Study On Atmospheric Pressure Cascade Discharge Plasma And Material Surface Treatment

The low-temperature plasma generated by atmospheric pressure glow discharge(APGD)has been widely used in research fields such as surface modification,biomedicine,and thin film deposition.The discharge process does not require a vacuum system,so it is suitable for continuous processing in the industry.However,compared with low-pressure discharge,APGD requires a higher breakdown voltage,and the stability and uniformity of the discharge are not easy to control,which limits application range of APGD.Therefore,the development of atmospheric pressure plasma sources and the research of plasma characteristics have become the current research hotspots of low-temperature plasma.The repetition frequency of the APGD excitation source is in the range of kilohertz to megahertz.When it is in the megahertz range,the various active particles such as electrons and ions generated in the discharge will not be annihilated in one radio frequency(RF)cycle,compared to kilohertz APGD,its sustain voltage and breakdown voltage are relatively low,the discharge plasma density is higher,and its chemical reactivity is higher.The kilohertz range APGD can improve the stability of the discharge through sub-microsecond high-voltage pulses.The discharge requires a higher breakdown voltage,and the pulse discharge intensity generated is higher than that of the RF discharge.In this paper,the kilohertz pulse discharge and the megahertz RF discharge are combined to form a cascade discharge mode to obtain a high-density and stable discharge plasma.First,a two-dimensional fluid numerical simulation model was established to explore the spatial distribution of particles in the pulse modulated(PM)RF discharge process,study the electron generation process during the PM RF discharge initiation process,and the electron dissipation process in the PM RF power off phase.It reveals the influence of the number and spatial distribution of the residual electrons on the ignition process of PM RF discharges.In the experimental research,by designing the parallel plate discharge structure,the pulse discharge is introduced in the off phase of the PM RF discharge,and the coupling cascade discharge is formed on the time scale.First,the pulse and RF cascade discharge is studied.The time evolution of the discharge space distribution is studied from the front and side directions.It is found that a large number of high-energy electrons are generated in the area of the edge of the metal electrode during the discharge at the falling edge of the pulse,which enhances the intensity of the RF discharge and causes the asymmetric spatial distribution.The RF discharge was changed to PM RF discharge.By changing the time interval between pulse discharge and PM RF discharge,it was found that residual electrons generated by pulse discharge would accumulate on the edge of the metal electrode.These electrons assisted the ignition process of RF discharge.When the time interval between pulse discharge and PM RF discharge increases,the residual electrons in the discharge space will gradually dissipate,and the RF discharge will be uniformly ignited in the metal electrode area.This shows that the residual electrons generated by the pulse discharge can assist the ignition of the RF discharge and help improve the stability of the discharge.By adopting the jet cascade discharge device,the pulse discharge and the RF discharge jet coupling cascade discharge are studied,and it is found that by adjusting the width of the pulse voltage,the length of the pulse discharge jet can be adjusted,and the spatial cascade discharge mode is realized.When the width of the pulse voltage is short,the plasma bullets generated in the pulse discharge cannot reach the RF discharge area,failing to form a cascade discharge mode.At this time,the RF discharge is stable and the jet generated by the RF discharge is weak;As the pulse width of the pulse voltage increases,the plasma bullets generated by the pulse discharge are injected into the RF discharge area,and the injection of high-energy electrons increases the intensity of the RF discharge and enhances the length and intensity of the RF discharge jet;Continue to increase the width of pulse voltage,and the pulse discharge plasma bullet will pass through the RF discharge area,further enhancing the intensity of the RF discharge jet.Through the spatio-temporal evolution of the discharge jet at the nozzle,it is found that the length of the RF discharge jet increases from 2mm to 8 mm as the pulse width of the pulse voltage increases,and can be maintained for more than ten microseconds.Finally,the atmospheric pressure cascade discharge plasma jet was used to study the surface modification of polyester fibers and carbon fibers.The super hydrophobic properties of polyester fibers were successfully realized,and the fiber water contact angle was increased from 112° to 154°.During the surface treatment of carbon fiber,it was found that the RF discharge jet can increase the COOH or COOR oxygen-containing polar groups on the surface of the carbon fiber.With the enhancement of the RF discharge jet,more carboxyl groups are bonded to the surface of the carbon fiber.The pulse discharge plasma jet treatment of carbon fiber mainly increases the density of C-O single bonds.When the high-energy electrons generated by the pulse discharge and the RF discharge jet jointly treat the surface of the material,more oxygen-containing polar groups will be introduced into the surface of the carbon fiber,increasing the surface modification effect.