Small-sized High Power Density Plasma Discharge And Materials Treatment

The property investigation of plasma source is the basis of the industrial application of low-temperature plasma.With the characteristics of low power consumption,energy concentration and high reactivity,small-sized plasma has wide applications in local,fast and highly reactive processing.However,due to large specific surface area,the effective power density is difficult to improve,and the diagnosis methods are very limited for small-sized plasma.In this thesis,the characteristic and application researches of three types of small-sized high power density plasma,cathode spot plasma,liquid-phase plasma and millimeter ICP plasma are carried out.Firstly,the optical characteristics of cathode spot plasma have been studied using optical emission spectroscopy(OES)and high-speed camera based on arc descaling.The electron excitation temperature(Tex,)is obtained via Boltzmann fit.It is found the cathode material has a significant influence on the discharge parameters and instabilities.Texc increases with arc current.Compared with the discharge on clean surface,Texc is smaller,and the fluctuation of arc voltage,Texc,along with the OES light intensity reduces for discharge conducted on oxidized surface.Also,the spots present negligible change in trajectory.It is believed that the instabilities are mainly caused by the explosive electron emission of the cathode.The presence of oxides on cathode surface increases the ecton's existing probability,which leads to a relatively invariable emission state,and thus improves the stability for the spot plasma.The breakdown characteristics for high-voltage discharge under n-dodecane and heavy oil with a point-to-point electrode configuration have been investigated using an oscilloscope.The breakdown voltage increases with gap distance.Compared with n-dodecane,the breakdown voltage is lower for heavy oil.It is likely that the impurities in heavy oil enhance the likelihood of dielectric breakdown via improving nucleation of bubbles.Three electrode structures have been investigated for their influences on discharge under liquid.The reults indicate that using a constant small gap distance and avoiding carbon deposition between electrodes are required for a continuous discharge.The treatments of heavy oil and n-dodecane have been carried out through high voltage AC discharge under liquid,where a plasma reactor with small-gap point-to-plane electrodes is constructed.The products are checked by gas chromatography(GC),simulated distillation,viscometer,group analysis and scanning electron microscope(SEM).After plasma treatment for n-dodecane,the gas products include H2 with a volume fraction of 38%and gas hydrocarbons with mainly acetylene.Meanwhile,the high-boiling fractions in liquid residue increase slightly and also carbon particles with diameter 1-10 ?m are produced.As discharge is carried out in heavy oil,similar gas products with higher H2 fraction are obtained,and the viscosity of the residues is enhanced.The results indicate that the bonds of C-C and C-H in hydrocarbons can be easily cracked to form fragments and radicals owing to the heat and high-energy electrons in plasma under liquid.Recombination reactions of radicals lead to the formation of hydrogen and light hydrocarbons.Besides,the polymerization reactions may also exist,which produce heavier components.The optical characteristics of small-sized ICP plasma jet have been studied,where the discharge is maintained in high power density mode.The parameters of CH4/Ar plasma are diagnosed with OES,and the gas temperature is obtained by fitting CH's A2??X2? band using Lifbase software.As the discharge voltage increases,the C2 density increases while CH density slightly decreases,and the gas temperature has an increasing tendency due to intenser electron heating.With increasing the CH4 concentration,both C2 and CH densities increase,and the gas temperature becomes higher possibly due to more effective energy transfer via electronic-vibration and vibration-translation channels.By using Ar and CH4 as the working gas,the application study of small-sized ICP jet for deposition of carbon materials has been carried out As a result,a slightly yellow and soift film is deposited on silicon substrate.The Raman and infrared spectra indicate that the deposited materials are polymers mainly containing CH3 and CH2.