Showing papers in "Composites Part B-engineering in 2019"

Improved flame resistance and thermo-mechanical properties of epoxy resin nanocomposites from functionalized graphene oxide via self-assembly in water

Abstract: The development of a green and facile strategy for fabricating ecofriendly, highly effective flame retardants has remain a major challenge. Herein, supermolecular aggregates of piperazine (PiP) and phytic acid (PA) have been self-assembled onto the graphene oxide (GO) surface in water to fabricate functionalized GO (PPGO). The chemical structure and morphology of PPGO are determined by the X-ray photoelectron spectroscopy, transmission electron microscopy and scanning electron microscopy along with the energy dispersive spectroscopy. Due to the introduction of organic component onto the surface of graphene oxide, the adhesion between PPGO and the epoxy resin (EP) matrix is enhanced. As a result, the storage modulus (E′) of EP composites is increased in addition to a better dispersion of PPGO. Compared with the pure EP, the flame resistance of EP/PPGO is significantly improved, exhibiting a 42% decrease in peak heat release rate (pHRR), 22% reduction in total heat release (THR). The reduced flammability of EP is attributed to the synergistic effects afforded by the gas dilution effect of piperazine, char-forming promotion effect of phytic acid and the creation of "tortuous path" barrier effect of GO during burning. This work offers a green and facile approach for creating highly effective graphene-based flame retardants.

Effects of combined usage of GGBS and fly ash on workability and mechanical properties of alkali activated geopolymer concrete with recycled aggregate

Abstract: Geopolymer binder and recycled aggregates are considered as the two important sustainable ingredients in concrete production. Ground granulated blast furnace slag (GGBS) and fly ash combination based geopolymeric recycled aggregates concretes (GRAC) were prepared as a substitution of normal concrete in order to reduce the consumption of ordinary Portland cement and natural aggregates as well as to make use of demolished waste concrete. The aim of this study is to investigate the combined effects of GGBS and fly ash considering water-binder (W/B) ratio on the fresh and hardened properties of alkali activated GRAC. The slump, setting time, compressive strength, stress-strain relation, elastic modulus, Poisson's ratio, toughness and failure mode of alkali activated GGBS and fly ash based GRAC containing 100% recycled coarse aggregates were experimentally examined. In addition, the hydration mechanisms were studied by XRD and SEM. Finally, the failure mechanism of this kind of novel green concrete subjected to compression was revealed. The results show that GGBS and fly ash exhibits a superior synergetic effect on the performance of GRAC, i.e. fly ash and GGBS are mainly responsible for the workability and mechanical properties, respectively. GGBS/fly ash ratio has a significant influence on the fresh and hardened properties of GRAC. Moreover, the effect of W/B ratio on GRAC strongly depends on the GGBS/fly ash ratio. GGBS content at less than 25% was found to slightly influence the consistency and compressive strength of GRAC. The combination of 50% GGBS and 50% fly ash with a 0.5 W/B ratio could provide excellent mechanical performance and workability for GRAC.

Fabrication on the annealed Ti3C2Tx MXene/Epoxy nanocomposites for electromagnetic interference shielding application

Abstract: Few-layered Ti3C2Tx MXene was fabricated by ionic intercalation and sonication-assisted method, followed by thermal reduction at medium-low temperature. Then annealed Ti3C2Tx/epoxy electromagnetic interference (EMI) shielding nanocomposites were obtained by solution casting method. XRD, SEM, AFM and TEM indicated the successful preparation of few-layered Ti3C2Tx. FTIR, XPS and XRD showed that thermal reduction removed partial polar groups on the surface of Ti3C2Tx with no by-product. For a fixed Ti3C2Tx loading, compared with Ti3C2Tx/epoxy EMI shielding nanocomposites, the annealed Ti3C2Tx/epoxy EMI shielding nanocomposites exhibited relatively higher electrical conductivity and excellent EMI shielding effectiveness (SE). When the mass fraction of annealed Ti3C2Tx was 15 wt%, the annealed Ti3C2Tx/epoxy EMI shielding nanocomposites presented the optimal electrical conductivity of 105 S/m and EMI SE of 41 dB, 176% and 37% higher than that of 15 wt% Ti3C2Tx/epoxy EMI shielding nanocomposites. Furthermore, the 5 wt% annealed Ti3C2Tx/epoxy EMI shielding nanocomposites exhibited the optimal Young's modulus of 4.32 GPa and hardness of 0.29 GPa, respectively.

