Research On The Fabrication&Properties Of NaNbO₃-Ba_0.6(Bi_0.5K_0.5)_0.4TiO₃Lead-free Piezoelectric Ceramics

Lead-based piezoelectric ceramics have been widely used in actuators,transducers, sensors and other electromechanical devices due to their excellentpiezoelectric properties. But, lead-based ceramics contain a large amount of toxic leadoxide and thus the use of these ceramics has caused serious environmentalproblems.Therefore, there is a great need to develop “green” lead-free piezoelectricceramics with excellent properties and environmental friendliness.In this thesis, the new （1-x） NaNbO₃-Ba_0.6(Bi_0.5K_0.5)_0.4TiO₃+1mol%MnO₂（abbreviation for NN-BBKT） lead-free piezoelectric ceramics has been prepared by atraditional solid-state reaction method and their phase transition, crystalstructure,ferroelectric and piezoelectric properties were studied; and the effects ofMnO₂doping on the microstructure and electrical properties have been alsoinvestigated.The results show that: The （1-x） NaNbO₃-Ba_0.6(Bi_0.5K_0.5)_0.4TiO₃+1mol%MnO₂lead-free piezoelectric ceramics possess a pure perovskite structure and no secondaryphase. A low Ba0.6（Bi0.5K0.5）0.4TiO3（BBKT）doping level （x≥0.025） transforms theNaNbO3ceramics from antiferroelectric to ferroelectric. The ceramics with low BBKTconcentratoin are normal ferroelectrics, but the ferroelectric-paraelectric phase becomediffusive with x increasing. As the BBKT doping level increases, the Curie temperatureof the ceramics decreases, while the relative permittivity εrincreases monotonously.The ceramics have an orthorhombic at x≤0.075and then transform gradually to atetragonal structure with x further increasing, and finally change into a pseudocubicphase. A morphotropic phase boundary （MPB） is formed at0.075<x<0.15. It worthnote that, the ceramic has a very high Qmvalue of1862at x=0.05, and the ceramicswith x=0.10-0.175exhibit the optimum piezoelectric properties, giving piezoelectricconstants d₃₃=105-131pC/N and planar electromechanical coupling factor k_p=20.8-26.5%. The ceramics are transformed from tetragonal to pscudocubic phases by increasingthe doping level of MnO₂. After the addition of MnO₂, the Curie temperture TCof theceramics decreases and the ferroelectric-paraelectric phase transition at TCbecomesmore diffusive. Because of the donor and acceptor doping effects of Mn ionssimultancously, the piezoelectric constant, electromechanical coupling coefficient,relative permittivity and mechanical quality factor are enhanced considerably afteraddition of1mol%MnO₂. d₃₃=131pC/N、k_p=21.8%and Q_m=627.