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Nanocomposite electrical generator based on piezoelectric zinc oxide nanowires
Abstract
A nanocomposite electrical generator composed of an array of zinc oxide nanowires is considered. The electric potential distribution along zinc oxide nanowires is modeled using continuum mechanics and Maxwell's equations for the case of axial loading. A perturbation technique is used for decoupling the constitutive equations. The governing differential equations are solved using a finite difference method. It is shown that a gradient of electric potential exists along the axis of the zinc oxide nanowires. Maximum and minimum values of electric potential exist at the extreme ends along the nanowire length and have opposite signs. The positive and negative voltages are separated by a zero-valued electric potential at the middle of the nanowire. It is also shown that the electric potential is a strong function of shear stress at the interface of matrix-nanowire. The proposed system and loading configuration can generate up to 160% more electric potential than the values reported for the nanowire in the bended configuration, which results in a more sustainable energy source.
... Various energy harvesting nanodevices were fabricated based on connecting individual ZnO NWs [6,11]. Multiscale modeling assists in designing nanodevices which utilize ZnO NWs by predicting the overall property of the device as a function of its structural geometry [12]. ...
... Nanocomposite electrical generators (NCEGs) are one of the multiscale devices successfully built experimentally [11] and studied theoretically [12], where the load transfer mechanism between polymer matrix and ZnO NWs plays a key role in their performance. Efforts have been made to assess the load transfer between matrix and nanoscale reinforcement phase of nanocomposites, which includes experimental studies [13], molecular dynamics simulations [14,15], and continuum based models [16,17]. ...
... Two different mod- The radius of NW is ro, and origin of the coordinate system is considered to be at the center of NW which has a length 2L f . Reprinted with permission from [12]. Copyright 2010, American Institute of Physics. ...
A multiscale approach is pursued to develop a shear-lag model in combination with core-surface and core-shell models for capturing size-scale effect on mechanical properties of ZnO nanowire (NW)-reinforced nanocomposites. Surface effects are represented by a zero-thickness (finite-thickness) surface with different elastic modulus from the central part of NW. The molecular dynamics technique is utilized for calculating thickness of the shell in the core-shell model. Linear elasticity for an axisymmetric problem and the cylindrical coordinate system is used to find the closed form of governing equations. The effect of different parameters, including diameter and aspect ratio of NWs, is studied to demonstrate the application of the developed model. Numerical results disclose that NWs with a larger aspect ratio and a smaller diameter can carry a larger portion of applied stress and are preferable in designing high-performance nanocomposites. This result is in agreement with the reported computational and experimental data.
... C/ m 2 , e 33 = 1.22 C/m 2 , ϵ 33 = 0.079e-9 F/m. [52][53][54] In addition, the components of its elastic matrix are as C 11 = C 22 = 207 GPa, C 33 = 209.5 GPa, C 12 = 117 GPa, C 13 = C 23 = 106.1 GPa, and C 44 = 44.8 GPa. ...
... GPa. [52][53][54] ...
Sweeping developments in microelectromechanical systems and low power electronics have pushed the need for piezoelectric energy harvesters (PEHs). Increasing environmental and biocompatibility issues have drawn interest in lead‐free piezoelectric materials. In this paper, cantilever‐type PEHs at centimeter scale have been proposed to harvest the energies of low‐frequency vibrations caused by a human's movements. The proposed PEHs are made of a passive PDMS substrate sandwiched between two active nanocomposite layers with embedded piezoelectric zinc oxide (ZnO) nanowires. Moreover, two different morphologies including PEHs with constant and tapered thicknesses have been considered. The material properties of such piezoelectric nanocomposites are calculated by an electromechanical model. Afterward, these novel PEHs have been developed in COMSOL Multiphysics and their static and dynamic performances have been investigated. The static study shows that tapered PEHs endures lower stresses. However, the dynamic study discloses that the rectangular design outperforms the tapered one in electrical power and resonance frequency. It is found that the rectangular design can produce 4.1623 μW at the resonant frequency of 25.8 Hz.
... In comparison with other smart materials, piezoelectric reveals some important advantages such as high accuracy, generation of large forces and very fast response. Hence, piezoelectric characteristics have been of great interest and a lot of works have been devoted to investigate its different properties in different size scales [8][9][10][11][12][13]. ...
... the first term of the summations (i.e. m=1, n=1) in the assumed mechanical displacements and electrical potential(12) and substituting them into Eq. (8), the governing partial differential equations (PDEs) of equilibrium in terms of displacements and electrical potential reduce to the following ordinary differential equations (ODEs) ...
In the present study, a layerwise finite element method is utilized to solve the coupled elasticity and piezoelectricity equations to study a functionally graded shell panel integrated w i t h piezoelectric layers under electromechanical loading. The system of equations is reduced to ordinary differential equations w i t h variable coefficients by means of trigonometric function expansion in circumferential and longitudinal directions satisfying mechanical and electrical boundary conditions. These equations are solved using the Galerkin F E M and Newmark method. The results of stress, displacement, and electrical potential are presented and the effect of panel thickness and applied voltage on the structural behavior is investigated.
... For a more detailed discussion on different aspects related to the cylindrical approximation, it is possible refer to Refs. [32][33][34][35][36][37]. We want to derive an expression for the strain tensors and the free elastic energy in cylindrical coordinates: ...
The main intent of this paper is to present an exhaustive description of the most relevant mathematical models for the electromechanical properties of heterostructure quantum dots. Models are applied both to wurtzite and zincblende quantum dot due to the relevance they have shown for optoelectronic applications. In addition to a complete overview of the continuous and atomistic models for the electromechanical fields, analytical results will be presented for some relevant approximations, some of which are unpublished, such as models in cylindrical approximation or a cubic approximation for the transformation of a zincblende parametrization to a wurtzite one and vice versa. All analytical models will be supported by a wide range of numerical results, most of which are also compared with experimental measurements.
... The use of solar and wind energy is widespread, but both are intermittent sources of energy [1]. Nanogenerators have been on the rise to produce energy from the environment, but their output is greatly affected by the ambient conditions [2][3]. ...
... This method is useful in this context because it is a closed form mathematical model that is valid for piezoelectric nanowires in a passive matrix and has been shown to be relatively accurate against measured piezoelectric and elastic properties and FEA to find the elastic properties [49]. The material properties of ZnO NWs used for the calculation were based on sources [10,50,51,52] with the NW diameter assumed to be 5 nm and length assumed to be 600 nm and properties for epoxy were based on sources [10,53,54], both are shown in Table 1. ...
In this work, a lead-free piezoelectric nanocomposite material is proposed to be used as an actuator for electronic devices in microscales. Specifically, the use of the novel nanocomposite is examined as a micropump actuator with insulin delivery applications for individuals suffering from diabetes. This novel actuator replaces both the passive substrate and the active layer of traditional diaphragms. The active layer in traditional micropumps usually uses a piezoelectric lead zirconate titanate (PZT) layer due to its strong and desirable properties. PZT contains lead, which is toxic and environmentally hazardous. It is therefore desirable to use lead-free materials. The proposed nanocomposite material contains barium titanate (BaTiO3) nanoparticles, as a lead-free piezoelectric material, embedded in a piezoelectric polymeric matrix of polyvinylidene fluoride (PVDF). The active nanocomposite’s electromechanical properties were estimated using Eshelby’s approach. This actuator has been developed and tested with COMSOL Multiphysics, meeting requirements needed for an insulin micropump. The proposed micropump meets the requirements for insulin delivery including a flow rate of 0.1–20 µL/min and a backpressure of greater than 9.8 kPa. Moreover, a comprehensive study on the static, free vibration, and dynamic behavior of the proposed diaphragm has been performed. This work shows that a BaTiO3/PVDF nanocomposite actuator is able to provide means of actuating an insulin delivery micropump. This nanocomposite can be an alternative to traditional lead-containing actuators. The lead-free actuator is also eco-friendly and bio-compatible, which are important considerations when designing a safe and sustainable micropump, especially for medical applications.
... The use of solar and wind energy is widespread, but both are intermittent sources of energy [1]. Nanogenerators have been on the rise to produce energy from the environment, but their output is greatly affected by the ambient conditions [2][3]. ...
Ocean waves are a great source of energy. Researchers have proposed various designs for triboelectric Nanogenerators for harvesting energy from water waves. Our aim in this project is to develop a flexible triboelectric Nanogenerator which can function in
the harsh marine environment. Biomimicry is the study of nature to solve the daily problems of life. This has led to
new revolutionized systems encouraged by the solutions of Biology in the nano and macroscale. Man has studied nature to find solutions to his queries since human beings come into existence. Nature has evaluated many technical intricacies such as environmental risk tolerance, self-healing, self-organization, and hydrophobicity.
... Shao et al. (2010) proposed the simple continuum model for evaluating the distribution of electric potential generated in the cantilever nanorod bent by the uniform force applied at its tip. Momeni et al. (2010) developed the multi-physics analytical model to determine the electric potential of ZnO nanocomposite. From these studies it is found that the piezoelectric effect in nanostructures play a significant role on their working mechanisms, especially nanowire-based nanogenerators. ...
Owing to its unique multifunctional and scale-dependent physical properties, a graphene is emerged as a promising reinforcement to enhance the overall response of its nanotailored composite materials. Most recently, the piezoelectricity phenomenon in graphene sheets was found through interplay between different non-centrosymmetric pores, curvature and flexoelectricity concept. This has added new functionality to the existing non-piezoelectric graphene. An overview of the literature revealed that the graphene-reinforced polymer matrix nanocomposite-based structures find numerous nanoelectromechanical systems (NEMS) and allow researchers to tailor their mechanical, thermal and electrical properties as per requirements. Such a piezoelectric graphene reinforced in the polymer matrix may be called as “graphene-reinforced nanocomposite (GRNC)”. Surprisingly, the application of piezoelectric graphene for modelling of graphene-based structures is not explored yet and this has provided the motivation for this Thesis. Therefore, the purpose of present research is to model the GRNC-based beams, plates, wires and shells.
