Study On Preparation And Properties Of Potassium-sodium Niobate Based Lead-free Piezoelectric Ceramics

Potassium-sodium niobate (K0.5Na0.5NbO3, abbreviated as KNN) based lead-free piezoelectric ceramics are considered one of the best candidate materials to replace lead-based piezoelectric ceramics because of its good performance, high Curie temperature, excellent environmental compatibility. However, there is a certain gap compared with lead-based materials on electrical properties. In this dissertation, we take two approaches of improving the preparation of ceramics and adding a second component to regulate the structure in order to improve the electrical properties of KNN based lead-free piezoelectric ceramics. The main research contents are as follows:First, the potassium-sodium niobate based lead-free piezoelectric ceramics were fabricated by tape casting process using the plate-like K0.5Na0.5NbO3 (abbreviated as KNN) as template particles, and using the (K0.45Na0.55)0.98Lio.o2(Nb0.77Ta0.18Sb0.05)03-0.005BaZrO3 (abbreviated as KNNLST-BZ) as base powder. The influence of organic additive agent on the tape casting forming technology and the effects of sintering temperature, soaking time, sintering time, template content on the microstructure and piezoelectric properties of the textured KNNLST-BZ ceramics were investigated systematically. The results showed that the uniform, stabilized slurries and high quality green tape are obtained with 55 wt%powder (20 wt% template particles),45wt% solvent (alcohol, butanone),2.0wt% dispersing agent (TEA),3.0 wt% binder (PVB) and 4.0 wt% plasticizer(DBP). And the high performance of the textured KNNLST-BZ ceramics with the template content for 20wt% sintered at 1145℃ for 2h are obtained:piezoelectric constant d33=204 pC/N, transverse electromechanical coupling factor k31=23%, dielectric loss tanδ=6.35%, mechanical quality factor Qm=19.64, dielectric constant εT33/ε0=611.6, transverse stretching vibration frequency constant N1=1807, remnant polarization Pr=26μC/cm2, coercive fieldEc=1.0kV/mm. At the same time, the textured KNNLST-BZ ceramics after sintered 2 times at 1145℃ exhibit the better performance:d33=248pC/N, k31=33%, tan6=6.38%, Qm=16.76, εT33/ε0=913.7,N1=2244, Pr=38μC/cm2 and Ec=1.2kV/mm. With template content increasing, d33, k31 value of the textured KNNLST-BZ ceramics sintered at 1150℃ for 2h increase, then decrease when template content is more than 10%, the maximum value d33=286pC/N, k31=26% for template content is 10%.Second, (1-x)(K0.49Na0.50)(Nb0.95Sb0.05)O3-xBi0.5(Na0.82K0.18)0.5ZrO3 [(1-x)KNNS-xBNKZ, x=0-0.05] lead-free piezoelectric ceramics were synthesized by the solid state reaction method. The effects of different BNKZ content on the crystalline phase, micro structure and electrical properties of the (1-x)KNNS-xBNKZ ceramics were investigated. The results show that the orthorhombic-tetragonal in these ceramics appear to be at 0.02≤x≤0.03. The coexistence of orthorhombic-tetragonal-rhombohedral phases are observed when x=0.04. And the phase structure is transformed to rhombohedral phase when x=0.05. The high density and uniform grain of KNNS-BNKZ ceramics sintered at optimum sintering temperature about 1110℃ for 3h is obtained. The introduction of BNKZ can inhibit the grain growth, the average grain size of the ceramics is about 4～6μm when x=0.04, then reduce to about 1μm when x increases to 0.05, The piezoelectric constan d33 firstly increases and then decreases with the increasing of x doping amount. The optimized piezoelectric properties can be obtained for the ceramics with x=0.04 as follows: d33=420pC/N,kp=38%,Qm=27, εT33/ε0=2249, tanδ=3.56%, Pr=20μC/cm2, Ec=0.9kV/mm. The dielectric temperature spectrum indicated:with the increasing of BNKZ, Tc and To-T of the KNNS-BNKZ ceramics decrease gradually,Tc=215℃ and TO-T drops to room temperature when x=0.04.