## 2 Results and Discussion
respectively), the presence of CNTs seems to contribute to a lesser reduction in \(\Delta\)H values compared to the composites with the pure carbon matrices. It is worth noting that half of the carbon in the composite materials was replaced with nanotubes (0.5 g carbon and 0.5 g CNTs in carbon/CNTs/OD composites instead of 1 g carbon in carbon/OD composites). In the case of the CNT composites, the \(\Delta\)H values are closer to the calculated theoretical values according to each wt% OD loading. The presence of nanotubes enhances the thermal properties of octadecane probably due to the higher thermal conductivity of CNTs compared with AC, CCF and EG, facilitating more efficient heat transfer during the phase transitions of octadecane [22; 34]. In addition, it seems that the paraffin loss observed in earlier studies can be prevented by the addition of CNTs [20]. Figure 7 presents the thermal conductivity (TC) of the samples studied in this work at 300 K. As one can see from Figure 7, all samples exhibit low TC (0.10-0.28 W/m\(\cdot\)K\({}^{-1}\)) resulting in inadequate heat transfer that prevents heat from penetrating into the PCM interior, thereby reducing their effectiveness in thermal energy storage (TES) applications [7]. The thermal conduction of AC and EG was measured as 0.13 and 0.10 W/m\(\cdot\)K\({}^{-1}\), respectively, while the corresponding values of AC/OD and EG/OD were 0.29 and 0.21 W/m\(\cdot\)K\({}^{-1}\), respectively. As already reported [35] and verified from our findings, AC, a porous form of graphite with a complex and imperfect structure, has a higher surface reactivity and adsorption capacity, which enhances its ability to store heat, verified by its higher TC compared to EG. Compared to OD (with a TC of ~0.153 W mK\({}^{-1}\) in a liquid state [36]), the above-mentioned enhancement of both the AC/OD and the EG/OD samples in TC is mainly due to the many interconnected heat conduction channels of the carbon network structure, owing to the substitution of the air within the pores of AC (or EG) with OD [37], as already reported in the literature [38; 39]. The incorporation of multi-walled carbon nanotubes (MWCNTs) into activated carbon or expanded graphite has garnered significant attention in materials science and engineering due to the unique properties that MWCNTs bring to composite materials. One of the critical properties affected by this addition is thermal conductivity, which is essential for various applications, including energy storage, thermal management, and filtration systems. The addition of MWCNTs in activated carbon or expanded graphite combined with octadecane can lead to decreased overall thermal conductivity under specific conditions due to poor dispersion or interface effects that inhibit effective heat transfer pathways. As seen in Figure 7, the addition of MWCNTs in AC/OD or EG/OD leads to AC/CNTs/OD  
Figure 7: Thermal conductivity of studied samples (AC-based samples marked with blue stars, EG-based samples marked with red dots, at 300 K.  
or EG/CNTs/OD samples with reduced TC values, equal to \(\sim\)0.10 and \(\sim\)0.13 W/m\(\cdot\)K\({}^{-1}\), respectively. This behavior could be attributed to insufficient formation of MWCNTs' conductive pathways, poor dispersion leading to agglomeration, and ineffective interface interactions that hinder heat transfer, especially at low concentrations, where there may not be enough MWCNTs to create an effective network for heat conduction. The EMI shielding (\(SE\)) efficiency of our samples \(SE_{T}\) can be expressed as the sum of reflection (\(SE_{R}\)), absorption (\(SE_{A}\)) and multiple-reflection (\(SE_{M}\)) as follows [40]:  
\[SE_{T}(\text{dB})=SE_{R}(\text{dB})+SE_{A}(\text{dB})+SE_{M}(\text{dB}) \tag{1}\]  
Generally, multiple-reflection at internal interfaces inside the material can be excluded if \(SE>\) 10-15 dB.