## 3 Shell Materials  
The use of PCMs in their crude form has several drawbacks, such as low thermal conductivity and liquid leakage during solid-liquid phase changes, which limit their applications. Thus, many encapsulation techniques were developed to overcome these disadvantages and improve heat transfer. This approach allows not only to transform a  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Eutectic Mixture** & \begin{tabular}{c} **Mass Ratio** \\ **wt\%** \\ \end{tabular} & \
\begin{tabular}{c} **Onset Melting** \\ **Temperature (\({}^{\circ}\)C)** \\ \end{tabular} &
\begin{tabular}{c} **Melting** \\ **Latent Heat (\(\cdot\)g\({}^{-1}\))** \\ \end{tabular} & **Ref.** \\ \hline CA-LA & 64:36 & 19 & 163 & [73] \\ CA-MA & 78.39:21.61 & 25 & 123 & [74] \\ CA-PA & 891:1 & 28 & 145 & [74] \\ CA-SA & 94.47:5.53 & 30 & 156 & [74] \\ LA-PA & 77.51:22.49 & 38 & 151 & [74] \\ MA-SA & 76.29:23.71 & 48 & 148 & [74] \\ LA-MA & 63.63:36.37 & 34 & 130 & [74] \\ LA-SA & 86.51:13.49 & 41 & 164 & [74] \\ MA-PA & 64.96:35.04 & 45 & 152 & [74] \\ PA-SA & 62.99:37.01 & 54 & 179 & [74] \\ MA-SA & 64.36 & 44 & 182 & [75] \\ CA-PA & 76.5:23.5 & 23 & 156 & [76] \\ CA-MA-SA & 72.5:22.5:5.0 & 24 & 159 & [66] \\ CA-PA-SA & 79:13:8 & 20 & 129 & [71] \\ SA-PA-LA & 6.77:20.97:72.26 & 32 & 159 & [74] \\ LA-MA-SA & 57.5:34.3:8.2 & 29 & 140 & [77] \\ SA-PA-LA-MA & 11.86:18:54.5:6.24 & 27 & 128 & [72] \\ Tetradecane-octadecane & - & -4 & 228 & [38] \\ Tetradecane-docosane & - & 2–6 & 234 & [38] \\ Tetradecane-hexadecane & - & 2 & 156 & [38] \\ \hline \hline \end{tabular}
\end{table}
Table 2: Examples of PCMs eutectic mixtures and their properties.  
liquid into a pseudo-solid and thus protect it during its use but also improve its thermomechanical and morphological properties. The performance of microcapsules is greatly influenced by the chemical and physical properties of the shell, and therefore, the selection of raw materials depends on desired characteristics and the application. There are three different types of shell materials, i.e., organic, inorganic, and organic-inorganic hybrid shells.  
Organic materials are mainly melamine-formaldehyde (MF) resin, urea-formaldehyde (UF) resin, polyurea (PU), and acrylic resins poly(urea-urethane).  
Microcapsules with organic shells are characterized by their suitable structural flexibility and suitable reliability after repeated thermal cycles [78]. However, they exhibit poor chemical and thermal stabilities and undergo some shortcomings such as toxicity, flammability, and poor heat transfer performance [23]. The selection of the techniques depends on the physicochemical properties of core and coating materials (Figure 3), required properties, and the application fields.  
Recently, inorganic shell materials have gained more attention, including silica, titania, calcium carbonate, zinc oxide, and polystyrene (PS). Inorganic shell materials are recognized by excellent thermal stability, high thermal conductivity, mechanical durability, chemical inertness, and non-toxicity [44].  
The employment of hybrid shells consists of mixing organic and inorganic materials. The hybrid shells not only lead to overcoming the drawbacks but also combine the advantages of organic and inorganic materials. Therefore, the use of inorganic materials enhances mechanical rigidity, thermal stability, and thermal conductivity, while organic materials offer structural flexibility to microcapsules.