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15.6: Depolarisation

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    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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