Haiyan Ge

Facile synthesis and high d33 of single-crystalline KNbO3 nanocubes

Single-crystalline KNbO(3) nanocubes with orthorhombic phase were prepared in a large scale by a simple one-step molten salt route without using any surfactant as template; the nanostructures exhibited high piezoelectric properties such as d(33)=105 pC/N and k(p)=0.34 as piezoelectric materials.

The investigation of depoling mechanism of densified KNbO3 piezoelectric ceramic

Thermal depoling phenomena determine the upper temperature limit of the piezoelectric ceramics in application. In this paper, high-densified KNbO3 ceramics derived from single-crystalline nanostructures exhibited favorable piezoelectric constants d33, which varied from 105 to 80 pC/N over a broad temperature range from 25 to 225 °C. In situ x-ray diffraction combined with Raman spectra demonstrate clearly the transition sequence of crystallographic orientations during thermal depoling process. The interaction between defect dipoles D and spontaneous polarization inside domains Ps favored to preserve piezoelectric activity, while the spontaneous rotation of Ps induced by the phase transition resulted in the deliquesce of d33.

Synthesis and Piezoelectric Properties of KNbO3 Ceramics by Molten-Salt Synthetic Method

Potassium niobate (KNbO3) single-crystalline nanocubes have been successfully synthesized through a large-scale and facile molten-salt synthetic method. High-density KNbO3 ceramics can be obtained by the conventional sintering of the synthesized KNbO3 single-crystalline nanocubes without the addition of sintering additive. The crystal structure, microstructure, and dielectric and piezoelectric properties of these ceramics were characterized. The KNbO3 ceramics show a comparatively saturated polarization–electric field (P–E) hysteresis loop. The remnant polarization Pr and coercive field Ec are 23 µC/cm2 and 8 kV/cm, respectively. The KNbO3 ceramics also show enhanced piezoelectric properties in terms of the planar electromechanical coupling factor kp, the piezoelectric constant d33, and the mechanical quality factor Qm, which are 0.34, 105 pC/N, and 216, respectively.

The variation of Curie temperature and dielectric relaxor behaviour in the nominal (1-x)BaTiO3-xBiAlO3 system

BiAlO3 is a perovskite ferroelectric with a high Curie temperature above 520°C. However, it is difficult to prepare pure perovskite phase by the conventional method. In this experiment, BaTiO3 has been selected as host to incorporate BiAlO3, and the nominal binary system of (1-x)BaTiO3-xBiAlO3 has been investigated with regard to its Curie temperature (Tc) and dielectric relax behaviors. It was found that the pure perovskite phase can be retained below x = 0.08. However, the value of Tc in the system doesn’t follow the normal Vegard’s law, on the contrary, shows the decreasing trend with the increase of x. According to the compared experiments about Bi3+ and Al3+ doping in BaTiO3, it can be safely concluded that the combined substitution of Bi3+ and Al3+ played the dominant role to decrease the Tc and increase the relaxor behavior in (1-x)BaTiO3-xBiAlO3 system.

Size dependence of the polarization and dielectric properties of KNbO3 nanoparticles

Ferroelectric nanostructures have recently attracted much attention due to the continuous demand of miniaturizing devices and discovering novel phenomena. Therefore, in depth study of the ferroelectric, and dielectric properties at various particle sizes in nanoscale ferroelectrics is of technological importance. Unfortunately, research aimed at discovering and understanding the size effect on the electric properties of ferroelectric nanostructures are scarce. Here, we investigate the spontaneous polarization and dielectric constant at various sizes of nanocrystalline potassium niobates (KNbO3) by using powder X-ray diffraction (XRD) Rietveld refinement, Raman spectroscopy, and theoretical calculation. It is surprising to find that the spontaneous polarization Ps reaches the highest value of 40.3 μC cm−2 at 100 nm, and decreases with increasing grain size from 100 nm to 500 nm. The dependence of the dielectric constant on the grain size shows similar variation as that of spontaneous polarization. It is deduced that the large lattice distortion induced by lowering the grain size facilitates the increase of Ps, which in turn contributes mainly to the enhancement of the dielectric constant.

