Glow discharge refers toLow pressure gasShow inglowOfgas discharge Phenomenon, namely self-sustaining discharge in thin gas（Self excitationelectric conduction）Phenomenon.fromFaradayThe first discovery.It includes two transitional stages: sub normal glow and abnormal glow.Glow discharge is mainly used forNeon manostat、He Ne laserAnd other devices.
Glow discharge is a low pressure discharge phenomenon. The working pressure is generally lower than 10 mbar. Its basic structure is to place two parallel electrode plates in a closed container, use the generated electrons to excite neutral atoms or molecules, and the excited particles will release energy in the form of light when they fall back to the ground state from the excited state.
Discharge phase
Announce
edit
Glow discharge distribution area
glowThe discharge has two transition stages: sub normal glow and abnormal glowsectionConsisting ofcathodeStarting from the surface, the sequence is: ①Aston Dark area；②Cathode light layer; ③Cathode dark area（KruxDark area); ④Negative glow area; ⑤FaradayDark area; ⑥Positive column area; ⑦anodeDark area; ⑧Anodic light layer.The negative glow area, Faraday dark area and positive pillar area are the main areas.These light areas are the result of space ionization process and charge distribution, and are related to gas type, gas pressure, electrode material and other factors, which can be explained from the discharge theory.During glow discharge, more space charges with different signs gather near the two electrodes, thus forming obviouspotentialThe voltage drop is called cathode voltage drop and anode voltage drop respectively.The cathode voltage drop is the main component of the potential drop between electrodes. During normal glow discharge, the voltage between the two electrodes does not change with the current, which means it has the characteristics of voltage stabilization.
Simple glow discharge diagram
glowWhen dischargingBipolarelectric fieldUnder the action ofPositive ionRespectivelyanode、cathodeMovement and accumulation near the two polesSpace charge region。CationicDrift velocityIt is much smaller than the electron, so thecharge densityIt is much larger than the electron space charge area, so that the entire interelectrode voltage is almost all concentrated in the narrow area near the cathode.This is a prominent feature of glow discharge, and in normal glow discharge, the voltage between the two poles does not change with the current.
Near the cathode,Secondary electronThe electrons generated by emission have not been enough to make gas moleculesionizationOr excited kinetic energy, so the area next to the cathode is notluminescence。At the cathodePyrophoric region, the electron has obtained enough energycollisionThe gas molecule is ionized or excited to emit light.The formation of other dark and bright areas also depends mainly on the kinetic energy of electrons reaching the area and thepressure(Electronic and gas molecularInelastic collisionWill losekinetic energy）。
Development history
Announce
edit
1831-1835,M. FaradayStudyingLow atmospheric pressureFound during dischargeglowDischarge phenomenon and FaradayDark area。In 1858, J. Prucker discovered thatcathodeX-ray became the pioneer of particle radiation and atomic physics at the end of the 19th century.
Discharge study
Announce
edit
Atmospheric pressure glow discharge (APGD)
After nearly 20 years of development, low pressurelow temperature plasma Much progress has been made.However, its wide application is limited due to its operation needs vacuum pumping, large equipment investment, complex operation, and unsuitable for industrial continuous production.Glow discharge at low pressure can treat these materials, but there are some problems such as cost and treatment efficiency, which cannot be applied to the surface treatment of textiles on a large scale.For a long time, people have been trying to realize atmospheric pressure glow discharge (APGD).
In 1933, Von Engel of Germany reported the research results for the first time. Glow discharge was realized in atmospheric pressure hydrogen and air using cooled bare electrodes, but it was easy to transition to an arc, and must be ignited at low pressure, that is, it could not be separated from the vacuum system.In 1988, Kanazawa et al. reported the research results of obtaining stable APGD using helium at atmospheric pressure, and summarized three conditions for generating APGD through experiments: (1) the frequency of the excitation source should be above 1 kHz;(2) Dual media DBD is required;(3) Helium gas must be used.Since then, Okazaki in Japan, Massines in France and Roth in the United States have claimed to have achieved "APGD" at atmospheric pressure in some gases and gas mixtures by using the DBD method with power and media of different frequencies.In 1992, Roth team realized APGD in a 5mm helium gap, and claimed that APGD was also achieved in a few mm air gap. The main experimental conditions were that the humidity was lower than 15%, the gas flow rate was 50 l/min, the frequency was 3 kHz, and the power supply matched the load impedance.They believe that "ion capture" is the key to achieve APGD.Roth et al. explained APGD with the principle of ion capture, that is, when the frequency of the working voltage used is high enough to capture positive ions between the plates within half a cycle, but not high enough to capture electrons, space charges will be left in the gas gap, which will affect the discharge of the next half cycle, so that the required discharge field strength is significantly reduced, which is conducive to producing uniform APGD.They realized the "APGD" of Ar, He and air in a gas discharge plasma experimental device in the laboratory.In 1993, Okazaki team used metal wire mesh (wire diameter 0.035mm, 325 mesh) electrode as PET film (medium), 50Hz power supply（argon 、nitrogen、atmosphere）A lot of experiments have been done in the gap, and it is claimed that atmospheric pressure glow discharge has been realized.According to the number of current pulses and the difference of Lisajous figure (X axis is the applied voltage, Y axis is the discharge charge), they proposed a method to distinguish glow discharge and filament discharge, that is, if there is only one current pulse in each half cycle of applied voltage, and the Lisajous figure is two parallel oblique lines, it is glow discharge.If there are multiple current pulses in the half cycle and the Lisajous figure is obliqueparallelogram, it is filament discharge.The Massines Group in France, Radu Group in Canada and Golubovskii Group in Russia have also conducted relatively in-depth research on the formation mechanism of APGD.The Massines team carried out experimental research and numerical simulation on APGD of helium and nitrogen. In addition to measuring the applied voltage and discharge current, they took time-resolved discharge images with ICCD camera with exposure time of only 10ns, and recorded the emission spectra of discharge and other ionomers with time-space resolved spectrum measurement. Combined with one-dimensional numerical simulation of discharge process, they believed that,The uniform discharge in nitrogen still belongs to Townsend discharge, while the uniform discharge in helium is the real glow discharge or sub glow discharge.They also believe that the key to achieving uniform discharge at atmospheric pressure is to slowly develop a large number of electron avalanches under a lower electric field.Therefore, there must be a large number of seed electrons in the gap before the discharge starts, which can be provided by the long-life metastable state and its Penning ionization.According to the discharge image taken by ICCD with 10ns exposure, Radu's team found that glow discharge can be realized in the DBD gap of atmospheric pressure inert gases He, Ne, Ar and Krypton.
