Bimorph Structure Research Articles

Thin-film piezoelectric bimorph actuators with increased thickness using double Pb[Zr,Ti]O3 layers

We studied the effects of increasing the thickness of bimorph structures containing double layers of Pb(Zr,Ti)O3 (PZT) thin films. Thick PZT films allow high-voltage application (e.g. in terms of coercive electric field and breakdown voltage) and large generative force, which are effective in microelectromechanical systems (MEMS) applications. Improvement of the deposition conditions and electrode structures facilitates the fabrication of the thin-film PZT/PZT bimorph structure. The PZT/PZT bimorph with a total thickness of 5.8 µm, deposited using a radio-frequency (RF) magnetron sputtering system is much thicker than conventional bimorph structures. The characteristics of the two piezoelectric layers were similar to each other and revealed good electric and ferroelectric characteristics with a remanent polarization of 20 µC cm−2 and a coercive electric field of 50 kV cm−1. PZT/PZT bimorph cantilevers were fabricated in order to evaluate their characteristics for actuator applications. The residual stress of the bimorph cantilever was reduced by an annealing process. The vibration test on the fabricated bimorph cantilevers during electrical voltage application revealed a twice as large displacement as compared with a single layer actuation, and the piezoelectric coefficient value d31 was estimated to be −61 pm V−1, which is better than the value of conventional PZT/PZT bimorph cantilevers (−13 pm V−1). It was also shown that the influence of the temperature change is much less than that in unimorph structures with similar dimensions. The evaluated results demonstrate that the PZT/PZT bimorph structures are effective as MEMS devices.

Progress in carbon nanotube and graphene based artificial muscles

Carbon nanotube and graphene with excellent electrical, mechanical, electrochemical and electromechanical properties have been made remarkable progress and achievements in the fields of functional materials, energy and environment, especially in artificial muscles. Artificial muscles can be treated as actuation materials which converts different types of energy into mechanical energy. Among these actuation materials, carbon nanotube and graphene electrodes based ionic actuators have been intensively studied for their impressive large-deformation and air-working capability under low voltage stimulation. Their typical bimorph structure is composed of one ion-conductive electrolyte membrane laminated by two electron-conductive electrode membranes, which can bend back and forth due to the electrode expansion and contraction induced by ion motion under alternating applied voltage. In this review, we mainly summarize the recent progress in carbon nanotube and graphene electrodes based ionic actuators. Key factors which may significantly affect the actuation process are discussed in order to shed light on possible future research of the novel carbon nanotube and graphene electrodes based ionic actuators.

Piezoelectric Resonance Sensors of DC Magnetic Field

The operational principle, methods of calculation, and designs of the dc magnetic field sensors using the combination of the Ampere force, the piezoelectricity, and the acoustic resonance have been described. The prototypes of the sensors based on a lead zirconate titanate ring and bimorph structures with radial or bending oscillations excited by the alternative current have been fabricated and investigated. The sensitivity of the sensors reached 245 V/(A·T) and the measured field range was from ~10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-5</sup> T up to several Teslas.

Modeling and numerical simulation of static and dynamic behavior of multilayered plates with interface effects

In Multilayered structures the interface effects have a wide range of applications in aerospace, automotive and especially in civil engineering. The design and construction of these structures and the account for interface effects require special expertise in modeling, simulation and implementation. Many studies in this case were conducted to address these issues. The objective of this work is the modeling and numerical simulation of static and dynamic behaviors of beams and plates multilayered structures with different types of interfaces. The focus was on the prediction of the behavior of stresses; shears and displacements depending on thickness. The interface can be elastic or viscoelastic of small or large thickness. The state space method has been developed for this purpose. Various types of rolled arbitrary number of isotropic or anisotropic layers structures were considered. The three-dimensional behavior is obtained for different types of static and dynamic loading. The results were compared with those based on the model of Stroh and on the various existing theories of beams and plates. The methodological approach, developed here, will be applied to thick structures, functionally graded, bimorph or multilayer structures and possibly piezoelectric or viscoelastic layered structures with interface effect

Flexible and Robust Multilayer Micro-Vibrational Harvesters for High Acceleration Environments

This paper presents the fabrication and characterization of multilayer PVDF resonant micro-vibrational energy harvesters designed to withstand environments in which high levels of acceleration are present. The multilayer cantilevers are fabricated by combining two folded PVDF stacks into a multilayered, bimorph structure. This acts to increase the overall capacitance of the harvester, a problem that plaques PVDF cantilevers as a result of its low dielectric constant. Moderate powers (7 μW) are produced from the cantilevers even at high acceleration levels (20 g) due to the limited piezoelectric coefficient of PVDF; however, as a result of the high tensile strength and low elastic modulus of PVDF, the cantilevers are able to survive extremely high accelerations (> 4000 g) without breakage – a critical problem for harvesters based on brittle piezoelectric materials and substrates.

