Potassium nitrate is a chemical compound with the chemical formula KNO3. It is an ionic salt of potassium ions K+ and nitrate ions NO3−, is therefore an alkali metal nitrate, it occurs in nature as niter. It is a source of nitrogen. Potassium nitrate is one of several nitrogen-containing compounds collectively referred to as saltpeter or saltpetre. Major uses of potassium nitrate are in fertilizers, tree stump removal, rocket propellants and fireworks, it is one of the major constituents of gunpowder. In processed meats, potassium nitrate generates a pink color. Potassium nitrate, because of its early and global use and production, has many names. Hebrew and Egyptian words for it had the consonants n-t-r, indicating cognation in the Greek nitron, Latinised to nitrum or nitrium. Thence Old French had Middle English nitre. By the 15th century, Europeans referred to it as saltpeter and as nitrate of potash, as the chemistry of the compound was more understood; the Arabs called it "Chinese snow". It was called "Chinese salt" by the Iranians/Persians or "salt from Chinese salt marshes".
Potassium nitrate has an orthorhombic crystal structure at room temperature, which transforms to a trigonal system at 129 °C. Potassium nitrate is moderately soluble in water; the aqueous solution is neutral, exhibiting pH 6.2 at 14 °C for a 10% solution of commercial powder. It is not hygroscopic, absorbing about 0.03% water in 80% relative humidity over 50 days. It is not poisonous. Between 550–790 °C, potassium nitrate reaches a temperature dependent equilibrium with potassium nitrite: 2 KNO3 ⇌ 2 KNO2 + O2 The earliest known complete purification process for potassium nitrate was outlined in 1270 by the chemist and engineer Hasan al-Rammah of Syria in his book al-Furusiyya wa al-Manasib al-Harbiyya. In this book, al-Rammah describes first the purification of barud by boiling it with minimal water and using only the hot solution the use of potassium carbonate to remove calcium and magnesium by precipitation of their carbonates from this solution, leaving a solution of purified potassium nitrate, which could be dried.
This was used for the manufacture of gunpowder and explosive devices. The terminology used by al-Rammah indicated a Chinese origin for the gunpowder weapons about which he wrote. At least as far back as 1845, Chilean saltpeter deposits were exploited in Chile and California, USA. A major natural source of potassium nitrate was the deposits crystallizing from cave walls and the accumulations of bat guano in caves. Extraction is accomplished by immersing the guano in water for a day and harvesting the crystals in the filtered water. Traditionally, guano was the source used in Laos for the manufacture of gunpowder for Bang Fai rockets; the most exhaustive discussion of the production of this material is the 1862 LeConte text. He was writing with the express purpose of increasing production in the Confederate States to support their needs during the American Civil War. Since he was calling for the assistance of rural farming communities, the descriptions and instructions are both simple and explicit.
He details the "French Method", along with several variations, as well as a "Swiss method". N. B. Many references have been made to a method using only straw and urine, but there is no such method in this work. Turgot and Lavoisier created the Régie des Poudres et Salpêtres few years before the French Revolution. Niter-beds were prepared by mixing manure with either mortar or wood ashes, common earth and organic materials such as straw to give porosity to a compost pile 4 feet high, 6 feet wide, 15 feet long; the heap was under a cover from the rain, kept moist with urine, turned to accelerate the decomposition finally leached with water after one year, to remove the soluble calcium nitrate, converted to potassium nitrate by filtering through potash. LeConte describes a process using only urine and not dung. Urine is collected directly, in a sandpit under a stable; the sand itself is dug out and leached for nitrates which were converted to potassium nitrate using potash, as above. From 1903 until the World War I era, potassium nitrate for black powder and fertilizer was produced on an industrial scale from nitric acid produced using the Birkeland–Eyde process, which used an electric arc to oxidize nitrogen from the air.