... [285][286][287][288][289][290] For piezoelectric nanogenerators (PENGs), force can be exerted perpendicularly or parallelly to the axis of the nanowire. PENGs can also be categorized by the geometric configuration, namely, Vertical nanowire Integrated Nanogenerator (VING), 291 Lateral nanowire Integrated Nanogenerator (LING), 292 and Nanocomposite Electrical Generator (NEG), 293 and fabric-like geometrical configuration. Fiber-shaped PENGs could be realized by growing ZnO nanowires on Kevlar fibers in the radial direction or by growing ZnO on PVDF-coated conductive fibers. ...
Wearable devices are drawing increasing attention in both academia and industries in that they can offer unprecedented information related to human health in real-time and human-machine interactions, which is expected to enable a paradigm shift in the digital world. For this shift to occur, green and sustainable energy technology for powering flexible wearable devices is a roadblock. This paper is dedicated to reviewing the cutting-edge wearable power generation methodologies, for which we discuss their Pros and Cons, their underlying physics, and general design/evaluation criterion. Sensor types, materials, processing technology, power consumption, and methods of testing the stretchability and flexibility of wearable devices are also summarized. Based on application scenarios in healthcare, industrial inspection, structural monitoring, armed forces and consumer electronics, an integrated system architecture of the wearable, flexible system is presented. Finally, future perspectives of wearable technologies are outlined by covering the aspects of all-in-one printable wearable electronics, self-powered self-awareness wearable system, hybrid-integrated SoC (System on Chips) for flexible electronics, fiber electronics-enabled smart textiles, and IoT (Internet of Things)-enabled self-contained system towards full life cycle monitoring.
... Shao et al. (2010) proposed the simple continuum model for evaluating the distribution of electric potential generated in the cantilever nanorod bent by the uniform force applied at its tip. Momeni et al. (2010) developed the multi-physics analytical model to estimate the electric potential of ZnO nanocomposite. Recently, Shingare and Kundalwal (2020) studied the static response of graphene-based composite nanobeams subjected to end-point load with different boundary conditions considering the flexoelectric effect. ...
In this work, an analytical model was developed to study the distribution of electric potential in a graphene reinforced nanocomposite (GRNC) nanowire. The electromechanical responses such as electric potential and deflection of cylindrical GRNC cantilevered nanowire were investigated. Moreover, the conservative fully coupled finite element (FE) models were developed to validate the analytical predictions. Analytical solution shows that the piezoelectric potential in the GRNC nanowire depends on the transverse force, but it is not a function of the force acting along its axial direction. The electric potential in the tensile and compressive sections of nanowire is antisymmetric along its cross-section, making it as a “parallel plate capacitor” for nanopiezotronics applications such as nanogenerator and piezoelectric field effect transistor due to potential drop across the nanowire which assists as the gate voltage. The predictions of potential distributions across the GRNC nanowire considering piezoelectricity show better agreement with FE predictions. Outcomes of the current work reveal that the flexoelectric effect on the electromechanical behavior of GRNC nanowire is noteworthy and cannot be ignored.
... These nanowires are utilized as the nanofillers of polymer-based smart piezoelectric nanocomposites. The resulted smart piezoelectric nanocomposites are ideal and applicable materials for smart electromechanical devices with an enormous range of multi-functionality applications such as energy harvesting and fluid delivery systems (Agrawal and Espinosa 2011;Momeni, Odegard, and Yassar 2010;Moradi-Dastjerdi, Meguid, and Rashahmadi 2019a). Furthermore, the piezoelectric ZnO or GaN NW/ polymer nanocomposites enjoy different advantages in comparison with conventional piezoceramics. ...
The use of piezoelectric nano-fillers in a non-piezoelectric matrix is a very attractive proposition for developing smart nanocomposite materials with desired electro-mechanical properties. The eco-friendly nanowires (NWs) of zinc oxide (ZnO) and gallium nitride (GaN) are the two candidates used to introduce smart piezoelectric nanocomposite materials. Specifically, in this first effort, we examined the electro-mechanical performance of newly developed composite plates reinforced by piezoelectric ZnO NW and GaN NW of varied volume fractions. The static deflections and natural frequencies of the newly developed bimorph piezoelectric nanocomposite plates subject to electro-mechanical loads are analyzed using a mesh-free method in conjunction with the shape functions of MLS. Using third order shear deformation theory (TDST), the coupled electro-mechanical governing equations for the considered smart plates are obtained and numerically integrated. The effects of the electro-mechanical loading and plate thickness on static deflection and natural frequencies of piezoelectric plates are investigated and discussed. Our predictions reveal that the application of electrical input to the plates can induce greater deflection than the ones introduced by mechanical loads and that ZnO NWs offer greater deflection than GaN NW. However, dynamic analysis indicates that GaN NW-reinforced plates have higher natural frequencies than those reinforced by ZnO NW.
... The recombination emission between the electrons and holes is resonantly transferred to Mn 2þ ions (⑤), which pumps their electrons from the ground state to higher energy levels (⑥). Finally, after a non-radiative relaxation process (⑦), a red ML is then observed due to the 4 T 1 ( 4 G) to 6 A 1 ( 6 S) transition of Mn 2þ (⑧) [22,[31][32][33]. Nevertheless, we believe that the study on the mechanism of ML is still in its infancy. ...
... In addition, the material properties of ZnO NW with the diameter d =50 nm and length l = 600 nm, according to Refs. [35][36][37], are: It is worth noting that, the material properties of the piezoelectric ZnO NW/Epoxy nanocomposite were estimated using a method based on a micromechanical model reported by Tan and Tong [38]. ...
This paper is concerned with the thermo-electro-dynamic modeling of a novel micro fuel delivery system (MFDS) that is accurate, low cost and ecofriendly. To avoid the use of brittle piezoelectric materials, we propose to use a two-chamber micro-device with a bimorph piezoelectric nanocomposite diaphragm containing Zinc Oxide Nanowires (ZnO NWs). The numerical study of our dynamic model is facilitated by mesh-free method and higher order shear deformation theory (HSDT) for varied temperatures and applied electro-mechanical loads. The elasticity modulus of the polymeric matrix is taken to be temperature-dependent. The static behavior and the natural frequency of the proposed smart nanocomposite diaphragm are analyzed for different volume fraction of ZnO NWs, diaphragm thickness, and operating temperature. We further examined the effect of the aforementioned parameters as well as the exciting electrical potential on the delivered flowrate and backpressure of the newly proposed MFDS. The results show that the dynamic behavior of the nanocomposite piezoelectric diaphragm is governed by the volume fraction of ZnO NWs. It also demonstrates the potential use of piezoelectric ZnO NWs nanocomposite in the diaphragm of a MFDS.
... Conversely, the strain-induced positive piezo-charges at the p-n junction create a dip in the local band structure, leading to the accumulation of electrons from the n-type side, and thus modify carrier transport characteristics. As discussed above, it's clear that the strain-induced piezopotential can modify the local band structure near the interfacial region so that to effectively tune/control the generation, separation, recombination, and transport of charge carriers [42][43][44]. ...
Recently, there has been an increasing research interest in the emerging fields of piezophotonics, which is the great interesting physics responsible for numbers of important technologies such as light source, smart sensor and mechanoelectronics. Piezophotonic effect is the coupling between the piezoelectric polarization and the photonic excitation in crystal that has a non-central symmetry. The strain-induced piezopotential can stimuli the photon emission without additional energy excitation such as light and electricity, which also offers great new opportunities of manipulating and fabrication of flexible optoelectronic devices. In this review, we will give a detailed description of the piezophotonic effect including its theoretical fundamental and practical applications. The piezophotonic light emission in doped ZnS CaZnOS, SrAl2O4 and LiNbO3 attract great interesting during the past years, and researchers have executed many scientific inquiries into flexible/stretchable optoelectronic devices. Until now, significant breakthroughs have been achieved on piezophotonic e-signature system, visible wearable electronic devices and multi-physical coupling devices. Certainly, rapid innovations in this field will be quite significant to the future of human life.
... Ethylene-vinyl acetate incorporated with organically modified nanofiller clay showed worsened electrical properties owing to moisture absorption dependent on the aspect ratio of the nanofiller, and it was found that a higher aspect ratio led to greater moisture absorption [3]. Moreover, the aspect ratio of the nanoparticles strongly affects the stress distribution within the matrix reinforcement, and this stress can further affect the diffusion rate of absorbed moisture [7][8][9]. ...
The use of nanocomposites as dielectric materials is expected to lead to improved electrical performance. However, recent research has shown that moisture absorption can cause a deterioration in the electrical performance of nanocomposites. Although it is generally accepted that hydroxyl groups attached to nanoparticle surfaces are the main cause of moisture absorption, the impact of this absorption on the electrical properties of nanocomposites is still not fully understand. In this paper, a series of measurements, including thermogravimetric analysis, DC breakdown, surface potential decay and space charge, are conducted with the aim of determining the impact of moisture absorption on the electrical properties of polyethylene/silica nanocomposites. The results show that the loading ratio of nanosilica and the humidity of the conditioning environment determine the amount of absorbed moisture. According to the Zhuravlev model, the main contribution to the deterioration in electrical properties of nanocomposites comes from the large amount of moisture absorbed in multilayer form. It is found that the loading ratio of nanosilica is the most significant factor in reducing DC breakdown strength.
... A wind-driven pyroelectric NG has been also recently reported by Xie et al. 15 Alongside experimental evolution, theoretical and analytical efforts have also been undertaken to demonstrate the generation of electric potential in piezoelectric materials. 16,17 Boxberg et al. 18 reported that the piezoelectric property of compound semiconductor core−shell nanowires (NWs) might be utilized for the separation of photon-generated electron− hole pairs and, thereby, for photovoltaic applications. ...
... zinc oxide [2,4], lead zirconate titanate (PZT) [5], bismuth sodium titanate (Bi1/2Na1/2)TiO3(BNT)-barium zirconate titanate Ba(Ti,Zr)O3 (BZT) [6], polyvinylidene fluoride (PVDF), and so forth. The materials with a noncentrosymmetric crystal are polarized between atoms and excited charge carried when subjected to an external force [7]. Among them, ZnO is a good material exploiting for PENGs because of biocompatible, biodegradable, and biosafe [8]. ...
