Not that recent
Although the first practical piezoelectric devices emerged little more than three decades ago they are becoming increasingly commonplace and can now be found in a diverse array of devices and applications. With new materials and designs constantly emerging, developments in piezoelectric technology focus not only on achieving more desirable operational characteristics but on improving environmental performance too.
First demonstrated by Pierre and Jacques Curie in the latter half of the 19th Century, the piezoelectric effect is a phenomenon in which certain crystalline materials generate an electrical charge when exposed to mechanical stress. Inversely, these types of materials exhibit dimensional change when presented with an electrical field.
This effect is linear: the greater the deformation, the higher the charge developed and vice versa. This means that piezoelectric materials are ideally suited to function as electromechanical transducers, such as those found in medical and industrial applications of ultrasonics (see article on ultrasonics in April 2014 e-tech). Today they are typically found in sensor systems and, increasingly, in energy harvesting applications (see article on energy harvesting in this e-tech).
Naturally occurring piezoelectric materials include quartz and tourmaline but manufactured ceramic materials such as barium titanate and the more commonly used lead zirconate titanates (PZT) are also produced.
These ceramics, so-called ferroelectric materials, can be rendered piezoelectric through a process of polarization by applying a strong electrical charge to the material, usually at an elevated temperature (2-3 kV/mm at 80°C – 140°C).
As with many ceramic materials, piezoelectric ceramics are hard, chemically inert and can be formed into almost any shape required. Mechanically they are similar to commonly found insulators and they are also impervious to atmospheric conditions.
Production of piezoelectric ceramics typically begins with a powder consisting of oxides of lead, titanium and zinc. This powder is then compacted into a mould before sintering at temperatures of 1 000°C – 1 300°C, which allows the material to develop its polycrystalline structure.
After finishing and polarization, characteristics such as capacitance and resonant frequency are determined by the dimensions of the product and the material used.
An alternative production method uses a process akin to printing – screen or pad – to deposit a layer of piezoelectric material on a substrate of the desired shape. According to its proponents, this process supports further miniaturization of piezoelectric devices, as well as enabling focused transducers to be developed using curved substrates.
Applications for piezoelectric materials
Piezoelectric materials are often employed in applications requiring measurement. Frequently these involve basic physical tenets such as force, acceleration or pressure. Piezoelectric transducers are ideal for converting such qualities into electrical signals.
Given their suitability as electromechanical transducers, it’s no surprise to see such materials in numerous sensor applications such as those found in the ultrasonic measurement of distance in air – as exemplified by aids in your car that help you to reverse – but also in materials-testing equipment, in accelerometers and pressure sensors, and in medical equipment. They are also employed in spark generators, for example those used in an electronic ignition cigarette lighter.
There are two main types of sensor: axial and bending. In axial sensors the force is applied parallel to the direction of polarization (known as d33 mode), while in bending sensors the force is applied perpendicular to the polarization (so-called d31 mode).
A number of other applications require displacements beyond what is possible with transducers operating in d33 or d31 mode. In such cases a flexure or cantilever element such as a bimorph – two bonded strips of PZT – can be used.
Piezoelectric ceramics fall into two broad categories: hard and soft. So-called ‘hard’ ceramics are capable of handling high levels of electrical excitation and mechanical stress and are suitable for use in high voltage or high power applications. Soft ceramics display high sensitivity and permittivity (i.e. level of resistance encountered when forming an electric field in the material) but are vulnerable when heated beyond their operating range under high power conditions. These soft ceramics are typically found in low power applications such as sensors, receivers and low power generators.
Exploiting the inverse piezoelectric effect these materials are also found in atomizers, cleaning equipment and as low-power actuators. Piezoelectric motors are unaffected by energy efficiency losses that limit the miniaturization of electromagnetic motors and do not produce electromagnetic noise. Piezoelectric actuators may therefore be employed in controlling hydraulic valves or acting as small-volume pumps.
One key area of piezoelectric materials development is focused on new applications and new materials to improve sensitivity, durability and operational performance. For example, a UK manufacturer launched a new range of air in-line and occlusion sensors for the medical sector in September last year.
Capable of delivering non-invasive air bubble detection and measuring pressure changes in tubes leading into the body, the devices provide a precise means of monitoring safety-critical events in medical products such as infusion pumps, dialysis equipment and other fluid-handling applications.
The company says the technology has been developed in response to the increasingly large variations in tube sizes and materials used in the medical market for drug delivery and fluid management.
The development followed the March 2013 launch of PZT5K1 a new high density and low porosity piezoelectric material by the company suitable for applications in medical instrumentation and energy harvesting, among others.
Some of the new materials that are being considered for piezoelectric ceramics are ones that do not contain lead, which present problems on account of its toxicity as well as potential challenges associated with its final disposal. A manufacturer explains that this advance is likely to occur within the decade, but warns that the performance of lead-free materials is "not yet anywhere near where it needs to be in terms of sensitivity".
Setting standards and upcoming challenges
Within the IEC, most International Standards for piezoelectric technology, with the exception of those for piezoelectric transducers, which are prepared by TC 29: Electroacoustics, and TC 87: Ultrasonics, are developed by TC 49: Piezoelectric, dielectric and electrostatic devices and associated materials for frequency control, selection and detection.
It is clear that piezoelectric materials are becoming more diverse, more sophisticated and more effective. International Standards will evolve to ensure further advances in the piezoelectric domain.