Showing papers on "Composite number published in 2007"

Graphene-silica composite thin films as transparent conductors.

Abstract: Transparent and electrically conductive composite silica films were fabricated on glass and hydrophilic SiOx/silicon substrates by incorporation of individual graphene oxide sheets into silica sols followed by spin-coating, chemical reduction, and thermal curing. The resulting films were characterized by SEM, AFM, TEM, low-angle X-ray reflectivity, XPS, UV−vis spectroscopy, and electrical conductivity measurements. The electrical conductivity of the films compared favorably to those of composite thin films of carbon nanotubes in silica.

In situ growth of mesoporous SnO2 on multiwalled carbon nanotubes : A novel composite with porous-tube structure as anode for lithium batteries

Abstract: A novel mesoporous-nanotube hybrid composite, namely mesoporous tin dioxide (SnO2) overlaying on the surface of multiwalled carbon nanotubes (MWCNTs), was prepared by a simple method that included in situ growth of mesoporous SnO2 on the surface of MWCNTs through hydrothermal method utilizing Cetyltrimethylammonium bromide (CTAB) as structure-directing agents. Nitrogen adsorption–desorption, X-ray diffraction and transmission electron microscopy analysis techniques were used to characterize the samples. It was observed that a thin layer tetragonal SnO2 with a disordered porous was embedded on the surface of MWCNTs, which resulted in the formation of a novel mesoporous-nanotube hybrid composite. On the base of TEM analysis of products from controlled experiment, a possible mechanism was proposed to explain the formation of the mesoporous-nanotube structure. The electrochemical properties of the samples as anode materials for lithium batteries were studied by cyclic voltammograms and Galvanostatic method. Results showed that the mesoporous-tube hybrid composites displayed higher capacity and better cycle performance in comparison with the mesoporous tin dioxide. It was concluded that such a large improvement of electrochemical performance within the hybrid composites may in general be related to mesoporous-tube structure that possess properties such as one-dimensional hollow structure, high-strength with flexibility, excellent electric conductivity and large surface area.

Template-free hydrothermal synthesis of CuO/Cu2O composite hollow microspheres

Abstract: CuO/Cu2O composite hollow microspheres with controlled diameter and composition were prepared without the addition of templates and additives by hydrothermal synthesis using Cu(CH3COO)2·H2O as a precursor. Increasing the precursor concentration from 0.02 to 0.2 M increased the diameter of the composite hollow microspheres from 500 nm to 5 μm. Moreover, the content of Cu2O in the composite hollow microspheres increased with increasing the reaction time or/and precursor concentration to produce a range of composite hollow microspheres with Cu2O contents from 20 to 80 wt %. A localized Ostwald ripening mechanism was proposed to account for the formation of CuO/Cu2O composite hollow microspheres. The photocatalytic activity experiment indicated that the prepared CuO/Cu2O composite hollow microspheres exhibited a higher photocatalytic activity for the photocatalytic decolorization of methyl orange aqueous solution under the visible-light illumination than the single phase CuO or Cu2O samples.

High Dielectric Performance of Polymer Composite Films Induced by a Percolating Interparticle Barrier Layer

Abstract: The mechanical flexibility and tunable properties of polymer-based materials make them attractive ones for a lot of applications. Exploring polymer-based dielectrics, such as ones used for capacitors and charge-storage applications, with high dielectric constant (high-j) has recently aroused considerable interest. Especially, motivated by higher function and further miniaturization of electronics, embedding (or integrating) polymer-based capacitors into the inner layers of organic printed circuit boards (PCBs) allows packaging substrate miniaturization and better electrical performance, which is a key for organic-based system-on-package technologies. But as capacitors, the relative dielectric constant j of general polymers (being good insulators) is too low (e.g., j< 5).Thus, a key issue is to substantially raise the dielectric constant of the polymers while retaining low dielectric loss. A few strategies have been developed to raise the j of polymer-based materials. A common approach is to add high-j ceramic fillers (e.g., BaTiO3) into a polymer. High loading of the ceramic fillers in the polymer composite, usually over 50 vol %, can increase j by about ten times relative to the polymer matrix, but dramatically decreases the adhesion of the composite (and increases its porosity) thus deteriorating the adaptability between the composite and the organic circuit boards. Another strategy is to fabricate percolative composite capacitors by using conductive fillers (e.g., metal particles). As the volume fraction f of the fillers increases to the vicinity of the percolation threshold fc, j of the composites can be dramatically enhanced as described by the well-known power law

