1.
Diffusion
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Diffusion is the net movement of molecules or atoms from a region of high concentration to a region of low concentration. This is also referred to as the movement of a substance down a concentration gradient, a gradient is the change in the value of a quantity with the change in another variable. The word diffusion derives from the Latin word, diffundere, which means to spread out, a distinguishing feature of diffusion is that it results in mixing or mass transport, without requiring bulk motion. Thus, diffusion should not be confused with convection, or advection, an example of a situation in which bulk flow and diffusion can be differentiated is the mechanism by which oxygen enters the body during external respiration. The lungs are located in the cavity, which expands as the first step in external respiration. This expansion leads to an increase in volume of the alveoli in the lungs and this creates a pressure gradient between the air outside the body and the alveoli. The air moves down the gradient through the airways of the lungs and into the alveoli until the pressure of the air. The air arriving in the alveoli has a concentration of oxygen than the “stale” air in the alveoli. The increase in oxygen concentration creates a concentration gradient for oxygen between the air in the alveoli and the blood in the capillaries that surround the alveoli, oxygen then moves by diffusion, down the concentration gradient, into the blood. The other consequence of the air arriving in alveoli is that the concentration of carbon dioxide in the alveoli decreases and this creates a concentration gradient for carbon dioxide to diffuse from the blood into the alveoli. The pumping action of the heart then transports the blood around the body, as the left ventricle of the heart contracts, the volume decreases, which increases the pressure in the ventricle. This creates a gradient between the heart and the capillaries, and blood moves through blood vessels by bulk flow. The concept of diffusion is widely used in, physics, chemistry, biology, sociology, economics, however, in each case, the object that is undergoing diffusion is “spreading out” from a point or location at which there is a higher concentration of that object. In the phenomenological approach, diffusion is the movement of a substance from a region of high concentration to a region of low concentration without bulk motion, according to Ficks laws, the diffusion flux is proportional to the negative gradient of concentrations. It goes from regions of higher concentration to regions of lower concentration, some time later, various generalizations of Ficks laws were developed in the frame of thermodynamics and non-equilibrium thermodynamics. From the atomistic point of view, diffusion is considered as a result of the walk of the diffusing particles. In molecular diffusion, the molecules are self-propelled by thermal energy. Random walk of small particles in suspension in a fluid was discovered in 1827 by Robert Brown, the theory of the Brownian motion and the atomistic backgrounds of diffusion were developed by Albert Einstein
2.
Solution
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In chemistry, a solution is a homogeneous mixture composed of two or more substances. In such a mixture, a solute is a substance dissolved in another substance, the mixing process of a solution happens at a scale where the effects of chemical polarity are involved, resulting in interactions that are specific to solvation. The solution assumes the characteristics of the solvent when the solvent is the fraction of the mixture. The concentration of a solute in a solution is the mass of that solute expressed as a percentage of the mass of the whole solution, a solution is a homogeneous mixture of two or more substances. The particles of solute in a solution cannot be seen by the naked eye, a solution does not allow beams of light to scatter. The solute from a solution cannot be separated by filtration and it is composed of only one phase. Homogeneous means that the components of the form a single phase. Heterogeneous means that the components of the mixture are of different phase, the properties of the mixture can be uniformly distributed through the volume but only in absence of diffusion phenomena or after their completion. Usually, the present in the greatest amount is considered the solvent. Solvents can be gases, liquids or solids, one or more components present in the solution other than the solvent are called solutes. The solution has the physical state as the solvent. If the solvent is a gas, only gases are dissolved under a set of conditions. An example of a solution is air. Since interactions between molecules play almost no role, dilute gases form rather trivial solutions, in part of the literature, they are not even classified as solutions, but addressed as mixtures. If the solvent is a liquid, then almost all gases, liquids, here are some examples, Gas in liquid, Oxygen in water Carbon dioxide in water – a less simple example, because the solution is accompanied by a chemical reaction. Liquid in liquid, The mixing of two or more substances of the same chemistry but different concentrations to form a constant, alcoholic beverages are basically solutions of ethanol in water. Solid in liquid, Sucrose in water Sodium chloride or any other salt in water, solutions in water are especially common. Counterexamples are provided by liquid mixtures that are not homogeneous, colloids, body fluids are examples for complex liquid solutions, containing many solutes
3.
Temperature
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A temperature is an objective comparative measurement of hot or cold. It is measured by a thermometer, several scales and units exist for measuring temperature, the most common being Celsius, Fahrenheit, and, especially in science, Kelvin. Absolute zero is denoted as 0 K on the Kelvin scale, −273.15 °C on the Celsius scale, the kinetic theory offers a valuable but limited account of the behavior of the materials of macroscopic bodies, especially of fluids. Temperature is important in all fields of science including physics, geology, chemistry, atmospheric sciences, medicine. The Celsius scale is used for temperature measurements in most of the world. Because of the 100 degree interval, it is called a centigrade scale.15, the United States commonly uses the Fahrenheit scale, on which water freezes at 32°F and boils at 212°F at sea-level atmospheric pressure. Many scientific measurements use the Kelvin temperature scale, named in honor of the Scottish physicist who first defined it and it is a thermodynamic or absolute temperature scale. Its zero point, 0K, is defined to coincide with the coldest physically-possible temperature and its degrees are defined through thermodynamics. The temperature of zero occurs at 0K = −273. 15°C. For historical reasons, the triple point temperature of water is fixed at 273.16 units of the measurement increment, Temperature is one of the principal quantities in the study of thermodynamics. There is a variety of kinds of temperature scale and it may be convenient to classify them as empirically and theoretically based. Empirical temperature scales are historically older, while theoretically based scales arose in the middle of the nineteenth century, empirically based temperature scales rely directly on measurements of simple physical properties of materials. For example, the length of a column of mercury, confined in a capillary tube, is dependent largely on temperature. Such scales are only within convenient ranges of temperature. For example, above the point of mercury, a mercury-in-glass thermometer is impracticable. A material is of no use as a thermometer near one of its phase-change temperatures, in spite of these restrictions, most generally used practical thermometers are of the empirically based kind. Especially, it was used for calorimetry, which contributed greatly to the discovery of thermodynamics, nevertheless, empirical thermometry has serious drawbacks when judged as a basis for theoretical physics. Theoretically based temperature scales are based directly on theoretical arguments, especially those of thermodynamics, kinetic theory and they rely on theoretical properties of idealized devices and materials
4.
Absolute zero
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Absolute zero is the lower limit of the thermodynamic temperature scale, a state at which the enthalpy and entropy of a cooled ideal gas reaches its minimum value, taken as 0. The corresponding Kelvin and Rankine temperature scales set their zero points at absolute zero by definition, in the quantum-mechanical description, matter at absolute zero is in its ground state, the point of lowest internal energy. And a system at absolute zero still possesses quantum mechanical zero-point energy, the kinetic energy of the ground state cannot be removed. Scientists and technologists routinely achieve temperatures close to zero, where matter exhibits quantum effects such as superconductivity and superfluidity. At temperatures near 0 K, nearly all molecular motion ceases and ΔS =0 for any adiabatic process, in such a circumstance, pure substances can form perfect crystals as T →0. Max Plancks strong form of the law of thermodynamics states the entropy of a perfect crystal vanishes at absolute zero. The Nernst postulate identifies the isotherm T =0 as coincident with the adiabat S =0, although other isotherms, as no two adiabats intersect, no other adiabat can intersect the T =0 isotherm. Consequently no adiabatic process initiated at nonzero temperature can lead to zero temperature, a perfect crystal is one in which the internal lattice structure extends uninterrupted in all directions. The perfect order can be represented by translational symmetry along three axes, every lattice element of the structure is in its proper place, whether it is a single atom or a molecular grouping. For substances that exist in two crystalline forms, such as diamond and graphite for carbon, there is a kind of chemical degeneracy. The question remains whether both can have zero entropy at T =0 even though each is perfectly ordered, using the Debye model, the specific heat and entropy of a pure crystal are proportional to T3, while the enthalpy and chemical potential are proportional to T4. These quantities drop toward their T =0 limiting values and approach with zero slopes, for the specific heats at least, the limiting value itself is definitely zero, as borne out by experiments to below 10 K. Even the less detailed Einstein model shows this curious drop in specific heats, in fact, all specific heats vanish at absolute zero, not just those of crystals. Likewise for the coefficient of thermal expansion, maxwells relations show that various other quantities also vanish. Since the relation between changes in Gibbs free energy, the enthalpy and the entropy is Δ G = Δ H − T Δ S thus, as T decreases, ΔG and ΔH approach each other. Experimentally, it is found that all spontaneous processes result in a decrease in G as they proceed toward equilibrium, if ΔS and/or T are small, the condition ΔG <0 may imply that ΔH <0, which would indicate an exothermic reaction. However, this is not required, endothermic reactions can proceed spontaneously if the TΔS term is large enough, moreover, the slopes of the derivatives of ΔG and ΔH converge and are equal to zero at T =0. e. An actual process is the most exothermic one, one model that estimates the properties of an electron gas at absolute zero in metals is the Fermi gas
5.
