15.3: Polarisation

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The polarisation is defined as the dipole moment per unit volume:

\[ P = \frac{\sum \mu}{V} \]

In a piezoelectric which is not ferroelectric, there is no spontaneous polarisation. An applied stress therefore, will generate a polarisation in every unit cell of the crystal, assuming the crystal is homogenous. This polarisation is therefore the same throughout the crystal, and will cause a charge to be developed on the surfaces of the piezoelectric, due to the large number of small charges moving. If the piezoelectric is placed in a closed circuit and subjected to a stress, then a current will be recorded, produced by the movement of charge from one face of the crystal to another.

The polarisation can be described as the charge per unit area developed on the surface, as by the equation:

\[ P = \frac{Q}{A} \]

This polarisation is directly proportional to the stress applied, as described by the equation:

\[ P = d \sigma \]

where P = polarisation, d = piezoelectric coefficient, σ = stress.

However, while this is a direct effect, the stress can be multi-axial, so d can be an array of coefficients. (Also called a 3rd rank tensor , but the meaning of this is beyond the scope of this TLP.)

The reverse effect can also be seen if an electric field is applied to a piezoelectric. In a reverse process to the movement of atoms causing a dipole moment, the application of an electric field causes a dipole moment to be created in order to oppose the field. This dipole moment is created by the motion of atoms. This may result in the contraction or expansion of the unit cell. As this occurs throughout the crystal, there is a large change overall, which changes the shape of the crystal. (It must be noted however, that as there are a very large number of unit cells in the typical crystal, the actual shape change is small. The maximum strain usually seen is about 0.1%.)

This effect is described by the equation:

\[ \varepsilon = d E \]

where ε = strain, d = piezoelectric coefficient, E = electric field.