Aramid fibers are famous for their high specific strength and energy absorption properties and have been intensively used for soft body armor and ballistic protection. However, the use of aramid fiber reinforced composites is barely observed in structural applications. Aramid fibers have smooth and inert surfaces that are unable to form robust adhesion to polymeric matrices due to their high crystallinity. Here, a novel method to effectively integrate aramid fibers into composites is developed through utilization of aramid nanofibers. Aramid nanofibers are prepared from macroscale aramid fibers (such as Kevlar®) and isolated through a simple and scalable dissolution method. Prepared aramid nanofibers are dispersible in many polymers due to their improved surface reactivity, meanwhile preserve the conjugated structure and likely the strength of their macroscale counterparts. Simultaneously improved elastic modulus, strength and fracture toughness are observed in aramid nanofiber reinforced epoxy nanocomposites. When integrated in continuous fiber reinforced composites, aramid nanofibers can also enhance interfacial properties by forming hydrogen bonds and π-π coordination to bridge matrix and macroscale fibers. Such multiscale reinforcement by aramid nanofibers and continuous fibers results in strong polymeric composites with robust mechanical properties that are necessary and long desired for structural applications.
Fiber reinforced polymer composites are becoming ubiquitous in modern structures, due to their light weight, high specific strength, and ability to be tailored for a specific application. The increase in the commercial adoption and feasible applications of composite materials has motivated researchers to develop the next generation of composites. These next generation composites aim to integrate more structural and nonstructural properties into the structure with the goal of increasing the efficiency of the system as a whole. There have been many efforts in modifying or replacing structural fiber and matrix phases with active materials. However, this methodology usually affects the structural properties of the composite and limits their practical applications. Here, we present a new approach for the development of multifunctional fiber reinforced polymer composites. In this method, piezoelectric nanostructures (ZnO nanowires and barium titanate textured films) are integrated at the interface between structural fibers and matrix phase. Since the load transfer between reinforcement phase and polymer matrix happens at the interfacial region, the active phase at the interface results in a composite with unique properties. In this study we examined the vibration damping and energy harvesting of the fabricated composites. The nanostructured interface showed a great potential as a damping mechanism and energy harvesting constituent in these composites. The large amount of stress concentration in this region resulted in increased damping properties and sustainable energy harvesting performance. This research introduces a route for integrating responsive properties into structural composites by utilizing functional nanostructured interfaces.
Recently, the reinforcement-matrix interface of fiber reinforced polymers has been modified through grafting nanostructures – particularly carbon nanotubes and ZnO nanowires – on to the fiber surface. This type of interface engineering has made a great impact on the development of multiscale composites that have high stiffness, interfacial strength, toughness, and vibrational damping – qualities that are mutually exclusive to a degree in most raw materials. Although the efficacy of such nanostructured interfaces has been established, the reinforcement mechanisms of these multiscale composites have not been explored. Here, strain transfer across a nanowire interphase is studied in order to gain a heightened understanding of the working principles of physical interface modification and the formation of a functional gradient. This problem is studied using a functionally graded piezoelectric interface composed of vertically aligned lead zirconate titanate nanowires, as their piezoelectric properties can be utilized to precisely control the strain on one side of the interface. The displacement and strain across the nanowire interface is captured using digital image correlation. It is demonstrated that the material gradient created through nanowires cause a smooth strain transfer from reinforcement phase into matrix phase that eliminates the stress concentration between these phases, which have highly mismatched elasticity.
Piezoelectric nanowires, in particular zinc oxide (ZnO) nanowires, have been vastly used in the fabrication of electromechanical devices to convert wasted mechanical energy into useful electrical energy. Over recent years, the growth of vertically aligned ZnO nanowires on various structural fibers has led to the development of fiber-based nanostructured energy harvesting devices. However, the development of more realistic energy harvesters that are capable of continuous power generation requires a sufficient mechanical strength to withstand typical structural loading conditions. Yet, a durable, multifunctional material system has not been developed thoroughly enough to generate electrical power without deteriorating the mechanical performance. Here, a hybrid composite energy harvester is fabricated in a hierarchical design that provides both efficient power generating capabilities while enhancing the structural properties of the fiber reinforced polymer composite. Through a simple and low-cost process, a modified aramid fabric with vertically aligned ZnO nanowires grown on the fiber surface is embedded between woven carbon fabrics, which serve as the structural reinforcement as well as the top and the bottom electrodes of the nanowire arrays. The performance of the developed multifunctional composite is characterized through direct vibration excitation and tensile strength examination.
