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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. [ABSTRACT FROM AUTHOR]

Abstract: A sandwich composite is typically designed to possess high bending stiffness and low density and consists of two thin and stiff skin sheets and a lightweight core. Due to the high stiffness-to-weight and strength-to-weight ratios, sandwich composite materials are widely used in various structural applications including aircraft, spacecraft, automotive, wind-turbine blades and so on. However, sandwich composite structures used in such applications often suffer from poor acoustic performance. Ironically, these superior mechanical properties make the sandwich composites “excellent” noise radiators. There is a growing interest in optimizing and developing a new sandwich composite which will meet the high stiffness-to-weight ratio and offer improved acoustic performance. The focus of this study is to investigate the structural–vibrational performance of carbon-fiber face sheet sandwich composite beams with varying core materials and properties. Core materials utilized in this study included Nomex and Kevlar Honeycomb cores, and Rohacell foam cores with different densities and shear moduli. The structural–vibrational performance including acoustic and vibrational damping properties was experimentally characterized by analyzing the wave number response, and structural damping loss factor (η) from the frequency response functions, respectively. It was observed that the relationship between the slopes of the wave number data for frequencies above 1000Hz is inversely proportional to the core material’s specific modulus (G/ρ). The analysis also showed the importance of using a honeycomb core’s effective properties for equal comparison to foam-cored sandwich structures. Utilizing analytical modeling, the loss factors of the core materials (β) was determined based upon the measured structural loss factors (η) for a frequency range up to 4000Hz. It was determined that low shear modulus cores have similar material damping values to structural damping values. However as the core’s shear modulus increases, the percent difference between these values is found to increase linearly. It was also observed that high structural damping values correlated to low wave number amplitudes, which correspond to reductions in the level of noise radiation from the structure. [Copyright &y& Elsevier]

Abstract: Due to their higher strength-to-weight and stiffness-to-weight ratios compared to metals, fiber reinforced composite materials are a great alternative for use in many structural applications. However these properties lead to poor acoustic performance as composite materials are excellent noise radiators. This is particularly true for sandwich composite structures. Therefore the focus of this study is to investigate the effect of a core thickness change on the vibrational properties of Rohacell foam/carbon-fiber face sheet sandwich composite beams. Four different foam core thicknesses were explored, using a combination of experimental and analytical methods to characterize sound and vibrational properties of the sandwich beams. First, the wave number responses of the beams were obtained, from which coincidence frequencies were identified. Second, from the frequency response functions the structural damping loss factor, η, was determined using the half-power bandwidth method. Experimental and analytical results show that the relationship between core thickness and coincidence frequency is non-linear. A drastic increase in coincidence frequency was observed for the sandwich beam with the thinnest core thickness due to the low bending stiffness. Moreover this low bending stiffness results in low damping values, and consequently high wave number amplitude responses at low frequency ranges (<1000Hz). [Copyright &y& Elsevier]

Porous carbon materials have been widely used as electrodes for various energy storage and conversion devices, particularly for supercapacitors. Herein, a bamboo-derived hierarchical porous carbon (BHPC) is directly prepared under air atmosphere via an eco-friendly, one-step, and easily-scalable salt-templating strategy using ZnCl 2 /KCl salt mixture as a pore-directing solvent. The obtained BHPC material exhibits large specific surface area (1296 m2 g−1) and large total pore volume (1.26 cm3 g−1) which make the BHPC material applicable for supercapacitor as an electrode material. Electrochemical performance evaluated in a 6 M KOH electrolyte in a three-electrode system indicates a high specific capacitance of 394 F g−1 at 1 A g−1 and a good rate capacity with 76.14% capacitance retention at 20 A g−1. The as-prepared symmetric supercapacitor delivers a high energy density of 11 Wh kg−1 at a powder density of 126 W kg−1, and an outstanding lifespan with 81% capacitance retention over 10,000 cycles. These results further open new possibilities for tailoring the surface properties (i.e., heteroatom doping, surface modification, metal oxides incorporation, etc.) towards energy storage and conversion applications such as electrocatalysts for oxygen reduction reaction/hydrogen evolution reaction, lithium-ion batteries, and supercapacitors, as well as wastewater treatment, and electromagnetic wave absorption applications. [Display omitted] [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

