CHEN Jiaqi, XIAO Huan, LIN Yongde, YANG Chunrong, PENG Ping

(Fujian University of Technology, Fuzhou 350108, Fujian, China)

Extended Abstract:[Background and purpose] Piezoelectric materials are widely used in the fields of actuators, transducers and sensors. Lead zirconate titanate (PZT) has been dominant in the market, due to its excellent piezoelectric properties. However, the toxicity of lead poses serious environmental and health risks. Bismuth sodium titanate has attracted much attention for its high remanent polarization and high coercive field, which is expected to be one of the lead-free alternatives to PZT. In addition, the multifunctionality of lead-free ferroelectric materials has become a hot topic in recent years, especially the ferroelectric-optical coupling effect. Rare earth ion doping is an effective approach to realizing the multifunctionality in ferroelectric materials, due to their complex energy level structures which can endow excellent luminescent properties. In this study, (Bi_0.5Na_0.5)(Ti_0.995RE_0.005)O₃ (BNT-0.005RE, RE=Er, Pr, Sm) ceramics were prepared by using the traditional solid-state method, while their phase structure, dielectric properties, ferroelectric properties and photoluminescence properties were systematically explored.[Methods] BNT-0.005RE ceramics were prepared by using the traditional solid-state method. The raw materials included Bi₂O₃ (99.999%), Na₂CO₃ (99.5%), TiO₂ (99.8%), Er₂O₃ (99.99%), Pr₆O₁₁ (99.9%) and Sm₂O₃ (99.9%). These powders were weighed according to the stoichiometric composition and mixed in ethanol, with zirconia balls as the ball-milling medium for 6 h. The mixtures were then calcined at 850 ℃ for 4 h, followed by ball-milling for 10 h. After drying, 8 wt.% polyvinyl alcohol (PVA) was added as a binder and the powders were pressed into discs with a diameter of 12 mm and a thickness of 1 mm. Finally, the samples were sintered for 2 h in a covered alumina crucible at temperatures of 1140–1160 ℃. Phase composition of these ceramics was analyzed using an X-ray diffraction (Bruker D8 Advanced XRD, Germany) with Cu-Kα radiation. Surface microstructure of the ceramics was characterized by using scanning electron microscopy (SEM; S-3400N, Japan), while the average grain size was calculated using the Nano Measurer software. Ferroelectric properties (P-E) were measured by using a ferroelectric analyzer (aix ACCT TF 2000, Germany). Dielectric temperature properties of the samples were tested from room temperature to 200 ℃, using an LCR meter (TH2832, Tonghui, Changzhou, Jiangsu). Photoluminescence excitation (PLE) and photoluminescence (PL) spectra were recorded using the FLS980 fluorescence spectrometer (Edinburgh Instruments).[Results] According to the XRD patterns of BNT and BNT-0.005RE ceramics, it can be concluded that the rare earth ions have entered the BNT lattice. A (1–11) splitting peak at a low angle near the (111) peak is present, while the (200) peak is a single peak, indicating that the samples are of the rhombohedral R3c structure. In BNT ceramics, the rhombohedral R3c phase is regarded as a long-range ferroelectric ordered phase, which is conducive to the acquisition of strong ferroelectricity. As can be observed from the SEM images and grain size distribution profiles, the grain size was significantly reduced, after the doping with rare earth ions. Dielectric constant (εr) increases and the dielectric loss (tanδ) rises after the doping with rare earth ions, which may be related to the reduction in grain size of the ceramics. The depolarization temperature of the ceramic samples decreases after doping with rare earth ions, as compared with that of pure BNT ceramics. This may be ascribed to the decrease in chemical orderliness, leading to reduction in the ferroelectric-relaxor phase transition temperature. Moreover, the doping with rare earth ions significantly increase the remanent polarization of the ceramics, with the 0.005Pr sample exhibiting the highest remanent polarization (Pr) of 40.4 μC·cm⁻², an increase of about 80% as compared with the pure component. This is attributed to the solid solution of a small quantity of rare earth ions in the BNT lattice, which enhances lattice motion, leading to lattice distortion and local inhomogeneity, thereby enhancing the polarization at external electric fields and improving ferroelectric properties. Furthermore, due to the luminescence activity of the rare earth ions, all the rare earth-doped samples emitted visible light under near-ultraviolet excitation, exhibiting strong green (Er) red (Pr) and orange-red light (Sm), respectively.[Conclusions] Rare earth ions have entered the BNT lattice without the formation of second phases, while the samples exhibit rhombohedral R3c structure, which is conducive to the acquisition of strong ferroelectricity. The doping with rare earth ions reduces the grain size of the ceramics and increases the dielectric constant while increasing the dielectric loss, but it also lowers the depolarization temperature. The ferroelectric properties of the ceramics are significantly enhanced after rare earth doping, with the remanent polarization of the 0.005Er, 0.005Pr and 0.005Sm ceramics increasing from 22.4 μC·cm⁻² of the pure component to 39.7 μC·cm⁻², 40.4 μC·cm⁻² and 39.9 μC·cm⁻², respectively. This is attributed to the increase in lattice distortion and local inhomogeneity, which increases the polarization. All the samples doped with rare earth ions exhibit strong luminescent characteristics, with the ceramics doped with Er, Pr and Sm emitting green, red and reddish-orange light, respectively, under near-ultraviolet excitation. Therefore, rare earth ion doping provides a design strategy for multifunctional ferroelectric devices.

Key words: BNT-based ceramics; rare earth doping; ferroelectricity; dielectric properties; luminescence