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DoITPoMS Teaching & Learning Packages Piezoelectric Materials Spontaneously polarised piezoelectrics (on the macro scale)
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Spontaneously polarised piezoelectrics (on the macro scale)

Now, ferroelectric materials possess multiple domains. For background on this, read the TLP on Ferroelectric Materials.

To make it simple, we will only consider single crystal ferroelectrics. These, when first made, have domains of the form:

Diagram of single crystal ferroelectric domains

If a mechanical stress is applied to the ferroelectric, then there are domains which will experience an increase in dipole moment and some which will experience a decrease in dipole moment. Overall, there is no net increase in polarisation. This makes BaTiO3 useless as a piezoelectric unless it is put through some additional processing. This process is called poling. An electric field is applied to the ferroelectric as it passes through its Curie temperature, so as its spontaneous polarisation develops, it is aligned in a single direction:

Diagram of aligned single crystal ferroelectric domains

All of the domains in the piezoelectric have a dipole moment pointing in the same direction, so there is a net spontaneous polarisation. Now, when a mechanical stress is applied, the polarisation will increase:

Diagram of aligned single crystal ferroelectric domains under mechanical stress

or decrease:

Diagram of aligned single crystal ferroelectric domains under mechanical stress

but still remain pointing in the original direction. This makes ferroelectrics into useful piezoelectrics.