The cobalt and aluminum codoped into zinc oxide (ZnO) crystals are studied for enhancing the output voltage of piezoelectric-based nanogenerators (PENGs). The nanogenerators were simply fabricated from ZnO nanofibers (NFs) synthesized by electrospinning machine. The electrospinning processes were conducted at a flow rate of 4 μL/min and at various sintering temperature of 450, 500, 550, and 600°C. The results show that the highest output voltage of PENGs was obtained at a sintering temperature of 500°C, i.e. 127 mV. Therefore, PENGs fabricated from ZnO NFs codoped with aluminum and cobalt are challenging for the next generation of self-powered devices. The output voltage of PENGs at sintering temperatures of 450°C, 550°C, and 600°C are 75 mV, 105 mV, and 85 mV, respectively. The PENGs which fabricated from ZnO NFs was codoped with aluminum and cobalt will generate high output voltage, and are interesting candidates for exploiting in self-power devices.
... For example, the maximum power output of the model with the flexoelectric effect is almost twelve times that of the classical model in some cases. Nanocomposite electrical generators based on ZnO nanowires embedded in an epoxy matrix were also modeled and quantitatively analyzed with varying aspect ratios and diameters [125,126], and the surface effects were further incorporated through a core-surface model [127]. Numerical results indicated that the maximum generated voltage is related to the diameter of nanowire and an optimum aspect ratio for each nanowire diameter was determined for the energy generator. ...
Piezoelectric nanomaterials (PNs) are attractive for applications including sensing, actuating, energy harvesting, among others in nano-electro-mechanical-systems (NEMS) because of their excellent electromechanical coupling, mechanical and physical properties. However, the properties of PNs do not coincide with their bulk counterparts and depend on the particular size. A large amount of efforts have been devoted to studying the size-dependent properties of PNs by using experimental characterization, atomistic simulation and continuum mechanics modeling with the consideration of the scale features of the nanomaterials. This paper reviews the recent progresses and achievements in the research on the continuum mechanics modeling of the size-dependent mechanical and physical properties of PNs. We start from the fundamentals of the modified continuum mechanics models for PNs, including the theories of surface piezoelectricity, flexoelectricity and non-local piezoelectricity, with the introduction of the modified piezoelectric beam and plate models particularly for nanostructured piezoelectric materials with certain configurations. Then, we give a review on the investigation of the size-dependent properties of PNs by using the modified continuum mechanics models, such as the electromechanical coupling, bending, vibration, buckling, wave propagation and dynamic characteristics. Finally, analytical modeling and analysis of nanoscale actuators and energy harvesters based on piezoelectric nanostructures are presented.
... The piezoelectric material is a particular material which has a non-centrosymmetric crystalline form [1]. Research on piezoelectric materials including the ZnO material for nanogenerators has been intensively done especially on how to enhance their performance. ZnO material is very interesting to study and continue to be developed because of the availability abundant and chemically stable [2]. ...
This study reports the effect of co-doping cobalt and aluminum on the properties of ZnO-fiber piezoelectric materials. Synthesizing the ZnO-fiber piezoelectric material consisted of four steps: (1) making precursor solutions by dopingaluminum and cobalt, (2) creating green fibers with an electrospinning machine, (3) sintering the fibers, and (4) testing the piezoelectricity properties. The precursor materials used were polyvinyl acetate (PVA), zinc-acetate (ZnAc), AlCl3, and CoAc. The ZnO fibers were produced in an electrospinning machine with a distance between needle tip and collector of 8 cm and a flow rate of precursor solutions at 4 µL/min. The sintering was conducted at temperatures of 400, 450, 500, 550, and 600°C for 4 hours. Piezotest was used to measure the piezoelectricity properties of ZnO or d33. The results show that the maximum value of d33 was obtained in co-doping cobalt:aluminum at a ratio of 75:25 at the sintering temperature of 500°C, which amounted to −4.1 pC/N. The ZnO-fiber-based nanogenerators co-doped with cobalt and aluminum was capable of producing energy by 218 nW/cm².
... The metal-semiconductor contact is one of the common structures used in semiconductor optoelectronics and electronics. 58,59 A Schottky barrier is created at the metal-semiconductor junction The piezopotential distribution along a ZnO nanowire under c-axial strain calculated with numerical methods. The nanowire grows along the c-axis. ...
Wurtzite structured materials such as InN, CaN, ZnO, and CdSe simultaneously possess piezoelectric, semiconducting, and photoexcitation properties. The piezo-phototronic effect utilizes the piezo-polarization charges induced in the vicinity of the interface/junction to regulate the energy band diagrams and modulate charge carriers in the optoelectronic processes, such as transport, generation, recombination, and separation. This article reviews recent progress in piezo-phototronic effect enhanced photodetectors, starting from the fundamental physics, following the development from a single nanowire device to a large-scale photodetector array for illumination imaging. The piezo-phototronic effect provides a promising approach to improve the performance of the wurtzite structured material-based photodetectors. It may have potential applications in optical communication, optoelectronic devices, and multifunctional computing systems.
... Subsequent realizations have significantly improved the performance of the ZnO-based piezogenerators. Three different configurations have been introduced: the lateral NW integrated nanogenerator (LING) , the nanocomposite electrical generator (NEG) (Momeni et al 2010) and the vertical NW integrated nanogenerator (VING) (Xu et al 2008). Interest in the use of flexible substrates to fabricate NW piezogenerators was first evidenced by a new prototype based on cyclic stretching/releasing of a single piezoelectric wire packaged on a flexible substrate. ...
Ambient energy harvesting using piezoelectric nanomaterials is today considered as a promising way to supply microelectronic devices. Since the first demonstration of electrical energy generation from piezoelectric semiconductor nanowires in 2006, the piezoelectric response of 1D-nanostructures and the development of nanowire-based piezogenerators have become a hot topic in nanoscience. After several years of intense research on ZnO nanowires, III-nitride nanomaterials have started to be explored thanks to their high piezoelectric coefficients and their strong piezogeneration response. This review describes the present status of the field of piezoelectric energy generation with nitride nanowires. After presenting the main motivation and a general overview of the domain, a short description of the main properties of III-nitride nanomaterials is given. Then we review the piezoelectric responses of III-N nanowires and the specificities of the piezogeneration mechanism in these nanostructures. Finally, the design and performance of the macroscopic piezogenerators based on nitride nanowire arrays are described, showing the promise of III-nitride nanowires for ultra-compact and efficient piezoelectric generators.
... The revealed structural transformation introduces new ways for producing 1D nanomaterials such as laser ablation of nanosheets to make nanotubes that have a wide range of applications such as sensors, 42 actuators, 43 energy harvesting devices, 44,45 and composites. [46][47][48] ...
Reducing the dimensions of materials to atomic scales results in a large portion of atoms being at or near the surface, with lower bond order and thus higher energy. At such scales, reduction of the surface energy and surface stresses can be the driving force for the formation of new low-dimensional nanostructures, and may be exhibited through surface relaxation and/or surface reconstruction, which can be utilized for tailoring the properties and phase transformation of nanomaterials without applying any external load. Here we used atomistic simulations and revealed an intrinsic structural transformation in monolayer materials that lowers their dimension from 2D nanosheets to 1D nanostructures to reduce their surface and elastic energies. Experimental evidence of such transformation has also been revealed for one of the predicted nanostructures. Such transformation plays an important role in bi-/multi-layer 2D materials.
... Lately, piezoelectric materials based nanostructures have attracted much attention for applications in nanoelectromechanical systems (NEMS) where they serve as nanogenerators that could convert mechanical vibrations into electricity to power nanoscale devices without batteries. [1][2][3][4][5][6][7][8] Several lead-based materials are in use so far. But, due to environmental issues like recycling and disposal of lead in electronic products and its toxicity, lead-based piezoelectric materials are banned worldwide. ...
Nanocrystalline Ba0.85Ca0.15Zr0.1Ti0.9O3 (BCZT) powder was synthesized via the complex oxalate precursor route at a relatively low temperature (800°C/5h). The phase formation temperature of BCZT at nanoscale was confirmed by thermogravimetric (TG), differential thermal analysis (DTA) followed by X-ray powder diffraction (XRD) studies. Fourier Transform Infrared (FTIR) spectroscopy was carried out to confirm the complete decomposition of oxalate precursor into BCZT phase. The XRD and profile fitting revealed the coexistence of cubic and tetragonal phases and was corroborated by Raman study. Transmission electron microscopy (TEM) carried out on 800°C and 1000°C/5h heat treated BCZT powder revealed the crystallite size to be in the range of 20 – 50 nm and 40 – 200 nm respectively. The optical band gap for BCZT nanocrystalline powder was obtained using Kubelka Munk function and was found to be around 3.12 ± 0.02 eV and 3.03± 0.02 eV respectively for 800°C (20 – 50 nm) and 1000°C/5h (40 – 200 nm) heat treated samples. The piezoelectric properties were studied for two different crystallite sizes (30 and 70 nm) using piezoresponse force microscope (PFM). The d33 coefficients obtained for 30 nm and 70 nm sized crystallites were 4 pm/V and 47 pm/V respectively. These were superior to that of BaTiO3 nanocrystal (≈ 50 nm) and promising from the technological/industrial applications view point.
... Furthermore, the piezoelectric effect that converts mechanical strain into electric power can be utilized as an energy harvesting source in a smarter way in which human motion, low-frequency seismic vibrations, acoustic noise, vibration from a transportation vehicle and machine vibrations act as sources of mechanical strain [10]. In recent years, alongside experimental developments, several theoretical and analytical attempts have also been taken to demonstrate the generation of electric potential in piezoelectric materials using momentum, stress tensor and electric displacement vector equations [11][12][13]. ...
A high-performance flexible piezoelectric hybrid nanogenerator (HNG) based on lead-free
perovskite zinc stannate (ZnSnO3) nanocubes and polydimethylsiloxane (PDMS) composite
with multiwall carbon nanotubes (MWCNTs) as supplement filling material is demonstrated.
Even without any electrical poling treatment, the HNG possesses an open-circuit voltage of 40 V
and a short-circuit current of 0.4 μA, respectively, under repeated human finger impact. It has
been demonstrated that the output volume power density of 10.8 μWcm−3 from a HNG can drive
several colour light emitting diodes (LEDs) and a charge capacitor that powers up a calculator,
indicating an effective means of energy harvesting power source with high energy conversion
efficiency (∼1.17%) for portable electronic devices.
