1.Introduction

Since the beginning of the 21st century,atmospheric pressure plasma Engineering technology has gradually shown its great value in industrial productions.Because atmospheric pressure plasma is wildly used in many application fields such as surface treatment of materials,air purification,biomedicine and film deposition,atmospheric pressure plasma technology is honored as ‘a platform technology in the 21st century’[1–9].

So far,plasma industry has been used for more than 100 years.For example,the ozone generated by the discharge process is used to disinfect the water supply system,and the arc plasma is used to weld the plasma.The illumination and display of the plasma are carried out by the luminous characteristics of the plasma[10–13].

Generally,however,low temperature plasma is generated assisted by means of a vacuum system that makes the production process greatly complicated and increases the operating costs.To solve the problem of use of vacuum equipment,research interests have gradually shifted from the generation of low-pressure plasma to the generation of atmospheric pressure plasma over the past decades.

Atmospheric pressure glow discharge(APGD)plasma has advantages such as moderate electron density and high energy density[14].Therefore,it is more suitable for the surface modification of materials than other type of discharge at atmospheric pressure,and many methods have been explored to achieve APGD since 1933[15–25].In 1933,von Engle et al[15]generated normal glow discharges in air and in other gases at 1atm using a direct current(DC)power source or a radio frequency power source with assisted by vacuum equipment.Besides,Okazaki et al[16,17],Massines et al[18–20]and Roth et al[21–23]and other groups achieved APGD under some special conditions in argon or nitrogen,and some even in air,too.

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By controlling the discharge order,and based on the design concept of providing initial electrons from a lateral area to a weak electric field area,we designs the hamburger-electrode(as the structure formed by electrodes and the dielectric is like a hamburger,here we called it hamburger-electrode).

2.Methods of achieving APGD in air

The difficulty of APGD in air is the effective control of plasma density n produced by discharge.According to Townsend theory,the plasma density n produced by discharge is expressed as

where E is the applied electrical field in the discharge gap,n0is the initial electron density,dis the distance between discharge electrodes,pis the ambient gas pressure,and AandBare constants related to external factors such as gas type,pressure or the like.For atmospheric pressure air discharge,p,AandB are predetermined values[36].Therefore,the key parameters that determine the plasma density nareE,n0andd.

Generally,in order to obtain APGD in air,the distance dbetween discharge electrodes must be small enough(nanometer magnitude)to suppress the development of electron avalanche.However,to make the glow discharge plasma to be used more effectively,increasing d has been the goal for many researchers.Therefore,in order to achieve a larger spacing glow discharge,the electric field E between the electrodes and the initial electron density n0must be taken into account.That is,in the case where the electric field Eor the initial electron density n0is too small,the discharge phenomenon hardly occurs while in the case that both are too large,sharp filament discharge is formed must be small enough(nanometer magnitude)to suppress the development of electron avalanche.However,to make the glow discharge plasma to be used more effectively,increasing d has been the goal for many researchers.Therefore,in order to achieve a larger spacing glow discharge,the electric field E between the electrodes and the initial electron density n0must be taken into account.If that is,in the case where the electric field E or the initial electron density n0is too small,the discharge phenomenon would hardly not occurs;while in the case that if both are too large,a sharp filament discharge is formed.

According to(1),the plasma densityn is capable of effectively controlled by appropriate initial electron density n0 and low electric field E.Therefore,the method of dielectric barrier discharge(DBD)is generally used to suppress γ function during the discharge process in order to reduce the plasma density n and the absorbing electron capacity of electret materials(e.g.,polytetra fluoroethylene (PTFE))is used to provide initial electrons,so that the APGD plasma in air is formed in alternating current(AC)electric field.However,the initial electron density provided by electret materials is usually small.Accordingly,to increase the initial electron density n0at the initial stage of the discharge,pre-discharging or other methods are necessary means for forming APGD plasma in air.

It is generally considered that corona discharge is formed under the condition of non-uniform electric field,while glow discharge is formed under uniform electric field.However,for the atmospheric air,it is difficult to form a glow discharge with a large gap under uniform electric field condition because of higher breakdown electric field strength at atmospheric pressure.Therefore,it is very important to explore the possibility of forming glow discharge under the condition of DBD by constructing non-uniformly distributed electric field between appropriate electrodes.

3.Characteristics of DBD in non-uniform electric field

Discharge with a rod–rod contact electrode structure is typical DBD in non-uniform electric field.Here the characteristics of DBD in non-uniform electric field with using the rod–rod contact electrode structure are discussed.

3.1.Experimental conditions and discharge phenomenon

In the experiment,a power supply with high frequency and high voltage AC voltage made by our group is adopted whose schematic diagram is shown as in figure 1.A 220 V AC voltage is applied to the power supply through input terminal and is rectified to a DC voltage by a bridge circuit.After the ripple wave of the DC voltage is reduced by a filter capacitance,the DC voltage is chopped by full control devices MOSFET such that a low voltage and high frequency AC voltage is obtained.Then the AC voltage is prompted by high frequency transformer and the high frequency and high voltage AC voltage is obtained with frequency of 20kHz and maximum output voltage of 10 kV.The discharge voltage U is measured with a Tektronix P6015A high voltage probe and the discharge current I is measured by acquiring the voltage across a resistance R connected in the discharge circuit.The waveforms of discharge voltage and current are recorded with the Tektronix digital oscilloscope TDS1012B-SC.The photographs are taken by Nikon COOLPIX P100.The electric field　distributions of different electrode　structures are obtained by using Maxwell 3D software.

