Tip Deflection and Blocking Force of Soft PZT-Based Cantilever RAINBOW Actuators

A piezoelectric/electrostrictive RAINBOW actuator is a monolithic bending device consisting of an electromechanically active layer and a reduced passive layer formed in a high-temperature reduction treatment. When the piezoelectric or electrostrictive layer is driven under an electric field or when the environmental temperature changes, bending deflection is produced because of the constraint of the reduced inactive layer or because of the thermal expansion coefficient difference of the two layers. In this study, general analytical expressions relating tip deflection, blocking force, and equivalent moment with an applied electric field and temperature change are derived for a cantilevered RAINBOW actuator. It is shown that optimal actuator performance can be achieved in the RAINBOW actuator by choosing a suitable thickness ratio of the reduced layer to the PZT layer. A series of RAINBOW cantilever actuators have been experimentally prepared from high-density, soft, lead zirconate titanate (PZT) ceramics. Different reduction layer thickness is obtained by adjusting the processing parameters, such as reduction temperature and time. The measured results on tip deflection and blocking force agree well with theoretical prediction under a weak electric field. However, when a high driving electric field is used, deviation is observed, which can be attributed to a nonlinear piezoelectric response and a nonlinear elastic behavior associated with soft PZT materials under high driving electric fields.     

Field-dependent nonlinear piezoelectricity: a focused review

ABSTRACT

This contribution presents a multidisciplinary review of the so-called field-dependent nonlinear piezoelectricity. It starts with an introduction that poses the literature analysis framework, through defining this operational (that is often met in practice) piezoelectric field-dependent nonlinearity. Indeed, the latter is a less known phenomenon although it is inherent to stress-free actuation responses of corresponding smart materials, actuators and structures. Then, related experimental observations from piezoelectric materials, actuator devices and smart structures tests are multidisciplinary surveyed for understanding the underlying mechanisms of the encountered field-dependent nonlinearity. Next, empirical material and numerical structural modelling and simulation approaches are critically reviewed from, respectively, the constitutive and finite element analysis points of view. Summary conclusions and few future directions for research are finally provided as a closure. It is worth mentioning that, although it is concise (retains only experiments and experimentally-correlated models and simulations), this critical review covers the last three decades period which is almost the whole age of the piezoelectric materials, actuators and smart structures research field.     

Properties of Lead Zirconate Titanate Thick-Film Piezoelectric Actuators on Ceramic Substrates

Lead zirconate titanate (PZT) is a piezoelectric material that can sense or respond to mechanical deformations and can be used in ceramic electro-mechanical systems (C-MEMS). The microstructural, electrical, and piezoelectric characteristics of thick PZT films on low-temperature cofired ceramics (LTCC) and alumina substrates were studied. The PZT composition was prepared with low-melting-point additives in order to decrease the sintering temperature and to be compatible with thick-film technology. The integration of the PZT thick-film materials on ceramic substrates could lead to degradation of the PZT's characteristics due to the interactions between an active PZT layer and a substrate, particularly with glassy LTCC material. To minimize the interactions with LTCC substrates, an intermediate PZT barrier layer was integrated. The value of the piezoelectric coefficient d ₃₃ was found to be up to 120 pC/N on an alumina substrate and approximately 50 on an LTCC substrate. Based on these results, a cantilever-type actuator was designed and fabricated on alumina substrates. Under an applied voltage of 200 V, the maximum tip deflection was about 5 μm.     

A ZnO thin-film driven microcantilever for nanoscale actuation and sensing

Zinc oxide (ZnO) thin film as a piezoelectric material for microelectromechanical system (MEMS) actuators and sensors was evaluated. ZnO thin films were deposited using radio frequency (RF) magnetron sputtering. Process parameters such as gas ratio, working pressure, and RF power were optimized for crystalline structure. The ZnO thin films were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). Good quality of ZnO thin films was further confirmed by a high transverse piezoelectric coefficient d ₃₁. A microcantilever was then designed, fabricated, and characterized. Design formulas of resonant frequency, actuation, and sensing sensitivities were derived. The resonant frequency was determined by an impedance analyzer. Tip deflection on nanometer level was demonstrated with the cantilever used as an actuator. The actuation sensitivity was found to be 12.2 nm/V. As a sensor, the cantilever was calibrated against a reference accelerometer. The sensing sensitivity was characterized to be 46 mV/g. The characterization results were compared with design specifications. The differences were caused mainly by thickness control in etching. This study showed that ZnO is a promising piezoelectric material for MEMS actuators and sensors in terms of excellent process compatibility and good piezoelectric performance.     

