One question about piezoelectric

[athosanian]

hello, I have a question about piezoelectric material.
In general piezoelectric material is ferroelectric which has spontaneous polarization. The spontaneous polarization should produce depolarization filed in the material leading to some voltage across the electrodes of the material. But I measured some piezoelectric plates across electrodes and found zero voltage. I know that is true and there should be something wrong with my thinking. but whre is it? Thanks for help!

 

[mfb]

How did you measure the voltage? With a voltmeter, you cannot measure anything different from 0 in a static setup - it is conducting, and you cannot violate energy conservation.
In addition, I would expect that your material itself is conducting as well, and electrons can cancel the voltage.

 

[athosanian]

hello, sir, I very appreciate your reply. I think that is a point. I use a conventional volt-meter or oscilloscope to measure the voltage. So the accumulated charges will dissipate when the electrodes are connected to volt-meter and they dissipate quickly. Thus we cannot measure the DC voltage from the peizo even there is a DC depolarization field exists in it.

 

[athosanian]

By the way, I still want to confirm whether there is spontaneous polarization in the piezo materials. I know the piezo materials need to be poled under high temperature and electric field before applications. So if there is spontaneous polarization, comes my following confusing.
Given a piezo disk with polarization along the thickness direction, with upper and bottom surface silver coated as electrodes. the spontaneous polarization of piezo disk is denoted as P0. When disk is in such a state, without any stress applied on it and the upper and bottom electrodes are connected. Then the stress and electric field in the piezo disk is zero. so from the constitutive equation:
　　　　　 D=ε*E+d*T
(where D is electric displacement, ε is electric permittivity, E is electric field, d is piezoelectric coefficient and T is stress.), the D is 0 in the piezo disk in such a state. However, because of spontaneous polarization in the piezo disk the D should not be 0, it should be P0 ! What is wrong with above thinking? I hope some help.