Thermal Conductivity Of Composite Materials Research Articles

A Randomly Distributed Filler Model for Heat Conductivity Prediction in Filled Composite Materials Considering Fillers Aggregation

Polymeric heat exchangers are widely used in various industries. However, their low thermal conductivity has limited their practical applications. Adding fillers with high thermal conductivity to the base materials has been recognized as an efficient way to increase the thermal conductivity of polymers. Filler contents have great impacts on the overall effective thermal conductivity of composite materials. Increasing filler content is the most direct and efficient way to prepare materials of super-high thermal conductivity. Previous researches were confined to volumetric filler content less than 20%. Higher filler contents were not considered due to possible filler aggregation. In this study, the effects of filler contents higher than 20% are investigated, with simultaneous considerations of random distribution of fillers in the base material and fillers aggregation. The interactions between the fillers, and between the fillers and the base material, are studied to predict the effective thermal conductivity of the composite material. It is found that the thermal conductivity of composite material grows rapidly with filler content. It has a sudden increase when the filler volume content increases beyond 30%. Under such circumstance, new “larger fillers” are formed by the aggregation of small fillers. They have tremendous impacts on the thermal conductivity of composites, impacts that were not disclosed by previous models for thermal conductivity predictions.

Calculating the Thermal Conductivity of Filled Fluoroplastics by Methods of Generalised Conductivity Theory

An examination was made of the potential use of analytical methods of generalised conductivity theory for the interpolation and extrapolation of the results of measurements and prediction of the effective thermal conductivity of composite materials – fluoroplastics with different powder fillers.

A generalized correlation for predicting the thermal conductivity of composite materials

Enhancing the thermal conductivity of polymer materials is actively being attempted by applying metal/carbon-based micro-/nanoparticles with higher thermal conductivity. In this study, a generalized correlation for predicting the thermal conductivity of composite materials is proposed based on an extensive FEM numerical study. The effects on the effective thermal conductivity of composite materials were investigated in regard to the thermal conductivity ratio between a particle and the matrix material, the particle volume fraction, and the thermal contact resistance between the particle surface and the matrix. The results indicate that the effective thermal conductivity increases with either the particle volume fraction or the conductivity ratio in general. This trend changes when the thermal contact resistance is present and greater than a threshold value. Based on the results, a generalized correlation is proposed as a function of the three aforementioned non-dimensional parameters. The results cover wide ranges of the parameters and can be widely utilized for predicting the thermal conductivity of composite materials.

Performance of thermal lattice Boltzmann and finite volume methods for the solution of heat conduction equation in 2D and 3D composite media with inclined and curved interfaces

In the present study, the accuracy of predicted effective thermal conductivity of composite materials, determined by the finite volume method (FVM) and the thermal lattice Boltzmann method (TLBM) using single relaxation time (SRT), is investigated. In addition, computational efforts required by both solution methods are also compared. Two- and three-dimensional heat conduction problems in two-phase domains with a high thermal conductivity ratio are considered, where oblique and curved interfaces are represented by the staircase approximation. The discretisation is carried out on a uniform mesh. For TLBM, different stencils are employed and a wide range of relaxation frequencies, corresponding to relative magnitudes of thermal conductivities of different phases, is investigated. For comparison, wherever available, the analytical expression for the effective thermal conductivity is considered. Otherwise, the reference effective thermal conductivity is determined using a commercial software employing an appropriate body-fitted mesh. The present results show that the accuracy of TLBM depends on the employed stencil, relaxation frequency and arrangement of two materials in the computational domain. It is also observed that TLBM can be even more accurate than FVM, although the accuracy of FVM is independent of individual values of thermal conductivities. For the same convergence criterion, however, TLBM requires a higher number of iterations than FVM, although TLBM shows a potential for speed-up.

The Estimation of Thermal Conductivity for Alumina-Epoxy Composite Material with High Filling Volume Fraction

In this study, the transport theorem of phonons and electrons is utilized to create a model to predict the thermal conductivity of composite materials. By observing or assuming the dopant displacement in the matrix, a physical model between dopant and matrix can be built, and the composite material can be divided into several regions. In each region, the phonon or electron scattering caused by boundaries, impurities, or U-processes was taken into account to calculate the thermal conductivity. The model is then used to predict the composite thermal conductivity for several composite materials. It shows a pretty good agreement with previous studies in literatures. Based on the model, some discussions about dopant size and volume fraction are also made.