Modeling of Piezoelectric Bimorph Nano-Actuators With Surface Effects

Two-dimensional (2D) equations of piezoelectric bimorph nano-actuators are presented which take account of the surface effect. The surface effect of the bimorph structure is treated as a surface layer with zero thickness. The influence on the plate's overall properties resulted from the surface elasticity and piezoelectricity is modeled by a spring force exerting on the boundary of the bulk core. Using the derived 2D equations, the anti-parallel piezoelectric bimorph nano-actuators of both cantilever and simply supported plate type are investigated theoretically. Numerical results show that the effective properties and the deflections of the antiparallel bimorph nano-actuators are size-dependent. The deflection at the resonant frequency achieves nearly 50 times as that under the static driving voltage.

Unusual process-induced curl and shrinkage of electrospun PVDF membranes

A significant shrinkage of about 17% of poly(vinyliedene fluoride) (PVDF) fibrous membranes processed by electrospinning is reported and analyzed for the first time in this article. Such shrinkage leads to a time dependent curl of the membrane, probed by image analysis and tensile test measurements. Crystalline structure of PVDF is analyzed through X-ray diffraction and FTIR analyses while solvent evaporation is monitored by weight lost measurements. The electrospinning process induces a crystal structure change of PVDF from non-polar α phase to polar β phase exhibiting piezoelectric properties. The curl was modeled taking into account the bimorph structure of the polymer membrane deposited onto the aluminum substrate with the electric field acting on the PVDF through the piezoelectric effect. Besides, just after the processing, the membranes exhibited a nearly 15% weight loss ascribed to the evaporation of the solvents entrapped within the solid fibrous membranes. The piezoelectricity of the β-phase together with solvent evaporation may be responsible for the observed contraction inducing the curl, the second mechanism of evaporation being predominant.

Designs for miniaturization of bending actuator utilizing hydrogen storage alloy

Bending behavior of a miniature actuator utilizing Pd–Ni hydrogen storage alloy (HSA) has been investigated on hydriding and dehydriding process. The actuator has a bimorph structure consisting of the HSA thin sheet and copper plating on one side of the sheet. Palladium sputtering on the back surface of the HSA significantly improves bending characteristics due to the catalytic action of palladium. The maximum bending velocity is obtained in the HSA with 10μm thick Cu-plating. The bending actuators having the same aspect ratio (length to width) and different dimensions show the same bending velocity and the critical aspect ratio of the actuator free from the constraint by deformation in width direction is around five. The bending behavior can be controlled by adjusting the surrounding hydrogen pressure.

Admittance matrix of a trapezoidal piezoelectric heterogeneous bimorph

Bimorph structures are a standard method for transforming the high force of piezoelectric materials into a large deflection. In micro electromechanical systems (MEMS) applications, it is preferable to use structures consisting of a passive substrate (usually silicon) and one or more piezoelectric layers on the top. Such structures are called heterogeneous bimorphs or enakemesomorphs. In some MEMS applications- for example, for use as acoustic transducers-it is desirable to arrange such heterogeneous bimorphs in a circular shape, which results in trapezoidal cantilever structures. In this paper, an analytic dynamic description of such actuators is obtained. The resulting model is proved to be compatible with existing models for heterogeneous bimorphs with constant width. A comparison to a finite element analysis model of an exemplary layout shows divergences wholly within the same range as found for published models for constant-width structures.

Analytical modeling and experimental verification of vibration-based piezoelectric bimorph beam with a tip-mass for power harvesting

Power harvesting techniques that convert vibration energy into electrical energy through piezoelectric transducers show strong potential for powering smart wireless sensor devices in applications of structural health monitoring. This paper presents an analytical model of the dynamic behavior of an electromechanical piezoelectric bimorph cantilever harvester connected with an AC–DC circuit based on the Euler–Bernoulli beam theory and Hamiltonian theorem. A new cantilevered piezoelectric bimorph structure is proposed in which the plug-type connection between support layer and tip-mass ensures that the gravity center of the tip-mass is collinear with the gravity center of the beam so that the brittle fracture of piezoelectric layers can also be avoided while vibrating with large amplitude. The tip-mass is equated by the inertial force and inertial moment acting at the end of the piezoelectric bimorph beam based on D'Alembert's principle. An AC–DC converting circuit soldered with the piezoelectric elements is also taken into account. A completely new analytic expression of the global behavior of the electromechanical piezoelectric bimorph harvesting system with AC–DC circuit under input base transverse excitation is derived. Moreover, an experimental energy harvester is fabricated and the theoretical analysis and experimental results of the piezoelectric harvester under the input base transverse displacement excitation are validated by using measurements of the absolute tip displacement, electric voltage response, electric current response and electric power harvesting.

