Homogenization Of Composite Materials Research Articles

Influence of Silicification on the Structural and Biological Properties of Buffer‐Mediated Collagen Hydrogels

AbstractA buffer‐mediated gelation route for collagen hydrogels that allows the formation of homogeneous composite and hybrid materials with various silica sources (i.e., colloidal silica and soluble silicates) at high concentration (up to 25 × 10−3 M) is described. Most significant improvement in rheological properties and proliferation of primary adult human dermal fibroblasts was obtained for the silicate‐based hybrid materials. A similar trend was observed in composite materials incorporating 14 nm SiO2 nanoparticles, although to a much lesser extent, whereas larger colloids (80 and 390 nm) did not significantly impact mechanical stability and cell behavior. Modification of 80 nm particles surface with amine groups weakens the collagen‐mineral interface, resulting in the decrease of material stability and leading to particle aggregation during the course of cell proliferation experiments.

Development of Petroleum-Based Carbon Composite Materials Containing Graphite/silicon Particles and Their Application to Lithium Ion Battery Anodes

ABSTRACT: Herein, a novel preparation method of highly homogeneous carbon-silicon composite materials waspresented. In contrast to conventional solvent evaporation method, a milled silicon-graphite or itsoxidized material were directly reacted with petroleum-derived pitch precursor. After thermal reactionunder high pressure, pitch-graphite-silicon composite was prepared. Carbon-graphite-silicon compositewere prepared by an air-oxidization and following carbonization. From energy dispersive spectroscopy,it was observed that small Si particles were highly embedded within carbon, which was confirmedby disappearance of Si peaks in Raman spectra. Furthermore, X-ray diffraction and Raman spectrarevealed that carbon crystallinity decreased when the strongly oxidized silicon-graphite was added,which was probably due to oxygen-induced cross-linking. From the anode application in lithiumion batteries, carbon-graphite-silicon composite anode displayed a high capacity (565 mAh g − 1 ), agood initial efficiency (68%) and an good cyclability (88% retention at 50 cycles), which wereattributed to the high dispersion of Si particles within cabon. In case of the strongly oxidized silicon-graphite addtion, a decrease of reversible capacity was observed due to its low crystallinity.Keywords : Pitch-graphite-Si composite, Petroleum pitch, Anode in lithium ion batteries

Parameter identification in viscoelastic strain models for composite materials by analyzing impulsive loading of shells of revolution

An analytical-experimental method for identifying the material constants and functions in the constitutive relations of viscoelastic strain for homogeneous composite materials is proposed. The method is based on the minimization of the discrepancy between the results of numerical and experimental modeling of nonstationary deformation processes in shells of revolution made of the materials under study. The approach was tested and its adequacy was shown in problems of determining the rigidity and rheological characteristics of composite materials from the results of comparative analytical-experimental study of nonstationary deformation of spherical and cylindrical shells under impulsive loading.

Nanoveneers: An Electrochemical Approach to Synthesizing Conductive Layered Nanostructures

We report herein a facile electrochemical approach to synthesizing various layered composite films of nanomaterials and conducting polymers, called nanoveneers. Layered structures of polypyrrole film with single-walled carbon nanotubes (SWNTs), graphene, and Au nanoparticles have been obtained by electropolymerization of pyrrole molecules on a heavily doped silicon wafer preloaded with target conductive nanomaterials. A free-standing, transparent, and highly conductive composite film was achieved after peeling off from a silicon wafer. Different from traditional homogeneous composite materials, such kinds of nanoveneers combined to the best extent the structural continuity and processability of conducting polymers with the high conductivity and functionality of discontinuous SWNTs, graphene, and other nanomaterials. The layered electrochemical deposition provides a great freedom for constructing various nanostructures with well-controlled geometry and thus physicochemical properties, as demonstrated by SWNT/polypyrrole nanoveneers. These nanoveneers are particularly attractive in areas of chemical sensors, labels, transparent electronics, and optoelectronics.

Determining the model parameters of nonlinear deformation of isotropic and composite materials from the results of an experimental-computational analysis of impulsively loaded circular plates

A method for identifying the material parameters of the constitutive relations of viscoelastic and elastoplastic deformation for isotropic and homogeneous composite materials is developed based on minimizing the mismatch between the results of numerical and experimental modeling of unsteady deformation of structural elements made of the materials studied. The method is tested and shown to be effective in determining the viscoelastic and elastoplastic characteristics of models for the nonlinear deformation of impulsively loaded isotropic and composite circular plates.

