In the previous two blog posts the physical basis of piezoelectricity and the main groups of materials were presented, focusing on the selection of a material for a specific purpose. In this blog post we discuss in what configuration piezoelectric materials can be used and illustrate some example device structures.
First recall from the first blog post on the topic the two main groups of piezoelectric materials: non-centrosymmetric crystal lattices and poled substances. For simplicity in the following discussion we do not treat them separately, as their operating principle is very similar.
Consider a piezoelectric material with a dipole moment (red arrow) as illustrated below:
Note that the dipole can be oriented in any direction to allow various device functions. Usually when the direction is not explicitly given, it is assumed to be along the +z direction. Now take a block of this material, and imagine voltage being applied between opposing faces.
First assume the voltage is applied between faces that are perpendicular to z, with a positive voltage at +z (note here that due to convention difference between countries, the arrow for the voltage is omitted on purpose):
The voltage generates an electric field, that points towards the voltage drop, i.e. towards -z (in all notations). The charges comprising the dipole try to move in the field, positive charges along the field, negative charges backwards. In this case, this means the charges moving towards one another, compressing the unit cell, and therefore the material itself. A reverse voltage reverses the effect and elongates the material. See the gif for periodic sinusoidal excitation.
More interesting is excitation of the sides, let’s say the faces normal to y:
In this case, an applied voltage (and electric field) still wants to align the dipoles in the direction of the field, moving the positive charges towards –y and the negative ones towards +y. This results in a shear deformation of the material. Again, reversing the voltage reverses the effect. Similar observations can be made for the x direction. The animation illustrates the phenomenon in 2D.
In transducers, such as a Tonpilz or Langevin structure, piezoceramic disks are stacked on top of each other (blue and cyan in the figure) and excited by a voltage that is aligned with the poling direction, making the stack periodically contract and expand along the poling, and generating a high frequency acoustic wave. Check out our example of a Tonpilz Transducer 3D – CAD!
However, for sensing and actuation purposes, where structural deformation is more important than energy conversion efficiency, shear-mode devices are used:
The poling is along the axis of the beam (+x) in this case, and the voltage is applied on top and bottom of the piezoelectric material (which is colored blue). This corresponds to the second case we discussed (but rotated), and shearing occurs, moving the tip of the whole structure vertically. This can be used in atomic force microscopy or accelerometers: in both cases the tip is moved, and the generated voltage measured via the direct piezoelectric effect.