KNN-based piezoelectric ceramic multilayer actuators with Ni internal electrodes

On the road of lead-free piezoelectric ceramics into practical applications, the study of Ni-internal-electrode (K, Na)NbO3-based (KNN-based) multilayer actuators (MLAs) is an important part, possessing the advantages of environmentally friendly and low cost. The Ni-internal-electrode KNN-based MLAs with different layer numbers and layer thicknesses were fabricated via the tape casting method and sintered in the reducing atmosphere. The piezoelectric layers consist of the main KNN-based phase and a trace amount of second phase Mn4Nb2O9. The element diffusion between the Ni electrodes and KNN-based grains is tiny, indicating that Ni is suitable for co-firing with KNN-based ceramics. After sintering, the compressive stress perpendicular to the thickness direction and the "relative tensile stress" parallel to the thickness direction are retained in the MLAs, bringing influences on the piezoelectric and dielectric properties of KNN-based materials. Compared with the bulk ceramics, the prepared MLAs significantly reduce the driving voltages and increase the displacement outputs, which are more applicable to the scenarios of miniaturization and portability. Particularly, the 46-layer actuator shows high displacements of 1580 and 2737 nm under the voltages of 100 and 200 V, respectively. However, the inverse piezoelectric coefficient d(33)* for each layer of MLAs is still lower than that of the bulk ceramics, indicating that the piezoelectric properties of the KNN-based materials are suppressed. To give full play to the piezoelectric properties of KNN-based materials, more attentions should be paid to the design of reasonable electrode structure and the development of internal electrode paste for MLAs.