During World War I the newly industrialized Haber process was combined with the Ostwald process after 1915, allowing Germany to produce nitric acid for the war after being cut off from its supplies of mineral sodium nitrates from Chile. Potassium nitrate can be made by combining potassium hydroxide. NH4NO3 + KOH → NH3 + KNO3 + H2O An alternative way of producing potassium nitrate without a by-product of ammonia is to combine ammonium nitrate, found in instant ice packs, potassium chloride obtained as a sodium-free salt substitute. NH4NO3 + KCl → NH4Cl + KNO3 Potassium nitrate can be produced by neutralizing nitric acid with potassium hydroxide; this reaction is exothermic. KOH + HNO3 → KNO3 + H2O On industrial scale it is prepared by the double displacement reaction between sodium nitrate and pota
An oxide is a chemical compound that contains at least one oxygen atom and one other element in its chemical formula. "Oxide" itself is the dianion of an O2 -- atom. Metal oxides thus contain an anion of oxygen in the oxidation state of −2. Most of the Earth's crust consists of solid oxides, the result of elements being oxidized by the oxygen in air or in water. Hydrocarbon combustion affords the two principal carbon oxides: carbon monoxide and carbon dioxide. Materials considered pure elements develop an oxide coating. For example, aluminium foil develops a thin skin of Al2O3 that protects the foil from further corrosion. Individual elements can form multiple oxides, each containing different amounts of the element and oxygen. In some cases these are distinguished by specifying the number of atoms as in carbon monoxide and carbon dioxide, in other cases by specifying the element's oxidation number, as in iron oxide and iron oxide. Certain elements can form many different oxides, such as those of nitrogen.
Due to its electronegativity, oxygen forms stable chemical bonds with all elements to give the corresponding oxides. Noble metals are prized because they resist direct chemical combination with oxygen, substances like gold oxide must be generated by indirect routes. Two independent pathways for corrosion of elements are oxidation by oxygen; the combination of water and oxygen is more corrosive. All elements burn in an atmosphere of oxygen or an oxygen-rich environment. In the presence of water and oxygen, some elements— sodium—react to give the hydroxides. In part, for this reason and alkaline earth metals are not found in nature in their metallic, i.e. native, form. Cesium is so reactive with oxygen that it is used as a getter in vacuum tubes, solutions of potassium and sodium, so-called NaK are used to deoxygenate and dehydrate some organic solvents; the surface of most metals consists of hydroxides in the presence of air. A well-known example is aluminium foil, coated with a thin film of aluminium oxide that passivates the metal, slowing further corrosion.
The aluminum oxide layer can be built to greater thickness by the process of electrolytic anodizing. Though solid magnesium and aluminum react with oxygen at STP—they, like most metals, burn in air, generating high temperatures. Finely grained powders of most metals can be dangerously explosive in air, they are used in solid-fuel rockets. In dry oxygen, iron forms iron oxide, but the formation of the hydrated ferric oxides, Fe2O3−x2x, that comprise rust requires oxygen and water. Free oxygen production by photosynthetic bacteria some 3.5 billion years ago precipitated iron out of solution in the oceans as Fe2O3 in the economically important iron ore hematite. Oxides have a range of different structures, from individual molecules to polymeric and crystalline structures. At standard conditions, oxides may range from solids to gases. Oxides of most metals adopt polymeric structures; the oxide links three metal atoms or six metal atoms. Because the M-O bonds are strong and these compounds are crosslinked polymers, the solids tend to be insoluble in solvents, though they are attacked by acids and bases.
The formulas are deceptively simple. Many are nonstoichiometric compounds; some important gaseous oxides. Examples of molecular oxides are carbon monoxide. All simple oxides of nitrogen are molecular, e.g. NO, N2O, NO2 and N2O4. Phosphorus pentoxide is a more complex molecular oxide with a deceptive name, the real formula being P4O10; some polymeric oxides depolymerize when heated to give molecules, examples being selenium dioxide and sulfur trioxide. Tetroxides are rare; the more common examples: ruthenium tetroxide, osmium tetroxide, xenon tetroxide. Many oxyanions are known, such as polyoxometalates. Oxycations are rarer, some examples being nitrosonium and uranyl. Of course many compounds are known with other groups. In organic chemistry, these include many related carbonyl compounds. For the transition metals, many oxo complexes are known as well as oxyhalides. Conversion of a metal oxide to the metal is called reduction; the reduction can be induced by many reagents. Many metal oxides convert to metals by heating.
