Wenyi Zhu

Effects of noble gases on diamond deposition from methane‐hydrogen microwave plasmas

The deposition of diamond films by microwave plasmas has been studied in gaseous mixtures of methane, hydrogen, and noble gases. Plasma diagnostic results are compared with growth rates and Raman spectra of the films. The noble gases, which influence the degree of excitation or reactant molecules by energy transfer or charge transfer from their excited and ionic states, are active in the deposition process by inducing additional ion‐molecule and excited atom‐molecule reactions. As a result, enhanced deposition rates have been observed. Small oxygen additions along with the noble gases can suppress the formation of nondiamond carbon phases, leading to an effective way to rapidly deposit diamond films at high methane concentrations while still retaining minimal nondiamond carbon components in the films.

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...

Fabrication and characterization of xBiGaO3–(1−x)PbTiO3: a high temperature reduced Pb-content piezoelectric ceramic

Abstract Bismuth gallium based crystalline solutions, x BiGaO 3 –(1− x )PbTiO 3 (BG–PT), were studied as novel material group of low lead content perovskite ceramics. X-ray diffraction (XRD) was utilized to investigate the crystal structure and phase evolution. A pure perovskite phase with tetragonal symmetry was obtained for x ≤0.25. Differential scanning calorimetry (DSC) revealed that a first order solid-to-solid transformation occurs at 495 °C for x =0.2. The dielectric properties were investigated at room and elevated temperatures. Our results show that BG–PT is a good candidate high-temperature piezoelectric ceramic, with reduced lead content.

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.

Flexure mode flexoelectric piezoelectric composites

We present a flexure mode composite design to generate steep transverse strain gradient to exploit large flexoelectric coefficient μ1122 of (Ba,Sr)TiO3 (BST) ceramics. Very strong direct piezoelectric effect was observed in composites due to the flexoelectricity. In a single unit composite, sharp low frequency ( 2000 pC/N as the result of enhancement of strain gradient at resonance. Giant nonresonance d33 well beyond piezoelectric single crystal about 4350 pC/N was measured at a temperature around Curie temperature of BST ceramic in a six unit three layer composite.

Electrical properties of sol-gel-derived Pb(Zr0.52Ti0.48)O3 thin films on a PbTiO3-coated stainless steel substrate

Lead zirconate titanate (PZT) thin films were deposited on stainless steel (SS) substrates by a sol-gel spin-on technique, and crystallized by a low-temperature annealing process. A lead titanate thin coating was deposited between the PZT film and SS substrate in order to decrease the annealing temperature. X-ray diffraction revealed that amorphous PZT thin layers crystallized into a perovskite phase on annealed at 550 °C for 0.5 h. No second phase formation, due to chemical reactions with the substrate, was observed. For films with a thickness of 1.68 μm, the dielectric constant, tan δ, remnant polarization and coercive field were determined to be 280, 0.07, 35 μC/cm2, and 99 kV/cm, respectively. The transverse piezoelectric constant d31 was measured using a wafer flexural technique.

Dielectric properties of (100) textured thick Pb(ZrxTi1−x)O3 films with different Zr/Ti atom ratios

Near 4-μm-thick Pb(ZrxTi1−x)O3 (PZT) films with Zr/Ti ratios of 60/40, 52/48, and 45/55 were coated onto platinized silicon substrates by using 2-methoxyethanol based sol-gel spin-on techniques with a special thermal treatment process. The scanning electron microscopy observations show the columnar growth of grains. The analysis of x-ray diffraction data indicates that all PZT films exhibit (100) texture. The dielectric constants and dissipation factors of the films were measured at elevated temperatures and frequencies. It is found that Curie points of 60/40, 52/48, and 45/55 films are at 350, 375, and 422 °C, respectively. All these films exhibit high dielectric constants and remnant polarizations. A permittivity of 1658 and remnant polarization of 35 μC/cm2 had been achieved for the 60/40 films. No enhancement of the dielectric constant was observed in films with a Zr/Ti ratio close to morphotropic phase boundary. The high dielectric constant observed in films with the higher Zr content was explained b...

FLEXOELECTRIC COMPOSITE — A NEW PROSPECT FOR LEAD-FREE PIEZOELECTRICS

Flexoelectricity describes the physical phenomenon of the generation of electric polarization from mechanical strain gradient in solid insulators. In common dielectric materials, the flexoelectric coefficient is trivially small ~10-10 C/m. In Ba(Sr,Ti)O3 (BST) ceramics, flexoelectric coefficient up to 10-4 C/m was observed. Such high coefficient makes it possible to design high piezoelectric response flexoelectric composites. In this letter, we will demonstrate that the newly designed flexoelectric composites could have piezoelectric properties better than conventional piezoelectric materials.

Direct evidence of ferroelastic participation in 180° polarization switching and fatigue for 111 oriented rhombohedral ferroelectric 0.955 Pb(Zn1/3Nb2/3)O3:0.045 PbTiO3 single crystals

Direct elastic deformation measurements taken during polarization reversal in 111 oriented 0.955 Pb(Zn1/3Nb2/3)O3–0.045 PbTiO3 single crystals confirm the observation by W. Cao [Ferroelectrics 290, 107 (2003)] that a ferroelastic reorientation of the domain polarization vectors is involved, and the observed strain changes are consistent with the model he proposed. First cycle nonrecoverable fatigue occurring in our crystals and the associated changes of shape in polarization and strain hysteresis are suggested to be due to microcracking associated with the large very rapid strain changes associate with the ferroelastic switching mode.