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

abstract

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.

keywords

TEMPERATURE DISTRIBUTION; INTERNUCLEAR DISTANCES; MICROSTRUCTURE; CHALLENGES; STATE

subject category

Materials Science; Physics

authors

Serrazina, R; Gomes, M; Vilarinho, R; Pereira, L; Dean, JS; Reaney, IM; Senos, AMOR; Vilarinho, PM; Moreira, JA

Groups

G2 - Photonic, Electronic and Magnetic Materials

Projects

CICECO - Aveiro Institute of Materials (UIDB/50011/2020)

CICECO - Aveiro Institute of Materials (UIDP/50011/2020)

Sinterizacao FLASH de oxidos funcionais livres de chumbo para o processamento sustentavel de materiais para aplicacoes em energia e relacionadas. (FLASH)

Collaboratory for Emerging Technologies, CoLab (EMERGING TECHNOLOGIES)

acknowledgements

The authors give a special acknowledgment to Artur Sarabando and Dr Rosario Soares for the data acquisition and analysis of residual stress evaluation by XRD. To Dr Alex Tkach for the discussion of XRD data. This work was developed within the scope of the projects CICECO-Aveiro Institute of Materials, UIDB/50011/2020 & UIDP/50011/2020, financed by national funds through the FCT/MEC and when appropriate co-financed by FEDER under the PT2020 Partnership Agreement and from IFIMUP, with ref. NORTE-01-0145-FEDER-022096 & UID/NAN/50024/2019. Additionally, the work was financed by Portugal 2020 through European Regional Development Fund (ERDF), in the frame of Operational Competitiveness and Internationalization Programme (POCI), in the scope of the project FLASH sintering of lead free functional oxides towards sustainable processing of materials for energy and related applications - FLASH, POCI-01-0247-FEDER-029078. Ricardo Serrazina and Mariana Gomes acknowledge the financial support of, respectively, FCT (SFRH/PD/BD/128411/2017) and the project PTDC/NAN-MAT/28538/2017.