Metals are "won" from their oxides by chemical reduction, i.e. by the addition of a chemical reagent. A common and cheap reducing agent is carbon in the form of coke; the most prominent example is that of iron ore smelting. Many reactions are involved, but the simplified equation is shown as: 2 Fe2O3 + 3 C → 4 Fe + 3 CO2Metal oxides can be reduced by organic compounds; this redox process is the basis for many important transformations in chemistry, such as the detoxification of drugs by the P450 enzymes and the production of ethylene oxide, converted to antifreeze. In such systems, the metal center transfers an oxide ligand to the organic compound followed by regeneration of the metal oxide by oxygen in the air. Metals that are lower in the reactivity series can be reduced by heating alone. For example, silver oxide decomposes at 200 °C: 2 Ag2O → 4 Ag + O2 Metals that are more reactive displace the oxide of the metals that are less reactive. For example, zinc is more reactive than copper, so it displaces copper oxide to form zinc oxide: Zn + CuO → ZnO + Cu Apart from metals, hydrogen can displace metal oxides to form hydrogen oxide
Iron oxide or ferric oxide is the inorganic compound with the formula Fe2O3. It is one of the three main oxides of iron, the other two being iron oxide, rare; as the mineral known as hematite, Fe2O3 is the main source of iron for the steel industry. Fe2O3 is attacked by acids. Iron oxide is called rust, to some extent this label is useful, because rust shares several properties and has a similar composition. To a chemist, rust is considered an ill-defined material, described as hydrated ferric oxide. Fe2O3 can be obtained in various polymorphs. In the main ones, α and γ, iron adopts octahedral coordination geometry; that is, each Fe center is bound to six oxygen ligands. Α-Fe2O3 is the most common form. It occurs as the mineral hematite, mined as the main ore of iron, it is antiferromagnetic below ~260 K, exhibits weak ferromagnetism between 260 K and the Néel temperature, 950 K. It is easy to prepare using both thermal precipitation in the liquid phase, its magnetic properties are dependent on many factors, e.g. pressure, particle size, magnetic field intensity.
Γ-Fe2O3 has a cubic structure. It is metastable and converted from the alpha phase at high temperatures, it occurs as the mineral maghemite. It is ferromagnetic and finds application in recording tapes, although ultrafine particles smaller than 10 nanometers are superparamagnetic, it can be prepared by thermal dehydratation of gamma iron oxide-hydroxide. Another method involves the careful oxidation of iron oxide; the ultrafine particles can be prepared by thermal decomposition of iron oxalate. Several other phases have been claimed; the β-phase is cubic body-centered, at temperatures above 500 °C converts to alpha phase. It can be prepared by reduction of hematite by carbon, pyrolysis of iron chloride solution, or thermal decomposition of iron sulfate; the epsilon phase is rhombic, shows properties intermediate between alpha and gamma, may have useful magnetic properties. Preparation of the pure epsilon phase has proven challenging due to contamination with alpha and gamma phases. Material with a high proportion of epsilon phase can be prepared by thermal transformation of the gamma phase.
