Journal of Advanced Dielectrics

The characteristics of the electromechanical response observed in an ionic-electroactive polymer (i-EAP) are represented by the time ($t$) dependence of its bending actuation ($y$). The electromechanical response of a typical i-EAP — poly(ethylene oxide) (PEO) doped with lithium perchlorate (LP) — is studied. The shortcomings of all existing models describing the electromechanical response obtained in i-EAPs are discussed. A more reasonable model: $y=y_{max}{e}^{-\tau ∕t}$ is introduced to characterize this time dependence for all i-EAPs. The advantages and correctness of this model are confirmed using results obtained in PEO-LP actuators with different LP contents and at different temperatures. The applicability and universality of this model are validated using the reported results obtained from two different i-EAPs: one is Flemion and the other is polypyrrole actuators.

Perovskite solid solution ceramics of PZN–PT–BT with several compositions were synthesized by solid-state reaction method. X-ray diffraction results indicate pure perovskite structures for all samples. Dielectric measurements show that the implicit Ba(Zn${}_{1∕3}$Nb${}_{2∕3}$)O₃(BZN) composition could broaden the diffusion phase transition (DPT) peaks and lower the ${\epsilon }_{max}$ and $T_{max}$ values. The observed dielectric properties may be caused by the increased cation disorder due to the substitution of barium onto the A-site and titanium onto the B-site.

In this paper, we demonstrate a method of designing all-dielectric metamaterial frequency selective surface (FSS) with ceramic resonators in spatial arrangement. Compared with the traditional way, spatial arrangement provides a flexible way to handle the permutation and combination of different ceramic resonators. With this method, the resonance response can be adjusted easily to achieve pass/stop band effects. As an example, a stop band spatial arrangement all-dielectric metamaterial FSS is designed. Its working band is in 11.65–12.23GHz. By adjusting permittivity and geometrical parameters of ceramic resonators, we can easily modulate the resonances, band pass or band stop characteristic, as well as the working band.

An ultra-thin metasurface is proposed to realize wideband polarization-independent anomalous reflection. The sub-wavelength resonator can produce different resonance modes, which are the result of the combined effect of dielectric and the metallic structure. The gradient metasurface is done by six discrete orientation of the local sub-wavelength resonator which provides a phase gradient. The simulation and measured results show that 9GHz bandwidth of the anomalous reflection is achieved.

The influence of temperature on the dielectric properties of sol-gel routed spin-coated molybdenum trioxide (MoO${}_3)$ thin film has been investigated. Prepared films were annealed at temperatures 250${}^{\circ }$C, 350${}^{\circ }$C and 400${}^{\circ }$C. The phase transformation from amorphous to $\alpha$-orthorhombic phase with preferential orientation (0 2 2) has been found by XRD for the film annealed above 250${}^{\circ }$C. The vibration modes of $\alpha$-orthorhombic MoO₃ have been examined by Raman spectrum. The predominant Raman’s band of $\alpha$-orthorhombic MoO₃ thin film has been found at the frequency range 1000–600cm${}^{-1}$. Using the UV–Vis spectrum, the band gap of the film is found to be 3.3–3.8eV. The surface morphology of the MoO₃ films has been examined by scanning electron microscope. The AC conductivity measurement of the MoO₃ film has been carried out in the frequency range 10–10⁶ Hz. The frequency dependence of the impedance has been plotted in the complex plane. The variation of the capacitance and dielectric constant of MoO₃ film with respect to temperature and frequency has been analyzed. Tunability of capacitance and figure of merit of the film are also determined.

(Sr${}_{0.5}$Ba${}_{0.5}$)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 (Sr${}_{0.5}$Ba${}_{0.5}$)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 (Sr${}_{0.5}$Ba${}_{0.5}$)Nb₂O₆ powders reached as low as 1200${}^{\circ }$C, and the size and morphology of the powders were controlled by temperature. By adjusting the calcination temperature, we obtained (Sr${}_{0.5}$Ba${}_{0.5}$)Nb₂O₆ powders with uniform sizes of 20nm to 500nm. Then, dense (Sr${}_{0.5}$Ba${}_{0.5}$)Nb₂O₆ ceramics were successfully prepared using these ultrafine powders. Finally, an enhanced electrocaloric effect (ECE) value of 0.35${}^{\circ }$C was obtained at 100kV/cm.

Barium-doped Ca₂Nb₂O₇ ceramics were prepared in the form of Ca${}_{2-x}$Ba_xNb₂O₇ (0 $\le x\le$ 0.6) by solid-state reaction. The solubility limit of barium in Ca₂Nb₂O₇ was found to be $x=0.4$ based on X-ray diffraction and Raman spectroscopy analysis. When $x\le$ 0.4, Ca${}_{2-x}$Ba_xNb₂O₇ solid solutions with a monoclinic perovskites-like layered structure (PLS) were formed, whereas beyond $x=0.4$, a secondary phase CaNb₂O₆ was detected. The dielectric properties obviously depended on the barium substitution. With increasing barium amount, the dielectric constant increased from 33.5 for pure Ca₂Nb₂O₇ to 38.6 for $x=0.4$ ($f=1$MHz). The Curie temperature ($T_c)$ of the $x=0.1$ sample was 1280$\pm$5${}^{\circ }$C. The phase transition was confirmed to be the second order.

Lead-free piezoelectric thin films of 0.055BaZrO₃–0.935(K${}_{0.45}$Na${}_{0.5}$Li${}_{0.05}$)NbO₃–0.01(Bi${}_{0.5}$Na${}_{0.5}$)TiO₃ (BZ–KNLN–BNT) were fabricated on (100)-LaNiO₃/SiO₂/Si substrates. The Pechini (polymeric precursor) method was carried out to prepare the complicated multi-component precursor solution. This method was suited for controlling the complex components accurately. The films crystallized at 700${}^{\circ }$C showed a (100)_pc-oriented perovskite structure, whose grain size was about 200nm and thickness was about 700nm. The Curie temperature $T$_c of the films was 292${}^{\circ }$C that was near to that of the bulk ceramics. The preparation method used in this work provided a possibility for the application of multi-component lead-free piezoelectric films.