Piezoelectric materials have the capability to convert electrical energy into mechanical energy, and vice versa. Piezo stems from the ancient Greek πιέζειν (piezein), which means ‘to press’ or ‘to squeeze’.
Piezoelectric materials are among these ‘invisible’ materials that are widespread around us, although they are unknown to the public at large. Mobile phones, automotive electronics, medical technology and industrial systems are only a few areas where ‘piezo’ is indispensable. Echoes to capture the image of an unborn baby in a womb make use of piezo. Even in a parking sensor at the back of our car piezo is present.
Convert signals into action – and vice versa
What makes ‘piezo’ a phenomenon that can be applied so abundantly? Well, it is the nature of the material itself: it has the capability to change shape – for example become shorter or wider – by applying an electric voltage over it. This change in shape is not very big – generally in the micron range – but it occurs very fast, within milliseconds. Furthermore it is highly reproducible, and accurate in the nanometer range. Piezo also works the other way around: compressing or otherwise deforming the material generates an electric charge. So the piezo material is a smart system in itself.
Piezoelectric actuators – devices that convert an electrical signal into an ‘action’ such as a physical displacement – play an important role in high tech systems, and also in high tech manufacturing technology. As do piezoelectric sensors – devices that convert a mechanical action into an electrical signal.
Due to the fact that piezoelectric materials are able to change in size very accurately, they can be found in various applications. Not only in inkjet printers, but also in loudspeakers, for example. Furthermore, since piezoelectric materials can position objects in an extremely accurate way, they are applied in scanning tunneling microscopes (STM) to keep the needle close to the sample, or in wafer steppers for making integrated circuits (ICs).
The basics of piezoelectricity
Let’s have a closer look at the interaction between the electrical and the mechanical state in piezo materials, taking the most commonly used piezoelectric material PZT as an example. PZT is an abbreviation of lead-zirconate-titanate, which is a ceramic oxide where the crystal lattice consists of lead (Pb), zirconium (Zr), titanium (Ti) and oxygen (O) ions. Let’s zoom in to the microscopic scale and consider how the individual atoms are arranged in a periodic lattice, i.e. in a crystal.
Below the so-called Curie temperature, PZT has a tetragonal crystal structure. Positively charged Pb ions are positioned on the corners of the tetragonal unit cell – which is in fact a cube that is extended in one direction – whereas negative O ions are positioned at the centres of the unit cell’s faces. The position of the positively charged Zr and Ti ions – one Zr or Ti ion for each unit cell – is somewhat in the centre of the unit cell.
If the PZT lattice would be essentially cubic, these Zr/Ti ions would be exactly in the cell’s centre. However, as the unit cell is elongated in one direction, the favourable and stable position of the Zr/Ti ion deviates somewhat from this central position. Due to this shift of the Zr/Ti ion’s position, which is not equal to the centre of charge for the negatively charged O ions, PZT has a natural, permanent electric dipole. While the unit cell itself is electrically neutral, it has two charged ‘poles’ inside – a positive pole at the Zr/Ti position, and a negative pole at the oxygen ions centre of charge – which together form a dipole. The existence of this built-in dipole is essential for the material’s piezoelectric behaviour.
An external electric field that is applied to this cell pushes the positive ions (such as the Zr/Ti ions) in the direction of that field, and pulls the negative ions in the opposite direction. As the spacing between the opposite charged ions is changed, so is the shape of the unit cell and, hence, of the entire material. This is known as the inverse piezoelectric effect.
The response of the piezoelectric material to the electric field is virtually instantaneous. This is especially relevant when such a material is placed in an oscillating (AC) field that drives the charges between two alternative configurations. A response to a megahertz field (106 s-1) occurs in microseconds (10-6 s).
To get some feeling of the deformation of piezo materials: it can’t be observed by the naked eye, as a PZT material will change only 0.1 % of its original dimension when an external electric field is applied to it. Although it is a small effect, it is a linear effect and that is ideal for positioning a probe with sub-micron or nanoscale precision as in a scanning tunneling microscope.
The direct piezoelectric effect works the other way around. By pressing or squeezing the PZT, the ions move relative to each other. As a result the size of the dipole changes, as well as the charge on the surface of the material. Direct and inverse piezoelectricity is reversible, with only minimal energy losses.
Echoscopy – seeing by (not) hearing
Echoscopy, also known as ultrasound imaging, is a typical example of the use of piezoelectric materials. Echoscopy has gained an important position within medical diagnostics. Ultrasound – sound just beyond what can be heard by the human ear – is used to see what is inside the body. A gynaecologist uses echoscopy to see how an unborn baby develops in a womb, and a cardiologist uses this technique to detect a leaking heart valve or to determine the flow velocity in blood vessels. Echoscopy normally uses a small, handy-size device that is being moved over the human skin to see what is inside the body, so this non-invasive technique is painless and easy to use.
The echoscopic device sends sound waves with a frequency in the megahertz range to a part of the body that has to be investigated. This ultrasound has the ability to penetrate liquids and soft body tissues. At the interface between soft and hard tissue, for example the edge of an organ, these ultrasound waves are being reflected to some extent, giving spatial information about the internal body parts. This reflection or ‘echo’ is then caught by the device and visualised on a monitor using imaging software. The time span between emitting and receiving the sound waves is characteristic for the distance to the reflecting body tissue, as the speed of sound in the human body is known. Since the difference between the properties of body tissue is often small, the receiving part of the device has to be very sensitive.
Within the echoscopic device, piezoelectric transducers produce the ultrasound waves by transforming high frequent AC voltage into sound. Sensors receive the returning sound waves and transform them into an electrical signal for further processing into an image. In practice within the compact echoscopic devices, sending and receiving occurs with the same component, where the returning sound waves are received in the ‘breaks’ between the emitted sound waves.