Showing papers in "Journal of Materials Science in 2016"

High-performance fiber-reinforced concrete: a review

Abstract: In recent years, an emerging technology termed, “High-Performance Fiber-Reinforced Concrete (HPFRC)” has become popular in the construction industry. The materials used in HPFRC depend on the desired characteristics and the availability of suitable local economic alternative materials. Concrete is a common building material, generally weak in tension, often ridden with cracks due to plastic and drying shrinkage. The introduction of short discrete fibers into the concrete can be used to counteract and prevent the propagation of cracks. Despite an increase in interest to use HPFRC in concrete structures, some doubts still remain regarding the effect of fibers on the properties of concrete. This paper presents the most comprehensive review to date on the mechanical, physical, and durability-related features of concrete. Specifically, this literature review aims to provide a comprehensive review of the mechanism of crack formation and propagation, compressive strength, modulus of elasticity, stress–strain behavior, tensile strength (TS), flexural strength, drying shrinkage, creep, electrical resistance, and chloride migration resistance of HPFRC. In general, the addition of fibers in high-performance concrete has been proven to improve the mechanical properties of concrete, particularly the TS, flexural strength, and ductility performance. Furthermore, incorporation of fibers in concrete results in reductions in the shrinkage and creep deformations of concrete. However, it has been shown that fibers may also have negative effects on some properties of concrete, such as the workability, which get reduced with the addition of steel fibers. The addition of fibers, particularly steel fibers, due to their conductivity leads to a significant reduction in the electrical resistivity of the concrete, and it also results in some reduction in the chloride penetration resistance of the concrete.

Flexible electronics under strain: a review of mechanical characterization and durability enhancement strategies

Abstract: Flexible electronics incorporate all the functional attributes of conventional rigid electronics in formats that have been altered to survive mechanical deformations. Understanding the evolution of device performance during bending, stretching, or other mechanical cycling is, therefore, fundamental to research efforts in this area. Here, we review the various classes of flexible electronic devices (including power sources, sensors, circuits and individual components) and describe the basic principles of device mechanics. We then review techniques to characterize the deformation tolerance and durability of these flexible devices, and we catalogue and geometric designs that are intended to optimize electronic systems for maximum flexibility.

A review of carbon dots in biological applications

Abstract: Luminescent carbon-based nanomaterials have inspired tremendous research interests due to their tunable optical properties as well as superior biocompatibility. In this review, distinct light emission properties of carbon dots (CDs) derived from different synthesis methods are summarized. The optical properties of as-synthesized CDs can be further controlled by element doping and surface functionalization of CDs for tunable band gap. Due to their low cytotoxicity and tunable optical behaviors, luminescent CDs have been extensively studied for their potential biomedical applications, such as analytical sensors, and bioimaging devices. This review presents a comprehensive overview of the emerging luminescent CDs and their applications as biosensors and bioimaging agents. The challenges and perspectives in the near future are also discussed.

A review of hydrogel-based composites for biomedical applications: enhancement of hydrogel properties by addition of rigid inorganic fillers

Abstract: There is a growing demand for three-dimensional scaffolds for expanding applications in regenerative medicine, tissue engineering, and cell culture techniques. The material requirements for such three-dimensional structures are as diverse as the applications themselves. A wide range of materials have been investigated in the recent decades in order to tackle these requirements and to stimulate the anticipated biological response. Among the most promising class of materials are inorganic/organic hydrogel composites for regenerative medicine. The generation of synergetic effects by hydrogel composite systems enables the design of materials with superior properties including biological performance, stiffness, and degradation behavior in vitro and in vivo. Here, we review the most important organic and inorganic materials used to fabricate hydrogel composites. We highlight the advantages of combining different materials with respect to their use for biofabrication and cell encapsulation as well as their application as injectable materials for tissue enhancement and regeneration.