Processing and properties of carbon nanotubes reinforced aluminum composites

Abstract: Carbon nanotubes reinforced aluminum matrix composites were fabricated by isostatic pressing followed hot extrusion techniques. Differential scanning calorimetric, X-ray diffraction, field emission scanning electron microscopy, transmission electron microscopy has been carried out to examine the reaction condition of nanotubes and aluminum, and to analyze the composites structure. The effects of nanotubes content on mechanical properties of composites were investigated. Experimental results showed that nanotubes are homogeneously distributed in the composites. Some nanotubes act as bridges across cracks, others are pulled-out on fracture surfaces of composites. However, nanotubes react with aluminum and form Al4C3 phases when the temperature is above 656.3 °C. The nanotubes content affects significantly mechanical properties of composites. Meanwhile, the 1.0 wt.% nanotube/2024Al composite is found to exhibit the highest tensile strength and Young's modulus. The maximal increments of tensile strength and Young's modulus of the composite, compared with the 2024Al matrix, are 35.7% and 41.3%, respectively.

Fabrication and Mechanical Properties of Completely Biodegradable Hemp Fiber Reinforced Polylactic Acid Composites

Abstract: Biodegradable composite materials can be produced by the combination of biodegradable polymers and natural fibers. In this study, a new biodegradable composite of hemp fiber reinforced polylactic acid (PLA) was fabricated using the hot press method. Mechanical properties of composites with different fiber volume fractions were tested. The optimum fiber content was determined according to the test results. Effects of alkali treatment on the fiber surface morphology and the mechanical properties of the composites were investigated. Test results show that the composite with 40% volume fraction of alkali treated fiber has the best mechanical properties. The tensile strength, elastic modulus, and flexural strength of the composite with 40% treated fiber are 54.6 MPa, 8.5 Gpa, and 112.7 MPa respectively, which are much higher than those of PLA alone. The composites have lower densities, which were measured to be from 1.19 g/cm3 to 1.25 g/cm3. Specific strengths were also calculated. Surface morphologies of fibe...

Enhanced thermal conductivity of boron nitride epoxy‐matrix composite through multi‐modal particle size mixing

Abstract: Castable particulate-filled epoxy resins exhibiting excellent thermal conductivity have been prepared using hexagonal boron nitride (hBN) and cubic boron nitride (cBN) as fillers. The thermal conductivity of boron nitride filled epoxy matrix composites was enhanced up to 217% through silane surface treatment of fillers and multi-modal particle size mixing (two different hBN particle sizes and one cBN particle size) prior to fabricating the composite. The measurements and interpretation of the curing kinetics of anhydride cured epoxies as continuous matrix, loaded with BN having multi-modal particle size distribution, as heat conductive fillers, are highlighted. This study evidences the importance of surface engineering and multi-modal mixing distribution applied in inorganic fillered epoxy-matrix composite. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007

Towards a Fundamental Understanding of the Improved Electrochemical Performance of Silicon–Carbon Composites†

Abstract: Silicon–carbon composites consisting of Si particles embedded in a dense and non-porous carbon matrix are prepared by the pyrolysis of intimate mixtures of poly(vinyl chloride) (PVC) and Si powder at 900 °C under a flow of N2. In contrast to bare micrometer-sized (1–10 μm) and nanometer-sized (10–100 nm) Si powders, which show poor cycling behavior with almost no capacity remaining after 15 cycles, the texture of the composite is seen to greatly enhance the reversibility of the alloying reaction of Si with Li. For instance, a capacity of ca. 1000 mA h g–1 is achieved for 20 cycles (0–2.0 V vs. Li+/Li) for a silicon–carbon composite containing nanometer-sized Si particles. We also demonstrate that a mild manual grinding treatment degrades the cycling performance of the composites to levels as low as the parent Si, even though free Si is not released. The electrochemical measurements in conjunction with Raman spectroscopy data indicate that a huge stress is exerted on the Si domains by the in situ formed carbon. This carbon-induced stress is found to disappear during the milling of the composites, indicating that the carbon-induced pressure, along with the accompanying improvement in electrical connectivity, are the key parameters for the improved cycling behavior of Si versus Li.

