Abstract
The potassium sodium niobate, K0.5Na0.5NbO3, solid solution (KNN) is considered as one of the most promising, environment-friendly, lead-free candidates to replace highly efficient, lead-based piezoelectrics. Since the first reports of KNN, it has been recognized that obtaining phase-pure materials with a high density and a uniform, fine-grained microstructure is a major challenge. For this reason the present paper reviews the different methods for consolidating KNN ceramics. The difficulties involved in the solid-state synthesis of KNN powder, i.e., obtaining phase purity, the stoichiometry of the perovskite phase, and the chemical homogeneity, are discussed. The solid-state sintering of stoichiometric KNN is characterized by poor densification and an extremely narrow sintering-temperature range, which is close to the solidus temperature. A study of the initial sintering stage revealed that coarsening of the microstructure without densification contributes to a reduction of the driving force for sintering. The influences of the (K + Na)/Nb molar ratio, the presence of a liquid phase, chemical modifications (doping, complex solid solutions) and different atmospheres (i.e., defect chemistry) on the sintering are discussed. Special sintering techniques, such as pressure-assisted sintering and spark-plasma sintering, can be effective methods for enhancing the density of KNN ceramics. The sintering behavior of KNN is compared to that of a representative piezoelectric lead zirconate titanate (PZT).
Highlights
Piezoelectrics are an important group of functional materials with a wide range of applications, including sensors, actuators and transducer devices [1]
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We have reviewed the current knowledge about the sintering of KNN
Summary
Piezoelectrics are an important group of functional materials with a wide range of applications, including sensors, actuators and transducer devices [1]. The research into novel, lead-free materials has expanded tremendously over the past 15 years and, currently, three main groups of materials are being considered: K0.5Na0.5NbO3 (KNN)-based, BaTiO3 (BT)-based, and Bi1/2Na1/2TiO3 (BNT)-based piezoelectrics None of these materials can fully replace PZT in all applications, many compositions exhibit comparable or even better properties for specific requirements. The frequently reported drawbacks are related to processing: difficulties in obtaining high densities of sintered products, deviations from the stoichiometry and the subsequent formation of secondary phases, and difficulties in controlling the microstructure, see for example [15,17,25,33,34,35] Such phenomena can lead to inhomogeneous distributions of the applied electric fields, leakage currents, low breakdown fields, poor reproducibility and low piezoelectric performance of the ceramics, and should be carefully addressed.
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