A review of exfoliated graphite

Abstract: Exfoliated graphite (EG) refers to graphite that has a degree of separation of a substantial portion of the carbon layers in the graphite. Graphite nanoplatelet (GNP) is commonly prepared by mechanical agitation of EG. The EG exhibits clinginess, due to its cellular structure, but GNP does not. The clinginess allows the formation of EG compacts and flexible graphite sheet without a binder. The exfoliation typically involves intercalation, followed by heating. Upon heating, the intercalate vaporizes and/or decomposes into smaller molecules, thus causing expansion and cell formation. The sliding of the carbon layers relative to one another enables the cell wall to stretch. The exfoliation process is accompanied by intercalate desorption, so that only a small portion of the intercalate remains after exfoliation. The most widely used intercalate is sulfuric acid. The higher concentration of residue in unwashed EG causes the relative dielectric constant (50 Hz) of the EG to be 360 (higher than 120 for KOH-activated GNP), compared to the value of 38 for the water-washed case. An EG compact is obtained by the compression of EG at a pressure lower than that used for the fabrication of flexible graphite. Compared to flexible graphite, EG compacts are mechanically weak, but they exhibit viscous character, out-of-plane electrical/thermal conductivity and liquid permeability. The viscous character (flexural loss tangent up to 35 for the solid part of the compact) stems from the sliding of the carbon layers relative to one another, with the ease of the sliding enhanced by the exfoliation process.

Carbonation behavior of hydraulic and non-hydraulic calcium silicates: potential of utilizing low-lime calcium silicates in cement-based materials

Abstract: This paper presents a study on the carbonation behaviors of hydraulic and non-hydraulic calcium silicate phases, including tricalcium silicate (3CaO·SiO2 or C3S), γ-dicalcium silicate (γ-2CaO·SiO2 or γ-C2S), β-dicalcium silicate (β-2CaO·SiO2 or β-C2S), rankinite (3CaO·2SiO2 or C3S2), and wollastonite (CaO·SiO2 or CS). These calcium silicate phases were subjected to carbonation reaction at different CO2 concentration and temperatures. Thermogravimetric analysis (TGA) tests were performed to quantify the amounts of carbonates formed during the carbonation reactions, which were eventually used to monitor the degree of reactions of the calcium silicate phases. Both hydraulic and non-hydraulic calcium silicates demonstrated higher reaction rate in case of carbonation reaction than that of the hydration reaction. Under specific carbonation scenario, non-hydraulic low-lime calcium silicates such as γ-C2S, C3S2 and CS were found to achieve a reaction rate close to that of C3S. Fourier transformed infrared (FTIR) spectroscopy and scanning electron microscope (SEM) tests were performed to characterize the carbonation reaction products of the calcium silicate phases. The FTIR spectra during the early stage of carbonation reaction showed formation of calcium silicate hydrate (C–S–H) from C3S, γ-C2S, β-C2S, and C3S2 phases with a similar degree of polymerization as that of the C–S–H that forms during the hydration reaction of C3S. However, upon further exposure to CO2, these C–S–H phases decompose and eventually converted to calcium-modified (Ca-modified) silica gel phase with higher degree of silicate polymerization. Contradictorily, CS phase started forming Ca-modified silica gel phase even at the early stage of carbonation reaction. This paper also revealed the stoichiometry of the Ca-modified silica gel that formed during the carbonation reaction of the calcium silicate phases using the SEM/energy dispersive spectroscopy (EDS) and TGA results.

Viewpoint: metallurgical aspects of powder bed metal additive manufacturing

Abstract: Metal additive manufacturing has emerged as a new manufacturing option for aerospace and biomedical applications. The many challenges that surround this new manufacturing technology fall into several different categories. The paper addresses one of these categories, the physical mechanisms that control the additive manufacturing process. Physical mechanisms control the effects of processing parameters on microstructures and properties of additively manufactured parts. Some mechanisms might not have been recognized, yet, and for those that are currently known, detailed quantitative predictions have to be established. The physical mechanisms of metal additive manufacturing are firmly grounded in metallurgy, branching into laser physics and the physics of granular materials. Powder bed additive manufacturing is described from the powder storage to post-processing and elements of metallurgy are highlighted that are relevant for the different aspects of the additive manufacturing process. These elements include the surface reactions on powder particles, the heating and melting behavior of the powder bed, solidification, and post-processing. This overview of the different metallurgical aspects to additive manufacturing is intended to help guide research efforts and it will also serve as a snapshot of the current understanding of powder bed additive manufacturing.

Borate glasses for scientific and industrial applications: a review

Abstract: Research in borate glasses has started as a scientific curiosity and as an aid to explain the structure of oxide glasses in general. This effort led to a better understanding of the structure and unique properties of borate glasses. Although silicate and borosilicate glasses satisfy the vast majority of scientific and industrial needs, there are certain circumstances where they are not satisfactory. Furthermore, borate glasses offer certain advantages over silicate glasses which are not well known, neither well explored. However, certain characteristics of borate glasses such as their affinity to water requires that they are well selected, designed, or developed to satisfy the specifications of a given application. This review aims to explore and report some of the key properties of binary borate glasses. It also provides certain guidelines for several applications where the scientific literature is discussed in connection with industrial experience and technological needs.

