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Ta-modified potassium sodium niobate (KNN) ceramics are promising lead-free piezoelectric materials for sensing applications. In this study, the nominal composition $(\mathrm{K}_{0.47}\mathrm{Na}_{0.545})(\mathrm{Nb}_{0.55}\mathrm{Ta}_{0.45})\mathrm{O}_3$, reported in the literature to exhibit enhanced piezoelectric performance through polymorphic phase-transition (PPT) tuning, was investigated [1]. The ceramics were prepared by solid-state reaction and sintered at 1100–1250 °C in a loosely covered crucible. X-ray diffraction revealed pseudo-cubic symmetry without discernible orthorhombic–tetragonal peak splitting. The dielectric permittivity decreased from 548 to 362 over 30–400 °C at 10 kHz, with frequency dispersion and no distinct PPT anomaly. The measured piezoelectric coefficient was $d_{33}=72$ pC/N, compared with the literature-reported value of 333 pC/N. Polarisation–electric-field hysteresis loops exhibited a slim response, with $P_r/P_{\max}\approx0.22$ and an estimated internal bias field magnitude of approximately 2.91 kV/cm.
Taken together, these observations suggest a response consistent with an effective Nb/Ta phase-diagram composition near $x\approx0.60$, despite the nominal Ta fraction of $x=0.45$. This apparent discrepancy is hypothesised to arise from alkali volatilisation during sintering, which may alter A-site stoichiometry and thereby influence the dielectric and piezoelectric response [2]. As an additional assessment of acoustic response, the ceramic pellet was bonded to an aluminium beam using epoxy and evaluated using the Hsu–Nielsen pencil-lead-break (PLB) method. Two successive lead breaks were recorded using an oscilloscope, and the resulting power spectral density (PSD) profiles were analysed over 0–250 kHz. The smoothed spectra exhibited a Pearson correlation of 0.920 and a mean absolute difference of 3.34 dB, indicating preliminary repeatability for the tested pellet–beam assembly. A common spectral feature appeared near 40–50 kHz, while local maxima occurred near 148–153 kHz in the 100–200 kHz region. These findings demonstrate initial acoustic coupling and detection of transient stress-wave responses. Further compositional verification, repeated measurements across multiple specimens, and electromechanical-impedance measurements under baseline and controlled-damage conditions are required to establish the proposed volatilisation mechanism and assess the material's suitability for structural-health-monitoring applications.
References
[1] Sung, Y. S.; Baik, S.; Lee, J. H.; Ryu, G. H.; Do, D.; Song, T. K.; Kim, M. H.; Kim, W. J. Appl. Phys. Lett. 2012, 101, 012902.
[2] Wang, Y.; Damjanovic, D.; Klein, N.; Hollenstein, E.; Setter, N. J. Am. Ceram. Soc. 2007, 90, 3485–3489.
[3] de Almeida, V. A. D.; Baptista, F. G.; de Aguiar, P. R. Piezoelectric Transducers Assessed by the Pencil Lead Break for Impedance-Based Structural Health Monitoring. IEEE Sensors Journal 2015, 15(2), 693–702. https://doi.org/10.1109/JSEN.2014.2352171.
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| Figure caption | Figure 1. PSD comparison of two successive PLB tests on Ta-modified KNN, showing a common broad spectral maximum at 150–160 kHz and preliminary repeatability of the response. |