Download Time resolved current-voltage characteristics

GENERATION OF PIN-HOLE
DISCHARGES IN LIQUIDS
František Krčma, Zdenka Kozáková, Michal Vašíček,
Lucie Hlavatá, Lenka Hlochová
Faculty of Chemistry, Brno University of Technology
Czech Republic
Patrick Vanraes
Department of Applied Physics, Ghent University
Belgium
Electrical discharges in water
Physical properties of water
 Highly polar liquid with large relative permittivity (εr=81) and high dielectric
strength E > 1 MV/cm compared to ~ 30 kV/cm of air
 Relatively dense environment with high concentration of ions (H+, OH- etc.) that
determine electrolytic conductivity of water (i.e. resistance)
 Low mobility of ions compared to electrons => ions alter propagation of
discharge channel in water by compensation of space charge electrical field on
the streamer head
 Solution conductivity strongly affects electrical breakdown of water => high
requirements on power supply and reactor design
 To ensure electrical breakdown of water under moderate (reasonable) conditions
is necessary to use non-uniform electrode configurations
point-point, point-plane, wire-cylinder, pin-hole
Experimental devices and parameters
Devices:
Batch discharge reactor
HV source: DC, constant voltage
Experimental parameters:
High voltage: DC 1-3 kV
Discharge current: 90-250 mA
Input power: 90-300 W
Electrodes: planar, stainless steel, Pt
Dielectric diaphragm: PET, 0.25 mm, shapal ceramics 0.3 - 2 mm
Pin-hole: centred, initial diameter 0.2 - 1.0 mm
supporting electrolyte: NaCl, NaBr, NaNO3, Na2HPO4∙12H2O, Na2SO4, etc.
Optimal solution conductivity: 150-1300 μS∙cm-1
Pin-hole configurations
d is typically 0.1 – 2 mm
l ≈ d diaphragm discharge
l » d capillary discharge
d
l
Pin-hole configurations
d is typically 0.1 – 2 mm
l ≈ d diaphragm discharge
l » d capillary discharge
d
l
symmetric
asymmetric
Diagnostics
•
electrical measurements of voltage and current
•
sound generation
•
light generation – PMT, high speed camera, iCCD
•
optical emission spectroscopy – non-time resolved
Principle of pin-hole discharge formation
Theories of electrical discharge creation in liquids:
 thermal (bubble) theory – bubble formation due to Joule heating
 electron theory – analogy to Townsend´s theory in gases
Principle of pin-hole discharge formation
Theories of electrical discharge creation in liquids:
 thermal (bubble) theory – bubble formation due to Joule heating
 electron theory – analogy to Townsend´s theory in gases
positive streamers
negative streamers
̶
+
Principle of diaphragm discharge formation in liquids
Cathode space – positive plasma streamers
P = 75 W
P = 90 W
P = 120 W
Anode space – negative plasma streamers
positive streamers
̶
negative streamers
+
P = 160 W
P = 200 W
Mean current-voltage characteristics
V-A characteristic for gas (red line) and NaCl solution (blue crosses)
Mean current-voltage characteristics
gas
U
Rd
liquid
U
Rs Rd Rs
Rd « Rs
V-A characteristic for gas (red line) and NaCl solution (blue crosses)
sound [a.u.]
light [a.u.]
current [mA]
voltage [V]
Time resolved current-voltage characteristics - electrolysis
sound [a.u.]
light [a.u.]
current [mA]
voltage [V]
Time resolved current-voltage characteristics - bubbles
sound [a.u.]
light [a.u.]
current [mA]
voltage [V]
Time resolved current-voltage characteristics - bubbles
sound [a.u.]
light [a.u.]
current [mA]
voltage [V]
Time resolved current-voltage characteristics - breakdown
sound [a.u.]
light [a.u.]
current [mA]
voltage [V]
Time resolved current-voltage characteristics - breakdown
sound [a.u.]
light [a.u.]
current [mA]
voltage [V]
Time resolved current-voltage characteristics - breakdown
sound [a.u.]
light [a.u.]
current [mA]
voltage [V]
Time resolved current-voltage characteristics - breakdown
sound [a.u.]
light [a.u.]
current [mA]
voltage [V]
Time resolved current-voltage characteristics - discharge
sound [a.u.]
light [a.u.]
current [mA]
voltage [V]
Time resolved current-voltage characteristics - discharge
Mean current-voltage characteristics
discharge
discharge
breakdown
electrolyzis
bubbles
bubbles generation
electrolyzis
Mean current-voltage characteristics
Effect of solution kind – breakdown voltage
Diaphragm/capillary – bubbling voltage
d = 0.3 mm
Diaphragm/capillary – breakdown voltage
d = 0.3 mm
Diaphragm/capillary – breakdown voltage
l = 0.25 mm
Diaphragm/capillary – breakdown current
d = 0.3 mm
Discharge running in bubbles
plasma streamers
bubbles
Discharge running in bubbles
diaphragm 0.3/0.3 mm
bubble at pin, diameter 2 mm
discharge inside bubbles but plasma streamers propagate
into the liquid
Conclusions
• Pin-hole discharge is generated in bubbles (microbubbles) if
non fast pulsing voltage is applied
• Voltage of bubbles generation as well as the breakdown voltage
increase with the increase l/d parameter but there are some
limits
• The breakdown current is more or less independent on the pinhole length except very thin barriers
• Breakdown voltage decreases with the solution conductivity
increase but it is independent of the water solution kind
• Bubbles can generate significant sound even without discharge
• Streamers (streamer like channels) propagates from the bubble
into the liquid even at low voltage
This work was supported by Czech Ministry of Culture
project No. DF11P01OVV004
Thank you for your attention!!!