The transfer processes of solvent and dissolved substances in gas phase from aqueous solutions used as cathodes under the action of atmospheric pressure DC discharge were investigated.The electric field strength in plasma (E), cathode voltage drop (Uc) and rates of solution evaporation were measured. The transfer coefficients were calculated. The effect of transfer processes on plasma physical properties was experimentally studied. The threshold characteristics of cations transfer process were determined

Sirotkin, N. A.

Titov, V. A.

English

CTU Open Journal Systems (Czech Technical University, Prague / České vysoké učení technické v Praze)

Plasma Physics and Technology 3(3):126–130, 2016 © Department of Physics, FEE CTU in Prague, 2016EXPERIMENTAL STUDY OF THE LIQUID CATHODE COMPONENTSTRANSFER TO THE DC DISCHARGE PLASMA AT ATMOSPHERICPRESSURESirotkin N. A. ∗, Titov V. A.G.A. Krestov Institute of Solution Chemistry RAS, Ivanovo, Russia∗ alexsad8@yandex.ruAbstract. The transfer processes of solvent and dissolved substances in gas phase from aqueoussolutions used as cathodes under the action of atmospheric pressure DC discharge were investigated.Theelectric field strength in plasma (E), cathode voltage drop (Uc) and rates of solution evaporation weremeasured. The transfer coefficients were calculated. The effect of transfer processes on plasma physicalproperties was experimentally studied. The threshold characteristics of cations transfer process weredetermined.Keywords: DC electrical discharge, transfer processes, liquid cathode, cathode voltage drop, electricfield strength.1. IntroductionAtmospheric pressure discharges with liquid elec-trolyte electrodes are of interest as sources of activespecies for a variety of plasma-chemical applications[1–4] . The physical characteristics of liquid-cathodedischarge were investigated in [5–8]. The properties ofdischarges with liquid cathode depend on the transferof solvent and dissolved substances from the cathodeto gas phase. There are two mechanisms of solventtransfer to gas phase: thermal evaporation and ionsputtering. Sputtering is induced by ion bombard-ment and it is considered to be similar to the cathodesputtering in low pressure glow discharge [9]. Thisprocess can be characterized by a transfer coefficient— number of particles transferred from liquid to gasphase per one ion bombarding solution surface [10, 11].The transfer processes alters the composition of thegas phase and they lead to a change of dischargecharacteristics. The aim of this work is to study thetransfer processes of solution components in the atmo-spheric pressure discharge with liquid cathodes andto investigate the influence of transfer products andphysical characteristics of discharge.2. Experimental detailsSimplified scheme of experimental set-up is shown inFig. 1. Direct current (DC) discharge (i = 10–70 mA)was excited by applying high voltage between thesurface of aqueous cathode and copper anode whichplaced at the position 1–10 mm above the liquid sur-face. To excite the atmospheric pressure discharge theDC specially designed power supply with an 80 kWballast resistor was used. Output voltage was 1.5–2 kV.Discharge was ignited in the air. Water solutions ofcalcium, barium, magnesium, strontium and sodiumchlorides with concentrations of 0.1–0.5mol l−1 wereused as cathodes. The volume of the solution was230 ml. The discharge cell was equipped with a glasstrap to collect the condensate formed by evaporationof solution-cathode. Total rate of evaporation wasdetermined as the rate of condensate accumulationin the trap. The mass of condensate is measuredaccurate to 0.01 g.Figure 1. Experimental set-up.1 — HV-source, 2 — ballast resistor, 3 — electrodes,4 — discharge, 5 — solution, 6 — glass trap, 7 —optical fiber light guide, 8 — spectrometer.The concentrations of cations in condensate werefound by means of atomic absorption spectroscopy(spectrophotometer AAS-3, Carl Zeiss Jena, Ger-many). The concentrations of anions were found usinggravimetric method by means of the precipitation of126vol. 3 no. 3/2016 Liquid cathode components transfer to the DC discharge plasmachloride ions as AgCl and weighing of the precipi-tate. The electric field strength in plasma (E) andthe cathode drop voltage (Uc) were calculated fromdependences of the discharge voltage on the inter-electrode distance as described in [6]. The emissionspectra of the positive column (λ = 200–950 nm) wererecorded by the AvaSpec-2048 spectrometer (grating600 linesmm−1). Gas temperature was found fromthe intensity distribution in the radiation band ofN2(C3Πu −→B3Πg, 0–2) transition as described in[12]. The typical dimensions of the discharge weredetermined from photographs taken by digital cam-era Canon 1100D (exposure 1/500) with a objectiveCanon EF 50mm f/2.5 Compact Macro.3. Results and discussionsDischarge images of an atmospheric pressure dis-charges are shown in Fig. 2. The optical emissionspectroscopy measurements shows the resonance linesof metal atoms appeared in the spectra.Figure 2. Images of atmospheric pressure DC-discharge with liquid cathode (NaCl, c = 0.5mol l−1),i = 10mA (a) and 30 mA (b). 