What is vacuum 1



The word vacuum (from lat. vacuus "Empty", "free") is used in different meanings:

  • Colloquial: vacuum is a matter-free space.
  • Technology and Classical Physics: vacuum refers to the state of a fluid in a volume at a pressure that is significantly lower than atmospheric pressure under normal conditions.
  • Quantum physics: In quantum field theory, the state of lowest energy is with Vacuum condition designated.

History of the vacuum

The idea of ​​the vacuum probably comes from Leukippus or his student Democritus and was a supporting pillar of the worldview of Epicurean philosophy. They assumed that matter is made up of indivisible smallest particles (plural: atomoi), which move in empty space, i.e. in a vacuum, and only have the possibility of movement and interaction as a result of the emptiness of the space. This assumption was rejected above all by Aristotle and his academy, since Aristotle could not imagine a movement without a driving medium; one therefore imagined the space between the stars to be filled with an ether and postulated the so-called horror vacui: an aversion of nature to the void. The Platonic School also refused to accept the Non-beings to believe. In the Middle Ages, Aristotle was considered an authority. Although Giordano Bruno took up and defended it again, the idea of ​​a vacuum could only prevail with the first demonstrations.

The first earthly (or man-made) vacuum was created by Evangelista Torricelli with the help of a column of mercury in a curved glass tube. Blaise Pascal failed shortly afterwards with his famous attempt vide dans le vide prove for the first time in November 1647 that a vacuum can actually exist. The vacuum became popular through Otto von Guericke, the inventor of the air pump. In 1657 he hitched horses to two metal hemispheres (see Magdeburg hemispheres), from which he had previously sucked the air. The observed effect is not a property of the vacuum, but rather due to the pressure of the surrounding air.

Robert Williams Wood first observed the tunnel effect in a vacuum during the field emission of electrons in 1897, but was not yet able to correctly interpret this effect.

At the end of the 16th century it was still assumed that light could not propagate in a vacuum, but only in a medium, the so-called ether. Albert Abraham Michelson and Edward Williams Morley tried in vain to use an interferometer to prove the existence of such an ether. Due to the general acceptance of Albert Einstein's special theory of relativity from 1905, the ether concept is considered obsolete and the propagation of light in a vacuum has been proven.

The scattering tests by Ernest Rutherford in 1911 showed that alpha particles can cross a gold foil without resistance. This showed that the mass of atoms is concentrated in a tiny nucleus compared to their total size. Building on this, Niels Bohr designed a model according to which the electrons orbit the atomic nucleus like the planets the sun. So there seemed to be a vacuum inside and between the atoms. Although this view is still occasionally found in the literature, the interior of the atoms is now considered to be filled with the areas where the electrons (orbitals) are located.

Gerd Binnig and Heinrich Rohrer developed the scanning tunneling microscope in which controlled two-electrode tunneling in a vacuum is used. A patent application was filed for the process in 1979.

The vacuum in modern physics

In quantum field theory, the vacuum appears as a dynamic medium with diverse properties (see vacuum fluctuations). The cosmological constant, which is again necessary in today's cosmology, is said to have its origin in the vacuum fluctuations.

Properties of the vacuum

Measurement of vacuum quality

While a completely material-free space cannot be created, technical vacuums of various qualities can be created. In technology, a distinction is made between different qualities of the vacuum achieved according to the amount of remaining matter (measured by the pressure in Pa = Pascal or mbar = Millibar):

Pressure area Pressure in hPa (mbar) Molecules per cm3mean free path
Ambient pressure 1013,25 2,7·1019 68 nm
Low vacuum 300...1 1019...1016 0.1 ... 100 μm
Fine vacuum 1...10-3 1016...1013 0.1 ... 100 mm
Hohvacuum (HV) 10-3...10-7 1013...109 10 cm ... 1 km
UltraHohvakuum (UHV) 10-7...10-12 109...104 1 km ... 105 km
extrem Hohes Vakuum (XHV) >105 km

Occurrence and examples according to vacuum quality:

  • Rough vacuum: vacuum cleaner (> 0.5 bar), vacuum packaging, gas discharge lamps, incandescent lamps
  • Fine vacuum: low pressure gas discharge lamps
  • High vacuum: electron tubes, particle accelerators
  • Ultra-high vacuum: particle accelerator, near-Earth space
  • XHV: space

Measuring devices for determining the gas pressure in one vacuum is called Vacuum gauge.

