Coal is a combustible black or brownish-black sedimentary rock, formed as rock strata called coal seams. Coal is carbon with variable amounts of other elements. Coal is formed if dead plant matter decays into peat and over millions of years the heat and pressure of deep burial converts the peat into coal. Vast deposits of coal originates in former wetlands—called coal forests—that covered much of the Earth's tropical land areas during the late Carboniferous and Permian times; as a fossil fuel burned for heat, coal supplies about a quarter of the world's primary energy and two-fifths of its electricity. Some iron and steel making and other industrial processes burn coal; the extraction and use of coal causes much illness. Coal damages the environment, including by climate change as it is the largest anthropogenic source of carbon dioxide, 14 Gt in 2016, 40% of the total fossil fuel emissions; as part of the worldwide energy transition many countries use less coal. The largest consumer and importer of coal is China.
China mines account for half the world's coal, followed by India with about a tenth. Australia accounts for about a third of world coal exports followed by Russia; the word took the form col in Old English, from Proto-Germanic *kula, which in turn is hypothesized to come from the Proto-Indo-European root *gu-lo- "live coal". Germanic cognates include the Old Frisian kole, Middle Dutch cole, Dutch kool, Old High German chol, German Kohle and Old Norse kol, the Irish word gual is a cognate via the Indo-European root. Coal is composed of macerals and water. Fossils and amber may be found in coal. At various times in the geologic past, the Earth had dense forests in low-lying wetland areas. Due to natural processes such as flooding, these forests were buried underneath soil; as more and more soil deposited over them, they were compressed. The temperature rose as they sank deeper and deeper; as the process continued the plant matter was protected from biodegradation and oxidation by mud or acidic water.
This trapped the carbon in immense peat bogs that were covered and buried by sediments. Under high pressure and high temperature, dead vegetation was converted to coal; the conversion of dead vegetation into coal is called coalification. Coalification starts with dead plant matter decaying into peat. Over millions of years the heat and pressure of deep burial causes the loss of water and carbon dioxide and an increase in the proportion of carbon, thus first lignite sub-bituminous coal, bituminous coal, lastly anthracite may be formed. The wide, shallow seas of the Carboniferous Period provided ideal conditions for coal formation, although coal is known from most geological periods; the exception is the coal gap in the Permian -- Triassic extinction event. Coal is known from Precambrian strata, which predate land plants—this coal is presumed to have originated from residues of algae. Sometimes coal seams are interbedded with other sediments in a cyclothem; as geological processes apply pressure to dead biotic material over time, under suitable conditions, its metamorphic grade or rank increases successively into: Peat, a precursor of coal Lignite, or brown coal, the lowest rank of coal, most harmful to health, used exclusively as fuel for electric power generation Jet, a compact form of lignite, sometimes polished.
Bituminous coal, a dense sedimentary rock black, but sometimes dark brown with well-defined bands of bright and dull material It is used as fuel in steam-electric power generation and to make coke. Anthracite, the highest rank of coal is a harder, glossy black coal used for residential and commercial space heating. Graphite is difficult to ignite and not used as fuel. Cannel coal is a variety of fine-grained, high-rank coal with significant hydrogen content, which consists of liptinite. There are several international standards for coal; the classification of coal is based on the content of volatiles. However the most important distinction is between thermal coal, burnt to generate electricity via steam. Hilt's law is a geological observation, the higher its rank, it applies if the thermal gradient is vertical. The earliest recognized use is from the Shenyang area of China where by 4000 BC Neolithic inhabitants had begun carving ornaments from black lignite. Coal from the Fushun mine in northeastern China was used to smelt copper as early as 1000 BC.
