Y.G. Wang

Rotational piezoelectric wind energy harvesting using impact-induced resonance

To improve the output power of a rotational piezoelectric wind energy harvester, impact-induced resonance is proposed to enable effective excitation of the piezoelectric cantilevers vibration modes and obtain optimum deformation, which enhances the mechanical/electrical energy transformation. The impact force is introduced by forming a piezoelectric bimorph cantilever polygon that is fixed at the circumference of the rotating fans internal surface. Elastic balls are placed inside the polygon. When wind rotates the device, the balls strike the piezoelectric cantilevers, and thus electricity is generated by the piezoelectric effect. The impact point is carefully chosen to use the first bending mode as much as possible, and thus maximize the harvesting efficiency. The design enables each bimorph to be struck in a similar area and every bimorph is struck in that area at different moments. As a result, a relatively stable output frequency can be obtained. The output frequency can also be changed by choosing different bimorph dimensions, which will also make the device simpler and the costs lower. A prototype piezoelectric energy harvester consisting of twelve piezoelectric cantilevers was constructed. The piezoelectric cantilevers were made from phosphor bronze, the lead zirconium titanate (PZT)-based bimorph cantilever had dimensions of 47u2009mmu2009×u200920u2009mmu2009×u20090.5u2009mm, and the elastic balls were made from steel with a diameter of 10u2009mm. The optimal DC output power was 613u2009μW across the 20u2009kΩ resistor at a rotation speed of 200u2009r/min with an inscribed circle diameter of 31u2009mm.

Resonance magnetoelectric effect in Ni/Pb(Zr,Ti)O3/Terfenol-D trilayered composites with different mechanical boundary conditions

Magnetoelectric Ni/Pb(Zr,Ti)O3 (PZT)/Tb1−xDyxFe2−y (Terfenol-D) trilayered composites were made up of negative magnetostrictive/piezoelectric/positive magnetostrictive layers, and bonded to nonmagnetic glass plates to obtain three different mechanical boundary conditions: (i) both ends of sample traction free (F-F), (ii) one end clamped while the other traction free (C-F), and (iii) both ends of sample clamped (C-C). In these three modes, various experimental values of resonance frequencies were obtained in 1–140u2009kHz range, which agree well with the calculated ones. In the C-F mode six resonance frequencies exist, which may be useful for multifrequency operation. The low resonance frequency of the C-F mode can be used to decrease the eddy current loss of the magnetostrictive phase and increase the lifetime of the devices.

Large magnetoelectric effect in mechanically mediated structure of TbFe2, Pb(Zr,Ti)O3, and nonmagnetic flakes

Magnetoelectric (ME) effect has been studied in a structure of a magnetostrictive TbFe2 alloy, two piezoelectric Pb(Zr,Ti)O3 (PZT) ceramics, and two nonmagnetic flakes. The ME coupling originates from the magnetic-mechanical-electric transform of the magnetostrictive effect in TbFe2 and the piezoelectric effect in PZT by end bonding, instead of interface bonding. Large ME coefficients of 10.5 and 9.9u2002Vu2009cm−1u2009Oe−1 were obtained at the first planar acoustic and third bending resonance frequencies, which are larger than that of conventional layered TbFe2/PZT composites. The results show that the large ME coupling can be achieved without interface coupling.

Crystallization mechanism and its correlation with structural and soft magnetic properties of FeSiBPCu nanocrystalline alloys

Correlation of crystallization mechanism with the structural and soft magnetic properties of FeSiBPCu nanocrystalline alloys has been investigated on the basis of Johnson–Mehl–Avrami kinetic model, transmission electron microscopy, and extended random anisotropy model. Pre-existent α-Fe clusters in melt-spun alloys with a size much smaller than critical nucleus size crystallize at a steadily increasing nucleation rate owing to the chain effect of rising constituent fluctuation. This unique crystallization mechanism leads to the formation of fine and uniform nanocrystallites and therefore superior soft magnetic properties. In contrast, pre-existent α-Fe clusters with a size approximate to the critical nucleus size nucleate gradually at the initial stage of crystallization process driven by thermal activation over the energy barrier of nucleation, and pre-existent α-Fe nanocrystallites with a size larger than critical nucleus size grow directly at the initial stage of crystallization process. Grains formed at the initial stage enlarge further in the subsequent crystallization process, which gives rise to a continuous decrease of the nucleation rate and the deterioration of soft magnetic properties.

Zero-biased and resonant magnetoelectric effect in magnetostrictive/piezoelectric multilayered composites

Magnetoelectric (ME) effect was investigated in various multilayered composites with various magnetostrictive materials Ni, FeNi and FeCo prepared by electroless deposition. Compared with the homogeneous ME composites, Ni/PZT/Ni and FeCo/PZT/FeCo, the ME voltage coefficient αE,31 of graded ME composites with two different magnetostrictive materials exhibit significant hysteresis phenomenon as a function of the external dc magnetic field at low frequency. The zero-biased ME response has been observed and can be attributed to a built-in magnetic field. However, at longitudinal vibration mode the ME voltage coefficient has been weakened in graded ME composites. The zero bias ME coupling observed in composites with graded magnetostrictive materials can be used in magnetic sensors with high sensitivity.