Efficiency of the Needle Probe Test for Evaluation of Thermal Conductivity of Composite Materials: Two-Scale Analysis

Abstract The needle probe test, as a thermal conductivity measurement method, has become very popular in recent years. In the present study, the efficiency of this methodology, for the case of composite materials, is investigated based on the numerical simulations. The material under study is a two-phase composite with periodic microstructure of “matrix-inclusion” type. Two-scale analysis, incorporating micromechanics approach, is performed. First, the effective thermal conductivity of the composite considered is found by the solution of the appropriate boundary value problem stated for the single unit cell. Next, numerical simulations of the needle probe test are carried out. In this case, two different locations of the measuring sensor are considered. It is shown that the “equivalent” conductivity, derived from the probe test, is strongly affected by the location of the sensor. Moreover, comparing the results obtained for different scales, one can notice that the “equivalent” conductivity cannot be interpreted as the effective one for the composites considered. Hence, a crude approximation of the effective property is proposed based on the volume fractions of constituents and the equivalent conductivities derived from different sensor locations.

The effect of the prismatic filler arrangement and cross-sectional shape on the thermal conductivity of polymer composites

Incorporation of fillers with high thermal conductivity to base materials has been recognized as an efficient way to increase the thermal conductivity of composite materials. In this study, the effects of the prismatic filler arrangement and cross-sectional shape on the thermal conductivity of the composites were investigated. This research addresses these prob- lems by solving the heterogeneous two-dimensional heat conduction problem in the composite materials. A validated com- mercial software-FLUENT and finite volume method was used in the analysis. The close-packed, directional and random filler arrangements were considered. It was found that the prismatic filler arrangement and cross-sectional shape have great impacts on the thermal conductivity of the composite materials. Double Y shaped, I shaped, T shaped, elliptical, rhombic and rectangular cross-sectional fillers can greatly increase the thermal conductivity only in directional arrangement. The effects of circular, square, triangular and Y shaped cross-sectional are similar in directional and random arrangements. The close-packed arrangements for all cross-sectional shapes have small thermal conductivity. The double Y shaped, Y shaped and I shaped cross-sectional were found to be the best choice for composite materials regardless of the filler arrangements.

Effect of composition on thermal conductivity of MgO–Nd2Zr2O7 composites for inert matrix materials

Inert matrix fuels based on magnesium oxide (MgO) as the inert phase have been of historic interest due to its high thermal conductivity. Minor actinide-bearing phases possessing an A2B2O7 pyrochlore structure are also believed to contain a range of favorable attributes, suggesting a possible pairing with MgO to yield a high performance inert matrix fuel. The thermal diffusivity, heat capacity, and thermal expansion of MgO–Nd2Zr2O7 composites were measured from room temperature to 1273K, where the MgO phase content was varied from 40 to 70 volume percent. The thermal conductivity of each composition was calculated using these results and then compared to widely employed methods to approximate the thermal conductivity of composite materials based upon the properties of the constituent phases. Results suggest that use of either a rule of mixtures or geometric mean approximation for the thermal conductivity of composite systems such as this one would be subject to significant uncertainties when the constituent properties widely differ. A sigmoidal average of the upper and lower Hashin–Shtrikman bounds was found to be in good agreement with the thermal conductivity of the composites as determined experimentally.

Evaluation of Thermal Conductive Resistance at Organic–Inorganic Interface and Effect of Interfaces on Thermal Conductivity of Composite Materials

The effect of organic–inorganic interfaces on the thermal conductivity of composite materials was studied by evaluating thermal conductive resistance at the organic–inorganic interfaces experimentally. Polymer layers were sandwiched between thin ceramic plates to produce multilayer stacked samples. Through the comparison between the thermal conductivities of the multilayer stacked materials and those of single component materials for both polymers and ceramics, the thermal resistance at the interface between a polymer and a ceramic plate was evaluated. The results suggest that the contribution of thermal resistance becomes significant for actual composite materials because of the increase in the number of interfaces. The results also demonstrate that organic modifications of an inorganic surface could effectively reduce interfacial thermal resistance. The control of interfaces seems to be effective for improving the thermal conduction property of composite materials.

Dependence of effective thermal conductivity of composite materials on the size of filler particles

A well-known Maxwell’s model for evaluation of effective thermal conductivity has been modified incorporating a correction factor s. It is an important quantity, which influences the effective thermal conductivity of composite materials together with other parameters like volume fraction and thermal conductivities of the constituents. Parameter estimation technique is used to determine the value of s. Optimized value of s is used in the modified Maxwell’s model to estimate effective thermal conductivity of composite materials. The effective thermal conductivity of composite materials using an artificial neural network approach has also been calculated. Materials of different sizes of filler particles/matrix have been considered for calculation of the effective thermal conductivity. Results obtained using modified Maxwell’s model and artificial neural network technique have been compared with the experimental results available in the literature which shows a good fit.

