In anatomy, a crystallin is a water-soluble structural protein found in the lens and the cornea of the eye accounting for the transparency of the structure. It has been identified in other places such as the heart, in aggressive breast cancer tumors. Since it has been shown that lens injury may promote nerve regeneration, crystallin has been an area of neural research. So far, it has been demonstrated; the main function of crystallins at least in the lens of the eye is to increase the refractive index while not obstructing light. However, this is not their only function, it has become clear that crystallins may have several metabolic and regulatory functions, both within the lens and in other parts of the body. More proteins containing βγ-crystallin domains have now been characterized as calcium binding proteins with Greek key motif as a novel calcium-binding motif; some crystallins are active enzymes, while others show homology to other enzymes. The crystallins of different groups of organisms are related to a large number of different proteins, with those from birds and reptiles related to lactate dehydrogenase and argininosuccinate lyase, those of mammals to alcohol dehydrogenase and quinone reductase, those of cephalopods to glutathione S-transferase and aldehyde dehydrogenase.
Whether these crystallins are products of a fortuitous accident of evolution, in that these particular enzymes happened to be transparent and soluble, or whether these diverse enzymatic activities are part of the protective machinery of the lens, is an active research topic. The recruitment of protein that evolved with one function to serve a second, unrelated function is an example of an exaptation. Crystallins from a vertebrate eye lens are classified into three main types: alpha and gamma crystallins; these distinctions are based on the order in which they elute from a gel filtration chromatography column. These are called ubiquitous crystallins. Beta- and gamma-crystallins are similar in sequence and domains topology, thus have been grouped together as a protein superfamily called βγ-Crystallins; the α-crystallin family and βγ-crystallins compose the major family of proteins present in the crystalline lens. They occur in all vertebrate classes. In addition to these crystallins there are other taxon-specific crystallins which are only found in the lens of some organisms.
For example, alpha and delta crystallins are found in avian and reptilian lenses, the alpha and gamma families are found in the lenses of all other vertebrates. Alpha-crystallin occurs as large aggregates, comprising two types of related subunits that are similar to the small heat shock proteins in their C-terminal halves; the relationship between these families is one of classic gene duplication and divergence, from the small HSP family, allowing adaptation to novel functions. Divergence occurred prior to evolution of the eye lens, alpha-crystallin being found in small amounts in tissues outside the lens. Alpha-crystallin has chaperone-like properties including the ability to prevent the precipitation of denatured proteins and to increase cellular tolerance to stress, it has been suggested that these functions are important for the maintenance of lens transparency and the prevention of cataracts. This is supported by the observation that alpha-crystallin mutations show an association with cataract formation.
The N-terminal domain of alpha-crystallin is not necessary for dimerisation or chaperone activity, but appears to be required for the formation of higher order aggregates. Beta and gamma- crystallin form a separate family. Structurally and gamma crystallins are composed of two similar domains which, in turn, are each composed of two similar motifs with the two domains connected by a short connecting peptide; each motif, about forty amino acid residues long, is folded in a distinctive Greek key pattern. However, beta crystallin is an oligomer, composed of a complex group of molecules, whereas gamma crystallin is a simpler monomer. Crystallins at the US National Library of Medicine Medical Subject Headings alpha-Crystallins at the US National Library of Medicine Medical Subject Headings Lens Crystallin Crystal Structures by Christine Slingsby, Birkbeck College Crystallins: Molecule of the Month, by David Goodsell, RCSB Protein Data Bank
Solid is one of the four fundamental states of matter. In solids particles are packed, it is characterized by structural resistance to changes of shape or volume. Unlike liquid, a solid object does not flow to take on the shape of its container, nor does it expand to fill the entire volume available to it like a gas does; the atoms in a solid are bound to each other, either in a regular geometric lattice or irregularly. Solids cannot be compressed with little pressure whereas gases can be compressed with little pressure because in gases molecules are loosely packed; the branch of physics that deals with solids is called solid-state physics, is the main branch of condensed matter physics. Materials science is concerned with the physical and chemical properties of solids. Solid-state chemistry is concerned with the synthesis of novel materials, as well as the science of identification and chemical composition; the atoms, molecules or ions that make up solids may be arranged in an orderly repeating pattern, or irregularly.