Physical, chemical and thermodynamic properties

Light, particles, solids, electric, magnetic and gravitational fields spread in a vacuum; on the other hand, sound waves need a material medium and therefore cannot propagate in a vacuum. Thermal radiation can also propagate as an electromagnetic wave in a vacuum. On the other hand, the lowering of the pressure leads to a reduction in the material-bound heat transfer (conduction and convection).

The reduction of heat flow (convection) and heat conduction (conduction) (see lattice vibrations; phonons) is used in thermos flasks, Dewar vessels and for the thermal insulation of tanks for liquid gas (oxygen, argon, nitrogen, helium).

The high dielectric strength of high vacuum is used in vacuum capacitors in high-performance electronics and in the high-voltage part of evacuated X-ray tubes. However, when the pressure is reduced starting from normal air pressure, the dielectric strength initially decreases. The minimum dielectric strength in air is reached at a pressure of 1 mbar, where it is only approx. 0.3 kV / cm (at 1 bar: 20-40 kV / cm). If the pressure is further reduced in the direction of high vacuum, the dielectric strength increases again exponentially.
This is also used with vacuum circuit-breakers.
For high voltage applications, in addition to a good vacuum, it is necessary to make all edges round in order to avoid field emission.

Biological effects

The vacuum is not a living space, as living things depend on matter for their metabolism. However, many living things (bacterial spores, plant seeds and spores) can survive in a vacuum for a certain period of time.

Higher living beings cannot survive in a vacuum because the fluid in the body cells (including the blood) begins to boil due to the lowering of the boiling point. Even if they do not burst, this leads to embolism and death, see decompression sickness.

Manufacture of vacuum

A vacuum can be created on earth by freeing a closed cavity, the recipient, from the gas contained therein by means of suitable vacuum pumps. The simplest device is the water jet pump; it creates a rough vacuum that corresponds to the water vapor pressure at the prevailing water temperature (e.g. 23 hPa or mbar at 20 ° C).

Ultra high vacuum

In applied physics, several types of pumps are used to generate an ultra-high vacuum. First of all, mechanically operating pumps (e.g. rotary vane pumps, diaphragm pumps) are used to create a pre-pressure in the range of 10-2 until 10-3 Millibars generated. Depending on the size of the recipient and the pumping capacity of the pumps, this normally takes a few minutes. Next, turbomolecular pumps generate a high vacuum in the pressure range of around 10 in a process that takes at least several hours-7 mbar. This pressure can no longer be reduced without further aids, since the constant desorption of adsorbed water and other compounds, such as hydrocarbons, with low vapor pressure, even with infinitely long-lasting pumping power, prevents this.

The desorption processes are accelerated if 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 if possible significantly higher. The most important criterion for the temperature level is the temperature resistance of the built-in components, such as bushings for electrical connections and for viewing windows. Usual bakeout temperatures are between 130 ° C and over 200 ° C.

Most of the water, which is highly desorbing, is pumped out by means of the turbo-molecular pumps during the heating process, as is any carbon contamination. This process takes a minimum of 24 hours; in chambers with comparatively complex internal surfaces due to attached equipment, the heating is usually switched off after two to three days.

Non-mechanical pumps are used to achieve the ultra-high vacuum. An ion getter pump pumps through ionization and trapping of the residual gas molecules in titanium tubes in a pressure range of 1x10-7 Millibars to 10-10 Millibar. This shows that the pump output is only sufficient if the bakeout previously reduced the residual gas pressure sufficiently. A titanium sublimation pump works with titanium vapor that is thermally distributed into the chamber, which is characterized by high chemical reactivity and binds residual gas atoms to itself and the (cold) chamber wall, so that the residual gas pressure is consequently further reduced. The minimum residual gas pressure that can be achieved with this method described above is in the range of 10-11 Millibar.

A statistically significant part of the residual gas can now also be temporarily bound by cold traps on the lower part of the chamber and the chamber pressure to around 10-12 Millibars can be reduced in the short term with optimal function of all components involved.