Dielectric and piezoelectric properties of PVDF/PZT composites: A review

Smart materials, which exhibit piezoelectricity, find an eclectic range of applications in the industry. The direct piezoelectric effect has been widely used in sensor design, and the inverse piezoelectric effect has been applied in actuator design. Ever since 1954, PZT and BaTiO₃ were widely used for sensor and actuator applications despite their toxicity, brittleness, inflexibility, etc. With the discovery of PVDF in 1969, followed by development of copolymers, a flexible, easy to process, nontoxic, high density alternate with high piezoelectric voltage coefficient was available. In the past 20 years, heterostructural materials like polymer ceramic composites, have received lot of attention, since these materials combine the excellent pyroelectric and piezoelectric properties of ceramics with the flexibility, processing facility, and strength of the polymers resulting in relatively high dielectric permittivity and breakdown strength, which are not attainable in a single phase piezoelectric material. The current review article is an attempt to provide a compendium of all the work carried out with reference to PVDF‐PZT composites. The review article evaluates the effect of grain size, content and other factors under the purview of dielectric and piezoelectric properties while evaluating the sensitivity of the material for sensor application. POLYM. ENG. SCI., 55:1589–1616, 2015. © 2015 Society of Plastics Engineers     

Sample rotation angle dependence of graphene thickness measured using atomic force microscope

A precise measurement of graphene thickness is required for the design and development of nano-devices based on the material. Many factors affect this measurement when using scanning tunneling microscope (STM) and atomic force microscope (AFM), including the interaction between the scanning tip and ripples on graphene; such effects have not previously been explored. To investigate this, we measure the sample rotation angle dependence of graphene thickness as determined by contact mode and tapping mode AFM. The graphene thickness as determined by contact mode AFM follows a cosine modulus function of sample rotation angle, while tapping mode AFM reveals a constant graphene thickness, independent of sample rotation angle. For comparison, the AFM torsion signal is measured and follows a sine function of the sample rotation angle. All the measured sample rotation angle dependences can be explained by the interaction between linearly aligned ripples on graphene and the AFM tip in contact with the graphene.     

Bioinspired Poly(vinyl alcohol) Film Actuator Powered by Water Evaporation under Ambient Conditions

Soft humidity‐responsive materials are highly desirable for applications such as actuators, sensors, generators, and soft robots. However, it remains a huge challenge to develop a durable, cost‐effective, fast responsive version of such a smart material powered by water evaporation at ambient conditions. Herein, this challenge is addressed to demonstrate sustained response to humidity gradient from ambient water evaporation by using common poly(vinyl alcohol) (PVA) film as an actuator. The resultant PVA film displays strong mechanical properties in both dry and wet conditions, which cause rapid adsorption and desorption of water vapor to drive the film undergoing swift locomotion with flipping frequency of up to 65 r min^‐1. Based on these features, a mimosa inspired humidity‐responsive actuator is developed which is far superior in response speed and durability than real mimosa. Furthermore, it is demonstrated that the film actuator can convert water evaporation energy into electricity when attached to a piezoelectric element.     

Dielectric and mechanical optimization properties of porous poly(ethylene‐co‐vinyl acetate) copolymer films for pseudo‐piezoelectric effect

In this article, the authors present a porous copolymer film with pseudo‐piezoelectric effects as a new candidate material for sensing applications. Porous films of poly(ethylene‐co‐vinyl acetate) (EVA) with a thicknesses ranging from 160 to 310 μm are fabricated by a coextrusion chemical foaming process and charged using a high‐voltage contact charging process. Output performances (piezoelectric constant and relative permittivity) with related thermal/mechanical stability are specifically studied as a function of the film porosity and of the electrical charging process. The piezoelectric constant d₃₃ increases with the cell porosity and an interesting piezoelectric constant close to 5.1 pC/N is achieved with a porous EVA film containing 65% of porosity. Actual results are then discussed using a theoretical solid–gas multilayer model to describe and predict the pseudo‐piezoelectric effect of porous polymer materials. The originality of this work lies in the fact that all the steps leading to optimize pseudo piezoelectric films are included, and also in the use of EVA which is not a standard piezoelectric material. Therefore, this work is a contribution in the development of low‐cost piezoelectric materials with potential applications as sensor in different fields such as medical, security, environment, sport, and transport. POLYM. ENG. SCI., 59:1455–1461 2019. © 2019 Society of Plastics Engineers     