Plating on acrylonitrile–butadiene–styrene (ABS) plastic: a review

Abstract: ABS is an engineering plastic that has butadiene part uniformly distributed over the acrylonitrile-styrene matrix. It possesses excellent toughness, good dimensional stability, easy processing ability, chemical resistance, and cheapness. However, it suffers from inherent shortcomings in terms of mechanical strength and vulnerability to environmental conditions. Furthermore, it is non-conducting and easily fretted. Plating on ABS can serve to enhance the strength and structural integrity as well as to improve durability and thermal resistance resulting in metallic properties on the ABS material. ABS is described as the most suitable candidate for plating because it is possible to deposit an adherent metal coating on it by only the use of chemical pretreatment process and without the use of any mechanical abrasion. This article aims to review the history of ABS plastics, properties of ABS, processes and mechanisms of plating, and studies of plating on ABS involving mainly eco-friendly methods of plating by discussing the literature published in recent years. The details of electroplating of ABS carried out in the authors’ laboratory are also presented.

Graphene-based flame retardants: a review

Abstract: Graphene and its derivatives are potential flame retardant materials with good flame retardant performance; in particular, graphene as an adjuvant in combination with inorganic nanomaterials may be a promising candidate of flame retardant. This review describes the flame retardant mechanism, the development trend, and the classification of graphene-based flame retardants. It points out that graphene has attracted intensive interests in the fields of electronics, energy, and information, due to its excellent properties such as high thermal conductivity, good electron transport ability, and large specific surface area. In the meantime, graphene can change the pyrolysis as well as the thermal conductivity, heat absorption, viscosity and dripping of polymer during the combustion process. In other words, graphene can improve the thermal stability of polymer and delay its ignition, and it can also inhibit fire from spreading and reduce heat release rate.

Graphene oxides for removal of heavy and precious metals from wastewater

Abstract: Iron, chromium, lead, europium, silver, copper, strontium, cesium, zinc and nickel are some of the most frequently found heavy metal ions in wastewater and can cause serious health problems. Hence, their removal is essential from the environmental point of view. Recent studies show that graphene oxide (GO) can efficiently remove heavy metal ions from wastewater. A great deal of effort has been made to enhance the waste removal performance of GO using a variety of techniques. The performance of GO as an adsorbent agent to remove various organic pollutants, radioactive wastes and dyes has already been reviewed by various authors. However, the capability of GO and its derivatives to remove heavy metal ions has not been reviewed in detail. This paper reviews the wastewater removal efficiency and sorption mechanism of GO, functionalized GO and their composites for five of the most extensively studied heavy metal ions: Cr(III), Cr(VI), Cu(II), Pb(II) and Au(III). The waste removal kinetics of the adsorbents and the condition for the maximum adsorption are analysed for each of these heavy metal ions.

Nanostructured Al and Cu alloys with superior strength and electrical conductivity

Abstract: Mechanical strength and electrical conductivity are the most important properties of conducting metallic materials used in electrical engineering. Today, there is a growing need in this field for innovative conductor materials with improved properties. Meanwhile, the main issue is that high electrical conductivity and high strength are usually mutually exclusive due to physical nature of these properties. Alloying of pure metals results in significant increase of their mechanical strength, whereas electrical conductivity dramatically drops due to the scattering of electrons at solutes and precipitates. Recent studies have shown that intelligent nanostructural design in Al, Cu, and their alloys can improve combination of high mechanical strength with enhanced electrical conductivity. It was demonstrated that mechanical strength and electrical conductivity of these materials are primarily controlled by their microstructure, of which grain size, morphology of second phases, and their distribution, as well as dislocation structure, are the most important parameters. Rapid development of the state-of-the-art methods for the microstructural characterization at nano- and atomic scale has allowed a deeper insight into microstructure–properties relationship. The approach of intelligent nanostructural design of Al and Cu alloys has even enabled to increase the material strength with simultaneous improvement of its electrical conductivity. In this case, recent works on nanostructuring alloys by severe plastic deformation are of special interest, which gives rise to fundamental questions dealing with new mechanisms of strength and electrical conductivity as well as innovation potential of practical application of nanostructured materials. These issues are considered and discussed in the present progress article.

