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Thick electrode with high-areal-capacity is a practical and promising strategy to increase the energy density of batteries, but development toward thick electrode is limited by the electrochemical performance, mechanical properties, and manufacturing approaches. In this work, we overcome these limitations and report an ultrathick electrode structure, called fiber-aligned thick or FAT electrode, which offers a novel electrode design and a scalable manufacturing strategy for high-areal-capacity battery electrodes. The FAT electrode uses aligned carbon fibers to construct a through-thickness fiber-aligned electrode structure with features of high electrode material loading, low tortuosity, high electrical and thermal conductivity, and good compression property. The low tortuosity of FAT electrode enables fast electrolyte infusion and rapid electron/ion transport, exhibiting a higher capacity retention and lower charge transfer resistance than conventional slurry-casted thick electrode design.

Additive manufacturing (AM) of lightweight and energy-efficient composites using continuous carbon fibers and thermosetting polymers offers great opportunities for advancing composite manufacturing with design flexibility, low cost, reliability and repeatability, with potential use in a wide range of applications. However, to date there has been no AM technique reported to process continuous carbon fibers and thermosetting polymers for direct 3D printing. The temperature-dependent viscosity of thermosetting polymers suffers a significant decrease before the composite suddenly turns into a solid, making it difficult to infuse the polymer into the fiber structure and quickly cure the composite into a solid while retaining the desirable pattern during the additive manufacturing process. Here, we overcome these difficulties and report a dynamic capillary-driven AM approach, called localized in-plane thermal assisted (LITA) 3D printing, with a controllable viscosity and degree of curing of polymer to enable its fast and near-simultaneous infusion and curing to implement in situ solidification of composites into arbitrary shapes. Using the LITA technique, we developed a robotic system consisting of a uniquely designed printing head and an automated robot arm, yielding a 3D printer that enables us to print a composite on 2D and 3D substrates or in free space. The printed composite had high fiber volume fraction (60%) and degree of curing (95%) with high mechanical strength (810 MPa) and modulus (108 GPa).

The need for developing energy storage devices with high energy and power densities has motivated tremendous effort in the research of supercapacitors. Major challenges still exist in improving the working voltage window and specific capacitance of the devices. Here, we report a systematic effort in achieving remarkable supercapacitor performance. Firstly, a flexible CNT film/double side carbon tape/CNT film@MnO2 (CDC@MnO2) positive electrode with a large voltage window of 0–1.2 V was fabricated via a facial one-step water bath process. The large interfacial area created by the CDC sandwich structure enabled a high MnO2 mass loading up to 6.6 mg cm−2, resulting in a high area capacitance of 1.1 F cm−2 at a current density of 1 mA cm−2. Secondly, inspired by the superior electrochemical performance of the CDC@MnO2 positive electrode, a CDC@Fe2O3 film with area capacitance up to 339.1 mF cm−2 was fabricated as the negative electrode. Thirdly, a flexible asymmetric supercapacitor was successfully assembled which possesses a large voltage window of 0–2.1 V, a high volume capacitance of 6.8 F cm−3, and a high energy density of 4.1 mWh cm−3 at the power density of 22.3 mW cm−3.

Stretchable conductive fibers have attracted significant attention due to their ability to be directly woven into or stitched onto fabrics, making them ideal for use in the design of integrated wearable strain sensors. Here, we report on a highly stretchable multi-walled carbon nanotube (MWCNT)/Thermoplastic Polyurethane (TPU) fiber produced via a wet spinning process. The effects of MWCNT content and alignment on the structural, mechanical, electrical and strain-sensing properties of the composite fibers were investigated. The highest conductivity (6.77 S cm−1), tensile strength (28 MPa) and maximum elongation at break (565%) were obtained by controlling the MWCNT content. Gauge factor (GF) values were also affected by the content and MWCNT alignment in the composite fibers, as these parameters determine the change in the effective contact area and number of conductive paths available during stretching. The well-aligned MWCNT/TPU fiber showed a high GF value of 5200. Wearable strain sensors capable of obtaining real-time mechanical feedback for various human motion detections with different GFs and working strain ranges could be realized by controlling the MWCNT concentrations in the TPU matrix.

This paper presents an investigation of the potential to use aligned discontinuous carbon fiber dry prepregs as electrodes in structural supercapacitors (SSCs). The high fiber-matrix interfacial bonding of the structural composite was achieved by adopting a solid polymer electrolyte, consisting of poly(vinylidene), lithium triflate, and epoxy. Processing of the SSC was carried out via dip-coating of the polymer electrolyte and then cured using a vacuum bag. The electrochemical performance of the SSCs was measured before and after mechanical loading. The microstructures of the SSCs as-fabricated and damaged under flexural loading were identified by μ-CT imaging. An SSC with a specific capacitance of 0.128 mF/cm2 (11.62 mF/g), a flexural strength of 47.49 MPa, and a flexural modulus of 8.48 GPa has been achieved, demonstrating significant improvements in mechanical properties over those of SSCs based on woven carbon fiber fabric-based electrodes. The mechanical behavior of the supercapacitors was evaluated by both quasi-static and cyclic flexural loading tests. The excellent electrochemical stability of the supercapacitors was validated by a capacitance retention of above 96% under galvanostatic charge-discharge cycling tests. The knowledge gained in this work will benefit future research in the optimization of SSC performance.