The Bio-inspired Study of Homogeneous Composite Materials

Wool is the most popular natural material. In the textile industries, the release of a lot of waste wool fibers and their products lead to the idea of regeneration of wool keratin materials. However, the most significant limitation may be the poor fracture resistance of neat keratin films. Greater failure resistance can be achieved by fiber-reinforced matrix when compared to the matrix materials alone. In this article, the fibrils of wool were used as the reinforcements in composite films, which can meet the requirement of biocompatibility. The fibrils morphology was investigated, that is, plain fibrils and fractal-tree structure fibrils. Finite element analysis and tensile tests were used to conduct a direct study on the mechanical properties of composites. It was found that the branched fibrils lead to both higher strength and fracture toughness for the composites than the plain fibrils.

Second-gradient plane deformations of ideal fibre-reinforced materials II: forming flows of fibre–resin systems when fibres resist bending

The continuum theory of ideal fibre-reinforced fluids, namely incompressible viscous fluids exhibiting some direction of inextensibility is extended to account for the fibre bending stiffness; namely a property that prevents discontinuity of the fibre slope under normal loading conditions. The principal kinematics of this new theoretical development is consistent with three-dimensional forming flows of fibre–resin systems though, for simplicity, formulation of relevant constitutive equations is confined within the framework of relevant plane flows. The theory adopts the macroscopic view that the resin matrix behaves as a viscous fluid but the resin and fibres form a homogeneous composite material. Consideration of the fibre bending resistance requires the inclusion of couple-stress and, hence, non-symmetric stress. The outlined theoretical developments are therefore relevant to polar-media behaviour; in this context, the anisotropic viscous fluids of interest become part of the material class of the so-called polar fluids. For plane flows of this type of fluids, a manner is also outlined in which the non-symmetric stress distributions sought can be determined by solving two simultaneous, first-order linear differential equations. Moreover, a relevant stress-resultants technique is adopted and extended appropriately to make possible complete determination of the kinematics dictating the creeping forming plane flow of the composite fluids of interest. Details of the mechanisms that capture fibre bending resistance are revealed and illustrated through a relatively simple example application. This considers and resolves the forming flow process of an ideal fibre-reinforced composite, moulded into a sharp corner under the action of an external line force.

Metamaterials and the homogenization of composite materials

Metamaterials and photonic crystals are often distinguished by the requirement that the former have features fine compared with the wavelength, and thus be homogenizable. We discuss the assumption implicit in this: that the metamaterial microstructure have well defined effective dielectric constant and magnetic permeability (concentrating on the former case). We illustrate subtleties in the calculation of effective dielectric constant in the particular case of square prisms composed of dielectric arranged in a square array in a host medium of another dielectric constant, commenting on poles, zeros, essential singularities and branch cuts of the effective dielectric constant function.

Effective thermal conduction in composite materials

The problem of determining the bounds and/or estimating the effective thermal conductivity (λ eff) of a composite (multiphase) system given the volume fractions and the conductivities of the components has been investigated. A comparison between the measured data and the results predicted by theoretical models has been made for seven heterogeneous samples. The tested models include those of the effective medium theory (EMT), Hashin and Shtrikman (HS) bounds, and Wiener bounds. These models can be used to characterize macroscopic homogeneous and isotropic multiphase composite materials either by determining the bounds for the effective thermal conductivity and/or by estimating the overall conductivity of the random mixture. It turns out that the most suitable one of these models to estimate λ eff is the EMT model. This model is a mathematical model based on the homogeneity condition which satisfies the existence of a statistically homogeneous medium that encloses inclusions of different phases. Numerical values of thermal conductivity for the samples that satisfy the homogeneity condition imposed by the effective medium theory are in best agreement with the experimentally measured ones.

