Chung Loong Choy

RETRACTED: On the microstructure of three-dimensional braided preforms

This paper describes a study of the microstructure of 3D braided preforms produced by the four-step 1 x 1 method. An analytical approach is employed in conjunction with experimental investigation to establish the relationship between the braid structure and braiding parameters. Microscopic evidence of the microstructure of preforms reveals the different configurations of the yarns in the interior, surface and corner regions of a braided preform. On the basis of microscopic observations, three types of microstructural unit cell models are established for the three regions. By using these three unit cell models, the structural geometry of the preforms is analyzed and the mathematical relationships among the structural parameters, such as the yarn packing factor, fiber orientation, fiber volume fraction, braiding pitch, are derived. From the preforms to the finial composites, if the RTM process is utilized, the unit cell size and shape will be changed. The modification of the relationships among the structural parameters has been made. Good agreement has been obtained between the calculated and measured values of the geometric characteristics of braided composite samples.

Mechanical analysis of 3-D braided composites by the finite multiphase element method

Abstract A finite multiphase element method (FMEM), in which the element comprises more than one kind of material, has been proposed to predict the effective elastic properties of 3-D braided composites. This method is based on the variational principle and our previous geometric model that assumes the existence of different types of unit cells in the three regions in a 3-D braided composite, i.e. the interior, surface and corner. The numerical procedure involved two steps. First, a fine local mesh at the unit cell level is used to analyze the stress/strain of each unit cell. Then, a relatively coarse global mesh is used to obtain the overall responses of the composite at macroscopic level. By using the stress volume averaging method, the effective elastic properties of the composite can be calculated under the prescribed uniform strain boundary conditions. Finally, the predicted stress/strain curves are compared with experimental results, demonstrating the applicability of the FME method.

A ring-shaped piezoelectric transformer operating in the third symmetric extensional vibration mode

Abstract In this study, a ring-shaped piezoelectric transformer operating in the third symmetric extensional vibration mode is proposed, and its characteristics are investigated. The transformer consists of a lead zirconate titanate (PZT) ceramic ring with a high mechanical quality factor which has an outer diameter of 12.7xa0mm, an inner diameter of 5.1xa0mm and a thickness of 1.2xa0mm. The transformer is poled along the thickness direction. The top surface is covered by two ring-shaped electrodes separated by a narrow annular gap, and the regions covered by the outer and inner ring electrodes serve as the input and output parts of the transformer, respectively. Its bottom surface is fully covered with a silver electrode. For a temperature rise of 20°C, the transformer has an output power of 1.8xa0W and a power density of 14.3xa0W/cm3. With a matching load resistance of 860xa0Ω, its maximum efficiency is 92.3%, and the maximum voltage gain is 1.9. When two ring-shaped transformers are connected in parallel, the total output power is significantly higher than that of a single ring-shaped transformer. However, the resultant maximum efficiency is slightly less than that of a single transformer, and the transformer with the lower voltage ratio determines the overall voltage gain of the parallel combination.

Piezoelectric and pyroelectric properties of PZT/P(VDF-TrFE) composites with constituent phases poled in parallel or antiparallel directions

Composites of lead zirconate titanate (PZT) powder dispersed in a vinylidene fluoride-trifluoroethylene copolymer [P(VDF-TrFE)] matrix have been prepared by compression molding. Three groups of polarized samples have been prepared by poling: only the ceramic phase, the ceramic and polymer phases in parallel directions, and the two phases in antiparallel directions. The measured permittivities of the unpoled composites are consistent with the predictions of the Bruggeman model. The changes in the pyroelectric and piezoelectric coefficients of the poled composites with increasing ceramic volume fraction can be described by modified linear mixture rules. When the ceramic and copolymer phases are poled in the same direction, their pyroelectric activities reinforce while their piezoelectric activities partially cancel. However, when the ceramic and copolymer phases are poled in opposite directions, their piezoelectric activities reinforce while their pyroelectric activities partially cancel.Composites of lead zirconate titanate (PZT) powder dispersed in a vinylidene fluoride-trifluoroethylene copolymer [P(VDF-TrFE)] matrix have been prepared by compression molding. Three groups of polarized samples have been prepared by poling: only the ceramic phase, the ceramic and polymer phases in parallel directions, and the two phases in antiparallel directions. The measured permittivities of the unpoled composites are consistent with the predictions of the Bruggeman model. The changes in the pyroelectric and piezoelectric coefficients of the poled composites with increasing ceramic volume fraction can be described by modified linear mixture rules. When the ceramic and copolymer phases are poled in the same direction, their pyroelectric activities reinforce while their piezoelectric activities partially cancel. However, when the ceramic and copolymer phases are poled in opposite directions, their piezoelectric activities reinforce while their pyroelectric activities partially cancel.

Compressive and flexural behavior of ultra-high-modulus polyethylene fiber and carbon fiber hybrid composites

Abstract With epoxy resin as the matrix, five types of unidirectional composites—an ultra-high-modulus polyethylene fiber (UHMPE) reinforced composite, a carbon-fiber-reinforced composite and three UHMPE carbon hybrid composites with different hybrid ratios—were fabricated. The compressive and flexural behavior of these composites was investigated and the failure modes were examined by scanning electron microscopy. The results show that the incorporation of a moderate amount of carbon fiber into a UHMPE-fiber-reinforced composite greatly improves the compressive strength, flexural modulus and flexural strength while the addition of a small amount of UHMPE fiber into a carbon-fiber-reinforced composite remarkably enhances the ductility with only a small decrease in the compressive strength. The mechanisms behind the enhancement of mechanical performance are discussed.