Review on slip transmission criteria in experiments and crystal plasticity models

Abstract: A comprehensive overview is given of the literature on slip transmission criteria for grain boundaries in metals, with a focus on slip system and grain boundary orientation. Much of this extensive literature has been informed by experimental investigations. The use of geometric criteria in continuum crystal plasticity models is discussed. The theoretical framework of Gurtin (J Mech Phys Solids 56:640–662, 2008) is reviewed for the single slip case. This highlights the connections to slip transmission criteria from the literature that are not discussed in the work itself. Different geometric criteria are compared for the single slip case with regard to their prediction of slip transmission. Perspectives on additional criteria, investigated in experiments and used in computational simulations, are given.

Magnetic nanoparticles: material engineering and emerging applications in lithography and biomedicine

Abstract: We present an interdisciplinary overview of material engineering and emerging applications of iron oxide nanoparticles We discuss material engineering of nanoparticles in the broadest sense, emphasizing size and shape control, large-area self-assembly, composite/hybrid structures, and surface engineering This is followed by a discussion of several non-traditional, emerging applications of iron oxide nanoparticles, including nanoparticle lithography, magnetic particle imaging, magnetic guided drug delivery, and positive contrast agents for magnetic resonance imaging We conclude with a succinct discussion of the pharmacokinetics pathways of iron oxide nanoparticles in the human body -- an important and required practical consideration for any in vivo biomedical application, followed by a brief outlook of the field

Semiconducting Sn-doped β-Ga2O3 homoepitaxial layers grown by metal organic vapour-phase epitaxy

Abstract: Sn-doped β-Ga2O3 epitaxial layers have been grown on (100) β-Ga2O3 substrates by metal organic vapour-phase epitaxy. Triethylgallium (TEGa), molecular oxygen (O2) and tetraethyltin (TESn) were used as Ga, O and Sn precursors, respectively. Layers grown at optimized temperature and chamber pressure, i.e. 850 °C and 5 mbar, had flat surfaces with a rms roughness of about 600 pm. Structural analysis by transmission electron microscopy revealed that the main defects in the layers were stacking faults and twin lamella. The incoherent boundaries of these defects are supposed to act as compensation and scattering centres, limiting the carrier mobility. Sn was homogeneously incorporated with a flat profile throughout the whole layer at concentration levels ranging from 2 × 1017 to 3 × 1019 cm−3 proportionally to the used TESn flux. All layers were electrically conductive. However, an unambiguous Hall effect was measurable only for Sn concentrations higher than 1 × 1018 cm−3, resulting in electron concentrations from 5 × 1017 to 2 × 1018 cm−3 at room temperature. For increasing free carrier concentrations, the electron mobility showed the tendency to increase from 10 to 30 cm2/Vs. The maximum mobility of 41 cm2/Vs, measured in a sample with free carrier concentration of 1 × 1018 cm−3, represents the highest value reported for β-Ga2O3 layers grown by MOVPE so far.

Morphing in nature and beyond: a review of natural and synthetic shape-changing materials and mechanisms

Abstract: Shape-changing materials open an entirely new solution space for a wide range of disciplines: from architecture that responds to the environment and medical devices that unpack inside the body, to passive sensors and novel robotic actuators. While synthetic shape-changing materials are still in their infancy, studies of biological morphing materials have revealed key paradigms and features which underlie efficient natural shape-change. Here, we review some of these insights and how they have been, or may be, translated to artificial solutions. We focus on soft matter due to its prevalence in nature, compatibility with users and potential for novel design. Initially, we review examples of natural shape-changing materials—skeletal muscle, tendons and plant tissues—and compare with synthetic examples with similar methods of operation. Stimuli to motion are outlined in general principle, with examples of their use and potential in manufactured systems. Anisotropy is identified as a crucial element in directing shape-change to fulfil designed tasks, and some manufacturing routes to its achievement are highlighted. We conclude with potential directions for future work, including the simultaneous development of materials and manufacturing techniques and the hierarchical combination of effects at multiple length scales.

