|Condensed matter physics|
|Phases · Phase transition · QCP|
The term comes from the Greek a ("without"), and morphé ("shape, form"). In some older articles and books, the term was used synonymously with glass. Today, "glassy solid" or "amorphous solid" is considered the overarching concept.[a] Polymers are often amorphous.
Amorphous materials have an internal structure comprising interconnected structural blocks that can be similar to the basic structural units found in the corresponding crystalline phase of the same compound. Whether a material is liquid or solid depends primarily on the connectivity between its building blocks; solids are characterized by a higher degree of connectivity than fluids.
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.
Amorphous materials will have some degree of short-range order at the atomic-length scale due to the nature of intermolecular chemical bonding.[b] 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, have difficulty distinguishing amorphous and crystalline structures at short-length scales.
Uses and observations
Amorphous thin films
Amorphous phases are important constituents of thin films. Thin films are solid layers of a few nanometres to tens of micrometres thickness that are deposited onto a substrate. So-called structure zone models were developed to describe the microstructure of thin films as a function of the homologous temperature (Th), which is the ratio of deposition temperature to melting temperature. 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.[c]
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.
Today, optical coatings made from TiO2, SiO2, Ta2O5 etc. (and combinations of these) in most cases consist of amorphous phases of these compounds. Much research is carried out into thin amorphous films as a gas separating membrane layer. 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.[d]
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.[e] 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.[f]
- Glass is considered a special case, as glass is an amorphous solid maintained below its glass transition temperature.
- See the structure of liquids and glasses for more information on non-crystalline material structure.
- For higher values, the surface diffusion of deposited atomic species would allow for the formation of crystallites with long-range atomic order.
- In the case of a hydrogenated amorphous silicon, the missing long-range order between silicon atoms is partly induced by the presence of hydrogen in the percent range.
- An initial amorphous layer was observed in many studies of thin polycrystalline silicon films.
- Experimental studies of the phenomenon require a clearly defined state of the substrate surface—and its contaminant density, etc.—upon which the thin film is deposited.
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Russian-language version: Fiz. Metal Metalloved (1969) 28: 653-660.
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- Buckel, W.; Hilsch, R. (1956). "Supraleitung und elektrischer Widerstand neuartiger Zinn-Wismut-Legierungen". Z. Phys. 146: 27–38. doi:10.1007/BF01326000. S2CID 119405703.
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- de Vos, Renate M.; Verweij, Henk (1998). "High-Selectivity, High-Flux Silica Membranes for Gas Separation". Science. 279 (5357): 1710–1711. Bibcode:1998Sci...279.1710D. doi:10.1126/science.279.5357.1710. PMID 9497287.
- Magnuson, Martin; Andersson, Matilda; Lu, Jun; Hultman, Lars; Jansson, Ulf (2012). "Electronic Structure and Chemical Bonding of Amorphous Chromium Carbide Thin Films". J. Phys. Condens. Matter. 24 (22): 225004. arXiv:1205.0678. Bibcode:2012JPCM...24v5004M. doi:10.1088/0953-8984/24/22/225004. PMID 22553115. S2CID 13135386.
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