Non-thermal plasma

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Non-thermal plasmas (often abbreviated as NTP ), also non-equilibrium plasmas , are plasmas that are not in thermal equilibrium . B. the temperatures of the contained types of particles ( neutral particles , ions , electrons ) differ significantly.

Basics

Thermal imbalance can have various causes in technical implementations:

At reduced pressures (e.g. less than 100 Pa = 0.001 bar ) the mean free path is so large that no significant energy transfer can take place between the particles, i.e. no thermal equilibrium can arise. However, the free electrons are selectively heated by coupling in electromagnetic waves . For the majority of gas particles, the temperature is at room temperature , but the plasma contains electrons having a very high average kinetic energy with temperatures of about 10000 K associated.
These high-energy electrons and the high-energy radiation from electron transitions are capable of initiating chemical reactions on surfaces or in areas close to the surface, which can even include the modification of the most stable chemical structures. At the same time, there is no thermal load on this surface, since the macroscopic temperature of the plasma is adapted to the environment.
  • Plasma parameters that vary greatly over time
In so-called dielectrically hindered discharges (discharges with dielectric barrier), plasmas are generated far from thermal equilibrium despite normal gas pressure. A high-frequency alternating voltage creates tiny discharge channels, so-called streamers, at different points with each period. Since the discharge channels only make up a fraction of the total discharge volume and the duration of the discharge is severely limited by the capacitive coupling, the mean gas temperature in the discharge remains low, i.e. close to room temperature.
Another example is pulsed plasmas d. H. Plasmas that are only excited for fractions of a second, d. H. in this also cannot reach thermal equilibrium.

application

In exhaust gas cleaning , non-thermal plasmas are specifically used to remove odors and certain hydrocarbons . The use of non-thermal plasmas for exhaust gas cleaning is one of the disposal processes, since the most complete possible oxidation of hydrocarbons is sought. These exhaust gas cleaning processes are used in particular in the food and animal feed industries. Non-thermal plasma - in combination with a biofilter  - has also been used successfully to reduce bioaerosols from the exhaust air of a composting plant . Non-thermal plasmas are also used in medical technology, for example in the treatment of poorly or non-healing wounds with the help of a so-called plasma pen, in which the antimicrobial effect of "cold plasma" is used. The technology can also be used to create conductor tracks on plastic substrates. In addition, the sterilization of PET bottles using non-thermal plasma is currently being tested.

The use of indoor air cleaning devices that operate on the basis of non-thermal plasma is viewed as critical because of the possible formation of ozone . In this way, newly formed air pollutants could be detected in the treated air .

See also

literature

  • VDI 2441: 2014-03 (draft) Process gas and exhaust gas cleaning using cold plasma methods - barrier, corona discharge, UV radiation. Berlin: Beuth Verlag ( summary online )

Individual evidence

  1. ↑ VDI 2441, p. 3.
  2. ↑ VDI 2441, pp. 3–4.
  3. ↑ VDI 2441, p. 5.
  4. ↑ VDI 2441, p. 2.
  5. ↑ VDI 2441, p. 13.
  6. ↑ VDI 2441, pp. 20-21.
  7. ↑ VDI 4255 sheet 2: 2009-12 Bioaerosols and biological agents; Sources of emissions and mitigation measures in farm animal husbandry; Overview (Bioaerosols and biological agents; Emission sources and control measures in livestock operations; Overview). Beuth Verlag, Berlin. P. 40.
  8. ↑ Plasma pen improves wound healing . VDI nachrichten , ISSN  0042-1758 , No. 47 of November 21, 2014, p. 24.
  9. ↑ https://3d-mid.de/cms/front_content.php?idcat=104
  10. ↑ Not too fast and not too slow . Process engineering, ISSN  0175-5315 , No. 1–2, 2010, p. 12.
  11. ↑ Henning Heberer, Eberhard Nies, Markus Dietschi, Angela Möller, Wolfgang Pflaumbaum, Marco Steinhausen: Considerations on the effect and toxicological relevance of NTP air purification devices. In: Hazardous substances - cleanliness. Air . 65, No. 10, 2005, ISSN  0949-8036 , pp. 419-424.
  12. ↑ Hans Jürgen Buschmann, Jörg Brandes, Vahid Ameri Dehabati, Jochen S. Gutmann: Indoor air - new possibilities for reducing pollution. In: Hazardous substances - cleanliness. Air. 74, No. 10, 2014, ISSN  0949-8036 , pp. 421-425.