Review: grain boundary faceting–roughening phenomena

Abstract: Similar to free surfaces, the grain boundaries (GBs) in metals, semiconductors and insulators can contain flat (faceted) and curved (rough) portions. In the majority of cases, facets are parallel to the most densely packed planes of coincidence sites lattice formed by two lattices of abutting grains. Facets disappear with the increasing temperature (faceting–roughening transition) and the increasing angular distance from coincidence misorientation. The temperature of GB faceting–roughening transition T R decreases with the increasing inverse density of coincidence sites Σ. In case of fixed Σ, T R decreases with the decreasing density of coincidence sites in the GB plane. The intersection line (ridge) between facets or between facets and curved (rough) portions of surfaces can be of first order (two different tangents in the contact point) or of second order (common tangent, continuous transitions). The rough (curved) portions of GB can also form the first-order rough-to-rough ridges (with two tangents). GB facets control the transition from normal to abnormal grain growth and strongly influence the GB migration, diffusion, wetting, fracture and electrical conductivity.

Preface to the 50th anniversary issue of the Journal of Materials Science

Abstract: In his book ‘‘The Coming of Materials Science’’ [1], the late Professor Robert Wolfgang Cahn FRS documented the rise of Materials Science as a distinct discipline from its roots in solid-state physics, metallurgy, polymer chemistry, inorganic chemistry, mineralogy, glass, and ceramic technology. The development of the discipline was a process in which he was intimately involved from 1942 when he enrolled at Trinity College, Cambridge to read Metallurgy, to his death from leukemia in 2007. His technical contributions were remarkably diverse, spanning such areas as: defects and deformation mechanisms, crystal growth, recrystallization, intermetallic compounds, and metallic glasses. It was these broad interests that led him to become such a staunch advocate for Materials Science as a discipline. Another of his passions was scientific editing, and he is remembered as much for his editing of several major journals and books as he is for his scientific research work. In 1964, Robert Cahn was invited by the publishers Chapman and Hall to establish the Journal of Materials Science as the first broad-spectrum Materials Science journal. As Chairman of Editors, he recruited a group of Editors with complementary areas of expertise, gave them independent powers of decision on submitted papers, and encouraged them to be pro-active in soliciting papers from key researchers on novel topics. This helped to ensure that the journal fulfilled its mission of being a broad-spectrum inclusive periodical that brought together work on different classes of materials using a wide variety of different experimental and theoretical approaches. The first issue appeared in April of 1966, and the Journal grew rapidly over the next few years to become a major international forum for research in Materials Science. In Robert Cahn’s memoirs [2], he said that the years he devoted to creating the Journal of Materials Science represented his single most important editorial contribution to the field. Over the last five decades, there have been many changes to the Journal, to the community that it serves, and to the field as a whole. The publishers have undergone several re-organizations and changes of ownership, and are now part of the Springer Nature group. Specialist sister journals have been spun off in the areas of Materials in Electronics and Materials in Medicine, while others such as Interface Science have been incorporated into the main journal. The community has broadened to encompass more biologists, chemists, and physicists due to developments in, for example, tissue engineering, nanostructured catalysts, and functional oxides, respectively. There have also been dramatic changes in the synthesis, processing, characterization, and computational modeling techniques available to materials scientists, and these advances have had a revolutionary effect on research directions in the field. Throughout, the Editors have maintained an open and flexible definition of Materials Science so that the Journal remains true to Robert Cahn’s original vision of a broadspectrum publication. This has been helped by stability in the leadership of the Journal with just four Chairmen/ Editors-in-Chief over the first 50 years: Professors Robert Cahn, William Bonfield, Rees Rawlings, and since 2004, C. Barry Carter. In 2006, the 40th anniversary of the founding of the Journal was marked with a special issue (volume 41, issue 3) comprising reviews, original research articles, and commentaries written by current and former Editors. The & Mark Aindow m.aindow@uconn.edu

Optimizing the preparation parameters of GO and rGO for large-scale production

Abstract: Preparation of graphene from chemical reduction of graphene oxide (GO) is recognized as one of the most promising methods for large-scale and low-cost production of graphene-based materials. Hummers’ method (KMnO4, NaNO3, H2SO4) is the most common method used for preparing GO. Excluding NaNO3 and optimizing the acids ratios of H2SO4/H3PO4 in order to improve the efficiency of the oxidation process is the target of this study. The chemical reduction of GO involves highly toxic reducing agent (hydrazine) that are harmful to human health and environment, and complicated surface modification is often needed to avoid aggregation of the reduced GO (rGO) during reduction process. An innocuous, safe, and efficient reducing agent (ascorbic acid/L.AA) was used for GO reduction. The measurements of the resultant graphene confirm the efficient removal of the oxygen-containing groups in GO. The 2 h-rGO-L.AA shows a higher C/O ratio (6.07) than 24 h-rGO-Hy. Since L.AA has excellent antioxidant activity and is widely available, it is reasonable to consider L.AA as a green, effective, and low-cost deoxygenation agent for mass production of rGO.

