Preparation And Electric Characteristics Of Polar Semiconductor/Relaxor Ferroelectric Composites

Piezoelectric ceramics have wide application in various fields, such as actuators, transducers, etc. However, up to now, the lead-based ceramics like Pb(Zr,Ti)O3 (PZT) have occupied the dominant place of commercialized application fields, which has caused serious environmental problems and done harm to human health due to the toxicity of lead. From the viewpoint of environmental consciousness and sustainable development, developing high-performance lead-free piezoelectric ceramics have received more and more attention all over the world in recent years. Among the reported lead-free piezoelectric materials, Bio.sNao.sTiO3 (BNT) based solid solutions are considered as one of the most possible lead-free candidates because of their good piezoelectric property, excellent reproducibility and high maximum dielectric constant temperature (Tm～300℃). However, the depolarization of BNT based solid solutions at around 100℃ (Td), which leads to disappearance of macroscopic ferro-/piezoelectric behavior and determines the up-limit working temperature, becomes a longstanding obstacle for their practical application. In this paper, following works aiming at overcoming the depolarization of BNT based solid solutions are carried out:1. The ferroelectric composites of Na0.5Bi0.5TiO3:xZnO (BNT:xZnO, x=0,0.1, 0.2,0.3,0.4) were synthesized by solid state reaction and the composite structure is confirmed by x-ray diffraction (XRD) and scanning electron microscopy (SEM) measurements. The temperature-dependent dielectric constant and loss indicate the Td increases with increasing x, and the depolarization disappears around x= 0.3. All samples show ferroelectric nature at room temperature confirmed by well-shaped polarization-electric field (P-E) loops and current-electric field (J-E) curves. Temperature-dependent ferro-/piezoelectric properties indicate the BNT solid solution tends to depolarize near 100℃, while the BNT:0.3ZnO composite keeps the well-shaped P-E loops up to 125℃. After thermal annealing at 125℃, the retained d33 of BNT solid solution and BNT:0.3ZnO composite is 9pC/N and 22pC/N, respectively, further confirming the deferred thermal depolarization in the composites.2. The ferroelectric composites of 0.94Na0.5Bi0.5Ti03-0.06BaTi03:xZnO (BNT-6BT:xZnO, x=0,0.1,0.2,0.3,0.4) were synthesized by solid state reaction. XRD, SEM and Raman measurements confirm the 0-3 type composite structure of BNT-6BT:xZnO, where the nano-sized ZnO has grown into larger ZnO particles locating at the boundaries between BNT-6BT grains. The temperature-dependent dielectric constant and loss indicate the Td increases with increasing x, and the depolarization disappears around x-0.3. All samples show ferro-/piezoelectric nature at room temperature confirmed by well-shaped P-E loops and butterfly-shaped bipolar S-E curves. Temperature-dependent ferro-/piezoelectric properties indicate the BNT-6BT solid solution tends to depolarize near 50℃, while the BNT-6BT:0.3ZnO composite keeps the well-shaped P-E loops and none-zero negative strain up to 125℃. After thermal annealing at 125℃, the retained d33 of BNT-6BT solid solution and BNT-6BT:0.3ZnO composite is 5pC/N and 80pC/N, respectively, further confirming the deferred thermal depolarization in the composites. The possible mechanism for deferred thermal depolarization of BNT-6BT:ZnO is discussed based charge-order model.