*2.1.1 Carbonic thermal reinforcements*

Carbonic thermal reinforcements are documented as inherently high-thermal conductivity materials based on well-connected thermal networks. They majorly include graphite powder (PG), expanded graphite (EG, which is obtained by heat-treating the expandable graphite at high temperature of 800–900°C for a few seconds), single-wall carbon nano-tubes (SWCNTs), multi-wall carbon nano-tubes (MWCNTs), and graphene nano-platelets (GNPs). The salient properties of the carbonic thermal reinforcement include the appropriate infusion-compatibility and unique morphology consisting of micro/nano-porous structures, as shown in **Figure 3**, rendering them both TCEs and SSs.

However, the disadvantageous features of carbonic thermal reinforcements also exist, for example, anisotropic thermal conductivity of graphite, reduced thermal conductivity of graphene when it is mixed, and segregation of carbon nano-tubes. In addition, duplex and triplex PWTCs can be synthesized with them via dry or wet-physical methods. For example, a triplex PWTC has been fabricated by dry-physical method, employing 80% of paraffin wax (melting temperature of 48–50°C, thermal conductivity of, and latent heat of 207 jg−1), 20% of EG and 5% of GNPs. In addition to EG, the incorporation of GNPs has further introduced effective results with enhanced thermal conductivity (around 5.9 Wm−1 K−1 at compress density of 505 kgm−3) and latent heat of 159 jg−1 [8], which is thus deemed to be feasible because the mass percent of all ingredients is somehow optimal. Nonetheless, graphene (also the graphene oxide) has been reported to have very high thermal conductivity (5000 Wm−1 K−1), but EG/PWTC/GNP could not correspondingly achieve that high thermal conductivity, implying a surprising phenomenon. The reason is attributed to the design challenge of GNPs, i.e., when GNPs are joined together to form a compound, the phase segregation effect appears causing to cleave the internal thermal networks due to which the thermal conductivity of compound GNPs is much reduced [10]. Another case study of triplex PWTC [11] has been presented, consisting of 50% paraffin wax (melting temperature of 60–61°C, thermal conductivity of 0.26 Wm−1 K−1, and latent heat of 223 jg−1), 10% of GP and

**Figure 3.** *Micro/nano-porous morphology of (a) EG [8], (b) GNPs [8] and (c) CNTs [9].*

40% of expanded perlite (EP). As-prepared GP/EP/PWTC has latent heat of 111.4 jg-1 and thermal conductivity of 1.34 Wm−1 K−1. The question arises on the latent heat which reduces from 223 jg−1 to 111.4 jg−1 which is mainly because of 50% of EP. In the meanwhile, the EP/PWTC neither achieves high thermal conductivity nor shape-stability, while upon incorporation of 10% GP, high thermal conductivity as well as effective shape-stability has been attained, as shown in **Figure 4** [11].

As a conclusive viewpoint, the need of thermal reinforcements should be wellunderstood together with avoiding the unimportant thermal reinforcements while synthesizing PWTCs, which otherwise can definitely lead to severe shortcomings particularly for energy storage applications where high thermal storage capability is preferred. Therefore, instead of such triplex PWTCs (e.g., EP/GP/paraffin wax), duplex PWTCs (e.g., GP/paraffin wax) may result in more viable outcomes.

The anisotropic thermal conductivity of EG has a great influence on thermophysical properties of the paraffin wax. The layered sheet/wall-like structure of EG is thus accountable for this effect, inhibiting the heat transfer when it is perpendicular to the graphite layered-structure, while expediting the heat transfer when it is parallel. Such anisotropic effects of has been studied recently for EG alone and EG/ PWTCs, demonstrating the much higher thermal conductivity in parallel direction than that of normal direction depending on various temperatures (**Figure 5**) [12].

#### **Figure 5.**

*Directional thermal conductivity of (a) EG and (b) EG/PWTC. P: Parallel direction, N: Normal direction [12].*

#### *Paraffin Wax-Based Thermal Composites DOI: http://dx.doi.org/10.5772/intechopen.97195*

These results imply that the directional effects should also be considered while implementing the EG/PWTCs in applications. In addition, particle size effect of EG is also important. It has been investigated recently that the large-sized and small-sized particles of EG have thermal conductivities of 0.5 Wm−1 K−1 and 3.23 Wm−1 K−1, respectively, which are capable of enhancing thermal conductivity of PWTCs in the same order [13]. In general, thermal conductivity of PWTCs increase with increasing the percent contents of thermal reinforcement. However, it is achieved on the penalty of latent heat reduction [13]. This is the reason that PWTCs need to be optimized by keeping the design parameters ahead, so that the ideal PWTCs can be created as claimed above in **Figure 2**.

Thermal stability of paraffin waxes is defined as the maximum temperature limit after which thermal decomposition begins and it is normally 150–170°C [9]. Evaluated with help of thermogravimetric analysis (in which the specimen material is evaporated at high temperature while simultaneously measuring the mass loss indicating the thermal decomposition), carbonic/PWTCs have been reported with enhanced thermal stability (around 190–200°C) [13] which is indeed attributed to the heat-withstanding strength of thermal reinforcements. The enhanced thermal stability also indicates the successful infusion of paraffin waxes into thermal reinforcements which then impart a kind of thermal protection during thermal processes.