It has been reported that vertical graphene (VG) has the capability to be a photocatalyst. In previous studies, the morphology evolution of the VG films synthesized by the plasma-enhanced chemical vapor deposition (PECVD) process has been investigated. This work focuses on the impact of the morphologies of the VG films on photocatalytic properties. Three morphologies of the VG films, VG with porous graphene (VG-P), 'forest-like' graphene (VG-F), and 'wall-like' vertical graphene (VG-W), are fabricated and the photodegradation properties are investigated. In order to enhance the photodegradation property of the VG films, the surface of the VG films with different morphologies is treated with oxygen plasma, in which the oxygen plasma treated VG-P, VG-F, and VG-W are named as VG-P-O, VG-F-O, and VG-W-O, respectively. As a result, the VG-P-O shows the best degradation performance of 82.6% for methylene blue and 86.7% for methyl orange, which is attributed to the desirable porous structure and the modified hydrophilicity. [Display omitted] [ABSTRACT FROM AUTHOR]

Abstract: This study investigates strain dependent energy dissipation characteristics in carbon nanofiber (CNF) reinforced carbon fiber epoxy composites (multi-scale composites) by characterizing their viscoelastic properties and vibrational damping response. The air damping effect on the energy dissipation characteristics is also examined. The viscoelastic properties of epoxy containing two weight fractions (3 and 5wt%) of added CNFs were characterized using dynamic mechanical analysis. Carbon fiber layers were then infiltrated with the two epoxy resins containing the CNFs to form multi-scale composites. A strain dependent loss factor behavior of the multi-scale composites was observed in the dynamic cyclic testing due to CNF’s stick–slip friction, showing a 53% increase in loss factor for the composites containing 5wt% CNFs. The beam vibration test results also indicated an improvement in loss factor for the multi-scale composite beams relative to those without the CNF addition in the first two resonant frequencies. The multi-scale composite beams exhibit an increase in loss factor, up to 43%, at high amplitude excitation, while a reduction in loss factor was seen at low amplitude. These observed strain dependent damping characteristics seem to result from both the stick–slip friction and the air damping effect. [Copyright &y& Elsevier]

Although equiatomic CoCrFeNi medium-entropy alloys (MEAs) show excellent ductility, their strength is not high enough for the use in various engineering applications. It is necessary for their strength to be improved, while maintaining their great ductility. In this study, the in-situ TiC x N y nanoparticle reinforced equiatomic CoCrFeNi MEA matrix nanocomposites were fabricated by laser powder bed fusion (LPBF) technique and the addition of sub-micron TiC particles. Interestingly, the LPBF-fabricated CoCrFeNi MEAs have been found to exhibit both poor ultimate tensile strength (UTS) (∼582 MPa) and uniform elongation (δ u) (∼11.7 %), attributed to the formation of hot cracks. However, the in-situ formation of TiC x N y nanoparticles could eliminate the formation of hot cracks. The near defect-free nanocomposites with substantial sub-grains were fabricated, and their UTS and δ u were increased by 74.4 % (∼1015 MPa) and 92.3 % (∼22.5 %), respectively. The microstructure of as-built nanocomposites was mainly composed of FCC phase and TiC x N y nanoparticles, and a few M 23 C 6 nanoparticles were found. The high strength of as-built nanocomposites was mainly attributed to Orowan and dislocation strengthening mechanisms. To the best of our knowledge, this is the first study to introduce high strength and great ductility into CoCrFeNi MEAs via the in-situ formation of TiC x N y nanoparticles with the optimized LPBF process. [Display omitted] • It is the first time that hot cracks were found in LPBF-fabricated CoCrFeNi medium entropy alloys. • The in-situ formation of TiC x N y nanoparticles could eliminate the generation of hot cracks. • Well-bonded interfaces were observed between TiC x N y nanoparticles and FCC matrix. • This is the study to introduce high strength and ductility into CoCrFeNi medium entropy alloys. [ABSTRACT FROM AUTHOR]