Materials whose constituents are arranged in a regular pattern are known as crystals. In some cases, the regular ordering can continue unbroken over a large scale, for example diamonds, where each diamond is a single crystal. Solid objects that are large enough to see and handle are composed of a single crystal, but instead are made of a large number of single crystals, known as crystallites, whose size can vary from a few nanometers to several meters; such materials are called polycrystalline. All common metals, many ceramics, are polycrystalline. In other materials, there is no long-range order in the position of the atoms; these solids are known as amorphous solids. Whether a solid is crystalline or amorphous depends on the material involved, the conditions in which it was formed. Solids that are formed by slow cooling will tend to be crystalline, while solids that are frozen are more to be amorphous; the specific crystal structure adopted by a crystalline solid depends on the material involved and on how it was formed.
While many common objects, such as an ice cube or a coin, are chemically identical throughout, many other common materials comprise a number of different substances packed together. For example, a typical rock is an aggregate of several different minerals and mineraloids, with no specific chemical composition. Wood is a natural organic material consisting of cellulose fibers embedded in a matrix of organic lignin. In materials science, composites of more than one constituent material can be designed to have desired properties; the forces between the atoms in a solid can take a variety of forms. For example, a crystal of sodium chloride is made up of ionic sodium and chlorine, which are held together by ionic bonds. In diamond or silicon, the atoms share form covalent bonds. In metals, electrons are shared in metallic bonding; some solids most organic compounds, are held together with van der Waals forces resulting from the polarization of the electronic charge cloud on each molecule. The dissimilarities between the types of solid result from the differences between their bonding.
Metals are strong and good conductors of both electricity and heat. The bulk of the elements in the periodic table, those to the left of a diagonal line drawn from boron to polonium, are metals. Mixtures of two or more elements in which the major component is a metal are known as alloys. People have been using metals for a variety of purposes since prehistoric times; the strength and reliability of metals has led to their widespread use in construction of buildings and other structures, as well as in most vehicles, many appliances and tools, road signs and railroad tracks. Iron and aluminium are the two most used structural metals, they are the most abundant metals in the Earth's crust. Iron is most used in the form of an alloy, which contains up to 2.1% carbon, making it much harder than pure iron. Because metals are good conductors of electricity, they are valuable in electrical appliances and for carrying an electric current over long distances with little energy loss or dissipation. Thus, electrical power grids rely on metal cables to distribute electricity.
Home electrical systems, for example, are wired with copper for its good conducting properties and easy machinability. The high thermal conductivity of most metals makes them useful for stovetop cooking utensils; the study of metallic elements and their alloys makes up a significant portion of the fields of solid-state chemistry, materials science and engineering. Metallic solids are held together by a high density of shared, delocalized electrons, known as "metallic bonding". In a metal, atoms lose their outermost electrons, forming positive ions; the free electrons are spread over the entire solid, held together by electrostatic interactions between the ions and the electron cloud. The large number of free electrons gives metals their high values of electrical and thermal conductivity; the free electrons prevent transmission of visible light, making metals opaque and lustrous. More advanced models of metal properties consider the effect of the positive ions cores on the delocalised electrons.
As most metals have crystalline structure, those ions are arranged into a periodic lattice. Mathematically, the potential of the ion cores can be treated by various models, the simplest being the nearly free electron model. Minerals are
Diffusion is the net movement of molecules or atoms from a region of higher concentration to a region of lower concentration. Diffusion is driven by a gradient in chemical potential of the diffusing species. A gradient is the change in the value of a quantity e.g. concentration, pressure, or temperature with the change in another variable distance. A change in concentration over a distance is called a concentration gradient, a change in pressure over a distance is called a pressure gradient, a change in temperature over a distance is called a temperature gradient; the word diffusion derives from the Latin word, which means "to spread way out.” A distinguishing feature of diffusion is that it depends on particle random walk, results in mixing or mass transport without requiring directed bulk motion. Bulk motion, or bulk flow, is the characteristic of advection; the term convection is used to describe the combination of both transport phenomena. An example of a situation in which bulk motion and diffusion can be differentiated is the mechanism by which oxygen enters the body during external respiration known as breathing.