Additive manufacturing of natural fiber reinforced polymer composites: Processing and prospects

Abstract: Throughout the world there have been alarming concerns over the use of nonrenewable resources during manufacturing of goods and associated environmental legislations. Therefore, the use of natural materials and fabrication of composites therefrom, particularly, development of natural fiber reinforced polymer composites is gaining significant attention. Although natural fiber reinforced composites (NFRCs) show strong application prospects, various materials and processing related challenges needs to be addressed to achieve long-term stability and performance. In this review, we attempted to provide an overview of different types of natural fibers, their characteristics and properties enabling them to be used as reinforcing agents in different polymers. Then the unique requirement of fiber surface modification to achieve enhanced fiber-matrix bonding is discussed. The article also discusses conventional processing routes and critical issues associated with NFRCs processing. The use of different additive manufacturing (AM) technologies in processing polymer composites is also discussed. At the end, we have critically analyzed the challenges and opportunities associated with AM of NFRCs.

Enhanced thermal conductivities and decreased thermal resistances of functionalized boron nitride/polyimide composites

Abstract: Fluorine-containing polyimide (PI) with low dielectric constant (e) value was firstly synthesized by a polycondensation reaction of 4, 4-(hexafluoroisopropyl) diphthalic anhydride (6FDA), 1, 3, 4-triphenyldiether diamine (APB) and 1, 3-bis(3-aminopropyl) tetramethyldisiloxane (GAPD). γ-glycidoxypropyltrimethoxysilane (KH-560) and aminopropylisobutyl polyhedral oligomeric silsesquioxane (NH2-POSS) were synchronously performed to functionalize the surface of boron nitride (f-BN) fillers, which were then utilized as thermally conductive fillers to fabricate the corresponding f-BN/PI composites. NH2-POSS was successfully grafted on the surface of BN fillers. All the f-BN/PI composites presented better thermal conductivities, dielectric and thermal properties than those of BN/PI composites at the same addition of BN fillers. The obtained thermal conductivity coefficient (λ) of the f-BN/PI composites with 30 wt% f-BN was 0.71 W/mK, higher than that of BN/PI composites with 30 wt% BN (λ of 0.69 W/mK). Modified Hashin-Shtrikman model demonstrated that f-BN possessed relatively lower thermal resistance with PI matrix. Meantime, the corresponding e and dielectric loss tangent (tanδ) value of the f-BN/PI composites was decreased to 3.32 and 0.004, respectively, lower than that of BN/PI composites with 30 wt% BN (e of 3.77 and tanδ of 0.007). Moreover, the corresponding heat resistance index (THRI) and glass transition temperature (Tg) of the f-BN/PI composites was further enhanced to 280.2 °C and 251.7 °C, respectively.

Mechanical characterization of FDM 3D printing of continuous carbon fiber reinforced PLA composites

Abstract: Additive manufacturing of fiber reinforced composites is of great interest in various industrial applications. In this study, an innovative extruder is designed and manufactured for fused deposition modeling (FDM) 3D printers in order to produce continuous fiber reinforced thermoplastic (CFRT) composites. There are some challenges along this way such as making tension in fiber, fiber surface preparation, printing temperature and feed rate to produce a composite part with good quality. These challenges are discussed in detail. The main advantage of this extruder is that it can be mounted on the available FDM 3D printers and consequently there is no need to design a new chassis. In order to assess the quality of products, standard tensile and three-point bending specimens made of pure poly lactic acid (PLA) and carbon fiber reinforced PLA are printed and tested under quasi-static loading. Experimental results show significant improvements of tensile and bending properties of PLA. Morphological analysis is also conducted to study the bonding between the carbon fiber and PLA.

Enhanced microwave absorption performances of polyaniline/graphene aerogel by covalent bonding

Abstract: In this paper, covalently bonded polyaniline (PANI)/graphene aerogel (GA) was synthesized using hydrothermal and in-situ polymerization techniques. The chemical bonding between PANI and GA and the micromorphological features of the aerogel were investigated employing several characterization methods, which included Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), Brunauer-Emmett-Teller (BET) analysis, field emission scanning electron microscopy (FESEM), and transmission electron microscopy (TEM). Due to combination of appropriate impedance match, synergistic effects, and hierarchical nanostructures of PANI and GA, the hybrids exhibited obviously improved microwave absorption performances, compared to GA. The PANI/GA showed the strongest reflection loss (RL) of −42.3 dB at 11.2 GHz with a matching thickness of 3 mm, and the corresponding absorption bandwidth (RL

3D reduced graphene oxide/FeNi3-ionic liquid nanocomposite modified sensor; an electrical synergic effect for development of tert-butylhydroquinone and folic acid sensor