Diamond tips for automated electrical probing inside a scanning electron microscopy system

Pyramidal tips made from boron doped diamond have become the ultimate choice for electrically measuring semiconductor device structures in electrical atomic force microscopy (AFM). An advanced measurement setup with diamond probing units directly integrated inside a scanning electron microscopy (SEM) system is highly wanted as this allows for accurate tip positioning compared to the optical microscope of a standard AFM and enables also multiple tip measurements. Therefore, we have developed highly conductive in-plane diamond tips with a triangular shape and attached them to Ni cantilevers. We have established a LabVIEW-based setup enabling automated electrical measurements inside a SEM system using stick-and-slip motion nanomanipulators and a parameter analyzer system. To our best knowledge, this paper presents first 1- and 2-tip electrical measurements of microfabricated diamond probes inside a SEM system. Measurements of Si staircase and Ge structures are shown and compared to scanning spreading resistance (SSRM) results. Our work demonstrates that doped diamond tips clearly outperform common tungsten probe needles enabling nanoprobing experiments which were impossible so far. Based on our results, we predict that doped diamond is going to be the standard tip material not only for electrical AFM but also for nanoprobing of semiconductor materials.     

Preparation and characterization of ZnO microcantilever for nanoactuation

ABSTRACT: Zinc oxide [ZnO] thin films are deposited using a radiofrequency magnetron sputtering method under room temperature. Its crystalline quality, surface morphology, and composition purity are characterized by X-ray diffraction [XRD], atomic force microscopy [AFM], field-emission scanning electron microscopy [FE-SEM], and energy-dispersive X-ray spectroscopy [EDS]. XRD pattern of the ZnO thin film shows that it has a high c-axis-preferring orientation, which is confirmed by a FE-SEM cross-sectional image of the film. The EDS analysis indicates that only Zn and O elements are contained in the ZnO film. The AFM image shows that the film's surface is very smooth and dense, and the surface roughness is 5.899 nm. The microcantilever (Au/Ti/ZnO/Au/Ti/SiO2/Si) based on the ZnO thin film is fabricated by micromachining techniques. The dynamic characterizations of the cantilever using a laser Doppler vibrometer show that the amplitude of the cantilever tip is linear with the driving voltage, and the amplitude of this microcantilever's tip increased from 2.1 to 13.6 nm when the driving voltage increased from 0.05 to 0.3 Vrms. The calculated transverse piezoelectric constant d31 of the ZnO thin film is -3.27 pC/N. This d31 is high compared with other published results. This ZnO thin film will be used in smart slider in hard disk drives to do nanoactuation in the future.     

原子力显微镜对材料表面微摩擦性能研究进展

原子力显微镜(AFM)被广泛应用于材料表面微摩擦学的研究,利用AFM针尖对样品表面进行扫描,来模拟在微摩擦中存在的点面接触的情况。综述国内外研究发现:微摩擦性能受多因素的影响,摩擦力与表面形貌呈现相同的周期性,随表面湿度增加先增大后减小,并与表面分子基团类型、相对滑动速度、表面势的改变有很大关系。     

Microscopic study of electrical properties of CrSi<Subscript>2</Subscript> nanocrystals in silicon

Semiconducting CrSi₂ nanocrystallites (NCs) were grown by reactive deposition epitaxy of Cr onto n -type silicon and covered with a 50-nm epitaxial silicon cap. Two types of samples were investigated: in one of them, the NCs were localized near the deposition depth, and in the other they migrated near the surface. The electrical characteristics were investigated in Schottky junctions by current-voltage and capacitance-voltage measurements. Atomic force microscopy (AFM), conductive AFM and scanning probe capacitance microscopy (SCM) were applied to reveal morphology and local electrical properties. The scanning probe methods yielded specific information, and tapping-mode AFM has shown up to 13-nm-high large-area protrusions not seen in the contact-mode AFM. The electrical interaction of the vibrating scanning tip results in virtual deformation of the surface. SCM has revealed NCs deep below the surface not seen by AFM. The electrically active probe yielded significantly better spatial resolution than AFM. The conductive AFM measurements have shown that the Cr-related point defects near the surface are responsible for the leakage of the macroscopic Schottky junctions, and also that NCs near the surface are sensitive to the mechanical and electrical stress induced by the scanning probe.     

