Antiferroelectric (AFE) materials with perovskite structure have attracted great interest due to their distinctive structural complexity and useful physical properties. So far, most studies have focused on polycrystalline materials. Compared with the preparation of ceramics and films, the growth of Pb-based AFE single crystals with high quality is a more challenging task because of the high melting point, incongruently melting and the volatilization of Pb-containing components at high temperatures. In order to develop new antiferroelectric materials for energy storage applications and understanding the mechanisms of phase transitions and domain structure, single crystals of a new AFE-AFE solid solution of PbHfO3-Pb(Mg1/2W1/2)O3 (PHf-PMW) were successfully grown for the first time by the high-temperature solution growth method using the mixtures of PbO-B2O3 and Pb3O4-B2O3 as complex flux, respectively, and the grown crystals were characterized in terms of their crystal structure, dielectric properties and domain structure. It was found that Pb3O4 as the main flux component is more favorable than PbO as it lowers the melting point of the system and provides a more stable growth, leading to PHf-PMW crystals of larger size, better quality, higher optical transparency and less defects or inclusions. The structural analysis by XRD shows an orthorhombic Pbam symmetry at room temperature for the grown crystals, which is of antiferroelectric nature. By comparing the sequence and temperature of the phase transitions to the PHf-PMW ceramics, the compositions of the grown crystals are estimated to be 0.99PHf-0.01PMW and 0.985PHf-0.015PMW, respectively. Compositional segregation is observed in the grown crystals, which can be attributed to the incongruently melting character on the one hand, and the differences in the ionic radius and electronegativity of the oxygen anion and various B-site cations in the crystal lattice on the other hand. The observation and analysis of the ferroic domains by polarized light microscopy based on optical crystallography reveal domain structures consisting of two sets of domains with extinction directions parallel to the 〈110〉cub direction, confirming the orthorhombic symmetry. This work provides new experimental strategies in growing the high-quality antiferroelectric crystals of complex perovskite structure and a better understanding of their structure and properties for both fundamental studies and potential applications in optoelectronics and energy storage.
Focus on Mechanics of Electro/Magneto-Active Materials and Structures
Faxin Li, Peking University, China
Wei Hong, South University of Science and Technology, China
The IUTAM (International Union of Theoretical and Applied Mechanics) Symposium on Mechanics of Electro/Magneto-Active Materials and Structures – co-chaired by Professor Christopher Lynch and Professor Daining Fang – was successfully held at Peking University, Beijing, China, from August 26–30th, 2018. Thirty-nine invited talks and eighteen posters were presented at this Symposium.
This Focus Collection presents selected manuscripts from this conference, and contributions to this issue are strictly on an invitational only basis.
Papers
Inorganic ferroelectrics which have low tensile strength and poor ductility are difficult to be bent. In this paper, in order to realize great toughness, the inorganic ferroelectric film was deposited on flexible muscovite (Mica) substrate. For efficiency and convenience, sol-gel method is adopted to fabricate the Bi3.15Nd0.85Ti3O12 (BNT) thin-film capacitor on flexible Mica substrate with SrRuO3 (SRO) electrode layer. The remnant polarization of the Pt/BNT/SRO/CFO/Mica capacitor achieves ∼38 μC cm−2, which is larger than most BNT films prepared on hard substrates. No obvious fatigue phenomenon is observed for the BNT capacitor with 1010 switching cycles. The most important is that the Pt/BNT/SRO/CFO/Mica capacitor still owns relatively stable electrical properties when it is bent to 5 mm radius, showing an excellent bending performance for this structure. The flexible, anti-fatigue ferroelectric capacitor prepared in this paper is expected to get a wide range of application in flexible logic and elements for information storage, data processing and communication.
The aim of this study is development of an improved PVDF film printing system. First, solution-cast PVDF films of three types are prepared by dropping and drying PVDF/acetone solution, PVDF/dimethylformamide (DMF) solution, and PVDF/hexamethylphosphoric triamide (HMPA) solution droplets on a glass plate. Second, after constructing an in-house Raman spectroscopy measurement system, the crystalline structure of solution-cast PVDF films is measured using the system. Measurement results demonstrate that the measurement system can be used to distinguish between α-phase, β-phase, and γ-phase in the PVDF sample. Third, after a piezoelectric polymer dual head printer system (P–p dual head printer system) equipped with a syringe pump head and an inkjet head is developed, a circle type PVDF film is printed on a glass plate using the P–p dual head printer system. The printed PVDFs show that the P–p dual head printer system can print fine 2D moulding.
The design of miniaturized, long-lasting power supplies for portable internet of things (IoT) equipment has been identified as a critical problem constraining further development of the IoT. A promising methodology is able to resolve the problem by harvesting dissipated energy such as vibration in the environment or in a system to form self-powered microsystems. In this work, the performance of vibration energy harvesting using a magnetostrictive iron–cobalt/nickel (FeCo/Ni)-clad plate cantilever was examined both theoretically and experimentally. The experimental results indicate that the output power of the FeCo/Ni-clad plate cantilever shows significant improvement in comparison to a single FeCo plate, as a result of the inverse magnetostrictive properties of the FeCo and Ni layers in response to tension or compression. Finite element analysis illustrates how the unidirectional and identical magnetic induction in the upper and lower sides (i.e. the FeCo and Ni layers) gives rise to this enhanced output. The analysis also demonstrates the cancellation of the positive and negative magnetic induction within the interior of the single FeCo plate. This study not only provides insights into the magnetic features of a FeCo/Ni-clad plate but also proposes a feasible method for realizing industrial applications.
