It's amazing what can be produced from materials, given the right processing route. Brittle ceramics or cement can be transformed into strong, tough materials in a variety of unexpected shapes. Ceramic springs are a good example of what is possible. Such springs have been around as scientific novelty items for nearly two decades, produced by a special processing technology. An early example, made at ICI in the early 1980s, is the cement spring, which achieved celebrity status.
The Functional Materials Group at the University of Birmingham (UK) is carrying out speculative research work on ceramic springs and has arrived at some exciting potential applications for these devices, until now regarded merely as an interesting peculiarity. If a piezoelectric ceramic material such as PZT (lead zirconate titanate) is used to form a spring, all sorts of useful things start to become possible, including a revolutionary new method for making audio speakers. Such ideas and devices all rely on some clever ceramics processing and the phenomenon of piezoelectricity.
Viscous Polymer Processing VPP
The ICI industrial research group responsible for the cement spring also made other ceramic springs as a by-product of research into what became known as viscous polymer processing, or VPP. This technique creates a very viscous plastacine - like material, composed of fine ceramic powder particles dispersed in a polymer and solvent gel structure. The material is formed using very high shear forces to break apart any agglomerates in the powder, which are the main cause of defects in brittle materials such as cement and sintered ceramics.
This results in dense, very uniform and mechanically strong ceramics. Viscous processed materials can be typically twice as strong as those processed conventionally. In addition, because the material is highly plastic in its `green' formable state, products of all shapes can be made, while retaining the advantages of high strength and easy processing. Springs of various forms are ideal for demonstrating this process, as a spring structure can be visibly highly stressed in a uniform way, usually by hand. The sight of a ceramic material being elastically deformed to an apparently large degree changes many people's view of the capabilities of ceramics.
In addition to VPP, piezoelectricity is a key factor in the new work at Birmingham. Piezoelectricity is an electromechanical phenomenon common to many different materials, but usually only when they are in single crystal form. The property is related to the alignment of atoms in a crystalline lattice, which results in a net polarisation and a consequent link between electric field and material strain. The most widely known piezoelectric material is quartz, slivers of which are found in every digital watch and computer, producing very stable vibrations at high frequencies.
However, the most common piezoelectrics are based on polycrystalline ceramics. After conventional ceramic processing these materials can be “poled” to imitate the single crystal effect - usually to a much greater degree. The poling process, analogous to that used in magnetic materials, uses a high electric field and an elevated temperature to align the microscopically small piezoelectric domains within the polycrystalline material. This produces a net polarisation in the direction of the field. The residual polarisation after removal of the field allows smaller electric fields to produce mechanical strains in the direction of poling.
Applying an electric field across a piece of piezoelectric material will cause a slight corresponding strain to be developed in the direction of the field. Conversely, applying a strain causes an electric field to be set up across the material. This is illustrated for a simple cylindrical piezoelectric device in figure 1. The strains involved in the operation of a typical device are very small, generally no larger than 0.1%.
Such solid blocks are the basis for piezoelectric gas igniters - a large impact on a block generates a voltage sufficient to create a spark. There are also many uses in high frequency applications such as ultrasonic probes, in which the displacements involved are inherently very small, or for extremely accurate micro- and nano-positioning, such as for atomic force microscopy.
Figure 1. When a voltage is applied across a poled electroded piezoelectric device the material expands in the direction of the field and contracts perpendicular to the field. When a force is applied to the piezoelectric, an electric field is generated.
Piezo-Sounders in Smoke Alarms
Amplification techniques can be used to get more apparent strain out of the materials. The most common device used to amplify the effect is the Unimorph (Figure 2.) This is a plate of piezoelectric material stuck to a metal disc. If the piezoelectric layer attempts to contract or expand under the electric field, the disc flexes, producing movement of the centre of the disc relative to the edge.
This is how the piezo-sounder works, an extremely widely used ceramic piezoelectric device found in telephones, smoke alarms, high frequency tweeters' and many other items.
Figure 2. A “Unimorph” bending device – when a thin disc of piezoelectric is bonded to a metal substrate, applying a voltage causes the device to bend and produces amplified movement or sound in the direction perpendicular to the plate. The lower graphic shows a “Bimorph” bender – making a sandwich structure with an internal electrode allows one half to push while the other pulls, giving twice the movement of a Unimorph
Video Read Heads
For more technical applications, piezoelectric Bimorphs are made (figure 2). These are mirrored Unimorphs, in which the electrodes are engineered so that while one layer pushes, the other pulls, causing an increase in bending. These are used in applications in which more movement is needed but with small forces. The most common application is the video read head adjuster, which eliminates interference when freeze-framing a video recording.
Piezoelectric Springs and Loudspeakers
For extra actuation, things start to get difficult with ceramic piezoelectrics. Making a longer bender device does allow for increased actuation, but at the expense of reduced forces and increased fragility. A possible way round the problem is to create a more flexible structure such as a spring.
Before thinking of springs as actuators, it is easier to look at a piezoelectric spring generating electrical signals, as opposed to movement. Springs like this are formed in the green ceramic state by winding an extruded tube on to a former of the required diameter and pitch. The spring is then sintered on a refractory ceramic former of a size that allows for sintering shrinkage.
Applying electrodes to the former and outer surfaces of the tube and poling the material in the radial direction produces a vibration-sensing device. As the spring is very flexible, the resonance can be tuned to frequencies as low as 10Hz, with an appropriate seismic mass. The device then becomes a competitor to the more conventional electromagnetic geophone. The actuation effect is not great, and the spring cannot provide much in the way of force or displacement, unless at resonance.
With the correct electrode configuration, displacements as large as several millimetres can be achieved from such spring. However, this type of device is also inefficient, though is a big improvement on the simple spring form. This is because the material between the electrodes is not all activated to the same degree, and the material underneath the electrodes simply hinders movement.
So what kind of piezoelectric spring can generate the sort of movements that may be useful? One potentially groundbreaking device is a revolutionary new digital loudspeaker proposed by a new company 1…Limited. The loudspeaker is made up of an array of small, identical, independently driven transducers, each of which has a piezoelectric spring inside (Figure 3.). The active part of the transducer is in the form of a bender, similar to a bimorph but coiled round an inner core containing a bearing and moveable core.
Figure 3. A cut-away diagram of the 1…Limited digital loudspeaker driver unit.
When actuated, the bender attempts to coil or uncoil, which affects the pressure inside the linear bearing. If this coiling is made to be greater at one end than the other, or if the internal core is tapered, the core will be forced to move along the axis of the coil in line with the applied voltage. The action is, similar to that produced by a hand squeezing a wet bar of soap - the harder you squeeze, the faster the soap shoots out. The key thing is that a small movement in the `squeeze' produces a much larger movement in the core. This movement generates movement of the air next to it, and therefore produces noise. Through a trick of human audio perception, an array of these devices produces a coherent sound image away from the speaker. This is in some ways analogous to a television screen, made up of many tiny elements which in themselves are meaningless but when put together create an image.
From a laboratory curiosity with no apparent applications, piezoelectric ceramic springs have become an exciting new interdisciplinary research area. These structures show great potential for new types of actuators and sensors, and these springs, in their various forms, could greatly expand the realm of piezoelectric materials and make them an even more integral part of modem technology.