Dielectric barrier discharge

The silent electric discharge (also dielectric barrier discharge, English Dielectric Barrier Discharge, DBD) or plasma discharge is an AC gas discharge in which at least one of the electrodes is electrically isolated from the gas chamber by means of a galvanic isolation dielectric.

Declaration and characteristics

A gas-or air-filled space between the insulating coated electrodes can be ionized or passes (similar to a low-temperature plasma glow discharge ) in a plasma state, when an alternating voltage across the electrodes generates a sufficient field strength in the gas space. By the displacement currents, the discharge is maintained even through the insulation and it may be continuous electrical power to be transferred into the plasma. You can think appropriate arrangements as a capacitor with an inhomogeneous dielectric, which is why (some incorrect ) also speaks of capacitive excitation or electrodeless excitation. The field strength is inversely proportional to the dielectric constant and therefore the gas is always higher than in the dielectric. However, the surface of the dielectric is charged by ion bombardment, and ultraviolet radiation, which has to be utilized depending on the application, either or avoided.

DBE have the following properties:

  • The discharge can be either in the form of many filaments (micro discharges ) or occur as a homogeneous discharge. In the case of a homogeneous type, a discharge is observed haze, which extends across the entire discharge volume.
  • There are approximately only accelerates electrons, since the discharge duration is so low that the heavy ions due to learn by their inertia, only a little momentum.
  • The discharge extinguishes as soon as the applied electric field is compensated by the accumulated before the dielectric electrical charge.
  • The duration of a discharge is in the range of a few nanoseconds; The transport of ions is suppressed.
  • It may be cold plasma produced, especially as the gas temperature is mainly determined by the ion temperature.

To produce a homogeneous discharge, a pulsed excitation is beneficial. The DBE will be acted upon with uni- or bipolar pulses with pulse durations of a few microseconds down to few tens of nanoseconds and amplitudes in the single digits kilovolt range. The pulse-pause ratio is usually very small and is less than ten percent.

The high electrical alternating voltage ( several kilovolts ) of high frequency ( about 10 to 1000 kHz) or high frequency pulses can be generated with resonant converters with high efficiency.

Advantages depending on the application:

  • No metallic electrodes in the discharge space, thus no metallic impurities or electrode wear
  • High efficiency because the electrodes have no charge carriers from entering or ( omission of the cathode fall, no thermionic emission required)
  • Dielectric surfaces may be modified at low temperatures and chemically activated

An advantage is also working on a normal air atmosphere.

The frequency range is indeed bounded above, effective electric excitation circuits employ semiconductors to a few 100 kHz and generators with electron tubes at 10 ... 100 MHz. Similarly, but also method with magnetron work in the ISM band 2.4 ... 2.5 GHz. However, it is to note that the possible non-equilibrium plasma in DBE is mainly achieved by the pulse excitation. In contrast to continuous excitation, for example, a sine or square wave signal, the pulse mode on a small pulse-to- pause ratio (duty cycle ). After the excitation and induction of the plasma state, the charge carriers formed in the gas can again be removed and a efficiency - damaging thermalize the plasma is prevented during the break.

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

  • Parallel plates: Townsend regime to the so-called streamer breakdown
  • Wire mesh on Dielectric: corona discharge to so-called streamer corona
  • Peak-to- disk: streamer corona to glow discharge
  • Ionisation in the form of an insulator tube by high voltage and corona discharge

Applications

Environmental

  • Trink-/Abwasseraufbereitung with ozone
  • Paper bleaching with ozone as well as functional surfaces
  • Treatment of waste gases (for example, plasma torch)

Measurement

Use in gas chromatography as a Barrier Discharge Ionization Detector (BID) with cold plasma discharge. This detector uses the high-energy photons from the helium plasma for ionization of the sample molecules. Since practically all substances (except neon and helium itself) have a lower ionization potential, this detector can be described as universal. The Japanese company Shimadzu has developed the principle of the ionization barrier plasma discharge and this technique saved since 2013 by numerous patents exclusively.

Material and Surfaces

  • Clean, oxidation, 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 is superior to iodine and alcohol.

Light and radiation technology

Production of light and ultraviolet optical radiation:

  • Lamps with fluorescent phosphors
  • Plasma screens (excitation colored phosphors with ultraviolet light )
  • Generation of vacuum ultraviolet ( VUV) optical radiation with excimer lamps

Excitation of gas lasers.

Operation

Ballasts for gas discharge lamps, dielectric barrier are usually power radio frequency generators which comprise a transformer for the output voltage boost. The simplest ballasts generate a low or high frequency continuous sinusoidal voltage. Per half sine wave, it usually comes to several lamp strikes. An alternative to provide generators with square-wave output voltage signal represents the lamp strikes take place here at times of high voltage slew rates and thus outside the plateau voltages. The applying power electronic topologies are based - as with other operating devices for gas discharge lamps - on half or full bridge driven resonant circuits. The lamp capacity is usefully used as a capacitive part of the resonant circuit. In continuous operating units thus stored in the capacity of the lamp power does not need to be recovered. It remains in the resonance circuit and only the consumed by the lamp strikes the active power to be coupled into the resonant circuit. The advantageous for the lamp efficiency pulsed operation of DBE lamps is detrimental to the efficiency of the pulse mode device because the lamp usually has a very low power factor ( typically 10 %) and thus 90 % of the expenditure necessary to achieve the ignition energy back out of the resonant circuit must be removed. Typical topologies are the Flyback converter ( flyback ) and resonant half-bridge circuits half bridge. A flexible topology that combines these two approaches circuit, is presented in and and can be used for DBE_Lampen variable capacitance. Provides an overview of possible topologies and control concepts for the pulsed operation DBE

Electric discharge in gases ISM-Band Shimadzu Corp. Gaslaser
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