Wearable devices have attracted increasing attention for development of personal healthcare. In this study, three-dimensional porous carbon aerogels reinforced polydimethylsiloxane nanocomposites with controllable and hierarchical open, semi-open and closed cell structures were developed for multifunctional wearable heating and sensing devices. This investigation reveals that the microstructures of the aerogels play a critical role in determining nanocomposites properties, particularly their heating and sensing performances. As thermotherapy heaters, the nanocomposite with semi-open cell structure is observed with the highest energy transduction efficiency (equilibrium temperature ∼138.9 °C under only 5 V) compared to the nanocomposites with open and closed cell structures, due to the well-defined conductive network and structural stability. As stimuli-responsive sensors, compared to the nanocomposite with closed cell structure, the nanocomposites with open and semi-open cell structures are observed with higher sensitivity (gauge factor ∼369.03) and much better repeatability, benefiting from their structural integrity. Finally, the nanocomposite with semi-open cell structure was investigated for practical potential on human body. Experimental results demonstrated the uniform temperature distribution and reliable sensitivity as a multifunctional wearable device. Therefore, by controlling and optimizing the microstructure of carbon aerogels, the nanocomposites with tailored microstructure could be exploited for various engineering applications including emerging multifunctional wearable devices. Image 1 [ABSTRACT FROM AUTHOR]

Inspired by desert resurrection plant, Rose of Jericho, this study developed a new porous material with programmable multiple-stimuli responsive features. The first one is reversible shape-memory transformation of experiencing extremely large deformations upon dehydration and rehydration; superior to the plant, the aerogels display controllable responsiveness based on moisture amount with fast actuation. The second one is switchable flexibility depending on the polarity of exposed fluids; these aerogels are reversibly compressible with outstanding flexibility in polar solvents but sustain incompressibility with high stiffness in nonpolar solvents. The third one is programmable shape morphing capability driven by imbibition of liquid droplet; mimicking the movement of the plant but in different mechanisms, the aerogels can evolve into programmed V-shape or U-shape by simply controlling the liquid imbibition location. These multiple-stimuli responsive features arise from the unique ensemble of the porous morphology, and the interactions between solvent molecules and the chemical structures of the porous material engineered by two-step crosslinking mechanism. It can be foreseen that the synergistic effect of the intriguing features could allow for the applications of the aerogels as smart sensors/actuators, soft robotics, and biomedical devices, all of which typically require smart adaptability to external stimuli and/or extreme environments. Image 1 [ABSTRACT FROM AUTHOR]

Development of lightweight porous structure with high compressibility and mechanical durability is in great demand to promote graphene-based materials from fundamental research to engineering applications. In this study, graphene oxide (GO)-based sponges with hierarchical porous structures were synthesized by a simple freeze-drying method. By controlling and optimizing their structures, the lightweight GO-based sponges can offer excellent structural integrity even after being compressed at 95% strain, indicating the extremely high compressibility. The outstanding mechanical durability were also demonstrated by cyclic compression testing at 50% strain for 10,000 cycles. Considering the severe dye pollution problem, the sponges were investigated for methylene blue adsorption. It was found that the sponges with high selectivity during adsorption can have the maximum adsorption capacity up to 476 mg g−1. More importantly, the sponges were demonstrated with extraordinary recyclability along with a very simple recycling process and also high recycling efficiency (above 86% after the 10th cycle). It was experimentally demonstrated that if their morphological structures are carefully designed and optimized, the GO-based porous structures can exhibit the ensemble of outstanding compressibility and durability, and equally importantly the excellent adsorption performance. Therefore, it could show great promise of such nanomaterial-based sponges as excellent adsorbents for wastewater purification. Image 1 [ABSTRACT FROM AUTHOR]