Dye-sensitized solar cells based on nanocomposite of polyaniline/graphene quantum dots

Abstract: In this study, we demonstrate a new kind of Pt-free counter electrode for dye-sensitized solar cells (DSSCs). Polyaniline–graphene quantum dots (PANI–GQDs) nanocomposite, with the advantages of low cost and simple preparation, was prepared by in situ electrochemical polymerization of an aniline monomer in the presence of GQD and it shows good catalytic activity in promoting tri-iodide reduction. The fluorine-doped tin oxide (FTO) coated glass was immersed into the solution of the aniline and GQD during the polymerization of PANI. The PANI–GQD nanocomposite was in situ deposited onto the surface of FTO glass. Formation of PANI and PANI–GQD films was confirmed by FE-SEM, TEM, XRD, and FT-IR analysis. The DSSC composed of the PANI–GQD nanocomposite electrode exhibits an energy conversion efficiency of 1.6 %. The presence of the synergistic effect of PANI and GQD led to the higher electrochemical catalytic activity of PANI–GQD nanocomposite than that of pristine PANI. As a result, better photovoltaic performance was observed for DSSCs based on the PANI–GQD electrode as compared to that of the DSSC sample based on the PANI electrode.

Synergistic effect of expandable graphite and intumescent flame retardants on the flame retardancy and thermal stability of polypropylene

Abstract: In this article, the investigation mainly focuses on the synergistic effect of expandable graphite (EG) and intumescent flame retardants in improving the flame retardancy of polypropylene (PP). The ratio of melamine polyphosphate (MPP) and dipentaerythritol (DPER) has been studied by limiting oxygen index (LOI) and vertical burning test (UL-94) tests. The results demonstrate that the optimal ratio of MPP and DPER in flame-retarding PP was 3/1. Except that, the incorporation of EG into the PP/MPP/DPER system can greatly improve the flame-retardant properties of PP materials. When the content of EG is 10 wt%, LOI value of PP composites reaches 33.2 % and obtains V-0 rating in UL-94 tests. Furthermore, Fourier transform infrared spectra (FTIR) indicate that the main char-formation process of this system occurs at 350–400 °C. And char residue for PP composites after combustion was systematically analyzed by FTIR and X-ray photoelectron spectroscopy spectra. Based on these facts, a potential condensed flame-retardant mechanism was primarily proposed.

Molecular dynamics simulations of the structure and mechanical properties of silica glass using ReaxFF

Abstract: Assessment of the empirical reactive force field ReaxFF to predict the formation of amorphous silica from its crystalline structure and the determination of mechanical properties under tension using molecular dynamics simulations is presented. Detailed procedures for preparing amorphous silica from crystalline silica are presented and the atomic structure is in good agreement with experimental results. Tensile properties of silica are predicted over a wide range of strain rates (2.3 × 108 s−1–1.0 × 1015 s−1) allowing comparison with results reported in the literature for other force fields. Quasi-static modulus obtained from power-law fitting of the low-stain rate modulus predicted by ReaxFF is in good agreement with experimental results. A transition strain rate of approximately $$ 2.5 \times 10^{11} {\text{s}}^{ - 1} $$ is identified where modulus increases rapidly to a plateau level. Tensile strength also increases significantly in this range of strain rate and plateaus at the theoretical upper bound for silica. A detailed study is presented to understand the mechanisms associated with strain rate effects on the overall stress–strain response of silica. Bond breakage which evolves into void growth leading to failure is predicted to occur at approximately 27 % strain for all strain rates. Stress relaxation simulations indicates that the transition strain rate occurs when the characteristic time for high-strain rate loading and stress relaxation times are the same order. The effects of cooling rate and temperature on the structure and the stress–strain response of the silica glass are also investigated. Low-cooling rate and low-cooling temperature enhance the properties of silica.