Effect of a bias field on the dielectric properties of 0.69Pb(Mg1/3Nb2/3)O3?0.31PbTiO3 single crystals with different orientations

The microdomain–macrodomain transformations and phase transitions in 0.69Pb(Mg1/3Nb2/3)O3–0.31PbTiO3 single crystals were investigated by measuring the temperature dependence of relative permittivity as a function of crystal orientation and applied dc bias. A rhombohedral–tetragonal phase transition in the original crystal has been confirmed by powder x-ray diffraction at temperatures from 25 to 180 °C. The results show that microdomain relaxor ferroelectric states in the original crystals could be transformed to macrodomain ferroelectric states by applying a dc bias along any of the , and directions. It is also proposed that an electric field applied along the or direction could induce an orthorhombic or a monoclinic ferroelectric phase, respectively, between the rhombohedral and tetragonal ferroelectric phases in an intermediate temperature range.

Ferroelectric properties of (Ba0.5Sr0.5)TiO3/Pb(Zr0.52Ti0.48)O3/ (Ba0.5Sr0.5)TiO3 thin films with platinum electrodes

Multilayered Pt/(Ba0.5Sr0.5)TiO3/Pb(Zr0.52Ti0.48)O3/(Ba0.5Sr0.5)TiO3/Pt (BST/PZT/BST) thin films with different thicknesses of the BST layers were prepared by the pulsed-laser deposition method. The existence of a BST layer between the PZT and Pt electrode can greatly improve the fatigue properties of the PZT film. However, the heterostructure with thicker BST layers exhibits lower remnant polarization because of a lower electric field applied on the PZT layer. So, the thickness of BST layers should be decreased to decrease the working voltage of the multilayered film. A heterostructure with very thin BST layers (thickness ∼7.5 nm) has good ferroelectric properties, such as high remnant polarization and rare fatigue resistance after 1010 switching cycles. A possible reason for the effect of BST is that the BST layer can absorb oxygen vacancies or other point defects from the PZT layer and greatly improve its fatigue properties.

A standing wave-type noncontact linear ultrasonic motor

In this study, a novel standing wave-type noncontact linear ultrasonic motor is proposed and analyzed. This linear ultrasonic motor uses a properly controlled ultrasonic standing wave to levitate and drive a slider. A prototype of the motor was constructed by using a wedge-shaped aluminum stator, which was placed horizontally and driven by a multilayer PZT vibrator. The levitation and motion of the slider were observed. Assuming that the driving force was generated by the turbulent acoustic streaming in the boundary air layer next to the bottom surface of the slider, a theoretical model was developed. The calculated characteristics of this motor were found to agree quite well with the experimental results. Based on the experimental and theoretical results, guidelines for increasing the displacement and speed of the slider were obtained. It was found that increasing the stator vibration displacement, or decreasing the gradient of the stator vibration velocity and the weight per unit area of the slider, led to an increase of the slider displacement. It was also found that increasing the amplitude and gradient of the stator vibration velocity, or decreasing the weight per unit area of the slider and the driving frequency, gave rise to an increase of the slider speed. There exists an optimum roughness of the bottom surface of the slider at which the slider speed has a maximum.

Thermal hysteresis in the permittivity and polarization of lead zirconate titanate/vinylidenefluoride-trifluoroethylene 0-3 composites

The permittivities of 70/30 vinylidene fluoride-trifluoroethylene copolymer and lead zirconate titanate/vinylidene fluoride-trifluoroethylene 0-3 composites have been measured at 1 kHz over the temperature range of 20 to 120/spl deg/C. Thermal hysteresis was observed for the 0-3 composites and abrupt changes in the permittivities occurred at different temperatures upon heating and cooling. Good agreement was found between the measured permittivities and the predictions of the Bruggeman model. Implications of the thermal hysteresis behavior on the poling of 0-3 composites are also discussed.

The effects of composition gradients of BaxSr1-xTiO3 thin films on their microstructures, dielectric and optical properties

Abstract Ferroelectric thin films of Ba x Sr 1− x TiO 3 (BST) with artificial gradients in composition normal to the growth surface were prepared on Si and fused quartz substrates by a sol–gel technique. Auger electronic spectroscopy observation shows that composition gradients of Ba and Sr are gradual and uniform without obvious interfaces between the different layers. X-ray diffraction analysis reveals that polycrystalline and perovskite formed at 730xa0°C. Annealing at higher temperature enhanced grain growth, the lattice parameter decreases with increasing annealing temperatures. The small signal dielectric constant and dielectric loss at 100 kHz were found to be 300 and 0.02. The graded thin film shows a relatively high tunability of 35% at an applied field of 240 kV cm −1 . The optical properties were determined at room temperature in the wavelength range of 200–2000 nm from spectrophotometer measurements of the transmittance. The transmittance of the graded thin films increased, and a much weaker and flatter absorption band was found in the absorption spectra.