Thermal Conductivity Of Composite Materials Research Articles

The Effective Thermal Conductivity of Composite Materials with Spherical Dispersed Phase

The derivation of effective thermal conductivity (ETC) for composite materials is a long-standing thermal transport problem. Based on the thermal resistance model and integration method, two analytical solutions of the ETC of composite materials, in which the dispersed phase consists of spheres in cubic arrangements, were derived either neglecting or considering the effect of radiation. As the effect of radiation in porous composite materials is considered, the theoretical result indicates that the effect of radiation cannot be neglected as the temperature or porosity of composite materials is high. The differences between the theoretical results and experimental data of the ETC of porous composite materials are acceptable.

Prediction of particle size distribution effects on thermal conductivity of particulate composites

AbstractA material property that is of crucial importance for many industrial applications is the effective thermal conductivity. This property is a function of the thermal conductivities of components, their geometry, the spatial distribution of the components (including volume fraction and shape), the thermal resistances between phases, the possible contact between the filler particles and their size distribution law. The way the particles size distribution law influences the effective properties is not yet established, due to the fact that this law is not characterized in a unitary way. The numerous equivalent diameters (depending on the measurement technique) and the lack of a methodology of correspondence between them, complicates the solving of the problem regarding the size distribution dependence of the composite materials effective properties. The main goal of this paper was to introduce the predictions regarding the influence of the descriptors of the particle size distributions on the thermal conductivity of a composite material. It was found that the mean diameter is an inconsistent parameter for comparing the effective thermal conductivities of composite materials, and that the coefficient of variation offers an alternative criterion, as well as the Pearson's mode skewness.

Non-Linear Effect of Volume Fraction of Inclusions on The Effective Thermal Conductivity of Composite Materials: A Modified Maxwell Model

In this paper, non-linear dependence of volume fraction of inclusions on the effective thermal conductivity of composite materials is investigated. Proposed approximation formula is based on the Maxwell’s equation, in that a non-linear term dependent on the volume fraction of the inclusions and the ratio of the thermal conductivities of the polymer continuum and inclusions is introduced in place of the volume fraction of inclusions. The modified Maxwell’s equation is used to calculate effective thermal conductivity of several composite materials and agreed well with the earlier experimental results. A comparison of the proposed relation with different models has also been made.

Research on the Thermal Field and Active Water Cooling System Design of an Air-Core Compulsator

Compulsators are popular choices for high-end railgun power supplies. In order to maximize energy stores and power densities, compulsators are designed as an air-core prototype. However, new problems have also been brought out by the new structure. Due to the poor thermal conductivity of composite materials and the high field current needed to maintain the magnetic potential, one of the principal limiting factors for achieving continuous discharges is the high winding temperature rise. In this paper, on the basis of electromagnetic analysis, the losses of the coils and compensating shield could be calculated first, and then, a transient 3-D finite-element thermal analysis was performed for both the stator and rotor. Finally, for the purpose of stabilizing the hottest spot temperature of the coils at a given level, an active cooling system has been designed and simulated. The presented method can be applied to other compulsators having the same thermal issues.

Predicting the thermal conductivity of composite materials with imperfect interfaces

This paper compares the predicted values of the thermal conductivity of a composite made using the equivalent inclusion method (EIM) and the finite element method (FEM) using representative volume elements. The effects of inclusion anisotropy, inclusion orientation distribution, thermal interface conductance, h, and inclusion dimensions have been considered. Both methods predict similar overall behaviour, whereby at high h values, the effective thermal conductivity of the composite is limited by the inclusion anisotropy, while at lower h values, the effect of anisotropy is greatly diminished due to the more dominant effect of limited heat flow across the inclusion/matrix interface. The simulation results are then used to understand why in those cases where it has been possible to produce CNF reinforced Cu matrix composites with a large volume fraction of well dispersed CNFs, the measured thermal properties of the composite have failed to meet the expectations in terms of thermal conductivity, with measured conductivities in the range 200–300 W/m K. The simulation results show that, although degradation of the thermal properties of the CNFs and a poor interfacial thermal conductance are very likely the reasons behind the low conductivities reported, great care should be taken when measuring the thermal conductivity of this new class of materials, to avoid misleading results due to anisotropic effects.

Modeling of the effective thermal conductivity of composite materials with FEM based on resistor networks approach

In the present paper, a novel and efficient model was developed for predicting the effective thermal conductivity of the composite materials at different filler percentages. By introducing the relative radius as a parameter, the effective thermal conductivity can be predicted precisely when the thermal properties of filler and matrix are prescribed. The model employed the resistor network strategy to achieve a highly efficient prediction during the overall conductivity calculation. To verify this model, a wide range of two-phase composites, including Cu/PP, AlN/PI and a self-developed thermal conductive adhesive, were analyzed. The model-based simulation values showed good agreement with the experimental results. Moreover, a discussion on the effects of the newly-introduced parameter was given. Finally, the relationship between the filler radius variation and the thermal conductivity of the composite was studied.