Marco Polo, the Italian who traveled to China in the 13th century, described coal as "black stones... which burn like logs", said coal was so plentiful, people could take three hot baths a week. In Europe, the earliest reference to the use of coal as fuel is from the geological treatise On stones by the Greek scientist Theophrastus: Among the materials that are dug because they are useful, those known as anthrakes are made of earth, once set on fire, they burn like charcoa
An ectotherm, is an organism in which internal physiological sources of heat are of small or quite negligible importance in controlling body temperature. Such organisms rely on environmental heat sources, which permit them to operate at economical metabolic rates; some of these animals live in environments where temperatures are constant, as is typical of regions of the abyssal ocean and hence can be regarded as homeothermic ectotherms. In contrast, in places where temperature varies so as to limit the physiological activities of other kinds of ectotherms, many species habitually seek out external sources of heat or shelter from heat. For home captivity as pet, reptile owners can use a UVB/UVA light system to assist the animals' basking behaviour. In contrast to ectotherms, endotherms rely even predominantly, on heat from internal metabolic processes, mesotherms use an intermediate strategy. In ectotherms, fluctuating ambient temperatures may affect the body temperature; such variation in body temperature is called poikilothermy, though the concept is not satisfactory and the use of the term is declining.
In small aquatic creatures such as Rotifera, the poikilothermy is absolute, but other creatures have wider physiological options at their disposal, they can move to preferred temperatures, avoid ambient temperature changes, or moderate their effects. Ectotherms can display the features of homeothermy within aquatic organisms, their range of ambient environmental temperatures is constant, there are few in number that attempt to maintain a higher internal temperature due to the high associated costs. Various patterns of behavior enable certain ectotherms to regulate body temperature to a useful extent. To warm up, reptiles and many insects find sunny places and adopt positions that maximise their exposure. In cold weather, honey bees huddle together to retain heat. Butterflies and moths may orient their wings to maximize exposure to solar radiation in order to build up heat before take-off. Gregarious caterpillars, such as the Forest Tent caterpillar and fall webworm, benefit from basking in large groups for thermoregulation.
Many flying insects, such as honey bees and bumble bees raise their internal temperatures endothermally prior to flight, by vibrating their flight muscles without violent movement of the wings. Such endothermal activity is an example of the difficulty of consistent application of terms such as poikilothermy and homiothermy. In addition to behavioral adaptations, physiological adaptations help ectotherms regulate temperature. Diving reptiles conserve heat by heat exchange mechanisms, whereby cold blood from the skin picks up heat from blood moving outward from the body core, re-using and thereby conserving some of the heat that otherwise would have been wasted; the skin of bullfrogs secretes more mucus when it is hot. During periods of cold, some ectotherms enter a state of torpor, in which their metabolism slows or, in some cases, such as the wood frog stops; the torpor might last overnight or last for a season, or for years, depending on the species and circumstances. Ectotherms rely on external heat sources such as sunlight to achieve their optimal body temperature for various bodily activities.
Accordingly, they depend on ambient conditions to reach operational body temperatures. In contrast, endothermic animals, as a rule, maintain nearly constant high operational body temperatures by reliance on internal heat produced by metabolically active organs or by specialized heat producing organs like brown adipose tissue; as a rule, ectotherms have lower metabolic rates than endotherms at a given body mass. As a consequence, endotherms rely on higher food consumption, on food of higher energy content; such requirements may limit the carrying capacity of a given environment for endotherms as compared to its carrying capacity for ectotherms. Because ectotherms depend on environmental conditions for body temperature regulation, as a rule, they are more sluggish at night and in early mornings; when they emerge from shelter, many diurnal ectotherms need to heat up in the early sunlight before they can begin their daily activities. In cool weather the foraging activity of such species is therefore restricted to the day time in most vertebrate ectotherms, in cold climates most cannot survive at all.