Since they have the ability to act as both a sensor and an actuator, piezoelectric materials have been described as a ‘natural’ smart material. With the development of piezoceramic fibers and fabrics, functional composites consisting of these fibers within a polymeric matrix will allow designers to construct electromechanical devices having unique or superior characteristics. An essential need of these designers is the ability to predict the piezoelastic response of such composites. Recently, an analytical model has been developed to predict the effective piezoelastic properties of woven, braided and unidirectional piezoceramic fiber/polymer matrix composites. Piezoceramic fiber composites offer the possibility of achieving property gradients within an electromechanical device by varying the volume fraction of fibers throughout the matrix. This may prove helpful in solving some problems seen in the development of very large displacement actuators. Existing designs utilize the longitudinal strain of a piezoelectric element by coupling it to an inactive layer (unimorph) or a piezoelectric layer straining in the opposite direction (bimorph), which results in bending. While typical bender actuators utilize piezoelectric elements having a basic structural shape, such as disk, rod or plate, advances in ceramics processing techniques, such as the Fused Deposition of Ceramics (FDC) have allowed the production of piezoceramic elements of novel geometries. Current research has focused on utilizing these novel geometries to maximize actuator displacement and results show very large displacements are possible when using spiral shaped actuators. Additional studies indicate that spiral actuators using a bimorph configuration have tremendous potential. The performance of these layered or laminated actuators is dependent on the adhesive bond between the layers. Deterioration of the bond results in a rapid decrease in the actuator properties. It is thought that the ability to process property gradients within a device, deleterious stress concentrations between layers can be minimized.

Four-dimensionally (4D) printed origami structures have great potential for applications in actuators and reconfigurable devices by taking the advantages of 3D printing technology and shape memory polymers. This study focuses on the shape recovery progression of 4D printed laminated Miura-origami tessellations and tubes under compressive load-induced unfolding and folding. Recovery forces of the specimens are characterized by dynamic mechanical analysis (DMA) experiments. The shape recovery behavior and recovery force are significantly influenced by the shape recovery temperature and loading pattern. The high shape recovery capability of the specimens are signified by the shape recovery ratio of over 94% and volume changes of up to 289%. Lastly, the actuator application of a 4D printed laminated Miura-origami structure has been demonstrated.

Wire-shaped supercapacitors (WSC) have attracted tremendous attention for powering portable electronic devices. However, previously reported WSC suffered from a complicated fabrication process and high cost. The objective of this study is to develop a facile and scalable process for the fabrication of high energy density WSC. We coupled the wet-spinning assembly with an in situ electrodeposition technique to prepare carbon nanotube (CNT)-based composite fibers. The charge balance between the electrodes was realized by controlling the deposition time of the pseudocapacitive materials. A wire-shaped asymmetric supercapacitor (WASC) was fabricated by twisting MnO2/CNT fiber cathode and PPy/CNT fiber anode with LiCl/PVA electrolyte. The flexible MnO2/CNT//PPy/CNT WASC operated in a broadened voltage range of 0–1.8 V exhibited a high capacitance of 17.5F cm−3 (10.7F g−1). In addition, it delivered a maximum energy and power densities of 7.88 mWh cm−3 (4.82 Wh kg−1) and 2.26 W cm−3 (1382 W kg−1), respectively. The WASC device demonstrated satisfactory cycling stability with 86% capacitance retention, and its Coulombic efficiency remained at 96% after 5000 charge–discharge cycles. The contributions of the diffusion-controlled insertion and the surface capacitive effect were theoretically quantified to investigate the energy storage mechanism. The fabrication approaches hold potential for the construction of cost-effective and high-performance WSC.

High reflection loss, wide bandwidth and low density are desirable attributes of electromagnetic wave absorption materials. Here, an ultra-thin MnO2/Fe3O4/carbon nanotube film has been synthesized in two steps using solvothermal deposition and electrochemical deposition. The Fe3O4 nanoparticles and the MnO2 sheets form a nano-scale core-shell structure on the surface of the carbon nanotube film. A series of this hierarchical composite film with different shell structures and MnO2 weight ratios were successfully synthesized. Their unique micro-structure greatly enhances the electromagnetic wave absorption capability of the composites. Instrinsic ferromagnetism is confirmed by the magnetic hysteresis loops. The strong couplings among dielectric loss, hysteresis loss and multiple scattering resulted in the reflection loss up to −42.2 dB with the bandwidth of 7.1 GHz at 10% MnO2 weight ratio for a flexible composite film of 1 mm thick.

