Plasma in in Nature and Technology

The designation "Plasma" has been introduced by the scientist and nobel price winner David Langmuir (1881 - 1957) in the year 1928 for a state of matter which has been unknown before. Plasma is also considered to be a fourth state of matter.

solid
Atoms or molecules of the substance are fixed in their positions
liquid
Atoms and molecules can move and change their positions within the substance
gaseous
Atoms and molecules can move free so that they can also leave the ambient of the substance.
plasma
The electrons of the atoms received sufficient energy to surpass the attracting atomic forces and are able to leave the atom.

The word "Plasma" is greek origin and means "entity"

 

Plasma is a state of matter of high energetic stimulation where because of their extreme stimulation electrons are able to escape from the atomic shell, so that positive charges ions and free electrons result

 

 

Natural Plasma

For humans Plasma is a very strange and obscure state of matter, even if by far the biggest amount of matter in the universe is plasma. All active stars, i.e. all stars where nuclear fusion is active, consist from plasma.

The only natural plasma on earth is found

  • in polar lights
  • in flashes

i.e. usually far away from persons.

Technical Plasma

Even though plasma is so exotic in nature, it is not very difficult to generate it by technical means. The generation by extremely high temperatures which is dominating in nature technically is of minor relevance. It takes place in very hot flames. Technical plasma is generated either by

  • High voltage generation
  • High frequency generation

 

 

 

Plasma Stimulation by High Voltage Generation

 

The essential factor is a high electric field strength. Those can be achieved allready with moderate voltage if the generating electrodes are shaped to very sharp edge pins.

The high electric field strength deforms atoms and molecules, so that positive and negative centers of gravity are displaced so that strong electric dipols result. When the displacement of electrons and nucleus will be extreme, electrons may be completely lost and ionisation starts.

Plasma stimulation by High voltage generation can take place under atmospheric pressure or at low pressure. The following phenomena are plasma stimulated by High voltage:

  • Artificial and natural flashes
  • arc lamp
  • fluorescent lamps
  • Corona discharge
  • Atmospheric plasma

While gases are electrically isolating, plasma is conductive because of the positive ions and the free electrons. The high voltage causes high current nd high temperature. Therefore high temperatures are typical for atmospheric plasma. However there is a method to avoid them by coating at least one of the generating isolaters by a strong isolater (dielectric). View DBD

 

 

 

Plasma Stimulation by High Frequency Generation

In electric fields oszillating with high frequencies gas atoms and molecules can be stimulated to form electic dipols. Usind approlriate frequencies can cause the ionisation. High voltage is not required. Flat plate electrodes can be used to generate the plasma. This method however only can be applied und low pressure of no more than a few pascal.

The free electrons are oszillating with the frequency of the electric field. The much heavier ions however cannot oszillate with the high frequencies of much more than 10 kHz which need to be applied for plasma generation. Therefore temperature only slightly increases in a plasma stimulated by High frequency generation.

 

Plasma stimulation by High frequency generation under low pressure is of major relevance for technical Surface treatment by Plasma technology.

Low pressure plasma
Complete setup of a Low pressure plasma system. For detailed explanations to the components view Plasma Technology

 

Active species

In stars almost all matter is ionizized.

In a technical Plasma only a small fraction, rarely more than 1% og matter are ions. Except ions and free elctrons technical plasma contains additional components which cause plasma effects:

  • stimulated atoms:
    Stimulation of an electron not allways is sufficient to exit the atomic shell. Instead it is shifted to a higher energy level within the atomic shell. Such a valence electron is now positioned lonely on the highest energy level of the shell. Such electrons are only weakly connected to the atom and therefore can easily contribute to a chemical reaction by jumping to an atom, ion or molecule offerring a free space on a lower energy level.
  • UV-Radiation:
    Parallel to the ioniusation process which bis permanently continuing as long as generation is active ions also recombine with free electrons to return to the state of neutral atoms. The process causes the release of a high energy amount which results in high-energy UV radiation. This UV-radiation is ioniziding and acts very aggressive on matter, mainly on organic substance hit by the radiation.
  • Radicals:
    The most essential effect of UV radiation is the fact that it is able to crack molecular bindings. The resulting molecular fractions which also can be atoms are designated "radicals". The radicals have an unsatturated link at the cracking position, where easily atoms, molecules or other radicals can adhere. Therefore radicals are very reactive. Therefore they are unstable and immediately react to new molecules
    .

These reactive plasma components:

  • ions
  • stimulated atoms
  • radicals
  • UV-radiation

are Active Species of a plasma.

Parallel with the generation of stimulated atoms even the inverse process takes place, i.e. the return to its stable, non stimulated state of lower energy. The process is callled "Relaxation". Also the relaxation process releases energy, however on a lower level than during the process of recombination of ions. Therefore the energy level and consequently the frequency of the radiation released is lower and is within the visible range. The process of relaxation is responsible for the typical bright colour of the plasma which characterizes the process gas.

Due to the process of recombination and relaxation also many particles exist in a plasma in the state of atoms and molecules in their unstimulated base state.

 

 

A vacuum pump evacuates the plamsa chamber to a pressure of typically 1,1 - 1 mbar. Controlled by appropriate valves the process gas enters the plasma chamber.
Constant pressure. Evacuation and addition of process gas are continuing parralel. Plasma is not yet generated.
The High frequency plasma is generated. There are no substrates and no particles resulting from chemical reaction inside the plasma chamber.