Viscosity
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The viscosity of a fluid is a measure of its resistance to gradual deformation by shear stress or tensile stress. For liquids, it corresponds to the concept of thickness, for example. Viscosity is a property of the fluid which opposes the motion between the two surfaces of the fluid in a fluid that are moving at different velocities. For a given velocity pattern, the stress required is proportional to the fluids viscosity, a fluid that has no resistance to shear stress is known as an ideal or inviscid fluid. Zero viscosity is observed only at low temperatures in superfluids. Otherwise, all fluids have positive viscosity, and are said to be viscous or viscid. A fluid with a high viscosity, such as pitch. The word viscosity is derived from the Latin viscum, meaning mistletoe, the dynamic viscosity of a fluid expresses its resistance to shearing flows, where adjacent layers move parallel to each other with different speeds. It can be defined through the situation known as a Couette flow. This fluid has to be homogeneous in the layer and at different shear stresses, if the speed of the top plate is small enough, the fluid particles will move parallel to it, and their speed will vary linearly from zero at the bottom to u at the top. Each layer of fluid will move faster than the one just below it, in particular, the fluid will apply on the top plate a force in the direction opposite to its motion, and an equal but opposite one to the bottom plate. An external force is required in order to keep the top plate moving at constant speed. The magnitude F of this force is found to be proportional to the u and the area A of each plate. The proportionality factor μ in this formula is the viscosity of the fluid, the ratio u/y is called the rate of shear deformation or shear velocity, and is the derivative of the fluid speed in the direction perpendicular to the plates. Isaac Newton expressed the forces by the differential equation τ = μ ∂ u ∂ y, where τ = F/A. This formula assumes that the flow is moving along parallel lines and this equation can be used where the velocity does not vary linearly with y, such as in fluid flowing through a pipe. Use of the Greek letter mu for the dynamic viscosity is common among mechanical and chemical engineers. However, the Greek letter eta is used by chemists, physicists
6.
Flux
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Flux is either of two separate simple and ubiquitous concepts throughout physics and applied mathematics. Within a discipline, the term is used consistently. Both concepts have mathematical rigor, enabling comparison of the underlying math when the terminology is unclear, for transport phenomena, flux is a vector quantity, describing the magnitude and direction of the flow of a substance or property. In electromagnetism, flux is a quantity, defined as the surface integral of the component of a vector field perpendicular to the surface at each point. As will be clear, the easiest way to relate the two concepts is that the surface integral of a flux according to the first definition is a flux according to the second definition. The word flux comes from Latin, fluxus means flow, as fluxion, this term was introduced into differential calculus by Isaac Newton. One could argue, based on the work of James Clerk Maxwell, the specific quote from Maxwell is, In the case of fluxes, we have to take the integral, over a surface, of the flux through every element of the surface. The result of operation is called the surface integral of the flux. It represents the quantity which passes through the surface, according to the first definition, flux may be a single vector, or flux may be a vector field / function of position. In the latter case flux can readily be integrated over a surface, by contrast, according to the second definition, flux is the integral over a surface, it makes no sense to integrate a second-definition flux for one would be integrating over a surface twice. Thus, Maxwells quote only makes sense if flux is being used according to the first definition and this is ironic because Maxwell was one of the major developers of what we now call electric flux and magnetic flux according to the second definition. This implies that Maxwell conceived as these fields as flows/fluxes of some sort, given a flux according to the second definition, the corresponding flux density, if that term is used, refers to its derivative along the surface that was integrated. By the Fundamental theorem of calculus, the flux density is a flux according to the first definition. Given a current such as electric current—charge per time, current density would also be a flux according to the first definition—charge per time per area. Concrete fluxes in the rest of this article will be used in accordance to their acceptance in the literature. In transport phenomena, flux is defined as the rate of flow of a property per unit area, the area is of the surface the property is flowing through or across. Here are 3 definitions in increasing order of complexity, each is a special case of the following. In all cases the frequent symbol j, is used for flux, q for the quantity that flows, t for time
7.
Phase (matter)
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In the physical sciences, a phase is a region of space, throughout which all physical properties of a material are essentially uniform. Examples of physical properties include density, index of refraction, magnetization, a simple description is that a phase is a region of material that is chemically uniform, physically distinct, and mechanically separable. In a system consisting of ice and water in a jar, the ice cubes are one phase, the water is a second phase. The glass of the jar is another separate phase, the term phase is sometimes used as a synonym for state of matter, but there can be several immiscible phases of the same state of matter. Also, the phase is sometimes used to refer to a set of equilibrium states demarcated in terms of state variables such as pressure and temperature by a phase boundary on a phase diagram. Distinct phases may be described as different states of such as gas, liquid, solid. Useful mesophases between solid and liquid form other states of matter, distinct phases may also exist within a given state of matter. As shown in the diagram for iron alloys, several phases exist for both the solid and liquid states, phases may also be differentiated based on solubility as in polar or non-polar. A mixture of water and oil will separate into two phases. Water has a low solubility in oil, and oil has a low solubility in water. Solubility is the amount of a solute that can dissolve in a solvent before the solute ceases to dissolve. A mixture can separate into more than two phases and the concept of phase separation extends to solids, i. e. solids can form solid solutions or crystallize into distinct crystal phases. Metal pairs that are mutually soluble can form alloys, whereas metal pairs that are mutually insoluble cannot, as many as eight immiscible liquid phases have been observed. Mutually immiscible liquid phases are formed from water, hydrophobic solvents, perfluorocarbons, silicones, several different metals. Not all organic solvents are completely miscible, e. g. a mixture of ethylene glycol, phases do not need to macroscopically separate spontaneously. Emulsions and colloids are examples of immiscible phase pair combinations that do not physically separate, left to equilibration, many compositions will form a uniform single phase, but depending on the temperature and pressure even a single substance may separate into two or more distinct phases. Within each phase, the properties are uniform but between the two phases properties differ, water in a closed jar with an air space over it forms a two phase system. Most of the water is in the phase, where it is held by the mutual attraction of water molecules
8.
Molecule
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A molecule is an electrically neutral group of two or more atoms held together by chemical bonds. Molecules are distinguished from ions by their lack of electrical charge, however, in quantum physics, organic chemistry, and biochemistry, the term molecule is often used less strictly, also being applied to polyatomic ions. In the kinetic theory of gases, the molecule is often used for any gaseous particle regardless of its composition. According to this definition, noble gas atoms are considered molecules as they are in fact monoatomic molecules. A molecule may be homonuclear, that is, it consists of atoms of one element, as with oxygen, or it may be heteronuclear. Atoms and complexes connected by non-covalent interactions, such as hydrogen bonds or ionic bonds, are not considered single molecules. Molecules as components of matter are common in organic substances and they also make up most of the oceans and atmosphere. Also, no typical molecule can be defined for ionic crystals and covalent crystals, the theme of repeated unit-cellular-structure also holds for most condensed phases with metallic bonding, which means that solid metals are also not made of molecules. In glasses, atoms may also be together by chemical bonds with no presence of any definable molecule. The science of molecules is called molecular chemistry or molecular physics, in practice, however, this distinction is vague. In molecular sciences, a molecule consists of a system composed of two or more atoms. Polyatomic ions may sometimes be thought of as electrically charged molecules. The term unstable molecule is used for very reactive species, i. e, according to Merriam-Webster and the Online Etymology Dictionary, the word molecule derives from the Latin moles or small unit of mass. Molecule – extremely minute particle, from French molécule, from New Latin molecula, diminutive of Latin moles mass, a vague meaning at first, the vogue for the word can be traced to the philosophy of Descartes. The definition of the molecule has evolved as knowledge of the structure of molecules has increased, earlier definitions were less precise, defining molecules as the smallest particles of pure chemical substances that still retain their composition and chemical properties. Molecules are held together by covalent bonding or ionic bonding. Several types of non-metal elements exist only as molecules in the environment, for example, hydrogen only exists as hydrogen molecule. A molecule of a compound is made out of two or more elements, a covalent bond is a chemical bond that involves the sharing of electron pairs between atoms
9.
Potential energy
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In physics, potential energy is energy possessed by a body by virtue of its position relative to others, stresses within itself, electric charge, and other factors. The unit for energy in the International System of Units is the joule, the term potential energy was introduced by the 19th century Scottish engineer and physicist William Rankine, although it has links to Greek philosopher Aristotles concept of potentiality. Potential energy is associated with forces that act on a body in a way that the work done by these forces on the body depends only on the initial and final positions of the body in space. These forces, that are called potential forces, can be represented at every point in space by vectors expressed as gradients of a scalar function called potential. Potential energy is the energy of an object. It is the energy by virtue of a position relative to other objects. Potential energy is associated with restoring forces such as a spring or the force of gravity. The action of stretching the spring or lifting the mass is performed by a force that works against the force field of the potential. This work is stored in the field, which is said to be stored as potential energy. If the external force is removed the field acts on the body to perform the work as it moves the body back to the initial position. Suppose a ball which mass is m, and it is in h position in height, if the acceleration of free fall is g, the weight of the ball is mg. There are various types of energy, each associated with a particular type of force. Chemical potential energy, such as the energy stored in fossil fuels, is the work of the Coulomb force during rearrangement of mutual positions of electrons and nuclei in atoms and molecules. Thermal energy usually has two components, the energy of random motions of particles and the potential energy of their mutual positions. Forces derivable from a potential are also called conservative forces, the work done by a conservative force is W = − Δ U where Δ U is the change in the potential energy associated with the force. The negative sign provides the convention that work done against a force field increases potential energy, common notations for potential energy are U, V, also Ep. Potential energy is closely linked with forces, in this case, the force can be defined as the negative of the vector gradient of the potential field. If the work for a force is independent of the path, then the work done by the force is evaluated at the start
10.
