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dielectric strength of water
Posted by Donald D. Gray on 2/23/2004, 5:06 pm
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Re: dielectric strength of deionized water
Posted by somayeh taheri on 10/3/2005, 2:14 am, in reply to "dielectric strength of water" --Previous Message--
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Re: dielectric strength of water
Posted by Bert Hickman on 2/23/2004, 9:17 pm, in reply to "dielectric strength of water" 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: https://www.sandia.gov/media/z290.htm and 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: 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 water’s 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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molecular behaviour of water
Posted by reiko tropa on 2/9/2007, 5:24 am, in reply to "Re: dielectric strength of water" 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--
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Re: molecular behaviour of water
Posted by Bert Hickman on 2/14/2007, 10:28 pm, in reply to "molecular behaviour of water" 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
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overunity at resonance
Posted by reiko on 2/15/2007, 5:24 am, in reply to "Re: molecular behaviour of water" hello there: It just annoy me. I made simple energy calculations at series resonance circuit and I was surprised to find out that the total energy of the inductor and capacitor exceeds that of the input... Is this natural?
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Re: overunity at resonance
Posted by Bert Hickman on 3/19/2008, 10:59 am, in reply to "overunity at resonance"
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Re: overunity at resonance
Posted by David Kalendra on 3/19/2008, 10:38 am, in reply to "overunity at resonance"
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