materials Article Processing Optimization and Toxicological Evaluation of “Lead-Free” Piezoceramics: A KNN-Based Case Study Antonio Iacomini 1, * , Juan Antonio Tamayo-Ramos 2 , Carlos Rumbo 2 , Irem Urgen 2,3 , Marzia Mureddu 1 , Gabriele Mulas 1 , Stefano Enzo 1 and Sebastiano Garroni 1   Citation: Iacomini, A.; Tamayo-Ramos, J.A.; Rumbo, C.; Urgen, I.; Mureddu, M.; Mulas, G.; Enzo, S.; Garroni, S. Processing Optimization and Toxicological Evaluation of “Lead-Free” Piezoceramics: A KNN-Based Case Study. Materials 2021, 14, 4337. https://doi.org/10.3390/ ma14154337 Academic Editor: Csaba Balázsi Received: 30 June 2021 Accepted: 28 July 2021 Published: 3 August 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 Department of Chemistry and Pharmacy, University of Sassari and INSTM, Via Vienna 2, 07100 Sassari, Italy; m.mureddu6@studenti.uniss.it (M.M.); mulas@uniss.it (G.M.); enzo@uniss.it (S.E.); sgarroni@uniss.it (S.G.) 2 International Research Centre in Critical Raw Materials—ICCRAM, Universidad de Burgos, Plaza Misael Bañuelos s/n, 09001 Burgos, Spain; jatramos@ubu.es (J.A.T.-R.); crumbo@ubu.es (C.R.); iremurgen93@gmail.com (I.U.) 3 Department of Metallurgical and Materials Engineering, Meselik Campus, Eskisehir Osmangazi University, 26480 Eskisehir, Turkey * Correspondence: aiacomini@uniss.it; Tel.: +39-079-229558 Abstract: Due to the ever-increasing limitations of the use of lead-based materials, the manufacturing of lead-free piezoceramics with competitive piezoelectric properties and established nontoxicity is considered a priority for the scientific and industrial community. In this work, a lead-free system based on sodium potassium niobate (KNN), opportunely modified with MgNb 2 O 6 (MN), was prepared through a combination of a mechanochemical activation method and air sintering, and its toxicity was evaluated. The effect of the mechanical processing on the microstructure refinement of the processed powders was established by X-ray diffraction and the average crystallite size content of the Nb 2 O 5 species was evaluated. The experimental evidence was rationalized using a phenomenological model which permitted us to obtain the amount of powder processed at each collision and to optimize the activation step of the pre-calcined reagents. This influenced the final density and piezoresponse of the as-sintered pellets, which showed optimal properties compared with other KNN systems. Their toxicological potential was evaluated through exposure experiments to the pulverized KNN-based pellets, employing two widely used human and environmental cellular models. The in vitro assays proved, under the selected conditions, the absence of cytotoxicity of KNN-bases systems here studied. Keywords: lead-free piezoceramics; sodium potassium niobate; processing; toxicity assays; X-ray diffraction; mechanochemistry 1. Introduction Nowadays, piezoceramics represent an undeniable class of functional materials that are largely exploited in a wide range of key technologies, including imaging probes for medical diagnostics, new generations of sonars and wireless admittance monitoring sys- tems [1]. With exceptional electromechanical properties and mild processing conditions, highly dense PZT (lead, zirconate, titanate), with a general formula Pb(Zr x Ti 1−x )O 3 , is the most commonly used compound in the manufacturing of piezoceramics-based devices [2–4]. However, despite the excellent properties exhibited, the use of highly toxic lead oxide as starting reagent has led the industrial and scientific community to address their efforts in the development of competitive alternatives composed of lead-free materials [5]. Among the many lead-free based candidates, KNN (sodium, potassium, niobate), with a general formula of K x Na 1−x NbO 3 , has received great interest in the recent past, due to its high d 33 value (390–400 pC/N) coupled with a high Curie temperature (217–304 ◦ C) [6–10]. These promising properties, combined with the ever-increasing restrictions adopted against the use of lead in the manufacturing of costumer objects, have promoted fervent research into Materials 2021, 14, 4337. https://doi.org/10.3390/ma14154337 https://www.mdpi.com/journal/materials