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how electric conductivity affects the breakdown voltage of water
Posted by John Snow on 11/14/2004, 11:41 am In addition, I am still wondering,how the electric conductivity of water affect the breakdown of the water-air mixture and also water-solid(particles)-air(bubbles) mixture? Is there a modle to determine detailed influence? thank you so much!
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Re: how electric conductivity affects the breakdown voltage of water
Posted by Bert Hickman on 11/14/2004, 9:39 pm, in reply to "how electric conductivity affects the breakdown voltage of water" Hello John, Water behaves as a high voltage dielectric for only very short intervals (microseconds or less). This property allows ultra-pure, filtered and degassed water to be used as a high k (k ~80) self-healing dielectric for low impedance transmission lines and capacitors in high energy pulsed power work. The presence of gas bubbles on the surface of the metal electrodes will significantly reduce the breakdown voltage. The presence of soluble ionic contaminants are OK (as long as gas bubbles are not generated through electrolysis) since the applied voltage stress is evenly distributed through the solution. In fact, high energy high voltage water resistors are sometimes made with plastic tubing. Pure copper electrodes are used at each end, and the tubing is filled with water that's been doped with a small amount of copper sulphate. Generally speaking, the presence of suspended dirt or dust will significantly degrade the breakdown voltage of the any dielectric liquid, including water. I'm not aware of specific models or that predict the effects of suspended imurities in water. All of the water breakdown models that I've seen assume pure, clean DI water. In your experiment were you applying a high voltage pulse, or were you using 60 Hz or DC? Also, if you apply a relatively high current through an electrolyte, you may can create arcing at the interface between the water solution and the electrodes. Under the right conditions, you can create an underwater arc (plasma) at the electrode interface. Best regards, -- Bert --
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Re: how electric conductivity affects the breakdown voltage of water
Posted by John Snow on 11/15/2004, 2:13 am, in reply to "Re: how electric conductivity affects the breakdown voltage of water" I used a bipolar pulsed power(with a peak to peak voltage of 100kV,pulse repetition rate of 0~200Hz).Dicharge was carried out in a DBD configuration reactor with a wire to cylinder geometry electrode.Discharge electrode is metel rod while the grounded electrode is a reticulate stainless steel.Dicharge electrode was fully covered by a quartz glass tube.Air bubbles was artificially bubbled into the water.Gap distance varied between 5~50 mm. And what's more, when solid dielectric particles and the artificially created air bubble simutaneously present in water, the medium system become much more complex. How does the polarization electric field of the dielectric particles affect the discharge of the bubbles? I am not so familar with this field.I occasionally find these phenomena,and feel that they are so interesting.I am not sure whether i have made me known. thank you for your reply! Best regards!
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Re: how electric conductivity affects the breakdown voltage of water
Posted by Bert Hickman on 11/15/2004, 10:28 pm, in reply to "Re: how electric conductivity affects the breakdown voltage of water" John, What was the pulse width used from the pulser and what were the rise and fall times? For very short pulse widths, the water will appear "mostly" as a dielectric with a k of about 80, the air bubbles will have a k of 1, and the quartz glass (k between 4 - 8 depending on blend) will only allow capacitive (displacement) curents to flow in the system. The result will be that most of the applied voltage stress during the pulse will appear across the air bubbles within the gap. The air will then break down under the concentrated electrical stress, becoming ionized (conductive). Once this occurs, the E-field is redistributed in the gap, most likely leading to breakdown of the water within the gap. BTW, I'm assuming that the quartz glass is thick enough so that it does not break down even if the water and air have done so. A similar air-water breakdown sequence would occur if you also had suspended particles in the water, only now the water would breakdown at a lower voltage than if it were pure. The suspended particles provide localized areas of concentrated E-field stress inside the water. These areas of higher stress will cause the water to break down at a lower voltage than pure water. BTW, putting an air gap inside any otherwise good liquid or solid dielectric can often result in breakdown of the combined system under pulsed or RF stress since the air usually breaks down first, which can then cause subsequent breakdown of the remaining dielectric material. Best regards, -- Bert --
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