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.
One-dimensional nanostructures such as nanowires, nanorods, and nanotubes with piezoelectric properties have gained interest in the fabrication of small scale power harvesting systems. However, the practical applications of the nanoscale materials in structures with true mechanical strengths have not yet been demonstrated. In this paper, piezoelectric ZnO nanowires are integrated into the fiber reinforced polymer composites serving as an active phase to convert the induced strain energy from ambient vibration into electrical energy. Arrays of ZnO nanowires are grown vertically aligned on aramid fibers through a low-cost hydrothermal process. The modified fabrics with ZnO nanowires whiskers are then placed between two carbon fabrics as the top and the bottom electrodes. Finally, vacuum resin transfer molding technique is utilized to fabricate these multiscale composites. The fabricated composites are subjected to a base excitation using a shaker to generate charge due to the direct piezoelectric effect of ZnO nanowires. Measuring the generated potential difference between the two electrodes showed the energy harvesting application of these multiscale composites in addition to their superior mechanical properties. These results propose a new generation of power harvesting systems with enhanced mechanical properties.
Lead zirconate titanate (PZT) microwires with applications in sensors, actuators, and energy harvesters are produced using hydrothermal synthesis. The synthesized microwires are relatively large with an average length of about 450 microns and an average width of 4 microns. Each of these individual PZT microwires can be integrated in smart systems as an active phase or be used as an independent smart material. In this paper, the synthesis procedure and characterization of these large microwires is demonstrated. The converse piezoelectric properties of the microwires are measured using digital image correlation after clamping and adding electrodes at each end of the microwire. It has been shown in the literature that digital image correlation can be used as a precise tool for rapid characterization of piezoelectric materials. Here, it is demonstrated that this technique can be applied to characterize the actual response of piezoelectric materials at the micron scale.
Piezoelectric materials due to their high electromechanical coupling properties are good candidates for energy harvesting applications by transforming mechanical energy to electrical power. The piezoelectric coupling coefficient of each material is dependent on its operating mode and higher coupling coefficient means higher efficiency in energy harvesting. In most of the piezoelectric materials, the d15 piezoelectric strain coefficient is the highest coefficient compared to the d33 and d31 coefficients. However complicated fabrication and evaluation of energy harvesting devices operating in the shear mode has slow down the research in this area. The shear piezoelectric effect can be induced during the steady state response of a thick cantilever composite beam due to the effect of shear force through the thickness. Here, a model based on the Timoshenko beam theory is developed to estimate the electric power output in a cantilever beam with a piezoelectric core subjected to the base excitation. The governing electromechanical equations as well as the output voltage and power frequency responses are derived for the piezoelectric sandwich beam. This model is applicable to different geometries and piezoelectric compositions in order to design an optimal shear energy harvester. At the end, the performance of this type of shear energy harvesters is compared to the typical cantilever bimorph energy harvesting beams with the same piezoelectric volume.
Here, Digital image correlation (DIC) is demonstrated to be an accurate tool for the noncontact, non-destructive and rapid characterization of the converse piezoelectric effect in bulk and thin films. The out-of-plane (d33) and in-plane (d31) piezoelectric strain coupling coefficients of PZT- 5H wafers are measured simultaneously by imaging the wafer’s cross section under free mechanical boundary conditions. The large piezoresponse at switching domains and nonlinear behavior of PZT-5H are visualized in strain-electric field butterfly loops. The results show DIC as a simple advantageous technique to use for the characterization of piezoelectric materials under the influence of any field and physical constraints.