Study On Preparation And Doping Effects Of Potassium Niobate-based Lead-free Piezoelectric Ceramics

Piezoelectric ceramics have been widely used for sensors, ultrasonic transducers and other electronic devices. Most of them are lead oxide–based piezoelectric ceramics, which contain more than 60 wt% lead. Lead is a very toxic substance, and the evaporation and contamination of toxic lead during the fabrication and disposal can cause a crucial environmental pollution. Therefore, it is necessary and urgency to develop lead–free piezoelectric ceramics to replace lead–based piezoelectric ceramics. (Na0.5K0.5)NbO3 (NKN) has been considered as a good candidate for lead–free piezoelectric ceramics due to its good piezoelectric properties. In this paper, 0.94(Na0.5K0.5)NbO3–xLiSbO3–yLiNbO3 (KNLNS) were prepared by the conventional mixed oxide route with normal sintering. The phase structure,piezoelectric and dielectric properties were investigated.0.94(Na0.5K0.5)NbO3–xLiSbO3–yLiNbO3 (NKNLS) ceramics were synthesized by traditional sintering process and effects of the LiSbO3 and LiNbO3 amounts on density, phase structure, microstructure and electric properties of NKNLS were investigated. X–ray diffraction (XRD) results showed that NKNLS can not form pure perovskite phase with excess LiNbO3 or LiSbO3 and the second phase K3Li2Nb5O15 with tetragonal tungsten bronze structure appeared, the pure perovskite phase only formed with proper amount of LiNbO3 and LiSbO3. Scanning electron microscope (SEM) observation indicated that the microstructure of NKNLS lead free ceramics exhibits apparent difference due to different amounts of LiSbO3 and LiNbO3. The 0.94(Na0.5K0.5)NbO3–xLiSbO3–yLiNbO3 lead free ceramics exhibits an excellent piezoelectric properties with x=0.03 and y=0.03. The effects of sintering temperature on the density, structure and electric properties of 0.94(Na0.5K0.5)NbO3–0.03LiSbO3–0.03LiNbO3(NKNLS–3) ceramics were also studied. XRD results showed that the crystal structure of the KNLNS ceramic were pure perovskite phase with tetragonal phase structure sintered at T≤1080℃, however, a K3Li2Nb5O15 phase with tetragonal tungsten bronze structure began to appear when the sintering temperature was higher than 1080℃. The optimum sintering temperature is 1080℃which was determined by measuring the density of the samples. SEM observation indicated that the sintering temperature had a great effect on the microstructure of the samples. The KNLNS–3 ceramics under the optimum sintering temperature showed excellent electric properties which as follows:ρ=4.29g/cm3,εr = 826, tanδ=0.049, d33 = 190pC/N, kp = 0.30. To further improve the electric properties of the ceramics, especially for Qm, KNLNS–3 ceramic was investigated by doping CuO and MnO2. The influence of CuO content on the phase structure, microstructure, dielectric and piezoelectric properties of the ceramics was investigated in detail. The XRD analysis results showed that all the ceramics were tetragonal phase. With the increasing of CuO content,εr kept decreasing all the time, Qm and tanδfirstly increased and then decreased. As CuO was 0.3mol%, the ceramics showed better electrical properties. The influence of MnO2 content on the phase structure, microstructure, dielectric and piezoelectric properties of the ceramics was also investigated in detail. The XRD analysis results showed that all the ceramics were tetragonal phase. With increasing the amount of MnO2,εr and d33 also decreased all the time, kp have little change. Qm increased at first and then decreased, whereas, tanδdecreased firstly, then increased. The ceramics with 0.15mol% MnO2 addition showed the optimized electrical properties.