More recently, three dimensional printing (3D Printing), also known as an additive manufacturing (AM), has been highlighted since it shows a great promise to realize almost any three dimensional parts or structures with computer aided design (CAD). Several different processes are available for 3D printing, which includes fused deposition modeling, selective laser sintering, stereolithography, photopolymerization, and etc. In particular, considerable attention is paid to the 3D printing technique with photopolymerization due to their high resolutions. Unfortunately, the 3D printed products with photopolymerization however possess poor mechanical properties. Understanding of this should be necessary for the advantages of the 3D printing to be fully realized. Here, this study experimentally investigates the mechanical properties of the 3D printed photopolymer through thermomechanical analysis and tensile testing. In this study, it is found that the printed specimens are not fully cured after the 3D printing with photopolymerization. DiBenedetto equation is employed to better understand the relationship between the curing status and tensile properties. In addition to the poor mechanical properties, anisotropic and size dependent tensile properties of the 3D printed photopolymers are also observed. Electron beam treatment is used to ensure the cure of the 3D printed photopolymer and the corresponding tensile properties are characterized and investigated.

Four-dimensional (4D) printing of multi-directionally reinforced preforms has tremendous potential for the development of next generation functional composites by using the capabilities of 3D printing technology, 3D textile preform design, and polymer shape memory behavior. This work demonstrates the shape memory behavior and recovery force of 4D printed circular braided tube preforms and their silicone elastomer matrix composites. The preforms were printed by fused deposition modeling using the shape memory polymer (SMP), polylactic acid (PLA). The effects of braiding angle, tube wall thickness, and shape recovery temperature on the shape memory behavior of 4D printed tube preforms and their silicone elastomer matrix composites have been characterized. Measurements of shape recovery forces of the preform and composite were conducted using dynamic mechanical analysis (DMA). The braided microstructural parameters and shape recovery temperature have a significant effect on the preform shape memory behavior. The introduction of the silicone elastomer matrix greatly enhances the shape recovery force, shape recovery ratio, as well as radial compressive failure load of the 4D printed preform/silicone elastomer matrix composite. Building on these results, a potential application for 4D printed textile functional composites is presented.

Easy shapeable highly porous and robust three dimensional (3D) nano-carbon architectures (3D NCA) are crucial for the practical applications of electrochemical energy storage devices. Here, a facile easy shapeable nitrogen-doped graphene coated 3D NCA exhibiting an ultra-high specific surface area, remarkable robustness, and excellent aqueous wettability is reported. A 3D single-walled carbon nanotube (SWCNT) hydrogel composed of isolated SWCNTs is first prepared, and then a thin polydopamine (pDA) layer is uniformly coated onto the fabricated 3D SWCNT hydrogel via an in situ polymerization of dopamine. A nitrogen-doped graphene-coated 3D NCA is obtained via pyrolysis of the pDA-coated 3D NCA. By decorating this highly porous nitrogen-doped 3D NCA onto helical micro carbon fibers, a highly stretchable (∼100% strain) wire-type supercapacitor (WTSC) is fabricated. The areal specific power and energy density of the WTSC are determined to be 2.59 mW cm−2 and 1.1 μWh cm−2, respectively. These values are remarkably larger than those previously reported WTSCs. Moreover, our WTSC maintains more than 91% of its capacitance after 10,000 stretch-release cycles at tensile strains of up to 50%. The combination of the easy shapeable, robust and highly porous nitrogen-doped 3D NCA paves a new way for the development of high-performance wearable textile-based energy devices.

This work aims to characterize the interfacial bonding strength between printed wires of acrylonitrile-butadiene-styrene (ABS), carbon nanotube reinforced ABS (CNTABS) and short carbon fiber reinforced ABS (CFABS) specimens fabricated by fused deposition modeling. The in-plane tensile shear test and double notch shear test methods using ±45° specimens were used. The in-plane tensile shear strength of CFABS specimen is close to that of CNTABS specimen, and both of them are greater than that of ABS specimen. Double notch shear strengths are smaller than in-plane tensile shear strength of CFABS specimens at all three printing speeds studied. Also, the shear strengths of CFABS specimens decrease with increasing printing speed and layer thickness. X-ray micro-computed tomography examination concluded that CFABS specimens have the largest porosity compared with ABS and CNTABS specimens at corresponding raster orientation, especially for ±45° specimens. Matrix fracture and fiber pull-out, as well as fiber-matrix interfacial debonding are the dominating failure modes of CFABS specimen.