Three-dimensional (3D) porous materials gain considerable attention in electronics and biomedical/environmental engineering owing to their high porosity, large surface area, and controllable morphological and mechanical properties. In this work, reversibly compressible feather-like lightweight 3D chitosan/graphene oxide (CS/GO) composite foams were prepared with varying the composition ratios of CS to GO using a freeze-drying method. Their structure was obtained via amide linkage formation between the CS amine groups and GO carboxylic acid groups throughout the foam. Chemical analysis of the foams was performed using Fourier-transform infrared spectroscopy, and their morphology was analyzed by field-emission scanning electron microscopy and crosslinking density measurements. Their mechanical features and static and dynamic viscoelastic properties were characterized by monotonic compression, stress-relaxation, and frequency sweep tests using a custom-built micro-indenter and dynamic mechanical analyzer. With increasing the CS concentration, their storage/loss modulus and crosslinking density increased, while their tan δ decreased. Their viscoelastic properties predicted using the proposed modified micromechanics models agreed with the experimental data. This study demonstrates a simple strategy to produce lightweight porous 3D CS/GO composite foams with biocompatibility, electrical conductivity, and decent mechanical and viscoelastic properties regulated by the composition. These foams are promising materials applicable in biomedical and environmental engineering and electrical devices. Image 1 [ABSTRACT FROM AUTHOR]

Abstract This study is a semi-empirical approach to effectively estimate the interfacial shear strength (IFSS) of short fiber-reinforced polymer composites by combining tensile and acoustic emission testing with finite element modeling. The short glass fiber–polycarbonate composites were fabricated using a solution mixing method and the tensile properties were characterized with a standard test. During the tensile tests, the debonding stresses at the interface between the glass fiber and polycarbonate were monitored with an acoustic emission sensor. Subsequently, a finite element model of the composites was developed using the experimentally obtained debonding stresses to estimate the IFSS of the composites. By employing this semi-empirical method, the IFSS was effectively determined and found to range from 83 to 126 MPa depending on the fiber orientation. This approach can be broadly applied to micron-sized short fibers and nanoscale fillers such as carbon nanotubes or graphene-reinforced composites. Highlights • Interfacial shear strength (IFSS) of short fiber reinforced polymer composites has been estimated by a semi-empirical approach. • Short glass fiber reinforced polycarbonate composites were fabricated by solution mixing. • The debonding stresses at the interface between glass fiber and polycarbonate were monitored by tensile and acoustic emission testing. • A finite element analysis of the short fiber composites was developed with the experimentally obtained debonding stresses. • Different IFSS values were estimated depending on the fiber orientation. [ABSTRACT FROM AUTHOR]

Abstract The static and dynamic compressive behaviors of glass fiber-reinforced polyurethane foams were characterized under cryogenic temperature conditions to help identify the essential design and parameters required to model the response of the foam accurately under the various loading conditions encountered in liquefied natural gas cargo containment systems. The material properties of polyurethane foams with varying concentrations and orientations of the glass fiber continuous strand mat were characterized by quasi-static, dynamic mechanical analysis (DMA), and low velocity impact testing. The testing results showed increased strength with increasing density and strain rate, as well as similar failure modes independent of the strain rate. In addition, the glass fibers dominated the DMA behavior and a glass transition temperature at or above 130 °C was observed for all foam varieties. Overall, the glass fiber reinforced polyurethane foam showed the typical static and dynamic compressive behaviors of foams at both room and cryogenic temperatures. Highlights • The static and dynamic viscous behaviors of the glass fiber-reinforced polyurethane foams (PUFs) were characterized by the quasi-static and dynamic tests. •The glass fiber-reinforced polyurethane foams were fabricated with varying concentrations and orientations of the glass fiber continuous strand mat (CSM). •Through the quasi-static test, it was observed that the fracture of the fibers may occur depending on the direction of CSM. •Low velocity impact test showed the rate dependent stress-strain relationship with the various densities. •The viscoelastic responses of the PUF specimens with different densities were examined by DMA test. [ABSTRACT FROM AUTHOR]

