Heterogeneity Challenges in Multiple-Element-Modified Lead-Free Piezoelectric Ceramics

Abstract

We report on a heterogeneity study, down to the atomic scale, on a representative multiple-element-modified ceramic based on potassium sodium niobate (KNN): 0.95(Na0.49K0.49Li0.02)(Nb0.8Ta0.2)O3–0.05CaZrO3 with 2 wt % MnO2. We show that different routes for incorporating the MnO2 (either before or after the calcination step) affect the phase composition and finally the functionality of the material. According to X-ray diffraction and scanning electron microscopy analyses, the ceramics consist of orthorhombic and tetragonal perovskite phases together with a small amount of Mn-rich secondary phase. The addition of MnO2 after the calcination results in better piezoelectric properties, corresponding to a ratio between the orthorhombic and tetragonal perovskite phases that is closer to unity. We also show, using microscopy techniques combined with analytical tools, that Zr-rich, Ta-rich and Mn-rich segregations are present on the nano and atomic levels. With this multi-scale analysis approach, we demonstrate that the functional properties are sensitive to minor modifications in the synthesis route, and consequently to different material properties on all scales. We believe that detecting and learning how to control these modifications will be a step forward in overcoming the irreproducibility problems with KNN-based materials.

Screen Printed Copper and Tantalum Modified Potassium Sodium Niobate Thick Films on Platinized Alumina Substrates

“…Note that the perovskite diffraction peaks in all thick films are slightly down-shifted compared to the ceramic. The larger perovskite lattice parameters in thick films compared to ceramic could be a consequence of several interdependent factors, such as different incorporation of Ta and Cu into the perovskite due to the different sintering temperature of films and ceramic [ 34 , 35 ], formation of O-vacancies [ 36 ] and altered K/Na ratio due to alkali losses [ 6 , 37 ].…”