This phase is metastable, transforming to the alpha phase at between 500 and 750 °C. Can be prepared by oxidation of iron in an electric arc or by sol-gel precipitation from iron nitrate. Additionally at high pressure an amorphous form is claimed. Recent research has revealed epsilon iron oxide in ancient Chinese Jian ceramic glazes, which may provide insight into ways to produce that form in the lab. Several hydrates of Iron oxide exists; when alkali is added to solutions of soluble Fe salts, a red-brown gelatinous precipitate forms. This is not Fe3, but Fe2O3·H2O. Several forms of the hydrated oxide of Fe exist as well; the red lepidocrocite γ-FeOH, occurs on the outside of rusticles, the orange goethite, which occurs internally in rusticles. When Fe2O3·H2O is heated, it loses its water of hydration. Further heating at 1670 K converts Fe2O3 to black Fe3O4, known as the mineral magnetite. FeOH is soluble in acids, giving 3+. In concentrated aqueous alkali, Fe2O3 gives 3−; the most important reaction is its carbothermal reduction, which gives iron used in steel-making: Fe2O3 + 3 CO → 2 Fe + 3 CO2Another redox reaction is the exothermic thermite reaction with aluminium.
2 Al + Fe2O3 → 2 Fe + Al2O3This process is used to weld thick metals such as rails of train tracks by using a ceramic container to funnel the molten iron in between two sections of rail. Thermite is used in weapons and making small-scale cast-iron sculptures and tools. Partial reduction with hydrogen at about 400 °C produces magnetite, a black magnetic material that contains both Fe and Fe: 3 Fe2O3 + H2 → 2 Fe3O4 + H2OIron oxide is insoluble in water but dissolves in strong acid, e.g. hydrochloric and sulfuric acids. It dissolves well in solutions of chelating agents such as EDTA and oxalic acid. Heating iron oxides with other metal oxides or carbonates yields materials known as ferrates: ZnO + Fe2O3 → Zn2 Iron oxide is a product of the oxidation of iron, it can be prepared in the laboratory by electrolyzing a solution of sodium bicarbonate, an inert electrolyte, with an iron anode: 4 Fe + 3 O2 + 2 H2O → 4 FeOThe resulting hydrated iron oxide, written here as FeOH, dehydrates around 200 °C. 2 FeO → Fe2O3 + H2O The overwhelming application of iron oxide is as the feedstock of the steel and iron industries, e.g. the production of iron and many alloys.
A fine powder of ferric oxide is known as "jeweler's rouge", "red rouge", or rouge. It is used to put the final polish on metallic jewelry and lenses, as a cosmetic. Rouge cuts more than some modern polishes, such as cerium oxide, but is still used in optics fabrication and by jewelers for the superior finish it can produce; when polishing gold, the rouge stains the gold, which contributes to the appearance of the finished piece. Rouge is sold as a powder, laced on polishing cloths, or solid bar. Other polishing compounds are often called "rouge" when they do not contain iron oxide. Jewelers remove the residual rouge on jewelry by use of ultrasonic cleaning. Products sold as "stropping compound" are applied to a leather stro
Close-packing of equal spheres
In geometry, close-packing of equal spheres is a dense arrangement of congruent spheres in an infinite, regular arrangement. Carl Friedrich Gauss proved that the highest average density – that is, the greatest fraction of space occupied by spheres – that can be achieved by a lattice packing is π 3 2 ≃ 0.74048. The same packing density can be achieved by alternate stackings of the same close-packed planes of spheres, including structures that are aperiodic in the stacking direction; the Kepler conjecture states that this is the highest density that can be achieved by any arrangement of spheres, either regular or irregular. This conjecture was proven by T. C. Hales. Highest density is known only in case of 2, 3, 8 and 24 dimensions. Many crystal structures are based on a close-packing of a single kind of atom, or a close-packing of large ions with smaller ions filling the spaces between them; the cubic and hexagonal arrangements are close to one another in energy, it may be difficult to predict which form will be preferred from first principles.
There are two simple regular lattices. They are called face-centered hexagonal close-packed, based on their symmetry. Both are based upon sheets of spheres arranged at the vertices of a triangular tiling; the fcc lattice is known to mathematicians as that generated by the A3 root system. The problem of close-packing of spheres was first mathematically analyzed by Thomas Harriot around 1587, after a question on piling cannonballs on ships was posed to him by Sir Walter Raleigh on their expedition to America. Cannonballs were piled in a rectangular or triangular wooden frame, forming a three-sided or four-sided pyramid. Both arrangements produce a face-centered cubic lattice – with different orientation to the ground. Hexagonal close-packing would result in a six-sided pyramid with a hexagonal base; the cannonball problem asks which flat square arrangements of cannonballs can be stacked into a square pyramid. Édouard Lucas formulated the problem as the Diophantine equation ∑ n = 1 N n 2 = M 2 or 1 6 N = M 2 and conjectured that the only solutions are N = 1, M = 1, N = 24, M = 70.