1 — anode glow, 2 —positive column of discharge, 3 — cathode glow.Metal atoms resonance lines appears in the spectraat discharge current exceeding the threshold value.The threshold current values were obtained by extrap-olation of experimental data of emission lines intensityof metal atoms (Fig. 3).It can be supposed that there is a threshold powerrequired for transfer of the solute into the gas phase.The maximum power input to solution by ion bom-bardment can be estimated as p = iUc and it increasewith the discharge current. The threshold current (i0)and p0 = i0U0 are shown in Fig. 4 as functions of themass of hydrated cations in solution, since the cationsof Na+, Mg2+, Ca2+, Sr2+, Ba2+ are surroundedby six water molecules forming the first hydrationshell [13]. The values of threshold current and thresh-old power increases with the mass of hydrated cations.Experiments have shown that the compositionof the liquid electrolyte cathode influences the val-ues of cathode voltage drop (Fig.5) and the electricfield strength in plasma (Fig.6). The maximum valuesof Uc and E are typical for discharge with water as aFigure 3. The resonance lines intensities of metalatoms in the emission spectra of the positive column.Figure 4. Threshold currents (i0) and threshold powers(p0) as functions of mass of hydrated cation.cathode. Smaller Uc and E values were observed forthe solutions of metal chlorides being used as cath-odes. The cathode voltage drop in the discharge withwater cathode is in a good agreement with the datafrom [5, 6]. The appearance of atoms with low ioniza-tion potentials in the gas-phase results in a decreasesof the electric field strength and the cathode poten-tial drop (Fig.7). Probably the product of transferprocesses influences on the balance of the chargedparticles in the plasma.Quantitative characteristic of the particles transferfrom the cathode in the plasma is the transfer co-efficient. Transfer coefficients (s) for components ofliquid cathodes were found using the rate of conden-sate accumulation in the trap, and the concentrationsof cations and anions in condensate. Experimentalresults showed that the transfer coefficients for waterare 300–500 molecule ion−1 independently on com-position of the liquid cathode, and it is in a goodagreement with the literature data [11]. The transfercoefficients increase slightly with discharge current(Table 1). High values of transfer coefficients allowsupposing that a significant part of molecules leave thesolution in clusters or microdrops. The gas tempera-127Sirotkin N. A. , Titov V. A. Plasma Physics and TechnologyDischarge current s(H2O)molecule ion −1(mA) NaCl MgCl2 CaCl2 SrCl2 BaCl215 300± 30 340± 20 340± 20 320± 10 330± 1020 350± 30 340± 20 360± 20 330± 15 330± 2030 340± 50 390± 30 360± 30 350± 20 380± 3040 400± 50 480± 30 470± 35 470± 20 460± 3050 450± 50 500± 40 500± 40 520± 30 510± 40Table 1. The transfer coefficients of water molecules for different liquid cathodes.Power s(Me) particle ion −1(W) NaCl MgCl2 CaCl2 SrCl2 BaCl28 0.005± 0.001 0.002± 0.001 — — —11 0.013± 0.004 0.062± 0.007 0.002± 0.001 — —14 0.034± 0.006 0.085± 0.007 0.012± 0.001 0.041± 0.005 —17 0.019± 0.005 0.086± 0.006 0.022± 0.003 0.082± 0.006 0.005± 0.00119 0.029± 0.006 0.072± 0.011 0.035± 0.006 0.096± 0.007 0.032± 0.00627 0.038± 0.007 0.065± 0.007 0.057± 0.009 0.153± 0.012 0.182± 0.01538 0.082± 0.006 0.098± 0.009 0.128± 0.012 0.132± 0.012 0.153± 0.012Table 2. The dependences of the transfer coefficients of cations on the power input to solution by ion bombardment.Figure 5. The cathode voltage drop of dischargeswith liquid cathodes in comparison with the litera-ture data [5, 6]. Concentrations of solutions — c =0.5mol l−1.ture in positive column is (1 600 ± 100)K and weaklydepends on the discharge current and the compositionof the electrolyte cathode. We assume that the gastemperature is in the positive column does not dependon the distance between the electrodes. Thus, thereis a high temperature region above solution surfaceresulting in thermal evaporation of the solvent. Thecontribution of thermal evaporation in total transferof water molecules to gas phase was estimated basedon experiments with the change of the polarity ofliquid electrode. The experiments showed that contri-bution of thermal evaporation was found to be 5–20%for different discharge conditions. Consequently, ionsputtering is the main reason for transferring the sol-vent molecules to plasma. Transfer coefficients forFigure 6. The electric field strength in plasma ofdischarges with liquid cathodes. Concentrations ofsolutions — c = 0.5mol l−1.cations increase with the power input to solution byion bombardment, they are directly proportional tothe mole fraction of the solute, and decrease withincreasing hydration energy of the cations (Table 2,Fig.8). The values of the transfer coefficients for an-ions are higher and depends on