Mechanism of thermal and electrical conductivity in polymer‐nanocarbon composites

AbstractThe electrical and thermal conductivity of nanocarbon filled polymers were studied by adding nanocarbon fillers (thermoexfoliated graphite (TEG) and dispersed TEG (nanoTEG)) to epoxy resin (ED) or polyethylene oxide (PEO). The content of filler in composite materials (CM) was (0.5–10) wt%. The temperature range of the investigations was (77–300) K for electrical conductivity and (150–423) K for thermal conductivity, respectively. It was found that electrical conductivity of CM obeys percolation dependence with low critical concentration cr∼(1–2) wt% (φcr∼(0.56–1.1) vol%) and characterised by a positive temperature coefficient. The thermal conductivity is approximately linear on nanocarbon content. The model describing the mechanisms of electrical and thermal conductivity of CM had been proposed. The basic parameters of this model are following: content of nanocarbon anisotropic particles and their distribution in polymer matrix; the contact thermal (electrical) resistance both between the filler particles and on the interface of two phases – polymer‐carbon filler.

Measuring and modeling the thermal conductivities of three-dimensionally woven fabric composites

The effect of a three-dimensional fiber reinforcement on the out-of-plane thermal conductivity of composite materials is investigated. Composite preforms with different fibers in the thickness direction were fabricated. After in fusion by using a vacuum-assisted resin transfer molding process, their through-thickness thermal conductivities were evaluated. The measured thermal conductivities showed a significant increase compared with those of a typical laminated composite. Although the through-thickness thermal conductivity of the samples increased with through-thickness fiber volume fraction, its values did not match those predicted by the simple rule of mixtures. By using finite-element models to better under stand the behavior of the composite material, improvements in an existing analytical model were performed to predict the effective thermal conductivity as a function of material properties and in-contact thermal properties of the composite.

Non-steady effective thermal conductivity of matrix composite materials with high volume concentration of particles

A new approach is proposed to investigate the non-steady effective thermal conductivity resulting from thermal waves in matrix composite materials with randomly distributed particles, and the interactions between the dense particles are considered. The theory of quasicrystalline approximation and Waterman’s T matrix formalism are employed to treat the multiple scattering of thermal waves from the particles in composites. The addition theorem for spherical Bessel functions is used to accomplish the translation between different coordinate systems. The Percus–Yevick correlation function widely applied in the molecular theory of liquids is applied to analyze the interaction between the densely distributed particles. The analytical expression of the Percus–Yevick correlation function is also given. Closed form solution of the effective propagation constants is obtained in the low frequency limit. Only numerical solutions are obtained at higher frequencies. Numerical examples show that the non-steady effective thermal conductivity of composite materials shows great difference when the frequency of thermal waves is different. Under different region of wave frequency, the effects of the volume fraction of particles and the material properties (thermal conductivity, specific heat and density) of the particles and matrix on the non-steady effective thermal conductivity are also examined.

Thermal conductivities of three-dimensionally woven fabric composites

This study investigates the effect of three-dimensional fiber reinforcement on the out-of-plane thermal conductivity of composite materials. Composite preforms 3D orthogonally woven with pitch carbon yarns and plied copper wires in thickness direction. After infusion, using a vacuum-assisted resin transfer molding process, the measured out-of-plane thermal conductivities of the resultant composites showed significant increase compared to a typical laminated uniaxially or biaxially reinforced composite. Although the through-thickness thermal conductivity of the samples increased with through-thickness fiber volume fraction, the values did not match those predicted by a simple rule of mixture. Using finite element models to better understand the behavior of the composite material, improvements to an existing analytical model were performed to predict the effective thermal conductivity as a function of the composite material properties and in-contact thermal material properties.

Prediction of Thermal Conductivity of Composite Materials

In this work, we developed a Web system for designing constitution and structure of a composite and predict its thermal conductivity. Two simulation methods, analytical method and finite element method, are available fitting for different requirements on computational efficiency and accuracy. This system has been used to predict the thermal conductivity of composites such as Mo/Al2O3, SiC/Al alloy, YSZ thermal barrier coatings, etc.. The results are in good agreement with the experimental data, so the reliability and effectiveness of the system has been proved.

Effective Thermal Conductivity of Composite Materials with 3-D Microstructures and Interfacial Thermal Resistance

This article presents a numerical scheme, based on an isoparametric second-order finite-element discretization of the unit cell heat conduction problem, to calculate the effective thermal conductivity of composite materials with general 3-D microstructures and interfacial thermal resistance. Representative numerical results for the effective conductivity of ordered arrays of spheres, prolate ellipsoids of revolution, and finite-length circular cylinders are presented and, when possible, validated against previous analytical predictions. Furthermore, the effective conductivity of a disordered multiparticle cell is calculated, showing that the present computational approach is a valuable tool for understanding the macroscopic thermal behavior of composite materials.

