Piezoelectric materials are a rapidly growing field of research and development, with potential applications in energy harvesting. Due to their outstanding electromechanical capabilities, Pb(Mg1/3Nb2/3)O3 (PMN) and Pb(Zr1-x,Tix)O3 (PZT) have been the two main piezoelectric materials studied over the last decades [1]. However, these two materials are lead-based compounds that should be replaced, due to environmental concerns about Pb and PbO toxicity [2], by (K,Na)NbO3, (Bi0.5Na0.5)TiO3 (BNT), and (Ba,Ca)(Zr,Ti)O3 (BCZT), the leading lead-free candidates [3, 4].

In particular, KNN is one of the most promising candidates to replace PZT [3, 4, 5, 6]. Dense KNN ceramics require relatively high sintering temperatures [7], which are incompatible with the volatility of alkaline elements. Alternate processing techniques are thus required, such as spark plasma sintering (SPS). SPS enables a significant reduction in sintering temperatures and duration compared to conventional sintering. Here, we present our studies towards the low temperature sintering of KNN ceramics.

First, SPS was used to sinter disk-shaped ceramics from nanometric commercial KNN powder (grain size < 100 nm). The effects of different SPS conditions on the final density, namely, sintering temperature (400-1000°C), pressure (40, 100 MPa), time (5-60 minutes), heating rate (5-400°/min) have been studied. In addition, the sintering environment was modified by changing the material at the interfaces between the KNN powder and the graphite mould. We were able to sinter KNN achieving relative densities higher than 95% with sintering temperatures lower than 700°C. As a result, the post-annealing usually performed after SPS was avoided; thus, reducing the global energy budget even further. The impact of the SPS experimental conditions on both the density and the dielectric properties are discussed. For sake of comparison with results from literature, a ceramic sintered by SPS at 1000°C, 100°/min, 40 MPa for 5 minutes and annealed at 1000°C for 8h was investigated. The piezoelectric coefficient d31 was evaluated using the following relationship , and considering ε11 = 600 (measured far from the resonance) obtaining d31 = -93 pC/N, which is comparable with literature [8, 9]. Furthermore, all low temperature sintered KNN ceramics are currently being poled and characterized.

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