Energy
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In physics, energy is the property that must be transferred to an object in order to perform work on – or to heat – the object, and can be converted in form, but not created or destroyed. The SI unit of energy is the joule, which is the transferred to an object by the mechanical work of moving it a distance of 1 metre against a force of 1 newton. Mass and energy are closely related, for example, with a sensitive enough scale, one could measure an increase in mass after heating an object. Living organisms require available energy to stay alive, such as the humans get from food. Civilisation gets the energy it needs from energy resources such as fuels, nuclear fuel. The processes of Earths climate and ecosystem are driven by the radiant energy Earth receives from the sun, the total energy of a system can be subdivided and classified in various ways. It may also be convenient to distinguish gravitational energy, thermal energy, several types of energy, electric energy. Many of these overlap, for instance, thermal energy usually consists partly of kinetic. Some types of energy are a mix of both potential and kinetic energy. An example is energy which is the sum of kinetic. Whenever physical scientists discover that a phenomenon appears to violate the law of energy conservation. Heat and work are special cases in that they are not properties of systems, in general we cannot measure how much heat or work are present in an object, but rather only how much energy is transferred among objects in certain ways during the occurrence of a given process. Heat and work are measured as positive or negative depending on which side of the transfer we view them from, the distinctions between different kinds of energy is not always clear-cut. In contrast to the definition, energeia was a qualitative philosophical concept, broad enough to include ideas such as happiness. The modern analog of this property, kinetic energy, differs from vis viva only by a factor of two, in 1807, Thomas Young was possibly the first to use the term energy instead of vis viva, in its modern sense. Gustave-Gaspard Coriolis described kinetic energy in 1829 in its modern sense, the law of conservation of energy was also first postulated in the early 19th century, and applies to any isolated system. It was argued for years whether heat was a physical substance, dubbed the caloric, or merely a physical quantity. In 1845 James Prescott Joule discovered the link between mechanical work and the generation of heat and these developments led to the theory of conservation of energy, formalized largely by William Thomson as the field of thermodynamics
11.
Entropy
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In statistical thermodynamics, entropy is a measure of the number of microscopic configurations Ω that a thermodynamic system can have when in a state as specified by some macroscopic variables. Formally, S = k B ln Ω, for example, gas in a container with known volume, pressure, and temperature could have an enormous number of possible configurations of the collection of individual gas molecules. Each instantaneous configuration of the gas may be regarded as random, Entropy may be understood as a measure of disorder within a macroscopic system. The second law of thermodynamics states that an isolated systems entropy never decreases, such systems spontaneously evolve towards thermodynamic equilibrium, the state with maximum entropy. Non-isolated systems may lose entropy, provided their environments entropy increases by at least that amount, since entropy is a function of the state of the system, a change in entropy of a system is determined by its initial and final states. This applies whether the process is reversible or irreversible, however, irreversible processes increase the combined entropy of the system and its environment. The above definition is called the macroscopic definition of entropy because it can be used without regard to any microscopic description of the contents of a system. The concept of entropy has found to be generally useful and has several other formulations. Entropy was discovered when it was noticed to be a quantity that behaves as a function of state and it has the dimension of energy divided by temperature, which has a unit of joules per kelvin in the International System of Units. But the entropy of a substance is usually given as an intensive property—either entropy per unit mass or entropy per unit amount of substance. In statistical mechanics this reflects that the state of a system is generally non-degenerate. Understanding the role of entropy in various processes requires an understanding of how. It is often said that entropy is an expression of the disorder, or randomness of a system, the second law is now often seen as an expression of the fundamental postulate of statistical mechanics through the modern definition of entropy. In other words, in any natural process there exists an inherent tendency towards the dissipation of useful energy and he made the analogy with that of how water falls in a water wheel. This was an insight into the second law of thermodynamics. g. Clausius described entropy as the transformation-content, i. e. dissipative energy use and this was in contrast to earlier views, based on the theories of Isaac Newton, that heat was an indestructible particle that had mass. Later, scientists such as Ludwig Boltzmann, Josiah Willard Gibbs, henceforth, the essential problem in statistical thermodynamics, i. e. according to Erwin Schrödinger, has been to determine the distribution of a given amount of energy E over N identical systems. Carathéodory linked entropy with a definition of irreversibility, in terms of trajectories
12.
Sintering
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Sintering is the process of compacting and forming a solid mass of material by heat or pressure without melting it to the point of liquefaction. Sintering happens naturally in mineral deposits or as a process used with metals, ceramics, plastics. The atoms in the materials diffuse across the boundaries of the particles, fusing the particles together, the study of sintering in metallurgy powder-related processes is known as powder metallurgy. An example of sintering can be observed when ice cubes in a glass of water adhere to each other, examples of pressure-driven sintering are the compacting of snowfall to a glacier, or the forming of a hard snowball by pressing loose snow together. The word sinter comes from the Middle High German sinter, a cognate of English cinder, the driving force for densification is the change in free energy from the decrease in surface area and lowering of the surface free energy by the replacement of solid-vapor interfaces. It forms new but lower-energy solid-solid interfaces with a decrease in free energy occurring. On a microscopic scale, material transfer is affected by the change in pressure, If the size of the particle is small, these effects become very large in magnitude. For properties such as strength and conductivity, the area in relation to the particle size is the determining factor. The variables that can be controlled for any material are the temperature. Through time, the radius and the vapor pressure are proportional to 2/3 and to 1/3. The source of power for solid-state processes is the change in free or chemical energy between the neck and the surface of the particle. The pore elimination occurs faster for a trial with many pores of uniform size, for the latter portions of the process, boundary and lattice diffusion from the boundary become important. Sintering is part of the process used in the manufacture of pottery. These objects are made from such as glass, alumina, zirconia, silica, magnesia, lime, beryllium oxide. Some ceramic raw materials have an affinity for water and a lower plasticity index than clay. Sintering is performed at high temperature, besides, second and/or third external force could be used. Commonly used second external force is pressure, so, the sintering that performed just using temperature is generally called pressureless sintering. Pressureless sintering is possible with graded metal-ceramic composites, with a nanoparticle sintering aid, a variant used for 3D shapes is called hot isostatic pressing
13.
Powder metallurgy
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Powder metallurgy is a term covering a wide range of ways in which materials or components are made from metal powders. PM processes can avoid, or greatly reduce, the need to use metal removal processes, thereby drastically reducing yield losses in manufacture, Powder metallurgy is also used to make unique materials impossible to get from melting or forming in other ways. A very important product of this type is tungsten carbide, WC is used to cut and form other metals and is made from WC particles bonded with cobalt. It is very used in industry for tools of many types and globally ~50. Other products include sintered filters, porous oil-impregnated bearings, electrical contacts, the PM press and sinter process generally consists of three basic steps, powder blending, die compaction, and sintering. Optional secondary processing such as coining or heat treatment often follows to obtain special properties or enhanced precision and this produces precise parts, normally very close to the die dimensions, but with 5-15% porosity, and thus sub-wrought steel properties. There are several other PM processes which have developed over the last fifty years. A preform made by the press and sinter method is heated and then hot forged to full density. Here the powder is filled into a mould, normally consisting of a metallic can of suitable shape, the can is vibrated, then evacuated and sealed. It is then placed in a hot press, where it is heated to a homologous temperature of around 0.7. This results in a part of full density with as-wrought or better. HIP was invented in the 1950-60s and entered production in the 1970-80s. In 2015, it was used to produce ~25, 000t/yr of stainless and tool steels and this part is then heated or otherwise treated to remove the binder to give a brown part. This part is then sintered, and shrinks by ~18% to give a complex, invented in the 1970s, production has increased since 2000 with an estimated global volume in 2014 of 12, 000t worth €1265millions. Powders employed in ECAS can avoid binders thanks to the possibility of direct sintering, without the need of pre-pressing, the most common of these technologies is hot pressing, which has been under use for the production of the diamond tools employed in the construction industry. Spark plasma sintering and electro sinter forging are two modern, industrial commercial ECAS technologies, additive manufacturing is a relatively novel family of techniques which use metal powders to make parts by laser sintering or melting. This is a process under development as of 2015. Sintering involves the production of a solid metal or ceramic piece from a starting powder
14.
Ceramic
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A ceramic is an inorganic, non-metallic, solid material comprising metal, non-metal or metalloid atoms primarily held in ionic and covalent bonds. This article gives an overview of ceramic materials from the point of view of materials science, the crystallinity of ceramic materials ranges from highly oriented to semi-crystalline, vitrified, and often completely amorphous. Most often, fired ceramics are either vitrified or semi-vitrified as is the case with earthenware, stoneware, varying crystallinity and electron consumption in the ionic and covalent bonds cause most ceramic materials to be good thermal and electrical insulators. With such a range of possible options for the composition/structure of a ceramic, the breadth of the subject is vast. 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 materials like silica, hardened, sintered. Later ceramics were glazed and fired to create smooth, colored surfaces, decreasing porosity through the use of glassy, ceramics now include domestic, industrial and building products, as well as a wide range of ceramic art. In the 20th century, new materials were developed for use in advanced ceramic engineering. The word ceramic comes from the Greek word κεραμικός, of pottery or for pottery, from κέραμος, potters clay, tile, 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 commonly, as the plural noun ceramics. A ceramic material is an inorganic, non-metallic, often crystalline oxide, nitride or carbide material, some elements, such as carbon or silicon, may be considered ceramics. Ceramic materials are brittle, hard, strong in compression, weak in shearing and they withstand chemical erosion that occurs in other materials subjected to acidic or caustic environments. Ceramics generally can withstand high temperatures, such as temperatures that range from 1,000 °C to 1,600 °C. Glass is often not considered a ceramic because of its amorphous character. However, glassmaking involves several steps of the process and its mechanical properties are similar to ceramic materials. Traditional ceramic raw materials include minerals such as kaolinite, whereas more recent materials include aluminium oxide. The modern ceramic materials, which are classified as advanced ceramics, include silicon 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, electrical, electronics industries. Crystalline ceramic materials are not amenable to a range of processing
15.
