Lead‐Free (K 0.5 Na 0.5 )NbO 3 Unmodified Piezoceramics: A Reduction in the Energetic Budget of Solid State Synthesis and Sintering

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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 (K 0.5 Na 0.5 )NbO 3 (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 ( d 33 = 115 pC/N, d 31 = − 22 pC/N, k p = 20%, k t = 40%) are exceptional when derived from such a straightforward solid‐state route.

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