Among the processing parameters of additive manufacturing, printing direction is of critical importance. While studies on effects of printing direction have so far mainly focused on mechanical properties of solid specimens, the present research is intended to demonstrate printing direction dependence of mechanical behavior of additively manufactured 3D preforms and their composites. Compressive behavior of additively manufactured 3D braid preforms and composites was investigated for three distinct printing directions (0°, 45° and Z-direction). Fused filament fabrication (FFF) of acrylonitrile-butadiene-styrene (ABS) filament and short carbon fiber/ABS (CF/ABS) filament was adopted. First, solid cube specimens were fabricated; the parts printed along 0° and 45° directions showed more fabrication-induced pores. Then, 3D braid preforms were fabricated and infused with silicone matrix. For preforms printed along 45° direction, inter-yarn adhesion was observed, which enhanced specimen initial modulus. On the other hand, Z-direction specimens showed higher structural ductility, due to inter-yarn slippage.

Residual stresses induced in the layer-by-layer fabrication process of additively manufactured parts have significant impact on their mechanical properties and dimensional accuracy. This work aims to characterize the residual stress and deformation in specimens based on unreinforced acrylonitrile-butadiene-styrene (ABS), carbon nanotube reinforced ABS and short carbon fiber reinforced ABS. The shrinkage and displacement fields were obtained, respectively, by thermal treatment as well as Digital Image Correlation observation of specimens before and after sectioning. The microstructure and porosity of additively manufactured specimens were also examined using X-ray micro-computed tomography. Specimen shrinkage and porosity content were significantly influenced by the process parameters of raster angle and printing speed, as well as material types. Faster printing speed led to larger porosity and residual stress, as well as higher shrinkage after specimen thermal treatment. Raster angle had a greater influence on specimen shrinkage and porosity as comparing to printing speed. Composite printing wires based on carbon nanotube and short carbon fiber in ABS greatly reduced specimen shrinkage and deformation, while increased the porosity, especially for carbon fiber reinforced ABS specimens.

The emerging wire-shaped supercapacitors (WSSs) have motivated tremendous research interests in energy storage devices. However, challenges still exist in the pursuit of high-rate performance WSS. Here, a graphene fiber made from chemical vapor deposition grown laminated graphene sheet is adopted to form a WSS which exhibits an exceptional rate capability with a relaxation time constant of 1.4 ms, smallest among WSSs reported to date. Furthermore, the resulting WSS can be charged-discharged at a high scan rate of 100 V s −1 while maintaining good capacitive behavior. When the scan rate increased by 5000 times, from 20 mV s −1 to 100 V s −1 , the graphene fiber based WSS maintains 38.6% of its original capacitance. After 10,000 galvanostatic charge-discharge cycles, the specific capacitance retains over 105%, demonstrating a long cycle stability.

A semi-continuum mechanical model has been developed for analyzing the wrinkling of graphene sheet supported by a soft substrate. An energy based method is adopted, in which the elastic energies of graphene sheet and substrate as well as the interfacial van der Waals energy are considered. The graphene sheet and substrate are modeled by continuum mechanical method. In order to estimate the interfacial interaction directly, molecular models of graphene sheet and Polydimethylsiloxane (PDMS) substrate are combined with the continuum models. Graphene wrinkling due to the stretch-then-release process has been examined. The numerical results reveal that substrate deformation in the thickness direction leads to the initial wrinkled morphology of graphene sheet, which is further enhanced by the nonuniform interfacial van der Waals interaction. Upon releasing the substrate, the wrinkle wavelength of graphene sheet with initial wrinkled morphology decreases linearly with the released strain.

AbstarctGraphene has sparked extensive research interest for its excellent physical properties and its unique potential for application in absorption of electromagnetic waves. However, the processing of stable large-scale graphene and magnetic particles on a micrometer-thick conductive support is a formidable challenge for achieving high reflection loss and impedance matching between the absorber and free space. Herein, a novel and simple approach for the processing of a CNT film-Fe3O4-large scale graphene composite is studied. The Fe3O4 particles with size in the range of 20–200 nm are uniformly aligned along the axial direction of the CNTs. The composite exhibits exceptionally high wave absorption capacity even at a very low thickness. Minimum reflection loss of −44.7 dB and absorbing bandwidth of 4.7 GHz at −10 dB are achieved in composites with one-layer graphene in six-layer CNT film-Fe3O4 prepared from 0.04 M FeCl3. Microstructural and theoretical studies of the wave-absorbing mechanism reveal a unique Debye dipolar relaxation with an Eddy current effect in the absorbing bandwidth.

The growing demand on flexible all-solid-state supercapacitors has accelerated the need for fundamental understanding of their response to extreme thermal environment. In this study, supercapacitors based on freestanding films composed of randomly oriented carbon nanotubes and H 2 SO 4 -PVA gel electrolyte were fabricated. The effect of temperature on their electrochemical behaviors was systematically investigated by using cyclic voltammogram, impedance spectroscopy and galvanostatic charge-discharge measurements at temperatures between −5 °C and 55 °C. The results indicated that the capacitance of the supercapacitor was enhanced by the increase of operating temperature while the internal resistance was reduced by virtue of the acceleration of transportation/adsorption of electrolyte ions and surface modification of the electrode. For instance, when charged with the current density of 0.2 mA cm −2 , the capacitance increased by 24.3% from −5 °C to 25 °C, and by 32.6% from 25 °C to 55 °C. Operating temperatures at or below 25 °C were found to facilitate the charge-discharge reversibility and long-term cycle stability. It has also been concluded that supercapacitors based on H 2 SO 4 -PVA gel electrolyte were not suitable for long term use at temperatures higher than 40 °C due to the aging of electrolyte.