Comparison study of macro and micro scale AC and DC conductivity measurements with Impedance Spectroscopy and Atomic Force Microscopy techniques applied in PBZT ceramics

Ferroelectric materials systematically enter into the structure of microelectronic devices. The ability to increase the packing density of the ferroelectric structures, and thus the piezoelectric coefficients of the final device, is primarily limited by the fact that such tiny ferroelectric structures may not preserve their microscopic properties at macroscopic scale. A problem of current interest in ferroelectric research is to get to know how to modify the domain structure and the piezoelectric properties of the material, if the polycrystalline material consists of grains and grain boundaries, in which electrical properties differ significantly. In this paper, we have combined the Impedance Spectroscopy (IS), as a method for detecting such inequality in the form of separated impedances, and Atomic Force Microscopy (AFM), as techniques for direct local engineering and investigation of grain and grain boundaries conductivity. We would like to present hitherto unreported connection between values of electrical parameters obtained by both methods.     

Unipolar Fatigue Behavior of BCTZ Lead‐Free Piezoelectric Ceramics

(Ba,Ca)(Ti,Zr)O₃ lead‐free piezoelectric ceramics have been considered to be one of the most potential lead‐free alternatives for PZT in the room‐temperature range. The stability of the piezoelectric performance during unipolar cycling is investigated in this study. It is found that the unipolar fatigue behavior is similar to soft PZT. Developments of bias field, offset polarization, asymmetry in strain, and dielectric hysteresis loops are observed during bipolar measurements. The changes are mainly contributed to the migration of charge carriers to the grain boundaries driven by the unscreened depolarization field. Redistribution of the accumulated charge carriers by bipolar electric cycling or thermal annealing can significantly recover the unipolar fatigued state. The unipolar strain response stabilized after 1000 cycles at 0.053% for an electric field of 0.6 kV/mm (d₃₃*= 883 pm/V), which is a good characteristic for actuator applications.     

Correlation Between Piezoelectric Properties and Phase Coexistence in (Ba,Ca)(Ti,Zr)O3 Ceramics

High piezoelectric properties are desired for lead‐free piezoelectric materials in consideration as a replacement for lead‐based materials in applications. Due to the high piezoelectric coefficient, (Ba_100?xCa_x) (Ti_100?yZr_y) O₃ (BCTZ) piezoelectric ceramics have been considered as a promising lead‐free alternate piezoelectric material. Here, six compositions were selected based on a prediction that all the compositions would have high piezoelectric coefficient at room temperature. The results confirmed all compositions exhibit well developed hysteresis loops and a large piezoelectric coefficient at room temperature. This is due to the coexistence of several phases where the major phase is likely to be orthorhombic and the second phase is proposed to be tetragonal. The phase transition was found to occur over a broad temperature range instead of at a specific temperature only. A relationship between the tetragonal–orthorhombic phase transition temperature and Ca²⁺ and Zr⁴⁺ content was proposed. This enables clear determination of BCTZ compositions with high piezoelectric coefficient at a desired operation temperature.     

Nanodomain Engineered (K,Na)NbO3 Lead‐Free Piezoceramics: Enhanced Thermal and Cycling Reliabilities

The growing environmental concerns have been pushing the development of viable green alternatives for lead‐based piezoceramics to be one of the priorities in functional ceramic materials. A polymorphic phase transition has been utilized to enhance piezoelectric properties of lead‐free (K, Na)NbO₃‐based materials, accepting the drawbacks of high temperature and cycling instabilities. Here, we present that CaZrO₃‐modified (K, Na)NbO₃ piezoceramics not only possess excellent performance at ambient conditions benefiting from nanodomain engineering, but also exhibit superior stability against temperature fluctuation and electrical fatigue cycling. It was found that the piezoelectric coefficient d₃₃ is temperature independent under 4 kV/mm, which can be attributed to enhanced thermal stability of electric field engineered domain configuration; whereas the electric field induced strain exhibits excellent fatigue resistance up to 10⁷ sesquipolar cycles. These findings render the current material an unprecedented opportunity for actuator applications demanding improved thermal and cycling reliabilities.     