Figure 1.Schematic diagram of power supply with high frequency and high voltage.

Figure 2.Rod–rod contact electrode structure.

Figure 2 shows the rod–rod contact electrode structure that includes a rod-shape high-voltage electrode and a rod shape grounding electrode contacted with each other in parallel,and both electrodes consist of a steel rod with an outer diameter of 8mm and a PTFE insulating layer with a thickness of 0.1 mm.

Figure 3(a)shows a simulation result for the electric field distribution in the electrode slit of one side in the case that a voltage of 3.12 kV is applied,and figure 3(b)shows analysis with respect to discharge development process.It is understood from figure 3 that the electric field intensity in a longitudinal direction of the air gap is almost constant.However,around the contact position of the high-voltage electrode and the grounding electrode,that is X = 0,there is the maximum electric field strength.The electric field strength in air gap decreases gradually with the increase of transverse distance X away from the contact position forming a one-dimensional transverse non-uniform electric field distribution.The non uniformity field determines that the discharge is firstly generated at the position having the maximum field strength when a certain voltage has been applied.Meanwhile,initial electrons are provided from stronger electric field discharge area to adjacent weaker electric field areas by diffusion effect such that the discharge is carried out from the strong electric field area to the weak electric field area in turn.Due to the presence of a large amount of initial electrons,the discharge is capable of generated at lower electric field strength,thereby effectively suppressing the development scale of the electron collapse.As a result,the possibility of occurrence of filament discharge is reduced.

When a voltage of 3.12 kV is applied between the two electrodes,the discharge phenomenon in the slit of the rod–rod contact electrode structure is shown as in figure 4(a).A glow band with a width of about 1.5 mm is formed between the slit of the two electrodes while there occurs no bright streamer.Therefore,stable APGD plasma in air is generated by the rod–rod contact electrode structure.

A rod–rod non-contact electrode structure is formed where the gap between the two electrodes is fixed at 0.5mm.Similarly,when the applied voltage is 3.12 kV,the discharge phenomenon　between　the electrodes is shown　a sin figure 4(b).The discharge is in a diffuse state and the light intensity of the area around the surface of the electrodes is much larger than the light intensity of the middle area of the air gap,therefore,the discharge belongs to typical anode glow discharge phenomenon.

After the switch is closed,according to KVL and VCR of the energy storage device,full response state equations of the circuit are expressed as:

Anode glow discharge,which is the phenomenon of typical corona glow discharge,is capable of being achieved either by non-uniform electric field such as shown in figure 4(b)or by uniform electric field.Because electron avalanche is developed from a cathode to an anode under the action of the electric field,in the discharge process,and the scale of the electron avalanche exponentially increases in the development process,the electron density near the anode is much larger than other regions.The increase in electron density leads to an increase in the number of photons due to the energy level transition,thereby increasing the luminous intensity near the anode.Moreover,the two electrodes alternately become as an anode or as a cathode under the action of AC power,so that the anode glow discharge is formed near both of the electrodes.The development distance of the electron avalanche increases as the discharge gap between two electrodes is further increased which results in an exponential increase in the plasma density in the air gap.Under the action of the magnetic field formed by the conduction current,the plasma channel shrinks and forms filament discharge.

To achieve glow discharge in a long air gap,an ideal way is to provide initial electrons from the lateral area of the discharge gap as shown in figure 3(b)so that electron avalanche is capable of directly being generated and developing at a position away from the surface of cathode to indirectly shorten the development distance of the electron avalanche.In addition,since the discharge occurs in the electric field with low field strength,the development rate and the scale of electron avalanche are reduced thereby the plasma density in the entire discharge channel being kept at a low level.Accordingly,the emergence of filament discharge is avoided due to reduction of the possibility of plasma contraction.

Figure 3.Discharge physical process in slit of rod–rod contact electrode structure.(a)Electric field distribution,(b)discharge development process.

Figure 4.Discharge phenomena of(a)rod–rod contact electrode structure and(b)rod–rod non-contact electrode structure(exposure time:0.5 s).

On the other hand,generally,the development rate of electron avalanche is large because the electric field with high field strength is required for discharge under atmospheric air conditions,and the discharge gap is not capable of being large,so that the development time of the electron avalanche is short,which lead to the absence of sufficient fusion between the different electron avalanches.Therefore,the distribution of each electron avalanche in the discharge space is independent and exhibits uneven distribution on the spatial scale.Moreover,because the development degree and the distribution of the electron avalanche are not exactly the same at different discharge times in the discharge process,the distribution at the time scale is not uniform.Therefore,it is more appropriate to characterize APGD with diffuse rather than uniformity.

Figure 5.Waveforms of discharge voltage and discharge current of the rod–rod contact electrode structure.

3.2.Electrical characteristics of discharge

The voltage current waveform in the discharge process of the rod–rod contact electrode structure is shown in figure 5 where discharge current consists of conduction currenticand displacement currentid.There are multiple current pulses in every half cycle of voltage,and each current pulse is oscillating with an oscillation period of about 100 ns.The maximum instantaneous pulse discharge current Imis 60 mA,which is at an mA level.

Figure 6.Equivalent circuit of discharge process.