Chemical reactor
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In chemical engineering, chemical reactors are vessels designed to contain chemical reactions. Also referred to as a vessel, the reactants contained are substances that change form after a chemical reaction. One example is a pressure reactor, the design of a chemical reactor deals with multiple aspects of chemical engineering. Chemical engineers design reactors to maximize net present value for the given reaction, normal operating expenses include energy input, energy removal, raw material costs, labor, etc. Energy changes can come in the form of heating or cooling, pumping to increase pressure, Chemical reaction engineering is the branch of chemical engineering which deals with chemical reactors and their design, especially by application of chemical kinetics to industrial systems. Most commonly, reactors are run at steady-state, but can also be operated in a transient state, when a reactor is first brought into operation it would be considered to be in a transient state, where key process variables change with time. Furthermore, catalytic reactors require separate treatment, whether they are batch, CST, or PF reactors, typical packed bed reactors consist of a chamber, such as a tube or channel that contains catalyst particles or pellets, and a liquid that flows through the catalyst. A chemical reactor may also be a bed, see Fluidized bed reactor. Chemical reactions occurring in a reactor may be exothermic, meaning giving off heat, or endothermic, a chemical reactor vessel may have a cooling or heating jacket or cooling or heating coils wrapped around the outside of its vessel wall to cool down or heat up the contents. The simplest type of reactor is a batch reactor, materials are charged in a batch reactor, and the reaction proceeds with time. A batch reactor does not reach a state, and control of temperature, pressure. Thus, a batch reactor has ports for sensors and material input and output, batch reactors are used in small-scale production and reactions with biological materials, such as in brewing, pulping and production of enzymes. In a CSTR, one or more fluid reagents are introduced into a tank equipped with an impeller while the reactor effluent is removed. The impeller stirs the reagents to ensure proper mixing, simply dividing the volume of the tank by the average volumetric flow rate through the tank gives the space time, or the average amount of time a discrete quantity of reagent spends inside the tank. Using chemical kinetics, the reactions expected percent completion can be calculated, some important aspects of the CSTR, At steady-state, the mass flow rate in must equal the mass flow rate out, otherwise the tank will overflow or go empty. While the reactor is in a transient state the model equation must be derived from the differential mass, the reaction proceeds at the reaction rate associated with the final concentration, since the concentration is assumed to be homogenous throughout the reactor. Often, it is beneficial to operate several CSTRs in series. This allows, for example, the first CSTR to operate at a higher reagent concentration, in these cases, the sizes of the reactors may be varied in order to minimize the total capital investment required to implement the process
16.
Catalysis
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Catalysis is the increase in the rate of a chemical reaction due to the participation of an additional substance called a catalyst. In most cases, reactions occur faster with a catalyst because they require less activation energy, furthermore, since they are not consumed in the catalyzed reaction, catalysts can continue to act repeatedly. Often only tiny amounts are required in principle, in the presence of a catalyst, less free energy is required to reach the transition state, but the total free energy from reactants to products does not change. A catalyst may participate in multiple chemical transformations, the effect of a catalyst may vary due to the presence of other substances known as inhibitors or poisons or promoters. Catalyzed reactions have an activation energy than the corresponding uncatalyzed reaction, resulting in a higher reaction rate at the same temperature. However, the mechanics of catalysis is complex. Usually, the catalyst participates in this slowest step, and rates are limited by amount of catalyst, in heterogeneous catalysis, the diffusion of reagents to the surface and diffusion of products from the surface can be rate determining. A nanomaterial-based catalyst is an example of a heterogeneous catalyst, analogous events associated with substrate binding and product dissociation apply to homogeneous catalysts. Although catalysts are not consumed by the reaction itself, they may be inhibited, deactivated, in heterogeneous catalysis, typical secondary processes include coking where the catalyst becomes covered by polymeric side products. Additionally, heterogeneous catalysts can dissolve into the solution in a system or sublimate in a solid–gas system. The production of most industrially important chemicals involves catalysis, similarly, most biochemically significant processes are catalysed. Research into catalysis is a field in applied science and involves many areas of chemistry, notably organometallic chemistry. Catalysis is relevant to aspects of environmental science, e. g. the catalytic converter in automobiles. Many transition metals and transition metal complexes are used in catalysis as well, Catalysts called enzymes are important in biology. A catalyst works by providing a reaction pathway to the reaction product. The rate of the reaction is increased as this route has a lower activation energy than the reaction route not mediated by the catalyst. The disproportionation of hydrogen peroxide creates water and oxygen, as shown below,2 H2O2 →2 H2O + O2 This reaction is preferable in the sense that the reaction products are more stable than the starting material, though the uncatalysed reaction is slow. In fact, the decomposition of hydrogen peroxide is so slow that hydrogen peroxide solutions are commercially available and this reaction is strongly affected by catalysts such as manganese dioxide, or the enzyme peroxidase in organisms
17.
Steel
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Steel is an alloy of iron and other elements, primarily carbon, that is widely used in construction and other applications because of its high tensile strength and low cost. Steels base metal is iron, which is able to take on two forms, body centered cubic and face centered cubic, depending on its temperature. It is the interaction of those allotropes with the elements, primarily carbon. In the body-centred cubic arrangement, there is an atom in the centre of each cube. Carbon, other elements, and inclusions within iron act as hardening agents that prevent the movement of dislocations that otherwise occur in the lattices of iron atoms. The carbon in steel alloys may contribute up to 2. 1% of its weight. Steels strength compared to pure iron is possible at the expense of irons ductility. With the invention of the Bessemer process in the mid-19th century and this was followed by Siemens-Martin process and then Gilchrist-Thomas process that refined the quality of steel. With their introductions, mild steel replaced wrought iron, further refinements in the process, such as basic oxygen steelmaking, largely replaced earlier methods by further lowering the cost of production and increasing the quality of the product. Today, steel is one of the most common materials in the world and it is a major component in buildings, infrastructure, tools, ships, automobiles, machines, appliances, and weapons. Modern steel is generally identified by various grades defined by assorted standards organizations, the noun steel originates from the Proto-Germanic adjective stakhlijan, which is related to stakhla. The carbon content of steel is between 0. 002% and 2. 1% by weight for plain iron–carbon alloys and these values vary depending on alloying elements such as manganese, chromium, nickel, iron, tungsten, carbon and so on. Basically, steel is an alloy that does not undergo eutectic reaction. In contrast, cast iron does undergo eutectic reaction, too little carbon content leaves iron quite soft, ductile, and weak. Carbon contents higher than those of steel make an alloy, commonly called pig iron, while iron alloyed with carbon is called carbon steel, alloy steel is steel to which other alloying elements have been intentionally added to modify the characteristics of steel. Common alloying elements include, manganese, nickel, chromium, molybdenum, boron, titanium, vanadium, tungsten, cobalt, and niobium. Additional elements are important in steel, phosphorus, sulfur, silicon, and traces of oxygen, nitrogen, and copper. Alloys with a higher than 2. 1% carbon content, depending on other element content, cast iron is not malleable even when hot, but it can be formed by casting as it has a lower melting point than steel and good castability properties
18.
Semiconductor
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Semiconductors are crystalline or amorphous solids with distinct electrical characteristics. They are of high electrical resistance — higher than typical resistance materials and their resistance decreases as their temperature increases, which is behavior opposite to that of a metal. The behavior of charge carriers which include electrons, ions and electron holes at these junctions is the basis of diodes, transistors and all modern electronics. Semiconductor devices can display a range of properties such as passing current more easily in one direction than the other, showing variable resistance. The modern understanding of the properties of a semiconductor relies on quantum physics to explain the movement of carriers in a crystal lattice. Doping greatly increases the number of carriers within the crystal. When a doped semiconductor contains mostly free holes it is called p-type, the semiconductor materials used in electronic devices are doped under precise conditions to control the concentration and regions of p- and n-type dopants. A single semiconductor crystal can have many p- and n-type regions, although some pure elements and many compounds display semiconductor properties, silicon, germanium, and compounds of gallium are the most widely used in electronic devices. Elements near the so-called metalloid staircase, where the metalloids are located on the table, are usually used as semiconductors. Some of the properties of materials were observed throughout the mid 19th. The first practical application of semiconductors in electronics was the 1904 development of the Cats-whisker detector, developments in quantum physics in turn allowed the development of the transistor in 1947 and the integrated circuit in 1958. There are several developed techniques that allow semiconducting materials to behave like conducting materials and these modifications have two outcomes, n-type and p-type. These refer to the excess or shortage of electrons, respectively, an unbalanced number of electrons would cause a current to flow through the material. Heterojunctions Heterojunctions occur when two differently doped semiconducting materials are joined together, for example, a configuration could consist of p-doped and n-doped germanium. This results in an exchange of electrons and holes between the differently doped semiconducting materials, the n-doped germanium would have an excess of electrons, and the p-doped germanium would have an excess of holes. The transfer occurs until equilibrium is reached by a process called recombination, a product of this process is charged ions, which result in an electric field. Excited Electrons A difference in electric potential on a material would cause it to leave thermal equilibrium. This introduces electrons and holes to the system, which interact via a process called ambipolar diffusion, whenever thermal equilibrium is disturbed in a semiconducting material, the amount of holes and electrons changes
19.
