Additive Manufacturing of High-Temperature Thermoplastic Polysulfone with Tailored Microstructure via Precipitation Printing.

Current additive manufacturing processes for polymers, including material extrusion, vat photopolymerization, material jetting, and powder bed fusion, have limitations in manufacturing high-temperature thermoplastics including narrow material selection, compromised mechanical properties, and potential degradation of materials during processing. Polysulfone (PSU) is a high-temperature thermoplastic with outstanding chemical resistance, flame retardancy, and toughness. However, besides injection molding, additive manufacturing of PSU has only been achieved through extrusion or solvent-cast three-dimensional (3D) printing without obtaining high mechanical properties. In this work, precipitation printing is applied to fabricate high-temperature thermoplastics such as PSU for the first time, where tailoring of the microstructure and mechanical properties is enabled through control of solvents and printing parameters. The printed PSU can either be dense and strong with 2.47 GPa Young's modulus and 70.6 MPa tensile strength or porous and highly anisotropic. After drying at a maximum temperature of 190 °C, both the printed dense and porous PSU samples have a glass-transition temperature of about 200 °C, which allows them to be used in high-temperature environments. Thus, precipitation printing provides an alternative approach to manufacture high-temperature thermoplastics like PSU with scalability and tailorable properties.

Highly Stretchable Printed Poly(vinylidene fluoride) Sensors through the Formation of a Continuous Elastomer Phase.

Stretchable piezoelectric stress/strain sensing materials have attracted substantial research interest in the fields of wearable health monitoring, motion capturing, and soft robotics. These sensors require operation under dynamic loading conditions with high strain range, changing strain/loading rates, and varying pre-stretch states, which are challenging conditions for existing sensors to produce reliable measurements. To overcome these challenges, an intrinsically stretchable poly(vinylidene fluoride) (PVDF) sensor is developed through the polymer blending of PVDF and acrylonitrile butadiene rubber (NBR). Through precipitation printing and vulcanization, the resulting PVDF/NBR blends exhibit strong ß phase PVDF and a blend morphology with submicron-level phase separation, but also strains up to 544%. Both the blend morphology and the mechanical properties indicate that this PVDF/NBR blend can be considered as a continuous elastomer phase above micron scale. After electric poling and adding electrodes, the PVDF/NBR blends have excellent piezoelectric properties to be used as both stretching mode strain sensors and compression mode stress/force sensors. The stretching mode sensors can measure strain up to 70% without strain rate and pre-stretch dependence, while the compression mode sensors have a loading-rate-independent linear voltage-stress relationship up to 4.8 MPa stress and a negligible pre-stretch dependence. Therefore, the PVDF/NBR sensors can provide accurate and reliable stress/strain measurements when attached to soft structures, which paves the way for sensing and calibration of soft robots under dynamic loading conditions.

Cellulose nanocrystal functionalized aramid nanofiber reinforced epoxy nanocomposites with high strength and toughness.

The mechanical properties of polymer nanocomposites can be improved by incorporating various types of nanofillers. The hybridization of nanofillers through covalent linkages between nanofillers with different dimensions and morphology can further increase the properties of nanocomposites. In this work, aramid nanofibers (ANFs) are modified using chlorinated cellulose nanocrystals (CNCs) and functionalized with 3-glycidoxypropyltrimethoxysilane to improve the chemical and mechanical interaction in an epoxy matrix. The integration of CNC functionalized ANFs (fACs) in the epoxy matrix simultaneously improves Young's modulus, tensile strength, fracture properties, and viscoelastic properties. The test results show that 1.5 wt% fAC reinforced epoxy nanocomposites improve Young's modulus and tensile strength by 15.1% and 10.1%, respectively, and also exhibit 2.5 times higher fracture toughness compared to the reference epoxy resin. Moreover, the glass transition temperature and storage modulus are found to increase when fACs are incorporated. Thus, this study demonstrates that the enhanced chemical and mechanical interaction by the CNC functionalization on the ANFs can further improve the static and dynamic mechanical properties of polymer nanocomposites.

Precipitation-Printed High-ß Phase Poly(vinylidene fluoride) for Energy Harvesting.

