The poling effect turns ferroelectrics into useful piezoelectrics. However, this means they can only be used within certain well defined limits. If piezoelectrics are used outside of these limits, the alignment of dipoles can disappear, leading to the depolarisation of the ferroelectric, and removing its piezoelectric properties. This can occur in a number of ways.
1. Thermal depoling
If the material is exposed to excessive heat, such that its temperature approaches
its Curie temperature, the dipole moments regain their unaligned state. At the
Curie temperature, a ferroelectric becomes entirely unaligned. In order to prevent
this occurring, it is sensible to use piezoelectrics well below their Curie
temperature.
2. Electrical depoling
A strong electric field, when applied in the reverse direction to the
already poled material, will lead to depoling. If an alternating field is used
to produce ultrasound waves (see later) the field will depolarise the piezoelectric
during the periods in which it is opposing the polarisation.
3. Mechanical depoling
If the stress placed on a piezoelectric is too high, it is possible to immediately
depolarise the piezoelectric as the atom positions are altered. This completely
ruins its properties.
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