Conjugate heat conduction in filled composite materials considering interactions between the filler and base materials

It is common to use fillers to increase the thermal conductivity of polymer materials. Filler conductivity and filler content have been identified as the dominating factors influencing the overall effective thermal conductivity of composite materials. The interactions between the fillers and the base materials have been seldom considered. This research addresses this problem by solving the heterogeneous two-dimensional conjugate heat conduction problem in the composite materials. The interactions between the fillers and the base materials are considered. It is found that the filler shapes and their orientation angles have great impacts on the effective thermal conductivity of the composite materials. Of the various shapes, the I shaped fillers are the best choice for composite polymer materials. The influences of filler shapes and angles explain the reason why there are large discrepancies between the measured data and the available correlations in literature, which were searched for many years for the accurate predictions of the effective thermal conductivity of composite materials. The slices fillers are effective only when they are distributed parallel to heat flow directions. Currently the I shaped and T shaped fillers are promising and they should be used for future materials fabrications.

Analytical expressions for effective thermal conductivity of composite materials with inclusions of square cross-section

Thermal conductivity of composite materials with periodically distributed cylindrical inclusions of square cross-section is investigated. Analytical interpolation formula for the effective thermal conductivity in transversal direction valid in the entire range of volume fraction of inclusions and value of their thermal conductivity is derived. The methods of asymptotic homogenization, boundary shape perturbation and asymptotically equivalent functions are used. Asymptotic expansions of the effective thermal conductivity in limiting cases of size of inclusions and their thermal conductivity are obtained. The analytical expressions are evaluated numerically and compared to other models. It is shown that the discrepancy between these results is very low in all cases.

RBF-based multiscale control volume method for second order elliptic problems with oscillatory coefficients

Many important engineering problems have multiple-scale solutions. Thermal conductivity of composite materials, flow in porous media, and turbulent transport in high Reynolds number flows are examples of this type. Direct numerical simulations for these problems typically require extremely large amounts of CPU time and computer memory, which may be too expensive or impossible on the present supercomputers. In this paper, we develop a high order computational method, based on multiscale basis function approach and integrated radial-basis-function (IRBF) approximant, for the solution of multiscale elliptic problems with reduced computational cost. Unlike other methods based on multiscale basis function approach, sets of basis and correction functions here are obtained through $C^2$-continuous IRBF element formulations. High accuracy and efficiency of this method are demonstrated by several one- and two-dimensional examples.

Nonlinear programming optimization of filler shapes for composite materials with inverse problem technique to maximize heat conductivity

Adding fillers to materials of low thermal conductivity has been recognized as an efficient way to increase the thermal conductivity of composite materials. The filler shapes have great impacts on heat conduction in the materials, which can be analyzed by a direct heat conduction problem. However what is the best shape? This study answers this inverse problem by a nonlinear optimization programming technique. The problem is first standardized to the optimization of a nonlinear objective function with nonlinear constraints. Then it is solved by SQP (sequence quadratic nonlinear programming) numerical scheme. The optimum filler shape parameters are obtained. The results are that the best shapes are “I” shapes. Further, the optimization process can be regarded as a filler growth process. Depending on the fillers thermal conductivity and filler volume contents, the finally formed optimum shapes can be classified into three categories: the handicapped dumbbells, the infant less developed “I” shapes, and adult fully developed “I” shapes. Composite materials made of paraffin wax and steel fillers of six shapes are prepared to validate the optimization results.

Fabrication and Properties of AlN/CNT Composite Ceramics

High thermal conductivity and dense AlN/CNT composite ceramics with additive Y2O3 were fabricated by spark plasma sintered (SPS) in this paper. The results indicated that CNT blocked the densification process in a certain extent, but in the sintering temperature of 1700°C, the relative density of composite ceramics were more than 97% with 1-10% volume fractions of CNT. The content of additive Y2O3 affected the thermal conductivity of AlN/CNT obviously. The thermal conductivity of composite material linearly decreased when the content of CNT increased and the tubular structure of CNT had been destructed during once sintering. The effects of the CNT structural damage to the thermal conductivity of composite ceramics were improved by adjusting the sintering process. The thermal conductivity of the composite ceramics with 3% volume fractions of CNT sintering in 1550°C/3min×3 was up to 145W/m•K, which higher than the single-phase AlN ceramics.

