Thermal Conductivity of Polymeric Composites: A

2013 IEEE International Conference on Solid Dielectrics, Bologna, Italy, June 30 – July 4, 2013

Thermal Conductivity of Polymeric Composites: A Review I.A. Tsekmes, R. Kochetov, P.H.F. Morshuis, J.J Smit Intelligent Electrical Power Grids Delft University of Technology Delft, the Netherlands Abstract—Polymers are the preferred insulating materials for several electrical applications due to their ease of production, light weight, and low cost. However, their low thermal conductivity constitutes a bottleneck and efforts are made in order to increase it. An approach to improve heat transfer through polymers is the inclusion of fillers with relatively high thermal conductivity. Investigation in this direction has been carried out by different research groups. In this paper, a reference to a number of different studies is made and the role of fillers in polymers is investigated. The main goal of this review is to determine any potential trends which govern and dictate the process of thermal conduction in polymeric composites including nanocomposites and microcomposites. Keywords—polymeric composites; thermal nanofillers; nanocomposites; microcomposites

I.

conductivity;

INTRODUCTION

Polymers are widely used as insulating materials. However, a lot of effort is put in order to improve their properties and especially their thermal conductivity. High thermal conductivity is desirable in order for heat to be efficiently dissipated. In this way, the operating temperature can be kept low, avoiding dielectric failures due to overheating. Polymers exhibit a low thermal conductivity because of their relatively low atomic density, weak interactions or chemical bonding, complex crystal structure, and high anharmonicity in their molecular vibrations [1]. Typical thermal conductivity values of some polymers are listed in Table I. Phonon transport is the main mechanism of heat conduction in most polymers. Phonons transfer heat energy through interactions with each other and with subatomic particles [2]. Lattice imperfections such as dislocations, voids, and impurities can introduce anharmonicities which result in phonon scattering. In a multi-phase system such as polymeric composites scattering also occurs when phonons propagate . TABLE I. THERMAL CONDUCTIVITY OF POLYMERS [3] Polymer Thermal Conductivity (W/mK) Low density polyethylene 0.28-0.32 High density polyethylene 0.38-0.58 Epoxy resin 0.17-0.21 Polypropylene 0.18-0.24 Phenol resin 0.24-0.29

through a boundary which separates one phase from another. Incorporation of fillers in electrical insulation polymers is a common approach to improve electrical, mechanical, and thermal properties. Improved electrical insulation systems can operate at higher temperatures and greater electrical stresses. The thermal conductivity of polymers has been traditionally enhanced by the addition of thermally conductive fillers including graphite, carbon black, carbon fibers, ceramic or metal particles [4]. Ceramics are vastly used as fillers due to their high thermal conductivity and electrical resistivity [5, 6]. In many publications, improvement in the thermal conductivity of polymers when fillers are included, has been reported [5, 7 - 12]. Here we are trying to take a step back and look at different studies which have been carried out so far. The main goal is to correlate all these investigations and summarize the main mechanisms which govern the determination of thermal conductivity in polymeric composites. In literature, a lot of thermal conductivity studies on polymeric nanocomposites with different base materials and fillers have been reported [13]. A direct comparison between different composites is difficult since a different manufacturing procedure is adopted by each group of researchers. Also, a comparison between the same materials, base and filler, might be meaningless as always there are differences between the same type of filler such as crystal structure, size, and shape. Nevertheless, the scope of this work is to investigate some general trends which affect the thermal conductivity of polymeric composites. II.

