Recently, Prof. Faxin Li’s group at Department of Mechanics and Engineering Science, College of Engineering, made a breakthrough in the field of piezoelectric materials. They proposed a novel periodically orthogonal poling method for piezoelectric ceramics, which turns the nonlinear switching strain to be reversible and realized a giant actuation strain of 0.6% in the most widely used PZT ceramics, about 4 times of that in conventional PZT. This work was published as a Featured Article on Journal of Applied Physics ( http://dx.doi.org/10.1063/1.4997940?. The AIP Scilight has post a special report with the title "Novel poling method in piezoelectric ceramics opens the door to the next-generation large-strain actuators" to introduce this work. ( http://scitation.aip.org/content/aip/journal/sci/2017/9/10.1063/1.5000153 ).
Piezoelectric ceramic based actuator has always been dominant in the Actuation area due to its quick response, high displacement resolution, electrical control, compact size, etc. The currently used piezoelectric actuators were mainly made of lead titanate zirconate (PZT) ceramics, which had been used in industries for over 50 years and its properties can be tailored in a wide range by doping. However, the typical actuation strain of PZT ceramics is only about 0.1-0.15%, which is much smaller than that of shape memory alloys, SMA (about 2-5%) and also smaller than that of giant magnetostrictive materials (Terfenol-D, about 0.2%).
In recent years, scholars had developed several methods to enhance the actuation strain of piezoelectric materials, which can mainly be classified to three types: 1) Developing relaxor ferroelectric crystals (such as PMN-PT, PZN-PT), enhancing the electrostrain via field induced transitions; 2) Developing lead-free piezoelectric ceramics, also enhancing the electrostrain via field induced transitions; 3) Realizing large electrostrain via reversible ferroelastic domain switching. However, all these methods cannot be applied in industries yet because of poor stability, large hysteresis, large field or large prestress required.
Enlightened from the actuation principle of shape memory alloys via the two-way shape memory effect, Prof. Faxin Li thinks that the reversible ferroelastic domain switching can also be controlled by the internal stresses in piezoelectric ceramics. However, the thermal-mechanical training process used in SMA cannot be extended to the brittle piezoelectric ceramics. After deep thinking, Prof. Faxin Li proposed a periodically orthogonal poling method for piezoelectric ceramics and realized the reversible domain switching using the interfacial stresses between the adjacent regions with different poling directions, leading to a giant actuation strain of about 0.6%. Furthermore, the giant actuation strain is very stable, which did not decrease but slightly increase after 10000 cycles of operation. Using an intersecting multi-layer design, the maximum actuation strain can always be employed in a piezoelectric stack actuator. The proposed periodically orghogonal poled PZT ceramics can be a promising candidate for the next-generation large-strain actuators.
Figure 1 Actuation strains in periodically orthogonal poled PZT and conventional PZT ceramics