Shu Ya Wu

Dielectric anomalies in (BaxSr1−x)4Nd2Ti4Nb6O30 ceramics with various radius differences between A1- and A2-site ions

Dielectric response of tetragonal tungsten bronze dielectrics (BaxSr1−x)4Nd2Ti4Nb6O30 was investigated over a broad temperature and frequency range, and the obvious composition-dependent dielectric anomalies with respect to x value were discussed in detail in association with the radius differences between A1- and A2-site ions. With decreasing the magnitude of radius difference between A1- and A2-site ions, the normal ferroelectric peak above 400K became weaker, and two relaxor peaks at lower temperatures became obvious. The low- and high-temperature relaxor behaviors, which followed well the Vogel-Fulcher relationship, were associated with the polar clusters caused by the off-center Nb∕Ti displacements and the incommensurate tilting modulation, respectively.

Giant dielectric response in two-dimensional charge-ordered nickelate ceramics

Dielectric relaxations of charge-ordered Ln1.5Sr0.5NiO4 (Ln=La and Nd) ceramics were investigated over a broad temperature range. The giant dielectric constant (over 70 000) with a low dielectric loss of ∼0.1 was determined at high frequencies (up to 5 MHz) over a broad temperature range. There are two dielectric relaxations in the vicinity of charge ordering temperatures. The thermal activated small polaronic hopping between two charge ordering temperatures should contribute to the giant dielectric response in the present ceramics. Compared to other giant dielectric constant materials, the present materials have the notable advantage for high frequency applications.

Dielectric and Magnetoelectric Characterization of CoFe2O4∕Sr0.5Ba0.5Nb2O6 Composites

Magnetoelectric composites were created through incorporating dispersed CoFe2O4 ferromagnetic grains into a Sr0.5Ba0.5Nb2O6 relaxor ferroelectric matrix. The two phases are chemically compatible and coexist in the dense, sintered composites. Unlike other relaxors, the Sr0.5Ba0.5Nb2O6-containing composites can be electrically poled. The apparent dielectric constant is enhanced by the conductivity in the ferrite phase and the space charge at the phase interface. A significant magnetoelectric effect was observed in all compositions investigated, with a maximum dE∕dH value of 24.8 mV/cm/Oe obtained for 0.4CoFe2O4∕0.6Sr0.5Ba0.5Nb2O6, which is much greater than that of magnetoelectric compounds and solid solutions. These results demonstrate that the Pb-free (Sr,Ba)Nb2O6 relaxor can be advantageously used as the matrix for ferromagnetic/ferroelectric composites in the future development of magnetoelectric materials.

Raman spectra of Nd∕Sn cosubstituted Ba6−3xSm8+2xTi18O54 microwave dielectric ceramics

The Raman spectra and dielectric properties of Nd∕Sn cosubstituted Ba6−3xSm8+2xTi18O54 (x=2∕3) microwave dielectric ceramics were discussed as the functions of composition and sintering time. The peaks in 753cm−1 were caused by the second order scatter. The peaks in 425 and 403cm−1 became sharper with prolonging sintering time, and this reflected the increased lattice defects. The shoulder peak near 292cm−1 was caused by the octahedral tilt when A site is Nd3+. The Raman shifts in 590, 520, 280, and 232cm−1 indicated no obvious change in position, but all peaks became sharper with prolonging sintering time. This indicated the increased ordering degree of A-site cations. With prolonging sintering time, the Qf factor (Q is the inverse of dielectric loss, tan δ, and f is the resonant frequency) increased, and the temperature coefficient of resonant frequency significantly decreased or became more negative, while the dielectric constant indicated no significant variation.

Infrared reflection spectra of Ba6−3xSm8+2xTi18O54 (x=0.5, 0.67, and 0.75) microwave dielectric ceramics

Infrared reflection spectra of the tungsten bronze-type Ba6−3xSm8+2xTi18O54 (x=0.5, 0.67, and 0.75) ceramics were measured and analyzed by a combined method using Kramers-Kroning relation and the classical oscillator model, and the selection rules and vibration modes of the tungsten bronze-type structure were investigated by a detailed group theory analysis. Microwave dielectric properties were calculated using the dispersion parameters obtained by the classical oscillator model. The error in reflectance due to the random scattering on sample surfaces indicated obvious effect on the calculated dielectric constant, but only slightly influenced the calculated Qf, and therefore, the present analysis provided a reasonable means to predict the intrinsic microwave dielectric loss. The predicted intrinsic Qf values (>30000GHz) were much higher than the experimental ones (∼10000GHz), and this suggested the great potential of significant improvement of Qf value in Ba6−3xSm8+2xTi18O54 ceramics by controlling the ex...

A novel sol–gel route to synthesize (Sr0.5Ba0.5)Nb2O6 ceramics with enhanced electrocaloric effect

(Sr0.5Ba0.5)Nb2O6 ultra-fine powders were synthesized by a novel sol–gel route, and the mechanism of the reaction was discussed. SrCO3, BaCO3, oxalate niobium and citric acid were used to initiate the sol–gel process, and ethylene glycol (EG) was added to further polymerize the cross-linking structure. The evolution of the (Sr0.5Ba0.5)Nb2O6 phase, the reaction process and the microstructures were investigated by X-ray diffraction (XRD), Raman spectroscopy, Fourier transform infrared spectroscopy, DSC-TG and scanning electron microscopy. The synthesis temperature of the (Sr0.5Ba0.5)Nb2O6 powders reached as low as 1200∘C, and the size and morphology of the powders were controlled by temperature. By adjusting the calcination temperature, we obtained (Sr0.5Ba0.5)Nb2O6 powders with uniform sizes of 20nm to 500nm. Then, dense (Sr0.5Ba0.5)Nb2O6 ceramics were successfully prepared using these ultrafine powders. Finally, an enhanced electrocaloric effect (ECE) value of 0.35∘C was obtained at 100kV/cm.

Low Temperature Synthesis of ZnNb2O6 Fine Powders by Wet-Chemical Processes

Low temperature synthesis of ZnNb 2 O 6 fine powders was conducted by several improved wet-chemical processes. ZnNb 2 O 6 fine powders could be obtained at 250°C from the K 8 Nb 6 O 19 and ZnCl 2 aqueous solutions with pH = 14, where ammonia was used as the mineralizer. The homogenous microstructures were observed for the ZnNb 2 O 6 fine powders after prolonging the reaction time to 24 h. ZnNb 2 O 6 fine powders could also be obtained by calcining the resultant dry solid suspension of K 8 Nb 6 O 19 and ZnCl 2 aqueous solution with H 3 BO 3 as the catalysis at 500°C in air for 2h. The improved methods to prepare the ultrafine ZnNb 2 O 6 powder made great progress in solving the problem of Zn volatilization.