Vacuum

The word vacuum [ va ː kuʊm ] (from the Latin vacuus, empty) means (largely) empty space ', ie the absence of matter. For generating a vacuum is particularly important to remove gas from said volume. Is the residual gas pressure only significantly lower than the atmospheric pressure, it is referred not correctly by a vacuum, but of a reduced or reduced pressure, or a vacuum.

• 4.1 ultrahigh vacuum

• 5.1 evacuating as separation methods (DIN 8580 )
• 5.2 Other technical applications
• 5.3 vacuum for preservation and for vacuum cooking
• 5.4 vacuum as a thermal insulator

Disambiguation

In colloquial language it is called vacuum at a largely empty space ( for example, vacuum packaging, vulgo " vacuumed "). The technique and classical physics use the term in this sense refers to the vacuum state of a gas in a volume at a pressure which is significantly lower than the atmospheric pressure under normal conditions. At extremely low absolute pressure is called high vacuum.

" Vacuum is, the state of a gas when the pressure in a container of the gas, and thus the number density is less than the outside or when the pressure of gas is lower than 300 mbar, i.e. less than the lowest atmospheric pressure on the ground surface occurring "

In quantum field theory the vacuum is the state with the lowest energy. In it, the particle numbers for all kinds of particles (field quanta) have the value zero. Nevertheless, this vacuum will appear due to the possibility that continuously excited virtual particles and are destroyed when a dynamic medium with chaotic vacuum fluctuations and a variety of other properties ( quantum vacuum).

Unlike the natural sciences, the philosophy sees vacuum as basically completely empty space.

History of Exploration

The idea of the vacuum is probably derived from Leucippus or his pupil Democritus and was a mainstay of the world picture of the Epicurean philosophy. This assumed that matter consists of indivisible smallest particles (plural: atomoi ) is constructed in the empty space, ie in vacuum, move and only due to the emptiness of space will have the opportunity for movement and interaction. This assumption was rejected by Aristotle and his academy, as a movement without driving medium appeared to be impossible. One therefore thought the space between the stars filled with an ether. It has been postulated against the emptiness a dislike of nature. This aversion was later referred to by the Latin term horror vacui. The Platonic school refused to believe in the non-being. In the Middle Ages, Aristotle was regarded as an authority. Although taken up again and defended by Giordano Bruno, the idea of ​​the vacuum could prevail only with the first demonstrations.

The first underground (or man-made ) vacuum was produced in 1644 by Evangelista Torricelli with the help of a mercury column in a curved glass tube. Blaise Pascal shortly after that show with his famous experiment vide dans le vide in November 1647 for the first time that a vacuum can actually exist. Popular was the vacuum by Otto von Guericke, the inventor of the air pump. He tensed in 1657 horses to two metal hemispheres (see Magdeburg hemispheres ), from which he had previously sucked out the air. However, the observed effect is due to no direct property of the vacuum, but rather by the pressure of the surrounding air.

Robert Williams Wood in 1897 first observed the tunnel effect in a vacuum at the field emission of electrons, this effect but could not interpret it correctly.

In the late 19th century was still assumed that light could not in a vacuum, but only in a medium called the ether, spread. With the Michelson -Morley experiment was tried in vain to prove the existence of such an ether. Due to the general acceptance of the theory of special relativity Einstein of 1905, the ether concept is considered obsolete and the propagation of light in a vacuum as proven.

The scattering experiments by Ernest Rutherford in 1911 showed that alpha particles can pass through a gold foil without resistance. This showed that the mass of atoms in a - is concentrated tiny core - compared to their full extent. Based on a model designed by Niels Bohr, according to which the electrons orbit the nucleus like the planets orbit. Inside the atoms and between them meaning a vacuum seemed to prevail. Although you can still occasionally encountered this view in the literature, the interior of the atoms is now completed as of the common areas of the electrons ( atomic orbitals ).

According to current understanding, but as described above and the vacuum is not empty, since even the quantum mechanical ground state has a finite energy density, which manifests itself, for example, the Casimir effect or spontaneous emission.

Properties

Characterization of the pressure ranges

While a completely matter- free space can not be produced, technical vacuums can be produced in different grades. To distinguish different qualities of the vacuum obtained after the amount of remaining material in the art. By default, the pressure in pascal (Pa) or millibars (mbar) is specified.

When Evacuating a vessel, the mechanical stress builds up to the rough vacuum on by the external air pressure. The limit for fine vacuum can be achieved with mechanical pumps or easy. In the area of ​​fine vacuum the mean free path reaches the typical dimensions of vacuum vessels, so that the viscous flow over the Knudsen flow in the molecular flow passes. The predominant type of flow is not only fundamental influence on the use of the vacuum, but also the vacuum generation and measurement itself in wide range of the high vacuum increases the time during which each point on the surface is taken in the middle one of a residual gas from one hour to one year, sufficient for many experiments. In the region of the vapor pressures of UHV construction materials begin to interfere, for example, of lead impurities in aluminum. An ideal vacuum is neither technically possible nor measurable.