Abstract: This paper reports an electroanalytical sensor developed based on reduced graphene oxide/FeNi3 (FeNi3/rGO)-ionic liquid (n-hexyl-3-methylimidazolium hexafluoro phosphate (HMPF6) carbon paste electrode (CPE) for determining the antioxidant additive tertbutylhydroquinone (TBHQ) in the presence of folic acid. The FeNi3/rGO synthesized by hydrothermal strategy and characterized by X-ray diffraction (XRD) and FESEM methods. The electrochemical response were found to be linearly symmetrical to TBHQ and folic acid concentrations in the range from 0.05 to 900 μM and 0.6–1100 μM with detection limits of 10.0 nM and 0.1 μM, respectively. The FeNi3/rGO/HMPF6/CPE showed high-quality for determination of TBHQ in the presence of folic acid in the real samples with the separation potential ∼600 mV.

Novel ligand functionalized composite material for efficient copper(II) capturing from wastewater sample

Abstract: Copper (Cu(II)) is a very toxic heavy metal that even at low concentration can affect living organisms. Therefore, designing effective materials with high selectivity and cost-efficiency is essential for the control capturing of toxic Cu(II) ions. This study was developed a ligand based composite material for simultaneous Cu(II) detection and removal from wastewater samples. The composite material was fabricated based on the ligand anchoring onto the mesoporous silica by direct coating approach. The application of Cu(II) detection and adsorption was measured at neutral pH region with exhibition of significant color visualization. The experiment conditions were optimized based on contact time, solution acidity, initial Cu(II) concentration and pH value and diverse metal salt concentrations. The results were revealed that the composite material was not affected with the existing foreign ions and the signal intensity was observed only toward the Cu(II) ion. The composite material was able to detected the low level Cu(II) ion as the detection limit was 0.25 μg/L and the adsorption of highest removal capacity was 171.33 mg/g. In addition, the diverse ions were not reduced the Cu(II) ion adsorption significantly, and the composite material has approximately no adsorption capacity for other ions at this pH. The elution of Cu(II) ions from the saturated composite material was desorbed successfully with 0.20 M HCl. The regenerated adsorbent that remained maintained the high selectivity to Cu(II) ions and exhibited almost the same sorption capacity as that of the original adsorbent. However, the sorption efficiency slightly decreased after ten cycles. Therefore, the proposed material offered a cost-effective material and may be considered a viable alternative for effectively toxic Cu(II) ion capturing from water samples without the need for sophisticated instrument.

Multifunctional application of carbon fiber reinforced polymer composites: Electrical properties of the reinforcing carbon fibers – A short review

Abstract: In most areas where weight reduction is crucial, carbon fiber reinforced composites (CFRPs) are an excellent choice. Carbon fiber, besides its structural role, can be applied for several secondary functions as well, based on its electrical properties; for example, it can be used for crosslinking, welding, as a sensor and it can also facilitate self-healing. By merging these functions, a multifunctional part or structure can be created. In this article, we review multifunctional application examples of reinforcing carbon fiber. The focus is on utilization: which additional function and what physical layout can be used with CFRP. In a summarizing table (Table 1), we classified the presented examples according to their secondary function, the material used and the physical layout. With the combination of different functions, important materials can be created for the energy and transportation industry, for autonomous vehicles and for Industry 4.0.

Nanocomposite sponges of sodium alginate/graphene oxide/polyvinyl alcohol as potential wound dressing: In vitro and in vivo evaluation

Abstract: Wound dressings with high flexibility, mechanical strength, and porosity have received increasing attention. Herein, nanocomposite sponges of sodium alginate/graphene oxide (GO)/polyvinyl alcohol were prepared by a freeze-thawing cyclic process and freeze-dried molding. The properties of the sponges were investigated including morphology, mechanical properties, swelling, water vapor permeability, in vitro norfloxacin (NFX) release, antibacterial property, and biocompatibility as well as in vivo wound healing in a mouse model. A GO concentration of 1 wt% resulted in the sponges with a homogeneously porous and interconnected network structure, which exhibited suitable water absorption, breathability, and mechanical properties. Hemolysis tests indicated that the prepared sponges were non-hemolytic material. The CCK-8 assay demonstrated that the presence of an appropriate amount of GO could promote the cell proliferation. The sponges displayed excellent bioavailability of NFX, with a sustained release behavior and a strong inhibitory effect against both Escherichia coli and Staphylococcus aureus. The in vivo evaluations demonstrated that the sponges enhanced wound healing. Therefore, these novel nanocomposite sponges are promising candidates for wound healing applications.

