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

K_0.5Na_0.5NbO₃（KNN）-based lead-free piezoceramics have become a lead-free piezoelectric ceramic system which is expected to replace PZT-based piezoelectric ceramics currently,because of its high piezoelectric constant and C urie temperature.In this paper,a series of doping modification research on KNN-based ceramics were carried out from three aspects,including the doping of late-transition metal oxides and rare-earth oxides,and the construction of new solid solutions.These works lay a theoretical and technical foundation for obtaining KNN-based lead-free piezoelectric ceramics with excel ent performances.The main contents were as follows:1.KNN-based lead-free piezoceramics doped with different late-transition metal oxides（In₂O₃,Bi₂O₃）were prepared by conventional solid state reaction.All the samples showed typical perovskite phase structure.The addition of In₂O₃ improved the fatigue resistance and the temperature stability.In³⁺prefered to occupy the A-site and then B-site with the increase of doping content,which were donor doping and then acceptor doping,and thus led to the different changes in the performance.Bi₂O₃ inhibited the growth of grains and obtained the best electrical properties when x=0.002:d₃₃=229 pC/N,k_p=31.08%,T_C=355°C,S_max=0.20%and d^*₃₃=392 pm/V.2.Differentrare-earthoxidesSc₂O₃,Pr₆O₁₁andNd₂O₃doped(K_0.44Na_0.52Li_0.04)(Nb_0.91Ta_0.05Sb_0.04)O₃（KNLNTS4）ceramics were prepared by the traditional solid phase method.Results showed that the proper amount of Sc₂O₃ doping improved the performance of KNN-based ceramics.The greatest unipolar strain of 0.13%(d₃₃*=448 pm/V)was obtained at applied electric field of 30 k V/cm for x=0.6,and the highest piezoelectric constant reached to 289 pC/N.Both Pr₆O₁₁ and Nd₂O₃ modified ceramics can form the polymorphic phase transition（PPT）at room temperature（RT）at x=0.3 with the optimal electrical properties(P_r=26.1μC/cm²,E_C=15.2 kV/cm,d₃₃=196pC/N,d₃₃*=390 pm/V for KNLNTS4-0.3Pr and P_r=25μC/cm²,E_C=13.8 kV/cm,d₃₃=228 pC/N,d₃₃*=350 pm/V for KNLNTS4-0.3Nd).Besides the excellent electrical properties,the KNLNTS-0.3RE samples exhibited a strong photoluminescence.3.The new solid solutions were formed by doping Bi_0.25Na_0.25NbO₃（BNN）into(Na_0.52K_0.44Li_0.04)(Nb_0.87Sb_0.08Ta_0.05)O₃（KNLNTS8）materials and further optimized their electrical properties of KNN-based materials.Results showed that when x<0.0025,the ceramics fited with the tetragonal phase structure by the（001）/（100）and（002）/（200）characteristic splitting peaks;when 0.0025≤x≤0.003,the new solid solutions were orthorhombic-tetragonal mixed phase;when x>0.003,the samples were dominated with tetragonal phase.The excellent comprehensive properties were achieved at x=0.0025:d₃₃=256 pC/N,T_C=354.27°C,k_p=43.43%,P_r=26.85μC/cm²,E_C=24.47 kV/cm.