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Mosaicity
In crystallography, mosaicity is a measure of the spread of crystal plane orientations. A mosaic crystal is an idealized model of an imperfect crystal, imagined to consist of numerous small perfect crystals (crystallites) that are to some extent randomly misoriented. Empirically, mosaicities can be determined by measuring ''rocking curves''. Diffraction by mosaics is described by the ''Darwin–Hamilton equations''. The mosaic crystal model goes back to a theoretical analysis of X-ray diffraction by Charles Galton Darwin, C. G. Darwin (1922). Currently, most studies follow Darwin in assuming a Gaussian distribution of crystallite orientations centered on some reference orientation. The ''mosaicity'' is commonly equated with the standard deviation of this distribution. Applications and notable materials An important application of mosaic crystals is in monochromators for x-ray and neutron radiation. The mosaicity enhances the reflected flux, and allows for some phase-space transf ...
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Crystallography
Crystallography is the branch of science devoted to the study of molecular and crystalline structure and properties. The word ''crystallography'' is derived from the Ancient Greek word (; "clear ice, rock-crystal"), and (; "to write"). In July 2012, the United Nations recognised the importance of the science of crystallography by proclaiming 2014 the International Year of Crystallography.UN announcement "International Year of Crystallography"
iycr2014.org. 12 July 2012
Crystallography is a broad topic, and many of its subareas, such as X-ray crystallography, are themselves important scientific topics. Crystallography ranges from the fundamentals of crystal structure to the mathematics of Crystal system, crystal geometry, including those that are Aperiodic crystal, not periodic or quasi ...
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Crystallite
A crystallite is a small or even microscopic crystal which forms, for example, during the cooling of many materials. Crystallites are also referred to as grains. Bacillite is a type of crystallite. It is rodlike with parallel Wikt:longulite , longulites. Structure The orientation of crystallites can be random with no preferred direction, called random Texture (chemistry), texture, or directed, possibly due to growth and processing conditions. While the structure of a single crystal is highly ordered and its crystal lattice, lattice is continuous and unbroken, amorphous solid, amorphous materials, such as glass and many polymers, are non-crystalline and do not display any structures, as their constituents are not arranged in an ordered manner. Polycrystalline structures and paracrystalline phases are in between these two extremes. Polycrystalline materials, or polycrystals, are solids that are composed of many crystallites of varying size and orientation. Most materials are ...
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X-ray Diffraction
X-ray diffraction is a generic term for phenomena associated with changes in the direction of X-ray beams due to interactions with the electrons around atoms. It occurs due to elastic scattering, when there is no change in the energy of the waves. The resulting map of the directions of the X-rays far from the sample is called a diffraction pattern. It is different from X-ray crystallography which exploits X-ray diffraction to determine the arrangement of atoms in materials, and also has other components such as ways to map from experimental diffraction measurements to the positions of atoms. This article provides an overview of X-ray diffraction, starting with the early #History, history of x-rays and the discovery that they have the right spacings to be diffracted by crystals. In many cases these diffraction patterns can be #Introduction to x-ray diffraction theory, Interpreted using a single scattering or kinematical theory with conservation of energy (#Ewald's sphere, wave vecto ...
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Charles Galton Darwin
Sir Charles Galton Darwin (19 December 1887 – 31 December 1962) was an English physicist who served as director of the National Physical Laboratory (NPL) during the Second World War. He was a son of the mathematician George Darwin and a grandson of Charles Darwin. Early life Darwin was born at Newnham Grange in Cambridge, England into a scientific dynasty. He was a son of mathematician Sir George Darwin and a grandson of Charles Darwin. His mother was Lady Darwin, Maud du Puy of Philadelphia, Pennsylvania. Darwin's elder sister was the artist Gwen Raverat, and his younger sister Margaret married Geoffrey Keynes, the brother of the economist John Maynard Keynes. His younger brother William Robert Darwin was a London stockbroker. Darwin was educated at Marlborough College (1901–1906) and then studied mathematics at Trinity College, Cambridge, graduating BA in 1910, later promoted to MA by seniority. Career He secured a post-graduate position at the Victoria University ...
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Gaussian Distribution
In probability theory and statistics, a normal distribution or Gaussian distribution is a type of continuous probability distribution for a real number, real-valued random variable. The general form of its probability density function is f(x) = \frac e^\,. The parameter is the Mean#Mean of a probability distribution, mean or expected value, expectation of the distribution (and also its median and mode (statistics), mode), while the parameter \sigma^2 is the variance. The standard deviation of the distribution is (sigma). A random variable with a Gaussian distribution is said to be normally distributed, and is called a normal deviate. Normal distributions are important in statistics and are often used in the natural science, natural and social sciences to represent real-valued random variables whose distributions are not known. Their importance is partly due to the central limit theorem. It states that, under some conditions, the average of many samples (observations) of a ...
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Monochromator
A monochromator is an optics, optical device that transmits a mechanically selectable narrow band of wavelengths of light or other radiation chosen from a wider range of wavelengths available at the input. The name is . Uses A device that can produce monochromatic light has many uses in science and in optics because many optical characteristics of a material are dependent on wavelength. Although there are a number of useful ways to select a narrow band of wavelengths (which, in the visible range, is perceived as a pure color), there are not as many other ways to easily select any wavelength band from a wide range. See #Applications, below for a discussion of some of the uses of monochromators. In hard X-ray and neutron radiation, neutron optics, crystal monochromators are used to define wave conditions on the instruments. Techniques A monochromator can use either the phenomenon of optical dispersion in a Prism (optics), prism, or that of diffraction using a diffraction gratin ...
