## 4 Improvement of Thermal Performance
### Thermal Conductivity Enhancement Techniques
#### 4.3.2 Enhancement of Thermal Conductivity with Nanoparticle Additives  
The enhancement of thermal conductivity of PCMs can be efficiently achieved with the fabrication of composite PCMs, which consist of the PCM material and a dispersion of high-thermal-conductivity additives. The PCMs mostly enhanced are organic PCMs such as paraffin, fatty acids, fatty acid ester, alcohols, and their eutectic mixtures. The additives can be carbon-based nanostructures [73; 74], metallic nanoparticles, or nonmetallic nanoparticles. Besides the nature of the additives materials, the size, the shape, and the aspect ratio are the main factors that define the thermal conductivity improvement.  
Carbon-based materials, such as carbon fibers (CF), carbon nanotubes (CNTs), graphene, graphite, and their derivatives, are widely being investigated for thermal conductivity enhancement, with very promising results. Carbon-based materials can be used as additives directly in the organic PCM, or they can be added inside a composite PCM, such as a form-stable PCM or a microcapsule, for further thermal conductivity improvement. CNTs' disadvantage is agglomeration, and as a result, CNTs show limited dispersion ability inside the PCM. This problem can be overcome with treatment of the CNTs, such as oxidation or doping, which weakens the intramolecular forces. On the other hand, carbon fibers show better dispersion ability, and they can be added efficiently without further treatment. Addition of graphite particles can also achieve great enhancement, although graphene nanoparticles are the most effective due to low interfacial thermal resistance and large aspect ratio [45; 74].  
Metallic nanoparticles can also be used as additives for the enhancement of thermal conductivity. The metallic nanoparticles that have been used are silver, copper, nickel, and magnesium nanoparticles. All of these metallic additives enhance efficiency of the thermal conductivity, but their practical use is limited because of a number of drawbacks that they show, such as low dispersion due to high agglomeration, low thermal stability, and high density [45; 50].  
Nonmetallic nanoparticles that can be added for thermal conductivity improvement are alumina (Al\({}_{2}\)O\({}_{3}\)), magnetite (Fe\({}_{3}\)O\({}_{4}\)), titania (TiO\({}_{2}\)), mesoporous SiO\({}_{2}\), aluminum nitride (AlN), boron nitride (BN), etc. Nanoalumina particles improve the thermal conductivity, and the heat transfer, and they reduce the supercooling effect, although nanoalumina, along with titania, suffers from low dispersion problems [51]. Samaneh Sami and Nasrin Etesami [75] controlled the aggregation of TiO\({}_{2}\) nanoparticles inside paraffin PCM by adding the surfactant reagent stearoyl lactylate.  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**PCM**} & \multirow{2}{*}{**Carrier**} & \multirow{2}{*}{**Preparation Method**} & \multicolumn{2}{c}{**Thermal Conductivity of Pure PCM(W/(m.K))**} & \multirow{2}{*}{**Thermal Conductivity of Encapsulated PCM (W/(m.K))**} & \multirow{2}{*}{**Year**} & \multirow{2}{*}{**Refs**} \\ \hline \begin{tabular}{c} Capric \\ Acid-Palmitic \\ Acid Eutectic \\ \end{tabular} & \begin{tabular}{c} Silica Xerogel/ \\ Exfoliated Graphite \\ nanoplatelets \\ \end{tabular} & Sol-gel & 0.22 & 0.70 & 2020 & [70] \\ \hline \multirow{4}{*}{\begin{tabular}{c} Lauric \\ Acid-based \\ \end{tabular} } & \(\succcurlyeq\) & Carbon & & & & & \\ \cline{1-1} \cline{3-6}  & \begin{tabular}{c} black/calcium \\ carbonate \\ powder \\ \end{tabular} & \begin{tabular}{c} Dispersion of \\ nanoincusions \\ in sonication \\ bath \\ \end{tabular} & 0.042 & 0.268 & & & & \\ \cline{1-1} \cline{3-6}  & \
\begin{tabular}{c} XPWCNTs/ \\ calcium carbonate \\ powder \\ \end{tabular} &
\begin{tabular}{c} Inversion of \\ nanoincusions \\ in sonication \\ bath \\ \end{tabular} & 0.033 & 0.024 & & & & \\ \hline \hline \end{tabular}
\end{table}
Table 6: Cont.