By virtue of enhanced design flexibility, additive manufacturing (AM) is a game-changing technology in processing piezoelectric ceramics with a wide range of applications, from transducers to energy harvesting. The current research presents vat photopolymerization (VPP)-based AM of potassium sodium niobate [(K, Na) NbO3], hereafter KNN, a promising candidate for the replacement of the lead-based piezoelectric ceramics. VPP relies on selective and layer-wise curing of a photopolymerizable ceramic suspension to form green bodies, followed by debinding and sintering to obtain dense ceramics. The crucial step of the process is the development of a stable ceramic suspension with high solid loading with rheological and optical properties suitable for fabrication.
The high solid loading of the suspension is imperative to improve the densification of KNN piezoceramics and reduce the shrinkage and probability of crack formation during the heat treatment step. However, increasing the solid content by a critical amount complicates the suspension recoating process due to increased viscosity and the curing behaviour and accuracy due to the intensified light attenuation and scattering.
This work investigates the effects of powder characteristics on the processing of KNN via the bottom-projection VPP-based AM technique. KNN powders made of two different synthesis methods, i.e., solid-state synthesis and spray pyrolysis, which own different shapes and sizes, were utilized to make highly loaded non-aqueous suspensions. Firstly, KNN powders were characterized in terms of morphology, particle size distribution, specific surface area, and phase composition. Then, the effects of powder type and solid loading on the rheological behaviour, e.g., flow curve, long-term stability, and thixotropic behaviour, of suspensions were scrutinized through rotational/oscillatory rheological measurements and sedimentation testing. The highest solid loading (up to   ?50 vol.%) was achieved using the solid-state KNN. The suspensions made of spray pyrolysis KNN powder exhibited the desired shear thinning behaviour. In contrast, a slight shear thinning-to-thickening transition was observed in suspensions made of solid-state KNN powder. Finally, the microstructure, density, and piezoelectric performance of the KNN printed and sintered samples were examined. Relatively high density for KNN (  ?93% of theoretical density) and reasonable piezoelectric response were achieved using either KNN powders.