Here N is the number of layers in the pyramidal stacking arrangement and M is the number of cannonballs along an edge in the flat square arrangement. In both the fcc and hcp arrangements each sphere has twelve neighbors. For every sphere there is one gap surrounded by six spheres and two smaller gaps surrounded by four spheres; the distances to the centers of these gaps from the centers of the surrounding spheres is √3⁄2 for the tetrahedral, √2 for the octahedral, when the sphere radius is 1. Relative to a reference layer with positioning A, two more positionings B and C are possible; every sequence of A, B, C without immediate repetition of the same one is possible and gives an dense packing for spheres of a given radius. The most regular ones are fcc = ABC ABC ABC... hcp = AB AB AB AB.... There is an uncountably infinite number of disordered arrangements of planes that are sometimes collectively referred to as "Barlow packings", after crystallographer William BarlowIn close-packing, the center-to-center spacing of spheres in the xy plane is a simple honeycomb-like tessellation with a pitch of one sphere diameter.
The distance between sphere centers, projected on the z axis, is: pitch Z = 6 ⋅ d 3 ≈ 0.816 496 58 d, where d is the diameter of a sphere. The coordination number of hcp and fcc is 12 and their atomic packing factors are equal to the number mentioned above, 0.74. When forming any sphere-packing lattice, the first fact to notice is that whenever two spheres touch a straight line may be drawn from the center of one sphere to the center of the other intersecting the point of contact; the distance between the centers along the shortest path namely that straight line will therefore be r1 + r2 where r1 is the radius of the first sphere and r2 is the radius of the second. In close packing all of the spheres share a common radius, r; therefore two centers would have a distance 2r. To form an A-B-A-B-... hexagonal close packing of spheres, the coordinate points of the lattice will be the spheres' centers. Suppose, the goal is to fill a box with spheres according to hcp; the box would be placed on the x-y-z coordinate space.
First form a row of spheres. The centers will all lie on a straight line, their x-coordinate will vary by 2r since the distance between each center of the spheres are touching is 2r. The y-coordinate and z-coordinate wil
A ceramic is a solid material comprising an inorganic compound of metal, non-metal or metalloid atoms held in ionic and covalent bonds. Common examples are earthenware and brick; the crystallinity of ceramic materials ranges from oriented to semi-crystalline and completely amorphous. Most fired ceramics are either vitrified or semi-vitrified as is the case with earthenware and porcelain. Varying crystallinity and electron composition in the ionic and covalent bonds cause most ceramic materials to be good thermal and electrical insulators. With such a large range of possible options for the composition/structure of a ceramic, the breadth of the subject is vast, identifiable attributes are difficult to specify for the group as a whole. General properties such as high melting temperature, high hardness, poor conductivity, high moduli of elasticity, chemical resistance and low ductility are the norm, with known exceptions to each of these rules. Many composites, such as fiberglass and carbon fiber, while containing ceramic materials, are not considered to be part of the ceramic family.
The earliest ceramics made by humans were pottery objects or figurines made from clay, either by itself or mixed with other materials like silica and sintered in fire. Ceramics were glazed and fired to create smooth, colored surfaces, decreasing porosity through the use of glassy, amorphous ceramic coatings on top of the crystalline ceramic substrates. Ceramics now include domestic and building products, as well as a wide range of ceramic art. In the 20th century, new ceramic materials were developed for use in advanced ceramic engineering, such as in semiconductors; the word "ceramic" comes from the Greek word κεραμικός, "of pottery" or "for pottery", from κέραμος, "potter's clay, pottery". The earliest known mention of the root "ceram-" is the Mycenaean Greek ke-ra-me-we, "workers of ceramics", written in Linear B syllabic script; the word "ceramic" may be used as an adjective to describe a material, product or process, or it may be used as a noun, either singular, or, more as the plural noun "ceramics".
