(Bi_0.5Na（0.5）)TiO₃-BaTiO₃Based Lead-free Ferroelectric Ceramics

(Bi0.5Na0.5)TiO3-BaTiO3based ceramics is one of the most promising lead-free ferroelectric ceramic systems. Because of the relatively excellent dielectric, ferroelectric and piezoelectric properties, this system has attracted much interest. In order to meets production needs, it is very important to modify the structure and properties of this solid solution. In this thesis, the dielectric, ferroelectric and the electric-field-induced strain properties of [(Bi0.5Na03)0.94Ba0.06]1-xSrxTiO3ceramics have been investigated together with the structure.Sr-substitution strongly affects the crystallographic structure, dielectric, ferroelectric, and field-induced strain properties of [(Bi0.5Na0.5)0.94Ba0.06]1-xSrxTiO3(x=0,0.025,0.05,0.1,0.15,0.2,0.25,0.3) ceramics. XRD patterns and refinements of XRD data show two kinds of symmetries in the ceramics:P4bmand R3c. For x≤0.05, P4bm and R3c co-exist in the ceramics for x<0.05and P4bm for x>0.05. The P4bm/R3c MPB at x=0.025is confirmed by the variation of Td with x and the measurement of field-induced strain. There are two dielectric anomalies in the permittivity v.s. temperature curve, corresponding to the permittivity-maximum temperature Tm and the depolarization temperature Td. Td indicates the minimum value at MPB, and Tm decreases slowly with increasing Sr content. The dielectric anomaly (around Tm) disappears, when Sr-substitution is more than25%. The dielectric frequency dispersion is observed around Td for all compositions, and all the dielectric peaks (around Td) could be fitted by the Volgel-Fulcher relationship. The saturated P-E hysteresis loops are determined further below Td, and the pinched P-E hysteresis loops are generally indicated between Td and Tm, while the pinched P-E hysteresis loops becomes very weak at x>0.25. Sr-substitution changes the stabilization of the nonpolar state above Td, and suppreses the electric field induced phase transition.The maximum values of S33(0.37%),Q33(0.031m4C-2) and Smax/Emax(535pm/V) of the ceramics are obtained at x=0.025composition. These three values slowly decrease with increasing Sr-substitution after getting the maximum at x=0.025. The field-induced strain mainly due to the electrostriction effect, the existence of MPB results in the best properties at x=0.025.