Multi-walled carbon nanotubes functionalized with a ultrahigh fraction of carboxyl and hydroxyl groups by ultrasound-assisted oxidation

Abstract: In this study, multifunctionalized multi-walled carbon nanotubes (MWCNTs) were functionalized through a novel three-step oxidation approach assisted by ultrasonication. We chose an oxidation system with three oxidation agents, concentrated sulfuric acid, potassium permanganate, and hydrogen peroxide. In comparison to non-ultrasonic processes, the effect of our ultrasound-assisted process on the carboxyl and hydroxyl content of the modified MWCNTs (m-MWCNTs-2) was discussed using Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy. The results verified that the ultrasound-assisted oxidation process is much more effective than those without ultrasonication; a higher hydroxyl content of 11.88 at.% and a carboxyl content of 8.82 at.% in the m-MWCNTs-2 were estimated. A Raman spectrum showed an increased value of I D/I G, which proved the presence of defect sites or the functional groups on the surface of the MWCNTs. Thermogravimetric analysis was found to semiquantitatively evaluate the amount of carboxyl and hydroxyl groups in the MWCNTs. A better dispersion of the m-MWCNTs-2 in ethanol or water was expected from a photograph, a particle size analysis, and field emission scanning electron microscopy. High-resolution transmission electron microscopy revealed that the diameter of the m-MWCNTs-2 was reduced to a lesser degree using the ultrasonication assistance than of those generated without ultrasonication.

Review: achieving superplastic properties in ultrafine-grained materials at high temperatures

Abstract: The mechanisms of superplasticity occurring in conventional materials, having grains sizes of the order of a few microns, are now understood reasonably well. However, very recent advances in the processing of ultrafine-grained (UFG) metals have provided an opportunity to extend the understanding of flow behavior to include UFG materials with submicrometer grain sizes. In practice, processing through the application of severe plastic deformation (SPD), as in equal-channel angular pressing (ECAP) and high-pressure torsion (HPT), has permitted the fabrication of relatively large samples having UFG microstructures. Since the occurrence of superplastic flow generally requires a grain size smaller than ~10 μm, it is reasonable to anticipate that materials processed by SPD will exhibit superplastic ductilities when pulled in tension at elevated temperatures. This review examines recent results that demonstrate the occurrence of exceptional superplastic flow in a series of UFG aluminum and magnesium alloys after ECAP and HPT. The results are analyzed to evaluate the superplastic flow mechanism and to compare with materials processed using different techniques. The critical issue of microstructural inhomogeneity is examined in two-phase UFG materials after SPD processing and the influence of microstructural homogeneity on the superplastic properties is also demonstrated.

Fe3O4-based core/shell nanocomposites for high-performance electrochemical supercapacitors

Abstract: In this work, magnetite (Fe3O4)-based core/shell composites, including Fe3O4@carbon (C), Fe3O4@polyaniline (PANI), and Fe3O4@C@PANI, are synthesized via a facile hydrothermal process. The as-prepared core/shell composites are characterized by transmission electron microscopy, X-ray diffraction powder, and Fourier transform infrared spectroscopy. The electrochemical performances of Fe3O4, Fe3O4@C, Fe3O4@PANI, and Fe3O4@C@PANI are investigated using cyclic voltammetry, galvanostatic charge–discharge measurement, and electrochemical impedance spectroscopy. The results show that the as-prepared nanomaterials are all typical pseudocapacitance capacitors. Carbon shell can significantly increase the electronic conductivity of electrode materials, reduce capacity loss, and improve the reversibility of Fe3O4. PANI coating layer can expressively enhance the specific capacitance. Synergistic effect of double shells improves the electrochemical property of Fe3O4. Fe3O4@C@PANI composites display the high capacitance of 322.5 F g−1 at 2.5 A g−1, and 87.8 % of which can still be maintained after 3000 charge–discharge cycles. The excellent electrochemical properties of Fe3O4@C@PANI evidence their potential application as supercapacitors in energy storage field.

Review: Overcoming the paradox of strength and ductility in ultrafine-grained materials at low temperatures

Abstract: Ultrafine-grained (UFG) materials with grain sizes in the submicrometer or nanometer range may be prepared through the application of severe plastic deformation (SPD) to bulk coarse-grained solids. These materials generally exhibit high strength but only very limited ductility in low-temperature testing, thereby giving rise to the so-called paradox of strength and ductility. This paradox is examined and a new quantitative diagram is presented which permits the easy insertion of experimental data. It is shown that relatively simple procedures are available for achieving both high strength and high ductility in UFG materials including processing the material to a very high strain and/or applying a very short-term anneal immediately after the SPD processing. Significant evidence is now available demonstrating the occurrence of grain boundary sliding in these materials at low temperatures, where this is attributed to the presence of non-equilibrium grain boundaries and the occurrence of enhanced diffusion along these boundaries.