Textile-based humidity sensors can be an important component of smart wearable electronic-textiles and have potential applications in the management of wounds, bed-wetting, and skin pathologies or for microclimate control in clothing. Here, we report a wearable textile-based humidity sensor for the first time using high strength (∼750 MPa) and ultratough (energy-to-break, 4300 J g–1) SWCNT/PVA filaments via a wet-spinning process. The conductive SWCNT networks in the filaments can be modulated by adjusting the intertube distance by swelling the PVA molecular chains via the absorption of water molecules. The diameter of a SWCNT/PVA filament under wet conditions can be as much as 2 times that under dry conditions. The electrical resistance of a fiber sensor stitched onto a hydrophobic textile increases significantly (by more than 220 times) after water sprayed. Textile-based humidity sensors using a 1:5 weight ratio of SWCNT/PVA filaments showed high sensitivity in high relative humidity. The electrical res...

A novel flexible nanoarchitecture was fabricated via the facile electropolymerization of a polyaniline (PANI) network on Fe3O4 particles grown axially on carbon nanotubes (CNTs) of a CNT film for supercapacitor electrode applications. The PANI chains serve as a protective shell to improve the structural stability of the Fe3O4 particles. The resulting supercapacitor exhibits a high specific energy density of 28.0 W h kg−1, high specific power density of 5.3 kW kg−1 and specific capacitance of 201 F g−1 at a scan rate of 20 mV s−1, surpassing the performances observed for many recently reported flexible supercapacitors. Furthermore, the CNT film-Fe3O4-PANI supercapacitor demonstrates excellent cyclic stability with only a 3.6% loss of its initial specific capacitance after 10 000 charge–discharge cycles. These results suggest that the CNT film-Fe3O4-PANI composite is very promising for next generation high-performance supercapacitors.

Here, we report a novel highly sensitive wearable strain sensor based on a highly stretchable multi-walled carbon nanotube (MWCNT)/Thermoplastic Polyurethane (TPU) fiber obtained via a wet spinning process. The MWCNT/TPU fiber showed the highest tensile strength and ultra-high sensitivity with a gauge factor (GF) of approximately 2800 in the strain range of 5–100%. Due to its high strain sensitivity of conductivity, this CNT-reinforced composite fiber was able to be used to monitor the weight and shape of an object based on the 2D mapping of resistance changes. Moreover, the composite fiber was able to be stitched onto a highly stretchable elastic bandage using a sewing machine to produce a wearable strain sensor for the detection of diverse human motions. We also demonstrated the detection of finger motion by fabricating a smart glove at the joints. Due to its scalable production process, high stretchability and ultrasensitivity, the MWCNT/TPU fiber may open a new avenue for the fabrication of next-generation stretchable textile-based strain sensors.

The concept of multifunctional hybrid composites has generated considerable research interest in the past two decades. A very significant advancement in this area has been the development of truly multifunctional components, which can simultaneously handle structural and nonstructural functionality in composites. Success in the development of stiff, strong and lightweight continuous fiber structural composites with the capability of energy storage has led to the evolution of “structural power composites.” Among the many structural power composites studied, significant progress has been made in recent years in structural supercapacitor (SSC) composites. The ability of SSCs to carry mechanical loads while simultaneously storing/releasing electrical energy has enabled their potential applications in many fields, such as portable electronics, electric vehicles and aerospace vehicles. In this review, we assess state-of-the-art advances in SSC composites in terms of the development of carbon fiber-based electrodes, solid polymer electrolytes and separators. The fabrication process and performance characterization of SSCs are discussed. Finally, the challenges and future opportunities in SSC composites research are presented.

Four-dimensional (4D) printed laminated structures, created by combining three-dimensional (3D) printing technology with shape memory polymer, have tremendous potential for applying in functional devices and sequentially controlled actuators. This work is aimed to study the tunable shape recovery behavior and recovery force of 4D printed laminated strips based on the microstructural design of raster angle and infill percentage. The effects of raster angle and infill percentage on the shape recovery properties of the printed specimens were investigated. The recovery force of the printed specimens was measured using dynamic mechanical analysis (DMA). The microstructural parameters had a significant influence on the shape recovery behavior and recovery force. Finally, the sequential shape-morphing behavior of 4D printed laminated strips using a single material was demonstrated.