The lungs are located in the thoracic cavity, which expands as the first step in external respiration. This expansion leads to an increase in volume of the alveoli in the lungs, which causes a decrease in pressure in the alveoli; this creates a pressure gradient between the air outside the body at high pressure and the alveoli at low pressure. The air moves down the pressure gradient through the airways of the lungs and into the alveoli until the pressure of the air and that in the alveoli are equal i.e. the movement of air by bulk flow stops once there is no longer a pressure gradient. The air arriving in the alveoli has a higher 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 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.
This creates a concentration gradient for carbon dioxide to diffuse from the blood into the alveoli, as fresh air has a low concentration of carbon dioxide compared to the blood in the body. The pumping action of the heart 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 pressure gradient between the heart and the capillaries, blood moves through blood vessels by bulk flow down the pressure gradient; as the thoracic cavity contracts during expiration, the volume of the alveoli decreases and creates a pressure gradient between the alveoli and the air outside the body, air moves by bulk flow down the pressure gradient. The concept of diffusion is used in: physics, biology, sociology and finance. However, in each case, the object, undergoing diffusion is “spreading out” from a point or location at which there is a higher concentration of that object. There are two ways to introduce the notion of diffusion: either a phenomenological approach starting with Fick's laws of diffusion and their mathematical consequences, or a physical and atomistic one, by considering the random walk of the diffusing particles.
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 Fick's laws, the diffusion flux is proportional to the negative gradient of concentrations, it goes from regions of higher concentration to regions of lower concentration. Sometime various generalizations of Fick's 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 random walk of the diffusing particles. In molecular diffusion, the moving 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. The concept of diffusion is applied to any subject matter involving random walks in ensembles of individuals. Biologists use the terms "net movement" or "net diffusion" to describe the movement of ions or molecules by diffusion.
For example, oxygen can diffuse through cell membranes so long as there is a higher concentration of oxygen outside the cell. However, because the movement of molecules is random oxygen molecules move out of the cell; because there are more oxygen molecules outside the cell, the probability that oxygen molecules will enter the cell is higher than the probability that oxygen molecules will leave the cell. Therefore, the "net" movement of oxygen molecules is into the cell. In other words, there is a net movement of oxygen molecules down the concentration gradient. In the scope of time, diffusion in solids was used. For example, Pliny the Elder had described the cementation process, which produces steel from the element iron through carbon diffusion. Another example is well known for many centuries, the diffusion of colors of stained glass or earthenware and Chinese ceramics. In modern science, the first systematic experimental study of di
Hardness is a measure of the resistance to localized plastic deformation induced by either mechanical indentation or abrasion. Some materials are harder than others. Macroscopic hardness is characterized by strong intermolecular bonds, but the behavior of solid materials under force is complex. Hardness is dependent on ductility, elastic stiffness, strain, toughness and viscosity. Common examples of hard matter are ceramics, certain metals, superhard materials, which can be contrasted with soft matter. There are three main types of hardness measurements: scratch and rebound. Within each of these classes of measurement there are individual measurement scales. For practical reasons conversion tables are used to convert between another. Scratch hardness is the measure of how resistant a sample is to fracture or permanent plastic deformation due to friction from a sharp object; the principle is that an object made of a harder material will scratch an object made of a softer material. When testing coatings, scratch hardness refers to the force necessary to cut through the film to the substrate.
The most common test is Mohs scale, used in mineralogy. One tool to make this measurement is the sclerometer. Another tool used to make these tests is the pocket hardness tester; this tool consists of a scale arm with graduated markings attached to a four-wheeled carriage. A scratch tool with a sharp rim is mounted at a predetermined angle to the testing surface. In order to use it a weight of known mass is added to the scale arm at one of the graduated markings, the tool is drawn across the test surface; the use of the weight and markings allows a known pressure to be applied without the need for complicated machinery. Indentation hardness measures the resistance of a sample to material deformation due to a constant compression load from a sharp object. Tests for indentation hardness are used in engineering and metallurgy fields; the tests work on the basic premise of measuring the critical dimensions of an indentation left by a dimensioned and loaded indenter. Common indentation hardness scales are Rockwell, Vickers and Brinell, amongst others.
