Fig. 2 shows the XRD patterns of CS

Fig. 2 shows the XRD patterns of CSAS and CS ceramics after sintering at 1130℃ for 4 h, and the synthesized KNN powders were also examined for comparison. For CS ceramics and CSAS ceramics with low water content (CSAS5), a pure perovskite phase with typical orthorhombic symmetry can be observed in the XRD patterns. Compared with that of the calcined powders, no noticeable secondary phase was formed which suggests that the addition of minor amount of water would not induce any noticeable phase transformation during CSAS procedure. However, very few amount of second phases in the XRD pattern of CSAS10 samples were found, identified as K4Nb6O17(JCPDS 14-0287), and a further increase of water content to 15 wt.% will increase the content of K4Nb6O17 phase. Therefore, for the fact that CSAS15 samples possess higher green density but lower sintered density if compared with CSAS5 sample, we realized that the formation of K4Nb6O17 second phase might be a more important reason for the decrease of ceramic density for the CSAS15 samples, and consequently be detrimental for the performance of KNN ceramics. As shown in Fig. 1(c), the piezoelectric coefficient (d33) first increases with added water amounts, and then decreases greatly due to the formation of secondary phase. The optimized d33 value (d33~130 pC/N) was achieved in the ceramics with water content of 10 wt%. Because of the high density, low second phase content and excellent electrical properties, the CSAS samples for the following research, if not specified, were all prepared by adding 10 wt% water for cold sintering.