Poly(vinylidene fluoride) (PVDF) possesses outstanding piezoelectric properties, which allows it to be utilized as a functional material. Being a semicrystalline polymer, enhancing the piezoelectric properties of PVDF through the promotion of the polar ß phase is a key research focus. In this research, precipitation printing is demonstrated as a scalable and tailorable approach to additively manufacture complex and bulk 3D piezoelectric energy harvesters with high-ß phase PVDF. The ß-phase fraction of PVDF is improved to 60% through precipitation printing, yielding more than 200% improvement relative to solvent-cast PVDF films. Once the precipitation-printed PVDF is hot-pressed to reduce internal porosity, a significant ferroelectric response with a coercive field of 98 MV m-1 and a maximum remnant polarization of 3.2 µC cm-2 is observed. Moreover, the piezoelectric d33 and d31 coefficients of printed then hot-pressed PVDF are measured to be -6.42 and 1.95 pC N-1, respectively. For energy-harvesting applications, a stretching d31-mode energy harvester is demonstrated to produce a power density of up to 717 µW cm-3, while a printed full-scale heel insole with embedded d33-mode energy harvesting is capable of successfully storing 32.2 µJ into a capacitor when used for 3 min. Therefore, precipitation printing provides a new method for producing high-ß phase PVDF and bulk piezoelectric energy harvesters with the advantages of achieving geometry complexity, fabrication simplicity, and low cost.

Thermally Stable Poly(vinylidene fluoride) for High-Performance Printable Piezoelectric Devices.

Piezoelectric polymers, such as poly(vinylidene fluoride) (PVDF) and its copolymers, can achieve large strains and high work density under external electrical fields. These materials are highly desirable in the development of electronic devices and intelligent structures. Here, we demonstrate that dehydrofluorination (DHF) can provide a versatile chemical modification of the PVDF homopolymer that yields thermally stable ferroelectricity. The DHF process significantly increases the fraction of planar chain conformation in the PVDF and results in higher piezoelectric coupling with a wider processing temperature range, compared to traditionally processed PVDF. The efficacy of DHF in promoting planar chain conformation is demonstrated through molecular simulation and further proven by experimental characterization. The induced piezoelectric phases by DHF were able to be preserved through high temperature treatments up to 200 °C. The dehydrofluorinated PVDF exhibits improved electromechanical coupling with a high piezoelectric strain coefficient of d31 = 25.12 ± 1.13 pC/N, which can be further improved to 35.12 ± 0.69 pC/N by common mechanical drawing. This high piezoelectric voltage coefficient leads to an excellent actuation and energy harvesting behavior with a power density of 21.96 mW/cm3 in a flexible undrawn PVDF energy harvester, which is 3.13 times higher than conventionally drawn PVDF. The versatile and scalable method for preparing PVDF polymers with high piezoelectric coupling will enable new manufacturing processes not currently compatible with PVDF homopolymers.

Vibration Damping Mechanism of Fiber-Reinforced Composites with Integrated Piezoelectric Nanowires.

Here, both piezoelectric and nonpiezoelectric nanostructures are used within fiber-reinforced composites to improve the damping capabilities of the host material. This work investigates and isolates the role of both piezoelectricity and the mechanical redistribution of strain on the damping properties of fiber-reinforced composites through the integration of a nanowire interphase between the fiber and matrix. Prior works have successfully studied and reported the effectiveness of modifying the surface of the reinforcing fibers in a composite material using nanowires and other nanostructured interfaces to increase mechanical damping, however, have yet to fully investigate the mechanism dictating the observed behavior. This study analyzes the effects of nonpiezoelectric nanowire interfaces in comparison to piezoelectric nanowire interfaces of the same microscale morphology. The damping properties of carbon fiber-reinforced composites containing both sets of nanowires are investigated via dynamic mechanical analysis over a range of temperatures as well as modal analysis at the first resonant frequency. The results conclusively indicate that a combination of both mechanical and piezoelectric effects contributes to the significant increase in damping properties of fiber-reinforced composites and quantifies the individual contributions.

Printed Nanocomposite Energy Harvesters with Controlled Alignment of Barium Titanate Nanowires.

