4 Piezoelectricity: motion from crystals
4.1 The piezoelectric effect
The phenomenon of piezoelectricity was first predicted and demonstrated in the late nineteenth century using naturally occurring materials. It has a vast number of applications, ranging from spark ignitors to inkjet printers. It is also utilised in timing circuits, where an oscillating electric field is used to make a quartz crystal resonate at its natural frequency. In MEMS, the effect is used to generate small-scale movements in a range of devices known as micro-actuators.
The effect is the direct interconversion of mechanical and electrical energy in a material. Only a few materials do this to an appreciable extent. Their response can be summarised briefly by saying that they change their physical shape when electrically stressed (by having a voltage applied across them); and they change their distribution of electrical charge (commonly known as their state of polarisation – I'll come to this later) when mechanically stressed (by being squeezed, bent, or pulled). The material's electrical response to mechanical stress is known as the ‘generator’ effect, and it is this that is described by the word ‘piezoelectric’ – the prefix ‘piezo-’ is from the Greek piezein, to press.
The generator effect is exploited in the spark ignitor. Here, a crystal is sufficiently stressed to generate a voltage across it large enough to make the air (between electrodes attached to it) break down. This creates a spark and is used for lighting stoves the world over. Conversely, a piezoelectric material will change its shape when its ambient electric field changes and this is known as the ‘motor’ effect. It is this effect that is deployed in some inkjet printers. A piezoelectric crystal is used as a piston to launch micro-droplets of ink across the short gap between the print head and the paper. Ink drops can be expelled very quickly (at a rate of tens of kHz) and at high velocity (several m s−1) enabling fast, accurate printing on the page. Figure 18 shows the effects.
This ‘motor’ effect then gives us the possibility of making something move, merely by the application of an electric field. That is to say, we can transduce an electrical signal into physical movement.