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A fan generates a flow of ambient air with a gas velocity up to 175m/s, heating elements are used to control the gas temperature in a range between 25°C and 75°C, a DC voltage of up to 30kV can be applied to the electrodes, a treatment compartment allows both static and dynamic treatments. In the dynamic mode, a polymer film moves along the plasma zone with a velocity up to 22 m/min (at present). |
The atmospheric pressure DC glow discharge has several advantages compared
to a corona or to a barrier (silent) discharge.
- Due to the high power density (30 W/cm3) it produces a much higher concentration of chemical activated particles (mainly atomic oxygen, ozone and activated nitrogen molecules).
- The plasma is almost as homogenous as a low pressure glow discharge.
The atmospheric pressure DC glow discharge is a nonthermal plasma. The setup is
simple because there is no need for vacuum or HF-devices. The treatment takes
place in down stream of the plasma zone without the danger of any back steaming.
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General scheme of the DC glow discharge at atmospheric pressure |
The atmospheric pressure DC glow discharge can be characterized by the applied
voltage, the gas velocity and the gas temperature. The voltage induces an electric
field in which electrons gain kinetic energy. They collide with gas particles and
ionize them if their energy is sufficiently high. With increasing voltage the
electrons gain more energy and the number of ionization events grows. Thus, the
current rises exponentially with the applied voltage. At a high concentration of
charge carriers streamers (small sparks) are formed. The corresponding voltage is
called critical voltage. In this case, it marks the transition between the corona
and spark discharge (US1). Under common conditions this takes place at a fairly low
current. Consequently, the corona discharge produces only a small concentration of
chemical activated particles. By using a special electrode design and a high flux
gas flow it is possible to establish a glow discharge. The critical voltage US1
becomes the transition between corona and glow discharge. The transition to the
spark discharge is displaced to higher values (US2) reaching a dramatically higher
current (power density).
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Voltage current characteristic of the DC glow discharge (parameter: T=20°C; v=90m/s; electrode gap=1.25cm), inset: schematic plot of transitions between corona, glow and spark discharge |