Lead zirconate titanate (PZT) is a widely used piezoelectric and ferroelectric material with applications ranging from actuators in fuel injectors to ferroelectric random-access memories. Hydrogen is known to cause degradation in most metals through a ductile-to-brittle transformation called hydrogen embrittlement. Similarly, piezoelectric materials have also been found to degrade through hydrogen exposure, not only due to embrittlement, but also through reduced polarity and Pb migration. This gives rise to the need of understanding the behavior of hydrogen in piezoelectric material. This research presents the initial results of a study which aims to simulate hydrogen diffusion in PZT using multi-scale simulation techniques. First-principles calculations were done to determine the possible hydrogen occupancy locations in the PZT lattice. For these locations we calculated the effect on polarization and the dimensions of the PZT lattice by dissolved hydrogen and predicting paths available for the escape of hydrogen atoms. These results will serve as a basis for future nudged elastic band calculations to determine the diffusion characteristics for the individual diffusion steps, which can in turn be used to predict the bulk diffusion characteristics with kinetic Monte Carlo simulations.
Ferroelectric switching near the crack tip in lead zirconate titanate ceramics leads to a change of both polarization and remanent strain, which affects substantially the stress field and fracture behavior. In particular, for tetragonal crystals the 90° domain reorientation is believed to enhance fracture toughness of the damaged piezoelectric ceramics. Current research models evolution of the domain reorientation processes during crack propagation by a micromechanical model using finite element analysis. The crack growth is numerically simulated by means of electromechanical cohesive elements along the prospective crack path. The study of these irreversible dissipative mechanisms makes the main subject of our numerical investigations. The computed 3D scattering of domain orientations in a mechanically loaded PZT-PIC151 CT-specimen is compared with in situ x-ray diffraction experiments in synchrotron (Jones et al 2007 Acta Mater. 55, 5538–5548). It is found that the preferred orientation and intensity of domain distribution in the mechanically loaded specimen depend on the in-plane position and are related to the projected deviatoric stresses/strains. In contrast to simulations with a fixed crack tip (Kozinov and Kuna 2018 Arch. Appl. Mech.), during crack propagation the maximal tensile stresses and region of highest intensity of domain reorientation are moved with the crack tip and smeared over a process zone. While the crack length increases, the domain switching belt causes a shielding effect leading to an apparent materials toughening. Such fully coupled, three-dimensional simulation of crack growth resistance curves is the first one in ferroelectrics.
Piezoelectric transducers have been widely employed for energy harvesting from vibration or kinetic energy sources. These systems, however, suffer from low energy density and consequently low power density at frequencies corresponding to common ambient vibrations. An alternative approach, using ferroelectric and ferroelastic switching offers potentially much greater energy density, at the cost of loss of linearity. Using a simple model of switching, a working cycle that could generate electrical energy from a harmonically varying source of stress is explored. The cycle uses depolarization by stress, followed by repolarization with combined electromechanical loading. A harvesting electric field and bias electric field are imposed to ensure a stable repeatable working cycle during the depolarization process and repolarization process, respectively. The bias electric field affects ferroelectric/ferroelastic switching, leading to a preferred direction of repolarization. By contrast, without bias electric field, stress alone would not trigger repolarization because of mechanically equivalent states with opposite polarization. The results illustrate that the bias electric field can be much lower than the harvesting electric field, requiring only a small electrical energy input during the cycle. Finally, the conversion efficiency of this cycle is estimated and improvements to the cycle are explored by adjusting the electrical and mechanical field amplitudes.
Soft tactile sensors have been widely used in wearable electronics and soft robotics. Among different sensing technologies, soft capacitive sensors receive much attention due to good signal repeatability, temperature insensitivity, relative simplicity of fabrication and adaptability of configuration. Many methods such as adding nano-fillers have been developed to increase the sensitivity of capacitive sensors. However, the existing methods usually sacrifice the transparency and reduces the flexibility. In this work, we propose to use dielectric gels to fabricate capacitive sensors for the first time. We synthesize dielectric gels by using ethylene carbonate and propylene carbonate as solvents and 2-Ethylhexyl acrylate and acryloyl morpholine as polymer networks. The synthesized dielectric gels are extremely soft, highly stretchable, and fully transparent to visible lights. The relative permittivity is as high as 30. We demonstrate that the sensitivity of the capacitive sensor made of the new dielectric gel increases about 6 times compared to the sensors made of VHB, PDMS, or Ecoflex. We design a touch sensor with the dielectric gel to control the LED, and also demonstrate the use of the dielectric gels as the transparent cover layer of a cell phone which may displace the traditional protective layer made of glass in future wearable electronics.
By means of the technic for measuring resonant converse magnetoelectric (CME) effect, the mechanical loss of PZT/Terfenol-D/PZT tri-layer symmetric magnetoelectric laminated composites under multi-physical field loads such as bias magnetic field, driven voltage and ambient temperature is systematically measured in this paper. The experimental results show that the mechanical loss in CME effect increases initially with the increase of the bias magnetic field, and approaches to a broad maximum value at 250–500 Oe. Further increasing bias magnetic field leads to the decrease of mechanical loss. The mechanical loss increases rapidly with the increase of driven voltage amplitude, especially in the medium magnitude of the bias magnetic field. While, there is little influence of voltage on the mechanical loss under the higher bias magnetic fields. Importantly, the mechanical loss increases monotonically as the increase of ambient temperature. The research results of the mechanical loss under multi-physics fields in this paper can provide a basic principle of CME improvement by reducing the mechanical loss. Therefore, this research is quite significant to the CME based applications, such as, magnetoelectric transducers, magnetoelectric microwave devices, multiferroic antennas and so on.