Bismuth-induced ferroelectric relaxor behavior in paraelectric LaAlO3

Perovskite-type dense ceramics (La0.9Bi0.1)AlO3 (LBAO) have been prepared through a solid-state reaction route. LaAlO3 is indeed a paraelectric, whereas a significant ferroelectric relaxor behavior is obtained in LBAO. The variations of ferroelectric hysteresis loops versus the applied field or frequency indicate the nature of the ferroelectric domain switching. A broad dielectric anomaly coupled with the shift in dielectric maxima toward a higher temperature with increasing frequency has been observed in LBAO, and the indicator of degree of diffuseness γ is about 1.86. Moreover, the dielectric relaxation, which follows the Vogel–Fulcher relationship with TVF = 706.9 K, f0 = 1.85 × 109 Hz, and Ea = 0.09 eV, further supports spin-glass-like characteristics. Compared with LaAlO3, the abnormal contraction of crystal cell is observed in LBAO, which does not follow the normal Vegard’s law. This can be attributed to the covalent hybridization of the Bi3+ ions with the surrounding O2- ions. The large local displ...

LOW-FIRING SOL–GEL DERIVED Mn-DOPED 0.92(K0.5Na0.5)NbO3–0.08LiNbO3 LEAD-FREE CERAMICS WITH ENHANCED PIEZOELECTRIC PROPERTIES

Mn-doped 0.92(K0.5Na0.5)NbO3–0.08LiNbO3 (0.92KNN–0.08LN) lead-free piezoelectric ceramics have been prepared by a developed sol–gel routes using Nb2O5 as Nb source. Due to the high quality of gel precursor, Mn-doped 0.92KNN–0.08LN nanopowders with the mean particle size about 20 nm were obtained at 500°C. The addition of Mn can promote the densification of nanopowders effectively at 950°C, and also influences in a pronounced way both the crystal structure and microstructure of the materials. With the increase of the amount of Mn addition, 0.92KNN–0.08LN ceramics are transformed from dominant orthorhombic to coexistence of the orthorhombic and tetragonal phases, and the grain sizes are enhanced simultaneously. Optimized parameters, such as d33 = 212 pC/N and kp = 0.46, were achieved in 0.2 Mn-doped 0.92KNN–0.08LN systems, which are good candidate material for lead-free piezoelectric devices.

A transmission electron microscopy study of submicrometer BiFeO 3 particle prepared by an improved solid state reaction

In this work, submicrometer bismuth ferrite (BiFeO3) particles have been prepared at the temperature of 750 °C by the improved solid state reaction, in which the quick sintering step is crucial in obtaining pure perovskite phase. The X-ray diffractometer (XRD) analysis indicates that the obtained BiFeO3 particles are rhombohedral structure, and the refined cell parameters are a=5.622Å and α=59.355° (R=8.35%), similar to the reported data (JCPDS Card 86–1518, a=5.632Å and α=59.356°). The microstructure has been observed using transmission electron microscope (TEM), and the image shows that BiFeO3 powders have the quasi spherical morphology with the mean particle size between 100–150nm. Further, the distinct lattice fringes reveal the nature of the single-crystalline BiFeO3 nanostructure, and the interplanar spacing is about 0.87nm, corresponding well to the double spacing of (012). Moreover, it is interesting to note that there existed the dislocation in BiFeO3 particle, which is believed to arise from the collision of different grains in growing process. The sintered compact bodies from BiFeO3 fine particles show the slim ferroelectric loops, and dielectric properties are frequency independent. The measured dielectric constant and loss tangent are 19.5 and 0.0025 at room temperature, showing the potential application in multiferroic devices.