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The purity of carbon nanotubes has a significant affect on the thermal conductivity of composite materials

噴霧熱分解法による球状酸化物フィラーの合成とその性質

Spherical SiO2 and Al2O3 fillers were synthesized by ultrasonic spray pyrolysis. The particles characteristics of these fillers were studied by SEM, TEM, XRD and a pycnometer. Crysalline state of both as-prepared fillers was amorphous. The morphology of SiO2 fillers was maintained at up to 900°C . The morphology of SiO2 fillers was changed to irregular shape after crystallization of cristobalite at 1100°C. On the other hand, the morphology of Al2O3 was retained after crystallization of α-alumina at 1100°C. By the surface modification of these fillers with a silane coupling agent, the composite materials of epoxy resin and fillers was successfully prepared. The Vickers hardness and thermal conductivity of composite materials were investigated. The use of fillers with a broad particle size distribution and spherical morphology led to an improvement in the packing fraction of fillers, Vickers hardness and thermal conductivity.

Thermal Conductivity of Composite Materials Having a Chaotic Structure

Thermal Conductivity of Composite Materials Having a Chaotic Structure

Thermal conductivity of particle filled polyethylene composite materials

In this study, the effective thermal conductivity of aluminum filled high-density polyethylene composites is investigated numerically as a function of filler concentration. The obtained values are compared with experimental results and the existing theoretical and empirical models. The thermal conductivity is measured by a modified hot-wire technique. For numerical study, the effective thermal conductivity of particle-filled composite was calculated numerically using the micro structural images of them. By identifying each pixel with a finite difference equation and accompanying appropriate image processing, the effective thermal conductivity of composite material is determined numerically. As a result of this study, numerical results, experimental values and all the models are close to each other at low particle content. For particle content greater than 10%, the effective thermal conductivity is exponentially formed. All the models fail to predict thermal conductivity in this region. But, numerical results give satisfactory values in the whole range of aluminum particle content.

Unified practical bounds for the thermal conductivity of composite materials

Two schemes for the thermal conductivity of three classes of composites—unidirectional continuous fibre composites, particulate composites and laminae sheets—are presented herein based on combination of parallel and series models. Comparison with experimental results reveals reasonable correlation even at extreme reinforcement-to-matrix thermal conductivity ratio. Results of the two sets of schemes further show that one is higher than the other. Hence, the two sets of solutions are named practical bounds due to their simplicity. By introducing a reinforcement parameter, the solutions for the three classes of composite materials are unified, and approximate solutions for another two classes of composite materials—unidirectional short fibre composites and aligned flake composites—are suggested based on the reinforcement parameter.

Effective thermal conductivity of heterogeneous materials: calculation methods and application to different microstructures

First of all, three different methods are presented for calculating the macroscopic thermal conductivity of composite materials: numerical methods, predictive analytical methods, and the bounds obtained by variational methods. Their predictions are then compared with measured results for several types of nuclear material having different microstructures and constituent conductivity ratios. The numerical calculations, which use the actual microstructure, are in principle the most accurate, but can generally be performed in two dimensions only, using the image of a section. The two-dimensional (2D) numerical results provide a good evaluation of the conductivity when the matrix and inclusions have similar conductivities. If this is not the case, they underestimate the true three-dimensional (3D) value; a 2D/3D conversion method must therefore be determined by the use of one of the models from one of the other categories. The predictive analytical results, obtained with idealised microstructures, are accurate provided the microstructure is simple or if the conductivities of the constituents are similar. The upper bounds obtained by variational methods correspond to microstructures close to the actual microstructure, and are the most accurate when the difference in conductivity between the constituents is large and the volume fraction of the inclusions is high. The choice of the most suitable model can be made by comparing the 2D model predictions with the results of the 2D numerical calculation results.

Thermal conductivity of composite material with coated inclusions: Applications to tetragonal array of spheroids

The aim of this article is to determine the effective thermal conductivity of a composite material made of ellipsoidal coated inclusions bonded to a homogeneous isotropic matrix. The distribution of inclusions is considered to be periodic, nevertheless complex pattern can be investigated. An analytical approximate method, which takes into account interactions between inclusions at finite concentration, is proposed to solve steady state heat conduction problems. This method is an extension of the cluster scheme proposed, for elastic materials, by A. Molinari and M. E. Mouden Int. J. Solids Struct., 33, 3131 (1996). The morphological and spatial distribution of inclusions is accounted for in the present approach. Through different examples, results are compared to existing analytical or numerical solutions.

A new theoretical equation for thermal conductivity of two‐phase systems

A new theoretical equation that describes the thermal conductivity of two-phase composite materials has been proposed. The Cheng-Vachon equation has been modified by introducing a new parameter named Pd max, permitting the new equation to describe the thermal conductivity of composite materials for a wide variety of filler shapes and states of dispersion. The new equation can describe the thermal conductivity of two-phase materials more accurately than any of the previous equations. Furthermore, this new equation will make it possible to evaluate the dispersion state of the discontinuous phase by measuring the thermal conductivity of the filled polymers or the polymer blends. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 72: 1689–1697, 1999