Silent electrical discharge

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The silent electric discharge (also dielectric barrier discharge , DBE, English dielectric barrier discharge , DBD ) or plasma discharge is an alternating voltage - the gas discharge , wherein at least one of the electrodes from the gas space through galvanic separation by means of a dielectric is electrically insulated.

Explanation and characteristics

A gas- or air-filled space between insulating coated electrodes can be ionized or enters a plasma state (low-temperature plasma similar to a glow discharge) if an alternating voltage generates sufficient field strengths at the electrodes in the gas space. The discharge is also maintained through the insulation by means of displacement currents and electrical power can be continuously transferred into the plasma. Corresponding arrangements can be imagined as a capacitor with an inhomogeneous dielectric, which is why one speaks of capacitive excitation or (somewhat incorrectly) electrodeless excitation. The field strength is inversely proportional to the dielectric constant and therefore always higher in the gas than in the dielectric. Nevertheless, the surface of the dielectric is stressed by ion bombardment and ultraviolet radiation, which, depending on the application, must either be exploited or avoided.

DBE have the following properties:

  • The discharge can occur either in the form of many filaments (micro-discharges) or as a homogeneous discharge. In the case of a homogeneous discharge, a kind of haze is observed that extends over the entire discharge volume.
  • Only electrons are approximately accelerated, since the discharge duration is so short that the heavy ions, due to their inertia, experience little momentum.
  • The discharge ceases as soon as the applied electric field is compensated by the electric charge accumulated in front of the dielectric .
  • The duration of a discharge is in the range of a few nanoseconds; the transport of ions is largely suppressed.
  • Cold plasma can be produced, especially since the gas temperature is largely determined by the ion temperature.

Pulsed excitation is advantageous for generating a homogeneous discharge. The DBE is applied with unipolar or bipolar pulses with pulse durations of a few microseconds down to a few tens of nanoseconds and amplitudes in the single-digit kilovolt range. The pulse-pause ratio is usually very small and is below ten percent.

The high electrical alternating voltage (a few kilovolts) with a high frequency (around 10 to 1000 kHz) or the high-frequency pulses can be generated with high efficiency using resonance converters .

Advantages depending on the application:

  • no metallic electrodes in the discharge space, thus no metallic contamination or electrode wear
  • high efficiency, as no charge carriers have to enter or exit the electrodes ( no need for a cathode , no glow emission necessary)
  • dielectric surfaces can be modified and chemically activated at low temperatures

Working in a normal air atmosphere is also an advantage.

The frequency range is not limited upwards, effective electrical excitation circuits work with semiconductors up to a few 100 kHz and for generators with electron tubes at 10 ... 100 MHz. However, processes with magnetrons in the ISM band 2.4 ... 2.5 GHz also work in a similar way . It should be noted, however, that the non-equilibrium plasma possible with DBE is primarily only achieved by pulse excitation. In contrast to continuous excitation, for example with a sine or square-wave signal , the pulse mode has a small pulse-to-pause ratio ( duty cycle ). After the excitation and creation of the plasma state , the charge carriers formed in the gas can be broken down again during the break and thermalization of the plasma that would damage the efficiency is prevented.

The electrode configurations of a silent discharge can vary greatly depending on the application:

Applications

environmental engineering

  • Generation of ozone for
    • Drinking / wastewater treatment
    • Cleaning (stench removal)
    • Paper bleaching
  • Treatment of exhaust gases (e.g. plasma torch)
  • Processing of functional surfaces

measuring technology

Used in gas chromatography as a Barrier Ionization Discharge Detector (BID) with cold plasma discharge. This detector uses the high-energy photons of the helium plasma to ionize the sample molecules. Since practically all substances (except neon and helium themselves) have a lower ionization potential, this detector can be described as universal. The Japanese company Shimadzu has further developed the principle of barrier ionization plasma discharge and has secured this technology exclusively through numerous patents since 2013.

Material and surfaces

  • Cleaning, oxidizing, etching, coating of surfaces

medicine

  • Dental treatment with plasma torch
  • Treatment of open legs in diabetes
  • Hand disinfection in hospitals
  • Surface disinfection of the skin, iodine and alcohol is superior.

Light and radiation technology

Generation of light and ultraviolet optical radiation:

  • Fluorescent lamps
  • Plasma screens (stimulation of colored phosphors with ultraviolet light)
  • Generation of vacuum ultraviolet (VUV) optical radiation with excimer lamps

Excitation of gas lasers .

business

Operating devices for dielectrically impeded gas discharge lamps are usually high-frequency power generators that contain a transformer to increase the output voltage. The simplest operating devices generate a low or high frequency continuous sinusoidal voltage. As a rule, several lamp ignitions occur per sine half-wave. Generators with a square-wave output voltage signal represent an alternative to this. The lamp ignitions take place here at times of high voltage rise rates and thus outside the plateau voltages . The power electronic topologies used are based - as is the case with control gear for other gas discharge lamps - on half- or full-bridge driven resonance circuits. The lamp capacitance is sensibly used as a capacitive part of the resonance circuit. In the case of continuously operating control gear, the energy stored in the capacity of the lamp does not have to be recovered. It remains in the resonance circuit and only the active power consumed by the lamp ignitions has to be coupled into the resonance circuit. The pulsed operation of DBE lamps, which is advantageous for lamp efficiency, is disadvantageous for the efficiency of the pulsed operating device, since the lamp usually has a very low power factor (typically 10%) and thus 90% of the energy required to achieve the ignition voltage again from the resonance circuit must be removed. Typical topologies are the flyback converter and resonant half bridge circuits . A flexible topology that combines these two circuit approaches is presented in and and can be used for DBE lamps with variable capacity. An overview of possible topologies and control concepts for pulsed DBE operation is provided.

A fairly new method for generating barrier discharges is the direct use of a piezoelectric transformer (PT) as an active dielectric component.

literature

Web links

Individual evidence

  1. ↑ Shuhai Liu, Manfred Neiger: Excitation of dielectric barrier discharges by unipolar submicrosecond square pulses . In: Journal of Physics D: Applied Physics . tape 34 , 2001, p. 1632–1638 ( [1] [accessed March 26, 2014]).
  2. ↑ Current controlled driver for a Dielectric Barrier Discharge lamp . In: Publication date June 21-24, 2010 . Power Electronics Conference (IPEC) 2010 International. Retrieved December 9, 2010.
  3. ↑ Resonance behavior of a pulsed electronic control gear for dielectric barrier discharges (PDF; 2.0 MB) In: Power Electronics, Machines and Drives (PEMD 2010), 5th IET International Conference on . Retrieved October 20, 2013.
  4. ↑ Patent application title: DEVICE FOR GENERATION OF VOLTAGE PULSE SEQUENCES IN PARTICULAR FOR OPERATION OF CAPACITIVE DISCHARGE LAMPS . In: Publication date 2005 . University of Karlsruhe. Retrieved May 23, 2011.
  5. ↑ Patent application title: Adaptive Drive for Dielectric Barrier Discharge (DBD) Lamp . In: Publication date 2008 . BRIARCLIFF MANOR, NY US. Retrieved December 9, 2010.
  6. ↑ Resonant Behavior of Pulse Generators for the Efficient Drive of Optical Radiation Sources Based on Dielectric Barrier Discharges . KIT scientific publishing. July 10, 2013. Retrieved October 20, 2013.