Aqueous dispersions of SWCNTs using pyrrolic surfactants for the electro-generation of homogeneous nanotube composites. Application to the design of an amperometric biosensor

A new approach for the fabrication of homogeneous single-walled carbon nanotube/polypyrrole composites is presented. Stable SWCNT dispersions were obtained using the electropolymerizable (11-pyrrolyl-1-ylundecyl)triethylammonium tetrafluoroborate as surfactant. For these dispersions, we used commercially available HiPco® SWCNTs (purified) as received without any further treatment. Highly homogeneous composite materials could be electrogenerated, which were analyzed by scanning electron microscopy showing thin SWCNT bundles embedded in the polymer matrix. The quality of the SWCNT composite was tested with regard to its performance for biosensor applications. Enzymes like glucose oxidase and tyrosinase were entrapped in this copolymer during electropolymerization. The resulting glucose and catechol biosensors exhibit a drastic increase of sensitivities and maximum currents compared with the same sensor setup without nanotubes.

Variational statement of deformation problems for a composite latticed plate with various types of lattices

The variational statement of various boundary value problems for tangential displacements and forces in a latticed plate with an arbitrary piecewise smooth contour is investigated. The lattice consists of several families of bars made of a homogeneous composite material with a matrix of relatively low shear stiffness. The energy method reduces the problem to the variational problem of minimizing the energy functional. The conditions on the plate contour are established under which the functional is minimal and positive definite, which ensures that the problem is well posed.

Discontinuous Galerkin methods for periodic boundary value problems

AbstractThis article considers the extension of well‐known discontinuous Galerkin (DG) finite element formulations to elliptic problems with periodic boundary conditions. Such problems routinely appear in a number of applications, particularly in homogenization of composite materials. We propose an approach in which the periodicity constraint is incorporated weakly in the variational formulation of the problem. Both H1 and L2 error estimates are presented. A numerical example confirming theoretical estimates is shown. © 2006 Wiley Periodicals, Inc. Numer Methods Partial Differential Eq 2007

Guidance varieties in photonic crystal fibers

We study in general the property of dielectric optical waveguides that are translationally invariant along the field propagation direction. The core and the cladding can be made of either homogeneous or two-dimensional (2D) composite material (or metamaterial). When the metamaterial involved exhibits 2D periodicity, the waveguides are so-called photonic crystal fibers (PCFs). Guidance varieties in such waveguides are studied by examining separately the optical properties of the bulk core and cladding materials as well as by explicit numerical calculation of the overall waveguide structure. Our analysis reveals several neglected modal characteristics of such waveguides. For example, we show that different modes can copropagate in some PCFs with different guidance mechanisms, i.e., index guidance and photonic bandgap guidance. Such a feature can offer additional flexibility in light-wave manipulation. We also show that there exists a cutoff wavelength for the fundamental index-guided mode in some PCFs, which breaks the convention in traditional step-index fibers where such a cutoff does not exist.

The influence of residual stresses implicated via cure volume shrinkage on CF/VEUH—composites

This paper presents a thermal finite element analysis (FEA) of a unidirectional carbon fibre reinforced vinylester urethane hybrid matrix system. The evolution of the thermal residual stresses due to the mismatch in the coefficient of thermal expansion (CTE) of the single components in the cooling phase have been investigated. Additionally, the cure volume shrinkage was implemented into the FE-model. The model allows the transition of the homogeneous unidirectional composite material properties on a microscopic spot, where the properties of the fibres and the matrix can be considered separately. It could be shown, that the cure volume shrinkage (CVS) has a dramatic effect on the fibre/matrix interface region due to radial compression stresses along the fibre. Further, this may lead to microcracking or fibre/matrix debonding before any kind of load is applied to the material.

Silicon oxide colloidal∕polymer nanocomposite films

The quarter-wavelength (λ∕4) acoustic matching layer, a vital component in medical ultrasonic transducer, can bridge the large acoustic impedance mismatch between the piezoelectric material and the human body. Composite materials are widely used as matching materials in order to cover the wide acoustic impedance range that cannot be accomplished by using a single-phase material. At high frequencies (>50MHz), the λ∕4 matching layers become extremely thin so that the fabrication of homogeneous composite material matching layers becomes very challenging. A method is reported in this letter to fabricate sol-gel silicon oxide colloidal∕polymer composite film on silicon substrate, in which the particle size of silicon oxide colloidal is between 10 and 40 nm. The acoustic impedance of the nanocomposite films versus aging temperature has been measured at the desired operating frequency.

