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From my understanding, after a piezoelectric crystal is poled, all the dipoles are roughly aligned inside it, so that the piezoelectric effect is not negligible. However, wouldn't that result in a higher density of charges at the surface of the crystal without any strain applied, so there would be a permanent voltage between the surfaces of the crystal? I believe if I touch a piezoelectric crystal at rest I don't get an electric shock, so it feels like there isn't any electric field inside. How can that be possible if all the dipoles are aligned?

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From my understanding, after a piezoelectric crystal is poled, all the dipoles are roughly aligned inside it, so that the piezoelectric effect is not negligible. However, wouldn't that result in a higher density of charges at the surface of the crystal without any strain applied, so there would be a permanent voltage between the surfaces of the crystal?

As I recall reading somewhere, what you say would be true except that in typical real world environments the charged surfaces of a relaxed piezoelectric crystal will become neutralized over time because they will attract oppositely charged dust and other particles.

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  • $\begingroup$ Since there are charged particles on the surface that neutralize the internal field and measuring voltage is done form the surface of the crystal, wouldn't a voltage be measurable from the oppositely charged particles? Moreover, if I apply a constant strain on a piezoelectric crystal for a long period of time, would that cause a neutralization of the field over time, even if there is technically a strain applied? $\endgroup$ – sebasket Apr 17 '19 at 18:49
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    $\begingroup$ Since the surfaces have been neutralized by oppositely charged particles, I don't see how a non-zero voltage could be measured across them. As for applying a sudden strain on a piezoelectric crystal so that the effective charge on the surfaces change, yes, I would think that the surfaces would again become neutralized over time. Don't know what typical time constants for neutralization of the surface is, though. $\endgroup$ – Samuel Weir Apr 17 '19 at 20:01
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    $\begingroup$ Here's a mention of surface charge neutralization in piezoelectrics: "Piezoelectric sensors have disadvantages, too. The surface charge produced by an applied force might be neutralized easily by charges from the environment (airborne charges), by current leakage (due to a non-zero conductivity of the dielectric) or just by the input resistance of the connected electronics (discussed further in Section 8.3). This makes the sensor behave as a high-pass filter for input signals, impeding pure static measurements." - sciencedirect.com/topics/engineering/piezoelectric-sensor $\endgroup$ – Samuel Weir Apr 17 '19 at 20:04

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