Microstructural changes in (K 0.5Na 0.5)NbO 3 ceramics sintered in various atmospheres

Abstract

(K 0.5Na 0.5)NbO 3 is a potential lead-free piezoelectric ceramic, but often suffers from abnormal grain growth. Previous work on BaTiO 3 and SrTiO 3 has shown that abnormal grain growth can be suppressed by controlling the sintering atmosphere. In the present work, (K 0.5Na 0.5)NbO 3 was sintered in atmospheres ranging from O 2 to H 2 and the effect on grain growth behaviour studied. Sintering in reducing atmospheres causes a delay in the onset and a reduction in the amount of abnormal grain growth. The effect of sintering atmosphere on grain growth behaviour can be explained using the 2D nucleation-controlled theory of grain growth. Changes in the grain shape during sintering in reducing atmospheres indicate a reduction in the edge free energy of (K 0.5Na 0.5)NbO 3 caused by an increase in the concentration of oxygen vacancies. This decreases the critical driving force necessary for rapid grain growth and causes a transition from abnormal to pseudo-normal followed by abnormal grain growth.