Piezoelectricity is an electric charge accumulating in certain solid materials in response to mechanical stress. Andy Pye reviews the materials and recent applications.
The piezoelectric effect was originally discovered in 1880 by French physicists Jacques and Pierre Curie, and before World War II, researchers discovered that certain ceramic materials could be made piezoelectric when subjected to a high polarising voltage, a process analogous to magnetising a ferrous material.
One of the first practical applications of the technology was made in the 1920s by another Frenchman, Langevin, who developed a quartz transmitter and receiver for underwater sound – the first sonar. But it was only in the 1950s that manufacturers begin to use the piezoelectric effect in industrial sensing applications.
Piezoelectric materials can convert mechanical vibrations into electric energy. Sensors using the piezoelectric effect can measure changes in pressure, sound, acceleration, temperature, strain, or force by converting them to an electrical charge. They can also be used to generate high voltages, electronic frequency generation, microbalances, to drive an ultrasonic nozzle, and ultrafine focusing of optical assemblies. It is the basis of scanning probe microscopies, yet also has everyday uses, such as acting as an ignition source for cigarette lighters, push-start propane barbecues, and in quartz watches. It has been successfully used in medical, aerospace, nuclear instrumentation, as a tilt sensor in consumer electronics or a pressure sensor in the touch pads of mobile phones.
Piezeoelectric sensors can be applied in places where a defined but not necessarily constant state of vibration exists. In the automotive industry, piezoelectric elements are used to monitor combustion in engines; here, the sensors are either directly mounted into additional holes into the cylinder head, or the spark/glow plug is equipped with a built-in miniature piezoelectric sensor. On the human body, they are used where blood pressure, breathing or heartbeat are constantly creating momentum.
Working with a device that slightly resembles a microscopically tiny tuning fork, researchers at the University of Tsukuba in Japan have recently developed coupled microcantilevers that can make mass measurements on the order of nanograms with only a 1% margin of error, potentially enabling the weighing of individual molecules in liquid environments.
There are three basic classes of piezoelectric materials used in microfabrication:
* natural piezoelectric substrates, such as quartz single crystals
* piezoelectric ceramics, such as lithium niobate, gallium arsenide, zinc oxide, aluminium nitride and lead zirconate-titanate (PZT)
* polymer-film piezoelectrics, such as polyvinylidene fluoride (PVDF).
PZT ceramic, the original piezoelectric material of choice, has a piezoelectric constant/sensitivity that is roughly two orders of magnitude higher than those of the natural single crystal materials and can be produced by inexpensive sintering processes. PZT materials are solid solutions of lead zirconate and lead titanate, often doped with other elements to obtain specific properties. The piezoeffect in piezoceramics is “trained” (known as poling), so their high sensitivity degrades over time. This degradation is highly correlated with increased temperature.
PZT sensors exhibit most of the characteristics of ceramics, namely a high elastic modulus, brittleness and low tensile strength. The material itself is mechanically isotropic, and by virtue of the poling process, is transversely isotropic as far as piezoelectric properties are concerned in the plane normal to the poling direction.
Aluminium nitride (AlN) is another option. Compared to PZT, it possesses more favourable mechanical properties, is lead-free, more stable and biocompatible. Moreover, it is virtually no problem to integrate AlN layers into conventional manufacturing processes for microelectronics.
The less-sensitive, natural, single-crystal materials (gallium phosphate, quartz, tourmaline) have a higher – when carefully handled, almost unlimited – long term stability. Gallium arsenide-based amplifiers and filters are already available on the market and this new discovery opens up new ways of integrating antennas on a chip along with other components. Working with the National Physical Laboratory and Cambridge-based dielectric antenna company Antenova, a team of Cambridge University researchers has found that, at a certain frequency, thin films of piezoelectric materials not only become efficient resonators, but efficient radiators as well, meaning that they can be used as aerials.
There are also new single-crystal materials commercially available such as Lead Magnesium Niobate-Lead Titanate (PMN-PT). These materials offer improved sensitivity over PZT but have a lower maximum operating temperature and are currently more expensive to manufacture.
PVDF polymer films
In 1969, very high piezo-activity was observed in the polarised fluoropolymer, polyvinylidene fluoride (PVDF). Piezo PVDF film is a flexible, lightweight, tough engineering plastic available in a wide variety of thicknesses and large areas. It has low density and excellent sensitivity, and is mechanically tough. The compliance of piezo film is 10 times greater than the compliance of ceramics. When extruded into thin film, piezoelectric polymers can be directly attached to a structure without disturbing its mechanical motion. Piezo film is well suited to strain sensing applications requiring very wide bandwidth and high sensitivity. As an actuator, the polymer’s low acoustic impedance permits the efficient transfer of a broadband of energy into air and other gases.
