The Inferences of ZnO Additions for LKNNT Lead-Free Piezoelectric Ceramics CHIEN-MIN CHENG 1, CHING-HSING PEI 1, MEI-LI CHEN 2, *, KAI-HUANG CHEN 3, * 1 Department of Electronic Engineering, Southern Taiwan University of Science and Technology, Tainan, Taiwan, R.O.C. 2 Department of Electro-Optical Engineering, Southern Taiwan University of Science and Technology, Tainan, Taiwan, R.O.C. 3 Department of Electronics Engineering and Computer Science, Tung-Fang Design Institute, Kaohsiung, Taiwan, R.O.C. *Corresponding author. Abstract By the conventional solid-state liquid-phase sintering technique, Li (K Na ) (Nb 0.9 Ta 0.1 )O 3 + x wt% ZnO (x = 0, 0.2, 0.4, 0.6, 0.8, 1, 3) lead-free piezoelectric ceramics were fabricated. The addition of ZnO liquid-phase sintering promoters could improve the grain-growth of LKNNT ceramics significantly and its inferences were investigated detailed in this paper. The crystal phases and micro-structures were analyzed by means of the X-ray diffraction and scanning electronic microscopy, respectively. Using the impedance analyzer, the dielectric constant, loss tangent, Curie temperature, phase transition point, and electromechanical coupling factor were measured. And the piezoelectric constants were measured by the d 33 meter. Compared to pure LKNNT ceramic (sintered at 1090  C, d 33 = 279 pC/N, and k p = 0.46), for x = 0.6 specimen, even though the optimal d 33 and k p values were only 272 pC/N and k p = 0.44, but the optimal sintering temperature have been improved from 1090  C to 1020  C successfully. Keywords Liquid-phase sintering, Lead-free, Piezoelectric, ZnO. Results and Discussion It is clear that all the ceramics possess a perovskite structure in which the tetragonal symmetry is dominant at room temperature, though a slight increase of the orthorhombic symmetry is observed with increasing ZnO content. From the 44  ~47  figure presented, we can observe that as 0.6 wt% ZnO was added, (002) and (200) peaks reveal obvious coexistence of orthorhombic and tetragonal phases for 0.6 wt% specimen. Figure 2. SEM images of 1020  C-sintered LKNNT + (x)ZnO ceramics. It can be observed that x = 0.6 specimen reveals the most uniform grains than others, and the average grain size is about 4  m. As x 0.6. It can be observed that for pure LKNNT (the theoretical density is g/cm 3 ), maximum relative density (0.97) can be obtained as the sintering temperature is 1090  C. Whereas increasing the ZnO contents, the relative density increases to a saturated and maximum value of 0.98 for x = 0.4, and also increases to for x = 0.6 (the theoretical densities are 4.55 g/cm 3 and g/cm 3 for x = 0.4 and x = 0.6, respectively), however, their optimal sintering temperature had be decreased from 1090  C (x = 0) to 1020  C (x = 0.4, 0.6) effectively due to these little ZnO additions. But for the ZnO contents further increase to x = 1 and x = 3, the relative density starts to decrease (0.962 for x = 1 and only for x = 3) due to the inferences of ZnO second phases, non-uniform grains, and pores. Figure 4. Piezoelectric constant (d 33 ) as a function sintered temperature for the LKNNT + (x)ZnO ceramics. Figure 5. The electromechanical coupling factor (k p ) as a function sintered temperature for the LKNNT + (x)ZnO ceramics. It is shown that for x = 0 and sintering temperature is 1090  C, the maximum d 33 value is 279 pC/N. Whereas increasing ZnO contents, the maximum d 33 value is 265 pC/N for x = 0.4 and 272 pC/N for x = 0.6, respectively. However, owing to the inferences of ZnO liquid-phase sintering promoters, their optimal sintering temperature all had been decreased to only 1020  C significantly. For most specimens, the d 33 values will first increase, and then reach to saturated maximum values gradually as sintering temperature is higher than 1020  C, and finally start to decrease as it higher than 1090  C. Especially, as x = 0.6, which exhibits the best d 33 value (272 pC/N) than other ZnO-doped LKNNT ceramics. Figure 6. The dielectric constant for the LKNNT + (x)ZnO ceramics as measured at 1KHz / 25~500  C. Figure 7. The Curie temperature vs. ZnO contents of the LKNNT + (x)ZnO ceramics (measured at 1KHz). X=0.2X=0.4X=0.6X=0.8X=1 ε max T C ( o C) T O-T ( o C) Table 1. Properties of the LKNNT + (x)ZnO ceramics. Conclusions Lead-free LKNNT + (x)ZnO piezoelectric ceramics were successfully prepared by adding little amount of ZnO. The structures, dielectric and piezoelectric properties of LKNNT + (x)ZnO ceramics were investigated. All samples show pure perovskite phase with typical orthorhombic symmetry. Compared to pure LKNNT ceramic (sintered at 1090  C, d 33 = 279 pC/N, and k p = 0.46), even though the optimal d 33 and k p values were only 272 pC/N and k p = 0.44 for x = 0.6 specimen, but the optimal sintering temperature have been improved from 1090  C to 1020  C effectively. Figure 1. XRD patterns of 1020  C-sintered LKNNT + (x)wt% ZnO ceramics. Figure 3. Relative Density variation as a function sintered temperature for the LKNNT + (x)ZnO ceramics. It is clear that for x = 0 and sintering temperature is 1090  C, the maximum k p value is Whereas increasing ZnO contents, the maximum k p value is 0.43 for x = 0.4 and 0.44 for x = 0.6, respectively. However, owing to the inferences of ZnO liquid-phase sintering promoters, their optimal sintering temperature all had been decreased to only 1020  C significantly. The k p values will first increase, and then reach to saturated maximum values gradually as sintering temperature is higher than 1020  C, and finally start to decrease as it higher than 1090  C. Especially for x = 0.6 specimen, its k p value (k p = 0.44) is better than others but for x = 0 (k p = 0.46). The T C values, which are concluded in Table 1 and Fig. 7. The Curie temperature (T C ) in this study is observed to be gradually increasing with the increase of ZnO content, and maximum relative dielectric constant (  max ) is 5081 for x = International Conference on Innovation, Communication and Engineering October , 2015, Xiangtan, Hunan, P.R. China