Qing-Ming Wang

Broadband piezoelectric energy harvesting devices using multiple bimorphs with different operating frequencies

This paper presents a novel approach for designing broadband piezoelectric harvesters by integrating multiple piezoelectric bimorphs (PBs) with different aspect ratios into a system. The effect of 2 connecting patterns among PBs, in series and in parallel, on improving energy harvesting performance is discussed. It is found for multifrequency spectra ambient vibrations: 1) the operating frequency band (OFB) of a harvesting structure can be widened by connecting multiple PBs with different aspect ratios in series; 2) the OFB of a harvesting structure can be shifted to the dominant frequency domain of the ambient vibrations by increasing or decreasing the number of PBs in parallel. Numerical results show that the OFB of the piezoelectric energy harvesting devices can be tailored by the connection patterns (i.e., in series and in parallel) among PBs.

Nonlinear piezoelectric behavior of ceramic bending mode actuators under strong electric fields

The nonlinear electromechanical behavior of cantilevered piezoelectric ceramic bimorph, unimorph, and reduced and internally biased oxide wafer actuators is studied in a wide electric field and frequency range. It is found that under quasistatic condition, linear relationships between actuator tip displacement-electric field, and blocking force-electric field are only valid under weak field driving. With increasing the driving field, electromechanical nonlinearity begins to contribute significantly to the actuator performance because of ferroelectric hysteresis behavior associated with piezoelectric lead zirconate titanate (PZT)-type ceramic materials. The bending resonance frequencies of all these actuators vary with the magnitude of the electric field. The decrease of resonance frequency with electric field is explained by the increase of elastic compliance of PZT ceramic due to elastic nonlinearity. Mechanical quality factors of the actuators also depend on the magnitude of electric field strength. No ...

Electromechanical coupling and output efficiency of piezoelectric bending actuators

Electromechanical coupling mechanisms in piezoelectric bending actuators are discussed in this paper based on the constitutive equations of cantilever bimorph and unimorph actuators. Three actuator characteristic parameters, (e.g., electromechanical coupling coefficient, maximum energy transmission coefficient, and maximum mechanical output energy) are discussed for cantilever bimorph and unimorph actuators. In the case of the bimorph actuator, if the effect of the bonding layer is negligible, these parameters are directly related to the transverse coupling factor lest. In the case of the unimorph actuator, these parameters also depend on the Youngs modulus and the thickness of the elastic layer. Maximum values for these parameters can be obtained by choosing proper thickness ratio and Youngs modulus ratio of elastic and piezoelectric layers. Calculation results on four unimorph actuators indicate that the use of stiffer elastic material is preferred to increase electromechanical coupling and output mechanical energy in unimorph actuators.

Piezoelectric Energy Harvesting with a Clamped Circular Plate: Analysis

Energy harvesting using piezoelectric materials is not a new concept, but its small generation capability has not been attractive for mass energy generation. For this reason, little research has been done on the topic. Recently, increased interest in wearable computer concepts and remote electrical devices has provided motivation for more extensive study of piezoelectric energy harvesting. The theory behind cantilever-type piezoelectric elements is well known, but the transverse moving plate elements, which can be used in energy generation from pressure sources is not yet fully developed. The power generation in a pressure-loaded plate depends on several factors. Among them, the thickness of each layer is important, as is the electrode pattern used. In this article, two clamped circular plate structures, a fully electroded unimorph, and a so-called regrouped electrode unimorph, are modeled. These models are then used to calculate energy generation with varying thickness ratios. The results of this analysis are presented with an eye toward guidelines for design of useful energy harvesting structures.

Performance analysis of piezoelectric cantilever bending actuators

Abstract Bimorph and unimorph are two typical bending mode actuators, either consisting two piezoelectric layers or one piezoelectric layer and one elastic layer. In the case of bimorph actuator, when an electric field is applied to the piezoelectric layers, one layer expands while the other contract along length direction, producing a pure bending deformation. In unimorph actuator, when piezoelectric layer is driven to expand or contract, the elastic layer will resist this dimension change, both bending and stretching deformation will be resulted. These actuators can generates very large tip displacement, in the range of tens micron to several millimeters, depending on the geometrical dimensions of actuators. In this paper, to describe the performance of cantilever bimorph/unimorph actuators in quasi-static driving condition, analytical expressions are derived to relate bending resonance frequency, tip deflection, blocking force and equivalent moment with actuator geometrical dimensions. Youngs modulus,...