Transport phenomena
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In engineering, physics and chemistry, the study of transport phenomena concerns the exchange of mass, energy, and momentum between observed and studied systems. While it draws from fields as diverse as continuum mechanics and thermodynamics, while it draws its theoretical foundation from principles in a number of fields, most of the fundamental transport theory is a restatement of basic conservation laws. Thus, the different phenomena that lead to transport are each considered individually with the knowledge that the sum of their contributions must equal zero and this principle is useful for calculating many relevant quantities. For example, in mechanics, a common use of transport analysis is to determine the velocity profile of a fluid flowing through a rigid volume. Transport phenomena are ubiquitous throughout the engineering disciplines and it is now considered to be a part of the engineering discipline as much as thermodynamics, mechanics, and electromagnetism. Transport phenomena encompass all agents of change in the universe. Moreover, they are considered to be building blocks which developed the universe. However, the scope here is limited to the relationship of transport phenomena to artificial engineered systems, in physics, transport phenomena are all irreversible processes of statistical nature stemming from the random continuous motion of molecules, mostly observed in fluids. Every aspect of transport phenomena is grounded in two primary concepts, the laws, and the constitutive equations. The conservation laws, which in the context of transport phenomena are formulated as continuity equations, the constitutive equations describe how the quantity in question responds to various stimuli via transport. These equations also demonstrate the connection between transport phenomena and thermodynamics, a connection that explains why transport phenomena are irreversible. Almost all of physical phenomena ultimately involve systems seeking their lowest energy state in keeping with the principle of minimum energy. As they approach this state, they tend to achieve true thermodynamic equilibrium, at which point there are no longer any driving forces in the system, examples of transport processes include heat conduction, fluid flow, molecular diffusion, radiation and electric charge transfer in semiconductors. For example, in solid state physics, the motion and interaction of electrons, holes, another example is in biomedical engineering, where some transport phenomena of interest are thermoregulation, perfusion, and microfluidics. In chemical engineering, transport phenomena are studied in reactor design, analysis of molecular or diffusive transport mechanisms, and metallurgy. The transport of mass, energy, and momentum can be affected by the presence of external sources, the rate of cooling of a solid that is conducting heat depends on whether a heat source is applied. The gravitational force acting on a rain drop counteracts the resistance or drag imparted by the surrounding air, an important principle in the study of transport phenomena is analogy between phenomena. Energy, the conduction of heat in a material is an example of heat diffusion
20.
Cell biology
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Cell biology is a branch of biology that studies the different structures and functions of the cell and focuses mainly on the idea of the cell as the basic unit of life. This is done both on a microscopic and molecular level as it encompasses prokaryotic cells and eukaryotic cells, knowing the components of cells and how cells work is fundamental to all biological sciences, it is also essential for research in bio-medical fields such as cancer, and other diseases. Research in cell biology is closely related to genetics, biochemistry, molecular biology, immunology, and developmental biology. The study of the cell is done on a level, however, most of the processes within the cell are made up of a mixture of small organic molecules, inorganic ions, hormones. Approximately 75-85% of the volume is due to water making it an indispensable solvent as a result of its polarity. These molecules within the cell, which operate as substrates, provide an environment for the cell to carry out metabolic reactions. The cell shape varies among the different types of organisms, and are then classified into two categories, eukaryotes and prokaryotes. Cell biology focuses more on the study of cells, and their signalling pathways. This environment of the cell is made up of hydrophilic and hydrophobic regions which allows for the exchange of the above-mentioned molecules and ions. The hydrophilic regions of the cell are mainly on the inside and outside of the cell, the cell membrane consists of lipids and proteins which accounts for its hydrophobicity as a result of being non-polar substances. Therefore, in order for these molecules to participate in reactions, within the cell and they accomplish this process of gaining access to the cell via, osmotic pressure, diffusion, concentration gradients, and membrane channels. Inside of the cell are extensive internal sub-cellular membrane-bounded compartments called organelles, cytoplasm - contents of the main fluid-filled space inside cells, chemical reactions also happen in this jelly-like substance. Nuclear Architecture is important for dictating nuclear function, vacuole - contain cell sap or other storage material. Vesicle - small membrane-bounded spheres inside cells which transport substances, the growth process of the cell does not refer to the size of the cell, but instead the density of the number of cells present in the organism at a given time. Cell growth pertains to the increase in the number of cells present in an organism as it grows and develops, Cells are the foundation of all organisms, they are the fundamental unit of life. The growth and development of the cell are essential for the maintenance of the host, for this process the cell goes through the steps of the cell cycle and development which involves cell growth, DNA replication, cell division, regeneration, specialization, and cell death. The cell cycle is divided into four phases, G1, S, G2. The G phases – which is the growth phase - makes up approximately 95% of the cycle
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Amino acid
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Amino acids are organic compounds containing amine and carboxyl functional groups, along with a side chain specific to each amino acid. The key elements of an acid are carbon, hydrogen, oxygen. About 500 amino acids are known and can be classified in many ways, in the form of proteins, amino acids comprise the second-largest component of human muscles, cells and other tissues. Outside proteins, amino acids perform critical roles in such as neurotransmitter transport. In biochemistry, amino acids having both the amine and the acid groups attached to the first carbon atom have particular importance. They are known as 2-, alpha-, or α-amino acids and they include the 22 proteinogenic amino acids, which combine into peptide chains to form the building-blocks of a vast array of proteins. These are all L-stereoisomers, although a few D-amino acids occur in bacterial envelopes, as a neuromodulator, twenty of the proteinogenic amino acids are encoded directly by triplet codons in the genetic code and are known as standard amino acids. The other two are selenocysteine, and pyrrolysine, pyrrolysine and selenocysteine are encoded via variant codons, for example, selenocysteine is encoded by stop codon and SECIS element. N-formylmethionine is generally considered as a form of methionine rather than as a separate proteinogenic amino acid, codon–tRNA combinations not found in nature can also be used to expand the genetic code and create novel proteins known as alloproteins incorporating non-proteinogenic amino acids. Many important proteinogenic and non-proteinogenic amino acids also play critical roles within the body. Nine proteinogenic amino acids are called essential for humans because they cannot be created from other compounds by the human body, others may be conditionally essential for certain ages or medical conditions. Essential amino acids may also differ between species, because of their biological significance, amino acids are important in nutrition and are commonly used in nutritional supplements, fertilizers, and food technology. Industrial uses include the production of drugs, biodegradable plastics, the first few amino acids were discovered in the early 19th century. In 1806, French chemists Louis-Nicolas Vauquelin and Pierre Jean Robiquet isolated a compound in asparagus that was subsequently named asparagine, cystine was discovered in 1810, although its monomer, cysteine, remained undiscovered until 1884. Glycine and leucine were discovered in 1820, usage of the term amino acid in the English language is from 1898. Proteins were found to yield amino acids after enzymatic digestion or acid hydrolysis, in the structure shown at the top of the page, R represents a side chain specific to each amino acid. The carbon atom next to the group is called the α–carbon. Amino acids containing an amino group bonded directly to the alpha carbon are referred to as amino acids
22.
Semipermeable membrane
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The rate of passage depends on the pressure, concentration, and temperature of the molecules or solutes on either side, as well as the permeability of the membrane to each solute. Depending on the membrane and the solute, permeability may depend on size, solubility, properties. How the membrane is constructed to be selective in its permeability will determine the rate, many natural and synthetic materials thicker than a membrane are also semipermeable. One example of this is the film on the inside of the egg. Note that a membrane is not the same as a selectively permeable membrane. Semi-permeable membrane describes a membrane that allows particles to pass through. An example of a biological membrane is the lipid bilayer. A group of phospholipids arranged into a double-layer, the bilayer is a semipermeable membrane that is very specific in its permeability. The hydrophilic phosphate heads are in the layer and exposed to the water content outside. The hydrophobic tails are the hidden in the inside of the membrane. The phospholipid bilayer is the most permeable to small, uncharged solutes, protein channels float through the phospholipids, and, collectively, this model is known as the fluid mosaic model. In the process of osmosis, thin film composite membranes are used. These are semipermeable membranes manufactured principally for use in water purification or desalination systems and they also have use in chemical applications such as batteries and fuel cells. In essence, a TFC material is a molecular sieve constructed in the form of a film two or more layered materials. Prof. Sidney Loeb and Srinivasa Sourirajan invented the first practical synthetic semi-permeable membrane, another example of a semipermeable membrane is dialysis tubing. The semipermeable membrane is pertinent to cellular communication, a cell membrane consists of proteins and phospholipids. Signaling molecules send chemical messages to the proteins in the cell membrane, the signaling molecules bind to proteins, which alters the protein. A message is sent into to the cell and a cellular response is turned on or activated
23.
