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Interesting question! Even in total pure water, a small portion of water molecules are dissociated into H+ and OH- ions. This means that no amount of purification, deionization, or degassing can achieve a resistivity exceeding 80 MegOhm-cm. Adding a strong electrical field causes electrons to be stripped near the anode and injected near the cathode, leading to additional dissociation. However, simple electrolysis is not a problem (at least for 1 usec pulses at fields of 10 kV/mm) unless the resistivity is less than 1 MegOhm-cm. Even polished electrodes have microscopic surface irregularities - localized points of E-field concentration. Under sufficiently high electrical stress, micropoints on the cathode become sites of direct electron emission which forms a weak space charge within the water adjacent to the cathode electrode. The space charge region is strongly repelled from the cathode, generating a shock wave and microbubbles via cavitation. This permits gaseous ionization, accompanied by higher velocity electrons within the microcavities. Similarly, electrons are stripped from water molecules near the anode, leading to impact boiling and vapor microbubbles. Once microbubbles form, breakdown rapidly progresses via formation and growth of streamers and a leader, accompanied by a significant increase in displacement (capacitive) currents. Because of the large difference in relative permittivity between the water (~80) and gas (~1), virtually all of the voltage stress appears across the newly formed gas bubbles. High transient currents during discharge propagation lead to explosive vaporization of the electrode micropoints, adding additionally conductive metal vapor into the plasma of the developing discharge. Because of the higher currents, electrolysis of the water may aid in additional bubble formation, but only for longer (>10 usec) pulses. Leader and streamers preferentially form at the tops of microbubbles near the anode electrode, so degassing and/or pressurization can delay bubble formation (and breakdown). Analysis of the spectrum of the propagating streamer and leader discharges show the presence of H and OH radicals that are believed to be created through electron impact ionization at the gas-water boundary: e + H2O => e + H + OH. Monatomic O radicals have also been detected via emission spectroscopy. However, these radicals are NOT created by electrolysis, but by direct interaction between "hot" electrons within plasma channels and surrounding water. References and further reading: "Transient regime of pulsed breakdown in low-conductive water solutions", Anto Tri Sugiarto, Masayuki Sato and Jan D Skalny, J. Phys. D: Appl. Phys. 34 (2001) 3400–3406 "Generation of chemically active species by electrical discharges in water", P Sunka, V Babick, M Clupek, P Lukes, M Simek, J Schmidt and M Cernak, Plasma Sources Sci. Technol. 8 (1999) 258–265
: hello there,
:
: As we all know if water is exposed to high electric field between to
: conductors (like a parallel-plate capacitor with water as dielectric) ,
: its molecular polarity tends to allign itself. My question is that if we
: reach the dielectric breakdown of water (say 15KV per mm) is there a
: possibility that oxygen and hydrogen atoms will also breakdown (or what I
: mean separate like in electrolysis set-up) in the process? Please reply to
: my e-mail....Thanks and more power to all geeks outthere....
:
Hello Reiko,
"Water as an insulator in pulsed facilities ( Review)", Gerasimov, A., Chemistry and Materials Science and Russian Library of Science, Issue Volume 48, Number 2 / March, 2005
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