Rebound hardness known as dynamic hardness, measures the height of the "bounce" of a diamond-tipped hammer dropped from a fixed height onto a material. This type of hardness is related to elasticity; the device used to take this measurement is known as a scleroscope. Two scales that measures rebound hardness are the Leeb rebound hardness test and Bennett hardness scale. There are five hardening processes: Hall-Petch strengthening, work hardening, solid solution strengthening, precipitation hardening, martensitic transformation. In solid mechanics, solids have three responses to force, depending on the amount of force and the type of material: They exhibit elasticity—the ability to temporarily change shape, but return to the original shape when the pressure is removed. "Hardness" in the elastic range—a small temporary change in shape for a given force—is known as stiffness in the case of a given object, or a high elastic modulus in the case of a material. They exhibit plasticity—the ability to permanently change shape in response to the force, but remain in one piece.
The yield strength is the point. Deformation in the plastic range is non-linear, is described by the stress-strain curve; this response produces the observed properties of scratch and indentation hardness, as described and measured in materials science. Some materials exhibit both viscosity when undergoing plastic deformation, they fracture—split into two or more pieces. Strength is a measure of the extent of a material's elastic range, or elastic and plastic ranges together; this is quantified as compressive strength, shear strength, tensile strength depending on the direction of the forces involved. Ultimate strength is an engineering measure of the maximum load a part of a specific material and geometry can withstand. Brittleness, in technical usage, is the tendency of a material to fracture with little or no detectable plastic deformation beforehand, thus in technical terms, a material can be both strong. In everyday usage "brittleness" refers to the tendency to fracture under a small amount of force, which exhibits both brittleness and a lack of strength.
For brittle materials, yield strength and ultimate strength are the same, because they do not experience detectable plastic deformation. The opposite of brittleness is ductility; the toughness of a material is the maximum amount of energy it can absorb before fracturing, different from the amount of force that can be applied. Toughness tends to be small for brittle materials, because elastic and plastic deformations allow materials to absorb large amounts of energy. Hardness increases with decreasing particle size; this is known as the Hall-Petch relationship. However, below a critical grain-size, hardness decreases with decreasing grain size; this is known as the inverse Hall-Petch effect. Hardness of a material to deformation is dependent on its microdurability or small-scale shear modulus in any direction, not to any rigidity or stiffness properties such as its bulk modulus or Young's modulus. Stiffness is confused for hardness; some materials are stiffer than diamond but are not harder, are prone to spalling and flaking in squamose or acicular habits.
The key to understanding the mechanism behind hardness is understanding the metal
Glass-ceramics have an amorphous phase and one or more crystalline phases and are produced by a so-called "controlled crystallization" in contrast to a spontaneous crystallization, not wanted in glass manufacturing. Glass-ceramics have the fabrication advantage of glass, as well as special properties of ceramics; when used for sealing, some glass-ceramics do not require brazing but can withstand brazing temperatures up to 700 °C. Glass-ceramics have between 30% and 90% crystallinity and yield an array of materials with interesting properties like zero porosity, high strength, translucency or opacity, opalescence, low or negative thermal expansion, high temperature stability, machinability, resorbability or high chemical durability, bioactivity, ion conductivity, superconductivity, isolation capabilities, low dielectric constant and loss, high resistivity and break-down voltage; these properties can be tailored by controlling the base-glass composition and by controlled heat treatment/crystallization of base glass.
In manufacturing, glass-ceramics are valued for having the strength of ceramic but the hermetic sealing properties of glass. Glass-ceramics are produced in two steps: First, a glass is formed by a glass-manufacturing process; the glass is cooled down and is reheated in a second step. In this heat treatment the glass crystallizes. In most cases nucleation agents are added to the base composition of the glass-ceramic; these nucleation agents control the crystallization process. Because there is no pressing and sintering, glass-ceramics have, unlike sintered ceramics, no pores. A wide variety of glass-ceramic systems exists, e.g. the Li2O × Al2O3 × nSiO2 system, the MgO × Al2O3 × nSiO2 system, the ZnO × Al2O3 × nSiO2 system. The LAS system refers to a mix of lithium and aluminum oxides with additional components, e.g. glass-phase-forming agents such as Na2O, K2O and CaO and refining agents. As nucleation agents most zirconium oxide in combination with titanium oxide is used; this important system was studied first and intensively by Hummel, Smoke.
