Freeze
Solidifying referred to in the physics the transition of a substance from the liquid into the solid state of aggregation . In most cases this is done by cooling . The reverse process of solidification is melting . The solidification takes place in the solidification interval.
With pure substances and constant pressure , the solidification always takes place at a certain temperature , which is called the freezing point ( solidification line ). In the case of pure substances, it corresponds exactly to the melting point . Although the material during solidification heat outputs ( heat of solidification ), the temperature remains during the transition from liquid to solid constant ( latent heat ). When solidifying, many substances crystallize , which reduces the Brownian motion of the molecules . The molecules therefore have a lower energy in the solidified state than in the liquid, which is equivalent to an energy release.
Water and other aqueous solutions freeze when they turn into a solid state. Freezing is also colloquially referred to as the preservation of food by freezing it . In the case of alloys and glasses, solidification begins at the liquidus temperature and is completed at the solidus temperature.
General
Liquids can turn into solids for various reasons and in various ways. The solidification can be observed particularly frequently at constant pressure by cooling. If it is a pure substance , the liquid begins to solidify when the solidification temperature is reached and only continues to cool when solidification is complete. The solidification temperature is almost always identical to the melting temperature. A change in pressure at constant temperature can also lead to solidification. For most substances, this requires an increase in pressure, while with water and some other substances, a decrease in pressure can also lead to solidification. (See also anomaly of water ). Also the boiling and condensation can be caused by pressure change during solidification but significantly greater pressure changes are necessary. The relationship between temperature and pressure can be seen in phase diagrams . It can also be seen there that the solidification line , which separates the liquid area from the solid area in the diagram, is steeper than the melting line between liquid and gaseous.
With many mixtures (and thus also alloys ) there is a temperature range, the solidification interval, in which the substance is both solid and liquid. The solidification begins at the liquidus temperature and ends at the solidus temperature. Both depend on the mixing ratio or the proportion of alloying elements and can also be taken from phase diagrams.
Solidification can also occur through chemical reactions . This is the case if, for example, a liquid is transformed into a substance at room temperature with a solidification temperature above room temperature. The phenomenon also occurs in metallurgy: liquid metals react with oxygen and form oxides . For example, liquid aluminum solidifies at 660 ° C, while aluminum oxide is still solid at over 2000 ° C.
When solidifying, energy is released, the solidification heat . The same amount of energy is required to melt the substance again ( heat of fusion ).
Property changes
Numerous physical properties change suddenly when solidifying. Almost all physical properties change with the temperature of a body. As long as the body does not change its aggregate state, these properties usually do not change suddenly, but continuously and very slowly. Changes in volume, density and solubility are of particular importance.
Volume and density
Most substances reduce their volume when they solidify and therefore increase their density (mass per volume) because of the constant mass . The phenomenon is known as freezing shrinkage . Water, on the other hand, expands when it solidifies. This is why ice floats on liquid water, while most other substances are lost in the melt. The expansion when water freezes can cause glass bottles filled with water to burst. In winter, water seeps into small cracks in the road surface and rock, expands and thus enlarges these cracks, which is known as frost blasting . The thermal expansion, on the other hand, is related to the temperature and not to the physical state.
In the foundry , solidification shrinkage can damage the castings. Especially in the case of workpieces with complex shapes, the reduction of the casting is blocked by the mold, which can lead to cracks. These are known as hot cracks and can also occur during welding. In order to keep the shrinkage as low as possible, silicon is often added to cast alloys , since it expands when it solidifies and can thus partially compensate for the shrinkage.
solubility
The solubility also changes greatly upon solidification. In general, a substance is less and less able to dissolve another substance when it cools down. When solidifying, however, the solubility drops sharply. If impurities are dissolved in a substance, they can be removed by means of recrystallization or the electroslag remelting process, since the impurities mainly remain in the not yet solidified area. In the foundry, gases are often dissolved in the melt. If the melt cools quickly, these gases do not have enough time to escape from the melt and then remain in the casting where they form bubbles and pores, which reduces the strength of the castings. When these castings are then welded, the gases in the pores expand greatly and can damage the workpiece.
Occurrence in nature and applications
The freezing of water to ice is best known . In nature it occurs in winter, when lakes and other bodies of water freeze, and when snow and hail arise . Otherwise, water is occasionally frozen in the household, for example for making water ice (e.g. from fruit juices) and ice cubes , as well as for preserving food, by freezing it . After cooking, melted butter and other fats can solidify again.
Solidification during welding , soldering and casting is of great technical importance . The exact solidification process has a great influence on the hardness and strength of the castings and welded joints, which is why the solidification of metals has been particularly well researched.
Solidification is also used to increase the purity of chemical substances. Recrystallization is used in chemistry and the electroslag remelting process in metallurgy . Both are based, among other things, on the fact that the impurities preferentially remain in the liquid (melt) when they solidify.
When concrete and cement solidify , water is absorbed into the respective building materials, turning the pulpy starting material into a solid.
Solidification rock are types of rock that are formed when lava or magma solidify.
In the manufacture of some plastics, a liquid starting material is transformed into a solid at room temperature through a chemical reaction. Thermosets and polymerisation are important groups .
metallurgy
As a rule, alloys do not have a fixed temperature at which a phase transition from liquid to solid takes place, but a solidification area between solidus and liquidus .
See also
- Time-temperature conversion diagram - illustrates changes at high cooling rates
- Nucleation - the process of the formation of individual solid particles in the liquid
- Hot crack - failure to solidify melt during welding
- Electrofreezing
Individual evidence
- ↑ a b Rau / Ströbel: The metals - materials science with its chemical and physical principles . Verlag Neuer Merkur GmbH, 1999, ISBN 978-3-929360-44-8 , p. 95 ( limited preview in Google Book search).
- ↑ Herbert Windisch: Thermodynamics A textbook for engineers . Walter de Gruyter, 2014, ISBN 978-3-486-85914-0 , p. 125 ( limited preview in Google Book Search).
- ↑ Ulrich Harten: Physics for Physicians . Springer-Verlag, 2015, ISBN 978-3-642-55273-1 , pp. 156 ( limited preview in Google Book search).
- ↑ Günther Schulze: Die Metallurgie des Schweens, Springer, 4th revised edition, 2010, p. 117.
- ↑ For the influence on hardness and strength, see Günther Schulze: Die Metallurgie des Schweens , Springer, 4th revised edition, 2010, p. 24. For the importance of casting and welding, see ibid or Bührig-Polaczek, Michaeli, Spur: Handbuch Urformen, chapter 1.2.1 "Solidification" or Fritz, Schulze: Fertigungstechnik Springer, 11th edition, 2015, p. 1725.