Ultrasonic transducers based on undoped lead-free (K 0.5 Na 0.5 )NbO 3 ceramics Micka Bah a , Fabien Giovannelli a , Frederic Schoenstein b , Christophe Brosseau a , Jean-Robert Deschamps a , Frédéric Dorvaux a , Lionel Haumesser a , Emmanuel Le Clezio c , Isabelle Monot-Laffez a,⇑ a Université François Rabelais de Tours, CNRS, CEA, INSA CVL, GREMAN UMR 7347, IUT de BLOIS 15 rue de la chocolaterie, CS 32903, 41029 Blois Cedex, France b Laboratoire des Sciences des Procédés et des Matériaux, CNRS, LSPM–UPR 3407, Université Paris 13, Sorbonne Paris Cité, 99 Avenue J.B. Clément, 93430 Villetaneuse, France c Université de Montpellier, IES, CNRS, UMR 5214, F-34000 Montpellier, France article info Article history: Received 20 May 2015 Received in revised form 9 June 2015 Accepted 10 June 2015 Available online xxxx Keywords: (K 0.5 Na 0.5 )NbO 3 ceramic Sintering Piezoelectric properties Ultrasonic transducer abstract Lead zirconate titanate (PZT) ceramics are the dominant piezoelectric elements for non-destructive eval- uation (NDE) and ultrasonic transducers devices. However, the presence of lead content may impose the scientific community to develop lead-free ceramics, concerning human health and environmental safety. During the past ten years, many contributions have highlighted the potential properties of complex com- positions like LiNbO 3 , LiTaO 3 and LiSbO 3 in the lead-free (K 0.5 Na 0.5 )NbO 3 KNN system. In this context, for the first time, the practical applications and the effectiveness of simply undoped (K 0.5 Na 0.5 )NbO 3 (KNN) ceramics are investigated. KNN powder is prepared by conventional solid state mixed oxide route. Ceramics of this material are prepared using conventional sintering (CS) and spark plasma sintering (SPS). Thickness coupling factor k t of 44–46%, planar coupling factor k p of 29–45%, relative permittivity at constant strain e S 33;r of 125–243 and acoustic impedance Z of 23–30 MRay are obtained for these two kinds of undoped KNN ceramics. Both ceramics are used to build single-element ultrasonic transduc- ers. Relative bandwidth of 49–78% and insertion loss of 27 and 51 dB are obtained for SPS and CS transducers, respectively. These results are suitable for use in non-destructive evaluation. The effective- ness of undoped KNN is evaluated using the KLM model, and compared to standard PZT based probe. Finally, chemical aging test of undoped KNN has demonstrated its stability in water. Ó 2015 Elsevier B.V. All rights reserved. 1. Introduction One of the most common techniques to probe materials for non-destructive evaluation purposes is ultrasonic testing. This is due to the non-ionizing character of ultrasounds. Piezoelectric materials in this field of applications are mainly from a lead-based family of Pb(Zr,Ti)O 3 (PZT) due to their most useful piezoelectric properties [1,2]. They are used in the form of bulk ceramics, piezocomposites or thick films to make ultrasonic probes. However, the lead (Pb) contained in this material is harm- ful and dangerous. The removal of such hazardous substance in electronic equipments and electrical waste has become a real chal- lenge. Since 2003, the European Parliament has adopted directives to limit the production of wastes and to recycle the equipment [3,4]. The piezoelectric devices made of lead-based materials are still authorized in the EU but will be prohibited as soon as a replacement which offers equivalent performances is available. This means that new lead-free materials must be developed. So as to reach this goal, many studies have been focused on lead-free materials such as the alkali niobate family (KNbO 3 -KN and K 0.5 Na 0.5 NbO 3 -KNN) or (Bi,Na)TiO 3 -BNT family and their derivative compounds [5–10]. The most promising material among them is the KNN system, which has a morphotropic phase bound- ary (MPB) at 50 mol% of KNbO 3 and NaNbO 3 according to its phase diagram [11]. Quite recently, it has been shown that doped lead-free KNN materials can compete with lead-based materials (PZT4). Indeed, Saito et al. reported piezoelectric coefficient d 33 of 416 pC/N, planar coupling factor k p of 61% and dielectric constant of 2300 in textured (K 0.44 Na 0.52 Li 0.04 )(Nb 0.86 Ta 0.10 Sb 0.04 )O 3 (LF4T) ceramics [12]. These values are comparable to those of PZT4: d 33 of 410 pC/N, k p of 60% and dielectric constant of 1570. Moreover their Curie temperatures (T C ) are close (253 °C and 250 °C, respectively). However, limited data can be found about the ultrasonic trans- ducers based on lead-free KNN materials. Table 1 summarizes the transducer characteristics and performances obtained by different groups. Feuillard et al. [13] have made KNN-Sr based probe using http://dx.doi.org/10.1016/j.ultras.2015.06.007 0041-624X/Ó 2015 Elsevier B.V. All rights reserved. ⇑ Corresponding author. Tel.: +33 254552105. E-mail address: isabelle.laffez@univ-tours.fr (I. Monot-Laffez). Ultrasonics xxx (2015) xxx–xxx Contents lists available at ScienceDirect Ultrasonics journal homepage: www.elsevier.com/locate/ultras Please cite this article in press as: M. Bah et al., Ultrasonic transducers based on undoped lead-free (K 0.5 Na 0.5 )NbO 3 ceramics, Ultrasonics (2015), http:// dx.doi.org/10.1016/j.ultras.2015.06.007