Improving the 3D printability of high volume fly ash mixtures via the use of nano attapulgite clay

Abstract: The effect of nano-attapulgite clay (NC) addition on the fresh, hardened and microstructural properties of high volume fly ash (HVFA) mixes designed for 3D printing was investigated in this study. Experimental results showed that the addition of 0.1–0.5% (i.e. by mass of binder) NC increased the static yield stress of HVFA mortars without significantly affecting the apparent viscosity, due to particle re-flocculation and enhanced thixotropy. When compared to the control mix, the use of 0.5% NC also led to increased viscosity recovery and improved structural build-up at different resting times and shear rates. These advancements were useful for 3D concrete printing, during which the material should be extrudable and buildable layer-by-layer without any deformation. Successful implementation of mixes containing NC was demonstrated via the printing of a 20-layer structure. Further optimization of the NC content and associated HVFA mix design is recommended, depending on the geometrical and mechanical properties required.

Eco-friendly synthesis of Nd2Sn2O7–based nanostructure materials using grape juice as green fuel as photocatalyst for the degradation of erythrosine

Abstract: Highly photocatalytic active Nd2Sn2O7 nanostructures have been prepared by a novel, facile and eco-friendly approach with employing grape extract as green fuel, for the first time. The calcination temperature has been optimized to achieve the pure product with the best shape and grain size via an eco-friendly approach. FESEM, BET, XRD, DRS, FT-IR, TEM, PL and EDS have been applied to determine purity and investigate properties of the produced Nd2Sn2O7 nanostructures. It was found that utilization of grape juice at 500 °C could be favorable to produce homogeneous Nd2Sn2O7 nanostructures with the sphere-like shape and high purity. Nd2Sn2O7 structures have been applied for photocatalytic destruction of erythrosine contaminant under UV illumination and their photocatalytic efficiency has been compared. It was found that Nd2Sn2O7 nanostructures produced with applying grape juice as green and novel fuel at 500 °C, could display superior photocatalytic performance towards erythrosine destruction. By applying them as photocatalyst, 90% of erythrosine contaminant was destructed. Besides, Nd2Sn2O7-Nd2O3 nanostructure has been produced with employing grape juice as novel and green fuel to evaluate the possibility of coupling Nd2O3 into Nd2Sn2O7. Results demonstrated that coupling Nd2O3 into Nd2Sn2O7 could improve the photocatalytic efficiency in erythrosine destruction to 96%.

Recent advances in novel metallic honeycomb structure

Abstract: Innovative honeycomb-based structures have been paid substantial attention in recent years due to their superb mechanical performances and specific functions. The presented study made acomprehensive overview on the development of the innovative honeycomb-based structures in the past two decades, including filled-type, embedded, tandem, hierarchical, auxetics with Negative Poisson's Ratio (NPR), etc. Mechanical performances of these structures were commented with advantages and disadvantages, related on their geometric configurations, mechanical performance and dynamic response, respectively. The challenges as well as future directions were also analyzed. The achievements provide significant guidelines in designing of new generation light-weight honeycomb-based structures.

Cleaning the arsenic(V) contaminated water for safe-guarding the public health using novel composite material

Abstract: With increasing arsenic contamination scenes reported around the globe, the highly promising materials are important to remove the arsenic from industrial and specified contaminated for safe-guarding the public health. In this study, we have prepared ligand anchoring inorganic-organic frameworks based well-ordered, high surface area composite materials for selective arsenic(V) (As(V)) removal from contaminated water. Adsorption studies were evaluated in batch mode with the function of pH, time, initial As(V) concentration, counter-ions and reuses by suitable eluent. Before starting the As(V) adsorption, the composite materials were protonated using 2.0 M HCl. The materials exhibited high adsorption efficiency in low pH area, however; the neutral pH region (7.0) was selected to avoid the divalent anions effect as the monovalent arsenate species were dominated. In the neutral pH region, the basicity of divalent HAsO42− was much greater than the divalent SO42− and the divalent HAsO42− was easily made the strong hydrogen bonding with the protonated composite materials surface. Then the effects of monovalent and divalent anions were not affected in the selective As(V) adsorption by the composite materials. In addition, the other cations did not affect the As(V) adsorption as the data clarified. The composite material was highly matched to the Langmuir model with the monolayer coverage. In addition, the determined maximum adsorption capacity was 142.25 mg/g. The As(V) was eluted from the composite materials with 2.0 M HCl, and the regenerated to the original pattern. The regenerated materials were exhibited the same functionality and used successively in several adsorption-elution-reuses cycles. Therefore, the newly fabricated composite material is an interesting, efficient and economical sustainable composite material and could be used selective As(V) removal from real contaminated water.

