Multilayer piezoelectric actuators - structures and reliability

Piezoelectric and electrostrictive ceramic multilayer actuators (MLAs) offer a combination of large generative force, quick response time, high volumetric efficiency, and low cost while utilizing relatively simple E-field control. A stack of ceramic disks is one form of multilayer actuator which provides a relatively reliable device, while limitations of disk thickness restrict driving voltages. Co-fired MLAs fabricated using conventional multilayer ceramic capacitor processing are essential for ultimate large scale production and low cost. Four types of internal electrode configurations for co-fired MLAs have been developed in the course of time in reference to performance and reliability. MLC type electrode configurations offer layer thicknesses less than 25 μm and thus driving voltage of only approximately 20V, but high stresses developed as a result of inactive regions may lead to failure. Plate-through multilayer actuators eliminate internal stress concentration but introduce difficulties in controlling electrode silver migration. Consequently, MLC electrode structures with slits or gaps have been introduced to overcome the above problems, though adding complexity to the process. A new interdigital line electrode configuration offers large displacement through excitation of gaps between fine electrode lines, though reintroducing non-uniform field strength in a system. In addition to the structural differences, materials choice is an important issue for each application and improvement of reliability.