Bone, wood and quartz are all piezoelectric materials. But what is piezoelectricity? And where does this remarkable property come from?
Coupling between electrical and mechanical properties
Piezoelectricity is a phenomenon which means that there is a coupling between the electrical and the mechanical state of the material. When a piece of piezoelectric material is mechanically deformed, e.g. compressed, a current will flow and charge its faces. and vice versa, it will be deformed when exerted to an electrical field.
The phenomenon was discovered in 1880 by Paul-Jacques Curie, and his younger brother, Pierre, husband of the well-known Marie Curie. In addition to the materials already mentioned, there are many more materials that exhibit piezoelectric properties such as sucrose, silk, Rochelle salt, PVDF, and many ceramics. But why don’t all materials exhibit this property? The reason for that is that a for piezoelectricity to appear, the material must be crystalline but not have a symmetry center. Why, we shall soon see.
The piezoelectric effect originates in the rearrangement of net charges
The manifestation of the pressure-induced current has its origin in the rearrangement of charges within the material. In the equilibrium state, when the material is unloaded, the arrangement of charges within the material lattice is such that the unit cell is uncharged, Fig 1 A. However, when the material is mechanically deformed, there will be a charge redistribution within the unit cell. This redistribution will induce net charges on the faces of the unit cell and give rise to a net dipole moment, Fig 1B and 1C.
Figure 1. A schematic illustration of how piezoelectricity works. A) in equilibrium, the charges of the unit cell are distributed in such a way that there is no net dipole moment. B) When compressed, a net dipole moment arises in the vertical direction. C) when stretched, a net dipole moment arises in the horizontal direction.
The sum of the net charge contribution from all the unit cells will be an electrical polarization of the piece of material. This means that the applied mechanical force has induced a voltage over the material. For the polarization to happen, however, there must not be a symmetry center, Fig 2. As already mentioned, this requirement is only fulfilled by some materials.
Figure 2. A schematic illustration of the charge distribution in the unit cell of a material with a symmetry center. Both in the equilibrium state (A), and when mechanically deformed (B, C), the charge distribution is such that a net dipole moment never arises.
The opposite phenomenon, i.e. where the material changes shape as a voltage is applied over its faces, is called the converse piezoelectric effect, and this was not discovered until a year later, in 1881.
Piezoelectricity is widely used, and new applications await around the corner
Since the piezoelectric effect was discovered more than a century ago, it has spread into various applications and is now widely used. Areas include frequency control, in for example clocks, loudspeakers to generate sound, and microbalances, such as QCM, to monitor mass changes. But it doesn’t stop there. Now piezoelectricity is a candidate contributor to future sustainable energy supply. Light, wind and thermal have long been obvious sources, and thanks to the piezoelectric effect, vibration has now been added to the list. One way to use vibrational sources for power generation is, for example, to harvest human kinetic energy via street or pavement tiles. i.e. to convert the energy of footsteps to electricity. Energy generated this way could be used to run for example street lights or other low-voltage equipment in cities.
Piezoelectricity is a property of certain materials which induces an electric current when mechanical stressed. It is used in, for example, frequency control and current generation applications.
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