Evolution of phase composition and microstructure of sodium potassium niobate –based ceramic during pressure-less spark plasma sintering and post-annealing. Issue 8 (1st June 2019)

• Record Type: Journal Article
• Authors: Kuscer, Danjela; Kocjan, Andraž; Majcen, Maja; Meden, Anton; Radan, Kristian; Kovač, Janez; Malič, Barbara
• Is Part Of: Ceramics international. Volume 45:Issue 8(2019)
• Page Start: 10429
• Page End: 10437
• Publication Date: 2019-06-01
• Subjects: Microstructure–final -- Piezoelectric properties -- Niobates -- Spark-plasma sintering; Ceramics -- Periodicals; Céramique industrielle -- Périodiques; Ceramics; Periodicals; Electronic journals; 666
• DOI: 10.1016/j.ceramint.2019.02.102
• Languages: English
• ISSNs: 0272-8842

Abstract

Abstract: This study was designed to better understand the microstructural and phase evolution of lead-free sodium potassium niobate based piezoceramics with a nominal composition (K0.5 Na0.5 )0.99 Sr0.005 NbO3 (KNNSr) during pressure-less spark plasma sintering followed by post-annealing in oxygen. The as-sintered samples were dark-coloured and electrically conductive as a result of partial reduction of Nb 5+ to Nb 4+ and formation of oxygen vacancies confirmed by X-ray photoelectron and Raman spectroscopy. The Rietveld refinement analysis showed that the as-sintered samples contained two perovskite phases with monoclinic Pm unit cell and slightly different unit-cell parameters. The microstructure with sub-micrometre-sized grains unambiguously confirmed that rapid heating and short dwell time hindered the grain growth. We found that post-annealing the samples at 950 °C in oxygen led to improvement in functional properties. The samples became white-coloured, the both perovskite unit cells decreased as a result of re-oxidation, while the microstructure remained essentially unchanged. The KNNSr sintered at nominal sintering temperature of 1300 °C for 3 min and post-annealed possessed a relative density of 88% and dielectric and piezoelectric properties similar to those of the conventionally sintered samples. Our findings contribute to the understanding of pressure-less spark plasma sintering of sodium potassium niobate-based materials and suggest that arrested grain growth and minimisation of alkali evaporation not necessarily lead to dense ceramic.