After crystallization the dominant crystal phase in this type of glass-ceramic is a high-quartz solid solution. If the glass-ceramic is subjected to a more intense heat treatment, this HQ s.s. transforms into a keatite-solid solution. This transition is non-reversible and reconstructive, which means bonds in the crystal-lattice are broken and new arranged. However, these two crystal phases show a similar structure as Li could show; the most interesting properties of these glass-ceramics are their thermomechanical properties. Glass-ceramic from the LAS system is a mechanically strong material and can sustain repeated and quick temperature changes up to 800–1000 °C; the dominant crystalline phase of the LAS glass-ceramics, HQ s.s. has a strong negative coefficient of thermal expansion, keatite-solid solution as still a negative CTE but much higher than HQ s.s. These negative CTEs of the crystalline phase contrasts with the positive CTE of the residual glass. Adjusting the proportion of these phases offers a wide range of possible CTEs in the finished composite.
For today's applications a low or zero CTE is desired. A negative CTE is possible, which means, in contrast to most materials when heated up, such a glass-ceramic contracts. At a certain point between 60% and 80% crystallinity, the two coefficients balance such that the glass-ceramic as a whole has a thermal expansion coefficient, close to zero; when an interface between material will be subject to thermal fatigue, glass-ceramics can be adjusted to match the coefficient of the material they will be bonded to. Developed for use in the mirrors and mirror mounts of astronomical telescopes, LAS glass-ceramics have become known and entered the domestic market through its use in glass-ceramic cooktops, as well as cookware and bakeware or as high-performance reflectors for digital projectors. One notable use of glass-ceramics is in the processing of ceramic matrix composites. For many ceramic matrix composites typical sintering temperatures and times cannot be used, as the degradation and corrosion of the constituent fibres becomes more of an issue as temperature and sintering time increase.
One example of this is SiC fibres, which can start to degrade via pyrolysis at temperatures above 1470K. One solution to this is to use the glassy form of the ceramic as the sintering feedstock rather than the ceramic, as unlike the ceramic the glass pellets have a softening point and will flow at much lower pressures and temperatures; this allows the use of less extreme processing parameters, making the production of many new technologically important fibre-matrix combinations by sintering possible. Glass-ceramic from the LAS-System is a mechanically strong material and can sustain repeated and quick temperature changes. However, it is not unbreakable; because it is still a brittle material as glass and ceramics are, it can be broken. There have been instances where users reported damage to their cooktops when the surface was struck with a hard or blunt object. At the same time, it has a low heat conduction coefficient and can be made nearly transparent for radiation in the infrared wavelengths.
In the visible range glass-ceramics can be transparent, translucent or opaque and colored by coloring agents. Today, there are two major types of electrical stoves with cooktops made of glass-ceramic: A glass-c
A grain boundary is the interface between two grains, or crystallites, in a polycrystalline material. Grain boundaries are 2D defects in the crystal structure, tend to decrease the electrical and thermal conductivity of the material. Most grain boundaries are preferred sites for the onset of corrosion and for the precipitation of new phases from the solid, they are important to many of the mechanisms of creep. On the other hand, grain boundaries disrupt the motion of dislocations through a material, so reducing crystallite size is a common way to improve mechanical strength, as described by the Hall–Petch relationship; the study of grain boundaries and their effects on the mechanical and other properties of materials forms an important topic in materials science. It is convenient to categorize grain boundaries according to the extent of misorientation between the two grains. Low-angle grain boundaries or subgrain boundaries are those with a misorientation less than about 15 degrees. Speaking they are composed of an array of dislocations and their properties and structure are a function of the misorientation.