The wrinkling morphology resulted from the multi-buckling process of graphene sheet supported on a PDMS substrate is modeled by a continuum mechanics approach. The effects of randomly distributed interfacial debonded regions as well as adhesive and shear stresses between graphene sheet and substrate are considered. The buckling process has been examined. The present approach reveals the mechanism of multi-buckling in graphene sheet supported by an elastic substrate and can simulate the initiation, growth and saturation of the buckles. The value of adhesive stress controls the morphology of buckling in graphene. Higher values of adhesive stress lead to more uniformly distributed buckles with tiny sizes while lower values lead to large buckle sizes. This study also has demonstrated the capability in estimating the interfacial shear strength and adhesive stress.

An electromechanical response of the chemically reduced graphene oxide (rGO) fiber is reported. The rGO fiber has a tensile strength of 159 MPa, and an electrical conductivity of 170 S/cm. When a current is applied, an immediate load-drop, up to 82% of the original loading, can be observed. Such a load-drop is partially reversible when the current is withdrawn. Higher current density (5–20 mA) and higher relative humidity (40%–80%) result in larger load changes. Joule heating induced dynamic absorption and desorption of water molecules, which further induce the rGO sheet sliding and elastic deformation are responsible for this actuation.

Additive manufacturing features “direct” and “layer-by-layer” fabrication and has significantly facilitated the microstructural design and fabrication of a wide range of highly complex parts. To enable the application of additive manufacturing in major industries and composites, it is necessary to evaluate the microstructural features of additively manufactured parts. Among the various advanced characterization techniques, X-ray micro-computed tomography (μ-CT) showed unique advantages as a high resolution, nondestructive and 3D visualization and measurement technique for material characterization. In the research reported in this article, we have fabricated an array of multi-directional preforms and composites and have characterized their microstructural features via X-ray μ-CT. First, a solid specimen as well as 3D orthogonal and 3D braided preforms have been fabricated using fused filament fabrication (FFF) and inspected with X-ray μ-CT. Then, the fabricated preforms have been infused with silicone matrix and the multi-directional preforms and composites have been tested in compression at different strain levels, to reveal their damage evolution under compressive loading. The preliminary effort made in this research demonstrates the feasibility of characterizing microstructure of additively manufactured parts via X-ray μ-CT technique and enables an investigation of the microstructural features and damage evolution of multi-directional preforms and composites.

The emergence of stretchable electronic devices has attracted intensive attention. However, most of the existing stretchable electronic devices can generally be stretched only in one specific direction and show limited specific capacitance and energy density. Here, we report a stretchable isotropic buckled carbon nanotube (CNT) film, which is used as electrodes for supercapacitors with low sheet resistance, high omnidirectional stretchability, and electro-mechanical stability under repeated stretching. After acid treatment of the CNT film followed by electrochemical deposition of polyaniline (PANI), the resulting isotropic buckled acid treated CNT@PANI electrode exhibits high specific capacitance of 1147.12 mF cm(-2) at 10 mV s(-1). The supercapacitor possesses high energy density from 31.56 to 50.98 μWh cm(-2) and corresponding power density changing from 2.294 to 28.404 mW cm(-2) at the scan rate from 10 to 200 mV s(-1). Also, the supercapacitor can sustain an omnidirectional strain of 200%, which is twice the maximum strain of biaxially stretchable supercapacitors based on CNT assemblies reported in the literature. Moreover, the capacitive performance is even enhanced to 1160.43-1230.61 mF cm(-2) during uniaxial, biaxial, and omnidirectional elongations.

In contrast to conventional preforming techniques, additive manufacturing features direct and layer-by-layer fabrication, which provides viable new capabilities for the fabrication of reinforced composites. In this article, we explore the microstructural design as well as additive manufacturing and characterization of 3D orthogonal, short carbon fiber/acrylonitrile-butadiene-styrene (ABS) preforms and composite. First, an array of 3D orthogonal preforms is designed based on topological consideration and validated by fused filament fabrication of pure ABS wire; high fidelity between models and preforms is accomplished. Then, short carbon fibers are introduced into the designed 3D orthogonal preforms as reinforcement, using a short carbon fiber/ABS wire. Lastly, the compressive behavior of a 3D orthogonal, short carbon fiber/ABS preform and that of its silicone infused composite are characterized. The preform design methodology developed in this research as well as the preliminary effort made in composite fabrication and characterization demonstrates the feasibility of additive manufacturing of 3D orthogonal preform based fiber composites.

The concept of “structural power composites” has motivated considerable research interests in the past decade. The purpose of this research is to demonstrate the feasibility of fabricating a hybrid composite laminate composed of structural and functional components; the former is consisted of four layers Kevlar fabric/epoxy prepregs and the latter is a thin interleaf of carbon fiber/solid electrolyte supercapacitor. The excellent electrochemical and mechanical performance has been demonstrated for the model laminate with electrode-normalized value of specific capacitance, energy density and power density of 3.79 F cm−3, 3.37 × 10−4 Wh cm−3 and 0.04 W cm−3, respectively, as well as laminate flexural strength and flexural modulus of 192 MPa and 9.3 GPa, respectively. The mechanical behavior of the hybrid composite was characterized by static and cyclic flexural loading as well as X-ray micro-computed tomography (μ−CΤ) imaging. The electrochemical stability of the supercapacitor was validated by the high capacitance retention under galvanostatic charge-discharge cycling as well as cyclic flexural loading. Lastly, the potential of scaling up the number of imbedded supercapacitor for adjustable energy storage capacity has been demonstrated.