the mole fraction ofthe solute. The value of pH of solution decreases andconductivity of solution increases under the actionof discharge. This can be explained by by nitrogenoxides dissolving in the solution. Nitrogen oxides areformed in the plasma as a result of reactions betweenmolecules of the plasma gas. We did not observe asignificant change in the appearance of the dischargein experiments. We consider that the change of thesolution properties does not have a strong influenceon the discharge properties and the characteristics of128vol. 3 no. 3/2016 Liquid cathode components transfer to the DC discharge plasmaFigure 7. The dependences of the cathode voltagedrop and the field strength in plasma on the ionizationenergy of metal atoms.discharge are determined by the intensity of solutioncomponents transfer to the plasma.Figure 8. Transfer coefficients for cations and anionsas functions of the mole fraction of the solute.The concentration of water molecules NH2O in theplasma can be estimated using experimental data onthe rate of condensate accumulation in the trap (w). Ifwater molecules accumulation in the trap is a result oftheir random thermal motion, and accommodation co-efficient is equal to 1, the rate of the mass condensatechange in the trap can be calculated as:w = dmdt= 14NH2OvTSmH2O, (1)where vT is mean speed of molecules, S is squareof input opening of trap, mH2O is mass of watermolecule. The mole fraction of the water moleculesin the plasma does not exceed ∼ 0.02.4. ConclusionsIon bombardment of the liquid cathode results inthe transfer of solvent and dissolved substances fromliquid phase to plasma. The transfer processes alterthe composition of the gas phase and they lead toa change of the values of cathode voltage drop andthe field strength in plasma. Moreover, the lowestvalues of cathode voltage drop and strength of electricfield in plasma are observed for discharges with liquidcathodes where there are cations of the metals with lowionization potentials. Transfer coefficients for cationsand anions depend on power input to solution by ionbombardment, solution concentration and hydrationenergy of cations in solution. The values of thresholdcurrent and threshold power increases with the massof hydrated cations.AcknowledgementsThe reported study was funded by Russian Foundation forBasic Research according to the research project No.16−33− 60061mol_a_dk.References[1] A.G. Zakharov, A.I. Maksimov, and Yu.V. Titova.Physicochemical properties of plasma - solution systemsand prospects for their use in technology. Russ. Chem.Rev., 76(3):235–231, 2007.[2] Y. Yang, Y.I. Cho, and A. Fridman. Plasma Dischargein Liquid: Water Treatment and Applications. NewYork: CRC Press, 2012.[3] J. Ren, M. Yao, Wu Yang, Y. Li, and J. Gao. Recentprogress in the application of glow-discharge electrolysisplasma. Centr. Eur. J. Chem., 12(12):1213–1221, 2014.[4] M. Moreau, N. Orange, and M. G. J. Feuilloley.Non-thermal plasma technologies: New tools for bio-decontamination. Biotechnol. Adv., 26(6):610–617, 2008.[5] P. Bruggeman and C. Leys. Non-thermal plasmas inand in contact with liquids. J. Phys. D: Appl. Phys.,5(2):053001, 2009.[6] V. A. Titov, V. V. Rybkin, A. I. Maximov, and H.-S.Choi. Characteristics of atmospheric pressure air glowdischarge with aqueous electrolyte cathode. PlasmaChem. Plasma P, 25(2):503–518, 2005.[7] M. R. Webb, F. J. Andrade, G. Gamez, R. McCrindle,and G. M. Hieftje. Spectroscopic and electrical studiesof a solution-cathode glow discharge. JAAS,20(1):1218–1225, 2005.[8] P. Mezei and T. Cserfalvi. Critical review of publisheddata on the gas temperature and the electron density inthe electrolyte cathode atmospheric glow discharges.Sens., 12(5):6576–6586, 2012.[9] G. Ecker and K. G. Emeleus. Cathode sputtering inglow discharges. Proc. P. Soc. Sec. B., 67(7):546–6586,1954.[10] A. V. Khlyustova, N.A. Sirotkin, and A.I. Maximov.Transfer coefficients of cations and anions upon glowdischarge-induced nonequilibrium vaporization ofsolutions of alkaline earth metal salts. High Energ.Chem., 44(1):75–77, 2010.[11] A.M. Kutepov, A.G. Zakharov, A.I. Maksimov, andV.A. Titov. Physicochemical and engineering problemsin studies on plasma-solution systems. High Energ.Chem., 37(5):362–366, 2003.129Sirotkin N. A. , Titov V. A. Plasma Physics and Technology[12] V. A. Titov, V. V. Rybkin, S. A. Smirnov, A. I.Kulentsan, and H.-S. Choi. Experimental andtheoretical studies on the characteristics of atmosphericpressure glow discharge with liquid cathode. PlasmaChem. Plasma Process., 26(6):543–555, 2006.[13] J. Maehler and I. Persson. A study of the hydrationof the alkali metal ions in aqueous solution. Inorg.Chem., 51(1):425–438, 2011.130

Experimental Study of the Liquid Cathode Components Transfer to the DC Discharge Plasma at Atmospheric Pressure

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Experimental Study of the Liquid Cathode Components Transfer to the DC Discharge Plasma at Atmospheric Pressure

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