In lizards, for instance, most nocturnal species are geckos specialising in "sit and wait" foraging strategies. Such strategies do not require as much energy as active foraging and do not, as a rule, require hunting activity of the same intensity. From another point of view, sit-and-wait predation may require long periods of unproductive waiting. Endotherms cannot, in general, afford such long periods without food, but suitably adapted ectotherms can wait without expending much energy. Endothermic vertebrate species are therefore less dependent on the environmental conditions and have developed a higher variability in their daily patterns of activity. Possible confusion can arise from the difference in the terminology of biology. Whereas the thermodynamic terms "exothermic" and "endothermic" refer to processes that give out heat energy and processes that absorb heat energy
Calorimetry is the science or act of measuring changes in state variables of a body for the purpose of deriving the heat transfer associated with changes of its state due, for example, to chemical reactions, physical changes, or phase transitions under specified constraints. Calorimetry is performed with a calorimeter; the word calorimetry is derived from the Latin word calor, meaning heat and the Greek word μέτρον, meaning measure. Scottish physician and scientist Joseph Black, the first to recognize the distinction between heat and temperature, is said to be the founder of the science of calorimetry. Indirect calorimetry calculates heat that living organisms produce by measuring either their production of carbon dioxide and nitrogen waste, or from their consumption of oxygen. Lavoisier noted in 1780 that heat production can be predicted from oxygen consumption this way, using multiple regression; the dynamic energy budget theory explains. Heat generated by living organisms may be measured by direct calorimetry, in which the entire organism is placed inside the calorimeter for the measurement.
A used modern instrument is the differential scanning calorimeter, a device which allows thermal data to be obtained on small amounts of material. It involves heating the sample at a controlled rate and recording the heat flow either into or from the specimen. Calorimetry requires that a reference material that changes temperature have known definite thermal constitutive properties; the classical rule, recognized by Clausius and by Kelvin, is that the pressure exerted by the calorimetric material is and determined by its temperature and volume. There are many materials that do not comply with this rule, for them, the present formula of classical calorimetry does not provide an adequate account. Here the classical rule is assumed to hold for the calorimetric material being used, the propositions are mathematically written: The thermal response of the calorimetric material is described by its pressure p as the value of its constitutive function p of just the volume V and the temperature T. All increments are here required to be small.
This calculation refers to a domain of volume and temperature of the body in which no phase change occurs, there is only one phase present. An important assumption here is continuity of property relations. A different analysis is needed for phase change When a small increment of heat is gained by a calorimetric body, with small increments, δ V of its volume, δ T of its temperature, the increment of heat, δ Q, gained by the body of calorimetric material, is given by δ Q = C T δ V + C V δ T where C T denotes the latent heat with respect to volume, of the calorimetric material at constant controlled temperature T; the surroundings' pressure on the material is instrumentally adjusted to impose a chosen volume change, with initial volume V. To determine this latent heat, the volume change is the independently instrumentally varied quantity; this latent heat is not one of the used ones, but is of theoretical or conceptual interest. C V denotes the heat capacity, of the calorimetric material at fixed constant volume V, while the pressure of the material is allowed to vary with initial temperature T.
The temperature is forced to change by exposure to a suitable heat bath. It is customary to write C V as C V, or more as C V; this latent heat is one of the two used ones. The latent heat with respect to volume is the heat required for unit increment in volume at constant temperature, it can be said to be'measured along an isotherm', the pressure the material exerts is allowed to vary according to its constitutive law p = p. For a given material, it can have a positive or negative sign or exceptionally it can be zero, this can depend on the temperature, as it does for water about 4 C; the concept of latent heat with respect to volume was first recognized by Joseph Black in 1762. The term'latent heat of expansion' is used; the latent heat with respect to
A carbohydrate is a biomolecule consisting of carbon and oxygen atoms with a hydrogen–oxygen atom ratio of 2:1 and thus with the empirical formula Cmn. This formula holds true for monosaccharides; some exceptions exist. The carbohydrates are technically hydrates of carbon; the term is most common in biochemistry, where it is a synonym of saccharide, a group that includes sugars and cellulose. The saccharides are divided into four chemical groups: monosaccharides, disaccharides and polysaccharides. Monosaccharides and disaccharides, the smallest carbohydrates, are referred to as sugars; the word saccharide comes from the Greek word σάκχαρον, meaning "sugar". While the scientific nomenclature of carbohydrates is complex, the names of the monosaccharides and disaccharides often end in the suffix -ose, as in the monosaccharides fructose and glucose and the disaccharides sucrose and lactose. Carbohydrates perform numerous roles in living organisms. Polysaccharides serve as structural components; the 5-carbon monosaccharide ribose is an important component of coenzymes and the backbone of the genetic molecule known as RNA.
