Sublimation (phase transition)

As sublimation, rarely also sublimation (from the Latin sublimis = high airborne ',' sublime '), is referred to in the thermodynamics of the process of immediate transition of a substance from the solid to the gaseous state without liquefy before.

At the pressure and temperature conditions at which sublimation occurs, no liquid state of aggregation, such as in the phase diagram, there can be seen on the right. We call these conditions as sublimation printing and sublimation, or taken together as sublimation. This in turn is part of the sublimation curve of the phase diagram, which is given in the example by the phase boundary between solid and gas below the triple point.

The phase transition in the opposite direction to sublimation is called Resublimation or as a deposition or Desublimation. The Resublimationspunkt is identical to the sublimation point in pure substances. For mixtures must be noted that both may be different, and therefore, in this case, the direction of phase change is involved.

If there is a sublimation at atmospheric pressure, so meaning they are Normalsublimationstemperatur and tabulated the fabric with its value without additional sublimation printing with specify.

Any substance takes its sublimation at the so-called sublimation, that is the sum of the melting and heat of vaporization.

Examples

• Boron, carbon, iodine and arsenic, as well as organic compounds such as camphor go when heated under normal pressure directly in gaseous form over.

• Dry ice, frozen at -78.5 ° C carbon dioxide sublimes at heat and immediately passes into the gaseous state. In this case no liquid is formed under normal pressure conditions, such as during the heating of water ice: thereof, the name is derived from dry ice.

• Water also occurs at sufficiently cold and dry air at atmospheric pressure from its solid state of ice directly in gaseous form (water vapor) over. The reason for this is that the water vapor pressure is a partial pressure of air, and thus lower than the atmospheric pressure of air. Therefore, this phase transition takes place in the phase diagram for water in the range of sublimation from below the triple point, even if it is done " apparently " at atmospheric pressure. Thanks to this effect dries wet laundry under the appropriate conditions of frost on the outdoor air faster than indoors, the warmer air is already saturated with more or less water vapor.

Application

In material processing by means Laserstrahlsublimierschneiden occurs with high pulse power of the laser on the sublimation. The material melts not only on, but goes directly from a solid into the gaseous state. The result is a very clean cut without waste and frayed edges.

Other applications are the freeze-drying of foods, which is very gentle compared to other drying methods, and sublimation.

The sublimation is also an alternative to recrystallization, in the purification of products in the synthesis. Opposite the crystallization provides the sublimation some advantages: the products are often very clean and it can be even the smallest quantities easily in a sublimation sublime. An example of the application of sublimation of small quantities in radiochemical field for radiochemical purification of a nuclide, the authors present within the half -life determination of 79Se. The sublimation is mainly a laboratory method for cleaning fabric, such as ammonium chloride, ferrocene and pyrogallol. Disadvantage compared to crystallization, the comparatively difficult scaling ( scaling up ) is in the sublimation. In the chemical industry, the technical implementation of sublimation as a separation method, therefore, has only a minor role. As a refining process for naphthalene, phthalic anhydride, camphor, anthraquinone, salicylic acid, benzoic acid, uranium hexafluoride, and many metals sublimation is a technical application.