... Due to the high electro-mechanical energy conversion efficiency of piezoelectric materials, micro power piezoelectric generators have received much attentions in the past decades (4)(5)(6)(7)(8)(9). In particular, the applications of piezoelectric materials to harvest ambient energies, such as wind flow, water current, and raindrops, have become increasingly interesting. ...
Although there have been significant efforts in harvesting environmental energy, our environment is still full of wasted and unused energy. As clean, ubiquitous and sustainable energy source, acoustic energy is one of the wasted energies and is abundant in our life. Therefore, it is of great interest to investigate acoustic energy harvesting mechanism as an alternative to existing energy harvesters. In this study, in order to harvest acoustic energy, piezoelectric cantilever beams are placed inside a quarter-wavelength straight-tube resonator. When the straight-tube resonator is excited by an incident wave at its acoustic eigenfrequency, an amplified acoustic resonant wave is developed inside the tube and drives the vibration motion of the piezoelectric beams. The piezoelectric beams have been designed to have the same structural eigenfrequency as the acoustic eigenfrequency of the tube resonators to maximize the amount of the harvested energy. With a single beam placed inside the tube resonators, the harvested voltage and power become the maximum near the tube open inlet where the acoustic pressure gradient is at the maximum. As the beam is moved to the tube closed end, the voltage and power gradually decrease due to the decreased acoustic pressure gradient. Multiple piezoelectric beams have been placed along the centerline of the tube resonators in order to increase the amount of harvested energy. Due to the interruption of acoustic air particle motion caused by the beams, it is found that placing piezoelectric beams near the closed tube end is not beneficial. The output voltage of the piezoelectric beams increases linearly as the incident sound pressure increases.
... Until recently, several nanogenerators have been reported using BaTiO 3 , ZnSnO 3 , Pb(Zr,Ti)O 3 , Pb(Mg,Nb)O 3 -PbTiO 3 , and (K,Na)NbO 3 [6][7][8][9][10][11]. In particular, piezoelectric nanocomposite devices, in which piezoelectric nanostructures are mixed with flexible polymers, have exhibited relatively easy, cost-effective fabrication, and high-power generation [9][10][11][12][13]. In a flexible nanocomposite-based nanogenerator, important parameters to increase the output power include using long nanowires with high piezoelectricity and decreasing the dielectric constant of the nanocomposite [9]. ...
In a flexible nanocomposite-based nanogenerator, in which piezoelectric nanostructures are mixed with polymers, important parameters to increase the output power include using long nanowires with high piezoelectricity and decreasing the dielectric constant of the nanocomposite. Here, we report on piezoelectric power generation from a lead-free LiNbO3 nanowire-based nanocomposite. Through ion exchange of ultra-long Na2Nb2O6-H2O nanowires, we synthesized long (approximately 50 μm in length) single-crystalline LiNbO3 nanowires having a high piezoelectric coefficient (d33 approximately 25 pmV-1). By blending LiNbO3 nanowires with poly(dimethylsiloxane) (PDMS) polymer (volume ratio 1:100), we fabricated a flexible nanocomposite nanogenerator having a low dielectric constant (approximately 2.7). The nanogenerator generated stable electric power, even under excessive strain conditions (approximately 105 cycles). The different piezoelectric coefficients of d33 and d31 for LiNbO3 may have resulted in generated voltage and current for the e33 geometry that were 20 and 100 times larger than those for the e31 geometry, respectively. This study suggests the importance of the blending ratio and strain geometry for higher output-power generation in a piezoelectric nanocomposite-based nanogenerator.
PACS
77.65.-j; 77.84.-s; 73.21.Hb
Ferritic-martensitic steels, such as T91, are candidate materials for high-temperature applications, including superheaters, heat exchangers, and advanced nuclear reactors. Considering these alloys’ wide applications, an atomistic understanding of the underlying mechanisms responsible for their excellent mechano-chemical properties is crucial. Here, we developed a modified embedded-atom method (MEAM) potential for the Fe-Cr-Si-Mo quaternary alloy system—i.e., four major elements of T91—using a multi-objective optimization approach to fit thermomechanical properties reported using density functional theory (DFT) calculations and experimental measurements. Elastic constants calculated using the proposed potential for binary interactions agreed well with ab initio calculations. Furthermore, the computed thermal expansion and self-diffusion coefficients employing this potential are in good agreement with other studies. This potential will offer insightful atomistic knowledge to design alloys for use in harsh environments.
Nanogenerators are the backbone of self-powered systems and they have been explored for application in miniaturized biomedical devices, such as pacemakers. Piezoelectric nanogenerators (PENGs) have several advantages, including their high efficiency, low cost, and facile fabrication processes, which have made them one of the most promising nano power sources for converting mechanical energy into electrical energy. In this study, we review the recent major progress in the field of PENGs. Various approaches, such as morphology tuning, doping, and compositing active materials, which have been explored to improve the efficiency of PENGs, are discussed in depth. Major emphasis is given to material tailoring strategies and PENG fabrication approaches, such as 3D printing, and their applications in the biomedical field. Moreover, hybrid nanogenerators (HNG), which have evolved over the last few years, are discussed. Finally, the current key challenges and future directions in this field are presented.
With the fast development of nanoscience and nanotechnology in the last 30 years, semiconductor nanowires have been widely investigated in the areas of both electronics and optoelectronics. Among them, representatives of third generation semiconductors, such as ZnO and GaN, have relatively large spontaneous polarization along their longitudinal direction of the nanowires due to the asymmetric structure in their c-axis direction. Two-way or multiway couplings of piezoelectric, photoexcitation, and semiconductor properties have generated new research areas, such as piezotronics and piezo-phototronics. In this review, an in-depth discussion of the mechanisms and applications of nanowire-based piezotronics and piezo-phototronics is presented. Research on piezotronics and piezo-phototronics has drawn much attention since the effective manipulation of carrier transport, photoelectric properties, etc. through the application of simple mechanical stimuli and, conversely, since the design of new strain sensors based on the strain-induced change in semiconductor properties.
The research paper discusses the agility of nanogenerators. It firstly confers the basics of nanogenerators and describes the mechanism encouraging agility. It also describes the geometrical configuration along with the comparison of agile materials used for nanogenerators. Finally it compiles by discussing various applications of nanogenerators with its advantages and disadvantages.
Piezoelectric nanofibers are of great importance in their potential applications as smart fibers and textiles to bring changes to daily lives. By employing the technique of electrospinning, polyvinylidene fluoride (PVDF) nanofibers modified with polymethyl methacrylate (PMMA) and single-wall carbon nanotubes (CNTs) (referred to as CNT/PMMA/PVDF) are prepared. The electric field induced displacement of the as-prepared nanofibers is characterized by piezoresponse force microscopy. Compared with the pure PVDF nanofibers, the CNT/PMMA/PVDF nanofibers exhibit a great enhancement of about 196% for the electric field induced displacement, while increments of about 104% and 78% are obtained for the PMMA/PVDF and CNT/PVDF nanofibers, respectively. A structural analysis indicates that the hydrogen bonding between the O atom in the carbonyl group of PMMA and the hydrogen atom in the CH2 groups of PVDF, the promotion of the nucleation of crystallites by CNTs, work synergistically to produce the high electroactive response of the CNT/PMMA/PVDF nanofibers. Based on the high-performance nanofibers, a prototype of a flexible nanofiber generator is fabricated, which exhibits a typical electrical output of 3.11 V upon a repeated impact-release loading at a frequency of 50 Hz.
A NiO/GaN heterojunction piezoelectric generator was fabricated, and the improvement in device performance was analyzed. The electrical properties of NiO were varied by regulating the gas environment during sputtering. An optimized NiO layer was adopted for high piezoelectric voltage generation. Internal carrier screening was revealed to be the dominant mechanism degrading the piezoelectric performance, necessitating the suppression of carrier screening. The highly resistive NiO layer was advantageous in the suppression of carrier transport across the junction that screened the piezoelectric field. The maximum piezoelectric voltage and current density values obtained were 7.55 V and 1.14 μA cm⁻², respectively. The power obtained was sufficient to operate a light-emitting diode combined with a charging circuit.
This paper gives a detailed report of the evolution and potential applications of piezoelectric nanogenerators (PENGs). Various configurations, together with the operating principles and techniques used for the fabrication of PENGs, are discussed. A brief overview of the wide range of materials used for the design of PENGs that exhibit piezoelectric properties is also reported here. The optimization parameters to be considered while designing a PENG are also presented. PENGs are used in several application areas, which are collectively summarized.
Pyroelectricity is the ability of certain materials to generate an electric polarization when they are heated or cooled. To observe pyroelectricity, we can heat a crystal uniformly and observe the change in polarization. Piezoelectricity is found in useful applications such as the production and detection of sound, generation of high voltages, electronic frequency generation, microbalances, and ultrafine focusing of optical assemblies. The first practical application for piezoelectric devices was sonar, first developed during World War I. Piezoelectric devices found applications in many fields. The development of piezoelectric actuators for fuel injection systems is a popular application of the near past, active noise and vibration reduction is a current activity, and the use of piezoelectric materials for energy harvesting in vibrating structures is one possible future trend. Endeavors for enhancing the piezoelectricity of the individual nanowire also led to the development of other piezoelectric materials based on wurtzite structure.
A single-layer zinc oxide (ZnO) nanorod array-based micro energy harvester was designed and integrated with a piezoelectric metacapacitor. The device presents outstanding low-frequency (1-10 Hz) mechanical energy harvesting capabilities. When compared with conventional pristine ZnO nanostructured piezoelectric harvesters or generators, both open-circuit potential and short-circuit current are significantly enhanced (up to 3.1 V and 124 nA cm⁻²) for a single mechanical knock (∼34 kPa). Higher electromechanical conversion efficiency (1.3 pC/Pa) is also observed. The results indicate that the integration of the piezoelectric metacapacitor is a crucial factor for improving the low-frequency energy harvesting performance. A double piezoelectric-driven mechanism is proposed to explain current higher output power, in which the metacapacitor plays the multiple roles of charge pumping, storing and transferring. An as-fabricated prototype device for lighting an LED demonstrates high power transference capability, with over 95% transference efficiency to the external load.
