Xiaoyong Wei

Composition and phase dependence of the intrinsic and extrinsic piezoelectric activity of domain engineered (1−x)Pb(Mg1/3Nb2/3)O3−xPbTiO3 crystals

The piezoelectric response of [001] poled domain engineered (1-x)Pb(Mg(13)Nb(23))O(3)-xPbTiO(3) (PMN-PT) crystals was investigated as a function of composition and phase using Rayleigh analysis. The results revealed that the intrinsic (reversible) contribution plays a dominant role in the high piezoelectric activity for PMN-PT crystals. The intrinsic piezoelectric response of the monoclinic (M(C)) PMN-xPT crystals, 0.31</=x</=0.35, exhibited peak values for compositions close to R-M(C) and M(C)-T phase boundaries, however, being less than 2000 pCN. In the rhombohedral phase region, x</=0.30, the intrinsic piezoelectric response was found to increase as the composition approached the rhombohedral-monoclinic (R-M(C)) phase boundary. The maximum piezoelectric response was observed in rhombohedral PMN-0.30PT crystals, being on the order of 2500 pCN. This ultrahigh piezoelectric response was determined to be related to the high shear piezoelectric activity of single domain state, corresponding to an ease in polarization rotation, for compositions close to a morphotropic phase boundary (MPB). The role of monoclinic phase is only to form a MPB with R phase, but not directly contribute to the ultrahigh piezoelectric activity in rhombohedral PMN-0.30PT crystals. The extrinsic contribution to piezoelectric activity was found to be less than 5% for the compositions away from R-M(C) and M(C)-T phase boundaries, due to a stable domain engineered structure. As the composition approached MPBs, the extrinsic contribution increased slightly (<10%), due to the enhanced motion of phase boundaries.

Microstructure and dielectric properties of (Nb + In) co-doped rutile TiO2 ceramics

The (Nb + In) co-doped TiO2 ceramics recently attracted considerable attention due to their colossal dielectric permittivity (CP) (∼100,000) and low dielectric loss (∼0.05). In this research, the 0.5 mol. % In-only, 0.5 mol. % Nb-only, and 0.5–7 mol. % (Nb + In) co-doped TiO2 ceramics were synthesized by standard conventional solid-state reaction method. Microstructure studies showed that all samples were in pure rutile phase. The Nb and In ions were homogeneously distributed in the grain and grain boundary. Impedance spectroscopy and I-V behavior analysis demonstrated that the ceramics may compose of semiconducting grains and insulating grain boundaries. The high conductivity of grain was associated with the reduction of Ti4+ ions to Ti3+ ions, while the migration of oxygen vacancy may account for the conductivity of grain boundary. The effects of annealing treatment and bias filed on electrical properties were investigated for co-doped TiO2 ceramics, where the electric behaviors of samples were found to...

Symmetry of flexoelectric coefficients in crystalline medium

Flexoelectric effect, which is defined as strain gradient–induced polarization or electric gradient–induced strain in crystalline solids, can be presented as a fourth-rank tensor. The symmetry of the flexoelectric coefficients in matrix form is studied. The results indicate that the direct flexoelectric coefficients should be presented in 3 × 18 form and the converse flexoelectric coefficients in 6 × 9 form, rather than 6 × 6 form, like elastic constants. In addition, non-zero and independent elements in the matrices have been calculated for 32 point groups and 7 Ci groups. These results will provide valuable reference to the theoretical and application studies of flexoelectric effect.

Electromechanical properties of Pb(In1/2Nb1/2)O3–Pb(Mg1/3Nb2/3)O3–PbTiO3 single crystals

The Pb(In1/2Nb1/2)O3–Pb(Mg1/3Nb2/3)O3–PbTiO3 (PIN-PMN-PT) crystals were studied as function of phase and orientation. The properties, including the Curie temperature TC, ferroelectric-ferroelectric phase transition temperature TR/O-T, coercive field, and piezoelectric/dielectric responses, were systematically investigated with respect to the composition of PIN-PMN-PT crystals. The Curie temperature TC was found to increase from 160 to 220 °C with ferroelectric-ferroelectric phase transition temperature TR-T and TO-T being in the range of 120–105 °C and 105–50 °C, respectively. The piezoelectric activity of PIN-PMN-PT crystals was analyzed by Rayleigh approach. The ultrahigh piezoelectric response for domain engineered [001] (1600–2200 pC/N) and [011] (830–1550 pC/N) crystals was believed to be mainly from the intrinsic contribution, whereas the enhanced level of piezoelectric and dielectric losses at the compositions around morphotropic phase boundaries (MPBs) was attributed to the phase boundaries motion.

Significantly enhanced recoverable energy storage density in potassium–sodium niobate-based lead free ceramics

Ceramic-based dielectric materials are regarded as the best candidates for advanced pulsed power capacitors because of their excellent mechanical and thermal properties. Nevertheless, lead-free bulk ceramics show relatively low recoverable energy storage density (Wrec < 2 J cm−3) owing to their low dielectric breakdown strength (DBS < 200 kV cm−1). In order to significantly increase Wrec, we proposed a strategy (compositions drive the grain size to submicrometer) to improve the DBS of lead-free ceramics. In this work, (1 − x)(K0.5Na0.5)NbO3–xSrTiO3 (KNN–ST) ceramics were chosen as a representative to verify the validity of this strategy. The (1 − x)KNN–xST ceramics (x = 0.15 and 0.20) with submicrometer grains (about 0.3 μm) were prepared using pressureless solid state sintering. A large Wrec (4.03 J cm−3) and DBS (400 kV cm−1 with a thickness of 0.2 mm) were achieved for 0.85KNN–0.15ST ceramics. The value of 4.03 J cm−3 is superior to all other Wrec in lead-free bulk ceramics and 2–3 times larger than that of other lead-free bulk ceramics. A large Wrec (3.67 J cm−3) and energy storage efficiency (72.1%) were simultaneously achieved for 0.80KNN–0.20ST ceramics. The results confirm that the (1 − x)KNN–xST ceramics (x = 0.15 and 0.20) are desirable materials for advanced pulsed power capacitors. The findings in this study could push the development of a series of KNN-based ceramics with enhanced DBS and Wrec in the future. On the other hand, this work could broaden the applications of KNN materials in a new field.