Dynamic mechanical analysis of carbon/epoxy composites for structural pipeline repair

Abstract: The viscoelastic behavior of a carbon fiber/epoxy matrix composite material system used for pipeline repair has been evaluated though dynamic mechanical analysis. The effects of the heating rate, frequency, and measurement method on the glass transition temperature ( T g ) were studied. The increase in T g with frequency was related to the activation energy of the glass transition relaxation. The activation energy can be used for prediction of long term performance. The measured tan delta peak T g ’s of room temperature cured and post-cured composite specimens ranged from 60 to 129 °C. Analysis of T g data at various cure states was used to determine use temperature limits for the composite repair system.

In situ Fenton reagent generated from TiO2/Cu2O composite film: a new way to utilize TiO2 under visible light irradiation.

Abstract: TiO2/Cu2O composite is prepared by a simple electrochemical method and coated on glass matrix through a spraying method. The obtained composite is characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The effect of TiO2/Cu2O composite films with different ratio of TiO2 and Cu2O on photodegradation of the dye methylene blue under visible light is investigated in detail. It is found that the photocatalytic activity of TiO2/Cu2O composite film with the presence of FeSO4 and EDTA is much higher than that for the similar system with only TiO2 and Cu2O film respectively. Without the presence of FeSO4 and EDTA, there is no degradation for methylene blue. The exploration of the optimized parameters for the degradation of methylene blue by using TiO2/Cu2O composite film as catalyst under visible light was also carried out. The most significant factor is the amount of TiO2 in the composite, and the second significant factor is the concentration of FeSO4. During the degradation of methylen...

Novel Method to Fabricate SiO2/Ag Composite Spheres and Their Catalytic, Surface-Enhanced Raman Scattering Properties

Abstract: This paper presents a novel method for the fabrication of SiO2/Ag composite spheres with the aid of the reducing and stabilizing function of polyvinylpyrrolidone (PVP). In this approach, [Ag(NH3)2]+ ions were first adsorbed on the surfaces of silica spheres via electrostatic attraction between the silanol groups and ions; these [Ag(NH3)2]+ ions adsorbed on silica spheres were then reduced and protected by PVP to obtain SiO2/Ag composite spheres. Neither additional reducing agent nor core surface modification was needed; the particle size and the coverage degree of silver nanoparticles on the silica spheres could be easily tuned by altering the concentration of the precursor-[Ag(NH3)2]+ ions. UV−visible spectrometer analysis showed these composite spheres had very good catalytic property; Raman spectrometer measurement showed that these composite spheres exhibited excellent surface-enhanced Raman scattering (SERS) performance.

Superparamagnetic composite colloids with anisotropic structures.

Abstract: We report a unified set of emulsion polymerization processes for the synthesis of a series of spherical and nonspherical magnetite−polystyrene (Fe3O4−PS) composite colloids with the superparamagnetic cores concentrically or eccentrically positioned inside polystyrene shells. The interfacial tension between the inorganic core and monomer-swollen polymer shell and the effect of swelling and phase separation have been employed to tune the morphology and structure of the composite colloids. The uniform size, anisotropic structure/shape, and the additional magnetic control make these colloidal particles ideal building blocks for the fabrication of novel functional devices and materials.

Processing and mechanical properties of SiC reinforced cast magnesium matrix composites by stir casting process

Abstract: Elemental Mg and Mg-alloy (AZ91D) based composites reinforced with 15 vol.% silicon carbide (SiC) particulates (average particle size 15 μm and 150 μm) were synthesised by stir casting technique. Particle distribution, particle–matrix interfacial reaction, hardness and mechanical properties in the as cast as well as T4 heat-treated conditions were investigated. The composite materials show uniform distribution of SiC particulates. The average grain size decreases with the presence of SiC particulates and the grain size further decreases as the particle size decreases. The AZ91D alloy composite shows an increase in hardness and elastic modulus compared to monolithic alloys. The improvement in elastic modulus of composite containing 15 μm size SiC particles is significantly higher than the composite with 150 μm size particles. The ultimate tensile strength and ductility of composite materials were reduced compared to unreinforced alloy.

