Large Electromechanical Response in Lead-Free La-Doped BNKT-BST Piezoelectric Ceramics

摘要

Piezoceramics 0.99[(Bi_(0.5)Na_(0.4)K_(0.1))_(1-x)La_xTiO_3] - 0.01 [Ba_(0.7)Sr_(0.3)TiO_3] (BNKT-BST-La_x, x = 0-0.030) were synthesized using a conventional solid-state reaction method. X-ray diffraction revealed a phase transition from a tetragonal to cubic phase at x ≥ 0.005. The maximum dielectric constant as well as the depolarization temperature (T_d) decreased with increasing La content. La addition interrupted the polarization and strain hysteresis loops and demonstrates that the ferroelectric order of the BNKT-BST ceramics lead to a reduction in the remnant polarization and coercive field. However, the destabilization of the ferroelectric order is accompanied by a significant increase in the unipolar strain which is highest at x = 0.020 with a value of ～0.39% and corresponding normalized strain, d~*_(33) (= S_(max)/ E_(max)) of 650 pm/V. It was observed that the unipolar strain of x = 0.020 is very temperature insensitive up to 125℃, even at 125℃ the d~*_(33) is as high as ～431 pm/V. Moreover, an electric-field-dependent XRD was conducted to identify the main source of the high strain and a recoverable transformation from cubic to a rhombohedral-tetragonal mixed phase was observed. The recoverable field-induced phase transformation is suggested to be the main cause for the obtained large strain at x = 0.020 in the BNKT-BST-La_x ceramics.