Investigations On Ferroelectric Domain And Fatigue Properties Of BNT-based Lead-free Piezoelectric Materials

Lead-free environmental friendly piezoelectric ceramics with the composition near the morphotropic phase boundary have gained extensive attentions because of their high performance, for example,(1-x)Bi0.5Na0.5Ti O3-x Ba Ti O3(BNT-BT) lead-free system.As is known to us, structure and switching of ferroelectric domain have direct impact on both the properties of piezoelectric materials and the applications, so it is essential to understand the laws of the ferroelectric domain transformation under different conditions. In addition, pure BNT-BT ceramics would appear severe fatigue phenomenon which hinders its practical applications. Therefore, the development of lead-free piezoelectric materials with high-performance and stable electrical properties becomes an important and hot research topic. In present work, focusing on the ternary piezoelectric ceramic(0.935-x)BNT-0.065BT-x Sr Ti O3(BNBSTx), we studied the composition, temperature, electric fielddependence of the ferroelectric domain structure and fatigue behavior. The main results are as follows:1. Through in-situ observation of domain structure of BNSBTx ceramics, it was found that ferroelectric domain transformed from multidomain state to non-ferroelectric state with the concentration x increasing, accompanied with the degradation of ferroelectric polarization. On the other hand, with the temperature increasing from 25 oC to 150 oC, the domain of all compositions gradually became ambiguous and finally vanished. The temperature corresponding to disappearance of ferroelectric domain decreased with the increase of Sr Ti O3(ST) concentration. 2. The phase transformation of BNBSTx under external electric field was studied via in-situ polarization. A low electric field could induce the irreversible phase transformation from rhombohedral to tetragonal phase under low concentration x. The critical electric field for phase transformation increased with the ST concentration increasing. The reversible domain evolution is considered as the major reason for the large strain response.3. The BNBST with a high ST content owning an ergodic relaxor state exhibited fatigue-free behavior after 106 cycles. Furthermore, unexpected almost fatigue-free behavior was also observed in BNBST with a low ST concentration which has a typical ferroelectric long-range order. Based on the data of the complex impedance spectra and XRD, we assume that the clearly observed fatigue-resistant behavior in BNBST solid solution should be mainly attributed to the lower defect density. The different fatigue response in Ec was considered to be related to the field-induced phase transition process.