Osmosis
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It may also be used to describe a physical process in which any solvent moves across a semipermeable membrane separating two solutions of different concentrations. Osmosis can be made to do work, osmotic pressure is defined as the external pressure required to be applied so that there is no net movement of solvent across the membrane. Osmotic pressure is a property, meaning that the osmotic pressure depends on the molar concentration of the solute. Osmosis is a process in biological systems, as biological membranes are semipermeable. Permeability depends on solubility, charge, or chemistry, as well as solute size, water molecules travel through the plasma membrane, tonoplast membrane or protoplast by diffusing across the phospholipid bilayer via aquaporins. Osmosis provides the means by which water is transported into. The turgor pressure of a cell is maintained by osmosis across the cell membrane between the cell interior and its relatively hypotonic environment. Jean-Antoine Nollet first documented observation of osmosis in 1748, the word osmosis descends from the words endosmose and exosmose, which were coined by French physician René Joachim Henri Dutrochet from the Greek words ἔνδον, ἔξω, and ὠσμός. Osmosis is the movement of a solvent across a membrane toward a higher concentration of solute. In biological systems, the solvent is water, but osmosis can occur in other liquids, supercritical liquids. When a cell is submerged in water, the water pass through the cell membrane from an area of low solute concentration to high solute concentration. For example, if the cell is submerged in saltwater, water molecules out of the cell. If a cell is submerged in freshwater, water molecules move into the cell, when the membrane has a volume of pure water on both sides, water molecules pass in and out in each direction at exactly the same rate. There is no net flow of water through the membrane, the mechanism responsible for driving osmosis has commonly been represented in biology and chemistry texts as either the dilution of water by solute or by a solutes attraction to water. Both of these notions have been conclusively refuted, the diffusion model of osmosis is rendered untenable by the fact that osmosis can drive water across a membrane toward a higher concentration of water. The bound water model is refuted by the fact that osmosis is independent of the size of the solute molecules—a colligative property—or how hydrophilic they are, one fact to take note of is that heat from the surroundings is able to be converted into mechanical energy. Osmotic pressure is the cause of support in many plants. The osmotic entry of water raises the pressure exerted against the cell wall, until it equals the osmotic pressure
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Metabolism
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Metabolism is the set of life-sustaining chemical transformations within the cells of living organisms. These enzyme-catalyzed reactions allow organisms to grow and reproduce, maintain their structures, usually, breaking down releases energy and building up consumes energy. The chemical reactions of metabolism are organized into metabolic pathways, in one chemical is transformed through a series of steps into another chemical. Enzymes act as catalysts that allow the reactions to proceed more rapidly, enzymes also allow the regulation of metabolic pathways in response to changes in the cells environment or to signals from other cells. The metabolic system of a particular organism determines which substances it will find nutritious, for example, some prokaryotes use hydrogen sulfide as a nutrient, yet this gas is poisonous to animals. The speed of metabolism, the rate, influences how much food an organism will require. A striking feature of metabolism is the similarity of the metabolic pathways. These striking similarities in metabolic pathways are likely due to their appearance in evolutionary history. Most of the structures that make up animals, plants and microbes are made from three classes of molecule, amino acids, carbohydrates and lipids. These biochemicals can be joined together to make such as DNA and proteins. Proteins are made of amino acids arranged in a linear chain joined together by peptide bonds, many proteins are enzymes that catalyze the chemical reactions in metabolism. Other proteins have structural or mechanical functions, such as those that form the cytoskeleton, Proteins are also important in cell signaling, immune responses, cell adhesion, active transport across membranes, and the cell cycle. Lipids are the most diverse group of biochemicals and their main structural uses are as part of biological membranes both internal and external, such as the cell membrane, or as a source of energy. Lipids are usually defined as hydrophobic or amphipathic biological molecules but will dissolve in organic solvents such as benzene or chloroform, the fats are a large group of compounds that contain fatty acids and glycerol, a glycerol molecule attached to three fatty acid esters is called a triacylglyceride. Several variations on this structure exist, including alternate backbones such as sphingosine in the sphingolipids. Steroids such as cholesterol are another class of lipids. Carbohydrates are aldehydes or ketones, with hydroxyl groups attached. Carbohydrates are the most abundant biological molecules, and fill numerous roles, such as the storage and transport of energy, the basic carbohydrate units are called monosaccharides and include galactose, fructose, and most importantly glucose
25.
Pulmonary alveolus
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An alveolus is a hollow cavity found in the lung parenchyma, and is the basic unit of respiration. Lung alveoli are the ends of the tree, branching from either alveolar sacs or alveolar ducts. Alveoli are particular to mammalian lungs, different structures are involved in gas exchange in other vertebrates. The alveolar membrane is the gas-exchange surface, carbon dioxide rich blood is pumped from the rest of the body into the alveolar blood vessels where, through diffusion, it releases its carbon dioxide and absorbs oxygen. The alveoli are located in the zone of the lungs, at the ends of the alveolar ducts and alveolar sac. They provide total surface area of about 75 m2, a typical pair of human lungs contain about 700 million alveoli, producing 70m2 of surface area. Each alveolus is wrapped in a mesh of capillaries covering about 70% of its area. An adult alveolus has an diameter of 200 micrometres, with an increase in diameter during inhalation. The alveoli consist of a layer and an extracellular matrix surrounded by small blood vessels called capillaries. In some alveolar walls there are pores between alveoli called Pores of Kohn, the alveoli contain some collagen and elastic fibres. The elastic fibres allow the alveoli to stretch as they are filled with air during inhalation and they then spring back during exhalation in order to expel the carbon dioxide-rich air. There are three types of cell in the alveolar wall–two types of alveolar cell and a phagocyte. Surfactant is continuously released by exocytosis and it forms an underlying aqueous protein-containing hypophase and an overlying phospholipid film composed primarily of dipalmitoyl phosphatidylcholine. The phagocytes are macrophages, that destroy foreign material, such as bacteria, the wall of each alveolus, is lined by thin, flat, type I cells, contains numerous capillaries, and is the site of gas exchange, which occurs by diffusion. The relatively low solubility of oxygen necessitates the large surface area. Weaving between the capillaries and helping to support them is a fabric of elastic and collagenous fibres. The collagen fibres, being rigid, give the wall firmness. The other type II cells in the wall are granular cells, which secrete surfactant
26.
Mammal
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Mammals are any vertebrates within the class Mammalia, a clade of endothermic amniotes distinguished from reptiles by the possession of a neocortex, hair, three middle ear bones and mammary glands. All female mammals nurse their young with milk, secreted from the mammary glands, Mammals include the largest animals on the planet, the great whales. The basic body type is a quadruped, but some mammals are adapted for life at sea, in the air, in trees. The largest group of mammals, the placentals, have a placenta, Mammals range in size from the 30–40 mm bumblebee bat to the 30-meter blue whale. With the exception of the five species of monotreme, all modern mammals give birth to live young, most mammals, including the six most species-rich orders, belong to the placental group. The largest orders are the rodents, bats and Soricomorpha, the next three biggest orders, depending on the biological classification scheme used, are the Primates, the Cetartiodactyla, and the Carnivora. Living mammals are divided into the Yinotheria and Theriiformes There are around 5450 species of mammal, in some classifications, extant mammals are divided into two subclasses, the Prototheria, that is, the order Monotremata, and the Theria, or the infraclasses Metatheria and Eutheria. The marsupials constitute the group of the Metatheria, and include all living metatherians as well as many extinct ones. Much of the changes reflect the advances of cladistic analysis and molecular genetics, findings from molecular genetics, for example, have prompted adopting new groups, such as the Afrotheria, and abandoning traditional groups, such as the Insectivora. The mammals represent the only living Synapsida, which together with the Sauropsida form the Amniota clade, the early synapsid mammalian ancestors were sphenacodont pelycosaurs, a group that produced the non-mammalian Dimetrodon. At the end of the Carboniferous period, this group diverged from the line that led to todays reptiles. Some mammals are intelligent, with some possessing large brains, self-awareness, Mammals can communicate and vocalize in several different ways, including the production of ultrasound, scent-marking, alarm signals, singing, and echolocation. Mammals can organize themselves into fission-fusion societies, harems, and hierarchies, most mammals are polygynous, but some can be monogamous or polyandrous. They provided, and continue to provide, power for transport and agriculture, as well as commodities such as meat, dairy products, wool. Mammals are hunted or raced for sport, and are used as model organisms in science, Mammals have been depicted in art since Palaeolithic times, and appear in literature, film, mythology, and religion. Defaunation of mammals is primarily driven by anthropogenic factors, such as poaching and habitat destruction, Mammal classification has been through several iterations since Carl Linnaeus initially defined the class. No classification system is accepted, McKenna & Bell and Wilson & Reader provide useful recent compendiums. Though field work gradually made Simpsons classification outdated, it remains the closest thing to a classification of mammals
27.
Oxygen
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Oxygen is a chemical element with symbol O and atomic number 8. It is a member of the group on the periodic table and is a highly reactive nonmetal. By mass, oxygen is the third-most abundant element in the universe, after hydrogen, at standard temperature and pressure, two atoms of the element bind to form dioxygen, a colorless and odorless diatomic gas with the formula O2. This is an important part of the atmosphere and diatomic oxygen gas constitutes 20. 8% of the Earths atmosphere, additionally, as oxides the element makes up almost half of the Earths crust. Most of the mass of living organisms is oxygen as a component of water, conversely, oxygen is continuously replenished by photosynthesis, which uses the energy of sunlight to produce oxygen from water and carbon dioxide. Oxygen is too reactive to remain a free element in air without being continuously replenished by the photosynthetic action of living organisms. Another form of oxygen, ozone, strongly absorbs ultraviolet UVB radiation, but ozone is a pollutant near the surface where it is a by-product of smog. At low earth orbit altitudes, sufficient atomic oxygen is present to cause corrosion of spacecraft, the name oxygen was coined in 1777 by Antoine Lavoisier, whose experiments with oxygen helped to discredit the then-popular phlogiston theory of combustion and corrosion. One of the first known experiments on the relationship between combustion and air was conducted by the 2nd century BCE Greek writer on mechanics, Philo of Byzantium. In his work Pneumatica, Philo observed that inverting a vessel over a burning candle, Philo incorrectly surmised that parts of the air in the vessel were converted into the classical element fire and thus were able to escape through pores in the glass. Many centuries later Leonardo da Vinci built on Philos work by observing that a portion of air is consumed during combustion and respiration, Oxygen was discovered by the Polish alchemist Sendivogius, who considered it the philosophers stone. In the late 17th century, Robert Boyle proved that air is necessary for combustion, English chemist John Mayow refined this work by showing that fire requires only a part of air that he called spiritus nitroaereus. From this he surmised that nitroaereus is consumed in both respiration and combustion, Mayow observed that antimony increased in weight when heated, and inferred that the nitroaereus must have combined with it. Accounts of these and other experiments and ideas were published in 1668 in his work Tractatus duo in the tract De respiratione. Robert Hooke, Ole Borch, Mikhail Lomonosov, and Pierre Bayen all produced oxygen in experiments in the 17th and the 18th century but none of them recognized it as a chemical element. This may have been in part due to the prevalence of the philosophy of combustion and corrosion called the phlogiston theory, which was then the favored explanation of those processes. Established in 1667 by the German alchemist J. J. Becher, one part, called phlogiston, was given off when the substance containing it was burned, while the dephlogisticated part was thought to be its true form, or calx. The fact that a substance like wood gains overall weight in burning was hidden by the buoyancy of the combustion products
28.