In the process of DBD using an AC power supply,when the voltage applied between the electrodes reaches a certain value such that the field strength of the external applied electric field Eain the discharge space reaches the breakdown field strength,discharge is generated.The discharge allows charged particles in the discharge space do directional movements under the effect of the electric field so that electrons accumulate on the surface of the insulating material on the side of the instantaneous anode,thereby the accumulation forming electric fieldEchargein the discharge space having a field direction opposite to the external applied electric field Ea.The discharge terminates when the difference between the strengths of the electric filedEchargeand the external applied electric fieldEais smaller than the breakdown field strength.Before the applied voltage reaches to the maximum,the field strength of the external applied electric fieldEacontinues to increase after the discharge terminated.Therefore,when the difference between the field strengths of the electric fieldEa and the electric fieldEchargeis larger than the breakdown field strength again,the discharge occurs again in the discharge space.The discharge phenomenon occurs many times in every half cycle of voltage.Accordingly,the current waveform is measured with many short-term pulse current pulses as shown in figure 5.

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The discharge process of the rod–rod contact electrode structure is equivalent to a circuit as shown in figure 6 where the discharge process is simulated through closing and breaking the branch switch in the equivalent circuit.Closing the branch switch simulates the case that discharge occurs between electrodes and breaking the branch switch simulates the case that non-discharge occurs between the electrodes.In figure 6,AC stands for AC voltage power supply,L and R respectively represent the inductance and resistance of the power supply,Cdis a dielectric capacitance,R1is an equivalent resistance of the discharge path,andCgis a gap capacitor.

市政工程项目一般都是具有特殊的工程特点，项目的预算资金通常都是提前垫付，然后使得工程的运作非常顺利，那么就需要对工程造价全过程进行合理地管理，使得流动资金可以被科学合理地运用，那么这就需要对工程的款项及时结算，这有利于工程项目的资金回笼和流动，加速资金周转，降低企业的运营成本。

针对下一代交换组网的相关研究以及发展下一代交换组网过程中对技术的应用是当前我国电信运营商重点研究的课题，也是电信运营领域高度重视的内容，为了推动电信事业的发展，就要针对软交换以及IMS技术进行系统的分析，在对比研究中形成深刻的认识。

whereuacis the voltage of the AC voltage power supply,icis the conduction current,udis the voltage of the dielectric capacitanceCdandugis the voltage of the dielectric capacitanceCg.Thus,the oscillation period T of the conduction currenticis conducted as

where

丰富多彩的课堂，能够帮助教师在课堂教授课本知识的同时建立师生之间良好的互动关系。课堂活动的创设要注意灵活性、针对性和适宜性的特点，课堂活动作为一种创新的教学方法，能够活跃课堂的教学氛围，提高每个学生参与的积极性，使得语文教学更加高效和有针对性。课堂活动重在把握活动的针对性和目的性，科学的构建方法的采用和新颖活动内容的纳入会对学生产生更大的吸引力，学生参与活动的积极性提高了，才会积极主动地参与其中。

From the above equations,it is understood that the current waveform is directly related to the circuit parameters of the power supply and the shape of the electrode structure and other parameters.The changes of the inductanceL,the capacitancesCdandCg,and the resistanceRin the equivalent circuit affect the conduction current amplitude,time width and oscillation cycle.Therefore,the amplitude of the current pulse exhibited by the oscilloscope is not capable of exactly showing the current density of the discharge.Meanwhile,the pulse width is not capable of exactly representing the time of discharge.In other words,the current waveform exhibited by the oscilloscope does not exactly reflect the generation process of charged particles between the electrodes.The discharge current is capable of only being used as a reference for the studies of APGD in air and is not capable of being used as a discriminant parameter.The discharge process between electrodes can be changed by changing the parameters of the electrode and the power supply.

Multiple rod electrodes form a composite multi-rod contact electrode structure.The equivalent circuit of the composite multi-rod contact electrode structure is shown as in figure 7 where the composite multi-rod contact electrode structure is simulated by parallel multi-group structure of the branch circuit with the electrode parameters shown in figure 6.Since the time of each pulses in one discharge process between electrodes is very short,the discharge branch circuit of which the switch is closed is regarded as a current source whose excitation is(q is the charge of charged particles generated by discharge,t is time)shown in the dashed frame of figure 7.During discharge,the measured conduction currenticis caused byigcurrent source.

两个研究区域分别位于南京市的江宁区和栖霞区。仙林大学城位于南京市栖霞区西部(118°52′～118°59′ E、32°4′～32°8′ N)，紫金山东麓，该区域突出生态环境和人文内涵，其规划体现人与自然的和谐统一的理念，人与自然和谐统一的生态型大学城是该区域建设的重要目标。牛首山区域位于江苏省南京市江宁区(118°38′～118°48′E，31°45′～31°52′N)，该区域包含银杏湖、牛首山、大金山、蒋门山，生物种类丰富，生态系统保护较好，而且该区域位于南京市江宁区美丽乡村核心区域，是城市化发展与自然保护矛盾较为尖锐的区域，选择这两个区域作为研究对象对生态体育城市构建具有重要的参考意义。

Figure 7.The equivalent circuit of out-of-synchronization of discharge.

Figure 8.(a)Discharge phenomena(exposure time:0.5 s)and(b)waveforms of discharge voltage and current of a filament discharge.

In an ideal state,the electric field strengths at the same position of the multiple electrodes are same as each other and discharge processes occur at the same time.In practice,the discharge processes are not synchronized due to error of the mechanical processing and the placement position of the electrode structure.The discharge nonsynchronous is simulated by nonsynchronous of closing the branch switches of branch circuits in figure 7.That is,the discharge branch is simulated by the current source branch,while the undischarged branch is simulated by the branch circuit ofCdand Cg.Figure 7 shows that there is only one branch discharge,and the others do not discharge.

