Piezoelectricity is the ability of certain materials to generate an electric charge in response to mechanical stress. They also have the opposite effect – the application of electric voltage produces mechanical strain in the materials.
This effect makes piezoelectric materials effective in sensors and transducers used in the automotive and healthcare industries, and for environmental monitoring.
The word is derived from the Greek piezein, which means to squeeze or press.
Piezoelectricity is an established and safe technology.
What parameters are important in the manufacture of piezoelectric materials?
Lead-based bulk ceramics, such as the most commonly used Pb(Zr,Ti)O3 or PZT-based compositions, contain more than 60 mass % of lead. Lead is a toxic element, harmful to both people and the environment. Therefore manufacturing of PZT ceramics, by solid state synthesis involving calcining the mixture of constituent oxide powders and sintering of the resultant powder, should be performed with care. Evaporation of lead oxide during sintering may result in a loss of stoichiometry and consequent deterioration of functional properties and thus it should be reduced, usually by sintering in the presence of a packing powder.
What impact do manufacturing limitations have on the ability to integrate piezoelectric materials with others?
Miniaturisation of piezoelectric elements or devices may be achieved by integrating a piezoelectric ceramic in the form of a thick film on a conductive substrate, for example on platinized alumina. The thicknesses of a few µm to a few tens of µm are achieved by various techniques, such as screen printing or inject printing. Upon sintering of such heterostructures reactions at the interface of different materials may occur, and they can be reduced by introducing barrier layers or decreasing the sintering temperatures. Thermal expansion mismatch between the film and the substrate introduces stresses, which influence the structure and the functional properties of the films.
How can the electrical properties of piezoelectric materials be monitored?
A piezoelectric material transforms mechanical strain into voltage, the so-called direct piezoelectric effect. This is used in the energy-harvesting applications: we can step on, and, consequently, mechanically deform, a piezoelectric tile obtaining as a result a voltage generation that can be detected, for example, as light. This is also the principle of a Berlincourt type meter of the piezo-coefficient d33: quasi-static (i.e. with frequency of 100Hz or below) mechanical stress in a given direction causes a strain in the piezoelectric material that generates a voltage in the same direction, which measured and compared with the one generated by a reference material, gives us the piezoelectric coefficient. The so-called converse piezoelectric effect, the generation of a strain (expansion or compression) when the piezoelectric is subjected to an electric field of a given polarity, is used in the generation and detection of vibration in non-destructive testing by ultrasounds, industrial and medical, sonars, ultrasonic motors and a large range of applications. This effect is also used to monitor the piezoelectric properties. An alternating electric field produces a vibration in the piezoelectric that, depending on the frequency, can be detected as sound, as a pattern produced by laser interferometry due to the strain at the material surface or, when the vibration produces a resonance, by monitoring the complex impedance change of the material as a function of the frequency. The latter is the principle of the most widely used standards on measurement of the properties of piezoelectric materials.
Which piezoelectric material is capable of producing the highest voltage and most charge?
In the quasi-static regime, we can use the equations of piezoelectricity to estimate the voltage and charge delivered by a piezoelectric ceramic. In open circuit, the generated voltage is then proportional to the effective piezoelectric voltage constant (g), to the applied force amplitude and to the thickness of the piezoelectric ceramic. In short circuit, the generated charge is proportional to the effective piezoelectric strain constant (d) and the applied force amplitude. Thus a comparison can be made between different materials, looking only at the g and d piezoelectric constants. Of course, in experimental conditions, with a varying mechanical excitation, the peak value of the generated voltage depend on the rate of strain (and on the electrical load value) : a fast impulse mechanical excitation can generate an arbitrarily large current (or voltage).
In energy harvesting applications, which piezoelectric material is capable of producing the highest
electrical energy when submitted to a given mechanical vibration or deformation?
In quasi-static regime and in simple electrical load conditions, it is possible to compare differents piezoelectric materials, as in our response above. But in dynamic regime and with an arbitrary electrical load connected to the output of the piezoelectric device, the harvested power will depend on : the characteristics of the mechanical force (amplitude, frequency, rise time...) ; the type of piezoelectric harvester (bulk material, cantilever beam, nanowire composite...) ; the impedance of the electrical load. Some figures of merit (FoM) have been proposed a priori [Priya10], which differ from a posteriori figures of merit [Defosseux12, Andosca12] evaluated on real prototypes. These FoM can be used to make the best design choices as possible, in response to particular specifications.
[Andosca12] Sensors and Actuators A 178 (2012) 76 - 87
[Defosseux12] Sensors and Actuators A 188 (2012) 489 – 494
[Priya10] IEEE Trans. Ultrason. Ferroelectr. Freq. Control. 57 (2010) 2610 - 2612
Can piezoelectric materials be used in harsh environments, such as at high temperatures? Piezo materials, in particular piezo ceramics, are usually poled to exhibit the piezo effect, but can depole when heated above the maximum allowed operating temperature. There are, on the market place, high-voltage lead-zirconium-titanate cercamics, or other types, having a Curie temperature of 300°C which can be operated up to temperature as large as 150°C and in some cases up to 200°C; materials which can be employed into sensors to serve severe applications like monitoring engine’s pressure cycle if mounted into the harsh engine’s glowplug.
The Piezo Institute Members have written a number of useful authoritative texts covering piezo materials and technologies. Details may be found below.
About this book:
This book presents selected topics on processing and properties of ferroelectric materials that are currently the focus of attention in scientific and technical research.
Ferro-piezoelectric ceramics are key materials in devices for many applications, such as automotive, healthcare and non-destructive testing. As they are polycrystalline, non-centrosymmetric materials, their piezoelectricity is induced by the so-called poling process. This is based on the principle of polarization reversal by the action of an electric field that characterizes the ferroelectric materials.
This book was born with the aim of increasing the awareness of the multifunctionality of ferroelectric materials among different communities, such as researchers, electronic engineers, end-users and manufacturers, working on and with ferro-piezoelectric ceramic materials and devices which are based on them.
The initiative to write this book comes from a well-established group of researchers at the Laboratories of Ferroelectric Materials, Materials Science Institute of Madrid (ICMM-CSIC). This group has been working in different areas concerning thin films and bulk ceramic materials since the mid-1980s. It is a partner of the Network of Excellence on Multifunctional and Integrated Piezoelectric Devices (MIND) of the EC, in which the European Institute of Piezoelectric Materials and Devices has its origin.