A review of coir fiber reinforced polymer composites

Abstract: Natural fiber reinforced polymer composites play important roles in the production of eco-friendly materials because of their high modulus, meticulous strength and reduced carbon footprint on the environment. Coconut (cocos nucifera) is cultivated extensively in tropical countries for its fruits whereas the husks and shells are mostly disposed as waste. These portions of the coconut plant serve as potential resource for natural fibers which are used to reinforce polymer composites. This work gives an overview of current knowledge of coir fiber and coir fiber reinforced polymer composites. The preparation and production of different matrices reinforced with coir fibers and the mechanical structural and thermal properties of these composites have been studied by several researchers and are elucidated in this review. The major treatment techniques used to modify coir fibers were also discussed.

Molecular dynamics modeling of the structure, dynamics, energetics and mechanical properties of cement-polymer nanocomposite

Abstract: Efforts to tune the performance of organic/inorganic composites are hindered owing to a lack of knowledge related to the interfacial interaction mechanisms. Here we investigated the interfacial structure, dynamics, energetics and mechanical properties between calcium silicate hydrates (C-S-H) and polymers by molecular dynamics (MD) simulation. In this work, polyethylene glycol (PEG), polyvinyl alcohol (PVA) and polyacrylic acid (PAA) are intercalated into nanometer channel of C-S-H sheets to construct the model of polymer/C-S-H composite. In the interfacial region, the calcium ions near the surface of C-S-H play mediating role in bridging the functional groups in the polymers and oxygen in the silicate chains by forming Os-Ca-Op bond. In addition to ionic bonding, the bridging oxygen (C-O-C) in the PEG, hydroxyl (C-OH) in the PVA and carboxyl groups (-COOH) in the PAA provide plenty oxygen sites to form H-bonds with silicate hydroxyl, interlayer water and calcium hydroxyl in C-S-H substrate. The interfacial binding energy is dependent on polarity of functional groups in the polymers, the stability of the H-bond and Ca-O bond, ranking in the following order: E(PAA)> E(PVA) > E(PEG). The PVA with small number of H-bonds formed between oxygen in PVA and water molecules, resulting in increasing the mobility of confined water in the interlayer region. On the other hand, PAA and PVA, with strong polarity, can provide more number of non-bridging oxygen sites that widely distributed along the polymer chains to associate with more calcium ions and H-bonds. Furthermore, uniaxial tensile test is utilized to study the mechanical behavior of the composites. The incorporation of polymers, strengthening the H-bonds in the interfacial region and healing the defective silicate chains, can inhibit the crack growth during the loading process, which both enhance the cohesive strength and ductility of the C-S-H gel. In particular, the intercalated PAA increases the Young's modulus, tensile strength and fracture strain of C-S-H gel to 22.27%, 19.2% and 66.7%, respectively. The toughening mechanism in this organic/inorganic system can provide useful guidelines for polymer selection, design, and fabrication of C-S-H/polymer nanocomposites, and help eliminate the brittleness of cement-based materials from the genetic level.

Eco-friendly polymer composites for green packaging: Future vision and challenges

Abstract: At present, renewable and biodegradable biocomposites materials have drawn much attention as promising green materials in different domains of application such as intelligent food packaging, biomedical and drug delivery, bio-membranes, automotive, as well as in industrial composting applications. The current review deals with the advances in preparation methods and technical applications of these biocomposites. Different biomass materials obtained from renewable resources such as coffee grounds (CG), nanocellulose and date stones are developed to be used as smart reinforcing agents in biodegradable biopolymers for improving their overall properties. Conversely, some drawbacks are associated with the use of lignocellulosic materials as reinforcing agents, especially their high humidity absorption, poor wettability, and incompatibility with most biopolymers. Thus, novel processing techniques and different aspects are proposed in this review to produce high performance lignocellulosic reinforced materials with better properties. Facial and green modification of organoclay (OC) by antibacterial natural rosin and stearic acid to obtain toxicity-free expanded OC is also discussed. Green modification using OC can also be used as compatibilizing and reinforcing material for different incompatible biopolymers such as chitosan, carboxy methyl cellulose (CMC) and polylactic acid (PLA). Ultimately, the future vision on the challenges and the environmental issues towards CO2 emission which is associated to the risk assessment of these bionanomaterials are also discussed.

