Under a plasma, also called the fourth state of aggregation, one understands a (partially) ionized gas in which next to neutral gas molecules and fragments there is also the occurrence of free electrons and cations; and which, therefore is conductive. In addition, there are a large amount of excited molecular conditions, that under delivery from electro-magnetic radiation turns back to the ground state, which leads to the characteristic luminance of the plasma.
The required energy by the origination of a plasma can have very different sources. Just like in nature, plasma occurs, i.e. on the sun, as well as in lightnings and flames; but also in the atmospheric phenomenon known as polar light. By far the most frequent type of technical production of plasma is the concerted irradiation of electronic fields. Depending upon the frequency used, one decides thereby between alternating current (50 Hz), Audio (kHz)- Radio (MHz) frequency or microwave plasmas (GHz). Plasmas get used technically, i.e. in fluorescent tubes and, above all, in recent times in the surface technique.
A current differentiation
of plasmas is the division in hot (thermal) and cold (non-thermal) plasmas:
- In the case of thermal plasmas the pressure of the gases is relatively high, which raises the number of collisions between the particles (neutral, charges, excited, non-excited) and thereby encourages the conveyance of the energy between them. The result is a plasma, that finds itself in thermodynamic equilibrium, so that all the approximate particles show the same high energy, which we perceive as "hot".
- In comparison, non-thermal plasmas develop at reduced pressure (ca. 1-10 4Pa). Here, the mean free path is so large, that except through electron collision, no important criterion energy transmission between the particles can take place, so that no thermal equilibrium can emerge. Macroscopicly viewed, the system is placed at room temperature; but contains a known share of particles, namely electrons, that show very high energies (temperatures up to 10 5 C). These high energy electrons and the high energy radiation from electron migration are qualified in the induction of chemical reactions on surfaces, or rather in near-surface ranges, where even the modification of very stable structures can occur. Simultaneously, no thermal strain of this surface occurs, due to the fact that the macroscopic plasma temperature is adapted to the surroundings.
The atmospheric
pressure plasma technique, as well as the low-pressure
plasma technique, is being used on the Institute for Manufacturing
Technical Science and Applied Material Research (IFAM) for surface
modification; whereas, the
mode
of action here is always the same.
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