The reverse sublimation is a thermodynamic process in which an exothermic state change occurs from a gas to a solid without first becoming a liquid. It is also known by the names of regressive sublimation, desublimation, or deposition; the latter is the most used in school texts and encyclopedics.
Reverse sublimation is said to be an exothermic process because gaseous particles (atoms or molecules) must lose energy releasing heat to the environment; in such a way as to cool enough to form crystals, solidify or freeze on a surface.
The word 'deposition' (and not 'deposition') means that the particle is deposited from a gas phase without wetting the receiving surface. That is why inverse sublimation phenomena are often found on icy objects; as it happens with the frost deposited on the leaves or winter landscapes.
Such deposition is often detected by a thin layer of crystals; although it can also be made of an apparent powder or clay. By controlling this process, new multilayer materials can be designed, where each layer consists of a specific solid deposited by chemical or physical processes..
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Reverse sublimation, as its name alone reveals, is the opposite phenomenon to sublimation: it starts not from a solid that evaporates, but from a gas that solidifies or freezes..
If you reason molecularly, it will look amazing that a gas is able to cool itself, to the point that it doesn't even condense in the first place; that is, it goes to the liquid state.
A gas, highly disordered and diffuse, suddenly manages to rearrange its particles and establish itself as a solid (whatever its appearance).
By itself this will be kinetic and thermodynamically difficult, since it needs a support that receives the gas particles and concentrates them so that they interact with each other while losing energy; that is, while they cool down. This is where the surface exposed to the gas participates: serving as a support and heat exchanger.
The gas particles exchange heat with the colder or icy surface, so they slow down and little by little the first crystalline nuclei are formed. On these nuclei, colder than the surrounding gas, other particles begin to be deposited, which are incorporated into their structure..
The end result of this process is that a layer of crystals or solid ends up forming on the surface..
For reverse sublimation to happen, either of two conditions must ordinarily exist: the surface in contact with the gas must be below its freezing point; or the gas has to be supercooled, in such a way that as soon as it touches the surface it is deposited when it disturbs its goal stability.
On the other hand, deposition can also occur when the gas is hot. If the surface is cool enough, the high temperature of the gas will transfer to it suddenly and cause its particles to adapt to the structure of the surface..
In fact, there are methods where the surface does not even have to be cold, since it participates directly in a reaction with the gaseous particles that end up covalently (or metallic) deposited on it..
In the technology industry, a methodology that works from this beginning and is called chemical vapor deposition by combustion.
When a beer is so cold that when it is taken out of the refrigerator the glass of its bottle is covered in white, it is said that it is dressed as a bride.
The beer bottle offers the necessary surface area for the water vapor molecules, HtwoOr, crash and lose energy quickly. If the glass is black, you will notice how it becomes white out of nowhere, and you can tear it off with your fingernail to write messages or draw pictures on it..
Sometimes the deposition of humidity from the environment is such that the beer appears covered with white frost; but the effect does not last long, because as the minutes pass it condenses and moistens the hand of those who hold it and drink.
Similar to what happens on the walls of a beer, frost is deposited on the inside walls of some refrigerators. Likewise, these layers of ice crystals are observed in nature at ground levels; does not fall from the sky unlike snow.
The supercooled water vapor collides with the surface of leaves, trees, grass, etc., and ends up giving them heat, in order to cool down and be able to settle on them, and manifest in their characteristic and radiant crystalline patterns.
Until now, there has been talk of water; But what about other substances or compounds? If there are gaseous gold particles in a chamber, for example, and a cold and resistant object is introduced, then a layer of gold will be deposited on it. The same would happen with other metals or compounds, as long as they do not require an increase in pressure or a vacuum..
What has just been described is about a method called physical deposition, and it is used in the materials industry to create metallic coatings on specific parts. Now, the problem lies in how to obtain gaseous gold atoms without high energy consumption, since very high temperatures are required..
It is there where the vacuum enters, to facilitate the passage from the solid to the gas (sublimation), as well as the use of electron beams.
Soot on chimney walls is often cited as an example of physical deposition; although, the very fine carbon particles, already in a solid state, and suspended in the smoke, are simply deposited without undergoing a change of state. This leads to blackening of the walls.
If there is a chemical reaction between the gas and the surface, then it is a chemical deposition. This technique is common in the synthesis of semiconductors, in the coating of polymers by bactericidal and photocatalytic layers of TiOtwo, or to provide a mechanical protection material by coating them with ZrOtwo.
Thanks to chemical deposition, it is possible to have surfaces of diamonds, tungsten, tellurides, nitrides, carbides, silicon, graphenes, carbon nanotubes, etc..
Compounds that have the M atom that wants to be deposited, and that are also susceptible to thermal decomposition, can give M to the surface structure so that it sticks permanently.
That is why it is usually used organometallic reagents, which, when decomposed, give up the metal atoms without the need to obtain it directly from it; that is, it would not be necessary to use metallic gold, but rather a gold complex to create the desired golden “plating”..
Note how the initial concept of inverse sublimation or deposition ends up evolving according to technological applications.
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