A ceramic material is an inorganic, non-metallic crystalline oxide, nitride or carbide material. Some elements, such as carbon or silicon, may be considered ceramics. Ceramic materials are brittle, strong in compression, weak in shearing and tension, they withstand chemical erosion that occurs in other materials subjected to acidic or caustic environments. Ceramics can withstand high temperatures, ranging from 1,000 °C to 1,600 °C. Glass is not considered a ceramic because of its amorphous character. However, glassmaking involves several steps of the ceramic process, its mechanical properties are similar to ceramic materials. Traditional ceramic raw materials include clay minerals such as kaolinite, whereas more recent materials include aluminium oxide, more known as alumina; the modern ceramic materials, which are classified as advanced ceramics, include silicon carbide and tungsten carbide. Both are valued for their abrasion resistance and hence find use in applications such as the wear plates of crushing equipment in mining operations.
Advanced ceramics are used in the medicine, electronics industries and body armor. Crystalline ceramic materials are not amenable to a great range of processing. Methods for dealing with them tend to fall into one of two categories – either make the ceramic in the desired shape, by reaction in situ, or by "forming" powders into the desired shape, sintering to form a solid body. Ceramic forming techniques include shaping by hand, slip casting, tape casting, injection molding, dry pressing, other variations. Noncrystalline ceramics, being glass, tend to be formed from melts; the glass is shaped when either molten, by casting, or when in a state of toffee-like viscosity, by methods such as blowing into a mold. If heat treatments cause this glass to become crystalline, the resulting material is known as a glass-ceramic used as cook-tops and as a glass composite material for nuclear waste disposal; the physical properties of any ceramic substance are a direct result of its crystalline structure and chemical composition.
Solid-state chemistry reveals the fundamental connection between microstructure and properties such as localized density variations, grain size distribution, type of porosity and second-phase content, which can all be correlated with ceramic properties such as mechanical strength σ by the Hall-Petch equation, toughness, dielectric constant, the optical properties exhibited by transparent materials. Ceramography is the art and science of preparation and evaluation of ceramic microstructures. Evaluation and characterization of ceramic microstructures is implemented on similar spatial scales to that used in the emerging field of nanotechnology: from tens of angstroms to tens of micrometers; this is somewhere between the minimum wavelength of visible light and the resolution limit of the naked eye. The microstructure includes most grains, secondary phases, grain boundaries, micro-
Magnetic tape is a medium for magnetic recording, made of a thin, magnetizable coating on a long, narrow strip of plastic film. It was developed in Germany based on magnetic wire recording. Devices that record and play back audio and video using magnetic tape are tape recorders and video tape recorders respectively. A device that stores computer data on magnetic tape is known as a tape drive. Magnetic tape revolutionized reproduction and broadcasting, it allowed radio, which had always been broadcast live, to be recorded for or repeated airing. It allowed gramophone records to be recorded in multiple parts, which were mixed and edited with tolerable loss in quality, it was a key technology in early computer development, allowing unparalleled amounts of data to be mechanically created, stored for long periods, accessed. In recent decades, other technologies have been developed that can perform the functions of magnetic tape. In many cases, these technologies have replaced tape. Despite this, innovation in the technology continues, Sony and IBM continue to produce new magnetic tape drives.