In spite of their lower piezoelectric coefficients, these characteristics make PVDF films more attractive than PZT ceramics for sensor applications. Piezoelectric polymer film sensors are among the fastest growing of the technologies within the $18 billion worldwide sensor market. Like any new technology, there have been an extraordinary number of applications considered for the sensor.
The Young’s modulus of PZT is comparable to that of aluminium, whereas that of PVDF is approximately 1/12th that of aluminium. It is therefore much more suited to sensing applications since it is less likely to influence the dynamics of the host structure as a result of its own stiffness. It is also very easy to shape PVDF film for any desired application.
Push buttons for keyboards, keypads, and control panels with small areas have been made with cellular piezoelectric polymer films.
A major advantage of piezo film over piezo ceramic is its low acoustic impedance, which is closer to that of water, human tissue and other organic materials. A close impedance match permits more efficient transduction of acoustic signals in water and tissue. For example, the acoustic impedance of piezo film is only 2.6 times that of water, whereas piezo ceramics are typically 11 times greater.
Piezo film does have some limitations for certain applications. Compared to ceramics, it makes a relatively weak electromechanical transmitter, particularly at resonance and in low frequency applications. Also, if the electrodes on the film are exposed, the sensor can be sensitive to electromagnetic radiation, though good shielding techniques are available for high EMI/RFI environments.
New copolymers of PVDF, developed over the last few years, have expanded the applications of piezoelectric polymer sensors. These copolymers permit use at higher temperatures (135C) and offer desirable new sensor shapes, like cylinders and hemispheres. Thickness extremes are possible with copolymer that cannot be readily attained with PVDF. These include ultra-thin (200Å) spin-cast coatings that enable new sensor-on-silicon applications, and cylinders with wall thicknesses in excess of 1200 microns for sonar. The copolymer film has a maximum operating/storage temperature of 135C, while PVDF is not recommended for use or storage above 100C.
ElectroMechanical Film (EMFi)
While other polymers, like nylon and PVC exhibit the effect, none are as highly piezoelectric as PVDF and its copolymers. An interesting recent development is EMFi, which is a thin polypropylene (PP) material having a special cellular structure. The internal structure of EMFi is made be stretching the PP preform during the manufacturing process in longitudinal and transversal directions. The film is charged with a corona discharge method using electric field strength locally exceeding the film dielectric strength. Finally, the film is coated with electrically conductive electrode layers.
The EMFi material consists of three layers: smooth and homogenous surface layers and a dominant, thicker mid-section. The mid-section is full of flat gas voids separated by leaf-like PP-layers. The voids can be compared to large electrical dipoles that are easily compressed in thickness direction by externally applied pressure.
In tests, the sensitivity of the EMFi sensor in the normal force direction has been found to be approximately five-fold when compared to the corresponding value of the PVDF sensor. The higher sensitivity of the EMFi material is mainly due to the internal voided structure. However, due to the relatively large gas voids and local corona breakdowns, sensitivity varies in different parts of the film.
Because the base material of EMFi is inexpensive PP, it is applicable also for large area sensors, like floor monitoring systems. Compared to the EMFi material, piezoelectric polymers usually contain fluoride, which is a potentially toxic substance. The main advantage of PVDF, however, is its sensitivity to forces related to its length and width. This property creates many versatile applications for the material: for example, it can be utilised also as a shear stress sensor.
Ceramic piezoelectric materials are brittle and vulnerable to accidental breakage. They have poor ability to conform to curved surfaces and are very dense and stiff. These limitations have encouraged researchers to develop alternative methods of manufacturing piezoelectric ceramics.
The idea of using a composite material consisting of an active piezoelectric ceramic fibrous phase embedded in a polymeric matrix phase has been investigated by a number of researchers. Piezo ceramic fibres have been produced through a patented injection moulding process. In addition to added strength, the flexibility of the polymer matrix allows the piezoelectric ceramic fibres to have greater conformability to curved surfaces and provides a protective shell around the piezoelectric material. The polymer shell also allows the piezoelectric-fibre to withstand impacts and harsh environments far better.
The procedure combines PZT powder with a wax-based binder, and then the material is granulated as feedstock for the injection moulding process. Once this is completed the feedstock is heated to the specified viscosity and rapidly injected at high pressure into a cooled mould. Due to the incompressible nature of the material, when injected into the mould the fibres obtain a constant density throughout and remove of voids and internal defects that occur during dry pressing or low pressure forming methods. The homogeneous density of the material produces uniform microstructures, dimensions and electromechanical properties after firing.
Table 1 Comparison of piezoelectric materials[table “4” could not be loaded /]