Piezoelectric nanocomposites are commonly used in the development of self-powered miniaturized electronic devices and sensors. Although the incorporation of one-dimensional (1D) piezoelectric nanomaterials (i.e., nanowires, nanorods, and nanofibers) in a polymer matrix has led to the development of devices with promising energy harvesting and sensing performance, they have not yet reached their ultimate performance due to the challenges in fabrication. Here, a direct-write additive manufacturing technique is utilized to facilitate the fabrication of spatially tailored piezoelectric nanocomposites. High aspect ratio barium titanate (BaTiO3) nanowires (NWs) are dispersed in a polylactic acid (PLA) solution to produce a printable piezoelectric solution. The BaTiO3 NWs are arranged in PLA along three different axes of alignment via shear-induced alignment during a controlled printing process. The result of electromechanical characterizations shows that the nanowire alignment significantly affects the energy harvesting performance of the nanocomposites. The optimal power output can be enhanced by as much as eight times for printed nanocomposites with a tailored architecture of the embedded nanostructures. This power generation capacity is 273% higher compared to conventional cast nanocomposites with randomly oriented NWs. The findings of this study suggest that 3D printing of nanowire-based nanocomposites is a feasible, scalable, and rapid methodology to produce high-performance piezoelectric transducers with tailored micro- and nanostructures. This study offers the first demonstration of nanocomposite energy harvesters with spatially controlled filler orientation realized directly from a digital design.

Isolation of Aramid Nanofibers for High Strength and Toughness Polymer Nanocomposites.

The development of nanoscale reinforcements that can be used to improve the mechanical properties of a polymer remains a challenge due to the long-standing difficulties with exfoliation and dispersion of existing materials. The dissimilar chemical nature of common nanofillers (e.g., carbon nanotubes, graphene) and polymeric matrix materials is the main reason for imperfect filler dispersion and, consequently, low mechanical performance of their composites relative to theoretical predictions. Here, aramid nanofibers that are intrinsically dispersible in many polymers are prepared from commercial aramid fibers (Kevlar) and isolated through a simple, scalable, and low-cost controlled dissolution method. Integration of the aramid nanofibers in an epoxy resin results in nanocomposites with simultaneously improved elastic modulus, strength, and fracture toughness. The improvement of these two mutually exclusive properties of nanocomposites is comparable to the enhancement of widely reported carbon nanotube reinforced nanocomposites but with a cost-effective and more feasible method to achieve uniform and stable dispersion. The results indicate the potential for aramid nanofibers as a new class of reinforcements for polymers.

Barium Titanate Film Interfaces for Hybrid Composite Energy Harvesters.

Energy harvesting utilizing piezoelectric materials has become an attractive approach for converting mechanical energy into electrical power for low-power electronics. Structural composites are ideally suited for energy scavenging due to the large amount of mechanical energy they are subjected to. Here, a multifunctional composite with embedded sensing and energy harvesting is developed by integrating an active interface into carbon fiber reinforced polymer composites. By modifying the composite matrix, both rigid and flexible multifunctional composites are fabricated. Through electromechanical testing of a cantilever beam of the rigid composite, it reveals a power density of 217 pW/cc from only 1 g root-mean-square acceleration when excited at its resonant frequency of 47 Hz. Electromechanical sensor testing of the flexible multifunctional composite reveals an average voltage generation of 23.5 mV/g at its resonant frequency of 96 Hz. This research introduces a route for integrating nonstructural functionality into structural fiber composites by utilizing BaTiO3 coated woven carbon fiber fabrics with power scavenging and passive sensing capabilities.

Enhanced Interfacial Strength and UV Shielding of Aramid Fiber Composites through ZnO Nanoparticle Sizing.

Here, a simple zinc oxide (ZnO) nanoparticle sizing is reported for aramid fibers that simultaneously provides interfacial reinforcement and UV absorption to develop improved fiber-reinforced composites. Through a one-step nanoparticle deposition, the modified aramid fiber showed an increase in interfacial shear strength of 18.9% with the addition of ZnO nanoparticles when tested by single-fiber pullout. The aramid fibers were then treated with a hydrolysis process common to aramid fibers to oxidize the surface and elucidate the importance of oxygen functional groups at the interface. These oxidized fibers proved to further enhance the interface between the fiber surface and nanoparticle, leading to a 33.3% increase relative to the bare fiber. Additionally, due to the absorption properties of ZnO, the retainment of mechanical properties of coated fibers was determined after exposure to an artificial UV light source. After 24 h of exposure, fibers coated with ZnO nanoparticles retained 25% more tensile strength and 21% more modulus than uncoated bare fibers. This work shows that ZnO nanoparticles may serve as a novel, yet simple, multifunctional fiber sizing with which to increase the interfacial strength of aramid fiber composites and improve the resistance to UV irradiation, enabling stronger and more-durable structural fiber composites.