The related deoxyribose is a component of DNA. Saccharides and their derivatives include many other important biomolecules that play key roles in the immune system, preventing pathogenesis, blood clotting, development, they are found in a wide variety of processed foods. Starch is a polysaccharide, it is abundant in cereals and processed food based on cereal flour, such as bread, pizza or pasta. Sugars appear in human diet as table sugar, lactose and fructose, both of which occur in honey, many fruits, some vegetables. Table sugar, milk, or honey are added to drinks and many prepared foods such as jam and cakes. Cellulose, a polysaccharide found in the cell walls of all plants, is one of the main components of insoluble dietary fiber. Although it is not digestible, insoluble dietary fiber helps to maintain a healthy digestive system by easing defecation. Other polysaccharides contained in dietary fiber include resistant starch and inulin, which feed some bacteria in the microbiota of the large intestine, are metabolized by these bacteria to yield short-chain fatty acids.
In scientific literature, the term "carbohydrate" has many synonyms, like "sugar", "saccharide", "ose", "glucide", "hydrate of carbon" or "polyhydroxy compounds with aldehyde or ketone". Some of these terms, specially "carbohydrate" and "sugar", are used with other meanings. In food science and in many informal contexts, the term "carbohydrate" means any food, rich in the complex carbohydrate starch or simple carbohydrates, such as sugar. In lists of nutritional information, such as the USDA National Nutrient Database, the term "carbohydrate" is used for everything other than water, fat and ethanol; this includes chemical compounds such as acetic or lactic acid, which are not considered carbohydrates. It includes dietary fiber, a carbohydrate but which does not contribute much in the way of food energy though it is included in the calculation of total food energy just as though it were a sugar. In the strict sense, "sugar" is applied for sweet, soluble carbohydrates, many of which are used in food.
The name "carbohydrate" was used in chemistry for any compound with the formula Cm n. Following this definition, some chemists considered formaldehyde to be the simplest carbohydrate, while others claimed that title for glycolaldehyde. Today, the term is understood in the biochemistry sense, which excludes compounds with only one or two carbons and includes many biological carbohydrates which deviate from this formula. For example, while the above representative formulas would seem to capture the known carbohydrates and abundant carbohydrates deviate from this. For example, carbohydrates display chemical groups such as: N-acetyl, carboxylic acid and deoxy modifications. Natural saccharides are built of simple carbohydrates called monosaccharides with general formula n where n is three or more. A typical monosaccharide has the structure H–x–y–H, that is, an aldehyde or ketone with many hydroxyl groups added one on each carbon atom, not part of the aldehyde or ketone functional group. Examples of monosaccharides are glucose and glyceraldehydes.
However, some biological substances called "monosaccharides" do not conform to this formula and there are many chemicals that do conform to this formula but are not considered to be monosaccharides. The open-chain form of a monosaccharide coexists with a closed ring form where the aldehyde/ketone carbonyl group carbon and hydroxyl group react forming a hemiacetal with a new C–O–C bridge. Monosaccharides can be linked togeth
In nuclear chemistry, nuclear fusion is a reaction in which two or more atomic nuclei are combined to form one or more different atomic nuclei and subatomic particles. The difference in mass between the reactants and products is manifested as either the release or absorption of energy; this difference in mass arises due to the difference in atomic "binding energy" between the atomic nuclei before and after the reaction. Fusion is other high magnitude stars. A fusion process that produces a nucleus lighter than iron-56 or nickel-62 will yield a net energy release; these elements have the smallest mass per nucleon and the largest binding energy per nucleon, respectively. Fusion of light elements toward these releases energy, while a fusion producing nuclei heavier than these elements will result in energy retained by the resulting nucleons, the resulting reaction is endothermic; the opposite is true for nuclear fission. This means that the lighter elements, such as helium, are in general more fusible.