MICROCOMPOSITES

A microcomposite consists of a polymer matrix and microsized particles with typical size between 1 and 100 μm. In the case of high filler concentrations (larger than 30 vol. %), composites exhibit much higher thermal conductivity compared to neat polymers. In this case, two important parameters can be claimed that play a major role in determining the thermal conductivity of microcomposites, i.e. the thermal conductivity of fillers and interaction between them. Table II refers to the study of Kochetov [14] where composites of epoxy resin with alumina (Al2O3) and silica

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2013 IEEE International Conference on Solid Dielectrics, Bologna, Italy, June 30 – July 4, 2013 (SiO2) microfillers were investigated. It can be observed that for a lower volume concentration of Al2O3, the thermal conductivities of the composites are similar. Microfillers with high thermal conductivity and high filler concentration can increase the heat transfer rate as heat conduction mainly occurs through them. Huang et al. [8] investigated the thermal conductivity of boron nitride (BN) based poly(phenylene) sulfide composites. A non-linear relationship was found between the BN loading and composite thermal conductivity. This behaviour could be attributed to the interaction between the particles. Conductive chains in the direction of heat flow can lead to an increase of thermal conductivity. The probability of forming conductive particle chains at high filler concentrations was studied by Agari and Uno [15]. In high filler loading composites, conductive chains can be formed where the mean interparticle distance is smaller than in other parts, leading to a high thermal conductivity along these chains. A part of the study of Weidenfeller et al. [16] is summarized in Table III. The large difference in the thermal conductivity between the two composites was related to the different interconnectivity of the fillers as the results could not be explained by using only the particle properties. However, at low filler concentrations (less than 20 vol. %), the improvement of thermal conductivity is usually less significant. The low thermal conductivity of the polymer matrix mainly determines the composite thermal conductivity. III.

NANOCOMPOSITES

Nanofillers can be one (fibers), two (platelets) or three (spheres) dimensional without any shape limitation and their size should be below 100 nm at least in one dimension. In the case of nanocomposites, the distribution of fillers should be as homogeneous as possible and if agglomerations are present, their average size should be less than 100 nm. A number of composites have been produced and investigated by different research groups [12, 13]. Generally speaking, inclusion of nanofillers in polymers can lead to a higher thermal conductivity compared to their neat counterparts. An effort to approach the main mechanisms behind this observation is included in this work. A.

Effect of Particle Size and Shape

Kochetov [14] studied the effect of size of BN particles on the thermal conductivity of composites based on epoxy resin and the results are listed in Table IV. Although the average paTABLE II. Composite ER-Al2O3 ER-SiO2

EPOXY RESIN MICROCOMPOSITES @ 180C [7, 14] Avg. λepoxy λfiller λcomp. Vol.% Particle (W/mK) (W/mK) (W/mK) Size 31.2 0.17 20-30 4μm 0.67 45 0.17 0.7-1.7 20μm 0.72

TABLE III. Composite PP-copper PP-talc

POLYPROPYLENE MICROCOMPOSITES [16] λfiller λPP λcomp. (W/mK) (W/mK) (W/mK) 30 400 0.25 1.25 30 10.6 0.25 2.5

Vol.%

EPOXY RESIN - BN COMPOSITES @ 180C [14] Avg. Particle λcomp. λER Vol.% Shape (W/mK) (W/mK) Size 5.8 0.17 Spherical 70nm 0.240 5.8 0.17 Platelet 0.5μm 0.274 5.8 0.17 Spherical 1.5μm 0.242

TABLE IV. Composite ER-BN ER-BN ER-BN

rticle size of BN increases from 70 nm (nanocomposite) to 1500 nm (microcomposite), the thermal conductivity is nearly the same. The difference between the particles of 70 nm and 500 nm could be attributed to the shape and not to the size difference. In case of platelet-like particles, due to the higher aspect ratio, there is a smaller average distance between the particles. The reduced distance between the particles could be the reason for the more efficient heat transfer. Han et al. [9] investigated the thermal conductivity of epoxy resin composites with the addition of different size of BN fillers. BN-micro, BN-meso, and BN-nano were used. In the latter case, micronsized hexagonal agglomerations were observed. The authors observed that there is no significant difference in the thermal conductivity between the aforementioned composites. Therefore, they concluded that at low and moderate filler concentrations the filler size is not of crucial importance to the thermal conductivity of the composites. In both aforementioned cases, the size of BN particles does not significantly affect the heat transfer through the composites. On the contrary, particles with higher than one aspect ratio may affect the thermal conductivity in a positive way. A high aspect ratio can favour the formation of conducting networks by lowering the percolation threshold [14] and diminish the disadvantageous role of interfacial thermal resistance on the thermal transport [17]. However, a good dispersion of nanosize particles with a larger than one aspect ratio is much more difficult to be achieved due to the increased particle-to-particle interaction [18]. B.