Optimal determination of the elastic constants of woven 2D SiC/SiC composite materials

For homogeneous materials, the ultrasonic immersion method, associated with a numerical optimization process mostly based on Newton's algorithm, allows the determination of elastic constants for various synthetic and natural composite materials. Nevertheless, a principal limitation of the existing optimization procedure occurs when the considered material is at the limit of the homogeneous hypothesis. Such is the case of the woven bidirectional SiC matrix and SiC fibre composite material. In this study, we have developed two numerical methods for the determination of the elastic constants of the 2D SiC/SiC composite material (2D SiC/SiC). The first one is based on Newton's algorithm: the elastic constants are obtained by minimizing the square deviation between experimental and calculated velocities. The second method is based on the Levenberg–Marquardt algorithm. We show that these algorithms give the same results in the case of homogeneous anisotropic composite materials. For the 2D SiC/SiC composite material, the two methods, using the same measured velocities, give different sets of elastic constants. We then note that the Levenberg–Marquardt algorithm enables a better convergence towards a global set of elastic constants in good agreement with the elastic properties, which can be measured using classical quasi-static methods.

A unified approach to predict overall properties of composite materials

This paper presents a unified approach to predict the overall elastic properties of homogeneous and isotropic composite materials from the micromechanics point of view. Within the framework of the approach, the commonly used approaches such as dilute estimate, Voigt estimate (VE), Reuss estimate (RE), modified rule of mixture, self-consistent estimate (SCE), Mori-Tanaka estimate (MTE) and Wakashima-Tsukamoto estimate (WTE) are expressible in the same form solely with different parameters. Particularly, the Voigt bound (VB) and Reuss bound (RB), as well as the Hashin-Shtrikman bounds (HSBs), are easily obtained by properly choosing the homogeneous comparison material. Furthermore, in light of the present approach, tighter than Hashin-Shtrikman bounds are proposed that can also be regarded as two kinds of estimates.

Sensitivity analysis of homogenized characteristics for some elastic composites

The main goal of the paper is to present theoretical aspects and the finite element method implementation of sensitivity analysis in homogenization of composite materials with linear elastic components using the effective modules approach. The sensitivity analysis of effective material properties is presented in a general form for n-component periodic composite and is illustrated using the examples of periodic 1D as well as 2D heterogeneous structures. The sensitivity coefficients are determined for the effective Young’s modulus and the effective elasticity tensor components. The structural response functional for the fiber-reinforced elastic composite is proposed in the form of total strain energy resulting from some uniform strain state of the composite representative volume element (RVE). The results of sensitivity analysis presented in the paper confirm the usefulness of the homogenization method in computational analysis of composite materials and its application in composite optimization, identification, shape sensitivity studies and, after some probabilistic extensions, in stochastic analysis of random composites.

Synthesis and characterization of new polyaniline/nanotube composites

New polyaniline/nanotube (PANI/NT) composites have been synthesized by “in situ” polymerization processes using both multi-wall carbon nanotubes (MWNTs) and single-wall carbon nanotubes (SWNTs) in concentrations ranging from 2 to 50 wt.%. Although no structural changes are observed using MWNTs above a concentration of 20 wt.%, the in situ synthesis results in electronic interactions between nanotubes and the quinoid ring of PANI leading to enhanced electronic properties and thus to the formation of a genuine PANI/MWNT composite material. On the other hand, using SWNTs favors the formation of inhomogeneous mixtures rather than of a homogeneous composite materials, independent of the SWNT concentration. X-ray diffraction, Raman and transport measurements show the different behavior of both classes of nanotubes in PANI/NT materials. The difficulties in the formation of a true PANI/SWNT composite are related to the far more complex structure of the SWNT material itself, i.e. to the presence of entangled bundles of SWNTs, amorphous carbon and even catalytic metal particles.

Two-dimensional analysis of composite structures by the finite-difference time-domain method adopting scaling approach [EM shielding

In this paper, the electric voltage of a single-layer homogeneous and anisotropic composite material has been calculated developing a mathematical model based on the diffusion equation. The proposed model has been developed assuming the minimum wavelength of the source much longer than the greatest dimension of the analyzed structures and the charge relaxation time of the composite material smaller than any allowable rise time of the field source. The finite-difference time-domain method, based on an explicit scheme, has been used to solve the differential equations and a scaling approach has been adopted to reduce computing time. The obtained results carried have been compared with those obtained simulating an equivalent electrical circuit network using SPICE. A good agreement between these two methods has been observed.