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Neutron Radiation
Neutron radiation is a form of ionizing radiation that presents as free neutrons. Typical phenomena are nuclear fission or nuclear fusion causing the release of free neutrons, which then react with nuclei of other atoms to form new nuclides—which, in turn, may trigger further neutron radiation. Free neutrons are unstable, decaying into a proton, an electron, plus an electron antineutrino. Free neutrons have a mean lifetime of 887 seconds (14 minutes, 47 seconds). Neutron radiation is distinct from alpha, beta and gamma radiation. Sources Neutrons may be emitted from nuclear fusion or nuclear fission, or from other nuclear reactions such as radioactive decay or particle interactions with cosmic rays or within particle accelerators. Large neutron sources are rare, and usually limited to large-sized devices such as nuclear reactors or particle accelerators, including the Spallation Neutron Source. Neutron radiation was discovered from observing an alpha particle colli ...
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Phase-space Transformation
The phase space of a physical system is the set of all possible physical states of the system when described by a given parameterization. Each possible state corresponds uniquely to a point in the phase space. For mechanical systems, the phase space usually consists of all possible values of the position and momentum parameters. It is the direct product of direct space and reciprocal space. The concept of phase space was developed in the late 19th century by Ludwig Boltzmann, Henri Poincaré, and Josiah Willard Gibbs. Principles In a phase space, every degree of freedom or parameter of the system is represented as an axis of a multidimensional space; a one-dimensional system is called a phase line, while a two-dimensional system is called a phase plane. For every possible state of the system or allowed combination of values of the system's parameters, a point is included in the multidimensional space. The system's evolving state over time traces a path (a phase-space tra ...
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Pyrolitic Graphite
Pyrolytic carbon is a material similar to graphite, but with some covalent bonding between its graphene sheets as a result of imperfections in its production. Pyrolytic carbon is man-made and is thought not to be found in nature.Ratner, Buddy D. (2004). Pyrolytic carbon. In Biomaterials science: an introduction to materials in medicine'. Academic Press. p. 171–180. . Google Book Search. Retrieved 7 July 2011. Generally it is produced by heating a hydrocarbon nearly to its decomposition temperature, and permitting the graphite to crystalize (pyrolysis). One method is to heat synthetic fibers in a vacuum, producing carbon fibers. It is used in high temperature applications such as missile nose cones, rocket motors, heat shields, laboratory furnaces, in graphite-reinforced plastic, coating nuclear fuel particles, and in biomedical prostheses. It was developed in the late 1950s as an extension of the work on refractory vapor deposition of metals. Physical properties Pyrolytic g ...
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Primary Extinction
Primary or primaries may refer to: Arts, entertainment, and media Music Groups and labels * Primary (band), from Australia * Primary (musician), hip hop musician and record producer from South Korea * Primary Music, Israeli record label Works * ''Primary'' (album) by Rubicon (2002) * "Primary" (song) by The Cure * "Primary", song by Spoon from the album '' Telephono'' Other uses in arts, entertainment, and media * Primaries or primary beams, in E. E. Smith's science-fiction series ''Lensman'' * ''Primary'' (film), American political documentary (1960) Computing * PRIMARY, an X Window selection * Primary data storage, computer technology used to retain digital data * Primary server, main server on the server farm Education * Primary education, the first stage of compulsory education * Primary FRCA, academic examination for anaesthetists in the U.K. * Primary school, school providing primary education Mathematics * ''p''-group of prime power order * Primary decomposi ...
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Dynamical Theory Of Diffraction
The dynamical theory of diffraction describes the interaction of waves with a regular lattice. The wave fields traditionally described are X-rays, neutrons or electrons and the regular lattice are atomic crystal structures or nanometer-scale multi-layers or self-arranged systems. In a wider sense, similar treatment is related to the interaction of light with optical band-gap materials or related wave problems in acoustics. The sections below deal with dynamical diffraction of X-rays. Principle The dynamical theory of diffraction considers the wave field in the periodic potential of the crystal and takes into account all multiple scattering effects. Unlike the kinematic theory of diffraction which describes the approximate position of Bragg or Laue diffraction peaks in reciprocal space, dynamical theory corrects for refraction, shape and width of the peaks, extinction and interference effects. Graphical representations are described in dispersion surfaces around reciprocal ...
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Incoherent Scattering
Incoherent scattering is a type of scattering phenomenon in physics. The term is most commonly used when referring to the scattering of an electromagnetic wave (usually light or radio frequency) by random fluctuations in a gas of particles (most often electrons). The most well known practical application is known as incoherent scatter radar theory, a ground-based technique for studying the Earth's ionosphere first proposed by Professor William E. Gordon in 1958. A radar beam scattering off electrons in the ionospheric plasma creates an incoherent scatter return. When an electromagnetic wave is transmitted through the atmosphere, each of the electrons in the ionospheric plasma essentially acts as an antenna excited by the incoming wave, and the wave is re-radiated from the electron. Since the electrons are all moving at varying speeds as a result of ionospheric dynamics and random thermal motion, the reflection from each electron is also Doppler shifted. The receiver on the g ...
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