The conduction currenticin the main circuit is expressed as:

For this kind of electrode structure,the APGD in air is achieved in an air gap with a maximum distance of 8mm,which plays an important role in material treatment,etc.

In the case where a single branch discharges,the more undischarged branches,in the equivalent diagram shown in figure 7,the larger the number of parallel branches,the smaller the conduction currenticin the main circuit,and the oscillation period T increases.On the other hand,the more discharge occurs between the multiple electrodes,the more discharge branches in the equivalent circuit,thus the larger conduct currenticis measured.Moreover,the more discharge occurs between the multiple electrodes,the larger the discharge area in the composite multi-rod contact electrode structure.Therefore,the magnitude of the discharge area also affects the amplitude of the measured conduction current and the period of the oscillation.

When the air gap of the rod–rod contact electrode structure increases to 1.1mm and the applied voltage is 4.0 kV,the discharge phenomenon in the air gap and the corresponding waveforms of discharge voltage and current are shown in figure 8.

It is seen that there are obvious filaments in the discharge area,and the discharge currents have multiple discharge pulses in every half cycle of voltage,and the maximum amplitude of pulse current reaches 380mA.In the field of APGD,there is a perception that the discharge with only one current peak in each half cycle of the discharge voltage is the glow discharge and the discharge with multiple peaks current waveform in each half cycle of voltage is a filament discharge.This statement is not completely correct.The multi-pulse current waveform is due to the discontinuity of discharge which is caused by electric field which is opposite to the applied electric field and formed by the charged particle on the dielectric layer.The filaments in a filament discharge refers to the passage of high density plasma between the electrodes,which presents as bright filaments.One is the peak in the time scale,the other is the peak in the spatial scale,and there is no necessary link between them.As the description above,during the discharge process in the atmospheric air,the discharge channel is formed in a very short time,and a multiple discharge process is formed when the discharge voltage is high or the discharge gap is not uniform.Therefore,it can be considered that if the pulse peak current waveform in the discharge is very small,there is no very high density plasma channel and bright filament in the discharge space,and it shows the characteristics of diffuse,it can be considered as glow discharge.

4.Realization of APGD in air based on design of electric field

The electric field distribution in the discharge space plays an important role in the development of the spatial electron avalanches.Both the changes in transverse electric field intensity among different discharge channels and the variations in longitudinal electric field intensity along the same discharge channel have decisive effects on the number of the head electrons in the development of electron avalanches.And then they relate to the contraction or dispersion of the discharge channels,i.e.,the formation or suppression of the filamentary discharge.Based on the design of electric field,the field distribution in discharge space is changed by proposing reasonable electrode structures.In our previous studies,the　discharge　character is tics of the　point-contact electrode structure and the carbon fiber helical-contact electrode structure was respectively investigated,and an effective method of achieving atmospheric air glow discharge was obtained via the controlling of the electric field[27,28].

4.1.Point-contact electrode structure

The point-contact electrode structure is able to generate an electric field　distribution　with　two-dimensional spatial change,which is more conducive to the realization of APGD in the air.The electrode structure is shown in figure 9.A pair of electrodes with the same size is arranged in a vertical cross contact.The outer diameter of the electrodes is 1mm and the barrier dielectric is PTFE with a thickness of 0.25mm.

The spatial distribution of the electric field for the cross section of the two electrodes and the plane between the two electrodes at the voltage of 5kV is shown in figure 10.Local strong electric field is observed in the nanometer-scale gap,and the maximal strength of the electric field is 2.20 ×107V m−1.However,the small gap shortens the development of the electron avalanches,thus inhibiting the formation of filament discharge.The gap in the two-dimensional direction increases from inside to outside the contact point,but the field strength decreases gradually,which ensures that the development of electron avalanches will not be too fierce.Meanwhile,the gradient change of the electric field strength from inside to outside provides favorable conditions for the initial electrons in the nanometer-scale gap diffuse to surrounding space,resulting in a point-glow discharge with a lager region at a lower voltage.

Figure 9.The structure diagram of the point-contact electrode.

The discharge phenomenon of a point-glow is shown in figure 11(a).A soft discharge for this electrode structure occurs at the contact point in three-dimensional space with a good uniformity of produced plasma.In order to generate the glow discharge plasma in a larger area,the point-contact electrode structure is extended to obtain a mesh-like electrode stricture and form a discharge phenomenon shown as figure 11(b).In figure 11(b),the mesh size is 10 mm ×10 mm with 121 contact points.It can be widely applied to the air purification or tail gas treatment after combination of this electrode structure,with the characteristics of stable discharge and strong adaptability to the environment.

4.2.Carbon fiber helical-contact electrode structure

On the basis of the point-contact electrode structure,a new helical-contact electrode structure is proposed.It is characterized by the formation of the electric field distribution with three-dimensional spatial variations.

By winding the insulated electrode(a metal wire covered with an insulating layer)with a cluster of carbon fibers at a certain angle and bringing them in close contact to each other,a helical-contact electrode structure is formed,as shown in figure 12.

