Hydrothermal Synthesis Niobate Lead-free Piezoelectric Ceramic Powder And Its Properties

Piezoelectric ceramics are important functional materials, which are widely used in function devices, such as resonator, filter and sensor. However, the dominant piezoelectric ceramics, zirconium titanate (PZT) materials contain more than 70% PdO. The processes of its production and waste treatment make grate harm to human health and the environment. In order to protect the environment and ensure the sustainable development of the human society, people have been searching for the lead-free piezoelectric ceramics to replace PZT.Alkali Niobate are important lead-free piezoelectric ceramic materials, KNbO3-NaNbO3 (hereinafter referred to KNN), the most important of them, are expected as a promising to replace lead-based ceramic materials because they show much better piezoelectric performance such as high piezoelectric coefficients, high electromechanical coupling factors and Curie temperature. But preparation conditions of KNN ceramic had vital effection on the piezoelectric properties. Therefore, it was very hard to get KNN ceramics with compact structure and stability performance via the traditional solid-state method. At present, it had reported that powders prepared by hydrothermal synthesis show lower synthetic temperature and higher activity characteristic, which are hopeful to reduce the sintering temperature and improve density and properties of KNN ceramics.The present study is focused on preparing NaNbO3, KNbO3 and (Na0.5K0.5)NbO3 powders and ceramics by the hydrothermal synthesis method (HT) and the conventional solid-state method (CMO), respectively. Micromorphologies and phase structures of the two powders and the performance of ceramics are investigated.The major conclusions are as follows:(1) Uniform, small particle sizes and highly activity NaNbO3 powders with orthorhombic phase were successfully synthesized by HT method. It was found the synthetic temperature and alkali concentration were the key factors of the synthetic process. Fine NN powders were obtained under appropriate hydrothermal conditions (200℃,2.64 M NaOH concentration and heat preservation for 24 h). At the same time, NaNbO3 ceramic powders were obtained by CMO method, effects of synthesis methods on the micromorphology and phase structure of the powders were studied. The results of FTIR absorption spectra of them indicated that two kinds of samples have same components. The Raman spectra indicated HT-synthesized powders have small grain size with O3 orthorhombic structure, while CMO-synthesized powders have larger grain size with O1 orthorhombic structure. The phase structure and performance of ceramics made by the two kinds of powders were investigated, the results illustrated that the sintering temperature of HT-synthesized ceramics was 1340℃and the relative density was as high as 98.7%. However, for the CMO-synthesized ceramics, these values were 1360℃and 94.1%. Notably, the HT-synthesized ceramics obtained better electrical properties than that of CMO-synthesized ceramics, which were as follows:d33=41 pC/N-1, Kp=0.30,εm=1565(1 kHz) and Tc=378℃(2) KNbO3 ceramic powders were successfully prepared by HT method. The synthetic temperature, alkali concentration and heat preservation time were investigated. When [Nb2O5]= 0.33 M,160℃and above, the [OH-]≥7M, pure KNbO3 with perovskite phase can be synthesized. What's more, KNbO3 powders with pseudocubic phase could be obtained under the conditions of 180℃,9 M for 12 h. Meanwhile the CMO-synthesized KNbO3 powders were prepared and ceramics were made by the two kinds of powders. The appropriate sintering temperature of the HT ceramics was 960℃, which was lower 30℃than that of CMO ceramics. What's more, the relative densities of the ceramics were close very much. It could be seen from the temperature dependence ofεr of the ceramics the HT- ceramics possessed higherεm(11106), which corresponded to the phase transition from the tetragonal to orthorhombic phase (Curie temperature 424℃).(3) HT- synthesized K0.5Na0.5NbO3 (KNN) ceramics were made by the HT-synthesized NaNbO3, KNbO3 powders, and CMO-synthesized KNN ceramic were also obtained. The phase structure, micromorphology, dielectric and piezoelectric properties of the ceramics were investigated. It illustrated the appropriate sintering temperature of HT-KNN ceramics was 1070℃, which was lower 60℃than that of CMO ceramics. Therefore, the HT powders have higher sintering activities. In addition, the relative density of the HT ceramics was 3.77 g/cm3, and for CMO ceramics, that was 4.01 g/cm3. The electric properties of the two kinds of ceramics were close. For HT-ceramics, the properties were d33=100 pC/N,εm= 4432 and Tc= 415℃. The properties of CMO-ceramics were as follows:d33= 96 pC/N,εm= 4929 and Tc= 409℃.