Impedance studies of K0.5Na0.5NbO3 ceramics prepared from mechanically synthesized powders

 Over the past 60 years, lead zirconate titanate (PZT) has been the material of choice for piezoelectric applications. Recently, world-wide scientific research has been focused on replacement of PZT due its toxicity and... [ view full abstract ]

 Over the past 60 years, lead zirconate titanate (PZT) has been the material of choice for piezoelectric applications. Recently, world-wide scientific research has been focused on replacement of PZT due its toxicity and related environmental issues. One of the potential lead-free candidates is the xKNbO₃–(1−x)NaNbO₃ binary system. The K_0.5N_0.5NbO₃ composition exhibits relatively high Curie temperature and the highest electromechanical and ferroelectric response among the possible KNN compositions.

In this work (K_0.5Na_0.5)NbO₃ (KNN) ceramics have been fabricated from mechanochemically activated powders. Firstly, the substrates were activated in high energy mill (shaker type SPEX 8000 Mixer Mill) for different duration between 25 h and 100 h. Than the solid state reaction occurred at 550^oC and finally the uniaxially pressed samples were sintered at 1000^oC. Because the milling condition has the impact on ceramic morphology and properties the series of samples depending on milling time were prepared. Measurements of impedance spectroscopy were carried out at a wide temperature range. Based on this investidation the temperature dependencies of the resistance and capacitance of grains and grain boundaries have been determined. The ceramic properties has showed a strong corelation with the milling conditions. Grains and grain boundaries resistance increases with the high energy milling time, however the differences are significantly lower than observed by the other authors. A similar trend has been recorded in case of capacitance. Moreover the anomaly connected to the ferroelectric-paraelectric phase transition is visible on temperature dependencies of grains and grain boundaries capacitances. Those differences between properties of the ceramics obtained by classical method and mechanically activated route have been discussed.

Acknowledgements

The work was partially financed by Polish Ministry of Science and Higher Education within statutory activity.

Keywords: lead-free materials; perovskites; mechanical activation, KNN