Nonlinear dielectric properties of barium strontium titanate ceramics

Abstract In barium strontium titanate (BaxSr1−xTiO3) ceramics, obvious tunabilities of both dielectric permittivity and loss tangent under external DC bias field were observed. The temperature and frequency dependences of nonlinear dielectric properties were studied. Polarizing process was carried out to evaluate the influence of domain reorientation on dielectric nonlinearity in the ferroelectric polycrystalline samples. The nonlinear variation of dielectric properties under external DC bias field was tentatively explained.

Potassium–sodium niobate based lead-free ceramics: novel electrical energy storage materials

The development of lead-free bulk ceramics with high recoverable energy density (Wrec) is of decisive importance for meeting the requirements of advanced pulsed power capacitors toward miniaturization and integration. However, the Wrec (<2 J cm−3) of lead-free bulk ceramics has long been limited by their low dielectric breakdown strength (DBS < 200 kV cm−1) and small saturation polarization (Ps). In this work, a strategy (compositions control the grain size of lead-free ceramics to submicron scale to increase the DBS, and the hybridization between the Bi 6p and O 2p orbitals enhances the Ps) was proposed to improve the Wrec of lead-free ceramics. (K0.5Na0.5)NbO3–Bi(Me2/3Nb1/3)O3 solid solutions (where Me2+ = Mg and Zn) were designed for achieving large Ps, and high DBS and Wrec. As an example, (1 − x)(K0.5Na0.5)NbO3–xBi(Mg2/3Nb1/3)O3 (KNN–BMN) ceramics were prepared by using a conventional solid-state reaction process in this study. Large Ps (41 μC cm−2) and high DBS (300 kV cm−1) were obtained for 0.90KNN–0.10BMN ceramics, leading to large Wrec (4.08 J cm−3). The significantly enhanced Wrec is more than 2–3 times larger than that of other lead-free bulk ceramics. The findings in this study not only provide a design methodology for developing lead-free bulk ceramics with large Wrec but also could bring about the development of a series of KNN-based ceramics with significantly enhanced Wrec and DBS in the future. More importantly, this work opens a new research and application field (dielectric energy storage) for (K0.5Na0.5)NbO3-based ceramics.

High energy density in silver niobate ceramics

Solid-state dielectric energy storage is the most attractive and feasible way to store and release high power energy compared to chemical batteries and electrochemical super-capacitors. However, the low energy density (ca. 1 J cm−3) of commercial dielectric capacitors has limited their development. Dielectric materials showing field induced reversible phase transitions have great potential to break the energy storage density bottleneck. In this work, dense AgNbO3 ceramic samples were prepared successfully using solid state methods. Ferroelectric measurements at different temperatures reveal evidence of two kinds of polar regions. One of these is stable up to 70 °C, while the other remains stable up to 170 °C. The associated transition temperatures are supported by second harmonic generation measurements on poled samples and are correlated with the occurrence of two sharp dielectric responses. The average unit cell volume is seen to increase with increasing DC field and has been interpreted in terms of increasing levels of structural disorder in the system. At a high electric field the structure becomes ferroelectric with high polarization. This field induced transition exhibits a recoverable energy density of 2.1 J cm−3, which represents one of the highest known values for lead-free bulk ceramics.

Tunability and ferroelectric relaxor properties of bismuth strontium titanate ceramics

A systemic study was performed on bismuth strontium titanate (Sr1−1.5xBixTiO3, 0.04⩽x⩽0.25) ceramics. Dielectric properties were measured from 83to373K at different frequencies. A transformation from relaxor ferroelectrics to relaxor behavior was observed when x>0.10. Both diffuseness and relaxation degree increase as x rises. Tunability was found to increase with increasing x value until 0.07, and then decrease when Bi3+ content increases. Meanwhile, their figures of merit (defined as tunability/loss) shows a maximum at x=0.12. The change from relaxor ferroelectrics into relaxor behavior has a positive impact on the tunability of bismuth strontium titanate system.

Experimental realization of all-dielectric composite cubes/rods left-handed metamaterial

We present a novel all-dielectric left-handed metamaterial (LHM) in the X-band microwave regime. The LHM is formed by cubes and square rods, which are made of microwave ceramic with high permittivity and low loss. Both Mie theory and dielectric resonator theory are found to play important roles in generating negative effective parameters. The effective permeability is negative in the designed frequency range due to the first Mie resonance associated with the cubes, while the effective permittivity is negative below the plasma frequency owing to the second resonance mode of the square rods. The parameter retrieval method, transmission spectra method and wedge-shaped negative refraction method are used to demonstrate that the composite has a double-negative regime. Experiments are carried out to verify the designed all-dielectric LHM. The proposed method presents new ways to realize all-dielectric LHMs.