Brendan A. Patterson

Conformal BaTiO3 Films with High Piezoelectric Coupling through an Optimized Hydrothermal Synthesis

Two-dimensional (2D) ferroelectric films have vast applications due to their dielectric, ferroelectric, and piezoelectric properties that meet the requirements of sensors, nonvolatile ferroelectric random access memory (NVFeRAM) devices, and micro-electromechanical systems (MEMS). However, the small surface area of these 2D ferroelectric films has limited their ability to achieve higher memory storage density in NVFeRAM devices and more sensitive sensors and transducer. Thus, conformally deposited ferroelectric films have been actively studied for these applications in order to create three-dimensional (3D) structures, which lead to a larger surface area. Most of the current methods developed for the conformal deposition of ferroelectric films, such as metal-organic chemical vapor deposition (MOCVD) and plasma-enhanced vapor deposition (PECVD), are limited by high temperatures and unstable and toxic organic precursors. In this paper, an innovative fabrication method for barium titanate (BaTiO3) textured films with 3D architectures is introduced to alleviate these issues. This fabrication method is based on converting conformally grown rutile TiO2 nanowire arrays into BaTiO3 textured films using a simple two-step hydrothermal process which allows for thickness-controlled growth of conformal films on patterned silicon wafers coated with fluorine-doped tin oxide (FTO). Moreover, the processing parameters have been optimized to achieve a high piezoelectric coupling coefficient of 100 pm/V. This high piezoelectric response along with high relative dielectric constant (εr = 1600) of the conformally grown textured BaTiO3 films demonstrates their potential application in sensors, NVFeRAM, and MEMS.

Adhesive Force Measurement between HOPG and Zinc Oxide as an Indicator for Interfacial Bonding of Carbon Fiber Composites

Vertically aligned zinc oxide (ZnO) nanowires have recently been utilized as an interphase to increase the interfacial strength of carbon fiber composites. It was shown that the interaction between the carbon fiber and the ZnO nanowires was a critical parameter in adhesion; however, fiber based testing techniques are dominated by local defects and cannot be used to effectively study the bonding interaction directly. Here, the strength of the interface between ZnO and graphitic carbon is directly measured with atomic force microscopy (AFM) using oxygen plasma treated highly oriented pyrolytic graphite (HOPG) and an AFM tip coated with ZnO nanoparticles. X-ray photoelectron spectroscopy analysis is used to compare the surface chemistry of HOPG and carbon fiber and to quantify the presence of various oxygen functional groups. An indirect measurement of the interfacial strength is then performed through single fiber fragmentation testing (SFF) on functionalized carbon fibers coated with ZnO nanowires to validate the AFM measurements. The SFF and AFM methods showed the same correlation, demonstrating the capacity of the AFM method to study the interfacial properties in composite materials. Additionally, the chemical interactions between oxygen functional groups and the ionic structure of ZnO suggest that intermolecular forces at the interface are responsible for the strong interface.

Effect of zinc oxide nanowire length on interfacial strength of carbon fiber composites

Vertically aligned arrays of zinc oxide nanowires can serve as an adjustable interface for fiber composites through controllable synthesis techniques. When grown on carbon fiber surfaces as a fiber-matrix interphase of a composite, ZnO nanowires increase the surface area of interaction between fiber and matrix, and thus cause a greater interfacial shear strength of the composite. The ability to control the interfacial strength of this interphase through tailored morphologies enables the design of composite systems to specific applications. This report focuses on the controlled growth of ZnO nanowires and correlates the relationship between nanowire length and interfacial shear strength of the composite. Previous studies have focused on the effects of nanowire morphology on the interfacial strength; however, the data was limited to nanowire lengths < 1μm due to problems with nanowire uniformity and cleanliness [1]. Here, a new synthesis method is applied to the growth of zinc oxide nanowires on carbon fiber that enables the production of long, vertically aligned, uniform nanowires while maintaining the tensile properties of the fiber. The nanowires created by the new method are then compared to previous method nanowires by scanning electron microscopy imaging. Lastly, the interfacial shear strength of the fiber/polymer matrix composite is tested using single fiber fragmentation and correlated to the nanowire length of each method.Copyright

Modification of pullout behavior of kevlar fabric by zinc oxide nanowire reinforcement

The future development of body armor is to develop a lightweight, and wearable garment system without a loss of ballistic impact resistance. High performance fabrics, such as Kevlar, have been utilized for body armor due to their high energy absorption and lightweight characteristics. However, additional reinforcement is necessary for Kevlar fabric to meet the protection requirements for body armor against typical ballistic threats. Thick layers of fabric or embedded ceramic plates have been used to meet these requirements at the expense of increased weight of the armor and reduced mobility of the user. Thus, much research has been conducted on this topic to increase the ballistic impact resistance of Kevlar fabrics, mainly focused on the understanding and modeling of ballistic impact behavior. Due to the significant effect of damage mechanisms on ballistic impact response, these mechanisms should vastly be studied to better understand the ballistic impact response of Kevlar. When a projectile impacts a woven fabric, the imparted energy is dissipated through several damage mechanisms including tow pullout, local tow failure at the point of impact, and remote tow failure. Among those mechanisms, tow pullout is especially critical in the case of a penetrator with a blunt face impacting a fabric with non-penetrating velocities and is strongly influenced by friction between tows. In this work, we employed a novel method to increase the friction between Kevlar tows by synthesizing zinc oxide nanowires onto the fabric surface. As a result, vertically-aligned zinc oxide nanowires were grown on the fabric surface and tailored to achieve an optimum ballistic performance response reaching an enhancement of up to 390% in tow pullout peak load compared to untreated fabrics. Additionally, the effect of various surface functionalization processes and nanowire morphology is investigated so that an optimum process is developed for an efficient ballistic performance response.Copyright

Isolation of aramid nanofibers for high strength multiscale fiber reinforced composites

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.

Piezoelectric interfaces enabled energy harvesting and tailored damping in fiber composites

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.

Development of multifunctional fiber reinforced polymer composites through ZnO nanowire arrays

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.