Atmospheric pressure plasma
Atmospheric pressure plasma is often also called AD plasma or normal pressure plasma. The name comes from the fact that the pressure at which it is produced is approximately the same as that of the surrounding atmosphere, the so-called normal pressure. In contrast to low-pressure plasma or high-pressure plasma, there is no need for a reaction vessel to maintain the different pressure levels to atmospheric pressure.
Plasma is a partially ionized gas. Electric arcs, dielectric barrier, corona and piezoelectric direct discharges ionize gases at atmospheric pressures creating plasmas. The charged particles – electrons and ions – accelerate to very high energies. Only a small fraction of the gas molecules is turned into the energetic electrons and ions; the rest of the gas remains neutral and cold. Its temperature reaches only 50 C in the case of the piezoelectric direct discharge, and 250-450 C in the case of the arc discharge. At the same time, the very energetic electrons and ions collide with the gas molecules producing large quantities of short-lived chemical species, such as atomic H, N and O species, OH and ON radicals, ozone, nitrous and nitric acids, as well as various other molecules in metastable excited states. They make this plasma chemically very active.
Field of applications
Atmospheric pressure plasma is mainly used at low current density for surface activation and fine cleaning of plastic, metal or inorganic materials. In the industrial sector, plasma treatment is mainly used for the pre-treatment of surfaces prior to gluing, printing and painting processes. In the medical sector it is increasingly used for disinfection and sterilisation.
The advantage of treatment with atmospheric pressure plasma is that liquid chemicals can be completely dispensed with. Only electricity and a carrier gas are required. All in all, in this type of plasma generation compressed air or other harmless gases such as nitrogen or forming gas can be used as well as in the case of piezoelectric discharge noble gases and even ambient air.