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.

Lead-free 0.5Ba(Zr0.2Ti0.8)O3-0.5(Ba0.7Ca0.3)TiO3 nanowires for energy harvesting.

Lead-free piezoelectric nanowires (NWs) show strong potential in sensing and energy harvesting applications due to their flexibility and ability to convert mechanical energy to electric energy. Currently, most lead-free piezoelectric NWs are produced through low yield synthesis methods and result in low electromechanical coupling, which limit their efficiency as energy harvesters. In order to alleviate these issues, a scalable method is developed to synthesize perovskite type 0.5Ba(Zr0.2Ti0.8)O3-0.5(Ba0.7Ca0.3)TiO3 (BZT-BCT) NWs with high piezoelectric coupling coefficient. The piezoelectric coupling coefficient of the BZT-BCT NWs is measured by a refined piezoresponse force microscopy (PFM) testing method and shows the highest reported coupling coefficient for lead-free piezoelectric nanowires of 90 ± 5 pm V(-1). Flexible nanocomposites utilizing dispersed BZT-BCT NWs are fabricated to demonstrate an energy harvesting application with an open circuit voltage of up to 6.25 V and a power density of up to 2.25 µW cm(-3). The high electromechanical coupling coefficient and high power density demonstrated with these lead-free NWs produced via a scalable synthesis method shows the potential for high performance NW-based devices.

Growth of highly textured PbTiO3 films on conductive substrate under hydrothermal conditions.

Perovskite structure (ABO(3)) thin films have wide applications in electronic devices due to their unique properties, including high dielectric permittivity, ferroelectricity and piezoelectric coupling. Here, we report an approach to grow highly textured thick lead titanate (PbTiO(3)) filmson conductive substrates by a two-step hydrothermal reaction. Initially, vertically aligned TiO(2) nanowire arrays are grown on fluorine-doped tin oxide (FTO) coated glass, which act as template crystals for conversion to the perovskite structure. The PbTiO(3) films are then converted from TiO(2) NW arrays by diffusing Pb(2+) ions into the template through a second hydrothermal reaction. The dielectric permittivity and piezoelectric coupling coefficient (d(33)) of the PbTiO(3) films are as high as 795 at 1 kHz and 52 pm V−1, respectively. The reported process can also potentially be expanded for the assembly of other complex perovskite ATiO(3) (A = Ba, Ca, Cd,etc) films by using the highly aligned TiO(2) NW arrays as templates. Therefore, the approach introduced here opens up a new door to synthesize ferroelectric thin films on conductivesubstrates for application in sensors, actuators, and ultrasonic transducers that are important in various industrial and scientific areas.

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.

Morphology-controlled ZnO nanowire arrays for tailored hybrid composites with high damping.

Hybrid fiber reinforced composites using a nanoscale reinforcement of the interface have not reached their optimal performance in practical applications due to their complex design and the challenging assembly of their multiscale components. One promising approach to the fabrication of hybrid composites is the growth of zinc oxide (ZnO) nanowire arrays on the surface of carbon fibers to provide improved interfacial strength and out of plane reinforcement. However, this approach has been demonstrated mainly on fibers and thus still requires complex processing conditions. Here we demonstrate a simple approach to the fabrication of such composites through the growth of the nanowires on the fabric. The fabricated composites with nanostructured graded interphase not only exhibit remarkable damping enhancement but also stiffness improvement. It is demonstrated that these two extremely important properties of the composite can be controlled by tuning the morphology of the ZnO nanowires at the interface. Higher damping and flexural rigidity of these composites over traditional ones offer practical high-performance composites.

Scalable synthesis of morphotropic phase boundary lead zirconium titanate nanowires for energy harvesting.

Lead zirconium titanate (PZT) nanowires are synthesized using a scalable two-step hydrothermal reaction. The piezo-electric coupling coefficient of the PZT NWs shows the highest value for PZT nano-wires in the literature (80 ± 5 pm/V). A PZT-NW-based nanocomposite is fabri-cated to demonstrate an energy-harvesting application with an open-circuit voltage up to 7 V and a power density up to 2.4 µW/cm(3) .

Controlled synthesis of ultra-long vertically aligned BaTiO3 nanowire arrays for sensing and energy harvesting applications.

