| Sodium bismuth titanate(Na0.5Bi0.5TiO3, abbreviated as NBT), is a kind of A-site substituted perovskite-type ferroelectric with a relatively high Curie temerature (Tc =320℃). For its strong ferroelectrics at room temperature, NBT has been considered to be a promising candidate material for lead-free piezoelectric ceramics. In this dissertation, the new NBT-based lead-free piezoceramics have been prepared by adding third substance to form solid solution systems, doping with different ions in A-site and using new preparation techniques, respectively. The phase structure, microstructure, dielectric, piezoelectric and ferroelectric properties were carefully investigated. The relaxation phase transition characteristic for NBT-based ceramics were discussed in terms of experimental results.(1)(1-x)(Na0.5Bi0.5)0.74Sr0.26TiO3-xNaNbO3(NBSTNN100x) ceramics were prepared by adding NN to form solid solution systems. The results show: NN added did not destroy the crystal structure. With the increasing of the additive amount of NN, grain size increases. Dielectric peaks become broader and move towards low temperature, when x up to 0.30, only one dielectric peak can be seen, shows typically characteristic of relaxor ferroelectrics. The dielectric constantεr, dielectric loss tgδ, coercive field Ec, remanent polarization Pr and saturated polarization Ps all decreases with the increase of NN. When x=0.10, ceramic with properties, Tc of 110.9℃,εr of 2843.88, tgδof 0.0338, Ec of 1.52KV/mm, Pr of 8.05μc/cm2 and Ps of 18.42μc/cm2 were obtained.(2)(Na0.97–xKxLi0.03)0.5Bi0.5TiO3(NBTKL100x) multicomponent ceramics were obtained by doping same value ions K+ and Li+ in A-site. The temperature dependence of dielectric constant curve exhibit two dielectric anomalies at Td and Tm. With the increasing of the amount of K+, ceramics will occur transition with rhombohedral and tetragonal co-existed crystal structure, the growth of grains were suppressed, the maximum value of dielectric constantεm increases, ferroelectric-antiferroelectric transition temperature Td move towards low temperature while antiferroelectric- paraelectric transition temperature Tm move towards high temperature. Being a kind of typical relaxor ferroelectrics, the ceramics hasεr=6636.45, Tm=335.1℃, d33=80pC/N,Ec=1.40KV/mm, Pr=10.99μc/cm2, Ps=29.35μc/cm2, showing excellent integrated properties.(3)Rare Earth ion La3+ first doped into 0.97Na0.5Bi0.5TiO3-0.03K0.5Na0.5NbO3 (0.97NBLT-0.03KNN) binary solid solution system, then cold isostatic pressing technique was used to prepare compact ceramics with minicrystal structure. X-ray diffraction patterns indicate that La3+ enters into crystal lattice completely within the range of 0.015≤x≤0.105. With the increasing of La3+ doping amount, grain size becomes smaller and shows diffuse distribution, relaxor characteristic becomes more obvious, depolarization temperature Td decreases while Curie temperature Tm increases. At the same time, dielectric constantεr, dielectric loss tgδand piezoelectric constant d33 all gradually decreases. All samples possessed the characteristic of diffuse phase transition, frequency dispersion and large degree of diffusenessα, showing that the ceramics are typically relaxor ferroelectrics. By using cold isostatic pressing technique, the sintering temperature and growing up of grains can be suppressed, the intensity of ceramics and dielectric, piezoelectric properties are enhanced . When x=0.015,εr is 2246.22, tgδis 0.041, Tm is 316.7℃, d33 is 88pC/N which are the best dielectric and piezoelectric properties of 0.97NBLT-0.03KNN ceramics.
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