Electrodeposition based on 3D scaffold-templates is a unique and facile method to tune the microstructure of composite electrode materials for energy storage devices. In this work, we developed a green process to fabricate 3D hierarchical porous carbon scaffold-MnO2 (3D-HPCS@MnO2) nanohybrids with colony-like microstructures. Acting as conductive template and pyrolysis accelerant, the 3D-HPCS enables microstructural control of MnO2 electrodeposits and facilitates valence conversion from Mn2+ to Mn4+. Comparing with depositing on nickel foam, the morphology of MnO2 changed from nanospheres to nanowrinkles when depositing on the 3D-HPCS templates. Benefiting from abundant forest-like micro/mesopores, the resultant 3D-HPCS possesses high specific surface area (1627 m2 g−1) and rich heteroatom dopants (6.94 wt%), affording a high gravimetric specific capacitance (231.5 F g−1) with outstanding cycling performance (95% capacitance retention after 10,000 cycles). The assembled asymmetric supercapacitor based on HPCS//HPCS@MnO2 exhibits an energy density of 60.8 Wh kg−1 and a maximum power density of 20.7 kW kg−1. Our investigation has quantitatively proved that mesopores contribute more than micropores in increasing capacitance of HPCS and both surface capacitive and diffusion-controlled processes play roles in capacitive performance of HPCS@MnO2. Moreover, the influence of the morphology and surface functionality on the electrochemical performances of composite electrodes have been analyzed.

Carbon nanotube (CNT) films are well known for their ultra-low density, high porosity, tunable conductivity and a number of other desirable characteristics. Integrating CNT films into composite materials would endow these structural materials with multifunctionality while adding only negligibly to their weight. This review introduces the fabrication of CNT films and presents the state-of-the-art research advances of CNT-film-enabled functions of composite materials, such as resistive heating, lightning strike protection, electromagnetic interference shielding, and structural health monitoring. Future research on CNT-film-based multifunctional composite materials is also discussed.

Axial electromechanical properties after transverse compression of carbon nanotube (CNT) fibers were studied for their potential application in constructing lightweight structures. The structure-property relationship under two different compression modes was revealed due to their unique mechanical and electrical properties. Change of disentanglements and slippage between CNTs or CNT bundles during transverse compression significantly altered the axial tensile performance. The axial tensile strength of aerogel-spun and dry-spun CNT fibers increased by 20.6% and 11.9 % under individually compressed at 60 MPa and 90 MPa, respectively. The tensile moduli of both fibers increased by 0.05 GPa. Followed by reduction of tensile performance as compressive strain level increased. The corresponding electrical resistance of both deformed CNT fibers increased more rapidly than that of the uncompressed fibers. In the cyclic compression mode, the axial tensile strength and modulus decreased by 40% and 45.7% for the aerogel-spun CNT fibers and by 17% and 14.2% for dry-spun CNT fibers owing to permanent damage accumulation. Scanning electron microscope examinations showed that tensile failure mode after compression transitioned from slippage into a new mode based on the fracture of CNT bundles. The failure mechanisms were finally investigated by a theoretical model. Keywords: CNT fiber, Axial electromechanical properties, Transverse compression, Cyclic loading-unloading

Many stretchable conductive composite fibers exhibit high elongation at break, but most of them do not have a large workable strain range towing to the low conductivity when used as strain sensors. In this paper, we fabricated a highly conductive silver nanowire (Ag NW)/multi-walled carbon nanotube (MWCNT)/thermoplastic polyurethane (TPU) fiber via a wet-spinning process to improve the workable strain range of composite fibers. TPU was used as a matrix material to introduce superior stretchability. MWCNTs act as sensing elements and Ag NWs were used to increase conductivity. We investigated the effect of Ag NW content on the mechanical, electrical, and strain-sensing performance of the fiber-type strain sensors. The optimal content of Ag NWs extended the workable strain range as higher as 254% with an electrical conductivity of 0.803 S/cm. A weight-to-strain cloth sensor was assembled by writing Ag NW/MWCNT/TPU fibers in the coagulation solution. Furthermore, such composite fiber can be free-written into any designed pattern, which can be used to prepare fiber-based devices.