Metal organic framework (ZIF-67)-derived hollow CoS2/N-doped carbon nanotube composites for extraordinary electromagnetic wave absorption

Abstract: The hierarchical hollow framework involving interconnected highly conductive N-doped carbon nanotube networks and CoS2 particles were successfully prepared by metal-organic framework (MOF) derived method. After the two pyrolysis process in the atmosphere of reducing gas and inert gases, numerous carbon nanotubes interlaced on the surface of framework and CoS2 nanoparticles also attached on the surface. The electromagnetic parameters of CoS2/NCNTs composites can be well controlled by regulating the loadings of sample in sample-paraffin mixture. The results demonstrate that CoS2/NCNTs with 50% loadings show superior electromagnetic wave absorption properties in the wide frequency range, almost covering the whole X bands (8–12 GHz) only with a relative thin thickness of 1.6 mm. Hierarchical hollow structure and better impedance matching performance between N-doped carbon nanocube and CoS2 nanoparticles contribute to the enhancement of microwave absorption ability. Our work confirms that hollow framework CoS2/NCNTs composites can provide a novel idea for designing high-absorbability microwave absorbers.

Reinforcing carbon fiber epoxy composites with triazine derivatives functionalized graphene oxide modified sizing agent

Abstract: To improve the dispersion of graphene oxide (GO) nanosheets in sizing agent and to enhance the interfacial adhesion between GO and epoxy, GO nanosheets were chemically modified with cyanuric chloride (TCT) and diethylenetriamine (DETA). The functionalized GO (i.e. GO-TCT-DETA) with different contents was introduced into the carbon fiber (CF) interface by sizing process. The uniform distribution of GO sheets on CF surface and the enhancement of surface roughness were obtained. Moreover, significant enhancements (i.e., 104.2%, 100.2%, and 78.3%) of interfacial shear strength (IFSS), interlaminar shear strength (ILSS), and flexural properties were achieved in the composites with only 1.0 wt% GO-TCT-DETA sheets introduced in the fiber sizing. The GO-TCT-DETA in the interface region enhanced the stress being transferred effectively and the local stress concentrations being relieved. This study indicates that the utilization of functionalized GO is one of the alternative approaches for controlling the fiber-matrix interface and improving the mechanical properties of CF epoxy composites.

Significant improvement of thermal conductivities for BNNS/PVA composite films via electrospinning followed by hot-pressing technology

Abstract: Research and development on polymer composites with high thermal conductivities and ideal thermal stabilities have become one of the hot topics in functional composites. In this work, boron nitride nanosheets/polyvinyl alcohol (BNNS/PVA) thermally conductive composite films were respectively prepared from ball milling (BNNS/PVA-I), directional freeze-drying (BNNS/PVA-II) and electrospinning technology (BNNS/PVA-III), followed by hot-pressing method. BNNS/PVA-III thermally conductive composite films had better comprehensive properties than that of BNNS/PVA-(I-II) composite films. Thermal conductivity coefficient in vertical direction (λ⊥) of the BNNS/PVA-III composite films with 30 wt% BNNS was 1.031 W/mK, obviously higher than that of BNNS/PVA-I (λ⊥ of 0.573 W/mK) and BNNS/PVA-II (λ⊥ of 0.694 W/mK), respectively. And the λ in parallel direction (λ∥) was significantly enhanced to 18.630 W/mK, increased by 3101.0% and 1437.1% compared to that of BNNS/PVA-I (λ∥ of 0.582 W/mK) and BNNS/PVA-II (λ∥ of 1.212 W/mK), respectively. Meantime, the glass transition temperature (Tg) and heat-resistance index (THRI) of the BNNS/PVA-III thermally conductive composite films with 30 wt% BNNS was 42.7 °C and 144.5 °C, 7.0 °C and 1.9 °C higher than that of pure PVA matrix, respectively.

Printability region for 3D concrete printing using slump and slump flow test

Abstract: Rheological studies are important for successful 3D concrete printing. The main challenge for successful 3D concrete printing is the complex characteristic the materials should possess. It should be flowable enough to be pumped and extruded through the hose, as well as gaining sufficient strength and stiffness for buildability after the layer by layer deposition. Existing literature has various mixtures proposed for successful 3D concrete printing. Most of these studies used rheometers to measure the dynamic yield stress and plastic viscosity. As the measurement with rheometer is sensitive to the protocols and is controlled by the rheologists, as well as data processing if non-standardized measuring geometries are used, results could vary significantly. This study used standardized field-friendly protocols to measure the slump and slump-flow of the mortars. The pumpability and buildability are evaluated in terms of the pumpability index and maximum height printed before collapsing. These result together with the slump and slump-flow values are used to define the printable region.