In this paper, a nanostructured piezoelectric beam is fabricated using vertically aligned lead zirconate titanate (PZT) nanowire arrays and its capability of continuous power generation is demonstrated through direct vibration tests. The lead zirconate titanate nanowires are grown on a PZT thin film coated titanium foil using a hydrothermal reaction. The PZT thin film serves as a nucleation site while the titanium foil is used as the bottom electrode. Electromechanical frequency response function (FRF) analysis is performed to evaluate the power harvesting efficiency of the fabricated device. Furthermore, the feasibility of the continuous power generation using the nanostructured beam is demonstrated through measuring output voltage from PZT nanowires when beam is subjected to a sinusoidal base excitation. The effect of tip mass on the voltage generation of the PZT nanowire arrays is evaluated experimentally. The final results show the great potential of synthesized piezoelectric nanowire arrays in a wide range of applications, specifically power generation at nanoscale.
Highly crystalline zinc oxide (ZnO) nanowires (NWs) were synthesized through chemical bath deposition (CBD) method by using a simple seeding technique. The process includes dispersion of commercially available ZnO nanoparticles through spraying on a desired substrate prior to the CBD growth. A typical growth period of 16 h produced ZnO NW assemblies with an average diameter of ∼45 nm and lengths of 1–1.3 μm, with an optical band gap of ∼3.61 eV. The as-prepared ZnO NWs were photoactive under ultra violet (UV)illumination.Photodetector devices fabricated using these NW assemblies demonstrated a high photoresponse factor of ∼40 and 120 at room temperature under moderate UVillumination power of ∼250 μW/cm2. These findings indicate the possibility of using ZnO NWs, grown using the simple method discussed in this paper, for various opto-electronic applications.
A systematic evaluation of the piezoelectrically induced electric polarization vector and the associated potential on the application of mechanical strain to charge-free semiconductor nanowires with zincblende crystal structure is reported. It is found that the bending mode which is easier to realize in practice over stable compressional modes generates maximum piezo energy for these zincblende semiconductor nanowires. Also zincblende ZnO nanowires are found to be superior over zincblende AlN and GaN wires for piezo energy harvesting.
An analytical model of a compressed piezoelectric ZnO rod is presented using two approximations. In one approximation, the total volume of the model system is maintained a constant, i.e., when a compressive force is applied vertically on the ZnO system, it is allowed to be deformed in the lateral direction. In the second approximation, the top potential of the ZnO rod is used as a constant initially. Later, this is approximated as a function of the radius after obtaining the first solution to observe the radial effects on the ZnO system potential. In the model used in this study, a ZnO rod of 880 nm in diameter and 700 nm in length was compressed by a tensile force of 107 N/m2. The scaled magnitude of the potential (a two-dimensional contour map) shows a radial distribution of the potential. The radial effects are directly reflected on the piezoelectric potential and the device design is required to minimize such a potential variation to achieve better uniform device performance and efficiency
Wurtzite materials exhibit both semiconductor and piezoelectric properties under strains due to the non-central symmetric crystal structures. The three-way coupling of semiconductor properties, piezoelectric polarization and optical excitation in ZnO, GaN, CdS and other piezoelectric semiconductors leads to the emerging field of piezo-phototronics. This effect can efficiently manipulate the emission intensity of light-emitting diodes (LEDs) by utilizing the piezo-polarization charges created at the junction upon straining to modulate the energy band diagrams and the optoelectronic processes, such as generation, separation, recombination and/or transport of charge carriers. Starting from fundamental physics principles, recent progress in piezo-phototronic-effect-enhanced LEDs is reviewed; following their development from single-nanowire pressure-sensitive devices to high-resolution array matrices for pressure-distribution mapping applications. The piezo-phototronic effect provides a promising method to enhance the light emission of LEDs based on piezoelectric semiconductors through applying static strains, and may find perspective applications in various optoelectronic devices and integrated systems.
Flexoelectricity presents a strong size effect, and should not be ignored for nanodevices. In this paper, the flexoelectric effect is taken into account to investigate the electrostatic potential distribution in a bent flexoelectric semiconductive nanowire, and the numerical solution is obtained by using the finite difference method. The effect of donor concentration on the electrostatic potential are also investigated. The results show that, the flexoelectricity increases the value of the voltage on the cross section. The flexoelectric effect is varied with the size, i.e. when the radius of the nanowire is small the flexoelectric effect is significant. It is also shown that a lower donor concentration can increase the value of the voltage on the cross section. The results indicated that one can use the flexoelectricity to modify the transfer efficiency from mechanical energy to electric energy through doping and strain engineering.
A multiscale approach is pursued for modeling the size-scale effect on generated electric potential by nanocomposite electrical generators of ZnO nanowires. A core-surface model is used for capturing the effect of size-scale on elastic modulus of ZnO NWs. In this model, a surface with different elastic modulus as of the core of NW was considered. Using linear elasticity and axisymmetric configuration of this problem, closed form governing equations are derived in cylindrical coordinate system. Parametric studied are performed for sample cases to demonstrate application of the developed model. It is shown that ZnO nanowires with larger aspect ratio and smaller diameters have higher performance and can produce higher electric potential.
A nanoelectromechanical vibrational energy harvester is reported based on a ZnO nanorod array coupled to a microelectromechanical mass-spring system. A carpet of vertically aligned crystalline zinc oxide nanorods grown using a low temperature process was characterized to determine their piezoelectric response to mechanical excitation. An atomic force microscope was operated in the force spectroscopy mode to probe ZnO nanorods applying a force in the µN range. In contrast to previously published reports using lateral tip motion (C-AFM), the motion of the AFM tip in our experiment was perpendicular to the plane of the nanorods, which closely mimics the operation of the bulk-micromachined NEMS energy harvester. Voltage pulses with amplitudes ranging from hundreds of µV to few mV were observed. The novelty of this work lies in the integration of the ZnO NEMS and Si-MEMS structures, the use of nanoimprint lithography to achieve periodic, vertical placement of the ZnO nanorods, and the demonstration of piezoelectric voltage generation through low-temperature processed ZnO nanorod carpet.
Doped and undoped zinc oxide fibers were fabricated by electrospinning at various solution flow rates of 2, 4, and 6 μl/min followed by sintering at 550 °C. The nanogenerators (NGs) fabricated from the fibers were examined for their performance by applying loads (0.25–1.5 kg) representing fingers taps on the keyboard. A higher solution flow rate resulted in a larger fiber diameter, thus reducing nanogenerator voltage. The maximum power density for undoped zinc oxide-based and doped zinc oxide-based nanogenerators was 17.6 and 51.7 nW/cm2, respectively, under a load of 1.25 kg. Enhancing nanogenerator stability is a topic that should be investigated further.
This paper presents a flexible, integrated piezoelectric system that is simultaneously designed to operate as a self-powered touch sensor and as a charge pump and storage device for the long-term accumulation of energy over several pressure cycles. Rectification is achieved by stacking a piezoresistive polymer on top of a piezoelectric polymer, so that the conductivity of the former switches in phase with the generation of charge in the latter, upon application and release of stress. A thin flexible electrochemical capacitor is then stacked onto the structure to store the generated charge. The resulting integrated generator proves a robust device of general applicability, featuring a flexible and deformable nature, a large dynamic range, and simple manufacturing.
This work reports the generation of piezoelectric potential in uniaxially strained zinc oxide (ZnO) nanowires with Ohmic-, symmetric diode-, and rectifying Schottky-like silver-ZnO (Ag-ZnO) contacts. By controlling the synthesis process of the ZnO nanowires, one can control the transport properties of metal-ZnO interfaces. The measurements show the influence of the transport properties of the Ag-ZnO contact on the piezoelectric response of ZnO nanowires. Although the rectifying Schottky contact results in more effective energy harvesting, this is no necessity for operation of a ZnO nanogenerator. Uniaxially strained ZnO nanowires with Ohmic-like Ag-ZnO contacts can also generate measurable piezoelectric signals.
Power generation from lead zirconate titanate (PZT) piezoelectric fibers in the form of 1–3 composites under application of an external force was investigated. Green fibers consisting of PZT powder dispersed in a cellulose binder were made by the Viscous Suspension Spinning Process (VSSP). The composites were made by firing sheets of parallel green PZT fibers at 1270 °C, and then laminating the sintered sheets in epoxy. Composites of several PZT fiber diameters (15, 45, 120, and 250 μm), with the fiber volume fraction fixed at ∼0.4, were investigated. Transducers comprised of electrode and poled plates of the composites, in which the plate thickness direction was in the fiber axis direction, were made. Power generation experiments were conducted by dropping a 33 g stainless steel ball onto the electroded face of each transducer from a height of 10 cm and recording the output voltage on an oscilloscope. A peak voltage of 350 V corresponding to 120 mW of peak power was obtained. The output voltage and power was the highest for the transducers made with the smallest diameter fibers (15μm) and increased with increasing of transducer thickness. The average piezoelectric coefficient, d33, of the transducers was about 300 pC/N and decreased with decreasing transducer thickness. In this paper, the power generation capability and dielectric properties of the laminated 1–3 fiber composites are discussed.
Infrared dielectric function spectra and phonon modes of high-quality, single crystalline, and highly resistive wurtzite ZnO films were obtained from infrared (300–1200 cm−1) spectroscopic ellipsometry and Raman scattering studies. The ZnO films were deposited by pulsed-laser deposition on c-plane sapphire substrates and investigated by high-resolution x-ray diffraction, high-resolution transmission electron microscopy, and Rutherford backscattering experiments. The crystal structure, phonon modes, and dielectric functions are compared to those obtained from a single-crystal ZnO bulk sample. The film ZnO phonon mode frequencies are highly consistent with those of the bulk material. A small redshift of the longitudinal optical phonon mode frequencies of the ZnO films with respect to the bulk material is observed. This is tentatively assigned to the existence of vacancy point defects within the films. Accurate long-wavelength dielectric constant limits of ZnO are obtained from the infrared ellipsometry analysis and compared with previously measured near-band-gap index-of-refraction data using the Lyddane–Sachs–Teller relation. The ZnO model dielectric function spectra will become useful for future infrared ellipsometry analysis of free-carrier parameters in complex ZnO-based heterostructures. © 2003 American Institute of Physics.
