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Parallel Plate Super-Capacitor

  1. Oct 24, 2012 #1
    Just throwing this idea out there and wondering if there is any reason it would not work.

    In a parallel plate capacitor, the maximum voltage is controlled by the dielectric strength and the thickness of the dielectric. When using vacuum as a dielectric, there is a voltage dependant leakage current due to residual gas molecules and also due to thermionic emissions from both metal plates. My question is, if we could achieve a pure vacuum ( like performing this experiment in outer space ) and by cooling the plates to near absolute zero, could we reduce the leakage current between the plates to a sufficiently low value as to allow us to store a million volts or 10 million volts? in the capacitor. Are there any other factors that would limit the maximum voltage in the capacitor other than free gas and thermionic emissions?

    If this is the case, we could build a capacitor with a volume of a few litres, that could hold many megajoules of stored energy.
     
  2. jcsd
  3. Oct 24, 2012 #2

    mfb

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    The surfaces of the plates are not perfect, and every edge will give a high local field strength - at some point, you will get discharges, even with a perfect vacuum.
     
  4. Oct 25, 2012 #3

    ZapperZ

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    You should look up the phenomenon of "Field Emission". As long as you are applying a potential difference, this effect will occur, no matter the vacuum level or the temperature.

    Zz.
     
  5. Oct 26, 2012 #4
    Another factor is that there is no perfect vacuum......even in outer space there are non zero quantum fields...which lead to virtual particles...which eventually lead to real particles and conduction...but I'm guessing later than field emissions....
     
  6. Oct 30, 2012 #5

    Baluncore

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    Vacuum is a better insulator than air but a vacuum is not the best insulator. A dielectric with very strong bonds such as PTFE is a better insulator than vacuum. Something must keep the closely spaced capacitor plates separated since they have an electrostatic attraction to each other.

    The capacity is partially determined by the dielectric constant, so when it comes to the choice of a solid insulator it is best to use one with a high dielectric constant. Most common materials have dielectric constants between 2 and 5. Liquid water is about 80. Tantalum and titanium oxides can have extreme values of dielectric constant in the hundreds and in the thousands.

    If you move a capacitor's plates apart by doubling the insulation thickness then you halve the capacitance, but you double the voltage rating. If you want to store maximum energy then, since E = ½ * C * V2, it is best to investigate the use of higher voltages.
     
  7. Nov 1, 2012 #6

    mfb

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    You double the required volume, too, so you don't gain much in terms of energy per volume or mass (neglecting the electrodes, the energy density stays the same).
     
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