W.P. Cao

Enhanced dielectric properties of PVDF-HFP/BaTiO3-nanowire composites induced by interfacial polarization and wire-shape

Polymer based composites with high dielectric constant were successfully fabricated using BaTiO3 nanowires with high aspect ratio as inorganic filler and PVDF-HFP as polymer matrix. PVDF-HFP/BaTiO3 nanoparticles composites were also prepared as a contrast. The BaTiO3 nanowires were synthesized via one-step hydrothermal method. The BaTiO3 nanowires and its two types of composites were characterized by X-ray diffraction, Fourier transform infrared spectroscopy, scanning electron microscopy and transmission electron microscopy. For the composites, breakdown strength measurements, tensile tests and broadband dielectric spectroscopy analyses were also carried out. The results show that the two types of fillers are homogeneously dispersed in the matrix. The dielectric constant of composites filled by BaTiO3 nanowires is larger than that filled by BaTiO3 nanoparticles at the same content. Stronger interfacial polarization was found in PVDF-HFP/BaTiO3 nanowire composites. Two theoretical models were employed to predict the dielectric constants of composites, and the experimental data were consistent with the estimated trend. The enhanced dielectric properties of PVDF-HFP/BaTiO3 nanowire composites are attributed to the superior interfacial polarization and high aspect ratio of BaTiO3 nanowires.

Defect dipole induced large recoverable strain and high energy-storage density in lead-free Na0.5Bi0.5TiO3-based systems

In this letter, we propose an effective route to obtain large recoverable strain, purely electrostrictive effects and high energy-storage density by inducing defect dipoles into Na0.5Bi0.5TiO3 (NBT)-based relaxor ferroelectrics. It has been found that pinched and double polarization hysteresis loops with high maximum polarization (Pmax) and negligible remanent polarization (Pr) can be observed due to the presence of acceptor-induced defect dipoles. A large recoverable strain of 0.24% with very little hysteresis and high electrostriction coefficient of 0.022 m4 C2 with purely electrostrictive characteristics were acquired when 11 mol. ‰ Mn-doped. Meanwhile, a high recoverable energy density of 1.06 J/cm3 with excellent temperature stability was obtained at the same composition owing to the enlarged value of Pmax-Pr (36.8 μC/cm2) and relatively high electric field (95 kV/cm). Our achievement can open up the exciting opportunities for ferroelectric materials in high-precision positioning devices and high ele...

Giant piezoelectric properties of BZT–0.5BCT thin films induced by nanodomain structure

Ba(Zr0.2Ti0.8)O3–0.5(Ba0.7Ca0.3)TiO3 (BZT–0.5BCT) thin films were prepared from two ceramics targets, Ba(Zr0.2Ti0.8)O3 and (Ba0.7Ca0.3)TiO3, using dual-magnetron sputtering, and a LaNiO3 (LNO) seed layer was introduced between the film and Pt(111)/Ti/SiO2/Si substrates via a sol–gel technique. Domain structures were observed on the thin films by piezoelectric force microscopy (PFM), and then an autocorrelation function technique was applied to analyze the obtained PFM images. The mean sizes of the domains of the BZT–0.5BCT and (001)-oriented BZT–0.5BCT/LNO thin films are 69.2 nm and 151 nm at room temperature respectively. On the basis of our results, it is proven that the nanodomains have a large effect on the piezoelectricity of the thin films for MPB composition: the converse piezoelectric coefficient d33 is 258 pm V−1 for BZT–0.5BCT thin films and 122 pm V−1 for BZT–0.5BCT/LNO thin films. It is worth noting that not only the crystal orientation, but also the domain structures play critical roles in the piezoelectricity for the BZT–0.5BCT thin films. Furthermore, (001)-oriented BZT–0.5BCT/LNO thin films exhibit excellent dielectric properties and the dielectric constant (e ∼ 1046) is dramatically increased compared with the BZT–0.5BCT thin films (e ∼ 168). The C–V results indicate that both of the thin films are ferroelectric in nature.

Self-polarization induced by lattice mismatch and defect dipole alignment in (001) BaTiO3/LaNiO3 polycrystalline film prepared by magnetron sputtering at low temperature

Upward self-polarization phenomenon was observed and studied in the highly (001)-oriented BaTiO3/LaNiO3 polycrystalline film, and the polycrystalline film was in situ prepared by radio frequency magnetron sputtering at 400 °C without post treatment. The low temperature crystallization and high orientation of BaTiO3 polycrystalline film may be caused by the better compatibility between BaTiO3 and LaNiO3. The upward self-polarization phenomena was characterized by piezoelectric force microscopy (PFM), and the coupling effects of low crystallization temperature, high (001) orientation, large compressive strain gradient and preferred orientation of defect dipoles are considered to be responsible for the upward self-polarization phenomena in the BaTiO3/LaNiO3 polycrystalline film during the cooling process, especially when ferroelectric domains nucleate and grow near 120 °C.

Positive/negative electrocaloric effect induced by defect dipoles in PZT ferroelectric bilayer thin films

Double hysteresis loops induced by defect dipoles are obtained in PbZr0.52Ti0.48O3/PbZr0.8Ti0.2O3 ferroelectric bilayer thin films prepared by a sol–gel method. The migration of the defect dipoles related to the temperature and applied electric field is investigated in detail, which has a deep influence on the polarization due to the effect of the domain pinning caused by defect dipoles. Meanwhile, the electrocaloric effect based on double hysteresis loops induced by defect dipoles has been studied. The coexistence of a negative electrocaloric effect and positive electrocaloric effect has been achieved in PbZr0.52Ti0.48O3/PbZr0.8Ti0.2O3 bilayer films, ΔT = −8.4 K at 155 °C, and ΔT = 4.4 K at 45 °C. The amplifying effect of the applied electric field in the PZr0.8Ti0.2O3 layer films plays an important role in the excellent negative electrocaloric values. More interestingly, the coexistence of the negative electrocaloric and positive electrocaloric effect can be obtained by regulating the applied electric field, which is beneficial to enhancing the cooling efficiency during the cycling of the applied electric field.

Enhanced piezoelectric properties and constricted hysteresis behaviour in PZT ceramics induced by Li+–Al3+ ionic pairs

Recently, we proposed a new approach (Li+–Al3+ ionic pairs doping) to enhance the piezoelectric properties of BaTiO3 ceramic. Here we sequentially prove that Li+ and Al3+ can also enhance the piezoelectric properties of PbZr0.52Ti0.48O3 (PZT) ceramic. The X-ray diffraction results demonstrate the optimized distribution of Li+–Al3+ ionic pairs parallel to the [001] direction in the doped ceramics, as same as the results in BaTiO3-based ceramic. The doped ceramics exhibit constricted hysteresis loops, which is associated with the restoring force of Li+–Al3+ ionic pairs as a result of the Coulomb force between Li+ and Al3+ ions. The results show that the piezoelectric properties and corresponding temperature stability of the doped ceramics were enhanced significantly. The analysis and conclusion indicate that the piezoelectric mechanism coming from ionic pairs applies equally well to the PZT ceramic. Co-doping Li+ and Al3+ into the A-site is a preferable approach for the piezoelectric property enhancement of ABO3 ferroelectric ceramics.