Can heat (Phonons) generate an electric field in Piezocrystals?

[miniconfusion]

I was thinking. Since phonons are lattice vibrations, and are affected by the temperature of the lattice, can heating a piezocrystal cause it to vibrate and therefore put a small stress on it, causing it's electron density to shift, creating an electric field?

 

[fzero]

I was thinking. Since phonons are lattice vibrations, and are affected by the temperature of the lattice, can heating a piezocrystal cause it to vibrate and therefore put a small stress on it, causing it's electron density to shift, creating an electric field?

Yes, the phenomenon is known as pyroelectricity http://en.wikipedia.org/wiki/Pyroelectricity.

 

[miniconfusion]

But then why aren't all piezoelectric crystals pyroelectric crystals? Also, pyroelectric crystals only produce an electric field during changes in temperature, and not in the presence of a continued temperature. If piezoelectric crystals could produce an electric field from a temperature, the temperature would not have to be changing, but rather, that electric field would simply occur form a constant temperature.

 

[Modulated]

The reason the voltage disappears over time is because there are leakage currents, not because the temperature stops changing.

The difference between piezoelectricity and pyroelectricity, broadly speaking, is that both involve inducing an electric dipole pointing in a particular direction, and so only exist if the material is not too symmetrical. But when you squeeze a material, in some sense you choose a direction (the direction you squeeze it in!), whereas when you change the temperature, you don't affect any direction preferentially. Thus materials can be more symmetrical and still be piezoelectric than they can and be pyroelectric. (Of course there are mathematical ways of formalising this distinction – google for crystallographic point groups if you're interested.)