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Superconducting Capacitor for Energy Storage?

  1. May 31, 2013 #1
    Hi I was about to fall asleep last night and was thinking about breakdown voltage, when electrons are pulled from the surface of a conducting plate and form a channel that discharges a capacitor. This is called a vacuum arc, some other keywords are thermionic emission and work function.

    http://en.wikipedia.org/wiki/Vacuum_arc
    http://en.wikipedia.org/wiki/Thermionic_emission
    http://en.wikipedia.org/wiki/Work_function

    http://hyperphysics.phy-astr.gsu.edu/hbase/tables/photoelec.html

    The highest work function metal is platinum, so theoretically a platinum capacitor should store the most charge in a vacuum.

    But it got me thinking, what if there was a way to coax the electrons into staying inside the cathode by raising the work function somehow? There might be a few ways to do this: make the surface of the cathode very cold, very smooth, or with such a high band gap that electrons want to stay where they are.

    But I thought of another possibility. Perhaps Cooper pairs within a superconductor want to stay paired. For example, if electrons in the surface of the superconductor are feeling a high electric field and want to be ripped from the surface, maybe something about being paired with another electron would compel them to stay within the bulk of the superconductor. So at least theoretically, a superconductor should have a fairly high work function.

    But I don't know enough about the skin effect in superconductors to know how much of the charge is near the surface, especially in DC or steady state.

    http://en.wikipedia.org/wiki/Skin_effect

    I also don't know how high electric fields influence the skin effect.

    The best ultracapacitors right now either use a high surface area material like graphite or an electrolyte. But I'm wondering if anyone has studied how to increase the capacitance by increasing the breakdown voltage beyond 173 MV/m:

    http://en.wikipedia.org/wiki/Dielectric_strength

    If there was a way to catch the free electrons in the gap, maybe with a force like acceleration in a spinning capacitor, then the breakdown could be prevented because the electrons would fly off into space instead of going to the anode, so the ion channel would never form. But I imaging that any centripetal force on the electrons would be a tiny fraction of the electric field force.

    So sorry this is such a general question, but does anyone know of any experiments to raise the work function of a material? This link suggests that something strange happens when a superconductor passes the critical temperature and becomes an insulator:

    http://adsabs.harvard.edu/abs/1991fnps.confQT...M

    I'm wondering if there might be such a thing as a semisuperconductor that has a very high dielectric in the skin so the majority of the electrons stay in the bulk and can't cross to the surface.

    Any ideas on this would be greatly appreciated. Thanks for any help you can provide,

    Zack Morris
     
  2. jcsd
  3. Jun 1, 2013 #2

    f95toli

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    I am not sure I understand what you are asking, but capacitors built from superconductors are from an electrostatic point of view pretty much identical to one built from normal metals.
    After all, most of the limitations come from the dielectric used, not the electrodes.
    Vacuum capacitors do exist, but they are very "inefficient" in that they can't store much energy.
     
  4. Jun 2, 2013 #3
    Ya I think you are right that the plate material is not much of a factor compared to the dielectric strength. I originally thought that vacuum was the best you could do, because there is nothing present to conduct electricity, but benzene and teflon are perhaps 4 times better. So you really can prevent electrons from moving through something, and maybe there will be better dielectrics someday.

    I was trying to visualize if there might be a way to make a region of high potential near the surface of a conductor that the electrons couldn't cross. But I don't think electric fields add that way. If you put a small reverse field inside a larger field, you might make a region with a net field of zero, but the field doesn't extend outside the ends so you'd still have the problem of electrons escaping the backside of the plate holding the inner field. There might be a way to do it with an intrinsic field like inside of a diode, like maybe do CVD or ion implantation on the surface somehow, but my guess is that the boundaries of electric fields still happen even at a microscopic level, unless you can get down to the nanoscale and maybe other quantum effects creep in but I doubt it. What I mean is, trying to find unbounded electric fields would be like trying to find magnetic monopoles and probably isn't going to happen.

    So when it's all said and done, a very thin dielectric is the best way to increase capacitance, and that's what happened with electrolytic capacitors:

    http://en.wikipedia.org/wiki/Electrolytic_capacitor

    But I was hoping that someone was researching how to either increase the dielectric strength or the work function of the plate to keep the electrons trapped in the cathode. I was thinking that maybe Cooper pairs might raise the work function because the electrons might have a higher affinity for each other because the superconductor would conduct so much better than the dielectric. Basically using a quantum effect to create an imaginary electric field that acts kind of like an unbounded one even though one isn't present. I read an article that said Cooper pairs might be free of the influence of gravity because they're in an indeterminate state but it had a hint of flimflam to it:

    http://www.technologyreview.com/view/412674/if-superconducting-sheets-reflected-gravitational-waves/

    I only want to talk about real effects that are well understood but haven't been applied yet. Something else that might work would be a current of muons or something crazy like that where the current is bound more tightly to the atoms. But of course they are too hard to make and decay too fast, but sometimes it helps to look at things from a new perspective.
     
  5. Oct 19, 2015 #4
    I understand what you are saying, but I think you are little confused as to what capacitance is. If I understand correctly, you are wondering if superconducting plates on a vacuum capacitor would increase the breakdown voltage of the capacitor. ZMorris is correct when he said that the maximum voltage of a capacitor is due to the dielectric, but this case is different because the dielectric is space. If you are correct about your cooper pairs theory, you would be virtually increasing the electric constant of space itself. Since superconductors deal with in the realms of infinity, an apparently infinite (or at least really high) electric constant might be achieved. That is a really interesting idea that I think should be investigated.

    Where you go a little wrong is to confuse an increased electric constant with higher capacitance. Since capacitance is charge storage per volt, your hypothetical superconducting capacitor would still have the same capacitance as its resistive counterpart. However, since a high voltage could be achieved without breakdown, you would have increased storage potential. If the electric constant were to increase in orders of magnitude from the cooper pair effect, you may be able to overcome the fact that ZMorris pointed out i.e. that vacuum capacitors have low capacitance.

    All highly speculative, but it does have me thinking :)
     
  6. Oct 20, 2015 #5

    mfb

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    This thread is two years old.
    A superconductor still has normal unpaired electrons (actually, most of them), so you wouldn't prevent the normal breakdown at the normal electric field strength.
     
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