Effect of Particle Thermal Conductivity

The thermal conductivity of nanofillers is assumed to be in the same range as the thermal conductivity of the bulk material counterparts. However, the validity of this approach is doubtful. The thermal conductivity of fillers is affected by their crystal structure, impurities, structural imperfections, phonon scattering events, etc. and can be different than that of the bulk material [1]. Nevertheless, provided that the filler thermal conductivity is known, a correlation between filler and composite thermal conductivity is attempted to be done. In Table V, two different composites based on epoxy resin are examined [5, 7, 14]. Aluminum nitride (AlN) is used in the first one and magnesium oxide (MgO) in the second. In both cases the same filler concentration is used. The results suggest that there is no significant contribution of the intrinsic thermal conductivity of the filler to the composite thermal conductivity. In the case of AlN, one would expect much higher composite thermal conductivity, given the higher thermal conductivity of the filler, the larger average particle size (by a

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2013 IEEE International Conference on Solid Dielectrics, Bologna, Italy, June 30 – July 4, 2013 EPOXY RESIN BASED COMPOSITES @ 180C [5, 14, 19] Avg. λfiller λER λcomp. Particle Composite Vol.% (W/mK) (W/mK) (W/mK) Size ER-AlN 0.7 150-320 0.17 60nm 0.179 ER-MgO 0.7 45-50 0.17 22nm 0.175 TABLE V.

factor of 3) and the larger number of agglomerations. However, it can be observed that the results are close to each other. Therefore, it is not always the case that higher thermal conductivity of fillers (at least theoretically) leads to a higher composite thermal conductivity. C.

Effect of Surface Modification of Particles

The use of coupling agents has been adopted by a number of researchers in order to obtain a better interaction between fillers and matrix [20]. It seems that interfacial thermal resistance decreases, suppressing phonon scattering, when surface treated fillers are used. Comparing the results between composites with and without surface treated particles, the former usually show higher thermal conductivity than the latter. Kochetov [14], Irwin et al. [21] and Choudhury et al. [22] studied the effect of coupling agents on the thermal conductivity of polymeric composites. Composites of epoxy resin and BN were studied by Kochetov. The thermal conductivity of the composites is higher (by nearly 3%) when silane coupling agent is used. Polyamide nanocomposites were studied by Irwin. The composites with coated nanofillers exhibit a higher (by 11% on average) thermal conductivity than those with uncoated nanofillers and it was attributed to the improved filler-matrix interactions. Finally, Choudhury studied epoxy – AlN nanocomposites and a more efficient heat transfer (an average improvement of 13%) was found for the composites with surface treated nanofillers. The improvement in thermal conductivity, which has been reported, is believed to be related to the interfaces between fillers and matrix. A better adhesion between fillers and host material can suppress phonon scattering, leading to higher values of thermal conductivity. A number of models and theories concerning interfaces have been proposed to explain their role. Electrical and electromechanical properties of interfaces were approached by Lewis [23 - 25]. Tanaka [26] proposed the multi-core model for explaining the chemical and electrical properties of interfaces. Raetzke [27] replaced the term interface with the term interphase in order to give emphasis on the different structure of this layer. According to that model, the interphase properties differ from the properties of the remaining materials. All the aforementioned models try to describe the features of interfaces, however, none of these has been verified yet. D.

