Modelling the particle contact influence on the Joule heating and temperature distribution during FLASH sintering

FLASH sintering is a field-assisted technique that allows the densification of ceramics in a few seconds at temperatures significantly lower than those of conventional cycles. There is still discussion among the scientific community about the mechanism behind this sintering process, that has been typically attributed to Joule heating, defect creation and movement or liquid phase assisted sintering. Computational modelling can be a powerful tool in helping to explain and predict this process. Using potassium sodium niobate (KNN) as a case study, a lead-free piezoelectric, this work explores Finite Element Modelling to evaluate the dependence of Joule heating generation and temperature distribution as a function of the cubic particle orientation

The role of particle contact in densification of FLASH sintered potassium sodium niobate

Potassium sodium niobate, K0.5Na0.5NbO3 (KNN) is a lead‐free piezoelectric with the potential to replace lead zirconate titanate (PZT) in electromechanical applications. Due to its cuboid particle morphology and volatile elements, monophasic and dense ceramics are difficult to obtain via conventional sintering. In this work, isothermal FLASH sintering produced uniformly densified KNN ceramics at 900 °C, 200 °C lower than conventional sintering. Specific surface area (SSA) analysis of pre‐FLASH ceramics revealed that a 30 min isothermal hold at 900 °C, before the application of electric field, increased the contact area between particles and was crucial to promote uniform densification. Finite element modelling (FEM) revealed why density is more uniform when using isothermal heating compared with a constant heating rate, commonly used in FLASH sintering. These results extend our understanding of FLASH sintering and illustrate its relevance for the development of lead‐free piezoelectrics

Induced internal stresses and their relation to FLASH sintering of KNN ceramics

Electric field and current applied to an unsintered ceramic body are known to promote low temperature and extremely fast densification, in a process referred to as FLASH sintering. Under the current urgency of the green transition of manufacturing processes, FLASH sintering is a very promising technology for materials industry. Suitable FLASH conditions result in dense ceramics but many issues associated with the effect of electric field and current on local chemistry, structure, and microstructure remain to be understood. We have used FLASH sintering to produce K0.5Na0.5NbO3 (KNN), a lead-free compound suitable for piezoelectric applications. Using a combined X-ray diffraction and Raman spectroscopy study, here we show for the first time that, although the FLASH process may produce homogeneous ceramics with negligible concentration of secondary phase, macroscopic core-localized stresses remain which have significant consequences on the final properties of the sintered material. In addition, the internal stress state and its dependence on the local temperature during FLASH sintering are established by Finite Element Modelling (FEM). The identification of the fine structure of FLASH sintered materials is critical for understanding the unique properties developed under this sintering process and for its development as an alternative low thermal budget sintering technology