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Non crystalline solid

In condensed matter physics and materials science, an amorphous solid (or non crystalline solid) is a solid that lacks the long range order that is characteristic of a crystal. The terms "glass" and "glassy solid" are sometimes used synonymously with amorphous solid; however, these terms refer specifically to amorphous materials that undergo a glass transition. Examples of amorphous solids include glasses, metallic glasses, and certain types of plastics and polymers.

The term comes from the Greek a ("without"), and morphé ("shape, form").

Amorphous materials have an internal structure consisting of interconnected structural blocks that can be similar to the basic structural units found in the corresponding crystalline phase of the same compound. Unlike in crystalline materials, however, no long range order exists. Amorphous materials therefore cannot be defined by a finite unit cell. Statistical methods, such as the atomic density function and radial distribution function, are more useful in describing the structure of amorphous solids. Medium range order is then defined as the structural organization extending beyond the short range order, usually by 1 2 nm. For all of these substances, specific heat has a (nearly) linear dependence as a function of temperature, and thermal conductivity has nearly quadratic temperature dependence. These properties are conventionally called anomalous being very different from properties of crystalline solids. On the phenomenological level, many of these properties were described by a collection of tunneling two level systems. Nevertheless, the microscopic theory of these properties is still missing after more than 50 years of the research. Remarkably, a dimensionless quantity of internal friction is nearly universal in these materials. This quantity is a dimensionless ratio (up to a numerical constant) of the phonon wavelength to the phonon mean free path. Since the theory of tunneling two level states (TLSs) does not address the origin of the density of TLSs, this theory cannot explain the universality of internal friction, which in turn is proportional to the density of scattering TLSs. The theoretical significance of this important and unsolved problem was highlighted by Anthony Leggett.

Amorphous materials will have some degree of short range order at the atomic length scale due to the nature of intermolecular chemical bonding. Furthermore, in very small crystals, short range order encompasses a large fraction of the atoms; nevertheless, relaxation at the surface, along with interfacial effects, distorts the atomic positions and decreases structural order. Even the most advanced structural characterization techniques, such as X ray diffraction and transmission electron microscopy, can have difficulty distinguishing amorphous and crystalline structures at short size scales.

Due to the lack of long range order, standard crystallographic techniques are often inadequate in determining the structure of amorphous solids. A variety of electron, X ray, and computation based techniques have been used to characterize amorphous materials. Multi modal analysis is very common for amorphous materials.

Unlike crystalline materials which exhibit strong Bragg diffraction, the diffraction patterns of amorphous materials are characterized by broad and diffuse peaks. As a result, detailed analysis and complementary techniques are required to extract real space structural information from the diffraction patterns of amorphous materials. It is useful to obtain diffraction data from both X ray and neutron sources as they have different scattering properties and provide complementary data. Pair distribution function analysis can be performed on diffraction data to determine the probability of finding a pair of atoms separated by a certain distance. From these techniques, the local order of an amorphous material can be elucidated.

X ray absorption fine structure spectroscopy is an atomic scale probe making it useful for studying materials lacking in long range order. Spectra obtained using this method provide information on the oxidation state, coordination number, and species surrounding the atom in question as well as the distances at which they are found.

The atomic electron tomography technique is performed in transmission electron microscopes capable of reaching sub Angstrom resolution. A collection of 2D images taken at numerous different tilt angles is acquired from the sample in question, and then used to reconstruct a 3D image. After image acquisition, a significant amount of processing must be done to correct for issues such as drift, noise, and scan distortion. Fluctuation electron microscopy experiments can be done in conventional or scanning transmission electron microscope mode. According to these models, a necessary condition for the occurrence of amorphous phases is that (Th) has to be smaller than 0.3. The deposition temperature must be below 30% of the melting temperature.

Regarding their applications, amorphous metallic layers played an important role in the discovery of superconductivity in amorphous metals made by Buckel and Hilsch. The superconductivity of amorphous metals, including amorphous metallic thin films, is now understood to be due to phonon mediated Cooper pairing. The role of structural disorder can be rationalized based on the strong coupling Eliashberg theory of superconductivity.

Amorphous solids typically exhibit higher localization of heat carriers compared to crystalline, giving rise to low thermal conductivity. Products for thermal protection, such as thermal barrier coatings and insulation, rely on materials with ultralow thermal conductivity. The technologically most important thin amorphous film is probably represented by a few nm thin SiO2 layers serving as isolator above the conducting channel of a metal oxide semiconductor field effect transistor (MOSFET). Also, hydrogenated amorphous silicon (Si:H) is of technical significance for thin film solar cells.

In the pharmaceutical industry, some amorphous drugs have been shown to offer higher bioavailability than their crystalline counterparts as a result of the higher solubility of the amorphous phase. However, certain compounds can undergo precipitation in their amorphous form in vivo, and can then decrease mutual bioavailability if administered together.

Amorphous materials in soil strongly influence bulk density, aggregate stability, plasticity, and water holding capacity of soils. The low bulk density and high void ratios are mostly due to glass shards and other porous minerals not becoming compacted. Andisol soils contain the highest amounts of amorphous materials.

The occurrence of amorphous phases turned out to be a phenomenon of particular interest for the studying of thin film growth. The growth of polycrystalline films is often used and preceded by an initial amorphous layer, the thickness of which may amount to only a few nm. The most investigated example is represented by the unoriented molecules of thin polycrystalline silicon films. Wedge shaped polycrystals were identified by transmission electron microscopy to grow out of the amorphous phase only after the latter has exceeded a certain thickness, the precise value of which depends on deposition temperature, background pressure, and various other process parameters. The phenomenon has been interpreted in the framework of Ostwald's rule of stages that predicts the formation of phases to proceed with increasing condensation time towards increasing stability.