The extreme astrophysical event of a supernova can produce enough energy to fuse nuclei into elements heavier than iron. In 1920, Arthur Eddington suggested hydrogen-helium fusion could be the primary source of stellar energy. Quantum tunneling was discovered by Friedrich Hund in 1929, shortly afterwards Robert Atkinson and Fritz Houtermans used the measured masses of light elements to show that large amounts of energy could be released by fusing small nuclei. Building on the early experiments in nuclear transmutation by Ernest Rutherford, laboratory fusion of hydrogen isotopes was accomplished by Mark Oliphant in 1932. In the remainder of that decade, the theory of the main cycle of nuclear fusion in stars were worked out by Hans Bethe. Research into fusion for military purposes began in the early 1940s as part of the Manhattan Project. Fusion was accomplished in 1951 with the Greenhouse Item nuclear test. Nuclear fusion on a large scale in an explosion was first carried out on 1 November 1952, in the Ivy Mike hydrogen bomb test.
Research into developing controlled thermonuclear fusion for civil purposes began in earnest in the 1940s, it continues to this day. The release of energy with the fusion of light elements is due to the interplay of two opposing forces: the nuclear force, which combines together protons and neutrons, the Coulomb force, which causes protons to repel each other. Protons are positively charged and repel each other by the Coulomb force, but they can nonetheless stick together, demonstrating the existence of another, short-range, force referred to as nuclear attraction. Light nuclei are sufficiently small and proton-poor allowing the nuclear force to overcome repulsion; this is because the nucleus is sufficiently small that all nucleons feel the short-range attractive force at least as as they feel the infinite-range Coulomb repulsion. Building up nuclei from lighter nuclei by fusion releases the extra energy from the net attraction of particles. For larger nuclei, however, no energy is released, since the nuclear force is short-range and cannot continue to act across longer atomic length scales.
Thus, energy is not released with the fusion of such nuclei. Fusion powers stars and produces all elements in a process called nucleosynthesis; the Sun is a main-sequence star, and, as such, generates its energy by nuclear fusion of hydrogen nuclei into helium. In its core, the Sun fuses 620 million metric tons of hydrogen and makes 606 million metric tons of helium each second; the fusion of lighter elements in stars releases the mass that always accompanies it. For example, in the fusion of two hydrogen nuclei to form helium, 0.7% of the mass is carried away in the form of kinetic energy of an alpha particle or other forms of energy, such as electromagnetic radiation. It takes considerable energy to force nuclei to fuse those of the lightest element, hydrogen; when accelerated to high enough speeds, nuclei can overcome this electrostatic repulsion and brought close enough such that the attractive nuclear force is greater than the repulsive Coulomb force. The strong force grows once the nuclei are close enough, the fusing nucleons can "fall" into each other and the result is fusion and net energy produced.
The fusion of lighter nuclei, which creates a heavier nucleus and a free neutron or proton releases more energy than it takes to force the nuclei together. Energy released in most nuclear reactions is much larger than in chemical reactions, because the binding energy that holds a nucleus together is greater than the energy that holds electrons to a nucleus. For example, the ionization energy gained by adding an electron to a hydrogen nucleus is 13.6 eV—less than one-millionth of the 17.6 MeV released in the deuterium–tritium reaction shown in the adjacent diagram. The complete conversion of one gram of matter would release 9×1013 joules of energy. Fusion reactions have an energy density many times greater than nuclear fission. Only direct conversion of mass into energy, such as that caused by the annihilatory collision of matter and antimatter, is more energetic per unit of mass than nuclear fusion. Research into using fusion for the p
An endotherm is an organism that maintains its body at a metabolically favorable temperature by the use of heat set free by its internal bodily functions instead of relying purely on ambient heat. Such internally generated heat is an incidental product of the animal's routine metabolism, but under conditions of excessive cold or low activity an endotherm might apply special mechanisms adapted to heat production. Examples include special-function muscular exertion such as shivering, uncoupled oxidative metabolism such as within brown adipose tissue. Only birds and mammals are extant universally endothermic groups of animals. Certain lamnid sharks and billfishes are endothermic. In common parlance, endotherms are characterized as "warm-blooded"; the opposite of endothermy is ectothermy, although in general, there is no absolute or clear separation between the nature of endotherms and ectotherms. Many endotherms have a larger number of mitochondria per cell than ectotherms; this enables them to generate heat by increasing the rate at which they metabolize sugars.
