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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.... --Previous Message--
hello there,
: --Previous Message--
: What is the dielectric strength of water? How is it affected by
: temperature and contaminants?
:
:
: Hello Donald,
:
: For brief pulses (a few microseconds or less), pure water is an excellent
: dielectric. However, the water needs to be continually filtered, degassed,
: and deionized so that it has a resistivity of ~5-7 megohm-cm in order to
: work reliably as a dielectric in high energy pulsed power pulse systems.
: For short pulses, water combines high dielectric strength, high dielectric
: constant (~80), and is "self healing" in the event of an
: electrical breakdown. These properties allow pulsed power engineers to
: create compact, high-energy storage and transmission systems using water
: as the dielectric. For example, water is used as the dielectric in low
: impedance, high current, high voltage transmission lines that feed 20
: million ampere pulses into the center of the huge "Z Machine" at
: Sandia Laboratory - the world's largest pulse generator - see:
: http://www.sandia.gov/media/z290.htm and
: http://www.sandia.gov/pulspowr/facilities/zaccelerator.html
:
: The main challenge is to keep the water sufficiently pure and keep gas
: bubbles from forming on the electrodes. Since water is the "universal
: solvent", it easily becomes contaminated by impurities (dust and ions
: leaching from the container that increase its conductivity). These
: impurities must be continually removed since their presence always
: degrades the water's performance as a high voltage dielectric.
:
: For short pulses, J. C. Martin developed an empirical breakdown scaling
: relation for water and mineral oil under a uniform E-field over a range of
: voltages, pulse times, and electrode area based upon his work at Sandia.
: The relationship is as follows:
: F = k*(t^(-1/3))*(A^(-1/10))
:
: where: F = the peak breakdown field (in megavolts/cm)
: t = duration of applied voltage (in microseconds)
: A = area (in square cm)
: k = 0.3 for water (positive streamers the normal case)
: k = 0.6 for water (a special case where field enhancement is purposely
: adjusted to cause streamers to form preferentially from the negative
: electrode instead of the positive electrode)
:
: For example, solving for the positive streamer breakdown field (F) for 1
: square cm electrodes in water, stressed by a 1 microsecond pulse in water,
: we get F = 300 kV/cm. If we used a 100 nsec pulse, this would be expected
: to increases to 646 kV/cm, and almost 1.4 million volts for a 10 nsec
: pulse. Breakdown behavior changes with longer (>10 microsecond) pulses,
: since ionic conduction may begin to alter the E-field distribution within
: the gap. Considerably more detail can be found in "High Power
: Switching" by Ihor M. Vitkovitsky, ISBN 0442290675 and Introduction
: to High Power Pulse Technology by S. T. Pai and Qi Zhang, ISBN
: 9810217145.
:
: Breakdown within water begins as streamers that initiate from points of
: field enhancements (bubbles, small projections, or particles on the
: electrodes). As noted above, streamers will form and propagate more easily
: from the positive electrode in a uniform field. I am not aware of explicit
: data relating breakdown strength to water temperature. However, increasing
: the waters temperature will reduce the water's density and increase ion
: mobility these factors may tend to decrease the dielectric strength.
: Increasing the applied pressure will significantly increase the breakdown
: voltage, possibly because it makes initial bubble formation (which seems
: to be necessary for slow streamer formation) more difficult.
:
: Best regards,
:
: -- Bert --
:
:
:
:
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