Investigation of mechanical properties and shrinkage of ultra-high performance concrete: Influence of steel fiber content and shape

Abstract: Use of steel fibers in ultra-high performance concrete (UHPC) plays a significant role in enhancing strength and toughness and restraining shrinkage. This paper investigates the effect of steel fiber content and shape on mechanical strength, toughness, and autogenous and drying shrinkage of UHPC. Three steel fiber shapes, including straight, corrugated, and hooked fibers, with volume fraction ranging from 0 to 3% were employed. Compressive, flexural, and fiber-matrix bond strengths were evaluated. A statistical quadratic model and the Composite Theory were employed to predict the flexural strength of UHPC. Test results indicated that the increase in fiber volume can enhance the compressive and flexural strengths of UHPC and reduce shrinkage. The optimum fiber content for strength and shrinkage was found at 2%, beyond which the strength was slightly increased and the shrinkage was slightly decreased. For a given fiber content, the use of hooked fibers was most efficient in improving fiber-matrix bond and flexural strengths and reducing shrinkage. The flexural strengths of UHPC made with various fiber contents and shapes can be predicted using the proposed quadratic model and the Composite Theory. The latter considers the primary parameters affecting performance, including bond strength, matrix properties, and fiber characteristics. Finally, several models were used to simulate autogenous shrinkage behavior of UHPC and optimal models were found.

Mechanical properties and deformation behaviour of early age concrete in the context of digital construction

Abstract: Digital construction is gradually opening unlimited possibilities for building and concrete industry. The key secret for a robust print process lies in our understanding of the processing technology and material fresh properties, in addition to developing novel measurement and control techniques. This paper aims to gain a better understanding of early age mechanical properties of 3D printable materials and improve it for the requirement of large scale concrete printing. Experimental investigations were carried out to measure green strength and stiffness of fresh fly ash-cement mortar with applied 3D optical metrology. The compressive green strength was linked with material yield strength evolution and later, modified with nanoclay for higher buildability properties. Nanoclay addition deceased the layer deformation due to significant increase in Young's modulus and to estimate this uncontrolled deformation, a mathematical function was formulated, which subsequently validated by comparison to printing experiments.

Preparation of magnetically retrievable CoFe2O4@SiO2@Dy2Ce2O7 nanocomposites as novel photocatalyst for highly efficient degradation of organic contaminants

Abstract: CoFe2O4@SiO2@Dy2Ce2O7 magnetic nanocomposite as recyclable photocatalyst has been prepared for the first time. The cobalt ferrite component was produced through combustion route with the aid of grape juice as novel and green fuel, and the silica component was prepared through sol-gel route with the aid of 2,2-dimethyl-1,3-propanediamine as new basic agent as well as Dy2Ce2O7 component has been synthesized through a modified Pechini route with the aid of 2,2-dimethyl-1,3-propanediamine as novel pH regulator. The as-fabricated CoFe2O4@SiO2@Dy2Ce2O7 nanostructures were characterized by FESEM, DRS, TEM, EDX, XRD, VSM and BET. This is the first try on the survey of photocatalytic efficiency of CoFe2O4@SiO2@Dy2Ce2O7 nanostructures. Role of kind of irradiation source, catalyst loading and kind of contaminant has been described upon improving performance of CoFe2O4@SiO2@Dy2Ce2O7 nanostructures. The results indicated that the fabricated CoFe2O4@SiO2@Dy2Ce2O7 could be potentially utilized as efficient and favorable kind of recyclable photocatalyst for removal of water contaminants.

A method to predict the ultimate tensile strength of 3D printing polylactic acid (PLA) materials with different printing orientations

Abstract: 3D Printing is widely used in scientific researches and engineering applications, ranging from aerospace to biomedicine. However little is known about the mechanical properties of 3D printing materials. In order to promote the mechanical analysis and design of 3D printing structures, the ultimate tensile strength of FDM PLA materials with different printing angles were studied theoretically and experimentally. A theoretical model was firstly established to predict the ultimate tensile strength of FDM PLA materials based on transverse isotropic hypothesis, classical lamination theory and Hill-Tsai anisotropic yield criterion, and then verified by tensile experiments. Compared with previous models, this model provided two kinds of in-plane shear modulus calculation methods, so the calculation results were more reliable. The specimens, designed according to the current plastic-multipurpose test specimens standard ISO 527-2-2012, were printed in seven different angles ( 0 ∘ , 15 ∘ , 30 ∘ , 45 ∘ , 60 ∘ , 75 ∘ , 90 ∘ ) with three layer thicknesses (0.1 mm, 0.2 mm, 0.3 mm) for each angle. The relative residual sum of squares between theoretical data and experimental data were all close to zero, so the results that the theoretical model can accurately predict the ultimate tensile strength of FDM materials for all angles and thicknesses were confirmed. It was also found that the ultimate tensile strength decreased as the printing angle becomes smaller or the layer becomes thicker. This theoretical model and experimental method can also be applied to other 3D printing materials fabricated by FDM or SLA techniques.