In contrast the properties of high-angle grain boundaries, whose misorientation is greater than about 15 degrees, are found to be independent of the misorientation. However, there are'special boundaries' at particular orientations whose interfacial energies are markedly lower than those of general high-angle grain boundaries; the simplest boundary is that of a tilt boundary where the rotation axis is parallel to the boundary plane. This boundary can be conceived as forming from a single, contiguous crystallite or grain, bent by some external force; the energy associated with the elastic bending of the lattice can be reduced by inserting a dislocation, a half-plane of atoms that act like a wedge, that creates a permanent misorientation between the two sides. As the grain is bent further and more dislocations must be introduced to accommodate the deformation resulting in a growing wall of dislocations – a low-angle boundary; the grain can now be considered to have split into two sub-grains of related crystallography but notably different orientations.
An alternative is a twist boundary where the misorientation occurs around an axis, perpendicular to the boundary plane. This type of boundary incorporates two sets of screw dislocations. If the Burgers vectors of the dislocations are orthogonal the dislocations do not interact and form a square network. In other cases, the dislocations may interact to form a more complex hexagonal structure; these concepts of tilt and twist boundaries represent somewhat idealized cases. The majority of boundaries are of a mixed type, containing dislocations of different types and Burgers vectors, in order to create the best fit between the neighboring grains. If the dislocations in the boundary remain isolated and distinct, the boundary can be considered to be low-angle. If deformation continues, the density of dislocations will increase and so reduce the spacing between neighboring dislocations; the cores of the dislocations will begin to overlap and the ordered nature of the boundary will begin to break down.
At this point the boundary can be considered to be high-angle and the original grain to have separated into two separate grains. In comparison to low-angle grain boundaries, high-angle boundaries are more disordered, with large areas of poor fit and a comparatively open structure. Indeed, they were thought to be some form of amorphous or liquid layer between the grains. However, this model could not explain the observed strength of grain boundaries and, after the invention of electron microscopy, direct evidence of the grain structure meant the hypothesis had to be discarded, it is now accepted that a boundary consists of structural units which depend on both the misorientation of the two grains and the plane of the interface. The types of structural unit that exist can be related to the concept of the coincidence site lattice, in which repeated units are formed from points where the two misoriented lattices happen to coincide. In coincident site lattice theory, the degree of fit between the structures of the two grains is described by the reciprocal of the ratio of coincidence sites to the total number of sites.
In this framework, it is possible to draw the lattice for the 2 grains and count the number of atoms that are shared, the total number of atoms on the boundary. For example, when Σ=3 there will be one atom of each three that will be shared between the two lattices, thus a boundary with high Σ might be expected to have a higher energy than one with low Σ. Low-angle boundaries, where the distortion is accommodated by dislocations, are Σ1; some other low-Σ boundaries have special properties when the boundary plane is one that contains a high density of coincident sites. Examples include high-mobility boundaries in FCC materials. Deviations from the ideal CSL orientation may be accommodated by local atomic relaxation or the inclusion of dislocations at the boundary. A boundary can be described by the orientation of the boundary to the two grains and the 3-D rotation required to bring the grains into coincidence, thus a boundary has 5 macroscopic degrees of freedom. However, it is common to describe a boundary only as the orientation relationship of the neighbouring grains.
The convenience of ignoring the boundary plane orientation, difficult to determine, outweighs the reduced information. The relative orientation of the two grains is described using the rotation
A boule is a single crystal ingot produced by synthetic means. A boule of silicon is the starting material for most of the integrated circuits used today. In the semiconductor industry synthetic boules can be made by a number of methods, such as the Bridgman technique and the Czochralski process, which result in a cylindrical rod of material. In the Czochralski process a seed crystal is required to create ingot; this seed crystal is dipped into the pure molten silicon and extracted. The molten silicon grows on the seed crystal in a crystalline fashion. A s the seed is extracted the silicon solidifies and a large, cylindrical boule is produced. A semiconductor crystal boule is cut into circular wafers using an inside hole diamond saw or diamond wire saw, each wafer is lapped and polished to provide substrates suitable for the fabrication of semiconductor devices on its surface; the process is used to create sapphires, which are used for substrates in the production of blue and white LEDs, optical windows in special applications and as the protective covers for watches