Microstructural changes in (K0.5Na0.5)NbO3 ceramics sintered in various atmospheres

(K0.5Na0.5)NbO3 is a potential lead-free piezoelectric ceramic, but often suffers from abnormal grain growth. Previous work on BaTiO3 and SrTiO3 has shown that abnormal grain growth can be suppressed by controlling the sintering atmosphere. In the present work, (K0.5Na0.5)NbO3 was sintered in atmospheres ranging from O2 to H2 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 (K0.5Na0.5)NbO3 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.