Introduction
Since the discovery of piezoelectric phenomenon (conversion of mechanical energy into electrical energy) by Nobel Prize winners Pierre and Jacques Curie in 1880, the application of piezoelectric science has also flourished, and various uses have been developed and practically used in numerous fields. Piezoelectric products are common not only in our day-to-day life, but also in industrial, medical, automotive and national-defense sectors. These materials are often used as “sensors,” because they generate voltage signals when subjected to mechanical stress or incoming ultrasonic signals. Conversely, mechanical stress, or strain, created when an electric field is applied to piezoelectric materials, is used to create “actuators,” that is an application of the inverse piezoelectric effect.
The popularity of multilayer piezoelectric ceramics (PZT) rose rapidly soon after the driving voltage was able to be reduced to below 100V, which was achieved by using precise technology allowing PZT layers to be less than 100 µm thick. The advantages of multilayer actuators include: (1) reproducible displacement at low drive voltages (2) fast response time (sub-microseconds) (3) size miniaturization (4) precision control of displacement.
The principle of the multilayer piezoelectric actuator is to integrate multiple thin-layers of piezoelectric ceramic materials in a parallel manner. Figure a) shows a schematic diagram of a single-layer element structure, and Figure b) shows a schematic diagram of a multi-layer element structure
E=V /d ………………………….(1)
Strain=Δt/t= d33E ……………..(2)
Δh=(strain per unit field) (electric field) ×(device thickness)=d33(V/d) (nd)=d33Vn …………(3)
Therefore, the displacement of the multilayer actuator is proportional to the applied voltage (V), the number of piezoelectric layers (n), and the d33 (axial) piezoelectric coefficient of the material.
Compared with traditional single-layer piezoelectric elements, multi-layer piezo ceramics are smaller in size and can generate greater displacement and output at lower voltages. The characteristics are compared in the Table 1 below.
| Property | Single layer piezoceramic element | Multilayer piezoceramic element |
| Cost | lower | higher |
| Drive voltage | ~ hundreds of volts | ~ tens of volts |
| Practicality | poor (high voltage drive) | good (low voltage drive) |
| Precision of operating displacement | less applicable | high precision |
| Size | low flexibility | high flexibility |
| Shape | square, disc, ring, rectangular, tubular, etc. | |
Table 1: Properties comparison of single-layer and multi-layer piezoelectric ceramic components
Applications
Owing to the advantages mentioned above, multilayer piezoelectric ceramic components are widely used in positioning, precise focusing of optical systems, vibration feedback, and sensors, such as deformation & vibration control, health monitoring, high-precision displacement, micro-pumps, medical applications, in and out flow control, fuel injector systems, etc.