KNN-based piezoelectric ceramic multilayer actuators with Ni internal electrodes

On the road of lead-free piezoelectric ceramics into practical applications, the study of Ni-internal-electrode (K, Na)NbO₃-based (KNN-based) multilayer actuators (MLAs) is an important part, possessing the advantages of environmentally friendly and low cost. The Ni-internal-electrode KNN-based MLAs with different layer numbers and layer thicknesses were fabricated via the tape casting method and sintered in the reducing atmosphere. The piezoelectric layers consist of the main KNN-based phase and a trace amount of second phase Mn₄Nb₂O₉. The element diffusion between the Ni electrodes and KNN-based grains is tiny, indicating that Ni is suitable for co-firing with KNN-based ceramics. After sintering, the compressive stress perpendicular to the thickness direction and the “relative tensile stress” parallel to the thickness direction are retained in the MLAs, bringing influences on the piezoelectric and dielectric properties of KNN-based materials. Compared with the bulk ceramics, the prepared MLAs significantly reduce the driving voltages and increase the displacement outputs, which are more applicable to the scenarios of miniaturization and portability. Particularly, the 46-layer actuator shows high displacements of 1580 and 2737 nm under the voltages of 100 and 200 V, respectively. However, the inverse piezoelectric coefficient d₃₃* for each layer of MLAs is still lower than that of the bulk ceramics, indicating that the piezoelectric properties of the KNN-based materials are suppressed. To give full play to the piezoelectric properties of KNN-based materials, more attentions should be paid to the design of reasonable electrode structure and the development of internal electrode paste for MLAs.     

Tip deflection calculations of small-diameter thin-walled piezoelectric tubes

Piezoelectric tubes with various dimensions were fabricated using the electrophoretic deposition (EPD) process. The tubes are aimed to be used as actuators in fiber-optic switches which require a tip deflection of at least 65 μm. To maximize the tip deflection of the tubes and to achieve multiple-direction switching, the tubes had an unique electrode arrangement and, therefore, new analytic equations were needed to analyze their tip deflection. Equations for thick-walled and thin-walled piezoelectric tubes were developed by modifying existing equations which could be used to estimate the performance of new tubes and to optimize their dimensions for different applications. Experimental verification showed that the new equations gave a good estimation of the new piezoelectric tubes’ deflection. The advantages and limitations of the modified equations are also discussed.     

Nanopatterning of graphene with crystallographic orientation control

The recent papers on the nanopatterning of graphene and cutting of graphene nanoribbons were reviewed. It was found that until now the simultaneous control of crystallographic orientation and of the ribbon width in the range of nanometers was possible only by scanning tunneling lithography. The cutting process by local anodic oxidation under the AFM tip is a similar process, but due to the different physical interaction mechanisms of the STM and AFM tip with the substrate, and due to the larger radius of the AFM tip, the resolution of AFM lithography is poorer. The various cutting processes based on mobile, catalytic nanoparticles yield trenches with well defined crystallographic orientation, but have a major drawback: the location of the nanoparticles and the control of the direction in which the cutting will start are currently not predictable. The first promising results of a solid phase reduction reaction of the SiO₂ substrate at the graphene edge indicate the possibility of developing a new type of lithography that will allow the realization of complex nanopatterns. Recent results pointing to the possibility of the engineered modification of graphene edges may prove useful to all lithographic processes.     

Optical characterization of surface roughness of diamond by spectroscopic ellipsometry

Vacuum ultraviolet spectroscopic ellipsometry (SE) was performed on high-temperature and high-pressure (HPHT) synthesized Ib diamond samples with different polishing grades of different surface roughness, R_a [nm], which was measured by atomic force scanning microscopy (AFM). Observations showed that SE spectra systematically changed as a function of surface roughness. From this experimental result, we estimated the ideal dielectric function (DF) of diamond with a flat surface. Using this estimated DF, the optical thickness of a roughness layer was evaluated only from the SE data. It was shown that SE is an effective means of evaluating the thickness of a roughness layer and also the degree of surface modification without damaging the diamond surface, unlike the tip scanning of AFM measurements.