Over time, magnetic tape made in the 1970s and 1980s can suffer from a type of deterioration called sticky-shed syndrome. It can render the tape unusable; the oxide side of a tape is the surface. This is the side that stores the information, the opposite side is a substrate to give the tape strength and flexibility; the name originates from the fact that the magnetic side of most tapes is made of iron oxide, though chromium is used for some tapes. An adhesive binder between the oxide and the substrate holds the two sides together. In all tape formats, a tape drive uses motors to wind the tape from one reel to another, passing over tape heads to read, write or erase as it moves. Magnetic tape was invented for recording sound by Fritz Pfleumer in 1928 in Germany, based on the invention of magnetic wire recording by Oberlin Smith in 1888 and Valdemar Poulsen in 1898. Pfleumer's invention used a ferric oxide powder coating on a long strip of paper; this invention was further developed by the German electronics company AEG, which manufactured the recording machines and BASF, which manufactured the tape.
In 1933, working for AEG, Eduard Schuller developed the ring-shaped tape head. Previous head designs were tended to shred the tape. Another important discovery made in this period was the technique of AC biasing, which improved the fidelity of the recorded audio signal by increasing the effective linearity of the recording medium. Due to the escalating political tensions, the outbreak of World War II, these developments in Germany were kept secret. Although the Allies knew from their monitoring of Nazi radio broadcasts that the Germans had some new form of recording technology, its nature was not discovered until the Allies acquired captured German recording equipment as they invaded Europe at the end of the war, it was only after the war that Americans Jack Mullin, John Herbert Orr, Richard H. Ranger, were able to bring this technology out of Germany and develop it into commercially viable formats. A wide variety of recorders and formats have been developed since, most reel-to-reel and Compact Cassette.
The practice of recording and editing audio using magnetic tape established itself as an obvious improvement over previous methods. Many saw the potential of making the same improvements in recording the video signals used by television. Video signals use more bandwidth than audio signals. Existing audio tape recorders could not capture a video signal. Many set to work on resolving this problem. Jack Mullin and the BBC both created crude working systems that involved moving the tape across a fixed tape head at high speeds. Neither system saw much use, it was the team at Ampex, led by Charles Ginsburg, that made the breakthrough of using a spinning recording head and normal tape speeds to achieve a high head-to-tape speed that could record and reproduce the high bandwidth signals of video. The Ampex system was called Quadruplex and used 2-inch-wide tape, mounted on reels like audio tape, which wrote the signal in what is now called transverse scan. Improvements by other companies Sony, led to the development of helical scan and the enclosure of the tape reels in an easy-to-handle videocassette cartridge.
Nearly all modern videotape systems use helical cartridges. Videocassette recorders used to be common in homes and television production facilities, but many functions of the VCR have been replaced with more modern technology. Since the advent of digital video and computerized video processing, optical disc media and digital video recorders can now perform the same role as videotape; these devices offer improvements like random access to any scene in the recording and the ability to pause a live program and have replaced videotape in many situations. Magnetic tape was first used to record computer data in 1951 on the Eckert-Mauchly UNIVAC I; the system's UNISERVO I tape drive used a thin strip of one half inch wide metal, consisting of nickel-plated bronze. Recording density was 100 characters per inch on eight tracks. Early IBM 7 track tape drives were floor-standing and used vacuum columns to mechanically buffer long U-shaped loops of tape; the two tape reels visibly fed tape through the columns, intermittently spinning the reels in rapid, unsynchronized bursts, resulting in visually striking action.
Stock shots of such vacuum-column tape drives in motion were used to represent "the computer" in movies and televis
A ferrite is a ceramic material made by mixing and firing large proportions of iron oxide blended with small proportions of one or more additional metallic elements, such as barium, manganese and zinc. They are both electrically non-conductive, meaning that they are insulators, ferrimagnetic, meaning they can be magnetized or attracted to a magnet. Ferrites can be divided into two families based on their resistance to being demagnetized. Hard ferrites have high coercivity, they are used to make permanent magnets for applications such as refrigerator magnets and small electric motors. Soft ferrites have low coercivity, so they change their magnetization and act as conductors of magnetic fields, they are used in the electronics industry to make efficient magnetic cores called ferrite cores for high-frequency inductors and transformers, in various microwave components. Ferrite compounds are low cost, being made of rusted iron, have excellent corrosion resistance, they are stable and difficult to demagnetize, can be made with both high and low coercive forces.