A Molecular Dynamics-Continuum Coupled Model for Heat Transfer in Composite Materials

The heat transfer problem in composite materials containing nanoscale interfaces is investigated. A molecular dynamics-continuum coupled model is developed to study heat transport from the macro- to the microscales. The model includes four major steps: (1) A reverse nonequilibrium molecular dynamics (RNEMD) method is used to calculate some physical parameters such as the thermal conductivities on the interface. (2) The homogenization method is applied to compute the homogenized thermal conductivities of composite materials. (3) The temperature field in the global structure of composite materials is computed with the multiscale asymptotic method for the macroscopic heat transfer equation. (4) A molecular dynamics-continuum coupled model is developed to reevaluate the temperature field of composite materials, in particular, the local temperature field near the interface. Numerical results in one-, two-, and three-dimensional structures of composite materials including the nanoscale interface are given. Good agreement is achieved between the numerical results of the proposed coupled algorithm and those of the full molecular dynamics simulation, demonstrating the accuracy of the present method and its potential applications in thermal engineering of composite materials.

Orthogonal Experimental Study on Thermal Insulation Mortar Containing Mixed Ceramic Sand and Vitrified Micro Bead Aggregates

Mixed light aggregates can effectively reduce the coefficient of thermal conductivity of composite materials. Through orthogonal experiments of thermal insulation mortar containing mixed ceramic sand and vitrified micro bead aggregates, we analyzed the law of influence of relevant factors on the dry bulk density, coefficient of thermal conductivity and compressive strength of mortar containing mixed ceramic sand and vitrified micro bead aggregates and provided basic data for further improvement of such thermal insulation mortar so as to promote the development and application of high performance thermal insulation materials.

Fabrication of single crystalline diamond reinforced aluminum matrix composite by powder metallurgy route

We have successfully fabricated highly densified aluminum (Al)-diamond composite materials by a simple hot press method. The thermal conductivity of the Al-diamond composite materials was measured. These materials had different types, sizes and fractions of diamond. These obtained values were discussed based on theoretically calculated values. The thermal conductivity of the composite materials, measured by Laser-Flash method, was found to have slightly increased compared to that of pure bulk Al. The obtained microstructures of the composite materials showed a lot of cleavage existing in the interface between the Al matrix and the diamond particles, which led to the low increment of the thermal conductivity. Moreover, Al-diamond bulk materials with different sintering temperatures in solid state, liquid phase, and transient region between solid and liquid of Al, have been synthesized.

Effect of coating on the microstructure and thermal conductivities of diamond–Cu composites prepared by powder metallurgy

Diamond–Cu composites from the direct combination of diamond and Cu show low thermal conductivities due to weak interface and high thermal resistance as a result of chemical incompatibility. In this paper, a new method is proposed to strengthen interfacial binding between diamond and Cu by coating strong carbide-forming elements, e.g., Ti or Cr on the surface of the diamond through vacuum micro-deposition. Interfacial thermal resistance of diamond–Cu composites is greatly decreased when diamond particles are coated by a Cr or Ti layer of a certain thickness before combining with Cu. Thermal conductivity is also increased several times. Cr coating can reduce more effectively interface thermal resistance between diamond and Cu than Ti coating. Moreover, it has a smaller negative impact on the thermal conductivity of the Cu matrix, resulting in higher thermal conductivity of Cr-coated diamond–Cu composites. Through the vacuum micro-deposition technology, Cr on the diamond particle surface is present in the form Cr 7C 3 near diamond and a pure Cr outer layer at 2:1. The optimum thickness is within 0.6–0.9 μm; at this depth, the thermal conductivities of 70 vol% diamond–Cu composites can be increased four times and reach as high as 657 W/m K. In this work, an original theoretical model is proposed to estimate the thermal conductivities of composite materials with an interlayer of a certain thickness. The predicted values from this model are in good agreement with the experimental values.

Experimental investigation of heat conduction in polyester–Al 2O 3 and polyester–CuO nanocomposites

This paper presents an experimental analysis of the thermal conductivity of nanocomposite systems composed of unsaturated polyester resin (UPR) as matrices and two different metal-oxides nanoparticles as fillers: alumina (aluminum oxide) and tenorite (copper oxide). The nanoparticles used were alpha-Al 2O 3 (30–40 nm) and CuO (30–50 nm). Samples were fabricated using simple molding and homogenization using magnetic stirring. Thermal conductivities were measured using a device that complies with ASTM norms C518-04 and E1530-06. Measurements were taken at three different temperatures (0 °C, 25 °C and 50 °C), for different sets of samples, varying the nanoparticle fraction used in composite systems. Finally, the experimental data are compared with traditional models for predicting the thermal conductivity of composite materials, showing that the traditional models underestimate the measured values.