Isotopic labeling
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Isotopic labeling is a technique used to track the passage of an isotope through a reaction, metabolic pathway, or cell. The reactant is labeled by replacing specific atoms by their isotope, the reactant is then allowed to undergo the reaction. The position of the isotopes in the products is measured to determine the sequence the isotopic atom followed in the reaction or the metabolic pathway. The nuclides used in isotopic labeling may be stable nuclides or radionuclides, in the latter case, the labeling is called radiolabeling. In isotopic labeling, there are ways to detect the presence of labeling isotopes, through their mass, vibrational mode. Mass spectrometry detects the difference in a mass, while infrared spectroscopy detects the difference in the isotopes vibrational modes. Nuclear magnetic resonance detects atoms with different gyromagnetic ratios, the radioactive decay can be detected through an ionization chamber or autoradiographs of gels. An example of the use of isotopic labeling is the study of phenol in water by replacing common hydrogen with deuterium, only the hydroxyl group was affected, indicating that the other 5 hydrogen atoms did not participate in these exchange reactions. An isotopic tracer, is used in chemistry and biochemistry to help understand chemical reactions and interactions, in this technique, one or more of the atoms of the molecule of interest is substituted for an atom of the same chemical element, but of a different isotope. The difference in the number of neutrons, however, means that it can be detected separately from the atoms of the same element. Nuclear magnetic resonance and mass spectrometry are used to investigate the mechanisms of chemical reactions, NMR and MS detects isotopic differences, which allows information about the position of the labeled atoms in the products structure to be determined. With information on the positioning of the atoms in the products. Radioactive isotopes can be tested using the autoradiographs of gels in gel electrophoresis, the radiation emitted by compounds containing the radioactive isotopes darkens a piece of photographic film, recording the position of the labeled compounds relative to one another in the gel. Isotope tracers are used in the form of isotope ratios. Isotopic tracers are some of the most important tools in geology because they can be used to understand complex mixing processes in earth systems, further discussion of the application of isotopic tracers in geology is covered under the heading of isotope geochemistry. Isotopic tracers are usually subdivided into two categories, stable isotope tracers and radiogenic isotope tracers, stable isotope tracers involve only non-radiogenic isotopes and usually are mass-dependent. In theory, any element with two isotopes can be used as an isotopic tracer. However, the most commonly used stable isotope tracers involve relatively light isotopes, a radiogenic isotope tracer involves an isotope produced by radioactive decay, which is usually in a ratio with a non-radiogenic isotope
29.
Kinetic isotope effect
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The kinetic isotope effect is the change in the rate of a chemical reaction when one of the atoms in the reactants is substituted with one of its isotopes. For example, kinetic isotope effects can be used to reveal whether a nucleophilic substitution reaction follows a unimolecular or bimolecular pathway, in the reaction of methyl bromide and cyanide, the observed methyl carbon kinetic isotope effect indicates an SN2 mechanism. Isotopic rate changes are most pronounced when the mass change is greatest. For instance, changing an atom to its isotope deuterium represents a 100% increase in mass, whereas in replacing carbon-12 with carbon-13. The rate of a reaction involving a C–H bond is typically 6–10 times faster than the corresponding C–D bond, isotopic substitution can modify the rate of reaction in a variety of ways. Heavier isotopes will lead to vibration frequencies, or, viewed quantum mechanically. With a lower zero-point energy, more energy must be supplied to break the bond, resulting in an activation energy for bond cleavage. A primary kinetic isotope effect may be found when a bond to the isotopically labeled atom is being formed or broken, interpretation of the leaving group kinetic isotope effects had been difficult at first due to significant contributions from temperature independent factors. A secondary kinetic isotope effect is observed when no bond to the substituted atom in the reactant is broken or formed in the rate-determining step of a reaction. For the aforementioned nucleophilic substitution reactions, secondary hydrogen isotope effects at the α-carbon provide a direct means to distinguish between SN1 and SN2 reactions. The magnitudes of such secondary isotope effects at the α-carbon are largely determined by the Cα-H vibrations, depending on the mass and energy of the reacting species, quantum mechanical tunneling may also make a large contribution to an observed kinetic isotope effect. Of the various treatments of kinetic isotope effects, Bigeleisens seems to be the most popular. His basic general equation for the kinetic isotope effects is given below. The subscripts H and D refer to hydrogen and deuterium substituted substrates, K H k D =3212 × e −12 The S factors are the symmetry numbers for the reactants and transition states. The M factors are the masses of the corresponding species. The ui factors are determined from the corresponding vibrational frequencies, νi, N and N‡ are the number of atoms in the reactants and the transition states, respectively. K H k D = S × M M I × E X C × Z P E The S factor is the ratio of the numbers for the various species. These symmetry numbers do not lead to isotopic fractionation, so the S factor can be set to be 1.000, the MMI factor refers to the ratio of the molecular masses and the moments of inertia
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Nuclear magnetic resonance
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Nuclear magnetic resonance is a physical phenomenon in which nuclei in a magnetic field absorb and re-emit electromagnetic radiation. NMR allows the observation of quantum mechanical magnetic properties of the atomic nucleus. Many scientific techniques exploit NMR phenomena to study physics, crystals. NMR is also used in advanced medical imaging techniques, such as in magnetic resonance imaging. The most commonly studied nuclei are 1H and 13C, although nuclei from isotopes of other elements have been studied by high-field NMR spectroscopy as well. A key feature of NMR is that the frequency of a particular substance is directly proportional to the strength of the applied magnetic field. Since the resolution of the technique depends on the magnitude of magnetic field gradient, many efforts are made to develop increased field strength. The effectiveness of NMR can also be improved using hyperpolarization, and/or using two-dimensional, three-dimensional and higher-dimensional multi-frequency techniques, the principle of NMR usually involves two sequential steps, The alignment of the magnetic nuclear spins in an applied, constant magnetic field B0. The perturbation of this alignment of the nuclear spins by employing an electro-magnetic, the required perturbing frequency is dependent upon the static magnetic field and the nuclei of observation. The two fields are chosen to be perpendicular to each other as this maximizes the NMR signal strength. The resulting response by the magnetization of the nuclear spins is the phenomenon that is exploited in NMR spectroscopy. NMR phenomena are also utilized in low-field NMR, NMR spectroscopy and MRI in the Earths magnetic field, in 1946, Felix Bloch and Edward Mills Purcell expanded the technique for use on liquids and solids, for which they shared the Nobel Prize in Physics in 1952. Yevgeny Zavoisky likely observed nuclear magnetic resonance in 1941, well before Felix Bloch and Edward Mills Purcell, russell H. Varian filed the Method and means for correlating nuclear properties of atoms and magnetic fields, U. S. Patent 2,561,490 on July 24,1951, Varian Associates developed the first NMR unit called NMR HR-30 in 1952. Purcell had worked on the development of radar during World War II at the Massachusetts Institute of Technologys Radiation Laboratory. His work during that project on the production and detection of radio frequency power, when this absorption occurs, the nucleus is described as being in resonance. Different atomic nuclei within a molecule resonate at different frequencies for the magnetic field strength. The observation of magnetic resonance frequencies of the nuclei present in a molecule allows any trained user to discover essential chemical and structural information about the molecule
31.
Spin echo
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In magnetic resonance, a spin echo is the refocusing of spin magnetisation by a pulse of resonant electromagnetic radiation. Modern nuclear magnetic resonance and magnetic resonance imaging make use of this effect, the first of these, relaxation, leads to an irreversible loss of magnetisation. However, the inhomogeneous dephasing can be removed by applying a 180° inversion pulse that inverts the magnetisation vectors, examples of inhomogeneous effects include a magnetic field gradient and a distribution of chemical shifts. If the inversion pulse is applied after a period t of dephasing, in simple cases, the intensity of the echo relative to the initial signal is given by e−2t/T2 where T2 is the time constant for spin-spin relaxation. Echo phenomena are important features of coherent spectroscopy which have used in fields other than magnetic resonance including laser spectroscopy. Echoes were first detected in nuclear magnetic resonance by Erwin Hahn in 1950, in nuclear magnetic resonance and magnetic resonance imaging, radiofrequency radiation is most commonly used. In 1972 F. Mezei introduced spin echo neutron scattering, a technique that can be used to study magnons and phonons in single crystals, the technique is now applied in research facilities using triple axis spectrometers. The spin echo effect was discovered by Erwin Hahn when he applied two successive 90° pulses separated by time period, but detected a signal, the echo. Six spins are shown above and these are not given the chance to dephase significantly, the size of the echo is recorded for different spacings of the two pulses. This reveals the decoherence which is not refocused by the π pulse, in simple cases, an exponential decay is measured which is described by the T2 time. Hahns 1950 paper showed that another method for generating spin echoes is to apply three successive 90° pulses, after the first 90° pulse, the magnetization vector spreads out as described above, forming what can be thought of as a “pancake” in the x-y plane. The spreading continues for a time τ, and then a second 90° pulse is applied such that the “pancake” is now in the x-z plane. After a further time T a third pulse is applied and an echo is observed after waiting a time τ after the last pulse. Hahn echos have also observed at optical frequencies. For this, resonant light is applied to a material with a broadened absorption resonance. Instead of using two spin states in a field, photon echoes use two energy levels that are present in the material even in zero magnetic field. Spin Choreography, Basic Steps in High Resolution NMR, spin Dynamics, Basics of Nuclear Magnetic Resonance. Principles of Pulse Electron Paramagnetic Resonance
32.