The carbon fiber is taken as a semiconducting material with a smaller curvature radius and a lower threshold electric field required for field emission.By using the carbon fiber electrodes in the asymmetrical single DBD,the number of second electrons can be reduced effectively.At the same time,the possibility of field emission is enhanced,thus causing the reduction of the initial discharge voltage and the formation of the discharge phenomenon in lower average electric field strength.

Figure 10.The electric field distribution of the point-contact electrode for(a)cross section of the two electrodes and(b)plane between the two electrodes.

Figure 11.Discharge phenomenon of(a)a point and(b)a mesh(exposure time:1 and 0.125 s).

Figure 12.The structure diagram of the carbon fiber helical-contact electrode.

The electric field distribution of the helical-contact electrode structure is shown in figure 13(a).The diameter of the metal wire,thickness of the dielectric,and diameter of the carbon fiber electrode are all set to 0.2mm(to conduct a better simulation study,the size of carbon fiber filaments is increased appropriately and the carbon fiber bundle is taken as an integrated whole),and the coil pitch of carbon fiber is 5mm.When the applied voltage is 1.8 kV,the whole electrode is wrapped by a strong electric field region distributed as a wavy pro file.The wrapping caused by the electric field distribution ensures that the charged particles are limited to a certain space.Figure 13(b)shows a partial distribution of the electric field vectors for the helical-contact electrode structure.It is observed that the fiber electrode is surrounded by the curved electric field vector lines.High electric field formed by this structure not only exists between the two discharge electrodes,but also emerges around the fiber electrode,even above of it.The charged particles generated in the vicinity of the contact points,under the effect of the outward expanding electric field vectors,will drift to the periphery where the electric field is relatively weak,avoiding the formation of filament discharge caused by excessive concentration of electrons near the contact points and achieving a glow discharge phenomenon surrounding the entire electrode.

After grasping relevant knowledge and skills,the students should beencouraged and guided togointothefield they arenot familiar with before.

“民主国家的公民须乐于以妥协方法解决他们的分歧。民主的所有条件中这是最重要的，因为没有妥协就没有民主。”[14]协同治理中的妥协理性作为调节参与各方利益冲突的重要保障，贯穿于整个社会管理的实践中，保证了协同治理的顺利实施。以妥协理性的方式调节利益冲突，是在理解和尊重人们各自的利益需求前提下，使冲突各方在一定的制度框架内，通过相互信任、信息沟通方式，实现利益整合，以平息冲突。

When the discharge power density F is 12.05 W cm−3,the variation law of chemical composition on AF surface with treatment time is shown in table 1.It is found that when t is 10s,C content on the AF surface decreased obviously,whereas the content of O and N increased from 20.26%and 2.21%to 29.66%and 4.06%,respectively.The results indicate that the high active plasma introduces not only a large number of oxygen functional groups,but also nitrogen functional groups which is not be introduced through traditional air plasma.And the amount and rate of active groups introduced on materials’surface are also significantly high.This is because the maximum electric field strength near the contact position between the SCE and the treated material can reach 1.4 × 107Vm−1under the premise of uniform discharge,which makes the energy of the active particles in the APGD plasma much higher than that in traditional DBD and effectively improves the modification effect and Efficiency of the plasma.

建立实例库，需要将已有几何参数、力学性能等完善的知识融入到施工升降机导轨架零件中生成实例模型。这些模型主要由Dfa文件的实例化建立，少数复杂模型可直接建立零件模型。在NX/KF中Dfa文件的获取方法有两种，一种是利用NX/KF语言直接编辑而成，比如施工升降机标准节的一些零件如主弦管、斜腹杆、螺栓等零件的建立。另一种方法是几何采用机制，利用已建好的导轨架零件模型使用知识融合模块中的Adoption来反求零件模型的知识，然后将零件模型转化为KF类，用户可以在KF导航器中添加数学公式、产生式规则、外部数据库等，进一步修改使其符合KF语法规则，然后保存为新的Dfa文件。

Figure 13.The electric field distribution of the carbon fiber helical-contact electrode.(a)Distribution of the spatial electric field.(b)Distribution of the electric field vectors.

Figure 14.Discharge phenomenon of the carbon fiber helical-contact electrode under a voltage of 1.8 kV(exposure time:0.083 s).

Figure 15.Waveforms of discharge voltage and current.

Figure 14 illustrates the glow discharge phenomenon of the carbon fiber helical-contact electrode in atmospheric pressure air.When the amplitude of applied voltage was 960 V,the luminescence first appeared in the vicinity of the carbon fiber electrode.When the applied voltage was up to 1.8kV,a three-dimensional and dispersion glow discharge emerged from the surface of the entire electrode,and was light blue in color.The glow diffused along the radial direction of the wire electrode and completely wrapped it with almost no dark areas,and the carbon fiber was also completely buried in the plasma.

Figure 15 shows the current–voltage waveform of steady discharge.It is seen that the currents corresponding to the positive and negative half cycles are evidently unequal.In the positive half periods of the voltage,the high-voltage electrode is the anode and the carbon fiber electrode is the cathode because the carbon fiber is grounded.During the discharge,the PTFE dielectric on the high-voltage electrode adsorbs electrons and inhibits the development of electron avalanches from the carbon fiber electrode.In contrast,the carbon fiber electrode takes in electrons and ensures that the electron avalanches generated from the insulated electrode are not too fierce near the anode,restraining the filament discharge.

It is seen that through the design of electric field,under the conditions of low voltage,forming an atmospheric air glow discharge in an open space is achieved.Moreover,this electrode structure effectively reduces the generation of ozone,and is very suitable for air purification and other applications.

