Glow discharge
A glow discharge is a gas discharge that occurs independently between two cold electrodes connected to a DC or AC voltage source at low gas pressure. The color of the glowing glow depends on the gas. The noble gas neon , which gives the color orange-red, is often used in indicator lamps .
application
The light emissions resulting from the discharge are used in fluorescent tubes and glow lamps for lighting or display.
In vacuum technology , a glow discharge is used to clean surfaces to be vaporized. For this purpose, a vacuum of 10 −2 to 10 −3 mbar is set in the recipient and a glow discharge is built up between a ring electrode and ground using a controllable high voltage source . The gas discharge ( plasma ) spreading throughout the recipient sublimes and oxidizes impurities adhering to the surfaces, e.g. B. organic compounds that are difficult to remove with other cleaning methods. In addition, the glow discharge is used as a source of excitation in carbon dioxide lasers .
Glow lamps appear in staircase light switches and as night lights (advantage: low power consumption with the usual mains voltages of 120 to 240 volts); Voltage testers (response voltage, low current through the human body, shockproof), machines (shockproof) and electric cookers since the 1960s (heatproof).
The heat generated by the glow discharge is used to move bimetal sheets : in the conventional starter for a fluorescent tube and in the optical simulation of a flickering candle flame. By feeding one of 10 digit-shaped electrodes, the corresponding digit figure lights up in a display tube.
Glow discharge can be used in spectrometers for material analysis for elements on the surface of samples and can also selectively penetrate to a depth of 0.15 mm. Glow discharge is a way of generating plasma in order to influence the polymerization of reactants or to change the surfaces of plastics, for example to make polyethylene printable.
Spatial structure of the discharge stages
Moving from the cathode to the anode , the glow discharge can be divided into eight successive layers:
- Closest to the cathode is the Aston dark room . It is quite thin, but easily visible if there are noble gases or hydrogen in the discharge tube.
- This is followed by a thin, reddish skin of light, which is called the first cathode layer or cathode glow skin .
- This is followed by a weaker zone, which is called the Hittorf or Crookes dark room or cathodic dark room .
- The brightest part of the discharge process is the negative glow light , which is clearly separated from the Hittorf dark room and becomes weaker towards the other side. The large voltage drop between the cathode and the onset of the glowing light is called the cathode drop . This voltage drop results from the ionization of the gas particles. The resulting positive ions (they drift 'slowly' towards the cathode) are responsible for the strong voltage drop (= cathode drop) between the cathode and the negative glowing light. The greater inertia (mass!) Of the positive ions creates an excess of positive charge carriers. The field strength between the negative glow light and the anode is reduced. The electrons are therefore accelerated less strongly and their ionization capacity decreases. There is a negative space charge in this area.
- The next lightless zone is called the Faraday dark room .
- This is followed by the positive column , which, depending on the pressure and gas filling, appears as a hanging light band or in the form of separate layers.
- The anodic glow light occurs near the anode .
- The anode dark room is located directly on the anode .
properties
1) Dependent discharge: When charge carriers hit the oppositely charged electrode, these are neutralized. For a steady flow of electricity, a constant new production of charge carriers is necessary. In the case of dependent unloading, this production takes place from the outside - in contrast to independent unloading, in which the production z. For example, by impact ionization occurs
2) glow discharge
3) arc discharge
In contrast to other forms of gas discharge, the temperature of the electrodes and walls remains low in the glow discharge, as only little heat is released due to the low current density and the associated impact of charge carriers.
Due to the low gas pressure typical of glow discharges, the mean free path of the electrons is greater than that of atmospheric discharges. This reduces the energy exchange between electrons and the heavier gas particles (atoms, molecules and ions ) during glow discharges , since the number of particle collisions decreases. The temperatures of the individual gas components therefore differ considerably from one another. If the mean energy of the electrons is converted into a temperature, the result is temperatures of 10 3 to 10 5 K. The temperature of the ions and neutral particles, on the other hand, remains close to room temperature. In this case, one also speaks of a non-thermal plasma .
The negative glow light and the above-described stratification of the positive column come about because the electrons between the individual layers are accelerated until they have built up the energy required to excite the gas. At the cathode, electrons are released by thermal emission or as secondary electrons by ions or photons and accelerated by the electric field. As long as their energy remains below the excitation energy of the gas, the collisions between the electrons and the neutral particles of the gas are essentially elastic. In the case of an elastic collision between two bodies with very different masses, the kinetic energy of the lighter collision partner (in this case the electron) is almost retained. If the energy of the electrons due to the acceleration in the field is so great that the excitation energy of the gas is reached, the gas particles are excited and the excitation electrons lose most of their kinetic energy ( inelastic collision ). The excited gas particles lose their excited state via optical radiation. The first luminescent layer is therefore the area where the electrons have built up the excitation energy required for the type of gas for the first time due to the acceleration in the electric field. The braked electrons are accelerated again from this area by the electric field until they lose their energy again due to the excitation of the gas particles. The various luminous layers are built up by this mechanism.
The shape of the positive column depends on the interaction of the electrons with the glass tube. The glass tube slows the electrons, which increases their recombination rate and reduces the electron density. This gives the remaining electrons enough energy to ionize more atoms. If the diameter of the tube is too large relative to the electrodes, no positive column will form. For this reason, there are no spherical fluorescent tubes . Tubes with a smaller diameter allow higher gas pressures and shorter lengths for the same operating voltage.
A clear development of the luminous layers is only possible if there are very defined excitation states in the gas. For this reason, gas mixtures should not be used for observing the phenomenon in the experiment, and the gas should have a simple electrical excitation structure, as e.g. B. is the case with noble gases .
literature
- Gerhard Franz: Low Pressure Plasmas and Microstructuring Technology. Springer, Berlin et al. 2009, ISBN 978-3-540-85849-2 ( preview ).
- Horst Stöcker (Ed.): Pocket book of physics. Formulas, tables, overviews. 4th corrected edition. Verlag Harry Deutsch, Thun et al. 2000, ISBN 3-8171-1628-4 .
- Wolfgang Demtröder: Experimental Physics 2: Electricity and Optics. Springer, Berlin 2009, ISBN 978-3-540-68210-3 .
Web links
- Lecture manuscript ( Memento from March 31, 2010 in the Internet Archive ) (Chapter 4.2 on glow discharge, pdf; 3.6 MB - www.ieap.uni-kiel.de)
See also
Individual evidence
- ↑ Analysis of thin and thick layers with pulsed radio frequency glow discharge spectrometers (RF GDOES) horiba.com, (c) 1996–2017, accessed October 26, 2017.
- ↑ Joachim Klotzbücher: Investigations into the long-term stability of plasma-activated plastic surfaces ( Memento of the original from June 30, 2012 in the Internet Archive ) Info: The archive link was automatically inserted and not yet checked. Please check the original and archive link according to the instructions and then remove this notice. Diploma thesis, Aalen University, October 20, 2007, accessed October 26, 2017.
