Amorphous solid

The amorphous material ( gr μορφή morphé " figure ", " form " with alpha privative, eg " without form " ) is called in physics and chemistry, a substance in which the atoms form no ordered structures, but an irregular pattern and only over short-range order, but do not have long-range order.

In contrast to amorphous hot regularly structured materials crystals.

Production

The classical method to produce the amorphous state, the "fast" cooling a melt or liquid. Condition for the amorphous state is that the atoms or molecules during cooling can not order regularly, that is, the viscosity must exceed a certain value and it may not come for crystallization. The critical cooling rate needed to avoid crystallization, is dependent on the material. Traditional glasses such as window glass may be cooled relatively slowly (e.g., 1 K per minute). In contrast, the most amorphous metals require a cooling rate above 1000 K per second. A related method is the vapor deposition on a cooled substrate. Attending the missing atoms time and mobility to assume the orderly form.

Another method is the preparation by disrupting the crystalline order with a strong mechanical deformation (e.g., ball milling ), bombardment. By ion irradiation or a strong

Not any material can be produced at normal temperature in an amorphous form.

Properties

Because the atoms have a low packing density, amorphous materials have almost always has a lower density than materials having a crystalline form. The amorphous state is metastable. On heating an amorphous substance, it can come in a more stable state to the spontaneous crystallization and thus to convert. If it does not come previously to crystallization, there is a direct transition to the liquid phase, without a classical phase transition.

Examples and Applications

Glass is a typical amorphous material. Quartz glass is the amorphous form of silica (SiO2). One of its crystalline forms is called quartz. Glass is produced by the addition of substances, so-called glass transducers prevent a uniform crystal lattice.

Amorphous metals are prepared by rapid solidification technology in the form of thin films industrially. Mainly used in this magnetic materials, soft magnetic alloys (Fe, Ni, Co) and amorphous brazing foil.

Amorphous silicon is a non-crystalline form of the pure semiconductor silicon, and is used mainly for thin-film solar cells.

Obsidian is a natural amorphous material of volcanic origin.

Amorphous thermoplastics ( plastics) such as polystyrene (PS ), polyvinyl chloride (PVC ) or polycarbonate (PC).