5.Realization of APGD in air based on controlling initial electrons

When the applied voltage is 6.4kV,the discharge begins,and when the voltage is 6.6kV,the phenomenon of anode glow is formed in the surface of the dielectric,as shown in figure 20(a).When the applied voltage increases to 7.2 kV,the whole air gap is full of the diffuse glow,as shown in figure 20(b).Further,this electrode structure achieves not only APGD in air in the 3.5 mm discharge gap,but also stable nitrogen APGDPJs with a diameter of 3.5 mm.

5.1.Line-plane electrode with a floating-voltage electrode structure

According to the phenomenon of electromagnetic induction,the floating conductor in the electric field will be induced to another voltage.If there is a certain voltage difference between a high-voltage electrode and a floating-voltage electrode　near　the　high-voltage　electrode,　discharge phenomenon can be produced between them.Using this principle,the line-plane electrode with the floating-voltage electrode shown in figure 16(a)is designed.The purpose is to provide initial electrons from one polarity of the two discharge electrodes,so that the discharge phenomenon are able to be generated in a weak electric field.

The outer diameter of the high-voltage electrode is 0.3 mm,and the thickness of the dielectric layer is 0.065mm.The outer diameter of the floating-voltage electrode is 1mm,and the thickness of the dielectric layer is 0.2 mm.The thickness of the barrier dielectric is 0.3 mm,and the grounding electrode is 0.2mm thick copper.All the dielectric materials are made of PTFE.The high-voltage electrode and the floating-voltage electrode must be close to each other,and their axes must be parallel to both the barrier dielectric and the grounding electrode.The air gap between the upper surface of the high-voltage electrode and the lower surface of the barrier dielectric is 2–8mm.

Figure 16.(a)Structure diagram and(b)distribution of the spatial electric field of the line-plane electrode with a floating-voltage electrode.

Figure 17.Discharge phenomenon of the ling-plane electrode with a floating-voltage electrode(exposure time:0.1 s).

When the high-voltage electrode is applied a voltage of 4.2 kV,the spatial electric field distribution of the electrode structure is shown in figure 16(b).It is seen that there is a strong electric field area between the high-voltage electrode and the floating-voltage electrode.The electric field strength between the high-voltage electrode and the barrier dielectric is relatively weak,and its value is about1.5　×　106V m−1.Since the electric field between the high-voltage electrode and the floating-voltage electrode is strong,the discharge phenomenon is generated firstly.The electrons produced by the discharge provide a large amount of the initial electrons for the large gap between the high-voltage electrode and the barrier dielectric,so that the air gap of the electric field strength of about1.5　×　106Vm−1is also able to generate discharge.The discharge phenomenon at a voltage of 4.2 kV is shown in figure 17.

It is seen from figure 17 that the plasma density around the high-voltage electrode at the bottom of the figure and the barrier dielectric at the top of the figure is larger than that of other areas,and the phenomenon of these larger plasma density areas is purple.While since the electric field strength in the middle areas of the air gap is low,the degree of ionization is low and the phenomenon is blue.The whole discharge phenomenon in the air gap is in a diffuse state.The discharge between the high-voltage electrode and floating voltage electrode generate a large number of plasmas,forming a strong luminescence phenomenon.While according to the section 3.1,the phenomenon of anode glow is formed near the surface of the barrier dielectric.Since the electric field strength in the middle the middle areas of the air gap is low,the development velocity of the electron avalanche is slow,and the collision ionization produced by the electrons coming from the discharge between the high-voltage electrode and the floating-voltage electrode is very small,thereby the filament discharge do not occur in this area.

Figure 18.Waveforms of discharge voltage and current of line-plane electrode with a floating-voltage electrode.

The waveforms of discharge voltage and current shown in figure 18 shows that the amplitude of current pulse is only in a mA level.And the amplitude of current pulse in the negative half cycle of voltage is slightly larger than that in the positive half cycle of voltage.According to the characters of this electrode structure,when the applied voltage is in the negative half cycle,the instantaneous cathode occurs in the high-voltage electrode.In turn,for the positive half cycle of voltage,the grounding electrode is the instantaneous cathode.Therefore,it is inferred that the number of initial electrons provided by discharge between the high-voltage electrode and the floating-voltage electrode is larger than that of the barrier dielectric.That is to say,the high-voltage electrode and the floating-voltage electrode are the key factors for the generation of diffuse discharge.If there is no floating-voltage electrode,the discharge voltage and the amplitude of current pulse increase,the phenomenon appears an intense filament discharge.

where U is the voltage of the inductance L and the resistor R in the main circuit,andCiis the total capacitance for each undischarged branch.

5.2.Hamburger-electrode structure

Figure 19.Hamburger-electrode structure.(a)Structure diagram.(b)Distribution of the spatial electric field.

Compared with in other gases,APGD plasma in air has higher application value.However,the APGD in air easily converts to the mode of filament discharge.Currently,an effective way of generating APGD plasma in air is adopting a nanosecond pulse power to shorten the duration of the electric field on discharge process[26–29].However,nanosecond pulse power supply has the disadvantages of high cost and large loss,and APGD is not capable of being achieved in air between a large-distance gap and an open space.In the present paper,based on our previous research results and published papers[30–35],the methods of achieving APGD in air without using the nanosecond pulse power are analyzed systematically.The aim is to provide references for the researchers of industrial applications of plasma.

