Structure And Properties Of Potassium Sodium Niobate-Based Lead-Free Piezoelectric Ceramics Sintered In Different Atmospheres

In this dissertation,lead-free piezoelectric?K,Na?NbO₃-Bi_0.5Na_0.5ZrO₃?KNN-BNZ?ceramics were investigated.The effect of Mn,Ta,Ca ZrO₃ dopants on microstructure and electrical peroperties of KNN-BNT ceramics sintered in different atmosphere was studied systematically.Solutions to improve temperature stability and electrical peroperties at the same time are discussed.The microstructure and electrical peroperties of Mn O doped 0.955K_0.5Na_0.5NbO₃-0.045Bi_0.5Na_0.5ZrO₃?KNN-0.045BNZ+x%MnO?ceramics sintered in air or reducing amtophere were investigated systematically.We clarified the mechanism of the occupying behavior of Mn ions on the anti-reduction properties of the system.The change of defects type under different MnO content was studied.Effect of different defects on piezoelectric properties was elaborated clearly.The CaZrO₃ dopant can improve the piezoelectric properties and temperature stabilityofthedesigned0.96K_0.46Na_0.54Nb_0.98Ta_0.02O₃-0.04Bi_0.5(Na_0.82K_0.18)_0.5Zr O₃)+0.2%mol Mn O ceramics with tetragonal?T?phase and orthorhombic?O?coexisting phases.Ca Zr O₃ dopant induced the temperature stability of inverse piezoelectric coefficient(d₃₃^*)to increase from 110 ? and 200 ?.1% CaZrO₃ doped 0.96K_0.46Na_0.54Nb_0.98Ta_0.02O₃-0.04Bi_0.5(Na_0.82K_0.18)_0.5ZrO₃)+0.2% mol MnO ceramics showed optimal piezoelectric properties(d₃₃=300 pC/N and d₃₃^*=500 pm/V at 25kV/cm),and the room piezoelectric strain?0.17%?at 35kV/cm varied less than 10%from room temperature to 120 ?.Effects of Ta₂O₅dopant on microstructures and electrical peroperties of mol Mn O ceramics sintered in reducing atmosphere were investigated systematically.In thermally stimulated depolarization current?TSDC?of the system,we consider the current density is derived from the oxygen vacancies(?V_O^??with single ionized state.Formation of(V_O^?was benefited to improve the insulation resistivity of ceramics.The ceramics at x=0.02 showed good resistance (³.54×10¹¹?·cm),the optimum piezoelectric properties(d₃₃=278pC/N,d₃₃^*=500pm/V at 25kV/cm)and favorable temperature stability?Te=120 ??.Effects of sintering temperature on microstructures,piezoelectric properties and temperaturestabilityof0.985(0.96K_0.46Na_0.54Nb_0.98Ta_0.02O₃-0.04Bi₍0.50(Na_0.82K_0.18)0.5ZrO₃)-0.015 CaZrO₃+0.2%mol MnO ceramics were investigated.An abnormal growth region in ceramics sintered at 1180-1190 ? was found with many fine grains surrounding around coarse grains.Such abnormal region was good for improving the inverse piezoelectric coefficient d₃₃^* of ceramics without deteriorating the temperature stability visibly.The multilayer piezoelectric devices with ten layers,each layer thickness of 27?m sintered in air?dimension of 8.3 mm×8.26 mm×0.6mm?were fabricated successfully and showing large displacement?690nm?and high temperature stability?175??.The structure,micro-morphology and piezoelectric properties of bulk ceramic materials and multilayer devices were studied,which provided an effective way to solve the contradiction between enhancing temperature stability and improving piezoelectric properties.