Chemical equilibrium
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In a chemical reaction, chemical equilibrium is the state in which both reactants and products are present in concentrations which have no further tendency to change with time. Usually, this results when the forward reaction proceeds at the same rate as the reverse reaction. The reaction rates of the forward and backward reactions are not zero. Thus, there are no net changes in the concentrations of the reactant, such a state is known as dynamic equilibrium. The concept of chemical equilibrium was developed after Berthollet found that chemical reactions are reversible. For any reaction mixture to exist at equilibrium, the rates of the forward and backward reactions are equal, conversely the equilibrium position is said to be far to the left if hardly any product is formed from the reactants. Since at equilibrium forward and backward rates are equal, k + α β = k − σ τ, K c = k + k − = σ τ α β By convention the products form the numerator. Equality of forward and backward reaction rates, however, is a condition for chemical equilibrium. Adding a catalyst will affect both the reaction and the reverse reaction in the same way and will not have an effect on the equilibrium constant. The catalyst will speed up both reactions thereby increasing the speed at which equilibrium is reached, although the macroscopic equilibrium concentrations are constant in time, reactions do occur at the molecular level. This is an example of dynamic equilibrium, equilibria, like the rest of thermodynamics, are statistical phenomena, averages of microscopic behavior. Le Châteliers principle gives an idea of the behavior of a system when changes to its reaction conditions occur. If a dynamic equilibrium is disturbed by changing the conditions, the position of equilibrium moves to partially reverse the change. For example, adding more S from the outside will cause an excess of products, and this can also be deduced from the equilibrium constant expression for the reaction, K = If increases must increase and CH 3CO−2 must decrease. The H2O is left out, as it is the solvent and its concentration remains high, a quantitative version is given by the reaction quotient. J. W. Gibbs suggested in 1873 that equilibrium is attained when the Gibbs free energy of the system is at its minimum value, what this means is that the derivative of the Gibbs energy with respect to reaction coordinate vanishes, signalling a stationary point. This derivative is called the reaction Gibbs energy and corresponds to the difference between the chemical potentials of reactants and products at the composition of the reaction mixture and this criterion is both necessary and sufficient. If a mixture is not at equilibrium, the liberation of the excess Gibbs energy is the force for the composition of the mixture to change until equilibrium is reached
33.
Brownian motion
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Brownian motion or pedesis is the random motion of particles suspended in a fluid resulting from their collision with the fast-moving atoms or molecules in the gas or liquid. This transport phenomenon is named after the botanist Robert Brown and this explanation of Brownian motion served as convincing evidence that atoms and molecules exist, and was further verified experimentally by Jean Perrin in 1908. Perrin was awarded the Nobel Prize in Physics in 1926 for his work on the structure of matter. Brownian motion is among the simplest of the stochastic processes. This universality is closely related to the universality of the normal distribution, in both cases, it is often mathematical convenience, rather than the accuracy of the models, that motivates their use. The Roman Lucretiuss scientific poem On the Nature of Things has a description of Brownian motion of dust particles in verses 113 –140 from Book II. He uses this as a proof of the existence of atoms, Observe what happens when sunbeams are admitted into a building and you will see a multitude of tiny particles mingling in a multitude of ways. Their dancing is an indication of underlying movements of matter that are hidden from our sight. It originates with the atoms which move of themselves, then those small compound bodies that are least removed from the impetus of the atoms are set in motion by the impact of their invisible blows and in turn cannon against slightly larger bodies. So the movement mounts up from the atoms and gradually emerges to the level of our senses, so that those bodies are in motion that we see in sunbeams, moved by blows that remain invisible. Although the mingling motion of dust particles is caused largely by air currents, while Jan Ingenhousz described the irregular motion of coal dust particles on the surface of alcohol in 1785, the discovery of this phenomenon is often credited to the botanist Robert Brown in 1827. Brown was studying pollen grains of the plant Clarkia pulchella suspended in water under a microscope when he observed minute particles, ejected by the pollen grains, executing a jittery motion. By repeating the experiment with particles of matter he was able to rule out that the motion was life-related. The first person to describe the mathematics behind Brownian motion was Thorvald N. Thiele in a paper on the method of least squares published in 1880. This was followed independently by Louis Bachelier in 1900 in his PhD thesis The theory of speculation, in which he presented an analysis of the stock. The Brownian motion model of the market is often cited. Albert Einstein and Marian Smoluchowski brought the solution of the problem to the attention of physicists and their equations describing Brownian motion were subsequently verified by the experimental work of Jean Baptiste Perrin in 1908. In this way Einstein was able to determine the size of atoms, in accordance to Avogadros law this volume is the same for all ideal gases, which is 22.414 liters at standard temperature and pressure
34.
Concentration
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In chemistry, concentration is the abundance of a constituent divided by the total volume of a mixture. Several types of mathematical description can be distinguished, mass concentration, molar concentration, number concentration, the term concentration can be applied to any kind of chemical mixture, but most frequently it refers to solutes and solvents in solutions. The molar concentration has variants such as concentration and osmotic concentration. To concentrate a solution, one must add more solute, or reduce the amount of solvent, by contrast, to dilute a solution, one must add more solvent, or reduce the amount of solute. Unless two substances are fully miscible there exists a concentration at which no further solute will dissolve in a solution, at this point, the solution is said to be saturated. If additional solute is added to a solution, it will not dissolve, except in certain circumstances. Instead, phase separation will occur, leading to coexisting phases, the point of saturation depends on many variables such as ambient temperature and the precise chemical nature of the solvent and solute. Concentrations are often called levels, reflecting the mental schema of levels on the axis of a graph. There are four quantities that describe concentration, The mass concentration ρ i is defined as the mass of a constituent m i divided by the volume of the mixture V, ρ i = m i V. The molar concentration c i is defined as the amount of a constituent n i divided by the volume of the mixture V, c i = n i V, however, more commonly the unit mol/L is used. The number concentration C i is defined as the number of entities of a constituent N i in a divided by the volume of the mixture V, C i = N i V. The volume concentration ϕ i is defined as the volume of a constituent V i divided by the volume of the mixture V, ϕ i = V i V. Being dimensionless, it is expressed as a number, e. g.0.18 or 18%, several other quantities can be used to describe the composition of a mixture. Note that these should not be called concentrations, normality is defined as the molar concentration c i divided by an equivalence factor f e q. Since the definition of the equivalence factor depends on context, IUPAC, the SI unit for molality is mol/kg. The mole fraction x i is defined as the amount of a constituent n i divided by the amount of all constituents in a mixture n t o t, x i = n i n t o t. However, the deprecated parts-per notation is used to describe small mole fractions. The mole ratio r i is defined as the amount of a constituent n i divided by the amount of all other constituents in a mixture
35.
Precipitation (chemistry)
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Precipitation is the creation of a solid from a solution. When the reaction occurs in a solution, the solid formed is called the precipitate. The chemical that causes the solid to form is called the precipitant, without sufficient force of gravity to bring the solid particles together, the precipitate remains in suspension. After sedimentation, especially when using a centrifuge to press it into a compact mass, Precipitation can be used as a medium. The precipitate-free liquid remaining above the solid is called the supernate or supernatant, powders derived from precipitation have also historically been known as flowers. When the solid appears in the form of fibers which have been through chemical processing. Sometimes the formation of a precipitate indicates the occurrence of a chemical reaction, if silver nitrate solution is poured into a solution of sodium chloride, a chemical reaction occurs forming a white precipitate of silver chloride. When potassium iodide solution reacts with lead nitrate solution, a precipitate of lead iodide is formed. Precipitation may occur if the concentration of a compound exceeds its solubility, Precipitation may occur rapidly from a supersaturated solution. Precipitation in solids is routinely used to synthesize nanoclusters, an important stage of the precipitation process is the onset of nucleation. The creation of a solid particle includes the formation of an interface, which requires some energy based on the relative surface energy of the solid. If this energy is not available, and no suitable nucleation surface is available, Precipitation reactions can be used for making pigments, removing salts from water in water treatment, and in classical qualitative inorganic analysis. Precipitation is also useful to isolate the products of a reaction during workup, ideally, the product of the reaction is insoluble in the reaction solvent. Thus, it precipitates as it is formed, preferably forming pure crystals, an example of this would be the synthesis of porphyrins in refluxing propionic acid. Thereafter, the precipitate may easily be separated by filtration, decanting, an example would be the synthesis of chromic tetraphenylporphyrin chloride, water is added to the DMF reaction solution, and the product precipitates. Precipitation is also useful in purifying products, crude bmim-Cl is taken up in acetonitrile, and dropped into ethyl acetate, another important application of an antisolvent is in ethanol precipitation of DNA. In metallurgy, precipitation from a solution is also a useful way to strengthen alloys. An example of a reaction, Aqueous silver nitrate is added to a solution containing potassium chloride
36.
Anomalous diffusion
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Physically, the MSD can be considered the amount of space the particle has explored in the system. Unlike typical diffusion, anomalous diffusion is described by a law, σr2 ~ Dtα. In a typical process, α =1. If α >1, the phenomenon is called super-diffusion, super-diffusion can be the result of active cellular transport processes. If α <1, the particle undergoes sub-diffusion, sub-diffusion has been proposed as a measure of macromolecular crowding in the cytoplasm. Anomalous diffusion was found in other biological systems, including heartbeat intervals. The daily fluctuations of climate variables such as temperature can be regarded as steps of a walker or diffusion and have been found to be anomalous. Of interest within the community, when an anomalous-type diffusion process is discovered. There are a number of frameworks which give rise to anomalous diffusion that are currently in vogue within the physics community. Anomalous Subdiffusion Is a Measure for Cytoplasmic Crowding in Living Cells, propagation of a chemical wave front in a quasi-two-dimensional superdiffusive flow. CS1 maint, Explicit use of et al, Chen, Wen, Sun, HongGuang, Zhang, Xiaodi, Korosak, Dean. Anomalous diffusion modeling by fractal and fractional derivatives, Sun, HongGuang, Meerschaert, Mark M. Zhang, Yong, Zhu, Jianting, Chen, Wen. A fractal Richards equation to capture the scaling of water transport in unsaturated media. Boltzmanns transformation, Parabolic law Anomalous interface shift kinetics
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