Aerosol filtration using electrospun cellulose acetate fibers

Abstract: Aerosol filtration using electrospun cellulose acetate filters with different mean fiber diameters is reported, and the results are compared with those for two conventional filter media, a glass fiber filter and a cellulose acetate microfiber filter. The performance of these filters was studied using two aerosols, one solid (NaCl) and one liquid (diethyl hexyl sebacate), under conditions of relatively high face velocity (45 cm/s). The experimental observations are compared to theoretical predictions based on single fiber filtration efficiency. Our results indicate that the mechanisms for single fiber filtration efficiency provide reasonable predictions of the most penetrating particle size (MPPS), in the range of 40–270 nm, percentage penetration from 0.03 to 70 %, and fiber diameter in the range from 0.1 to 24 µm. Using an analysis based on blocking filtration laws, we conclude that filtration by cake formation dominated in the case of NaCl aerosols on electrospun filter media, whereas filters with larger fiber diameter showed a transition in mechanisms, from an initial regime characterized by pore blocking to a later regime characterized by cake formation. The liquid aerosol did not exhibit cake formation, even for the smallest fiber diameters, and also had much smaller influence on pressure drop than did the solid aerosol. The electrospun filters demonstrated slightly better quality factors compared to the commercial glass fiber filter, at a much lower thickness. In general, this study demonstrates control of the properties of electrospun cellulose acetate fibers for air filtration application.

Size effect of graphene nanoplatelets on the morphology and mechanical behavior of glass fiber/epoxy composites

Abstract: We investigated the effect of graphene nanoplatelets (GnPs) with two different lateral dimensions on the morphology, flexural, and thermo-mechanical properties of multiscale GnPs/glass fiber/epoxy composites. First, 3 and 5 wt% of GnP-C750 (<1 µm in diameter) and GnP-5 (5 µm in diameter) were individually integrated into epoxy suspension through a combination of calendaring and sonication processes. The GnPs/glass fiber/epoxy composites were then fabricated by incorporating glass fibers into the GnPs/epoxy mixture. Results showed that the flexural modulus of the GnPs/glass fiber/epoxy composites was improved by 11.5 and 26.3 % with the addition of 5 wt% GnP-C750 and GnP-5, respectively. At the same filler content, the storage modulus of the glass/epoxy composites incorporated with GnP-C750 and GnP-5 at 30 °C was enhanced by 10.2 and 28.2 %, respectively. The flexural strength of the 3 wt% GnP-5-reinforced glass fiber/epoxy composite is 16.2 % higher than that of the glass fiber/epoxy composite. The dispersion results of GnPs in the composites and the interfacial interactions between fibers and modified matrix were evaluated by scanning electron microscopy.

Carbon fiber-reinforced epoxy filament-wound composite laminates exposed to hygrothermal conditioning

Abstract: This study focuses on the evaluation of the effect of hygrothermal conditioning on tensile, compressive, in-plane and interlaminar shear properties, and also on the viscoelastic characteristics of carbon fiber/epoxy laminates. Flat unidirectional laminates were manufactured by dry filament winding and cured under hot compression. The laminates were later exposed to hygrothermal conditioning in a chamber, following the recommendations of ASTM D5229M. All composite coupons were tested before and after conditioning. An analytical Fickian model was used to fit experimental data, showing very good estimates. Shear strength and modulus reduced to about 30 and 38 %, respectively. All specimens presented acceptable failure modes; shear specimens failed at the gage section with delaminations and fiber/matrix debonding, whereas short beam specimens failed via delaminations at the specimen mid-plane. Moisture penetration through the carbon/epoxy surface lead to interfacial debonding and matrix plasticization. Puck’s failure envelope accurately predicted failure under compressive and shear loading.

Facile synthesis of N-doped graphene aerogel and its application for organic solvent adsorption

Abstract: N-doped graphene aerogel with a three-dimensional interconnected network was fabricated by using graphene oxide and melamine via the hydrothermal self-assembly and thermal annealing process. The aerogels were characterized by means of scanning and transmission electron microscopy, Fourier transform infrared spectrum, X-ray photoelectron spectroscopy, elemental analyses, X-ray diffraction, Raman spectrum, and nitrogen adsorption/desorption measurement. The N-doped graphene aerogel showed high-specific surface area, superhydrophobic nature, excellent adsorption capacity for organic solvents, and adsorption recyclability. It would be a promising material for removal of organic contaminates from water.