静電気学会誌，37, 1（2013）22-27

J. Inst. Electrostat. Jpn.

論

文

Comparison between the Mechanism of Liquid Plasma Discharge Process in
Water and Organic Solution
Helena Oi Lun LI*, Jun KANG**, Kuniko URASHIMA ** and Nagahiro SAITO***Green Mobility Collaborative Research Center, Nagoya University** Graduate School of Engineering, Nagoya University(Received September 13, 2012; Accepted December 14, 2012)“Liquid in Plasma” has been demonstrated as a versatile process with high performance for many applications including
nanoparticle synthesis, water purification, organic compound decomposition as well as sterilization. The application ofelectrical discharge in water has been studied for many years; however, liquid discharge under organic solutions has yet
been thoroughly investigated. In this study, a detailed analysis of the fundamental characteristics of plasma discharges
under water and benzene were conducted. The solution plasma reactor was connected to a bipolar pulsed power supply
with a pair of tungsten electrodes under 280 ml of solution. The discharge voltage and current were in the order of 1 kVand 10 kA, and the characteristics were found to be different between the two liquids. The discharge in water occurredin three different phases: pre-discharge, the initiation and the formation of discharge, whereas the discharge in benzene
happened in two main steps: pre-discharging and the formation of discharge. By the observation of optical emission
spectrum, significant H radicals were generated in discharge under water, accompanied with the formation of otheractive species such as O and OH. As for the discharge under benzene, the majority of active species changed to C2, andfollowed by CH and H.

1．INTRODUCTION

and operating voltage/current ranges in the system:
pulsed corona (PCED) and pulsed spark (PSED) type

Liquid phase electrical discharge processes have beensystems use high voltage/ low current discharges aroundrecently studied in nanoparticle synthesis [1-5], water

1 J/pulse and 10J/pulse, respectively, whereas the pulsed

treatment [6-7], organic compound decomposition [8-9]arc (PAED) and the pulsed power (PPED) type systems

as well as sterilization [10-12]. High voltage electricaluse higher current discharges around 1 kJ/pulse with few

discharges directly in water (electrohydraulic discharge)

kV and larger energy per pulse with higher voltages,

initiating both chemical and physical processes. It injects

respectively [14].

energy directly into an aqueous solution through a

While the application of electrical discharge in waterplasma channel formed by a high-current/high voltage

has been studied for many years, liquid discharge underelectrical discharge between two submersed electrodesorganic solutions has yet been clearly investigated. The[6-7]. The liquid plasma is consisted of multiple phases:

plasma aimed to apply to current experiment was

plasma, gas and liquid phase. Also, the density of thebelonged to the spark type system: which occurred in theplasma and active species are different from eachrange of discharge voltage and current, respectively, 1-2

interface, from plasma to gas and gas to liquid phases.

kV and 1-20 kA.

These unique properties and novel reaction kinetics

In this study, plasma discharges under water andpromote fast reaction and high density of radicals. The

benzenewere

conducted.

Detailed

analysesofprocess generates UV radiation, hydrogen peroxide,

fundamental characteristics of the discharge werehydrogen, oxygen and other radicals and ions [13].

compared between the two liquids. Benzene (C6H6) isElectrohydraulic discharge can be divided into fourchosen to be a reference solution of organic solution as it

different types based on the amount of energy deposited

is a simple, non-polar, non-conductive organic solution.

Comparison between the Mechanism of Liquid Plasma Discharge Process in Water and Organic Solution（Helena Oi Lun LIら）

23

In addition, it is a potential liquid used for the synthesis

order to fully understand the process of plasma discharge,of carbon nanotube or nanoparticle in electrical plasma

primary voltage (from power supply), were graduallydischarge [15-16]. The discharge mechanism is expected

increased from 30 - 130 V with each increment voltage

to be significantly different compared to the water, whichof 10 V. The relationship between primary voltage,is polar and conductive in properties. This fundamental

discharge voltage and discharge current was summarizedknowledge can provide us information to control thein Fig 2. The primary voltage is the voltage indicated inmechanism for future carbon nanoparticle synthesis

bipolar power supply, where the discharge voltage andprocess.

current are the actual value measured at the tungsten

2. EXPERIMETNAL APPARATUS

electrodes by the oscilloscope. The discharge voltage
increased proportional to the increase of primary voltage.

