Thisreview addresses theoretical and fundamental aspects of thermal conductivity incomposite materials and the progress made over the last decade. It has beendemonstrated that the thermal conductivity parameter k is quite complex anddifficult to model for composite systems. Enhancing the thermal conductivity ofpolymer-based composites by incorporating thermally conductive fillers requiresa good understanding of the fundamental mechanism and what parameters to focuson. Many thermally conductive fillers have been studied in the literature, inefforts to improve the thermal conductivity of composites, but in most cases,to increase thermal conductivity by 10, a loading higher than 30% wt. isrequired. Currently, the challenge is to further improve thermal conductivitywith much lower loadings.In thisreview, several general parameters are revealed to be essential and directlyrelated to the thermal conductivity parameter.

First, crystallinity is one ofthe main parameters to consider. Defects in the crystalline structure leadinevitably to phonon scattering, i.e., a decrease in thermal conductivity. Actually,any change in the linearity or regularity of the morphological aspect of thefiller will tend to decrease intrinsic thermal conductivity.

Recent studies ata quantum scale helped to understand the complex mechanism of thermalconductivity a little better, but even if some progress has been made in thecomprehension of mechanism, it is now important to focus on how to reduce the”Kapitza resistance” and fully benefit from the extraordinary intrinsic thermalconductivity of grapheme or CNTs.Thermal conductivity is an anisotropic property; there-fore, theaspect ratio, as well as the length, size, diameter, and specific surface areaof the filler, are particularly important. From a general perspective, it seemsthat for each of these parameters, the higher, the better. How-ever, CNTs’diameters remain a topic of discussion in the literature regarding their impacton thermal conductivity. The dispersion state of nanoparticles does not appearto be essential for thermal conductivity, as demonstrated experimentally by theauthors.

The processing method is also quite important, especially regardingthe viscosity and the resulting porosity of the sample. In the composite, thealignment of anisotropic nanofillers will directly impact the thermalconductivity, increased in the filler direction. Experimentally, this type of elaborationis relatively difficult (magnetically or electrically), especially forindustries. Nevertheless, it will be crucial to focus in future work on thestructural and geometrical aspects of the materials, which are essentialparameters to increase thermal conductivity.  Finally, another way to improve the thermal conductivity ofpolymers is by reducing “thermal resistance”, mostly due to the filler/matrixinterfaces. Several types of functionalization were performed over the years.Recent studies demonstrated both the positive and the negative effects of thefunctionalization of fillers on thermal conductivity, increasing thermalconductance at the filler/matrix inter-face while decreasing the intrinsic thermalconductivity of the filler. The challenge is now to determine how to benefitfrom functionalization at the interface without any loss in intrinsic thermalconductivity of the filler.

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 To summarize, research on thermal conductivity has reached aninteresting point. Many intrinsic and experimental parameters influence theresulting thermal conductivity of the material. In other words, it has beenillustrated through these examples how complex the thermal conductivityparameter is and what compromise should be made to improve this property incomposites. Enhancing the thermal conductivity of composites or polymersrequires naturally thermally conductive fillers, but to achieve much highervalues of thermal conductivity, it is crucial to focus on how to improve heattransfers at the interfaces. Over the last decade, researchers were able toreach thermal conductivities comparable to the values of metals, usingorganic-based composites. Enormous progress has been made with carbon fibers incomposites, but some encouraging results were also obtained using micro- or Nano-particles.Through this review, we present the many important parameters for reachinghigher thermal conductivities.

While some progress has been made forcomposites, the challenges remain currently to determine how to benefit fullyfrom the intrinsic thermal conductivity of highly conductive fillers, such as grapheme,CNTs, and graphite and to achieve values as close as possible to thetheoretical ones.