Preparation And Properties Of (K_0.44Na_0.52Li_0.04)(Ta_0.2 Nb_0.8)O₃ -xCeO 2 Lead-free Piezoelectric Ceramics

Because of the excellent properties, PZT-based piezoelectric ceramics have been widely used. However, such PZT-based materials contain more than 60% the toxicy lead, thus causes unconsiderable pollution during its manufacturing and using as well as disposaling, thus lead-free piezoelectric materials has arisen more and more attentions in recent years. K0.5Na0.5NbO₃(KNN)-based lead-free piezoeletric ceramics has been expected to be the most potential candidate for lead-based materials because of its good piezoelectric properties and high Curie temperature.In this paper, the development of lead-free piezoelectric ceramics was introduced, the methods of improving the piezoelectric properties of KNN-based ceramics were discussed, lead-free piezoelectric ceramics (K_0.44Na_0.52Li_0.04) (Ta_0.2Nb_0.8)O₃-xCeO₂ (x = 0, 0.0025, 0.005, 0.0075, 0.01, 0.0125) were prepared by conventional sintering technique, and the resulting effects on the structure and properties were studied. The X-ray diffraction(XRD) patterns show that all the ceramics exhibit a pure perovskite structure. From the scanning electron microscopy(SEM) micrograophs, it can be seen that with the increase of CeO₂-doping the ceramics grain size decreases from 5μm to 2μm and the grain becomes uniform, thus the ceramics become dense. The density of the samples was measured using Archimedes method, the result show that as x increases the ceramic density first increase and then decrease. From the temperature dependences of the relative permittivity it can be seen that the ferroelectric orthorhombic-ferroelectric tetragonal phase transition temperature TO-T of all the ceramics was near room temperature and the phase transition temperature range was broadened and smoothed after adding CeO₂. The P-E loop of the samples shows that with the increasing of CeO₂-adding amount, the remnant polarization Pr first increase and then decrease, however the coercive field Ec first decrease and then increase. The piezoelectric coefficient d33 was measured by a piezo-d33 meter and the planar electromechanical coupling coefficient kp and Mechanical quality factor Qm were measured using resonance method. The results indicating that by adding CeO₂ the piezoelectric properties of the ceramics were improved, and with the increasing of CeO₂-adding amount, the piezoelectric coefficient d33, the planar electromechanical coupling coefficient kp as well as mechanical quality factor Qm first increases and then decreases.The ceramic samples exhibite the optimum properties when x = 0.0075: the densityρ= 4.88 g/cm3,the piezoelectric coefficient d33 = 182 pC/N,the planar electromechanical coupling coefficient kp=0.31,the ferroelectric orthorhombic- ferroelectric tetragonal phase transition temperature range is from 25℃to 100℃,the relative permittivityεr=590, the loss tangent tanδ=1.02%,the mechanical quality factor Qm = 28,the coercive field Ec = 1.2 kV/mm,the remnant polarization Pr=4μC/cm2。CeO₂ added (K_0.44Na_0.52Li_0.04)(Ta_0.2Nb_0.8) lead-free ceramics process a broad ferroelectric orthorhombic-ferroelectric tetragonal phase transition temperature range, good temperature stability, excellent piezoelectirc properties, low relative permittivity and small mechanical quality factor, thus it has a wide application prospect in ultrasound transducers.