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Amorphous ice is an amorphous solid form of water, meaning it consists of water molecules that are randomly arranged like the atoms of common glass. Everyday ice is a polycrystalline material. Amorphous ice is distinguished by its lack to short-range order. Amorphous ice is produced by cooling liquid water very quickly (around 1,000,000 K/s), so the molecules don't have enough time to form a crystal lattice.
Just as there are many different crystalline forms of ice (currently fourteen known), there are also different forms of amorphous ice, distinguished principally by their densities.
The key to producing amorphous ice is the rate of cooling. The liquid water must be cooled to its glass transition temperature (about 136 K or -137 deg C) in a matter of milliseconds to prevent the spontaneous nucleation of crystals. This is analogous to the production of ice cream, which must also be frozen quickly to prevent the growth of crystals in the mixture. The difference is that pure water forms crystals much more readily than the heterogeneous mixture of ingredients in ice cream, so amorphous water is more difficult to produce, requiring a physics lab rather than an ice cream churn.
Pressure is another important factor in the formation of amorphous ice, and changes in pressure may cause one form to convert into another.
Chemicals known as cryoprotectants can be added to water, to lower its freezing point (like an antifreeze) and increase viscosity, which inhibits formation of crystals. Vitrification without addition of cryoprotectants can be achieved by very rapid cooling. These techniques are used in biology for cryopreservation of cells and tissues.
Low-density amorphous ice
Low-density amorphous ice, also called LDA, vapor-deposited amorphous water ice, amorphous solid water (ASW) or hyperquenched glassy water (HGW), is usually formed in the laboratory by a slow accumulation of water vapor molecules (physical vapor deposition) onto a very smooth metal crystal surface under 120 K. In outer space it is expected to be formed in a similar manner on a variety of cold substrates, such as dust particles. It is expected to be common in the subsurface of exterior planets and comets.
Melting past its glass transition temperature (Tg) between 120 and 140 K, LDA is more viscous than normal water. Recent studies have shown the viscous liquid stays in this alternative form of liquid water up to somewhere between 140 and 210 K, a temperature range that is also inhabited by ice Ic. LDA has a density of 0.94 g/cm³, less dense than the densest water (1.00 g/cm³ at 277 K), but denser than ordinary ice (ice Ih).
Hyperquenched glassy water (HGW) is formed by spraying a fine mist of water droplets into a liquid such as propane around 80 K or by hyperquenching fine micrometer-sized droplets on a sample-holder kept at liquid nitrogen temperature , 77K, in a vacuum. Cooling rates above 104 K/sec are required to prevent crystallization of the droplets. At liquid nitrogen temperature, 77K, HGW is kinetically stable and can be stored for many years.
High-density amorphous ice
High-density amorphous ice (HDA) can be formed by compressing ice Ih at temperatures below ~140K. At 77 K, HDA forms from ordinary natural ice at around 1.6 GPa and from LDA at around 0.5 GPa (approximately 5,000 atm). At 77 K it can be recovered back to ambient pressure and kept indefinitely. At ambient pressure HDA has a density of 1.17 g/cm³.
Very-high-density amorphous ice
Very-high-density amorphous ice (VHDA), was discovered in 1996 by Mishima who observed that HDA became denser if warmed to 160 K at pressures between 1 and 2 GPa and has a density of 1.26 g/cm³ at ambient pressure . More recently, workers at the University of Innsbruck have suggested that this denser amorphous ice was a third amorphous form of water, distinct from HDA, and called it VHDA 
Amorphous ice is used in some scientific experiments, especially in electron cryomicroscopy of biomolecules. The individual molecules can be preserved for imaging in a state close to what they are in liquid water.
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Amorphous_ice". A list of authors is available in Wikipedia.|