This of the fundamental mechanism and what parameters

This
review addresses theoretical and fundamental aspects of thermal conductivity in
composite materials and the progress made over the last decade. It has been
demonstrated that the thermal conductivity parameter k is quite complex and
difficult to model for composite systems. Enhancing the thermal conductivity of
polymer-based composites by incorporating thermally conductive fillers requires
a good understanding of the fundamental mechanism and what parameters to focus
on. Many thermally conductive fillers have been studied in the literature, in
efforts to improve the thermal conductivity of composites, but in most cases,
to increase thermal conductivity by 10, a loading higher than 30% wt. is
required. Currently, the challenge is to further improve thermal conductivity
with much lower loadings.

In this
review, several general parameters are revealed to be essential and directly
related to the thermal conductivity parameter. First, crystallinity is one of
the main parameters to consider. Defects in the crystalline structure lead
inevitably to phonon scattering, i.e., a decrease in thermal conductivity. Actually,
any change in the linearity or regularity of the morphological aspect of the
filler will tend to decrease intrinsic thermal conductivity. Recent studies at
a quantum scale helped to understand the complex mechanism of thermal
conductivity a little better, but even if some progress has been made in the
comprehension of mechanism, it is now important to focus on how to reduce the
“Kapitza resistance” and fully benefit from the extraordinary intrinsic thermal
conductivity of grapheme or CNTs.

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Thermal conductivity is an anisotropic property; there-fore, the
aspect ratio, as well as the length, size, diameter, and specific surface area
of the filler, are particularly important. From a general perspective, it seems
that for each of these parameters, the higher, the better. How-ever, CNTs’
diameters remain a topic of discussion in the literature regarding their impact
on thermal conductivity. The dispersion state of nanoparticles does not appear
to be essential for thermal conductivity, as demonstrated experimentally by the
authors. The processing method is also quite important, especially regarding
the viscosity and the resulting porosity of the sample. In the composite, the
alignment of anisotropic nanofillers will directly impact the thermal
conductivity, increased in the filler direction. Experimentally, this type of elaboration
is relatively difficult (magnetically or electrically), especially for
industries. Nevertheless, it will be crucial to focus in future work on the
structural and geometrical aspects of the materials, which are essential
parameters to increase thermal conductivity.

 

 

Finally, another way to improve the thermal conductivity of
polymers is by reducing “thermal resistance”, mostly due to the filler/matrix
interfaces. Several types of functionalization were performed over the years.
Recent studies demonstrated both the positive and the negative effects of the
functionalization of fillers on thermal conductivity, increasing thermal
conductance at the filler/matrix inter-face while decreasing the intrinsic thermal
conductivity of the filler. The challenge is now to determine how to benefit
from functionalization at the interface without any loss in intrinsic thermal
conductivity of the filler.

 

To summarize, research on thermal conductivity has reached an
interesting point. Many intrinsic and experimental parameters influence the
resulting thermal conductivity of the material. In other words, it has been
illustrated through these examples how complex the thermal conductivity
parameter is and what compromise should be made to improve this property in
composites. Enhancing the thermal conductivity of composites or polymers
requires naturally thermally conductive fillers, but to achieve much higher
values of thermal conductivity, it is crucial to focus on how to improve heat
transfers at the interfaces. Over the last decade, researchers were able to
reach thermal conductivities comparable to the values of metals, using
organic-based composites. Enormous progress has been made with carbon fibers in
composites, but some encouraging results were also obtained using micro- or Nano-particles.
Through this review, we present the many important parameters for reaching
higher thermal conductivities. While some progress has been made for
composites, the challenges remain currently to determine how to benefit fully
from the intrinsic thermal conductivity of highly conductive fillers, such as grapheme,
CNTs, and graphite and to achieve values as close as possible to the
theoretical ones.