Transport Properties Of Composite Materials Research Articles

Assessing effective medium theories for conduction through lamellar composites

In this study, we developed a numerical finite-difference model for 2D transient diffusion in lamellar structures. The explicit application is heat transfer, but it could equally be applied to dielectric constants, magnetic susceptibilities, electron/ion conduction, and mass diffusion as well. The control volume contains two phases A and B. The phases have different transport parameters. The modeling aims to evaluate the effect of grain size, grain boundaries, and phase contrast on apparent transport properties of composite materials, such as laminates, polycrystalline materials, and block copolymers, by examining a progression of increasingly complex structures. To validate the model, effective transport parameters of parallel and perpendicular structures from the numerical model are compared to analytical expressions. Effective Medium Theory provides an analytical expression in the limit of many, small, randomly oriented grains. The impact of coarse grains on transport is investigated. Specifically, the model is used to examine how the apparent transport parameters trend from the limit of a homogeneous material to small randomly oriented grains containing two different phases. The effective thermal conductivity (averaged over many random structures) was found not to be a function of grain size. However, the standard deviation decreased exponentially with decreasing grain size, reaching less than 2% variation for transport through 15 grains. Thus, the appropriate Effective Medium prediction is reliable for a surprisingly few number of grains, and connectivity of the more conducting phase is important only in coarse grains with significant contrast.

Humidity dependence of transport properties of composite materials used for thermochemical heat storage and thermal transformer appliances

Abstract Water sorption thermochemical heat storage is a promising way to provide dwellings with renewable central heating. It requires the use of several cubic meters of materials per dwelling. Depending on the design of the heating system, specific heat and mass transfer issues occur. For instance, the heat transfer rate in reactive medium and the kinetics of sorption process determine the system thermal power. In addition, the moisture propagation during inter-seasonal storage must be understood. In this paper, the influence of the water mass uptake on the apparent thermal conductivity and apparent mass diffusivity of solid material were studied. The studied material was a composite of calcium chloride (CaCl2) encapsulated in mesoporous silica with a salt content of 40–43 wt.%. The thermal conductivity was measured by the transient hot bridge method and varied from 0.13 to 0.16 W m−1 K−1, having a threshold at 0.14 g/g of water mass uptake. The apparent water mass diffusivity was studied using a diffusion column. The water diffusivity – concentration dependency was established by using the modified Hall method. The apparent diffusion coefficient ranged from 3 × 10−10 to 2 × 10−8 m2 s−1 in experimental conditions.

Interfacial effect on physical properties of composite media: Interfacial volume fraction with non-spherical hard-core-soft-shell-structured particles.

Interfaces are known to be crucial in a variety of fields and the interfacial volume fraction dramatically affects physical properties of composite media. However, it is an open problem with great significance how to determine the interfacial property in composite media with inclusions of complex geometry. By the stereological theory and the nearest-surface distribution functions, we first propose a theoretical framework to symmetrically present the interfacial volume fraction. In order to verify the interesting generalization, we simulate three-phase composite media by employing hard-core-soft-shell structures composed of hard mono-/polydisperse non-spherical particles, soft interfaces, and matrix. We numerically derive the interfacial volume fraction by a Monte Carlo integration scheme. With the theoretical and numerical results, we find that the interfacial volume fraction is strongly dependent on the so-called geometric size factor and sphericity characterizing the geometric shape in spite of anisotropic particle types. As a significant interfacial property, the present theoretical contribution can be further drawn into predicting the effective transport properties of composite materials.

Cross-Property Relations between Mechanical and Transport Properties of Composite Materials

Cross-property relations are established in this paper between transport and mechanical properties of two-phase composite materials. In particular, the cross-property relations among the effective thermal conductivity (or chemical diffusivity), effective shear, and effective bulk moduli of two-phase composites are developed. Conventional isotropic composite mechanics models are employed to obtain effective properties of composite materials. The effective properties are expressed in terms of volume fraction of the inclusions, and then the volume fraction is used as the link to develop the expressions for the cross-property correlations. The present method can be used together with the recently developed composite damage mechanics method for evaluating cross-property correlations of distressed materials. Because mechanical properties of distressed materials have been investigated extensively, the cross-property relations provide a powerful method for evaluating transport properties of distressed materials without sophisticated experiments. Two numerical examples are shown for the cross-property correlations of distressed concrete under two-dimensional loading, where the isotropic damage is governed. Correct trends were obtained for the two numerical examples.