The solution plasma reactor was connected to a

The current, on the other hand, was increased in threebipolar pulsed power supply with a pair of tungsten

different stages. When the primary voltage was above

electrodes with diameter of 1 mm. The electrode gap

100V, the discharge occurred between the electrode anddistance was fixed at 0.5mm. The energy input to thethe plasma channel become conductive, and resulted in aplasma reactor was controlled by the primary voltage

significant rise of current. Detail analyses of water

input in the bipolar power supply with a frequency of 15

discharge will be carried out in the next section.

kHz. The discharge was conducted in 280 ml of tap
water and pure benzene.
Fundamental characteristics (discharge voltage andcurrent) were measured by a digital oscilloscope(Tektronix, TDS 3014B) via high voltage and current
probe. An optical fiber/PC computer-based optical
spectrometer (Ocean Optics Inst. Co. 200-800nm) wasused to measure the time dependent optical emission
spectrum through a horizontal Quartz lens window. The
window was located less than 1 cm away from thedischarge. The schematic of reactor is shown in Fig. 1.
Table 1: Properties of tap water and pure benzene
Conductivity

Boiling Point

S/cm

o

Water

280

100

Polar

BenzeneN/A

80.1

Non-Polar

Fig.1 Schematic of solution plasma reactor of 280ml ofsolution

PolarityC

3. RESULTS
3.1 EFFECT OF VOLTAGE ON WATER
PLASMA DISCHARGE
The mechanism of plasma discharge in tap water witha conductivity of 250 s was served as a reference ofbasic understanding of the liquid plasma mechanism. In

Fig. 2 Relationship of discharge voltage and current withrespect to primary voltage (power supply voltage)

静電気学会誌 第37巻 第 1 号（2013）24

3.2 FUNDAMENTAL CHARACTERISTICS
OF DISCHARGE UNDER WATER
The discharge under water were observed in threedifferent stages: (I) the pre-discharge, (II) the initiationand (III) the formation of water discharge. These stages
were corresponding to different ranges of voltage andcurrent. The summary of the discharge parameters forwater in different stages were obtained by 10 sets ofexperiments and the result was shown in Table 2. The

breakdown of water. In the physical phemomina, themicro-gap spark discharge in bubbles formed in stage 2
growed bigger with increasing the applied volage, andthese discharge bridged together and allowed a relative
high current (I > 10A) flowed through the plasma/gas
interface. In Fig. 5, it is observed that the current rised
sharply after voltage breakdown. Ringing of the current
was observed due to the specfic properties of the bipolar
power supply.

typcial voltage and current waveforms obtained from thepre-discharge, the initation and the formation of liquid
discharge are shown, respectively, in Fig.3, Fig.4 andFig.5.
In stage I, a small current (I < 1A) was flowing
between the electrodes since water is conductive. The
water was gradually heated by Joule heating, J  I2Rt
where I = conducting current, R = resistance of waterand t = time. At this stage, no bubbles, glow corona or
spark was observed.
In stage II, when the discharge votlage increased to 0.2
kV, more energy was applied to the liquid media and theheated water started to evaporate. Micro-bubbles wereformed around the electrodes. The high electric field ofFig. 3. Discharge curent and voltage waveforms of the
pre-discharge under water

1x105 V/m (calculated by E ~ V/d, where V is thedischarge voltage of 1.5 kV, d is the electrode diameter
of

1

mm)

first

introducedsmall

discharge

inmicro-bubbles. Thus the micro-gap spark discharge wasinitiated at the edge of both electrodes in the gas-liquidinterface. However, the electric field in current stage is
not high enough to cause the electrical breakdown inliquid water. Thus, no glow or spark was fully generated
between the electrodes. The physical phenomena
observed in this stage included rapid formation of small
bubbles. In Fig.4, the current waveform was distored dueto the current flow occurred between gas/liquid interface.
At stage III, the spark discharge was initiated when the
discharge voltage reached 1kV. The electrical field washigher than 1x 107 V/m and was able to cause electricalFig. 4 Discharge curent and voltage waveforms of the initiationof discharge under water

Comparison between the Mechanism of Liquid Plasma Discharge Process in Water and Organic Solution（Helena Oi Lun LIら）

25

Table 2. Summary of the conditions for three different stages indischarge under water
Discharge

Discharge

Voltage kV

Current A(I) Pre-discharge

0.1 – 0.2

<1

(II)

0.2 - 0.5

1 - 100.5 - 1

> 10

Initation of thedischarge(III) Formation of thedischarge

Fig. 6. Typical optical emission spectrum of plasma in waterwith various discharge gaps

3.4 FUNDAMENTAL CHARACTERISTICS
OF PLASMA DISCHARGE UNDERBENZENE LIQUID
The discharge in benzene was considered as arepresentative of the discharge occurred in a non-polar,
non-conductive organic solution. As expected, thedischarge mechanism was significant different to water,which has polar and conductive as main properties. The
discharge under benzene were observed in two different
Fig 5. Discharge curent and voltage waveforms of theformation of disharge under water