Accordingly, to sustain their higher metabolism, endothermic animals require several times as much food as ectothermic animals do, require a more sustained supply of metabolic fuel. In many endothermic animals, a controlled temporary state of hypothermia conserves energy by permitting the body temperature to drop nearly to ambient levels; such states may be brief, regular circadian cycles called torpor, or they might occur in much longer seasonal, cycles called hibernation. The body temperatures of many small birds and small mammals fall during daily inactivity, such as nightly in diurnal animals or during the day in nocturnal animals, thus reducing the energy cost of maintaining body temperature. Less drastic intermittent reduction in body temperature occurs in other, larger endotherms. There may be other variations in temperature smaller, either endogenous or in response to external circumstances or vigorous exertion, either an increase or a drop; the resting human body generates about two-thirds of its heat through metabolism in internal organs in the thorax and abdomen, as well as in the brain.
The brain generates about 16% of the total heat produced by the body. Heat loss is a major threat to smaller creatures, as they have a larger ratio of surface area to volume. Small warm-blooded animals have insulation in the form of fur or feathers. Aquatic warm-blooded animals, such as seals have deep layers of blubber under the skin and any pelage that they might have. Penguins have blubber. Penguin feathers serve both for insulation and for streamlining. Endotherms that live in cold circumstances or conditions predisposing to heat loss, such as polar waters, tend to have specialised structures of blood vessels in their extremities that act as heat exchangers; the veins are adjacent to the arteries full of warm blood. Some of the arterial heat is recycled back into the trunk. Birds waders have well-developed heat exchange mechanisms in their legs—those in the legs of emperor penguins are part of the adaptations that enable them to spend months on Antarctic winter ice. In response to cold many warm-blooded animals reduce blood flow to the skin by vasoconstriction to reduce heat loss.
As a result, they blanch. In equatorial climates and during temperate summers, overheating is as great a threat as cold. In hot conditions, many warm-blooded animals increase heat loss by panting, which cools the animal by increasing water evaporation in the breath, and/or flushing, increasing the blood flow to the skin so the heat will radiate into the environment. Hairless and short-haired mammals, including humans sweat, since the evaporation of the water in sweat removes heat. Elephants keep cool by using their huge ears like radiators in automobiles, their ears are thin and the blood vessels are close to the skin, flapping their ears to increase the airflow over them causes the blood to cool, which reduces their core body temperature when the blood moves through the rest of the circulatory system. The major advantage of endothermy over ectothermy is decreased vulnerability to fluctuations in external temperature. Regardless of location, endothermy maintains a constant core temperature for optimum enzyme activity.
Endotherms control body temperature by internal homeostatic mechanisms. In mammals, two separate homeostatic mechanisms are involved in thermoregulation—one mechanism increases body temperature, while the other decreases it; the presence of two separate mechanisms provides a high degree of control. This is important because the core temperature of mammals can be controlled to be as close as possible to the optimum temperature for enzyme activity; the overall rate of an animal's metabolism increases by a factor of about two for every 10 °C rise in temperature, limited by the need to avoid hyperthermia. Endothermy does not provide greater speed in movement than ectothermy —ectothermic animals can move as fast as warm-blooded animals of the same size and build when the ectotherm is near or at its optimum temperature, but cannot maintain high metabolic activity for as long as endotherms. Endothermic/homeothermic animals can be optimally active at more times during the diurnal cycle in places of sharp temperature variations between day and night and
Thermite is a pyrotechnic composition of metal powder, which serves as fuel, metal oxide. When ignited by heat, thermite undergoes an exothermic reduction-oxidation reaction. Most varieties are not explosive, but can create brief bursts of heat and high temperature in a small area, its form of action is similar to that such as black powder. Thermites have diverse compositions. Fuels include aluminium, titanium, zinc and boron. Aluminium is common because of low cost. Oxidizers include bismuth oxide, boron oxide, silicon oxide, chromium oxide, manganese oxide, iron oxide, iron oxide, copper oxide, lead oxide; the reaction called the Goldschmidt process, is used for thermite welding used to join rail tracks. Thermites have been used in metal refining, disabling munitions, in incendiary weapons; some thermite-like mixtures are used as pyrotechnic initiators in fireworks. In the following example, elemental aluminium reduces the oxide of another metal, in this common example iron oxide, because aluminium forms stronger and more stable bonds with oxygen than iron: Fe2O3 + 2 Al → 2 Fe + Al2O3The products are aluminium oxide, elemental iron, a large amount of heat.