The bending of a nonconducting piezoelectric ZnO nanowire is simulated by finite element method calculations. The top part is bent by a lateral force, which could be applied by an atomic force microscope tip. The generated electrical potential is ±0.3 V. This relatively high signal is, however, difficult to measure due to the low capacitance of the ZnO nanowire ( ∼ 4×10−5 pF) as compared to the capacitance of most preamplifiers ( ∼ 5 pF). A further problem arises from the semiconducting properties of experimentally fabricated ZnO nanowires which causes the disappearance of the voltage signal within picoseconds.
The arrays of piezoelectric, semiconducting ZnO nanowires (NW) grown on flexible plastic substrates can be used to convert mechanical energy into electrical energy using a conductive atomic force microscope. The estimated piezoelectric induced electrical power density of the NW array upon deflection of an AFM tip was on the order of milliwatts per centimeter squared, which is large enough to power a variety of MEMS, NEMS, and other nanoscale devices. The piezoelectric power generators that use ZnO NW arrays on flexible plastic substrates may be able to harvest energy from their environment for powering nanodevices and nanosystems. This flexible power source can find potential applications in implantable biosensors and biodetection, wireless self-powered sensors, and self-powering electronic devices.
The elastic properties of ZnO films deposited by rf magnetron sputtering on Al 2 O 3 substrates have been analyzed by means of an acoustic investigation technique. The phase velocities of a spectrum of acoustic modes propagating along the layered structure have been measured and the results exploited for determining the complete set of elastic constants of the film. The effective constants of the film are lower than those of the bulk material by amounts which depend on the elastic constant considered and range from -1.2% for c 3 3 to -24.8% for c 1 1 . The values obtained were used for determining the dispersion curves of acoustic modes propagating along ZnO layers deposited on fused quartz and silicon and showed good agreement with experimental results.
Hexagonal [0001] nonpassivated ZnO nanowires with diameters up to 2.8 nm are studied with density functional calculations. The authors find that ZnO nanowires have larger effective piezoelectric constant than bulk ZnO due to their free boundary. For ZnO nanowires with diameters larger than 2.8 nm , the effective piezoelectric constant is almost a constant. Surprisingly, the effective piezoelectric constant in small ZnO nanowires does not depend monotonically on the radius due to two competitive effects. Moreover, the quantum confinement effect results in larger band gaps of bare ZnO nanowires compared to that of bulk ZnO.
The harvesting of mechanical energy from ambient sources could power electrical devices without the need for batteries. However, although the efficiency and durability of harvesting materials such as piezoelectric nanowires have steadily improved, the voltage and power produced by a single nanowire are insufficient for real devices. The integration of large numbers of nanowire energy harvesters into a single power source is therefore necessary, requiring alignment of the nanowires as well as synchronization of their charging and discharging processes. Here, we demonstrate the vertical and lateral integration of ZnO nanowires into arrays that are capable of producing sufficient power to operate real devices. A lateral integration of 700 rows of ZnO nanowires produces a peak voltage of 1.26 V at a low strain of 0.19%, which is potentially sufficient to recharge an AA battery. In a separate device, a vertical integration of three layers of ZnO nanowire arrays produces a peak power density of 2.7 mW cm(-3). We use the vertically integrated nanogenerator to power a nanowire pH sensor and a nanowire UV sensor, thus demonstrating a self-powered system composed entirely of nanowires.
Zinc oxide nanowires, nanobelts, and nanoneedles were synthesized using the vapor-liquid-solid technique. Young's modulus of the nanowires was measured by performing cantilever bending experiments on individual nanowires in situ inside a scanning electron microscope. The nanowires tested had diameters in the range of 200–750 nm. The average Young's modulus, measured to be 40 GPa, is about 30% of that reported at the bulk scale. The experimental results are discussed in light of the pronounced electromechanical coupling due to the piezoelectric nature of the material.
Semiconductor nanowires are unique as functional building blocks in nanoscale electrical and electromechanical devices. Here, we report on the mechanical properties of ZnO nanowires that range in diameter from 18 to 304 nm. We demonstrate that in contrast to recent reports, Young's modulus is essentially independent of diameter and close to the bulk value, whereas the ultimate strength increases for small diameter wires, and exhibits values up to 40 times that of bulk. The mechanical behavior of ZnO nanowires is well described by a mechanical model of bending and tensile stretching.
We develop and solve a continuum theory for the piezoelectric response of one-dimensional nanotubes and nanowires, and apply the theory to study electromechanical effects in boron-nitride nanotubes. We find that the polarization of a nanotube depends on its aspect ratio, and a dimensionless constant specifying the ratio of the strengths of the elastic and electrostatic interactions. The solutions of the model as these two parameters are varied are discussed. The theory is applied to estimate the electric potential induced along the length of a boron-nitride nanotube in response to a uniaxial stress.
We have converted nanoscale mechanical energy into electrical energy by means of piezoelectric zinc oxide nanowire (NW) arrays. The aligned NWs are deflected with a conductive atomic force microscope tip in contact mode. The coupling of piezoelectric and semiconducting properties in zinc oxide creates a strain field and charge separation across the NW as a result of its bending. The rectifying characteristic of the Schottky barrier formed between the metal tip and the NW leads to electrical current generation. The efficiency of the NW-based piezoelectric power generator is estimated to be 17 to 30%. This approach has the potential of converting mechanical, vibrational, and/or hydraulic energy into electricity for powering nanodevices.
We have developed a nanowire nanogenerator that is driven by an ultrasonic wave to produce continuous direct-current output.
The nanogenerator was fabricated with vertically aligned zinc oxide nanowire arrays that were placed beneath a zigzag metal
electrode with a small gap. The wave drives the electrode up and down to bend and/or vibrate the nanowires. A piezoelectric-semiconducting
coupling process converts mechanical energy into electricity. The zigzag electrode acts as an array of parallel integrated
metal tips that simultaneously and continuously create, collect, and output electricity from all of the nanowires. The approach
presents an adaptable, mobile, and cost-effective technology for harvesting energy from the environment, and it offers a potential
solution for powering nanodevices and nanosystems.
The fracture strength of ZnO nanowires vertically grown on sapphire substrates was measured in tensile and bending experiments. Nanowires with diameters between 60 and 310 nm and a typical length of 2 um were manipulated with an atomic force microscopy tip mounted on a nanomanipulator inside a scanning electron microscope. The fracture strain of (7.7 +- 0.8)% measured in the bending test was found close to the theoretical limit of 10% and revealed a strength about twice as high as in the tensile test. From the tensile experiments the Young's modulus could be measured to be within 30% of that of bulk ZnO, contrary to the lower values found in literature.
The elasticity and piezoelectricity of zinc oxide (ZnO) crystals and single layers are investigated from the first-principles calculations. It is found that a ZnO thin film less than three Zn-O layers prefers a planar graphite-like structure to the wurtzite structure. ZnO single layers are much more flexible than graphite single layers in the elasticity and stronger than boron nitride single layers in the piezoelectricity. Single-walled ZnO nanotubes (SWZONTs) can exist in principle because of their negative binding energy. The piezoelectricity of SWZONTs depends on their chirality. For most ZnO nanotubes except the zigzag type, twists around the tube axis will induce axial polarizations. A possible scheme is proposed to achieve the SWZONTs from the solid-vapor phase process with carbon nanotubes as templates.
1. Introductory chapter.- 1.1. Conventional failure criteria.- 1.2. Characteristic brittle failures.- 1.3. Griffith's work.- 1.4. Fracture mechanics.- References.- 2. Linear elastic stress field in cracked bodies.- 2.1. Introduction.- 2.2. Crack deformation modes and basic concepts.- 2.3. Eigenfunction expansion method for a semi-infinite crack.- 2.4. Westergaard method.- 2.5. Singular stress and displacement fields.- 2.6. Method of complex potentials.- 2.7. Numerical methods.- 2.8. Experimental methods.- 2.9. Three-dimensional crack problems.- 2.10. Cracks in bending plates and shells.- References.- 3. Elastic-plastic stress field in cracked bodies.- 3.1. Introduction.- 3.2. Approximate determination of the crack-tip plastic zone.- 3.3. Small-scale yielding solution for antiplane mode.- 3.4. Complete solution for antiplane mode.- 3.5. Irwin's model.- 3.6. Dugdale's model.- 3.7. Singular solution for a work-hardening material.- 3.8. Numerical solutions.- References.- 4. Crack growth based on energy balance.- 4.1. Introduction.- 4.2. Energy balance during crack growth.- 4.3. Griffith theory.- 4.4. Graphical representation of the energy balance equation.- 4.5. Equivalence between strain energy release rate and stress intensity factor.- 4.6. Compliance.- 4.7. Critical stress intensity factor fracture criterion.- 4.8. Experimental determination of KIc.- 4.9. Crack stability.- 4.10. Crack growth resistance curve (R-curve) method.- 4.11. Mixed-mode crack propagation.- References.- 5. J-Integral and crack opening displacement fracture criteria.- 5.1. Introduction.- 5.2. Path-independent integrals.- 5.3. J-integral.- 5.4. Relationship between the J-integral and potential energy.- 5.5. J-integral fracture criterion.- 5.6. Experimental determination of the J-integral.- 5.7. Stable crack growth studied by the J-integral.- 5.8. Mixed-mode crack growth.- 5.9. Crack opening displacement (COD) fracture criterion.- References.- 6. Strain energy density failure criterion.- 6.1. Introduction.- 6.2. Volume strain energy density.- 6.3. Basic hypotheses.- 6.4. Two-dimensional linear elastic crack problems.- 6.5. Uniaxial extension of an inclined crack.- 6.6. Three-dimensional linear elastic crack problems.- 6.7. Bending of cracked plates.- 6.8. Ductile fracture.- 6.9. Failure initiation in bodies without pre-existing cracks.- 6.10. Other criteria based on energy density.- References.- 7. Dynamic fracture.- 7.1. Introduction.- 7.2. Mott's model.- 7.3. Stress field around a rapidly propagating crack.- 7.4. Strain energy release rate.- 7.5. Transient response of cracks to impact loads.- 7.6. Standing plane waves interacting with a crack.- 7.7. Crack branching.- 7.8. Crack arrest.- 7.9. Experimental determination of crack velocity and dynamic stress intensity factor.- References.- 8. Fatigue and environment-assisted fracture.- 8.1. Introduction.- 8.2. Fatigue crack propagation laws.- 8.3. Fatigue life calculations.- 8.4. Variable amplitude loading.- 8.5. Mixed-mode fatigue crack propagation.- 8.6. Nonlinear fatigue analysis based on the strain energy density theory.- 8.7. Environment-assisted fracture.- References.- 9. Engineering applications.- 9.1. Introduction.- 9.2. Fracture mechanics design philosophy.- 9.3. Design example problems.- 9.4. Fiber-reinforced composites.- 9.5. Concrete.- 9.6. Crack detection methods.- References.- Author Index.
