Component fraction effect on microstructure and properties of CoFe₂O₄−K_0.5Na_0.5NbO₃ composites

CoFe₂O₄−K_0.5Na_0.5NbO₃ (CFO−KNN) composites were sintered in air at 1100°C for 2 h. The relative density of sintered CFO−KNN varied with the KNN/CFO ratio and when the volume fraction of KNN reached 50% or more, the sintered composites had a relative density higher than both of the components (i.e., individually sintered KNN and CFO). The highest relative density (94%) was obtained with the composites of 80 vol% KNN. Transmission electron microscopy did not observe any secondary phase at the interface between CFO and KNN grains. However, elemental analyses showed traces of interdiffusion of Na, K, Co, and Fe ions, which were within the solubility limit of both lattices so that no segregation of other phase occurred. Nevertheless, the interdiffusion altered grain growth kinetics for both phases, resulting in the changes in grain sizes for both KNN and CFO in the composites. The composites of higher CFO content showed lower piezoelectric d₃₃ due to a higher DC conductivity that resulted in an incomplete poling of samples. The attainable magnetoelectric voltage coefficient was 3.6 mV cm⁻¹ Oe⁻¹, which was measured with the composites of 50 vol% KNN.