Fabrication and Characterization of Double-Layer Pb(Zr,Ti)O3 Thin Films for Micro-Electromechanical Systems

In this paper, we address the fabrication and characterization of bimorph structures with relatively thick double-layered Pb(Zr,Ti)O3 (PZT) thin films. The PZT/PZT layers are deposited by RF magnetron sputtering. Hysteresis loops of polarization and electrical field for the top and bottom PZT thin films revealed good ferroelectric characteristics with remanent polarization at approximately 20 µC/cm2 and a coersive electric field of about 100 kV/cm. The vibration tests of fabricated bimorph cantilevers during electrical voltage application revealed a twofold displacement compared with single layer driving, and the piezoelectric coefficient value d31 is estimated to be 13 pm/V. The residual stress difference between the top and bottom layers after the annealing process is calculated to be -0.32 MPa. For a further thickening of the bimorph structure, 6-µm-thick PZT/PZT is also sputtered. The thicker bimorph has a smaller residual stress difference, -30 MPa, between the two layers prepared without the annealing process. The evaluated results demonstrate that the PZT/PZT bimorph structures are applicable to micro-electromechanical systems (MEMS) devices.

Modeling a novel energy harvester working in Lame mode

A new square shaped piezoelectric bimorph structure for energy scavenging purposes has been proposed and simulated. It is derived from theoretical analysis that the output power of the structure is proportional to the value of the resonant frequency. The device working in Lame mode has a much higher resonant frequency of 2.39 MHz than devices working in ordinary bending modes, which is expected to significantly increase the energy output of radioisotope power generators (RPGs). The results of static analysis and dynamic response show that output voltage is linear with the applied load; the output power is quadratic with the applied load.

A micro-mechanical resonator with programmable frequency capability

Photo-thermal actuation has been used to program the resonant frequency of a VO2-coated SiO2 micro-bridge. The SiO2 micro-bridge had nominal length, width, and thickness of 300, 45, and 4.15 µm, respectively. The thickness of the VO2 coating was 150 nm. The changes in resonant frequency are caused by stress changes on the bimorph structure during the coating’s insulator-to-metal-transition. A total of 13 resonant frequency memory states ranging from 215.5 to 222.7 kHz were programmed by laser pulses of increasing energy in steps of 0.7 µJ, focused on the micro-bridge structure. The device was maintained at 60 °C during programming experiments, and the memory was reset by driving the temperature outside the hysteresis loop. After programming the device to a particular resonant frequency, the memory state was stored for more than 24 h as long as the sample was maintained at the pre-heating temperature of 60 °C.

Edge-released, piezoelectric MEMS acoustic transducers in array configuration

A sensitive, broad-bandwidth piezoelectric microelectromechanical systems (MEMS) transducer based on frequency interleaving of resonant transducers was designed and fabricated. A sputter-deposited piezoelectric zinc oxide (ZnO) thin film on the diaphragm is used to sense and generate acoustic pressure. A high compliance cantilever and spiral-beam-supported diaphragms are designed and built on the edge-released MEMS structure to release initial residual stress and to avoid in-plane tension when bent. Stress compensation has been achieved by adjusting the thickness of each layer of the cantilever and by compensating for the ZnO film's compressive stress with the bimorph structure of the spiral-beam. For a given pressure level and diaphragm size, the maximum strain on the spiral-beam-supported diaphragm is about an order of magnitude larger than that of a rectangular cantilever diaphragm. Also, the acoustic transducer built on the spiral-beam-supported diaphragm has a much higher sensitivity (but with less tolerance on the fabrication process variation and at the cost of lower usable bandwidth) than the one built on a rectangular cantilever diaphragm. By connecting many transducers in parallel, both the sensitivity and acoustic output were improved about 30 times. The interleaving of the transducers increased not only the sensitivity, but also broadened the useable bandwidth.