A novel approach for the synthesis of ultra-long (up to ∼45 µm) vertically aligned barium titanate (BaTiO3) nanowire (NW) arrays on an oxidized Ti substrate is developed. The fabrication method uses a two-step hydrothermal reaction that firstly, involves the growth of ultra-long aligned sodium titanate NW arrays and secondly, involves the transfer of these precursor sodium titanate NW arrays to BaTiO3 NW arrays while retaining the shape of the template nanowires. The ion-exchange during the second hydrothermal reaction in barium hydroxide solution results in the structural transformation from single-crystal sodium titanate NW arrays to BaTiO3 NW arrays. This synthesis approach is low-cost, scalable, and enables control over the morphology and aspect ratio of the resulting BaTiO3 NW arrays by tuning the hydrothermal reaction parameters. In addition to the synthesis methods reported here, the energy harvesting behavior of the BaTiO3 NW arrays is evaluated as a function of their aspect ratio and demonstrated to produce significant impact on the energy produced. The newly developed hydrothermal synthesis process for controlled growth of ultra-long, vertically aligned BaTiO3 NW arrays provides a promising method for their efficient utilization in nano-electromechanical system-based sensors, energy harvesters, and nano-scale electronic devices.

Relationship between BaTiO3 nanowire aspect ratio and the dielectric permittivity of nanocomposites.

The aspect ratio of barium titanate (BaTiO3) nanowires is demonstrated to be successfully controlled by adjusting the temperature of the hydrothermal growth from 150 to 240 °C, corresponding to aspect ratios from 9.3 to 45.8, respectively. Polyvinylidene fluoride (PVDF) nanocomposites are formed from the various aspect ratio nanowires and the relationship between the dielectric constant of the nanocomposite and the aspect ratio of the fillers is quantified. It was found that the dielectric constant of the nanocomposite increases with the aspect ratio of the nanowires. Nanocomposites with 30 vol % BaTiO3 nanowires and an aspect ratio of 45.8 can reach a dielectric constant of 44.3, which is 30.7% higher than samples with an aspect ratio of 9.3 and 352% larger than the polymer matrix. These results demonstrate that using high-aspect-ratio nanowires is an effective way to control and improve the dielectric performance of nanocomposites for future capacitor applications.

High-sensitivity accelerometer composed of ultra-long vertically aligned barium titanate nanowire arrays.

A configuration that shows great promise in sensing applications is vertically aligned piezoelectric nanowire arrays that allow facile interfacing with electrical interconnects. Nano-electromechanical systems developed using piezoelectric nanowires have gained interest primarily for their potential in energy harvesting applications, because they are able to convert several different sources of mechanical energy into useful electrical power. To date, no results have demonstrated the capability to use aligned piezoelectric nanowire arrays as a highly accurate nano-electromechanical system based dynamic sensor with a wide operating bandwidth and unity coherence. Here we report the growth of vertically aligned (~45 µm long) barium titanate nanowire arrays, realized through a two-step hydrothermal synthesis approach, and demonstrate their use as an accurate accelerometer. High sensitivity of up to 50 mV g(-1) is observed from the sensor composed of vertically aligned barium titanate nanowire arrays, thus providing performance comparable to many of the commercial accelerometer systems.

Vertically aligned arrays of BaTiO(3) nanowires.

Barium titanate (BaTiO3) nanowires have gained considerable research interest due to their lead-free composition and strong energy conversion efficiency. However, most research has focused on free-standing BaTiO3 nanowires, which are hard to apply for sensing and energy harvesting. Here, a novel method for the growth of vertically aligned BaTiO3 nanowire arrays on a conductive substrate is developed, and their electromechanical coupling behavior is directly evaluated to yield the strain coupling coefficient. The preparation of vertically aligned BaTiO3 nanowire arrays is based on a two-step hydrothermal reaction by first growing oriented rutile TiO2 nanowire arrays and then converting them to BaTiO3 while simultaneously retaining their morphology. A refined piezoelectric force microscopy (PFM) testing method is applied to demonstrate the piezoelectric behavior of BaTiO3 nanowires in the longitude direction. The piezoelectric response (d33 = 43 ± 2 pm/V) of the BaTiO3 nanowires is measured to demonstrate their potential application in sensors, energy harvesting, and micro-electromechanical systems.