Journal of Advanced Dielectrics

Single phase Li₂W₂O₇ with anorthic structure was prepared by the conventional solid-state reaction method at 550${}^{\circ }$C and the anorthic structure was stable up to 660${}^{\circ }$C. The dielectric properties at radio frequency (RF) and microwave frequency range were characterized. The sample sintered at 640${}^{\circ }$C exhibited the optimum microwave dielectric properties with a relative permittivity of 12.2, a quality factor value of 17,700GHz (at 9.8GHz), and a temperature coefficient of the resonant frequency of $-$232ppm/${}^{\circ }$C as well as a high relative density $\sim$94.1%. Chemical compatibility measurement indicated Li₂W₂O₇ did not react with aluminum electrodes when sintered at 640${}^{\circ }$C for 4h.

The polycrystalline samples of 0.8BiSm_xFe${}_{1-x}$O₃–0.2PbTiO₃ ($x=0.05,0.10,0.15$ and 0.20) were prepared by using the conventional solid-state reaction technique and sintered at high temperature (850${}^{\circ }$C). X-ray diffraction (XRD) confirms the distorted rhombohedral crystal structure for all the composites at room temperature. The surface morphology was checked by field-emission scanning electron microscope (FESEM) technique and homogeneous mixing of the components was confirmed by energy-dispersive analysis of X-ray (EDAX). The detailed study of dielectric properties of the composites reveals an increasing nature of dielectric constant (${\epsilon }_r$) and loss tangent (tan$\delta$) with the increase of temperature due to thermal activation. The Arrhenius plots of temperature dependence of AC conductivity yield the activation energy within the material at high-temperature range. The ferroelectric study shows that the remnant polarization decreases with the increase of Samarium (Sm) concentration.

Despite the promising characteristics of Dielectric Elastomel Actuator (DEA) as a practical soft actuator, the need of high voltage for its operation prevents the successful fabrication of a practical DEA, that is, the high voltage generation takes a bulky and costly power supply. Induction of complex shape change motion of DEA such as oscillatory shape change takes even a more bulky and costly multipurpose power supply. It is a serious practical issue to be overcome. In our latest study, however, we could build a simple DEA system which exhibited a relatively complex and autonomous oscillatory shape change merely under a constant voltage, though the voltage needed was high. This successful outcome must broaden the potential usefulness of DEA as a practical soft actuator.

Raman spectra of a complex perovskite system (Pb${}_{1-x}$Ba${}_x)$(Yb${}_{0.5}$Ta${}_{0.5}$)O₃, $x=0.0,0.1$ and 0.2 have been studied at low temperature in the range of 10–250K. The fascination of the system is that it undergoes a crossover from a highly ordered antiferroelectric ($x=0.0$) to ferroelectric ($x=0.1$) to disordered relaxor ferroelectric ($x=0.2$). The nature of splitting that prevails in F_2g ($\approx 60$cm${}^{-1}$), F_1g ($\approx 538$cm${}^{-1}$) modes and their gradual merging and disappearance with $x$ indicates an orthorhombic symmetry for $x=0.0$ and a pseudo-cubic structure for $x=0.2$. The mode at 420cm${}^{-1}$ suggests the existence of rhombohedral distortion in all the compositions. The soft mode behavior of the F_2g modes has been analyzed by fitting the data with the power law ${\omega }_s=A{(T_C-T)}^{\beta }$.

The influence of free surface and depolarizing field on structural phase transitions in thin ferroelectric films from an ordered state to a disordered one is investigated. The dependences of the order parameter on the distance from the free film surface are calculated. It is shown that with the presence of the depolarizing field and in its absence, the effective thickness of the surface layer depends on the temperature. Nearby the phase transition point, the thickness increases indefinitely. Calculations considering depolarizing field showed that the phase transition points for the bulk ferroelectrics and the film under given boundary conditions coincide. Also shown that in the absence of depolarizing field with mixed boundary conditions, the film thickness does not affect the order parameter, and in presence of the field, this influence is observed.

Presented results demonstrate importance of taking into account such a phenomenon as the solid solution decomposition at the boundaries separating coexisting phases in lead zirconate-titanate-based solid solutions with compositions belonging to the morphotropic boundary region of the “temperature–composition” phase diagram. It is shown that in the local decomposition of solid solutions in the vicinity of the boundaries separating the tetragonal and rhombohedral phases in lead zirconate-titanate-based solid solutions lead to the changes of the solid solution’s chemical composition and to the formation of segregates. It is also shown that the proper thermoelectric treatment of samples containing these segregates can give substantially higher values of piezoelectric parameters in the lead zirconate-titanate-based compounds.

Pyroelectric composites of triglycine sulfate (TGS)-polyvinylidene difluoride (PVDF) doped with graphene are studied. It is found that the graphene can effectively improve the polling efficiency and thermal property of the composites so that the infrared detective performance can be significantly improved. For example, by adding about 0.83 wt.% of graphene, the infrared detective property can be improved by more than 30%. It is also found that the size of the graphene plays a critical role on the property improvement. For example, the small-sized graphene prepared by ultrasonic exfoliation (UE) method is more effective than the big-sized graphene prepared by electrochemical exfoliation (EE) method.

Ba${}_{0.9}$Sr${}_{0.1}$TiO₃ ceramics with or without ZnO have been prepared by traditional solid state reaction method. The XRD analysis showed that the doped Zn${}^{2+}$ ions diffused into the BST crystal lattice, resulting in the variation of dielectric properties. Especially the dielectric constant at Curie point decreased with doping ZnO content when it is lower than 0.5mol%. Due to the promotion of sintering, doping ZnO can enhance the density of ceramics but increase grain size. However, ZnO is a kind of semiconductor and can lead to the decrease in electrical breakdown strength value.