Piezoelectric Clamped Beam Energy Harvester Using Vibration Caused by Centrifugal Force at High Wind Speeds

Abstract In this study, a piezoelectric wind energy harvester was vibrated that aims to convert high-speed wind energy into electrical energy using vibrations caused by centrifugal force. Vibrations induced by centrifugal force enabled effective distortion of the piezoelectric clamped beam and thus produced electric charge through the piezoelectric effect. A clamped beam was used rather than a conventional thin cantilever to harvest the wind energy in the proposed harvester. The centrifugal force was introduced by a pair of rotating eccentric turbines that are installed on two ball bearings on both sides of the piezoelectric unimorph. Benefiting from the rotating eccentric masses of these turbines, the harvester is capable of capturing wind energy in high speed wind environments. A prototype was set up to examine the effects of the wind speed and the structural parameters on the electrical output of the harvester. It is found that the harvester worked efficiently with wind applied from the axial directions in a 20–55u2006m/s speed range and produced a maximum open-circuit voltage of 47.2u2006V. When connected to an external load of 50u2006kΩ, the harvester showed a peak output power of 3.69u2006mW at a wind speed of 55u2006m/s.

Magnetic properties of structure ordered cores composited with Fe78Si9B13 amorphous and pure iron powders

Composite cores of Fe78Si9B13 amorphous and pure iron powder were prepared by cold pressing. Effective permeability, initial permeability and the magnetic flux density of cores can all be improved by increasing the content of pure iron powder. With and without an external magnetic field, the circularly orientated (COCPC) and non-oriented (NOCPC) composite powder cores were obtained. The effect of the shape anisotropy of the flake powders on magnetic properties of the composite powder cores was investigated. All investigated magnetic properties of COCPC cores are better than those of NOCPC cores with the same mixture ratio and the effect of structural ordering weakens as the content of pure iron powder increases. For cores with only amorphous powder the effective permeability below 100xa0kHz, the initial permeability and the magnetic flux density for Hmaxxa0=xa08xa0kAxa0m−1 of COCPC increase by 8.1, 15 and 10xa0%, respectively, compared to those of the NOCPC. Pressing magnetic powders under an external magnetic field to form ordered structure is suitable for optimal design of soft magnetic cores toward practical applications.

Atomic-scale mechanisms of annealing-induced coercivity modification in metallic glass

Amorphous Fe80Si9B11 ribbons were annealed at various temperatures below the crystallization temperature. Annealing-induced coercivity modification mechanisms at atomic scale in metallic glass were investigated by synchrotron X-ray diffraction. The peak intensity of the pair distribution function initially decreases before a sharp rise with the increasing annealing temperature. The peak positions move to large distances due to the enhanced thermal oscillations, then shift to short distances because of structural relaxation. Based on the synchrotron X-ray diffraction and Fe K-edge-extended X-ray absorption fine structure experiments, reverse Monte Carlo method was applied to extract the distribution of Fe-centered clusters. The variation of the distribution of Fe-centered clusters confirms the results from synchrotron X-ray diffraction. The thermal oscillations of the atoms lead to the relief of residual stress. Heating up to a temperature higher than 523xa0K reconstructs the distribution of internal stress and gives compressive stress an advantage. The variation of the distribution of atomic-scale hydrostatic stress upon annealing is responsible for the modification of the coercivity.

The enhanced ferromagnetic property of BiFeO 3 -YMnO 3 system

Single-phase multiferroic materials are important both in basic research and practical applications. In this research, solid solutions of (1−x) BiFeO3-xYMnO3 (x = 0, 0.05, 0.1) were synthesized by rapid liquid phase sintering method. The phase structures, leakage current density, magnetic and ferroelectric properties were investigated. All the solid solution samples show a perovskite phase structure. The addition of YMnO3 content was found to enhance the magnetization of the solid solutions, but the leakage current density was degenerated. Saturated electric hysteresis loop was obtained in the sample with 5% of YMnO3 fraction at room temperature, while at 10% of YMnO3 content the polarization loop present a leakage characteristic. The investigated materials may be promising as the diluted magnetic semiconductors in related applications.

Preparation and properties of BaTiO 3 / CoFe 2 O 4 composite ceramics

In this study, we report a general process to prepare BaTiO<inf>3</inf>, CoFe<inf>2</inf>O<inf>4</inf> nanoparticles and other oxides nanoparticles with complex compositions. BaTiO<inf>3</inf>, CoFe<inf>2</inf>O<inf>4</inf> nanoparticles with particle size less than 50nm were fabricated by a surfactant-assisted ball milling process followed a centrifugal separation. Furthermore, ferroelectric/ferrite composites of (1−x) BaTiO<inf>3</inf>/xCoFe<inf>2</inf>O<inf>4</inf> (x=0.1, 0.2, 0.3, 0.5) ceramics were then synthesized. The dependence of the ferroelectric and magnetic properties on the content of CoFe<inf>2</inf>O<inf>4</inf> has been investigated systematically. No interface reaction was observed in the multiferroic ceramics and the samples have been confirmed with excellent ferroelectric and ferromagnetic properties, of which the maximal remnant polarization value of 1.9µC/cm² and saturation magnetization of 35emu/g were measured at 10% and 30% of CoFe<inf>2</inf>O<inf>4</inf>, respectively. The dielectric properties of the composites have also been measured as the variation of ferroelectric/magnetic fractions.