Occurrence and examples after vacuum quality:

• The only vacuum ': vacuum cleaner ( > 0.5 bar)
• Rough vacuum: vacuum packaging, light bulbs, steam turbine condenser ( ≈ 0.03 bar ), intake system of a gasoline engine at idle (about 0.2 ... 0.3 bar)
• Fine vacuum: Low-pressure gas discharge lamps, modern precision pendulum clocks
• High vacuum: electron tubes, particle accelerators, electron microscopes
• Ultra-high vacuum: particle accelerator, near-Earth space, often in plants in the semiconductor industry
• Extremely high vacuum: Space

Measuring instruments for determining the gas pressure in a vacuum is called vacuum gauge.

Physical, chemical and thermodynamic properties

Light, particles, electric, magnetic and gravitational fields propagate in a vacuum; contrast, sound waves require a material medium and can not therefore in a vacuum spread. Thermal radiation can propagate as an electromagnetic wave in a vacuum. In contrast, the reduction of the pressure to reduce the matter bound heat transfer ( conduction and convection ) leads.

The reduction of heat flow ( convection) and heat transfer ( conduction ) (see lattice vibrations, phonons ) is applicable in jugs, dewar and heat insulation of tanks for liquefied petroleum gas ( oxygen, argon, nitrogen, helium).

The high electrical breakdown strength of high vacuum is used in vacuum capacitors of high-power electronics and high voltage part of the evacuated X-ray tubes. However, initially drops, starting from the normal air pressure, the breakdown strength at lowering the pressure. The minimum of the breakdown strength in air is achieved at a pressure of 1 mbar, where only about 0.3 kV / cm ( at 1 bar: 20-40 kV / cm). If the pressure is lowered further in the direction of high vacuum, the dielectric strength increases exponentially again. This is also utilized in vacuum circuit breakers. For high voltage applications it is necessary in addition to a good vacuum to make all the edges around to avoid field emission.

Biological effects

The vacuum is not a habitat since animals depend on matter for their metabolism. However, many living organisms ( bacteria, spores, plant seeds and spores ) can survive a period of time in a vacuum.

For a short time as the healthy person also higher organisms can withstand the vacuum, experiments with birds have been documented in the picture " The Experiment on a Bird in the Air Pump ". Contrary to the usual assumption that blood begins to boil in spite of the pressure difference immediately. Skin and tissue are usually able to the vapor pressure of the body fluids at less than 0.05 bar ( normal air pressure is 1 bar) to resist. Regardless, reduced pressure for decompression sickness or altitude sickness lead.

Generation

On the ground, a vacuum may be produced by a closed cavity, the recipient, is freed of the gas contained therein by means of suitable vacuum pumps. The simplest device is the water pump; it generates a low vacuum corresponding to (e.g., 23 hPa (or mbar) at 20 ° C) the water vapor pressure at the prevailing water temperature.

Ultra- high vacuum

In Applied Physics and surface chemistry several types of pumps are used to produce an ultra-high vacuum. First, with mechanically acting pumps (eg rotary vane pump, diaphragm pump or scroll pump ) a form in the recipient in the range of 0.01 to 1 mbar is generated. Depending on the size of the receptacle and of the pump power of the pump, this usually takes several minutes. Thereafter, turbo-molecular pumps produce in a continuous process up to several hours, a high vacuum at a base pressure of about 10-7 mbar. This pressure can not decrease without further aids, since the constant desorption of adsorbed water and other compounds with low vapor pressure (eg hydrocarbons ) prevents this.

The desorption processes are accelerated when the chamber is brought to a temperature by direct heating of the chamber walls and indirect thermal heating of the inner surfaces, which is at least above the boiling point of water, but preferably much higher. The most important criterion of the level of temperature is the temperature resistance of the installed components, such as bushings for electrical connections as well as for viewing window. Temperatures for this process called annealing are typically between 130 ° C. and about 200 ° C. Since vacuum equipment must resist the externally applied atmospheric oxygen even in this heat, they are often made ​​of stainless steel or glass, seals made of aluminum or PTFE.

Desorbing the highly water is largely evacuated during baking by means of the turbomolecular pump, as well as any hydrocarbon contamination. This process takes at least 24 hours in chambers with relatively complex inner- surface by an installed equipment will shut down the heater usually after two to three days.

To achieve the ultra-high vacuum mechanical pumps are not put to use. An ion getter pumps by ionization and trapping of residual gas molecules in titanium tubes in a pressure range of 10-10 to 10-7 millibar. This shows that the pump power is only sufficient if the baking has previously reduced the residual gas pressure is sufficient. A titanium sublimation pump operates via thermally distributed in the chamber titanium vapor, which is characterized by a high chemical reactivity and residual gas atoms binds to and the ( cold ) chamber wall so that consequently further reduced the residual gas pressure. The minimum achievable with this method, the above-described residual gas pressure in the range of 10-11 mbar.