Piezoelectric Energy Harvesting with a Clamped Circular Plate: Experimental Study

In a companion article, a model for a clamped circular unimorph piezoelectric plate has been developed for the purpose of analyzing the influence of geometric design parameters and electrode configuration on the amount of electrical energy that can be harvested from an applied pressure source. It has been shown that the ratio of layer thickness (piezoelectric layer to substrate layer) and electrode pattern have a significant effect on energy conversion for harvesting. Specifically, the theoretical analysis shows that regrouping of the electrodes (i.e., segmenting the electrode into a specific pattern) can lead to optimized energy harvesting in a clamped circular plate structure. This article provides experimental validation of these results. In this article, three circular plate piezoelectric energy generators (PEGs), one unmodified and two regrouped unimorph PEGs, were used to support the analysis.

Theoretical analysis of the sensor effect of cantilever piezoelectric benders

Piezoelectric bending mode elements such as bimorph and unimorph benders can be used as both actuation and sensing elements for a wide range of applications. As actuation elements, these devices convert electric input energy into output mechanical energy. As sensing elements, they convert external mechanical stimuli into electrical charge or voltage. In this article, the sensing effect of cantilever mounted piezoelectric bimorph unimorph and triple layer benders subjected to external mechanical excitations are discussed. General analytical expressions relating generated electric voltage (or charge) to the applied mechanical input excitations (moment M, tip force F, and body force p) are derived based on the constitutive equations of these bending devices. It is found that the clamping effect of each component in the bender devices decreases the dielectric constant. The bimorph bender has a higher voltage sensitivity than the unimorph or triple layer bender with the same geometrical dimensions. The depende...

Zinc oxide single-crystal microtubes

ZnO single-crystal microtubes were fabricated using an encapsulated microwave growth method. The ZnO crystals are grown in hexagonal hollow tube form with a well faceted end and side surfaces, which have cross-sectional dimensions of 100to250μm, lengths of 3–5mm, and wall thickness of 1–2μm. Under optical excitation, a strong near-band-edge emission was obtained at a peak wavelength of 377.8nm with a full width at half maximum of 11nm. The ZnO microtubes exhibited a highly selective UV light response with a cut-off wavelength at ∼370nm, and excellent electron field emission properties with an emission current density of 11mA∕cm2 at an applied field of ∼20V∕μm.

The performance of PEM fuel cells fed with oxygen through the free-convection mode

The feasibility and restrictions of feeding oxygen to a PEM fuel cell through free-convection mass transfer were studied through theoretical analysis and experimental testing. It was understood through the theoretical analysis that the free-convection mass-transfer coefficient strongly depends on the difference in mass fraction or concentration of oxygen at the cathode surface and in the quiescent air. Thus, the mass-transfer rate has a strong dependence on the oxygen concentration at the cathode surface, which can be viewed in terms of the relationship of the fuel cell current density and the fuel cell voltage. Through this analysis, the maximum applicable current density was derived, beyond which there will be an abrupt drop in the output voltage, which results in excessively low power in the fuel cell. Experimental tests were conducted for one PEM fuel cell stack and two single PEM fuel cell units. An excessive drop in output voltage was observed when the free-convection mass-transfer mode was utilized. It was also found that the orientation of the cathode surface affects the performance of the fuel cell, which is mainly due to the fact that the free-convection mass-transfer coefficient depends on the orientation of the involved mass-transfer surface, which is analogous to free-convection heat transfer.

Cell shape regulates collagen type I expression in human tendon fibroblasts

Understanding the relationship between cell shape and cellular function is important for study of cell biology in general and for regulation of cell phenotype in tissue engineering in particular. In this study, microcontact printing technique was used to create cell-adhesive rectangular and circular islands. The rectangular islands had three aspect ratios: 19.6, 4.9, and 2.2, respectively, whereas circular islands had a diameter of 50 microm. Both rectangular and circular islands had the same area of 1960 microm(2). In culture, we found that human tendon fibroblasts (HTFs) assumed the shapes of these islands. Quantitative immunofluorescence measurement showed that more elongated cells expressed higher collagen type I than did less stretched cells even though cell spreading area was the same. This suggests that HTFs, which assume an elongated shape in vivo, have optimal morphology in terms of expression of collagen type I, which is a major component of normal tendons. Using immunohistochemistry along with cell traction force microscopy (CTFM), we further found that these HTFs with different shapes exhibited variations in actin cytoskeletal structure, spatial arrangement of focal adhesions, and spatial distribution and magnitude of cell traction forces. The changes in the actin cytoskeletal structure, focal adhesion distributions, and traction forces in cells with different shapes may be responsible for altered collagen expression, as they are known to be involved in cellular mechanotransduction.