Effect of Agglomerations

Agglomerations of particles have been reported in literature to enhance the thermal conductivity of composites. Han [9] has reported an increase in thermal conductivity of epoxy based composites when agglomerates of particles are present. Evans et al. [17] developed a model to demonstrate the contribution of agglomerations to the thermal conductivity enhancement. They have shown based on thermal conduction

physics, that the thermal conductivity of nanocomposites can be increased when nanoparticles aggregate. E.

Free Volume

Information on the impact of nanoparticles on free volume is limited. Nelson [28] mentions three cases of clay-polymer nanocomposites where a decrease in free volume was observed. Also, surface modification of nanofillers has been reported to lead to a potential decrease in free volume in composites. Roy et al. [6] attributed the improvement in lifetime of composites with surface modified nanofillers to the formation of chemical bonds which increase the interfacial polymer density and minimize defects such as microvoids. Furthermore, Choudhury et al. [22] studied the density of epoxy - AlN composites with and without surface modified fillers. The results suggest that there is an increase in density when surface treated fillers are used. This behaviour can be attributed to the presence of less volume fraction of voids or pores in the composites with surface modified particles. IV.

DISCUSSION

Incorporation of microparticles, with relatively high thermal conductivity, into a polymer at high filler concentrations can result in significant enhancement of the composite thermal conductivity. The interaction between the particles and contribution of the high intrinsic thermal conductivity of the fillers are believed to lead to an improved heat transfer through the composite. At low microfiller volume fractions, the improvement in thermal conductivity is limited by the low thermal conductivity of the matrix. The conductivity of well dispersed and distributed nanoparticles at low filler concentrations in a composite seems not to significantly influence the thermal conductivity of the polymeric composite. It can be claimed that the low thermal conductivity of the matrix dominates. However, the way of incorporation of nanoparticles inside the polymer matrix seems to play an important role in determining the heat conduction in nanocomposites. Suitable surface modification of nanofillers improves the thermal conductivity of nanocomposites. A better filler – matrix adhesion could be the reason. It is postulated that the formation of chemical bonds between fillers and matrix decreases the interfacial thermal resistance and minimizes defects such as voids. In this way phonon-scattering is suppressed resulting in a more efficient heat transfer. It can be claimed that the contribution of particle size and particle thermal conductivity to the nanocomposite thermal conduction is limited. On the contrary, the aspect ratio of particles and agglomerations can affect to a high extent the effective thermal conductivity of nanocomposites. Due to their very high specific surface areas, a few percent nanofillers can self-assemble to produce skeleton-like superstructures especially when anisotropic fillers with high aspect ratio are used [29]. The interaction between fillers and matrix is not well understood yet. It is believed that interfacial regions are formed around the particles and affect to a great extent the .

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2013 IEEE International Conference on Solid Dielectrics, Bologna, Italy, June 30 – July 4, 2013 dielectric properties of nanocomposites. All the models developed in order to describe the properties of these interfaces have not been verified but they approach to interpret the improved properties that nanocomposites exhibit. In the case of nanocomposites, the interfacial area is much greater than in the case of microcomposites due to the high surface-to-volume ratio of the former [6]. So, for the same particle loading, nanocomposites will have a much greater interfacial area than microcomposites. Although a larger interaction zone is formed in nanocomposites, there is no significant difference in the thermal conductivity of nanocomposites and microcomposites at low filler concentrations. Further investigation is required to clarify the altered properties of these interfaces and the way in which they affect the effective thermal conductivity of composites. V.

CONCLUSIONS

So far, experimental data on the thermal conductivity of nanocomposites cannot directly verify the existence of interfacial layers or their significant contribution to the thermal conductivity enhancement. Other factors may overshadow the potential advantages that these layers can offer to the thermal conductivity of nanocomposites. The intention of this paper is to find some correlations between different composites in order to investigate how deeply we can understand the thermal transfer mechanisms in polymeric composites and the factors which affect their thermal conductivity. REFERENCES [1] [2]

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