Through cold traps at the lower part of the chamber further residual gas can now be temporarily bound and the chamber pressure can be reduced to about 10-12 mbar. If the entire chamber immersed in liquid helium, as under pressures of 10-16 mbar can be reached.

Applications

Technical vacuums are used in research, electron microscopy, in the melting of metallic materials and microelectronics manufacturing.

Quite often the vacuum in the heat treatment of metals ( hardening, tempering, nitriding, carburizing ) is used to prevent oxidation by oxygen, is in the air.

In the interior of electron tubes and picture tubes reigns high vacuum to keep the scattering of the electrons low. Remaining gas diffusing out later and remains tied with a getter.

There is double glazing, in which a vacuum instead of an inert gas between the panes. Because the two disks under the one-sided wedge on wheels air pressure deform, inconspicuous transparent spacers between the discs are distributed on the area required. Result is a relatively thin and light glazing with very low thermal conductivity. For comparison, the most triple glazing have a glass thickness of 4 12 4 12 4 = 36 mm; most double glazing assemblies of 4 16 4 = 24 mm. Triple glazing are systemic in 50 % heavier than double glazing and have two edge networks instead of a; they are therefore in the production depending on the window size is about 60-70 % more expensive.

Evacuation as a separation method (DIN 8580)

According to DIN 8580 manufacturing processes - Definitions, classification evacuation is one of the basic separation processes.

Freeze-drying removes water substances by being frozen and subjected to a vacuum. When freeze-drying as coffee, tea, vegetables, blood or biological products sublimation takes place, the ice goes directly into the gas phase, there is here no liquid phase, which could boil.

The crystallization process in the sugar production is carried out under vacuum to avoid caramelizing the dehydration by the lower boiling point of the sugar solution.

Also in the core step of plastination, the forced impregnation, vacuum is used to extract acetone or dichloromethane from the preparation.

Other technical applications

High vacuum is essential for the function of many technical devices, such as the electron tube. Even light bulbs and therefore the electric light became possible due to the vacuum. Especially with Edison's light bulb with carbon filament prevented the vacuum that the filament is burned (see also carbon filament lamp); later incandescent lamps were treated with a filling of nitrogen or other gas which does not support combustion, produced. The high vacuum electron tubes (including picture tubes ), X-ray tubes, magnetrons, electron sources, particle accelerators, vacuum fluorescent displays and the like. increases the mean free path of the electrons on a measure of the magnitude of the entire vessel so that hardly held collisions with residual gas and not disturb the particle.

When Vakuumfrittieren eg potato chips, it is all about, by the lower temperatures when frying the formation of harmful by-products of the Maillard reaction, such as to prevent or reduce acrylamide.

A rough vacuum is also frequently used to capture with the help of suction pads flat workpieces and / or transport.

Vacuum for preservation and for vacuum cooking

Another application is the packaging of food (vacuum packaging ), and other perishable products under vacuum. Vacuum is ideal because it is not a habitat, as a preservative method. The perishable means are enclosed in gas-tight plastic bags and are more durable by metabolism and oxidation processes are slowed by the absence of aging and decay process supporting oxygen in air.

Vacuum cooking vacuum packaged foods ( meat, vegetables, etc. ) can be cooked either in the water or in temperature-controlled steam at temperatures below 100 degrees and thus maintain structure and flavor better than at the usual cooking method.

In the household foods can be packed in bags and evacuated with vacuum devices, so that the foil bag rests against the packaged goods; therefore less oxygen gets to the food. In addition, the volume decreases. However, the Vacuum used may produce only a mediocre rough vacuum.

Another method is the canning / preserving. By cooking the foods are sterilized and possibly expelled gases contained. When filling the jars with food in liquid form, the air can be completely displaced from the glass. By the sealing rings, a better low vacuum is maintained for longer periods.

Vacuum as a thermal insulator

The low heat transfer in a vacuum is utilized in jugs and Dewar, also with vacuum tube collectors and rarely even in insulating glass units.

The vacuum of space

The ruling in space in interstellar space or in intergalactic space vacuum is more perfect than any producible on earth vacuum. However, the space is not completely empty, but contains an average of one particle per cubic centimeter, within voids but significantly less. Also there are static electric and magnetic fields, gravitational fields and electromagnetic waves (photons) and particle fluxes ( neutrinos, cosmic rays, particles) (see also Plenismus ).

Therefore artificial satellites and space probes are subject to special design requirements: the regulation of the heat balance ( internal heat sources and sunlight) can only be done by heat conduction and radiation, heat emission and uptake must partially variable absorbing or emitting or reflecting elements ( blinds, heat radiating body heat pipes ) can be guaranteed.

The sun shade can be due to the vacuum by radiation also targeted very low temperatures produce (eg for infrared and radio-wave radiation sensors).

To distinguish indoors in space, such as space stations, the term " open space " in use.