Nuts and Volts - November 2010

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PIEZOELECTRIC FILM

By John Iovine

The word piezo is derived from the Greek word piezein, meaning to press tight or squeeze. Add the second term electric and you’ve got squeezed electricity.

Piezoelectric materials develop a momentary electric potential (voltage) when the material is placed under a mechanical strain. Inversely, piezoelectric materials will strain internally or change shape dimensionally (slightly) when placed under a suitable electrical field. (See Figure 1.)

■ Completed circuit board connected to audio source and piezoelectric film speaker.

Naturally occurring piezoelectric materials are crystals. The piezoelectric effect was discovered by Pierre and Jacques Curie. In 1880, the Curie brothers published a paper on the piezoelectric phenomena and crystallographic structure of tourmaline, quartz, topaz, cane sugar, and Rochelle salt.

Since piezoelectric materials only develop momentary electrical potentials, in order for the electric potential generated to power an electrical device, the mechanical strain must oscillate to provide continual power to the device.

Figure 2 illustrates what the electrical response of a finger tap on piezoelectric film would look like on an oscilloscope. The initial pulse rapidly decays while oscillating.

Piezoelectric Film

In this article, we will experiment with a unique piezoelectric material: piezoelectric film. Piezoelectric film is made from polyvinylidene fluoride (PVDF) which is a non-reactive thermoplastic. The piezoelectric effect in PVDF material was first observed in 1969 by Kawai.

To make PVDF piezoelectric, the material must become anisotropic meaning the molecular structure of the plastic compound must be aligned to create a piezoelectric effect.

This is achieved using two processes during manufacturing. The heated PVDF material is extruded and stretched in the length direction into paper thin material.

This stretching causes some alignment of the internal plastic dipoles. To enhance the alignment of the dipoles, a high voltage “poling” voltage is applied to the surfaces of the plastic sheet material. This aligns most of the dipoles in the thickness direction that is perpendicular to the surfaces (see Figure 3).