Ultrasonic transducers based on undoped lead-free (K0.5Na0.5)NbO3 ceramics

• Electromechanical characterization of undoped KNN ceramics fabricated by conventional and spark plasma sintering.
• Modeling, fabrication and characterization of single-element transducer based on undoped KNN ceramics.
• Reactivity of KNN powder with water.
• Chemical aging effect of KNN on the thickness coupling factor (kt  ) and relative permittivity (ε33,rS).

Lead zirconate titanate (PZT) ceramics are the dominant piezoelectric elements for non-destructive evaluation (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 compositions like LiNbO3, LiTaO3 and LiSbO3 in the lead-free (K0.5Na0.5)NbO3 KNN system. In this context, for the first time, the practical applications and the effectiveness of simply undoped (K0.5Na0.5)NbO3 (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 ktkt of 44–46%, planar coupling factor kpkp of 29–45%, relative permittivity at constant strain ε33,rS 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 transducers. 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 effectiveness 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.