3.3 OPTICAL EMISSION SPECTRUM
FROM WATER DISCHARGE

stages: (I) pre-discharge and (II) formation of thedischarge. Typical voltage and current waveforms
obtained from the pre-discharge and the formation ofbenzene discharge are shown, respectively, in Fig.7 andThe optical emission spectrum observed under plasma

Fig.8. The generation of micro bubbles was not observed

discharge in water thoroughly investigated by previous

between the pre-discharge and the formation of plasma.

studies [10, 1718]. However, this experiment was repeated

It is suggested significant difference behavior inin order to act as a reference spectrum in current reactor

discharge is goverend by the conductivity of the solution

for the comparison of the spectrum obtained from

[6-7, 16]. Since benzene is a non-polar solution, almost

benzene in section 3.5. As agreed with previous studies,nothe active species including OH, H and O were observed

correspondingly low conducting current (< 0.05A) was

during the discharge period. This active species wererecorded in the pre-discharge stage with primary voltage

mostly generated by water dissociations. The major

increased from 30 to 90 V. The discharge started

active species were H, followed by atomic O and OH

simultaneously when the voltage was above 1.5 kV. With

radicals.

this amplitude of votlage, the electric field is highconductivity

can

be

detected.

In

Fig.7,

aenough to cause dielectric breakdown of benzene.
Dielectric breakdown is defined as a sharp reduction in静電気学会誌 第37巻 第 1 号（2013）26

electric resistance when the electric field higher than thebenzene’s dielectric constant (~ 106V/m). After the

Table 3. Summary of the conditions for two different stages indischarge under benzene

dielectric breakdown of benzene, the liquid become
conductive and allow a large current flow of 5 A.
Benzene was heated up by the large current (J  I2Rt)
and became gasified. The high electric field of 106 V/m

Discharge

Discharge

Voltage (kV)Current (A)(I) Pre-discharge

1 – 1.5< 0.05(II) Formation of

~ 1.5

4-8

the discharge(E ~ V/d) initiated the spark discharge intanesouly. The

3.5 OPTICAL EMISSION SPECTRUM OF
DISCHARGE UNDER BENZENE

summary of the discharge parameters for benzene indifferent stages is shown in Table 3.

The optical emission spectrum observed under plasma
discharge in benzene was shown in Fig. 9. Major emitted
wavelengths were found in the range of 470 - 480, 510 520 and 560 - 565 nm. The spectrum might becorresponded to the excited C2 emission spectrum
18 A weakerobtained from pure methane flame [19].

intensity of wavelength between 430 and 435 nm werealso inspected. It might be generated by the excited CH
radicals [20].
19 The peak of 657 nm was possible
contributed by active species of Hα.
Time (s)Fig. 7. Curent and voltage waveforms of the pre-discharge
under benzene

It is assumed this intensive energy input is sufficientfor the excitation of benzene molecules within theplasma channel [3,13]. When the benzene was gasified
and excited, the resonance electron structure was being
disturbed and then broken. The open chain structure wasfurther excited and finally, the active species such as C,
H, C2 and CH were generated in the plasma. It is
expected that these radicals were created in multiple
steps of dissociation and recombination [21].
20513nm C2

7000
6000

Time (s)Fig. 8. Curent and voltage waveforms of the formation ofliquid discharge under benzene

Intensity

5000
4000

560nm C2468nm C2

3000

657nm

2000

0

H

431nm CH

1000

200

400

600

Wave lenth

800

1000

1200

Fig. 9. Optical emission spectrum obtained from liquid
discharge under benzene

Comparison between the Mechanism of Liquid Plasma Discharge Process in Water and Organic Solution（Helena Oi Lun LIら）

4. CONCLUSION
A

detailed

comparison

of

the

fundamental

characteristics of discharge under water and benzene wascarried out in this study. The discharge mechasnim wasgoverned mainly by the conductivity of the solution. For
water which is polar and conductive properties, the liquid
is first heated by small conducting current and leads tobubble formation due to evaporation. When increasing
the discharge voltage, the high electric field intiated thedischarge in bubbles and a full discharge was formed
when these bubbles bridged together. For the discharge
phenomena in benzene, no conducting current wasobserved since it is non-polar and non-condcutive liquid.
The discharge occurred when dielectric breakdown ofbenzene occurred with a higher discharge voltage of
1.5kV. The liquid became conductive and allowed large
current flow. The current thus heated the liquid media
and leads to gasification of benzene. A full discharge wasformed simultaneouly since the eletric field was in therange of 106 V/m. In optical emission spectrum, major
active species was based on the solution property. For thedischarge under water, major active species such as H,
OH and O were generated based on water dissociation
chemistry. As for discharge under benzene, the major
active species were C2, CH and H.

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