Lead-Free KNN-based Piezoelectric Ceramics

In many commercial devices, there are a number of ways in which energy is wasted or dissipated. This waste energy can, in principle, be harvested by using the correct functional material. In the case of vibrational energy, the logical materials of choice are piezoelectric ceramics. However, all current commercial piezoelectrics contain lead oxide which is classed as a restricted material in environmental legislation. The main contenders for lead free piezoelectrics are based on K0.5Na0.5NbO3 (KNN) and Na0.5Bi0.5TiO3. The former however, has the advantage in that it is compatible with cheap Ni-based internal electrodes and thus it is feasible to manufacture low cost KNN based multilayer devices provided formulations do not deteriorate in the reducing condition required to suppress the formation of NiO. Consequently, KNN based lead free piezoelectric ceramics have been studied from the perspective of optimising their performance for multilayer actuators, potentially for energy harvesting applications. To this end, the defect chemistry of KNN has been investigated under different sintering conditions, dopants (acceptors: Mn2+, Ti4+, Sn4+ in KNN_50/50 ratio; Donor: Sr2+ in KNN_50/50 ratio; Ta5+ as an isovalent in KNN-51/49 ratio; and co-dopants: Bi3+ and Zr4+ in KNN_50/50 ratio) have been incorporated into KNN to enhance the piezoelectric performance and prototype multilayers of 10 and 16 layers with inner Pt electrodes have been fabricated to demonstrate the potential of 0.942KNN-0.058BNZ+ZrO2 for the fabrication of multilayer actuators. This lead free composition has the potential to replace PZT-4 and PZT-8 in piezoelectric devices for room temperature applications. To fabricate the multilayers, a novel Wet-Multilayer-Method (WMM) was also developed to overcome the issues of delamination during firing of MLCCs.