Abstract Anti-corrosion metal protection of the 2D structured graphene oxide (GO) was investigated in this study using its intrinsic impermeability against reactive molecules stemming from the small geometric pore size and π-orbital repelling fields of graphene. The electrophoretic deposition (EPD) of GO sheets on metals is an attractive coating method but the negatively-charged GO only allows the anodic EPD process, which makes it difficult to achieve high-quality coating layers due to gas bubbling and electrochemical oxidation of metals. Thus, we imposed the positive charge to GO sheets using p -phenylenediamine (PPD) and successfully carried out the cathodic EPD in the ethanol aqueous solution. The cathodic EPD of GO on cupper provided a linear growth rate with both deposition time and voltage providing the thickness over 6.0 μm in a void-free and robust feature, which could hardly be achieved by other coating methods. Converting the GO coating to the reduced GO (RGO) by thermal reduction, we successfully increased the RGO adhesion strength up to the highest adhesion grade of 5B in the cross-cut adhesion test. An excellent anti-corrosion protection capability of the developed RGO coating was demonstrated by a decreased corrosion current density and increased corrosion potential in potentiodynamic polarization analysis. Graphical abstract Image 1 [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Suberin is an aromatic-aliphatic cross-linked polyester structure which constitutes cell wall structures of cork. It is particularly interesting for its use of monomeric compounds towards renewable bio-based polymers. In this study, the extraction of suberin monomers was successfully done by green method using alkaline hydrolysis combined with mechanical grinding. As the mechanical grinding was applied along with hydrolysis process, the maximum yield of depolymerized suberin monomer (DSM) with relatively low energy and less time was acheived. The polarity and functionality of DSM extracted in this study also showed higher values compared to conventional reflux hydrolysis process. Polyesterification and curing behavior of DSM were examined with molecular weights and mechanical properties of the ensuing polyesters. Tensile properties of suberin-based polyesters are reported for the first time that the maximum strength was found to be 7.3 MPa while Young‘s modulus was found to be 105 MPa. Furthermore, all natural cork composites were fabricated which comprise suberin-based polyester as matrix and lignocellulosic residue as reinforcement, and also reported their significantly enhanced tensile properties showing the great potential as an alternative green polymer composites for various engineering applications. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

As three-dimensional (3D) printing technology is emerging as an alternative way of manufacturing, the high resolution 3D printing device often requires systems such as drop jetting printing of in situ UV-curable photopolymers. Accordingly, the key issue is process control and its optimization to ensure dimensional accuracy, surface roughness, building orientation, and mechanical properties of printed structures, which are based on the time- and temperature-dependent glass transition temperature ( T g ) of the resin system under UV-curing. In this study, the UV-cure kinetics and T g development of a commercially available UV-curable acrylic resin system were investigated as a model system, using a differential scanning photocalorimeter (DPC). The developed kinetic model included the limited conversion of cure that could be achieved as a maximum at a specific isothermal curing temperature. Using the developed model, the T g was successfully described by a modified DiBenedetto equation as a function of UV curing. The developed kinetic model and T g development can be used to determine the 3D printing operating conditions for the overlay printing and in situ UV curing, which could ensure high-resolution and high-speed manufacturing with various UV-curing materials. [ABSTRACT FROM AUTHOR]

In wearable and microelectronic devices, there is a rising need for ultrathin electromagnetic interference (EMI) shielding films with multifunction, such as Joule heating and photothermal conversion. MXene has shown promising applications in EMI shielding due to its excellent electrical conductivity, abundant surface functional groups, and layered structures. However, the self-stacking, poor mechanical strength and hydrophilicity of MXene hinder its applications. In this study, MXene/holey graphene (MX/HG) composite film is fabricated in a layered structure. The hydrogen bonds between HG and MXene improve the mechanical properties and prevent the self-stacking of MXene sheets. EMI shielding performances are investigated according to the monolayer MXene sheets' size and HG's mass ratio. As a result, the large lamella-based MX/HG composite film shows outstanding EMI shielding effectiveness (EMI SE) of 56.15 dB, excellent low-voltage-driven Joule heating (up to 100 °C at 3 V within 4 s), and high-efficiency photothermal conversion (up to 49.8 °C under 1000 W m−2 within 30 s) at a thickness of 5 μm. The multifunctional film achieves impressive EMI shielding, Joule heating, and photothermal conversion performances at a low thickness, showing great prospects in wearable and microelectronic device applications. • A free-standing MXene/holey graphene film is fabricated in a layered structure. • Shielding effectiveness of 56.15 dB is achieved at a low film thickness of 5 μm. • Excellent tensile stress of 120 MPa and a fracture strain of 3.5% are achieved. • The film shows outstanding Joule heating and photothermal conversion properties. [ABSTRACT FROM AUTHOR]