The role of particle morphology in the cold sintering of potassium sodium niobate (KNN) ceramic

• Autor(a) principal: Lin, Fábio Ye
• Data de Publicação: 2021
• Tipo de documento: Dissertação
• Idioma: eng
• Título da fonte: Repositório Científico de Acesso Aberto de Portugal (Repositórios Cientìficos)
• Texto Completo: http://hdl.handle.net/10773/34194
• topic: Cold sintering process; Lead-free piezoelectrics; Potassium sodium niobate; KNN; Particle size

Sintering has always been a critical step during the life cycle of technical ceramics. The high temperatures of conventional processing, which are reflected in high energy costs and CO₂ emission rates, have demanded strong efforts from the scientific community to identify more sustainable alternatives. In this sense, cold sintering (CSP) emerges as a promising technique, which combines pressure with a solvent at low temperatures (<300 ºC) to deliver highly densified ceramics. In this work, it was studied the cold sintering of a piezoelectric ceramic based on potassium and sodium niobate (KNN), a candidate for replacing lead based piezoelectrics. Herein, the study carried out was focused on the influence of the starting powders characteristics, an aspect scarcely reported in the literature, especially for KNN. The control of particle size and size distribution was achieved through different types of milling, conventional planetary (BM) and attrition milling (AM), or a combination of both. The resulting milled powder particles were further characterized in terms of crystal phase composition (DRX), particle morphology (SEM) and specific surface area (gas adsorption (BET isotherm)). The ceramics were produced in a cold sintering system at 250 ºC, 500 MPₐ, and assisted by a flux based on a potassium and sodium hydroxides mixture. We could demonstrate that it is possible to positively impact the cold sintering of KNN by acting on the characteristics of the starting powder, using an appropriate particle size distribution (particles with a bimodal size distribution centered on ~0.2 and 2 µm). In this case, ceramics with uniform microstructure and high relative densities up to 96%, which are comparable with those of conventionally produced ceramics (1120 ºC), were obtained. It was also observed that ceramics produced with finer starting particles, exhibiting aggregates, evidenced non-uniform microstructure, where the presence of porosity and local precipitates were identified, accounting for lower relative densities. Despite the high density achieved for some of the ceramics, it is still necessary to explore further other parameters, intrinsic and extrinsic to the powder, to improve the performance of the cold sintered ceramics including its mechanical resistance and electrical behavior which are still modest as compared to the corresponding properties of KNN ceramics obtained via conventional sintering.