As shown in figure 19(a),the hamburger-electrode mainly consists of a gas gap,two outer electrodes,an inner electrode,and a dielectric.The inner electrode is made of copper wires(with an outer diameter of 0.9 mm)with a PTFE thickness of 0.15 mm.The gas gap is 3.5 mm in diameter and 2mm long.The outer electrode comprise of a copper sheet(4mm × 1mm × 0.2 mm).The main structure characteristic of the hamburger-electrode is that the dielectric with a nonuniform thickness is located between the inner electrode and the outer electrodes.That is to say,it adopts the mode of nonuniform DBD to form a gas gap with a non-uniform length.The cylindrical conductor,with a smooth insulated layer,is chosen as the inner electrode,as it not only enlarges the discharge space,but also makes the surface of the inner electrode form a strong electric field.During discharging,the inner electrodes are connected to the high-voltage terminal of the discharge power and the outer electrodes are connected to the ground.

When the applied voltage of the inner electrode is 6.4kV,the electric field distribution in the center of the cross section of hamburger-electrode is shown in figure 19(b).It can be seen that the strength of electric field is a function of the discharge length,and the high strength is in the short gap and low strength is in the long gap.Discharge will be firstly generated in the short gap,where are at both borders of the inner electrode and the dielectric.When the discharge is generated in the short gap,the charged particle density in the short gap is higher than that in the long gap near the short gap.Thus,the charged particles will spread to the long gap with a lower electric field strength,and the discharge phenomenon is generated in the long gap.So the discharge will occur from the shorter gap to the longer gap in order,and eventually the whole air gap will be full of diffuse plasma.Additionally,we can know from the(1)that when the distance of discharged increases and the electric fieldEdecreases,the plasma density nis possible to be the same.Therefore,the hamburgerelectrode which provides initial electrons from the lateral area to the weak electric field area is bene ficial for generating the diffuse discharge phenomenon.

In order to generate discharge phenomenon in an electric field with low strength,it is necessary to provide a sufficient number of initial electrons for the weak electric field areas.Based on this,in the previous study,two electrode structure are proposed,which are able to provide the initial electrons from one polarity of the two discharge electrodes and from the lateral area of discharge gaps,respectively[29,30].

6.Application of APGD plasma in surface modification of materials

In the process of modification,polymers always participate in the generation of plasma as a part of the electrode structure.The surface properties of treated materials(such as morphology,structure defects,etc)will have an important influence on APGD.In addition,the modification effect is closely related to plasma parameters(such as plasma density,active particle energy,etc).Here,the surface modification of polymers such as aramid fiber(AF)and polyester are studied[31,32].

In order to generate APGD plasma on polymers’surface,a staggered contact electrode(SCE)structure shown as figure 21(a)was proposed.Each electrode in the SCE structure is made of stainless steel tube with an inner diameter of 6mm and an outside diameter of 8mm and PTFE insulation with a thickness of 0.2 mm.In the process of modification,treated material is placed between high-voltage electrode and two grounding electrodes and closely contacts with the surface of electrodes.When a certain voltage is applied,the electrodes will be rotated with the transmission of treated materials while the APGD plasma is generated on the surface of polymers,which accomplishes the continuous treatment of materials.

就以观赏石的分类而论，古典观赏石的分类因受制于科学技术和生产力条件的约束，就只有灵壁石、太湖石、昆石、英石四大类型为正宗，延续了上千年。

When discharge voltage is 3.6 kV,the discharge phenomena on one side of the slot regions between the electrode and the surface of AF is shown in figure 21(b).It can be seen that due to the fact that AF’s surface is not absolutely smooth,there are some dark regions in the slot.But the discharge is diffuse.The APGD plasma can be generated with AF as a dielectric barrier material.

Figure 20.Discharge phenomenon of the hamburger-electrode at different voltage(exposure time:2 s).

Figure 21.The treatment progress of matericals(exposure time:0.77 s).(a)Electrode structure.(b)Discharge phenomenon.

Table 1.Elements concentration on AF surface.

Chemical composition(at.%)Samples　C　O　N(1)Untreated　77.54　20.26　2.21(2)Treated for 5 s,12.05 W cm−3　73.02　23.06　3.92(3)Treated for 10 s,12.05 W cm−3　66.28　30.19　3.53

On the other hand,the special electric field distribution formed by the fiber electrode via the bending of electric field lines and formation of the non-uniform electric field with a longitudinal gradient change effectively suppresses the electron development in the process of impact ionization and is conducive to achieve the slow growth of electron avalanche in a single discharge channel,which provides the conditions for the glow discharge.

As shown in figure 22(a),the surface of polyester canvas is uneven,which longitude lines are convex and latitude lines are concave.Latitude lines are spaced in parallel and at equal distance.Longitude lines shuttle up and down between latitude lines and the arrangement of adjacent longitude lines is staggered.The longitudinal section of one braided unit chosen randomly on the canvas is shown in figure 22(b).It is found that the micro air gaps A and B can be formed by the polyester canvas and the electrode.The discharge pattern in micro air gaps depends on the electric field distribution between the electrodes.When the applied voltage is 5.6 kV,the electric field simulation results in micro air gaps is shown in figure 22(c).

Figure 22.Morphology characteristics and electric field distribution.(a)Morphology characteristics of polyester canvas.(b)Schematic diagram of structure.(c)Electric field distribution.

Figure 23.(a)Schematic diagram of structure and(b)discharge phenomena(exposure time:0.77 s).

