Lead-Free (K0.5Na0.5)NbO3 Unmodified Piezoceramics: A Reduction in the Energetic Budget of Solid State Synthesis and Sintering

Abstract

Replacement of the ubiquitous lead titanate zirconate piezoceramics is still a challenge involving sustainable materials and processes. An extended effort has been paid for the optimization of the properties of (K0.5Na0.5)NbO3 (KNN) lead-free piezoceramics at the expenses of high-energetic budget. A sustainable solid-state processing with potential for industrial scaling (knowledge-based, sustainable, reproducible, and cost-effective) is still lacking. In this work, an efficient solution is proposed using the well-known and noncontaminant attrition-ball-milling of the mixture of reagents for only 3 h, prior to the synthesis tests at increasing temperatures. After calcination at only 450 °C/2 h, 60 wt% KNN is achieved. Moreover, after 850 °C/2 h the calcined powder becomes a pure KNN. A second ball-milling of powder calcined at 700850 °C gives place to populations of nanostructured particles (380–40 nm, respectively). The milled powder calcined at 700 °C is pressureless sintered in air for 2 h at 1050 and 1100 °C, leading to coarse grained ceramics. Milled powder calcined at 750 and 850 °C, sintered at the same conditions, produces fine grain homogeneous microstructures. Ceramics show an orthorhombic Amm2 pure KNN perovskite phase, as seen by x-ray diffraction and Rietveld analysis. The piezoelectric values (d33 = 115 pC/N, d31 = −22 pC/N, kp = 20%, kt = 40%) are exceptional when derived from such a straightforward solid-state route.