Since the end of last century, many domestic institutions, such as Corona Laboratory, Tsinghua UniversityDalian University of Technology、North China Electric Power University、Xi'an Jiaotong University、Huazhong University of Science and TechnologyInstitute of Physics, Chinese Academy of SciencesHebei Normal UniversityHe has started the research on APGD.Because APGD has an attractive industrial application prospect in textile, coating, environmental protection, film materials and other technologies, the realization of glow discharge to generate low-temperature plasma at atmospheric pressure and in air has always been the research focus and hotspot of scholars at home and abroad.In 2003, the National Natural Science Foundation of China listed "atmospheric pressure glow discharge" as a national key research project.Some progress has also been made in the research of APGD. For example, he, Ne, Ar, Krypton inert gases have basically achieved APGD at atmospheric pressure, and the air has also achieved quasi "APGD" that looks more uniform with the eyes.
The research results and understanding of APGD are different from each other.The research on APGD is in the ascendant and has been widely valued by many universities and research institutions at home and abroad.As there is no recognized standard for atmospheric pressure glow discharge, the discharge phenomenon seen in many experiments is very similar to glow discharge, that is, there is a uniform "fog" discharge in visual characteristics, but no filament discharge can be seen. However, there is no consensus and conclusion on whether this discharge phenomenon belongs to glow discharge.
Glow discharge at sub atmospheric pressure (HAPGD)
As the atmospheric pressure glow discharge technology has been reported but the technology is not yet mature, there is no equipment available for industrial production.The sub atmospheric pressure glow discharge technology has been mature and applied to industrial production.The sub atmospheric pressure glow discharge can process various materials with low cost, short processing time, high atmosphere content of various gases, high power density and high processing efficiency.It can be applied to surface polymerization, surface grafting, metal nitriding, metallurgy, surface catalysis, chemical synthesis, surface modification of various powder, particle and sheet materials and surface treatment of textiles.The visual characteristics of glow discharge at sub atmospheric pressure show uniform fog discharge;When discharging, the voltage at both ends of the electrode is lowpower density Large;Materials such as textiles and carbon fibers are treated at temperatures close to room temperature without breakdown and combustion.The sub atmospheric pressure glow discharge technology can be used for surface hydrophilic treatment, surface grafting, surface polymerization, metal nitriding, metallurgy, surface catalysis, chemical synthesis and other processes of low-temperature materials, biomaterials, and profiled materials.Due to the glow discharge at sub atmospheric pressure, the atmosphere concentration of the treatment environment is high, and the energy of electrons and ions can reach more than 10eV.The efficiency of material batch treatment is 10 times higher than that of low pressure glow discharge.Can handle metal, non-metal, (carbon) fiberMetal fibre, particles, powders, etc.
application area
Announce
edit
glowThe main application of discharge is to useLuminescence effect(e.gThe neon lights、Fluorescent lamp）And normal glow dischargeVoltage stabilizing effect(e.gNeon manostat）。 Using the characteristics of the positive column region of glow discharge to generate laserHe Ne laser。
In recent years, glow discharge has also been widely used in sewage treatment, sterilization, surface modification of polymer materials, ion source of analytical instruments, etc.[1]
Due to its characteristics, glow generation is applied toEmission spectrum analysis, used for gas analysis and hard to excite element analysisExciting light source。stayglass tubeConnect a flat electrode at both ends to chargeinert gasAdd several hundred volts DC voltage, and glow discharge will be generated in the tube, with the current of 10-4~10-2A.Discharge form and gas propertypressure, discharge tube size, electrode material, shape and distance.Its application in emission spectrum can be used to detect the concentration of lead, etc.