Agro-hybrid Composite: The Effects on Mechanical and Physical Properties of Oil Palm Fiber (EFB)/Glass Hybrid Reinforced Polyester Composites:

Abstract: In this research, the combination of oil palm fiber and glass fiber as reinforcing fibers in polyester composites was evaluated The mechanical and physical properties of oil palm empty fruit bunch/glass hybrid reinforced polyester composites were studied Hybrid laminate composites with different weight ratios (w/w) of chopped strand mat (CSM) glass fibers: oil palm empty fruit bunch fiber (EFB) 3: 7, 5: 5, 7: 3, 9: 1 were prepared The hybrid effect of glass and EFB fibers on the tensile, flexural, impact, and hardness of the composites were investigated Water absorption and thickness swelling were also conducted In general the hybrid composites exhibited good properties compared to the EFB/polyester composites

Fusion of Seashell Nacre and Marine Bioadhesive Analogs: High-Strength Nanocomposite by Layer-by-Layer Assembly of Clay and L-3,4-Dihydroxyphenylalanine Polymer

Abstract: Nature has evolved materials that possess mechanical properties surpassing many man-made composites. Bones, teeth, spider silk, or nacre, are just a few well-known examples of biomaterials that exhibit exceptionally high tensile strengths, hardness, or toughness. These remarkable properties have driven scientists to study and model their architectures and compositions, from microto nanoscales, in the hope of developing analogous synthetic materials. Of these, probably the most studied is nacre. It is composed of 95 % brittle CaCO3 plates with just a few percent of organic “glue”, yet it is twice as hard and more than ca. 1000 times as tough as its constituent phases. These exceptional mechanical properties together with the macroscopic beauty and elegance of its nanoscale hierarchy serve as a model for design of high-performance materials. Preparation of artificial analogs of nacre has been approached by using several different methods and the resulting materials capture some of the characteristics of the natural composite. In our own work, we have used a layerby-layer (LBL) assembly technique to prepare a nanostructured analogue of nacre from inorganic nanometer-sized sheets of Na-Montmorillonite clay (C) and a polyelectrolyte, poly(diallyldimethylammonium chloride) (PDDA). The structure, deformation mechanism, and mechanical properties of this material were found to be comparable with those of natural nacre and lamellar bones (tensile strength, r= (100± 10) MPa, and Young’s modulus, Y = (11± 2) GPa). Contrary to other preparation techniques the LBL method is relatively simple and highly versatile in merging different functionalities into a single composite. At the same time, a vast array of available assembly components allows us to generate alternative designs as a means of understanding the different interactions necessary for preparation of nacrelike composites with application-tailored mechanical responses. LBL technique has proven to be an ideal method for preparation of multifunctional, nanostructured materials. Since its inception in 1990s, there has been a virtual explosion in the amount of scientific literature in this subject. Similarly, LBL assembly of clays was also pioneered and further studied in the 1990s by Ferguson’s group. Since then, the LBL technique has been found to be applicable for the preparation of superhydrophobic surfaces, sensors and semipermeable membranes, drug and biomolecules delivery, optically active and responsive films, fuel cells and photovoltaic materials, biomimetic and bioresponsive coatings, semiconductors, catalysts, and magnetic devices, to name a few. All of the potential applications mentioned above also require both control and improvement of mechanical properties. Using the mix-and-match approach to LBL films, that is, stratified multilayers, the mechanical properties can be incorporated in virtually any LBL functionality, if a convenient pair of LBL partners is available. Similarly to our work on nanostructured nacre, we have also previously shown that preparation of high-strength LBL composites from singleand multiwalled carbon nanotubes (CNTs). They demonstrated mechanical properties as high as: r= 220 MPa, and Y = 5 GPa, and are particularly suited for multifunctional stratified coatings with electrical conductivity. Having at hand versatility of the LBL technique and potential for use in a wide array of applications, we have set out to improve the mechanical properties of our composite further. Clay nanosheets possess exceptionally high mechanical properties, with Y calculated at ca. 250–260 GPa, which is two orders of magnitude greater than the mechanical properties of most clay nanocomposites achieved thus far. We have hypothesized that improving load transfer from the weak polymeric component to the inorganic nanosheets in our artificial nacre should increase the composite’s mechanical properties. This required a polymer that would have a potentially stronger interaction with the clay than the ionic bonds in PDDA/C. For inspiration we have turned to another exceptional biomaterial, the unusual protein adhesive secreted by mussels. C O M M U N IC A IO N