Analytical determination of characteristic frequencies and equivalent circuit parameters of a piezoelectric bimorph

Piezoelectric structures are nowadays used in many different applications. A better understanding of the influence of material properties and geometrical design on the performance of these structures helps to develop piezoelectric structures specifically designed for their application. Different equivalent circuits have been introduced in the literature to investigate the behaviour of piezoelectric transducers. The model parameters are usually determined from measurements covering the characteristic frequencies of the piezoelectric transducer. This article introduces an analytical technique for calculating the mechanical and electrical equivalent system parameters and characteristic frequencies based on material properties and geometry for a cantilever bimorph structure. The model is validated by measurements using a cantilever bimorph and fits the experimental results better than previous models. The model gives a full set of piezoelectric transducer parameters and is therefore well suited for further theoretical investigations of piezoelectric transducers for different applications. The results also show that even small manufacturing tolerances have a considerable effect on the system parameters and characteristic frequencies. This might lead to intolerable deviations, especially in dynamic applications and should be avoided by careful design and production.

Sensitivity study of surface stress biosensors based on ultrathin Si membranes

A study on the design of membrane based bimorph structures that can be used for the development of surface stress biosensors is presented. At first, the finite element analysis of cantilever based biosensors is evaluated utilizing analytical equations and published experimental results. The model that is selected simulates successfully the cantilevers' behavior and can be further expanded to more complicated structures. Therefore, we study the sensitivity of the membrane based surface stress biosensors and the effect that their geometrical parameters have on the sensor performance. Although membranes are less sensitive than the cantilevers, they attain interest in biological sensing because they can be used with capacitive readout, and thus have the potential for miniaturized sensor arrays. Based on the simulation results, we reveal the structure characteristics that should be considered in the design of the biosensors in the case of flat Si membranes.

Enhanced resonant magnetoelectric coupling in frequency-tunable composite multiferroic bimorph structures

We report on a giant tunable enhanced resonant magnetoelectric (ME) coupling in multiferroic magnetostrictive/piezoelectric composite bimorph structures. The approach uses a magnetic/electric field assisted stress-reconfigurable resonance to produce frequency tuning of up to 100%. The studies were performed by laser Doppler spectroscopy. We also show that this principle of a continuously tuned resonance might be used to improve sensitivity for ME magnetic sensors.

Bending and Rotation Movement Control of a Novel Actuator Utilizing Hydrogen Storage Alloys

A power actuator based on a great volume expansion on hydrogenation of hydrogen storage alloys (HSAs) has been developed. The actuator has a bimorph structure consisting of Pd-Ni alloy and Cu-plating to convert the volume change into bending motion. The techniques to control the bending and rotation motions of the actuator were investigated by adjusting alloy composition, shape and the amount of hydrogen absorbed in the HSA. It is found that Pd-Ni alloy actuators exhibit a cyclic bending motion on hydrogen absorption and desorption cycles and the bending behavior can be controlled by controlling the hydrogen pressure. When a ribbon shaped actuator was deformed into the “L” shape on the transverse section, a rotation motion was observed without modifying the basic bimorph structure of the actuator.

Large rotation angle micromirror based on hypocycloidal electrothermal actuators

A large rotation angle micromirror using hypocycloidal electrothermal actuators (HEAs) is presented. The mechanical rotation angle of the micromirror is ∼35° at 2.6 V in nonresonant mode. The Cr/Au coated square mirror plate is supported by two groups of HEAs in order to achieve single axis rotation. Four Al/Si bimorph structures are staggerly connected in parallel to form a HEA. This structure is experimentally demonstrated to increase the deflection angle. Moreover, the rotation axis keeps still and there is no lateral shifting effect. The −3 dB cutoff frequency was found to be about 29 Hz as the large signal frequency response.

715 MEMS-BASED EVENT-DRIVEN ON/OFF THERMOMETER FOR ANIMAL-HEALTH MONITORING APPLICATION

With rapidly growing threatens of bird flu and other diseases, there are strong interests in developing battery-free high-sensitivity thermometer for application in wireless sensor network-based animal-health monitoring system. We proposed new 3-D bimorph structure that enables passively digital sensing actuated by event-driven on/off mechanism. Tens and more such 3-D bimorphs are fabricated on a common substrate forming a micro thermometer. The array configuration potentially could have the sensitivity of 0.1℃ and even better. It also enables high fabrication yield and makes the package simplified. This paper would present the fabrication details and the related experimental results.