Yogoro Kato and Takeshi Takei of the Tokyo Institute of Technology synthesized the first ferrite compounds in 1930. Ferrites are ferrimagnetic ceramic compounds derived from iron oxides. Magnetite is a famous example. Like most of the other ceramics, ferrites are hard and poor conductors of electricity. Many ferrites adopt the spinel structure with the formula AB2O4, where A and B represent various metal cations including iron. Spinel ferrites adopt a crystal motif consisting of cubic close-packed oxides with A cations occupying one eighth of the tetrahedral holes and B cations occupying half of the octahedral holes, i.e. A2+B3+2O2−4. Ferrite crystals do not adopt the ordinary spinel structure, but rather the inverse spinel structure: One eighth of the tetrahedral holes are occupied by B cations, one fourth of the octahedral sites are occupied by A cations. and the other one fourth by B cation. It is possible to have mixed structure spinel ferrites with formula O4 where δ is the degree of inversion.
The magnetic material known as "ZnFe" has the formula ZnFe2O4, with Fe3+ occupying the octahedral sites and Zn2+ occupy the tetrahedral sites, it is an example of normal structure spinel ferrite. Some ferrites adopt hexagonal crystal structure, like barium and strontium ferrites BaFe12O19 and SrFe12O19. In terms of their magnetic properties, the different ferrites are classified as "soft", "semi-hard" or "hard", which refers to their low or high magnetic coercivity, as follows. Ferrites that are used in transformer or electromagnetic cores contain nickel, and/or manganese compounds, they are called soft ferrites. The low coercivity means the material's magnetization can reverse direction without dissipating much energy, while the material's high resistivity prevents eddy currents in the core, another source of energy loss; because of their comparatively low losses at high frequencies, they are extensively used in the cores of RF transformers and inductors in applications such as switched-mode power supplies and loopstick antennas used in AM radios.
The most common soft ferrites are: Manganese-zinc ferrite. MnZn have higher saturation induction than NiZn. Nickel-zinc ferrite. NiZn ferrites exhibit higher resistivity than MnZn, are therefore more suitable for frequencies above 1 MHz. For applications below 5 MHz, MnZn ferrites are used; the exception is with common mode inductors. Cobalt ferrite, CoFe2O4, is in between soft and hard magnetic material and is classified as a semi-hard material, it is used for its magnetostrictive applications like sensors and actuators thanks to its high saturation magnetostriction. CoFe2O4 has the benefits to be rare-earth free, which makes it a good substitute for Terfenol-D. Moreover, its magnetostrictive properties can be tuned by inducing a magnetic uniaxial anisotropy; this can be done by magnetic annealing, magnetic field assisted compaction, or reaction under uniaxial pressure. This last solution has the advantage to be ultra fast thanks to the use of spark plasma sintering; the induced magnetic anisotropy in cobalt ferrite is beneficial to enhance the magnetoelectric effect in composite.
In contrast, permanent ferrite magnets are made of hard ferrites, which have a high coercivity and high remanence after magnetization. Iron oxide and barium or strontium carbonate are used in manufacturing of hard ferrite magnets; the high coercivity means the materials are resistant to becoming demagnetized, an essential characteristic for a permanent magnet. They have high magnetic permeability; these so-called ceramic magnets are cheap, are used in household products such as refrigerator magnets. The maximum magnetic field B is about 0.35 tesla and the magnetic field strength H is about 30 to 160 kiloampere turns per meter. The density of ferrite magnets is about 5 g/cm3; the most common hard ferrites are: Strontium ferrite, SrFe12O19, used in small electric motors, micro-wave devices, recording media, magneto-optic media, telecommunication and electronic industry. Barium ferrite, BaFe12O19, a common material for permanent magnet applications. Barium ferrites are robust ceramics that are stable to moisture and corrosion-resistant.
They are used in e.g. loudspea