Low pressure plasma
Typical technical low-pressure plasmas are operated in the pressure range of a few Pascals , i.e. at pressures that are a factor of 10,000 lower than normal air pressure. A mean free path of the electrons which is greater than the Debye length is typical for low-pressure plasmas .
Low-pressure plasmas occur in space in glowing gas nebulae. Aurora borealis can be called low pressure plasmas.
Furthermore, low-pressure plasmas are used for a large number of coating tasks. Examples are:
- Consumer goods industry: lip gloss applicator, toys for children, cutlery baskets, hearing aids, pens, toothbrushes
- Medical technology: catheters, petri dishes, pipette tips, stents, dental implants
- Textile industry: yarn, fabrics, shoe soles
- Watch and jewelry industry: dial, gemstone
- Optics: glasses , lenses , intraocular lenses
- Automotive technology: headlights , mirrors , steering wheel
- Tools: Hardened surfaces
- Packaging industry: foils
- Beverage technology: inner coating of PET bottles with silicon dioxide or titanium pentoxide .
- Plasma medicine: Sterilization of medical instruments, especially heat and moisture sensitive devices such as endoscopes
- Electrical engineering: amorphous silicon solar cells , thin-film transistor screens, piezo elements
Another application of low-pressure plasmas is the refinement of surfaces. For example, if zirconium dioxide (a ceramic material) is exposed to a methane plasma, the oxygen atoms are replaced by carbon to a depth of a few micrometers. The resulting zirconium carbide has a silvery sheen and is just as scratch-resistant as the white zirconium dioxide.
During the plasma etching of PTFE, material is removed from the surface by means of a hydrogen plasma. The treatment takes place under a certain pressure in a vacuum chamber, where hydrogen gas is electrically excited. The PTFE is converted from the solid to the gaseous state of aggregation on the surface and can thus be easily sucked off by the vacuum pump. After the plasma treatment, the previously inert PTFE can be glued, overmoulded, painted or printed. The plasma process replaces the wet chemical etching of fluoropolymers.
In the case of low-pressure plasmas, due to the low pressure, the typical mean free path is so large that collision processes are “rare”. Accordingly, the different types of particles (heavy particles, electrons ) are not in thermal equilibrium, i. H. have different temperatures.
In technical low-pressure plasmas, electron temperatures of a few electron volts (several 10,000 K) are achieved through selective heating of the electrons , while the temperature of the neutral gas is slightly above room temperature. This means that thermally sensitive materials such as plastics can also be processed using low-pressure plasmas.
The plasma edge layers , i.e. the transition area from the plasma to the wall or to the material, play a major role in the technologically usable properties .
The excitation of the low pressure plasma can be implemented with different frequencies. A different suggestion is used depending on the application and problem. The most common frequencies are: 40–100 kHz ( long wave excitation ), 13.56 MHz ( short wave excitation , RFID band) and 2.45 GHz ( microwave excitation , WLAN band). Usually a frequency from one of the generally allocated ISM or SRD bands is used, as otherwise a separate allocation or license is required for frequency management .
- Gerhard Franz: Surface technology with low pressure plasmas. Coating and structuring in microtechnology . 2., completely reworked. Springer, Berlin a. a. 1994, ISBN 3-540-57360-7 .
- Michael A. Lieberman and Allan J. Lichtenberg: Principles of Plasma Discharges and Materials Processing . Wiley, New York et al. a. 1994, ISBN 0-471-00577-0 .
- Alfred Grill: Cold Plasma in Materials Fabrication. From Fundamentals to Applications . IEEE Press, New York 1994, ISBN 0-7803-4714-5 .