Nowadays, Lead Zirconia-Titanate (PZT) remains the primary piezo-ceramic in actuators, sensors or transducers applications. Lead toxicity has imposed its restriction few years ago and implies to find a substitute material.  Among the families of potential candidates to replace them, Niobate Sodium Potassium (K0.5Na0.5NbO3, KNN) has gained interests due to its high Curie Temperature (TC around 420°C) and KNN-based ceramics can exhibit performances reaching those of PZT, either in multi-components substituted KNN (by achieving Rhombohedral-Orthorhombic-Tetragonal phases co-existences) [1] or in highly textured KNN [2].

However, the high temperature and long dwelling time used to densified the KNN ceramics by conventional route can cause the volatilization of alkaline elements and inhomogeneous phases [3]. Spark Plasma Sintering (SPS) offers the opportunity to limit this effect by reducing both the sintering temperature and time to produce high densified ceramic up to 99 % [4]. Our previous reports on KNN-Ta showed that use both SPS and a specific precursor (Nb1-xTaxO5) to perform a Ta substitution about 20-30 %, near an Orthorhombic-Tetragonal transition, can achieved  homogeneous and highly 97 % dense ceramics with enhanced properties (kp = 59 %) compared to those fabricated with conventional precursors (kp = 42%) [5,6]. However, kt remain beyond 30%. In other studies, combination of Li and Ta substitution on conventionally sintered KNN displayed kp=40 %, but a nearly doubled d33 » 250 pC/N in comparison with pure KNN. This has been attributed to the proximity of a phase transition around room temperature [7]. Here, we have studied the relations between elaboration conditions, microstructure and piezoelectric properties (kt, kp, d33) using the combination of SPS process and Li /Ta substitution in KNN ceramics. SEM/EDX-WDX, XRD/Rietveld, XPS analysis were performed and interpreted. The dependence of ferroelectrics properties with the compositions was also studied as a function of substituents concentration and oxygen vacancies, in order to control the whole processing route and achieve optimized piezoelectric properties.
References:
[1]      H. Tao, et al., J. Am. Chem. Soc. 141 (2019) 13987–13994. https://doi.org/10.1021/jacs.9b07188.
[2]      B. Zhang, et  al., ACS Appl. Mater. Interfaces. 5 (2013) 7718–7725. https://doi.org/10.1021/am402548x.
[3]      Y. Wang, et al., J. Am. Ceram. Soc. 90 (2007) 3485–3489. https://doi.org/10.1111/j.1551-2916.2007.01962.x.
[4]      K. Wang, et al., J. Electroceramics. 21 (2008) 251–254. https://doi.org/10.1007/s10832-007-9137-z.
[5]      F. Jean, et al., Ceram. Int. 44 (2018) 9463–9471. https://doi.org/10.1016/J.CERAMINT.2018.02.163.
[6]      M. Dubernet, et al., J. Eur. Ceram. Soc. 42 (2022) 2188–2194. https://doi.org/10.1016/j.jeurceramsoc.2021.12.030.
[7]      P. Zhao, et al., J. Am. Ceram. Soc. 91 (2008) 3440–3443. https://doi.org/10.1111/j.1551-2916.2008.02629.x.