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This question is of a purely theorethical nature.

Applying pressure to a piezoelectric material creates voltage, and the inverse is also true. I have seen this likened to a spring, and was wondering how far you could take this concept.

Imagine the following scenario: You apply a voltage to cause a mechanical oscillation, then you use glass or plastic to encase and freeze it in an excited state. (I know it's impossible, the crystal itself would melt and the material would take some time to settle, but bear with me.)

Frozen in that state, question 1, would it retain its potential mechanical energy, and 2, if you were to break the glass would it still produce a voltage?

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  • \$\begingroup\$ What do you mean by freezing? Low temperature? \$\endgroup\$
    – AJN
    Commented May 29, 2021 at 8:51
  • \$\begingroup\$ "I know it's impossible, the crystal itself would melt and the material would take some time to settle" , why it should melt? And what's a frozen state for the crystal? \$\endgroup\$
    – Marko Buršič
    Commented May 29, 2021 at 8:53
  • \$\begingroup\$ Actually most ceramic capacitors are based on this effect! They employ PZT, the piezoelectric ceramic (similar to barium titanate.) As a result, the capacitance of ceramic caps is thousands of times larger than one might predict, and also, ceramic caps put out acoustic vibration during AC drive. WIth piezo dielectric, a "charged" capacitor is internally bent. As long as it remains disconnected, it will remain bent, and a voltage will persist on its terminals, same as any capacitor. (Embed it in glass, to keep it from discharging, same as any capacitor.) \$\endgroup\$
    – wbeaty
    Commented May 29, 2021 at 14:24
  • \$\begingroup\$ I believe that a piezo generates a voltage with a weak current only as it is flexed. If it is held frozen in a flexed position then after the voltage and weak current pulse, the voltage and current quickly leak away. \$\endgroup\$
    – Audioguru
    Commented May 29, 2021 at 15:31
  • \$\begingroup\$ @Audioguru Yes. The charges rebalance after some time. Similar to pyroelectric sensors. Once the internal equilibrium is reached its gone. \$\endgroup\$
    – DKNguyen
    Commented May 29, 2021 at 17:19

2 Answers 2

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I guess you wonder if the generated electric field stays if a piezo crystal was kept under a constant tension without letting it take back its original form.

The crystal would keep its voltage, but there are some amount of charged particles in the air and even in glass and on the crystal that gradually they would get organized by the field of the crystal to a formation which neutralizes the generated field and make it unobservable.

You can help it happen much sooner by connecting the surfaces of the crystal together for example with a voltmeter. You should see the crystal with metallized opposite surfaces as a capacitor which can be charged by letting a force make tension to the crystal but a slightest leak would discharge that capacitor sooner or later. New charge needs new deformation.

If the tension is released the collected charged particle formation disintegrates.

Unfortunately I do not know enough of material physics to be able to say does the crystal gradually get reorganized under the tension so that the tension and generated field disappears. I guess that a million years of hammering by thermal motion causes something.

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This is called an electret. Commonly used to bias a microphone capsule. "Freezing" in this case may be the freezing point of something like wax.

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  • \$\begingroup\$ I think an electret is basically a charged capacitor. Piezoelectric materials use a different physical principle. \$\endgroup\$
    – Supa Nova
    Commented May 29, 2021 at 13:59

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