It is found the electric field distribution in micro air gap A is symmetrical to gap B and there is the maximum electric field intensity of 1.2 × 107V m−1when X = 0 where the electrode is in contact with the polyester longitude line.The electric field distribution with intensive electric field in short gap while weak electric field in long gap is formed in micro air gap.The mutual restriction betweenEgandLginhibits the development of electron avalanche effectively and reduces the possibility of the transformation to the filament discharge,which is bene ficial to the generation of APGD plasma.

In order to analyze the discharge characteristics in micro air gaps,the metal part of upper electrode was replaced with a transparent conductive film ITO and the PTFE dielectric material for DBD was retained.As shown in figure 23(a),the discharge phenomenon was observed through the ITO film and PTFE material.When the applied voltage is 5.6 kV,the discharge phenomena is shown in figure 23(b).

从事生产、销售、进口、服务的单位和个人应当严格执行强制性标准的各项规定。不符合强制性标准的产品和服务禁止生产、销售、进口和提供。产品的生产者、销售者、进口商以及服务的提供者要有强制性标准意识。违反强制性标准的，将依法承担相应的法律责任。”[1]

The dark areas correspond to the contact regions between electrodes and longitude lines of polyester canvas,the bright areas correspond to the discharge phenomenon in micro air gaps.It can be seen clearly that the discharge in micro air gaps is diffused,the luminescence is nearly uniform,and there is no bright spot corresponding to the filament discharge,which reflects that the formation of quasi APGD has been achieved.The results indicate that the SCE can also be applied to the treatment of materials with non-smooth surface.

Table 2.Peeling strength on the surface of polyester canvas.

Treatment time　Peeling strength value Untreated　3.5 N mm−1 14 s　4.8 N mm−1 42 s　7.3 N mm−1

In order to analyze the modification effect of high activity APGD plasma on polyester canvas,the peel strength between treated polyester canvas and rubbers was obtained by optical tracking tensile testing machine.The peel angle is 180°,the size specification for adhesive bonding of test specimens is 200 × 25 mm2,the separation speed of collet is 100 mm min−1,and the stripping length is 125mm.The peel strength value of each sample is selected as the average value in stripping process.The detection results are shown in table 2.

3）计算由2）得到的一致集点数与集合元素总数之比，判断其是否大于指定阈值，若是，则此时一致集中的点为瞳孔的内点，其余点则为干扰点，否则，回到（1），直到找到满足条件的一致集；

It is found the treated polyester canvas has a Significant change compared to the untreated one,which means the bonding property of polyester canvas surface is improved.And the peel strength value will rise with the increase of treatment time.The peeling strength value of treated polyester canvas　increases　more　than　doubled　compared　with untreated ones.

7.Conclusion

In this paper,the means of generating APGD plasma in air is discussed,and the characteristics of DBD in non-uniform electric field are studied.The characteristics of APGD in air generated by the 4 kinds of electrode structures based on design of non-uniform electric field and the idea of controlling initial electrons are further analyzed.Finally,the application of contact electrode structure in the surface modification of polymer materials is introduced.The specific conclusions are as follows:

齐海峰也从一个小工人，一步步混到了厂办主任。官职升了，脾气还是没见长。妻管严就妻管严吧，半辈子不都吵过来了。

(1)The effective control of the three parameters of the electric field,the initial electron density and the discharge distance between the electrodes is the key to achieving APGD in air.Under the condition of not using nanosecond pulsed power supply,the APGD plasma in air is formed by adopting the DBD mode,using the absorbing electron capacity of electret materials to provide initial electrons and to end the discharge progress,and under the action of AC electric field.

(2)Non-uniform electric field DBD is an effective mean to achieve APGD in air.Discharge begins from the strong electric field area,gradually extends to the weak electric field area to avoid the occurrence of filament discharge,and eventually the entire discharge space are filled with diffuse glow.The waveform of discharge voltage and current are closely related to the discharge circuit parameters,and there is no necessary link between the if laments in the filament discharge and pulse current in current waveform.Under the condition of atmospheric air,the anode glow phenomenon is easier to form,and using the diffuse discharge to characterize the characteristics of APGD is more accurate.

(3)By analyzing the discharge characteristics of the pointcontact electrode structure and the carbon fiber helical contact electrode structure,it is found that the excitation of the strong electric field area is a necessary condition for the glow discharge in the whole area;through the design of electric field to form a two-dimensional and three dimensional space electric field,the development of electron avalanches in air-gap is suppressed effectively and a large space of APGD plasma in air are generated.Through the combination of electrode structures,a large areas of APGD plasma in air are generated.

(4)By analyzing the discharge characteristics of line-plane electrode with a floating-voltage electrode and hamburger electrode,it is found that the drift motion and the diffusion effect of initial electrons in the electric field are the key to generate the discharge at the low electric field strength.The glow discharge with millimeter gap can be formed in atmospheric pressure air by using the means of increasing the initial electron density,and an APGD plasma with a maximum air gap of 8mm is generated.

(5)By using the APGD plasma surface treatment device composed of the contact electrodes,the surface modification of high polymer materials such as AF and polyester,are studied and good effect of modifications is obtained.

The authors would like to thank the committee of The 18th National Conference on Plasma Science and Technology which was held in Xi’an,China.This paper is written based on the invited talk‘Exploration to Generate Atmospheric Pressure Glow Discharge Plasma in Air’presented by Professor Liu Wenzheng in the conference.

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