Applications

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

In the interior of electron tubes and picture tubes there is a high vacuum in order to keep the scattering of electrons low. Remaining and later diffusing gas residues are bound with a getter.

Evacuation as a separation process (DIN 8580)

According to DIN 8580 Manufacturing processes - terms, classification belongs Evacuate to the basic material separation processes.

Freeze-drying removes water from substances by deep-freezing them and exposing them to a vacuum. When freeze-drying coffee, tea, vegetables, blood or even biological preparations, sublimation takes place, the ice goes directly into the gas phase, there is no liquid phase that could boil.

The crystallization process in the sugar factory takes place under vacuum in order to save energy due to the lower boiling point of the sugar solution when the water is removed.

The core step of plastination, forced impregnation, also uses a vacuum to extract acetone or dichloromethane from the specimen.

Other technical applications

A high vacuum is the prerequisite for the functioning of all electron tubes (including picture tubes, X-ray tubes, magnetrons, electron beam sources, particle accelerators, vacuum fluorescent displays); This increases the free path of the electrons to a level in the order of magnitude of the system, so that there are hardly any collisions with gas residues.

When vacuum frying z. B. of potato chips, the main aim is to prevent harmful byproducts of the Maillard reaction such. B. to prevent or reduce acrylamide.

Vacuum as a preservation

Another area of ​​application is the packaging of food and other perishable products under vacuum. Because it is not a habitat, vacuum is suitable as a conservation method. The perishable substances are enclosed in gas-tight plastic casings and, due to the absence of atmospheric oxygen, which supports the aging and decomposition process, they have a longer shelf life by slowing down metabolic and oxidation processes.

In the household, food can be packed in bags and evacuated with vacuum sealers so that the bag film is attached to the packaged goods, which means that less oxygen reaches the food. In addition, the volume is reduced. However, the vacuum sealers used can only generate a moderate low vacuum.

Another method is canning / canning. Cooking sterilizes the food and expels any gases it may contain. When filling the mason jars with food in liquid form, the air can be completely displaced from the jar. The sealing rings allow a better rough vacuum to be maintained over longer periods of time.

Vacuum in space

The vacuum that prevails in space in interstellar space or in intergalactic space is better than any vacuum that can be produced on earth. However, space is not completely empty either, but contains an average of one particle per cm³. Static electric and magnetic fields, gravitational fields as well as electromagnetic waves (photons) and particle flows (neutrinos, cosmic rays, particles) also occur there (see also plenism).

Artificial satellites and space probes are therefore subject to special design requirements:
The control of the heat balance (internal heat sources and solar radiation) can only be done through heat conduction and radiation, heat emission and absorption must be guaranteed by partially variable absorbing, radiating or reflecting elements (blinds, heat radiating heat sinks, heat pipes).

In the shade of the sun, radiation can also be used to generate very low temperatures in a targeted manner due to the vacuum (e.g. for infrared and radio wave radiation sensors).

literature

  • Max Wutz, Hermann Adam, Wilhelm Walcher, Karl Jousten: Vacuum technology manual. theory and practice. Vieweg, ISBN 3-528-54884-3
  • Wolfgang Pupp, Heinz K. Hartmann: Vacuum technology. Fachbuchverlag Leipzig, ISBN 3-446-15859-6
  • Karin Wey, Ralph Jürgen Peters: History of vacuum technology. In: Vacuum in research and practice. 14 (3), pp. 180-183 (2002), ISSN 0947-076X
  • Heinz-Dieter Bürger: The history of vacuum cooling. In: Vacuum in research and practice. 16 (2), pp. 67-70 (2004), ISSN 0947-076X
  • Henning Genz: Nothing but nothing. The physics of the vacuum. WILEY-VCH Verlag, Weinheim 2004, ISBN 3-527-40319-1

swell

C. GRANDA, R.G. MOREIRA, S.E. TICHY (2004) Reduction of Acrylamide Formation in Potato Chips by Low-temperature Vacuum Frying, Journal of Food Science 69 (8), 405-411.

See also

  • Deutsches Museum München - Illustration of the Magdeburg hemispheres by Guericke
  • English FAQ: Explosive decompression and its effects on the body.

Category: vacuum technology