The reactants are powdered and mixed with a binder to keep the material solid and prevent separation. Other metal oxides can be used, such as chromium oxide, to generate the given metal in its elemental form. For example, a copper thermite reaction using copper oxide and elemental aluminium can be used for creating electric joints in a process called cadwelding, that produces elemental copper: 3 CuO + 2 Al → 3 Cu + Al2O3Thermites with nanosized particles are described by a variety of terms, such as metastable intermolecular composites, super-thermite, nano-thermite, nanocomposite energetic materials; the thermite reaction was discovered in 1893 and patented in 1895 by German chemist Hans Goldschmidt. The reaction is sometimes called the "Goldschmidt reaction" or "Goldschmidt process". Goldschmidt was interested in producing pure metals by avoiding the use of carbon in smelting, but he soon discovered the value of thermite in welding; the first commercial application of thermite was the welding of tram tracks in Essen in 1899.
Red iron oxide is the most common iron oxide used in thermite. Magnetite works. Other oxides are used, such as MnO2 in manganese thermite, Cr2O3 in chromium thermite, quartz in silicon thermite, or copper oxide in copper thermite, but only for specialized purposes. All of these examples use aluminium as the reactive metal. Fluoropolymers can be used in special formulations, Teflon with magnesium or aluminium being a common example. Magnesium/teflon/viton is another pyrolant of this type. Combinations of dry ice and reducing agents such as magnesium and boron follow the same chemical reaction as with traditional thermite mixtures, producing metal oxides and carbon. Despite the cold temperature of a dry ice thermite mixture, such a system is capable of being ignited with a flame; when the ingredients are finely divided, confined in a pipe and armed like a traditional explosive, this cryo-thermite is detonatable and a portion of the carbon liberated in the reaction emerges in the form of diamond.
In principle, any reactive metal could be used instead of aluminium. This is done, because the properties of aluminium are nearly ideal for this reaction: It is by far the cheapest of the reactive metals. For example, in December 2014, tin was US$19,830/metric ton, zinc was US$2,180/t and aluminium was US$1,910/t, it forms a passivation layer making it safer to handle than many other reactive metals. Its low melting point means that it is easy to melt the metal, so that the reaction can occur in the liquid phase and thus proceeds quickly, its high boiling point enables the reaction to reach high temperatures, since several processes tend to limit the maximum temperature to just below the boiling point. Such a high boiling point is common among transition metals, but is unusual among the reactive metals. Further, the low density of the aluminium oxide formed as a result of the reaction tends to cause it to float on the resultant pure metal; this is important for reducing contamination in a weld.
Although the reactants are stable at room temperature, they burn with an intense exothermic reaction when they are heated to ignition temperature. The products emerge as liquids due to the high temperatures reached —although the actual temperature reached depends on how heat can escape to the surrounding environment. Thermite does not require any external source of air, it cannot be smothered, may ignite in any environment given sufficient initial heat. It burns well while wet, cannot be extinguished with water—though enough water to remove sufficient heat may stop the reaction. Small amounts of water boil before reaching the reaction. So, thermite is used for welding underwater; the thermites are characterized by complete absence of gas production during burning, high reaction temperature, production of molten slag. The fuel should have high heat of combustion and produce oxides with low melting point and high boiling point; the oxidizer should contain at least 25% oxygen, have high density, low heat of formation, p