In condensed matter physics
and materials science
, an amorphous (from the Greek
''a'', without, ''morphé'', shape, form) or non-crystalline
solid is a solid
that lacks the long-range order
that is characteristic of a crystal
. In some older books, the term has been used synonymously with glass
. Nowadays, "glassy solid" or "amorphous solid" is considered to be the overarching concept, and glass the more special case: Glass is an amorphous solid stabilized below its glass transition
temperature. Polymers are often amorphous. Other types of amorphous solids include gel
s, thin film
s, and nanostructured materials such as glass.
Amorphous materials have an internal structure made of interconnected structural blocks. These blocks 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 elementary building blocks so that solids are characterized by a high degree of connectivity whereas structural blocks in fluids have lower connectivity.
In the pharmaceutical industry, the amorphous drugs were shown to have higher bio-availability than their crystalline counterparts due to the high solubility of amorphous phase. Moreover, certain compounds can undergo precipitation in their amorphous form ''in vivo
'', and they can decrease each other's bio-availability if administered together.
Even amorphous materials have some shortrange order at the atomic length scale due to the nature of chemical bond
ing (see structure of liquids and glasses
for more information on non-crystalline material structure). Furthermore, in very small crystal
s a large fraction of the atom
s are the crystal; relaxation of the surface and interfacial effects distort the atomic positions, decreasing the structural order. Even the most advanced structural characterization techniques, such as x-ray diffraction and transmission electron microscopy, have difficulty in distinguishing between amorphous and crystalline structures on these length scales.
Amorphous thin films
Amorphous phases are important constituents of thin film
s, which are solid layers of a few nanometre
s to some tens of micrometre
s thickness deposited upon a substrate. So-called structure zone models were developed to describe the micro structure and ceramics of thin films as a function of the homologous temperature
'' that is the ratio of deposition temperature over melting temperature.
Russian-language version: ''Fiz. Metal Metalloved'' (1969) 28: 653-660.
According to these models, a necessary (but not sufficient) condition for the occurrence of amorphous phases is that ''Th
'' has to be smaller than 0.3, that is the deposition temperature must be below 30% of the melting temperature. For higher values, the surface diffusion of deposited atomic species would allow for the formation of crystallites with long range atomic order.
Regarding their applications, amorphous metallic layers played an important role in the discovery of superconductivity
in amorphous metal
s 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, and the role of structural disorder can be rationalized based on the strong-coupling Eliashberg theory of superconductivity.
Today, optical coating
s made from TiO2
etc. and combinations of them in most cases consist of amorphous phases of these compounds. Much research is carried out into thin amorphous films as a gas separating membrane
The technologically most important thin amorphous film is probably represented by 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
, a-Si:H in short, is of technical significance for thin-film solar cells
. In case of a-Si:H the missing long-range order between silicon atoms is partly induced by the presence by hydrogen in the percent range.
The occurrence of amorphous phases turned out as a phenomenon of particular interest for studying thin-film growth.
Remarkably, 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 thin multicrystalline silicon films, where such as the unoriented molecule. An initial amorphous layer was observed in many studies.
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
that predicts the formation of phases to proceed with increasing condensation time towards increasing stability.
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.
Journal of non-crystalline solids
Category:Phases of matter
Category:Unsolved problems in physics