Study On Modifing And Mechanism Of KNN-based Lead-free Piezoceramics

Based on the ferroelectricity and high Curie temperature of the perovskite-type BiFeO3 (BF) and BiMnO3 (BM), and the substitute modifying effect of LiSbO3, the relationship among the composition, structure, phase transition, sintering process, piezoelectric and dielectric properties of KNN-LS-BF(BM) ceramic systems, which were prepared by using the traditional ceramic sintering process, was studied systematically. Several important conclusions can be summarized as follows:The relation between composition and crystalline structure, microstructure, piezoelectric properties, dielectric properties of (1-x)KNN-xBF system ceramics was systemically investigated. It was found that the morphotropic phase boundary(MPB) with orthorhombic and tetragonal co-existence crystalline structure for the ceramics lies in the range of x=0.01-0.02. The best piezoelectric properties of d33= 125 pC/N, kp=40%, Tc=405℃, To-t=185℃,εr= 432, tanδ= 2.93% were obtained at x= 0.008 which is near the MPB composition. The effects of sintering temperature and holding time on the microstructure, phase transition, density and piezoelectric properties of BF doped KNN ceramics were investigated. The 0.8at.% BF-doped KNN ceramics sintered at 1150℃for 2h exhibit excellent properties of p= 4.50 g.cm-3, d33= 132 pC/N, kp= 45%, Qm= 333.73 and tanδ= 2.39%.Studied on comparing the doping content, phase structure, phase transition, microstructure, piezoelectric and dielectric properties of (1-x)(0.95KNN-0.05LS)-xBF ceramics sintered on zirconia and alumina setter plate. Results showed that the piezoelectric properties of the ceramics sintered on the zirconia setter plate were higher than that of the ceramics sintered on the alumina setter plate. It was pointed out that the mechanism of this difference should attribute to whether the elements in the setter plate diffuse into (1-x)(0.95KNN-0.05LS)-xBF ceramics or not. What's more, the results also showed that the addition of Al element was useful for the enhance of the Curie temperature (Tc) of (1-x)(0.95KNN-0.05LS)-xBF ceramics. The relation between composition and crystalline structure, microstructure, piezoelectric properties, dielectric properties of (1-x)(0.95KNN-0.05LS)-xBF system ceramics was also systemically investigated. With increasing the content of BF, the phase structure of the ceramic changed from the co-existence phase structure including the orthorhombic and tetragonal phase to a single tetragonal phase, and the transition point was at x= 0.004. When the content of BF is 0.4at.%, the ceramics sintered at 1100℃for 3h showed good electrical properties:d33= 272 pC/N, kp= 53%, Tc= 345℃,εr= 1103, tanδ=1.89%.The relation between composition and crystalline structure, microstructure, piezoelectric properties, dielectric properties of (0.996-y) KNN-0.004BF-yLS system ceramics was also systemically studied. It was found that the MPB for the ceramics lies in the range of 0.02≤y≤0.04. With increasing the content of LS, the d33, kp andεr of the ceramics firstly increased and then decreased, while the tanδfirstly decreased and then increased. At the same time, both of To-t and Tc of the ceramics shifted to the low temperature with the increasing of LS content. The effects of sintering temperature on the microstructure, phase transition, density and piezoelectric properties of (0.996-y) KNN-0.004BF-yLS ceramics were also investigated. With increasing the sintering temperature, the piezoelectric properties of the ceramics firstly increased and then decreased, while the Tc firstly increased and then decreased. The ceramics sintered at 1100℃had the best piezoelectric properties: d33=297pC/N, kp=54%. BiFeO3 doped and undoped KNN-LS ceramics had a higher depolarizing temperature. The depolarizing temperature of the doped and undoped KNN-LS ceramics was 370 and 390℃, respectively. BiFeO3 doped KNN-LS ceramic show a dielectric constant-temperature behavior with a thermal hysteresis upon heating/cooling from 310-380℃which was corresponding to the temperature range of the mixed phase between tetragonal and cubic phases.The relation between composition and crystalline structure, microstructure, piezoelectric properties, dielectric properties of (1-x)(0.95KNN-0.05LS)-xBM system ceramics was systemically investigated. It was found that the MPB for the ceramics lies in the range of 0≤x≤0.008. When x= 0.01, the structure of the ceramics started to change from the co-existence structure to the tetragonal phase structure. With increasing the content of BM, the d33 and kp of the ceramics firstly increased and then decreased, while the tanδfirstly decreased and then increased. At the same time, Tc of the ceramics firstly increased and then decreased. The best piezoelectric properties of d33=213pC/N, kp= 40.8%, Tc= 370℃, tanδ= 2.6% were obtained at x= 0.004.The relation between composition, phase structure, phase transition, microstructure, piezoelectric properties and dielectric properties of (0.996-y) KNN-0.004BM-yLS system ceramics was also systemically studied. It was found that the MPB for the ceramics lies in the range of 0.03≤y≤0.05. With increasing the content of LS, the d33 and kp of the ceramics firstly increased and then decreased, while Tc of the ceramics shifted to the low temperature. The ceramics at x= 0.05 had the best piezoelectric properties:d33=230 pC/N,kp=42%.The mechanisms of BF(BM) in the KNN-LS ceramics were studied by contrast. The results showed that on the one hand, the effect of BF on the crystalline structure, phase transition, microstructure, piezoelectric and dielectric properties was similar with that of BM, and was differ from that of BM on the other hand. Comparing with BM-doping, the improving effect of BF-doping on the piezoelectric properties of KNN-LS ceramics was greater. The mechanisms of LS and BF(BM) in the pure KNN ceramics were also studied by contrast. The results showed that the effect of LS on KNN-LS-BF(BM) ceramics occupied the leading position. Studying on the frequency dependence of the impedance and phase angle for the ceramics showed that BF doped KNN ceramics showed the "hard" characteristics, while the LS doped KNN and KNN-BF(BM) ceramics showed "soft" characteristics.