The analysis of crack systems considered so far concerned only quasi-static situations in which the kinetic energy is relatively insignificant in comparison with the other energy terms and can be omitted. The crack was assumed either to be stationary or to grow in a controlled stable manner and the applied loads varied quite slowly. The present chapter is devoted entirely to dynamically loaded stationary or growing cracks. In such cases rapid motions are generated in the medium and inertia effects become of significant importance.
The mechanical resonance of a single ZnO nanobelt, induced by an alternative electric field, was studied by in situ transmission electron microscopy. Due to the rectangular cross section of the nanobelt, two fundamental resonance modes have been observed corresponding to two orthogonal transverse vibration directions, showing the versatile applications of nanobelts as nanocantilevers and nanoresonators. The bending modulus of the ZnO nanobelts was measured to be ∼52 GPa and the damping time constant of the resonance in a vacuum of 5×10-8 Torr was ∼1.2 ms and quality factor Q=500. © 2003 American Institute of Physics.
The interest in piezoelectric fiber composites with respect to the development of structurally integrated systems for fast actuation, vibration suppression, and acoustic control is founded on their adaptability to shape, stiffness, and actuation requirements. Layers of such material are laminated in order to form the walls of a single-cell closed cross-section beam, which is modeled in Timoshenko fashion with additional torsional warping. This allows to examine the interaction between active and load carrying functionalities and to analyze the influence of the diverse parameters from the micromechanics to the structural mechanics level. Active direct, extension coupled, and warping coupled torsion as well as combinations thereof are investigated and the abilities of these actuation mechanisms are compared.
A set of four tensors corresponding to Eshelby's tensor in elasticity are obtained for an ellipsoidal inclusion embedded in an infinite piezoelectric medium. These tensors, which describe the elastic, piezoelectric, and dielectric constraint of the matrix, are obtained from W. F. Deeg's solution to inclusion and inhomogeneity problems in piezoelectric solids. These tensors are then used as the backbone in the development of a micromechanics theory to predict the effective elastic, dielectric, and piezoelectric moduli of particle and fibre reinforced composite materials. The effects of interaction among inhomogeneities at finite concentrations are approximated through the Mori-Tanaka mean field approach. This approach, although widely utilized in the study of uncoupled elastic and dielectric behaviour, has not before been applied to the study of coupled behaviour. To help ensure confidence in the theory, the analytical predictions are proven to be self-consistent, diagonally symmetric, and to exhibit the correct behaviour in the low and high concentration limits. Finally, numerical results are presented to illustrate the effects of the concentration, shape, and material properties of the reinforcement on the effective properties of piezoelectric composites and analytical predictions are shown to result in good agreement with existing experimental data.
Cellulose nanofibers are known to possess aspect ratios larger than 200 and mechanical properties comparable to carbon nanotubes. Combined with other significant properties including low cost, low density, and biocompatibility, cellulose nanofibers are an attractive reinforcement material for nanocomposites. The load transfer between embedded fibers and matrix play a major role in designing nanocomposites with ultimate mechanical properties. In this work, we studied a general case where a simple axial loading exists along the axis of a cellulose fiber embedded in a polymer matrix. Then analytical relation between the applied load, the longitudinal stress along the fiber, and shear stresses along the interface of fiber and matrix was derived. It is shown that the maximum longitudinal stress occurs at the middle of the fiber, while maximum shear stress occurs at the extreme ends. Also, it is shown that the shear stress along the cellulose fibers can be approximated as a linear function of applied load. The derived relationships are useful for design of cellulose-based nanocomposites with enhanced mechanical properties.
Instability and buckling characterization of vertical well aligned single crystal of ZnO nanowires grown on SiC substrate was done quantitatively by nanoindentation technique. The critical load was found to be 477 μN and the corresponding buckling energy was 3.46×10−11 J. Based on the Euler model for long nanowire and Johnson model which is an extension of the Euler model for intermediate nanowire, the modulus of elasticity of single wire was calculated. Also, the critical buckling stress and strain were determined for the as grown single wire of ZnO. We found how the modulus of elasticity is dependent on the slenderness ratio.
Uniaxial tensile experiments were performed on single crystal zinc oxide nanowires with a custom microfabricated tool. The measured Young’s modulus is about 30%—40% of the bulk value for specimens with 200–400 nm in diameter, which cannot be explained with classical elasticity formulations. We discuss this anomaly in light of the enhanced electromechanical coupling due to static mechanical and isolated electrical boundary conditions that can significantly contribute to the softening of the material, irrespective of the length scale.
The piezoelectric constants of aluminum nitride and zinc oxide wurtzite-type nanowires have been calculated by first-principles approach and compared with the data obtained for bulk structures. The methods adopted here include the Hartree−Fock and density functional theory procedures in their periodic formulation. The piezoelectric response is seen to be higher in nanowires than in bulk. In zinc oxide wires, it is found that the piezoelectric constant is significantly enhanced by partial substitution of zinc by mercury.
Molecular dynamics simulations are performed to characterize the response of zinc oxide (ZnO) nanobelts to tensile loading. The ultimate tensile strength (UTS) and Young's modulus are obtained as functions of size and growth orientation. Nanobelts in three growth orientations are generated by assembling the unit wurtzite cell along the [0001], , and crystalline axes. Following the geometric construction, dynamic relaxation is carried out to yield free-standing nanobelts at 300 K. Two distinct configurations are observed in the [0001] and orientations. When the lateral dimensions are above 10 Å, nanobelts with rectangular cross-sections are seen. Below this critical size, tubular structures involving two concentric shells similar to double-walled carbon nanotubes are obtained. Quasi-static deformations of belts with and orientations consist of three stages, including initial elastic stretching, wurtzite-ZnO to graphitic-ZnO structural transformation, and cleavage fracture. On the other hand, [0001] belts do not undergo any structural transformation and fail through cleavage along (0001) planes. Calculations show that the UTS and Young's modulus of the belts are size dependent and are higher than the corresponding values for bulk ZnO. Specifically, as the lateral dimensions increase from 10 to 40 Å, decreases between 38–76% and 24–63% are observed for the UTS and Young's modulus, respectively. This effect is attributed to the size-dependent compressive stress induced by tensile surface stress in the nanobelts. and nanobelts with multi-walled tubular structures are seen to have higher values of elastic moduli (~340 GPa) and UTS (~36 GPa) compared to their wurtzite counterparts, echoing a similar trend in multi-walled carbon nanotubes.
The mechanical properties of individual zinc oxide (ZnO) nanowires, grown by a solid–vapour phase thermal sublimation process, were studied in situ by transmission electron microscopy (TEM) using a home-made TEM specimen holder. The mechanical resonance is electrically induced by applying an oscillating voltage, and in situ imaging has been achieved simultaneously. The results indicate that the elastic bending modulus of individual ZnO nanowires were measured to be ~58 GPa and the damping time constant of the resonance in a vacuum of 10−8 Torr was ~14 ms. A nanobalance was built and the mass of the nanoparticle attached at the tip of a nanowire was measured. The ZnO nanowires are promising in potential applications as nanocantilevers and nanoresonators.
An analysis is made of the effect of orientation of the fibres on the stiffness and strength of paper and other fibrous materials. It is shown that these effects may be represented completely by the first few coefficients of the distribution function for the fibres in respect of orientation, the first three Fourier coefficients for a planar matrix and the first fifteen spherical harmonics for a solid medium. For the planar case it is shown that all possible types of elastic behaviour may be represented by composition of four sets of parallel fibres in appropriate ratios. The means of transfer of load from fibre to fibre are considered and it is concluded that the effect of short fibres may be represented merely by use of a reduced value for their modulus of elasticity. The results of the analysis are applied to certain samples of resin bonded fibrous filled materials and moderately good agreement with experimental results is found.
Numerical unit cell models of 1-3 periodic composites made of piezoceramic unidirectional cylindrical fibers embedded in a soft non-piezoelectric matrix are developed. The unit cell is used for prediction of the effective coefficients of the periodic transversely isotropic piezoelectric cylindrical fiber composite. Special emphasis is placed on a formulation of the boundary conditions that allows the simulation of all modes of the overall deformation arising from any arbitrary combination of mechanical and electrical loading. The numerical approach is based on the finite element method and it allows extension to composites with arbitrary geometrical inclusion configurations, providing a powerful tool for fast calculation of their effective properties. For verification, the effective coefficients are evaluated for square and hexagonal arrangements of unidirectional piezoelectric cylindrical fiber composites. The results obtained from the numerical technique are compared with those obtained by means of the analytical asymptotic homogenization method for different volume fractions. Furthermore, the results are compared with other analytical and numerical methods reported in the literature.