Transport properties of densely packed composites. Effect of shapes and spacings of inclusions

Summary We analyze transport properties of ∞uid/solid and solid/solid composites containing flnite arrays of closely spaced rigid inclusions when a host medium is either an elastic matrix or an incompressible ∞uid. The appropriate choice of the number of inclusions and the symmetry of a periodicity cell allows us to introduce simple, yet physically relevant models so that efiective characteristics of homogenized media can be investigated analytically. For various applied loads and shapes of (polydisperse) inclusions we demonstrate the spatial non-uniformity of geometric conflgurations corresponding to either lowest dissipation rate (for ∞uid/solid composites) or to minimal stifiness (for solid/solid composites). In order to flnd the optimal conflgurations, we use a unifled framework based on asymptotic expansions in terms of inter-inclusion distances. Furthermore, we compare efiective transport properties of composite materials containing inclusions with either ∞at or curved boundaries.

Comment on "Walker diffusion method for calculation of transport properties of composite materials"

In a recent paper [C. DeW. Van Siclen, Phys. Rev. E 59, 2804 (1999)], a random-walk algorithm was proposed as the best method to calculate transport properties of composite materials. It was claimed that the method is applicable both to discrete and continuum systems. The limitations of the proposed algorithm are analyzed. We show that the algorithm does not capture the peculiarities of continuum systems (e.g., "necks" or "choke points") and we argue that it is the stochastic analog of the finite-difference method.

Walker diffusion method for calculation of transport properties of composite materials

The morphology of a multiphase microstructure greatly influences the macroscopic transport properties of the composite material. These properties are shown to be related to the diffusion coefficient of a random (nonbiased) walker. The proper diffusion rules are found by considering an isomorphic image of the microstructure in which distinct populations of walkers correspond to the phase domains, with the walker density of a population proportional to the transport coefficient of the corresponding domain. To demonstrate the method, it is applied to disordered two-phase percolating composites.

Quantum transport properties of composite materials by generalized molecular dynamics method in Wigner formulation of quantum mechanics

A new method for solving Wigner-Liouville-type equations and studying dynamics of quantum particles in composite materials and disordered system of scatterershas been developed. Approach combines both molecular dynamics and Monte Carlo methods. Numerical results have been obtained for the frequency dependences of tensors of electron conductivity and permitivity according to quantum Kubo formulas.

Effective transport properties of periodic composite materials

We study the scalar transport properties of composite materials composed of spherical inclusions arranged in a cubic lattice with a basis of two or more spheres. We use multipole expansions for the fields in and around each spherical inclusion and generalize an identity due to Rayleigh to provide a set of linear equations for the multipole coefficients. We use as examples the caesium chloride structure and a structure containing chains of spheres, studying the influence of geometry on the transport coefficients and asymptotic properties of fields when the spheres come close to touching.

Generalized combining rules for predicting transport properties of composite materials

A general relationship is presented to predict the effective transport properties (electrical conductivity, thermal conductivity, dielectric constants, magnetic permeability, and diffusion coefficients) of composite materials in terms of the properties and concentration of the components and the internal microstructure. Numerous existing relationships are obtained as special cases. Filler shapes ranging from platelet, particulate, and short-fiber, to continuous fiber are consolidated within the relationship. Particular emphasis is given to isotropic composites containing particulate fillers. Simplifications appropriate for metal-filled polymers are presented. An interpretation of structural parameters is proposed which accounts for sharp transitions associated with percolation mechanisms.

Extraction of structural information from measured transport properties of composites

We describe clearly Milton's bounds on the scalar transport properties of composite materials in the case when no geometrical information is known about the composite, but when a set of measured values of a transport property is available. We consider how the measured values may be used when they are subject to known errors. In this case, a statistical analysis is employed. We illustrate the various conclusions which may result from this analysis by considering six cases involving both two-and three-dimensional composites. In four of the six cases, estimates of the volume fraction of the composite in agreement with the known value are obtained. In the fifth case, the analysis pointed out a possible error in the experimental data, while in the sixth it revealed a critical dependence of transport properties on composite geometry due to filamentary linking between particles.

Variational bounds of transport properties of composite materials

We discuss here the relative merits, physical interpretations, limitations, and generalizations of various bounds of composite materials.

Variational Bounds of Transport Properties of Composite Materials

We discuss here the relative merits, physical interpretations, limitations, and generalizations of various bounds of composite materials.