickness of about 1 cm) and near infrared light (up to around 2.5 µm), and has many uses in technical optics as a consequence.

Properties of Clear Fused Quartz

(Based on information in Fused Quartz Catalogue Q-7A, General Electric Company)
- Density: 2.203 g/cm³
- Hardness: 7 (Modified Scale); 5.3–6.5 (Mohs Scale)
- Tensile strength: 48.3 MPa
- Compressive strength: >1.1 GPa
- Bulk modulus: ~37 GPa
- Rigidity modulus: 31 GPa
- Young's modulus: 71.7 GPa
- Poisson's ratio: 0.16
- Coefficient of thermal expansion: 5.5×10^-7 cm/(cm·K) (average from 20 °C to 320 °C)
- Thermal conductivity: 1.3 W/(m·K)
- Heat capacity: 45.3 J/mol
- Softening point: c. 1665 °C
- Annealing point: c. 1140 °C
- Strain point: 1070 °C
- Electrical resistivity: >10¹⁸ Ω×m
- Dielectric constant: 3.75 at 20 °C 1 MHz
- Dielectric loss factor: less than 0.0004 at 20 °C 1 MHz
- Index of refraction: 1.4585

Alumina

Aluminium oxide or aluminum oxide is a chemical compound of aluminium and oxygen with the chemical formula ₂₃. It is also commonly referred to as alumina in the mining, ceramic, and materials science communities. Alumina is generally available in two concentrations: 99.5% and 96%. Aluminium oxide is responsible for metallic aluminium's resistance to weathering. Metallic aluminium is very reactive with atmospheric oxygen, and a thin layer of aluminium oxide quickly forms on any exposed aluminium surface. This layer protects the metal from further oxidation. The thickness and properties of this oxide layer can be enhanced using a process called anodising. A number of alloys, such as aluminum bronzes, exploit this property by including a proportion of aluminum in the alloy to enhance corrosion resistance. Aluminium oxide is an excellent thermal and electrical insulator. In its crystalline form, called corundum, its hardness makes it suitable for use as an abrasive and as a component in cutting tools. Powdered aluminium oxide is frequently used as a medium for chromatography. The gems ruby and sapphire are mostly aluminium oxide, given their characteristic colors by trace impurities. In August 2004, scientists in the United States working for 3M developed a technique for making an alloy of alumina and rare earth elements to produce a strong glass called transparent alumina. Aluminium oxide was taken off the EPA's chemicals lists in 1988.

Industrial Fabrication Process

Aluminium oxide is the main component of bauxite, the principal ore of aluminium. Industrially, bauxite is purified to aluminium oxide via the Bayer process, and then converted to aluminium metal in the Hall-Heroult process. The bauxite ore is made up of impure Al₂O₃ + Fe₂O₃ + SiO₂. This is then purified by the Bayer Process: Al₂O₃ + 3H₂O + 2NaOH --(heated)--> 2NaAl(OH)₄. The Fe₂O₃ does not dissolve in the base. The SiO₂ dissolves as silicate Si(OH)₆^-6. Upon filtering, Fe₂O₃ is removed. With the addition of an acid, Al(OH)₃ precipitates. The silicate remains in solution. Then, Al(OH)₃ --(heated)--> Al₂O₃ + 3H₂O. The Al₂O₃ is of course, alumina.

External links

- [http://www.ilo.org/public/english/protection/safework/cis/products/icsc/dtasht/_icsc03/icsc0351.htm International Chemical Safety Card 0351]
- [http://physicsweb.org/article/news/8/8/9 PhysicsWeb article on Transparent alumina] Category:Aluminium compounds Category:Oxides ja:酸化アルミニウム