Developing wireless nanodevices and nanosystems are of critical importance for sensing, medical science, defense technology, and even personal electronics. It is highly desirable for wireless devices and even required for implanted biomedical devices that they be self-powered without use of a battery. It is essential to explore innovative nanotechnologies for converting mechanical energy (such as body movement, muscle stretching), vibrational energy (such as acoustic or ultrasonic waves), and hydraulic energy (such as body fluid flow) into electrical energy, which will be used to power nanodevices without a battery. This is a key step towards self-powered nanosystems. We have demonstrated an innovative approach for converting mechanical energy into electrical energy by piezoelectric zinc oxide nanowire (NW) arrays. The operation mechanism of the electric generator relies on the unique coupling of the piezoelectric and semiconducting properties of ZnO as well as the gating effect of the Schottky barrier formed between the metal tip and the NW. Based on this mechanism, we have recently developed a DC nanogenerator (NG) driven by the ultrasonic wave in a biofluid and a textile-fiber-based NG for harvesting low-frequency mechanical energy. Furthermore, a new field, ''nanopiezotronics'', has been developed, which uses coupled piezoelectric-semiconducting properties for fabricating novel and unique electronic devices and components. This Feature Article gives a systematic description of the fundamental mechanism of the NG, its rationally innovative design for high output power, and the new electronics that can be built based on a piezoelectric-driven semiconducting process. A perspective will be given about the future impact of the technologies.
The basis of the harvesting mechanism based on a Schottky diode and the signal sources in the case of ZnO nanowire (NW) generators was reported. ZnO NWs arrays were grown on bare c-cut α-Al203 single crystal substrates as well as on GaN-buffered α-Al20 3 single crystal substrates using the vapor liquid solid (VSL) method. Vertically aligned epitaxial p-type doped Si nanowires were grown on heavily doped p-type silicon (111) wafers using the VSS method. The contacted Si and ZnO NW arrays were measured by scanning a conductive tip with a nominal elastic constant of 2.5 Nm and using an CP-Research (VEECO) AFM microscope in contact mode. The results demonstrated that the measured signals might have different sources than the piezoelectric effect, most of them being features of the measuring instruments and set-up, and the energy is rather harvested from the instruments than from the nanowires.
The shear yield strength and the shear strength of a resin matrix increase almost linearly as the logarithm of the strain rate increases. This increasing tendency is almost the same at various temperatures. The strain rate temperature superposition held and an experimental equation was found to estimate the strain rate and temperature dependence of these shear properties. The strain rate and temperature dependence of the shear yield strength at the fibre-matrix interphase can be also estimated by the same equation. A strong quantitative relation was observed between the strain rate and temperature dependence of the shear properties of a resin matrix and that of the shear yield strength at the fibre-matrix interphase.
Relation between the elastic modulus and the diameter (D) of ZnO nanowires was elucidated using a model with the calculated ZnO surface stresses as input. We predict for ZnO nanowires due to surface stress effect: (1) when D≫20 nm , the elastic modulus would be lower than the bulk modulus and decrease with the decreasing diameter, (2) when 20 nm ≫D≫2 nm , the nanowires with a longer length and a wurtzite crystal structure could be mechanically unstable, and (3) when D≪2 nm , the elastic modulus would be higher than that of the bulk value and increase with a decrease in nanowire diameter.
A shear-lag model is developed for carbon nanotube-reinforced polymer composites using a multiscale approach. The main morphological features of the nanocomposites are captured by utilizing a composite cylinder embedded with a capped nanotube as the representative volume element. The molecular structural mechanics is employed to determine the effective Young’s modulus of the capped carbon nanotube based on its atomistic structure. The capped nanotube is equivalently represented by an effective (solid) fiber having the same diameter and length but different Young’s modulus, which is determined from that of the nanotube under an isostrain condition. The shear-lag analysis is performed in the context of linear elasticity for axisymmetric problems, and the resulting formulas are derived in closed forms. To demonstrate applications of the newly developed model, parametric studies of sample cases are conducted. The numerical results reveal that the nanotube aspect ratio is a critical controlling parameter for nanotube-reinforced composites. The predictions by the current analytical model compare favorably with the existing computational and experimental data.
In this paper, we present a nanoelectromechanical oscillator with a single semiconducting zinc oxide nanowire (ZnO) doubly clamped and suspended on two metal electrodes by which the piezoelectric property on the growth of the ZnO nanowire along the c-axis, [0001], is characterized by the resonant frequency shift of the oscillator. We report that the resonance of the nanowire oscillator can be detected in ambient air and the effective piezoelectric coefficient on the growth of a ZnO nanowire along the c-axis, [0001], is significantly larger than that of bulk (0001) ZnO.
An experimental and computational approach is pursued to investigate the fracture mechanism of [0001] oriented zinc oxide nanowires under uniaxial tensile loading. A MEMS-based nanoscale material testing stage is used in situ a transmission electron microscope to perform tensile tests. Experiments revealed brittle fracture along (0001) cleavage plane at strains as high as 5%. The measured fracture strengths ranged from 3.33 to 9.53 GPa for 25 different nanowires with diameters varying from 20 to 512 nm. Molecular dynamic simulations, using the Buckingham potential, were used to examine failure mechanisms in nanowires with diameters up to 20 nm. Simulations revealed a stress-induced phase transformation from wurtzite phase to a body-centered tetragonal phase at approximately 6% strain, also reported earlier by Wang et al. (1) The transformation is partial in larger nanowires and the transformed nanowires fail in a brittle manner at strains as high as 17.5%. The differences between experiments and computations are discussed in the context of (i) surface defects observed in the ZnO nanowires, and (ii) instability in the loading mechanism at the initiation of transformation.
Molecular dynamics simulations of ZnO nanowires under tensile loading were performed and compared with simulations of TiO(2) wires to present size-dependent mechanical properties and super ductility of metal oxide wires. It is shown that while large surface-to-volume ratio is responsible for their size effects, ZnO and TiO(2) wires displayed opposite trends. Although the stiffness of both wires converged monotonically to their bulk stiffness values as diameter increases, bulk stiffness represented the upper bound for ZnO nanowires as opposed to the lower bound for TiO(2) wires. ZnO nanowires relaxed to either completely amorphous or completely crystalline states depending on wire thickness, whereas a thin amorphous shell is always present in TiO(2) nanowires. It was also found that when crystalline ZnO nanowires are stretched, necking initiated at localized amorphous regions to eventually form single-atom chains which can sustain strains above 100%. Such large elongations are not observed in TiO(2) nanowires. Using the analogy of a clothesline, an explanation is offered for the necessary conditions leading to super ductility.
The bending Young's modulus of ZnO nanobelts was measured by performing three-point bending tests directly on individual nanobelts with an atomic force microscope (AFM). The surface-to-volume ratio has no effect on the bending Young's modulus of the ZnO nanobelts for surface-to-volume ratios ranging from 0.017 to 0.035 nm(2) nm(-3), with a belt size of 50-140 nm in thickness and 270-700 nm in width. The bending Young's modulus was measured to be 38.2 +/- 1.8 GPa, which is about 20% higher than the nanoindentation Young's modulus of 31.1 +/- 1.3 GPa. The ZnO nanobelts exhibit brittle fracture failure in bending but some plastic deformation in indentation.
Using a two-end bonded ZnO piezoelectric-fine-wire (PFW) (nanowire, microwire) on a flexible polymer substrate, the strain-induced change in I-V transport characteristic from symmetric to diode-type has been observed. This phenomenon is attributed to the asymmetric change in Schottky-barrier heights at both source and drain electrodes as caused by the strain-induced piezoelectric potential-drop along the PFW, which have been quantified using the thermionic emission-diffusion theory. A new piezotronic switch device with an "on" and "off" ratio of approximately 120 has been demonstrated. This work demonstrates a novel approach for fabricating diodes and switches that rely on a strain governed piezoelectric-semiconductor coupling process.
Strain sensors based on individual ZnO piezoelectric fine-wires (PFWs; nanowires, microwires) have been fabricated by a simple, reliable, and cost-effective technique. The electromechanical sensor device consists of a single electrically connected PFW that is placed on the outer surface of a flexible polystyrene (PS) substrate and bonded at its two ends. The entire device is fully packaged by a polydimethylsiloxane (PDMS) thin layer. The PFW has Schottky contacts at its two ends but with distinctly different barrier heights. The I- V characteristic is highly sensitive to strain mainly due to the change in Schottky barrier height (SBH), which scales linear with strain. The change in SBH is suggested owing to the strain induced band structure change and piezoelectric effect. The experimental data can be well-described by the thermionic emission-diffusion model. A gauge factor of as high as 1250 has been demonstrated, which is 25% higher than the best gauge factor demonstrated for carbon nanotubes. The strain sensor developed here has applications in strain and stress measurements in cell biology, biomedical sciences, MEMS devices, structure monitoring, and more.
We demonstrate a mechanical-electrical trigger using a ZnO piezoelectric fine-wire (PFW) (microwire, nanowire). Once subjected to mechanical impact, a bent PFW creates a voltage drop across its width, with the tensile and compressive surfaces showing positive and negative voltages, respectively. The voltage and current created by the piezoelectric effect could trigger an external electronic system, thus, the impact force/pressure can be detected. The response time of the trigger/sensor is approximately 10 ms. The piezoelectric potential across the PFW has a lifetime of approximately 100 s, which is long enough for effectively "gating" the transport current along the wire; thus a piezoelectric field effect transistor is possible based on the piezotronic effect.
Room-temperature ultraviolet lasing in semiconductor nanowire arrays has been demonstrated. The self-organized, <0001> oriented
zinc oxide nanowires grown on sapphire substrates were synthesized with a simple vapor transport and condensation process.
These wide band-gap semiconductor nanowires form natural laser cavities with diameters varying from 20 to 150 nanometers and
lengths up to 10 micrometers. Under optical excitation, surface-emitting lasing action was observed at 385 nanometers, with
an emission linewidth less than 0.3 nanometer. The chemical flexibility and the one-dimensionality of the nanowires make them
ideal miniaturized laser light sources. These short-wavelength nanolasers could have myriad applications, including optical
computing, information storage, and microanalysis.
An atomic force microscopy (AFM) based technique is demonstrated for measuring the elastic modulus of individual nanowires/nanotubes aligned on a solid substrate without destructing or manipulating the sample. By simultaneously acquiring the topography and lateral force image of the aligned nanowires in the AFM contacting mode, the elastic modulus of the individual nanowires in the image has been derived. The measurement is based on quantifying the lateral force required to induce the maximal deflection of the nanowire where the AFM tip was scanning over the surface in contact mode. For the [0001] ZnO nanowires/nanorods grown on a sapphire surface with an average diameter of 45 nm, the elastic modulus is measured to be 29 +/- 8 GPa.