Hydrothermal Preparation And Spark Plasma Sintering, Potassium Sodium Niobate Lead-free Piezoelectric Ceramics

Piezoelectric ceramics, the important functional materials, could be used as resonators, filters, transducers and so on. For more than half a century, lead-containing piezoceramics, represented by PZT, have always been taking the dominant role. However, PZT contains more than 60% of lead, which is harmful for humanity as well as environment during the manufacture and abandonment processes. In the view point of environment protection, people are always seeking for lead-free substitute.Alkaline niobate is important lead-free piezoelectric ceramics system, the most representative of which is NaNbO₃-KNbO₃ solid state solution. Although the research of NaNbO₃-KNbO₃ started in the 1950s, the PZT ceramics, which were discovered almost at the same time, showed much better piezoelectric performance. As a result, the alkaline niobate did not drawn much attention until 2004, when Saito et al. reported (K_0.5Na_0.5)NbO₃ based ceramics with outstanding piezoelectric properties comparative to PZT ceramics. For recent years, the (K, Na)NbO₃ based ceramics has been a hot research topic, and relative reports appeared in large amount, which mainly concerned improvement of piezoelectric performance by means of doping with other elements, such as Li, Ta, Sb and so on. However, most investigators adopted conventional synthesis process of solid state reaction. Despite the simpleness, the disadvantages of this process include inducing coarse grains distribution, inhomogeneous composition, and easy introduction of impurities during ball-milling process, which could deteriorate piezoelectric properties of the ceramics, and cause instability of production process. On the contrast, the hydrothermal method is a chemical synthesis process based on water solution, and has been widely used for the manufacture of commercial PZT, BaTiO₃ and other functional ceramics powders. And recent report indicated that the BaTiO₃ ceramics sintered from hydrothermal powders exhibited much more superior piezoelectric performance. Nevertheless, there were little reports concerning hydrothermal synthesis of (K, Na)NbO₃ powders. The present study is focused on the hydrothermal synthesis of (K, Na)NbO₃ powders and subsequent spark plasma sintering of these powders. The dielectrics, piezoelectric and ferroelectric properties of the obtaioned ceramics samples were also investigated.The major conclusions are as follows:The (K, Na)NbO₃ powders were successfully prepared by a hydrothermal method, and the influences of reaction temperature, reaction time, alkality of starting solution and amount of Nb₂O₅ were systematically investigated. It was revealed that improvement of reaction temperature was in favor of synthesis of single-phase perovskite, but prolonging reaction time could not convert two-phase perovskites into a single one. The alkality of starting solution, especially the KOH/NaOH ratio, had profound effects on the composition as well as microstructure of resultant powders. It was easy to synthesize single-phase NaNbO₃-KNbO₃ powders with compositions near either end members, but two-phase perovskites appeared around (K_0.5Na_0.5)NbO₃.The influences of additives on the hydrothermal synthesis of (K, Na)NbO₃ powders were studied preliminarily. Polyvinyl alcohol (PVA) and ethylene diamine tetra acetic acid (EDTA) were added to the starring solution. When KOH/NaOH ratio was 4.5/1.5, the total alkality was 6 mol/L, the reaction temperature was 220℃, and the reaction time was 24 hours, (K, Na)NbO₃ powders with perovskite phase were prepared. The result indicated that additives hindered growth of some crystal face, so the microstructure of resultant powders was changed a lot.The (K, Na)NbO₃ powders were successfully prepared using a alcohol-water solvothermal synthesis method, and the influences of alcohol amount, KOH/NaOH ratio and alkality of starting solution were investigated. The addition of alcohol improved the pressure of reaction vessel, which is in favor of synthesis of one single phase; however, compositions were changed at the same time. It was easy to synthesize single-phase NaNbO₃-KNbO₃ powders with compositions near either end members, but two-phase perovskites appeared around (K_0.5Na_0.5)NbO₃.Lead-free KNN ceramics with relative densities more than 99% have been prepared using hydrothermal precursors and spark plasma sintering (SPS). Although two similar perovskite phases appeared when K/Na ratio tended towards 1:1, the two-phase coexistence tendency was weakened by increasing hydrothermal reaction temperature, and consequently only one phase could be obtained after spark plasma sintering. Reasonably good ferroelectric and piezoelectric properties were obtained for the samples after post annealing, whose piezoelectric constant (d₃₃) reached 135 pC/N. The optimal remnant polarization (P_r) and mechanical quality factor (Q_m) were 26.2μC/cm² and 164, respectively, which were both twice as much as those of the samples using powders prepared from solid state reaction.