Deposition

Deposition, also known as desublimation, is a thermodynamic process, a phase transition in which gas transforms into solid. The reverse of deposition is sublimation. One example of deposition is the process by which, in sub-freezing air, water vapor changes directly to ice without first becoming a liquid. This is how snow forms in clouds, as well as frost and hoar frost on the ground. Another example is when frost forms on a leaf. For deposition to occur, thermal energy must be removed from a gas. When the leaf becomes cold enough, water vapor in the air surrounding the leaf loses enough thermal energy to change into a solid. Deposition in water vapor occurs due to the pureness of the water vapor. The vapor has no foreign particles, and is therefore able to lose large amounts of energy before forming around something. When the leaf is introduced, the supercooled water vapor immediately begins to condensate, but by this point is already past the freezing point. This causes the water vapor to change directly into a solid. Another example of physical deposition is the artificial process of physical vapor deposition, used to deposit thin films of various materials onto various surfaces. Deposition releases energy and is an exothermic phase change. [edit]Sublimation in manufacturing Semiconductors can be manufactured through PVD (physical vapor deposition). Here, a substance is vaporized in a vacuum chamber and then condensed on a substrate such as silicon dioxide. This process which only requires physical phase changes, is distinguished from CVD (chemical vapor deposition) which requires a vapor precursor to react chemically and leave a solid substance on the substrate. Abound Solar manufactures solar panels with a process that deposits semiconductor material through sublimation and deposition. A thermodynamic process is the energetic development of a thermodynamic system proceeding from an initial state to a final state. Paths through the space of thermodynamic variables are often specified by holding certain thermodynamic variables constant. A state function is a thermodynamic variable which depends only on the current state of the system, not the path taken to reach that state. Conversely a process function does depend on the path. Hoar frost (also calle radiation frost or hoarfrost or pruina) refers to the white ice crystals, loosely deposited on the ground or exposed objects, that form on cold clear nights when heat losses into the open skies cause objects to become colder than the surrounding air. A related effect is flood frost which occurs when air cooled by ground-level radiation losses travels downhill to form pockets of very cold air in depressions, valleys, and hollows. Hoar frost can form in these areas even when the air temperature a few feet above ground is well above freezing. Nonetheless the frost itself will be at or below the freezing temperature of water. Hoar frost may have different names depending on where it forms. For example, air hoar is a deposit of hoar frost on objects above the surface, such as tree branches, plant stems, wires; surface hoar is formed by fernlike ice crystals directly deposited on snow, ice or already frozen surfaces; crevasse hoar consists of crystals that form in glacial crevasses where water vapour can accumulate under calm weather conditions; depth hoar refers to cup shaped, faceted crystals formed within dry snow, beneath the surface. The name hoar comes from an Old English adjective for showing signs of old age, and is used in this context in reference to the frost which makes trees and bushes look like white hair. It may also have association with hawthorn when covered in its characteristic white spring blossom. Surface hoar is a cause of avalanches when it forms on top of snow. Conditions are ideal for the formation of hoarfrost on cold clear nights, with a very light wind that is able to circulate more humidified air around the snow surface. Wind that is too abrupt will destroy the crystals. When buried by subsequent snows they may remain standing for easy identification, or become laid down, but still dangerous because of the weakness of the crystals. In low temperatures surface hoar can also be broken apart and blown across the surface forming yukimarimo. Hoar frost also occurs around man-made environments such as freezers or industrial cold storage facilities. It occurs in adjacent rooms that are not well insulated against the cold or around entry locations where humidity and moisture will enter and freeze instantly depending on the freezer temperature.