4.2 The piezoelectric effect at the atomic scale
It has been mentioned above that by changing the state of polarisation of a piezoelectric material we can generate movement, and vice versa. Let's examine a little more deeply what is meant by ‘state of polarisation’ and how we can maximise its effect to get the best out of electrically controlled micro-actuators.
In order to electrically polarise a material we need, by definition, to cause a separation of charges within the material. The more we can do this the greater the degree of polarisation. Materials that utilise ionic bonds (see Box 7) lend themselves well to this, as they are made up of positively and negatively charged atoms (ions) held together by strong forces of electrostatic attraction. Not all piezoelectric materials are ionically bonded – some polymers are piezoelectric – but the effect is stronger in ionically bonded materials, and because these are hard, they are capable of producing large forces.
However, ionic bonding isn't the only item on the shopping list of characteristics for a useful piezoelectric material. It is also important that some of the individual ions in the material are free to move (to some degree). This allows the material's polarisation to be changed by the application of an electric field across it. This movement of charge must be tethered, though, because if the charges (whether they be ions or electrons) had complete freedom to move through the material, it would be a conductor and we wouldn't be able to sustain much of a voltage across it. So let me summarise. In order to have a material that exhibits a strong piezoelectric effect, it would be desirable to have an ionically bonded material that is insulating, but that allows some ions to move over a short distance and thus polarise the material.
Box 7 Ionic bonds
There are many different ways in which atoms can bond together to form solids. One of the most common bonds occurs when one atom donates one or more electrons to another. An atom which has lost or gained one or more electrons (that is to say, the number of electrons does not match the number of protons in the atom's nucleus) is referred to as an ion and it is from this that ionic bonds take their name.
A common example of this is NaCl, sodium chloride (common table salt), where each atom of sodium donates an electron to an atom of chlorine. The extra electron from the sodium atom becomes tightly bound to the chlorine atom, while the remaining electrons stay very tightly bound to the sodium atom. The net effect is that the resulting chlorine ion carries a negative charge, and the sodium ion a positive charge; the two stick together because they are oppositely charged (see Figure 19).
Ionically bonded materials are among the best insulators available. Insulators insulate because there are no free charges within them that can move in response to an applied voltage. In ionically bound materials, the electrons are all tightly bonded to atoms.