Facile synthesis of ellipsoid-like MgCo2O4/Co3O4 composites for strong wideband microwave absorption application

Abstract: As the high-speed advance of electronics industry, the facile synthesis of electromagnetic wave (EMW) absorbing materials with strong absorption capability, wide bandwidth, and thin thickness are urgently needed but remain challenging. Herein, hierarchical ellipsoid-like MgCo2O4/Co3O4 composites have been effectively synthesized by a facile hydrothermal method followed by calcination process. Meanwhile, by tuning the calcination temperature not only the morphology but also the crystalline structure (CoCO3+MgCo2(OH)6 → MgCo2O4+Co3O4) of the samples were attained. Especially, profiting from the distinctive anisotropic structure and the synergistic effect of dielectric and magnetic loss as well as good impedance matching, the hierarchical MgCo2O4/Co3O4 composites calcined at 450 °C exhibited excellent EMW absorption property. The minimum reflection loss (RLmin) of −48.54 dB at 12.96 GHz with the thickness of 2.6 mm as well as the broad effective bandwidth (RL

Electrostatic self-assembly synthesis of ZnFe2O4 quantum dots (ZnFe2O4@C) and electromagnetic microwave absorption

Abstract: Making effective usage of quantum size effect is a foregrounded strategy to design and fabricate excellent electromagnetic microwave absorption materials. In this research, ZnFe2O4 quantum dots were coated by hybrid amorphous carbon to form a sea islands structure by a facile electrostatic self-assembly synthetic technology. Simultaneously, the rejection of heterogeneous charges leads to the formation of quantum dots, by which the quantum size effects on dielectric and magnetic characteristic were investigated. Consequently, multiple hetero-interface and interfacial polarization was originated from polycrystalline feature of ZnFe2O4 with spinel and inverse spinel structures. In particular, the electromagnetic microwave absorption properties of ZnFe2O4 were greatly optimized, as the minimized reflection loss reached −40.68 dB at the frequency 11.44 GHz and thickness 2.5 mm, while the effective bandwidth corresponding was 3.66 GHz (from 9.87 to 13.52 GHz). The largest effective bandwidth was 4.16 GHz (from 8.08 to 12.24 GHz) with a thickness of 3 mm. It is suggested that high performance of microwave absorption of ZnFe2O4 quantum dots was well guided by the optimized impedance matching and attenuation constants.

Mesoporous carbon hollow microspheres with tunable pore size and shell thickness as efficient electromagnetic wave absorbers

Abstract: Carbon hollow microspheres with a uniform mesoporous shell (PCHMs) were designed and fabricated using a template-assistant method followed by a pyrolysis-etching process. Through tuning the pyrolysis temperature, PCHMs with various pore size and shell thickness can be obtained. In particular, the PCHMs carbonized at 650 °C (PCHMs-650) are composed of a mesoporous shell (thickness: 52 nm) and an interior void of 153 nm, endowing the materials with large surface area of 925.9 m2/g. The unique core-shell structure generated by carbon shell and void core is critical for the attenuation capability of EM energy. The composites containing 20 wt% PCHMs-650 exhibit favorable microwave absorbing performance with the minimum reflection loss (RLmin) of −39.4 dB at 3.6 mm. The broadest effective absorption bandwidth (EAB) can extend to 5.28 GHz (9.68–14.96 GHz) at only 2.6 mm. It is believed that PCHMs can be used as a promising absorber with lightweight, impressive bandwidth and strong absorption efficiency.

Comparison of physical and mechanical properties of PLA, ABS and nylon 6 fabricated using fused deposition modeling and injection molding

Abstract: The objective of this study was to compare the physical and mechanical performance of poly(lactic acid) (PLA), acrylonitrile butadiene styrene (ABS), and nylon 6 fabricated using fused deposition modeling (FDM) and conventional injection molding. It is found that different processing methods did not affect the viscosity of the samples, and the percentage difference for the density measurement is less than 4%. Water absorption of FDM samples is approximately 108% higher compared to those fabricated using the injection molding. The results also revealed that the FDM method did not strongly affect the degree of crystallinity of ABS, but it increased the degree of crystallinity of PLA and nylon 6. The tensile strength, Young's modulus, elongation at break, and impact strength of FDM samples were approximately 48%, 50%, 48%, and 78%, lower compared with the injection molded samples. The results presented can provide a guide to manufacturing the final products using FDM with the desired performance.