## 3 Results and Discussion
### Thermal Conductivity  
Thermal conductivity is a parameter that indicates the rate of heat transfer through materials. [PERSON] et al. (2018) found that the thermal conductivity of MgO at room temperature is 29.4 W\(\cdot\)m\({}^{-1}\) K\({}^{-1}\). Due to difficulties in accurately measuring the thermal conductivities of carbonates, the thermal conductivity of eutectic Na\({}_{2}\)CO\({}_{3}\)-K\({}_{2}\)CO\({}_{3}\) has not been experimentally studied. [PERSON] et al. (2018) have simulated the thermal conductivities of Na\({}_{2}\)CO\({}_{3}\) and K\({}_{2}\)CO\({}_{3}\) by molecular dynamics. The authors reported that the computed thermal conductivities of Na\({}_{2}\)CO\({}_{3}\) and K\({}_{2}\)CO\({}_{3}\) at \(\sim\)920 \({}^{\circ}\)C are 0.94 W\(\cdot\)m\({}^{-1}\) K\({}^{-1}\) and 0.98 W\(\cdot\)m\({}^{-1}\) K\({}^{-1}\), respectively. The samples studied in this work are composites physically synthetized using several components. Structural features like porosity may also significantly affect the overall thermal conductivity. For this reason, it would be difficult to estimate the theoretical thermal conductivity of the composites. This work reports the overall thermal conductivities of the composite PCMs with different concentrations of SiC at room temperature. Figure 2 shows the measured thermal conductivities of Sample S3 and Sample S6 to Sample S10. The experimental uncertainties are shown as error bars. The thermal conductivity of Sample S3 (without addition of SiC) measured at room temperature reaches 1.94 W\(\cdot\)m\({}^{-1}\) K\({}^{-1}\). One can see that the overall thermal conductivity of the composite PCM is enhanced when silicon carbide is added. One can also see that the thermal conductivity of the composite PCMs increases with increasing SiC content at first, but levels off at an SiC content level above \(\sim\)10%, where an enhancement of 17.53% is observed (Sample S8 gives a thermal conductivity of 2.28W\(\cdot\)m\({}^{-1}\) K\({}^{-1}\)). A further increase in SiC content gives no or a very small further increase in the thermal conductivity. The enhancements are 16.54% and 19.41% with an addition of 15 wt.% and 20 wt.% SiC, respectively. The effective thermal conductivity leveling off when the SiC addition is above 10 wt.% is attributed to an increasingly looser structure (more pores) of the composite with greater addition of SiC. This was observed in the SEM images and the porosity measurement, which is discussed in detail in Section 3.3. The increment in the porosity had a negative effect on the effective thermal conductivity of the composite due to very low thermal conductivity of air in the pores. When the amount of SiC addition reached a certain amount (\(\sim\)10% in this study), the negative effect of porosity increment was able to outweigh the benefits due to the high thermal conductivity of SiC. In addition, the energy density of the composite PCM decreased with an increasing amount of SiC addition. There would be, therefore, a compromise involving the maximum thermal conductivity enhancement with a minimal energy density decrease. As a result, Sample S8 with 10 wt.% SiC is considered as an optimal (or close to optimal) formulation in terms of the thermal conductivity enhancement and energy density reduction. In this section, thermal conductivities of the composites with different amount of SiC are discussed at room temperature. As thermal conductivity is temperature dependent and differs with different phases (e.g., liquid and solid phases), thermal conductivity of the composites at various temperatures is different. [PERSON] et al. (2018) have reported that thermal conductivity of a NaNO\({}_{3}\)/MgO/C composite PCM decreased with increasing temperature within the solid phase. [PERSON] et al. (2018) found a significant decrease in thermal conductivity when the PCM changed from the solid phase to the liquid phase. Therefore, it would be difficult to study the thermal conductivity around the working temperature, which is the phase change temperature.