John Y. Fu

Piezoelectric composite based on the enhanced flexoelectric effects

In this study, a piezoelectric composite is fabricated from a Ba0.67Sr0.33TiO3 (BST) composition and air, and measured at room temperature (24°C) where the BST is in its paraelectric centric phase. The charge separation mechanism is flexoelectricity, driven by the strain gradient introduced by the pyramidal boundary shapes of each BST building unit of the composite. In spite of the macro (millimeter) dimensions of the building units that inhibit a steep strain gradient, the composite is sufficiently active to be measured by using a conventional Berlincourt d33 meter. The measured values are in good agreement with expectation from the earlier measured flexoelectric coefficient μ11 of the BST and the finite element simulation results of the strain gradient in the pyramid BST units.

Experimental studies of the converse flexoelectric effect induced by inhomogeneous electric field in a barium strontium titanate composition

Recent experimental studies have shown that some inhomogeneously strained bulk solid dielectrics have large flexoelectric coefficients. Here we report the experimental observation of the converse flexoelectric effect, i.e., the elastic stress induced in a Ba0.67Sr0.33TiO3 (BST) composition trapezoid block under the inhomogeneous electric fields generated by its chosen boundary shape. This BST composition has a very high electric susceptibility value χ and a ferroelectric Curie point that is close to room temperature. The quantitative measurement of the flexoelectric coefficient μ11 at a temperature just above the Curie point indicates that an enhanced converse flexoelectric effect exists in the BST sample in good agreement with the earlier measured direct effect. This observed phenomenon agrees with the theoretical predictions of Tagantsev’s phenomenological model. Furthermore, our experimental results suggest that we may be able to develop a method to design some unusual but useful piezocomposites with m...

Gradient scaling phenomenon in microsize flexoelectric piezoelectric composites

The flexoelectric-type piezoelectric composites offer an alternative avenue for the development of piezoelectric ceramics, and since the flexoelectric response is diminished rather than enhanced in lead containing compositions, one of the merits of such composites is that those of highest sensitivity will be lead-free. The composites are fabricated by using certain nonpiezoelectric components with a texture symmetry which breaks up applied uniform fields, leading to the field gradients in the active flexoelectric components of the composites. Since these induced field gradients increase as the composite dimensions decrease, it is logical to expect that the piezoelectric performance of such composites would be enhanced with their reduced sizes. In this letter, we report the experimental studies that confirm such a gradient scaling phenomenon in two flexoelectric piezoelectric composites. The fabrication and measurement of the composites are discussed.

Strain gradient induced electric polarization in α-phase polyvinylidene fluoride films under bending conditions

The relationship between the applied elastic strain gradient and the induced electric polarization in the α-phase polyvinylidene fluoride films under bending conditions has been investigated. Our experimental studies have shown that the flexoelectric polarization is linearly proportional to the strain gradient and the corresponding direct flexoelectric response is strong. It is reasonable to believe that the physical mechanisms behind the flexoelectric effects in polymers and solid dielectrics are different.

Separate control of direct and converse piezoelectric effects in flexoelectric piezoelectric composites

The fabrication of piezoelectric composites driven by the flexoelectric charge separation has aroused considerable interest recently. In earlier oral presentations, we have posited that, in such composites, even though the direct and the converse flexoelectric effects are thermodynamically equivalent, it is possible to separately control the direct and converse piezoelectricities. Here, we would like to demonstrate how this separation can be accomplished through the control of the texture symmetry and that the result does not violate reciprocity as two different phases are involved in those two effects.

On the Flexoelectric Effects in Solid Dielectrics: Theories and Applications

We propose here a modified phenomenological model for the flexoelectric effects in solid dielectrics. These effects have been investigated from the viewpoint of equilibrium thermodynamics, and the mathematical expressions of their coupling coefficients are given. Finally, the possibilities of fabricating a new kind of flexoelectric piezoelectric composites by exploiting the flexoelectric charge separation mechanism are discussed.

Optical measurement of the converse piezoelectric d33 coefficients of bulk and microtubular zinc oxide crystals

Zinc oxide (ZnO) crystal is a semiconductor material; the direct measurement and the interpretation of its piezoelectric coefficients are challenging. In this letter, the authors investigate the piezoelectric properties (d33 coefficient) of a bulk ZnO sample by measuring its converse piezoelectric effect using an optical method. The experimental data agree with recent theoretical calculations. The piezoelectric properties of a ZnO hexagonal microtube, which has a highly symmetrical hexagonal tubular structure, are also studied. The results demonstrate that the microtubular ZnO has significantly different piezoelectric properties from that of ZnO bulk crystal, especially under the applied electric fields of low frequencies.

A fast and secure phase-multiplexing technique based on random speckles modulation in multimode ferroelectric single crystal fiber for volume holographic storage

In this paper, a new speckle based hologram multiplexing recording technique is proposed and tested. In this method, a multi-mode LiNbO3 single crystal fiber is employed to generate speckle patterns which are used as reference beams in hologram recording process. The speckle pattern generation can be precisely controlled by external E-field. Theoretically, this technique can generate thousands of decorrelated reference beams at given practical constraints. Its storage capacity can be scaled up as material properties are improved, making it well adapted to new material development. A theoretical analysis and numerical simulation of speckle pattern generation are also presented in this paper.

Displacement sensing with hetero-core fiber specklegram

In this paper we report the design and implementation of a multimode fiber sensing technique for displacement sensing. To exploit the spatial information contents for sensing, a multimode fiber specklegram sensor with a hetero-core fiber structure is used. The sensor utilizes the inner product of multimode fiber speckle fields, which is highly sensitive on the geometrical shape change of the sensing section. The sensitivity and the dynamic range of the displacement sensing are investigated for hetero-core structure fiber specklegram sensor and straight multimode fiber specklegram sensor. It’s found that the sensitivity of the hetero-core FSS offers sensitivity as high as 0.1 μm, with a dynamic range of about 3 μm, which is superior to straight structure multimode fiber FSS. Dynamic response of the hetero-core FSS for displacement sensing was also studied.

Frequency dependent electrooptic property of SBN single crystal

In electro-optic (EO) modulator devices ferroelectric crystals of strontium barium niobate (SBN) are attractive due to exceptional high EO coefficients and low half wave voltage. SBN single crystals grown by laser heated pedestal growth are investigated to explore frequency dependent EO property at low frequency and near resonant frequency range. The mechanism of its frequency dependence is discussed briefly.