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Thread: Piezoelectric Effect

  1. #1 Piezoelectric Effect 
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    I am finding it difficult to obtain empirical evidence, that a slowly applied stress to a piezoelectric will produce electricity.
    In other words, does anyone know if you apply a slowly increasing force to a thin film of piezoelectric material, will it produce a voltage?

    We are talking about 1 cycle over a period of a day.


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    Quote Originally Posted by lpresto15 View Post
    I am finding it difficult to obtain empirical evidence, that a slowly applied stress to a piezoelectric will produce electricity.
    In other words, does anyone know if you apply a slowly increasing force to a thin film of piezoelectric material, will it produce a voltage?

    We are talking about 1 cycle over a period of a day.
    That will be nearly impossible to show. You have to keep in mind that leakage paths will always exist, so there will be self-discharge that competes with piezoelectric generation. Discharge will win in your scenario.

    Is the aim of your setup merely to show the phenomenon, or to use the phenomenon for some other purpose?


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    I was thinking of sort of the same thing, could you explain where the leakage paths are coming from?
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    Quote Originally Posted by sapien View Post
    I was thinking of sort of the same thing, could you explain where the leakage paths are coming from?
    There are always leakage paths of one sort or another; there are no perfect insulators. These leakage paths can be in the bulk of the material (e.g., caused by structural defects as well as impurities), and they can also be on the surface of the material (e.g., from moisture and the ionic species found therein). Even capacitors that are designed to retain charge for long periods of time do not retain much after sitting unattended for a day. Piezoelectric devices are not normally so designed, and so their low-frequency cutoff point is typically measured in (inverse) seconds or minutes (at best), not days.

    Normally, these leakage paths can be neglected, so we don't think of them under ordinary circumstances. In this case, however, they will dominate. To appreciate the magnitude of the challenge, let's do an approximate calculation. Since I don't have details of the setup in question, I'll use illustrative values. You may substitute your own numbers and repeat the calculation. Let's say that the piezo device has a 1 microfarad capacitance. If you are willing to let the generated charge decay to 1/e of the initial value in a day, then the allowable resistance is about 86 Gohms. The piezo material itself must be sufficiently defect-free, and the surface sufficiently clean, that the net resistance they present is no smaller than that value. If the initial charge generates 1 volt, the peak allowable current is around 10 picoamperes. That level of leakage current is not trivially attained even if the bulk material is perfectly insulating; it corresponds to ~100,000 electrons/sec.

    To make matters even worse, you now have to consider how a measurement is to be made, even if you succeed in obtaining a defect-free piezo material with a clean surface. Your instrumentation must present a resistance that is sufficiently high not to perturb the measurement. That, in turn, requires picoampere-order currents. Such instrumentation exists, but this level of performance is not trivially attained, adding to the difficulty.

    ETA: To make matters even worse than that worse, standard signal-to-noise improvement tricks such as averaging cannot be invoked freely. There is a phenomenon known as 1/f noise that causes the failure of averaging beyond some window.
    Last edited by tk421; October 15th, 2013 at 02:01 PM.
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