Activation of SPS levers to optimize the microstructure and piezoelectric properties of low-temperature sintered KNN ceramics

In a global context of increasing environmental requirements, the transition towards lead-free piezoelectric materials and sustainable processes has become a strategic scientific and technological challenge. K 0.5 Na 0.5 NbO 3 (KNN) ceramics are a promising lead-free alternative, but low-temperature sintering while optimizing both microstructure and piezoelectric properties remains a major challenge. Starting with reactive undoped KNN nanopowder, we explore the unique features of Spark Plasma Sintering, such as pulse sequence and fast kinetics, to achieve controlled densification with improved microstructure at a temperature as low as 700°C. Using fast heating rate of 400°C/mn promotes grain growth at the expense of microstructural homogeneity. The modulation of the electrical power injected during sintering by tuning the ON/OFF pulse sequences is shown to be a relevant lever to improve the microstructure upon reducing the grain size distribution. The correlation between SPS parameters, microstructure and piezoelectric performances is an opportunity to design low-temperature piezoceramics while reducing energy costs.