Multidirectional textile preforms, such as three-dimensional (3D) angle-interlock woven fabric, orthogonal woven fabric and braided fabric, have been often used for developing lightweight and high-strength composites. Although multidirectional preforms for composites have been fabricated using traditional textile-forming techniques, some basic technological barriers still exist. Recent advancements in additive manufacturing (also known as 3D printing) have opened up new opportunities for manufacturing of multidirectional preforms and their composites. Furthermore, the functionalization of 3D preforms can greatly facilitate their application in the field of smart devices. The key step of the functionalization is to add the ‘time’ dimension to the 3D printed objects, with the resulting process known as four-dimensional (4D) printing. In this review article, we first briefly summarize the recent research work in the characterization of fabrication-induced defects when using a layer-by-layer deposition process and discuss the effect of processing parameters. Then, the various approaches in 4D printing for achieving shape morphing overtime are reviewed. Finally, some strategies for developing 4D printed multidirectional shape memory polymer-based preforms with high shape memory performance are summarized. Potential challenges and opportunities in the development of 3D printing and 4D printing are outlined. Keywords: Multidirectional preform, 3D printing, 4D printing, Mechanical property, Shape memory property

The four-dimensional (4D) printing technology enables the convergence of three-dimensional (3D) printing and shape memory polymer (SMP). The aim of this research is to demonstrate the synergistic effect between a spring steel strip (SSS) and a 4D printed thermoplastic SMP on enhancing the shape recovery properties. The recovery time of the SMP/SSS hybrid composite was shortened by 39% as compared to that of the SMP specimen. The corresponding recovery force of the SMP/SSS hybrid specimen at 64 °C was 9 N, 199 times of that of the non-hybrid SMP specimen. For optimizing the design of hybrid composite specimens, the structural parameters, such as hybrid stacking configuration, SMP infill percentage and SSS thickness, have been also investigated and their influences on the shape recovery properties of the hybrid composite specimens have been identified. The present results signify a promising approach to improve the performance of SMP based sensors and actuators using hybrid composites.

Current additive manufacturing methods present the potential to construct net-shape structures with complicated architectures, thus eliminating the need for multi-step processing and fasteners/joints. Combined with these features is the ability to ascribe material properties at the sub-millimeter scale, inspiring multi-material, functionally graded designs. These features make additive manufacturing an attractive option for composite materials development. In an effort to extend this family of technologies beyond nano- and micro-composites, we explore the additive manufacture of multi-directional composite preforms. This exercise has served to highlight the aspects of additive manufacturing critical to composite and general materials processing, as well as to demonstrate the high fidelity between modeled and additively manufactured structures. Within the scope of composites development, we review the state-of-the-art and discuss challenges facing the broad adoption of additive manufacturing for directionally reinforced composites processing.

Graphene films fabricated by chemical vapor deposition are promising electrode materials for stretchable energy storage devices. The buckled four-layer graphene on a polydimethylsiloxane film substrate subject to various applied tensile strains has been characterized by atomic force microscopy and micro-Raman mapping. The small redshift of 2D band and the indiscernible D band demonstrated that the tensile strains of up to 40% only induced a strain variation of less than 0.2% and did not cause any observable damage in the graphene film. This study has confirmed that the graphene film in the buckled state is suitable for its application as a stretchable electrode.

In spite of the recent rapid growth in stretchable electronic device research, efforts have been mainly focused on material selection, device geometric design and short-term performance characterization. The present research focuses on the long-term durability of electromechanical and electrochemical performance of buckled carbon nanotube fibers based stretchable conductors and supercapacitors under cyclic deformation. The damage mode and damage evolution as a function of fatigue deformation are identified. After 10,000 stretching-releasing cycles with mechanical deformation up to 40% strain, the conductivities of buckled dry spun and aerogel spun CNT fiber based stretchable conductors exhibit excellent stability and the resistances increase by only about 0.2% and 6%, respectively. The areal specific capacitances of buckled dry spun and aerogel spun CNT fiber based stretchable supercapacitors change, respectively, from 4.42 mF cm−2 to 3.60 mF cm−2, and from 8.16 mF cm−2 to 9.95 mF cm−2 at the scan rate of 50 mV s−1 after 10,000 deformation cycles.

Uniform carbon nano-fiber (CNF) films have been deposited onto carbon fabrics by sonication-assisted Electrophoretic Deposition (EPD). To examine the effect of sonication, the distributions of voltage and pH values in the deposition bath were measured during EPD process. The degree of uniformity of CNF deposition was analyzed from the morphology of the deposited CNFs by scanning electron microscopy. The neutralization of negative charged CNFs by hydrogen ions, which were generated by electrolysis of water at the anode (carbon fabric), is essential for the adsorption of CNFs onto carbon fabrics. In addition, sonication increases the movement of ions in the suspension and accelerates the neutralization of CNFs at the anode. Moreover, sonication overcomes the aggregation and settlement of CNFs at the bottom of the deposition bath caused by gravity. The bubbles, which were generated by electrolysis of water, in the vicinity of the carbon fabrics were removed by sonication, resulting in the improvement of CNFs deposition. Based upon the results of voltage and pH values distributions during the EPD and analysis of morphologies of CNF films, a mechanism of sonication-assisted EPD has been proposed.