Piezoelectric Actuator Materials
Below the so called Curie temperature Tc (see Table 1) the ionic lattice structure in the PZT crystallites becomes distorted and asymmetric (with an axis of polarity) and, additionally, exhibits spontaneous polarization. One result is that the discrete PZT crystallites become piezoelectric. However, the statistical distribution of the grain orientations in the ceramic will cause the macroscopic behavior to be nonpiezoelectric.
An additional property, the ferroelectric nature of the PZT material, will help to solve this problem. When an intense electric field is applied to the ceramic, the different lattice orientations of the individual ceramic grains can be permanently altered. As a result of this ‘‘poling process’’ the ceramic is accorded a net orientation of its internal, spontaneous polarization in the direction of the poling field and shows an overall piezoelectric effect. For some PZT ceramics, it is necessary to perform the poling process at elevated temperatures.
Table 1 shows the specifications of different PI Ceramic PZT materials.
The types PIC 151 and PIC 255 are the PI Ceramic standard actuator materials which are used for the PICA™-Stack and PICA™-Power actuators. These materials show the highest piezoelectric deformation constants, d33, d31 and d15 (see Table 1) and, consequently, the largest induced strain values at comparable fields. These compositions incorporate all our long-term experience in piezoelectronic actuator development, manufacturing and application.
PIC 151 is a modified lead zirconate titanate (PZT) ceramic with high permittivity, coupling factor and charge constant. It is thus well-suited for PICA™-Stack actuators and bender applications. Due to the high coupling factor and the low mechanical quality factor it is also recommended for low fregquency and pulsed ultrasonic applictions.
PIC 255 is a modified lead zirconate titanate (PZT) with a high curie temperature, coupling factor and charge constant. The material is optimized for actuator application under dynamic or high-temperature working conditions. Because of its high coercive field, PIC 255 can be used for bipolar-driving-mode applications as well as for PICA™-Shear actuators. Due to its high coupling efficiency, low mechanical quality factor and low temperature coefficient, it is also well suited low-power ultrasonic transducers, non-resonant boardband devices, sensors for load and sound transducers and is preferred for vacuum applications.
PIC 252 is a low-sintering modification of PIC 255, especially used for multilayer actuators. It is recommended for dynamic and/or high-temperature operating conditions due to its high curie temperature and low permittivity. This material will replace the currently used ceramic type in the near future.
Polymer coated stacked actuator (PICA™-Stack) and ceramic insulated cofired actuator (PICMA®)
Comparison of a long-travel, high-load piezoelectric actuator and a compact actuator for small loads
PICMA® and PICA™: Cofired and Stacked Piezoelectric Actuators
Two main types of piezo actuators are available: cofired PICMA® actuators requiring about 120 volts for full motion, and glued PICA™-Stack actuators, requring up to 1000 volts for full extension.
The maximum electrical field which can be recommended for reliable operation of PZT ceramics is on the order of 1 to 2 kV/mm. To keep the operating voltage within practical limits, actuators consist of thin layers of electroactive ceramic material which are electrically connected in parallel. The net positive displacement is the sum of the displacements of the individual layers. The thickness of the individual layers determines the maximum operating voltage of the actuator.
Glued PICA™-Stack piezoelectric actuators consist of separate ceramic discs with a thickness of 0.2 to 1.0 mm. These values, which are limited by the manufacturing technology, result in nominal driving voltages of up to 1000 V. In contrast, PICMA® actuators are manufactured using a cofiring technology. This advanced process allow for multilayer designs which have individual layer thicknesses of just 20 to 100 µm. Hence PICMA® actuators require nominal voltages of only 40 to 200 V.
Both types of piezoelectric actuators can be used for many applications: PICMA® actuators facilitate drive electronics design and can be produced at reasonable costs in standard sizes and large quantities. Due to its manufacturing technology, PICA™-Stack actuators can be designed with larger cross-sections for high-load applications. They can easily lift weights of up to several tons. Additionally, the PICA&tade;-Stack technology is very flexible in terms of special actuator shapes and sizes.
Layers in a stacked piezoelectric actuator
Layers in cofired monolithic piezoelectric actuator