*2.1.2 Silicate-based thermal reinforcements*

The silicate-based thermal reinforcements are regarded as low thermal conductivity materials, but their micro/nano-porous morphology (**Figure 6**) and consequential specific surface area allows them to impregnate with the paraffin waxes. More precisely, they come in class of clay-like materials and commonly termed as the clay minerals, for examples, expanded perlite (EP), kaolin clay, diatomite, palygorskite or attapulgite and vermiculate, etc., [14–16].

The silicate-based thermal reinforcements are available in abundance at cheap and economical rates, as well as they are notably non-toxic and have a good compatibility with the paraffin waxes based on which shape-stabilized PWTCs can be achieved. However, a large percent threshold of silicate-based thermal reinforcements is normally required to functionalize them as SS for PWTCs. In addition, for high thermal response of PWTCs with silicate-based thermal reinforcements, TCE in sufficient percent may be required; hence, the triplex PWTC cannot be avoided in that case, for example, carbonic TCEs have been incorporated [17]. In the meanwhile, the main point of focus, which is of great practical significance, is the consideration of applications of silicate/PWTCs. Normally, the most compatible features of silicate-based thermal reinforcements have been seen appropriate for the building materials, such as cement, gypsum and sand, etc., implying that

**Figure 6.** *Micro/nano-porous morphology of (a) EP [14] and (b) diatomite [15].*

thermal management of buildings can be done if PWTCs. In such applications, very high latent heat enabled by large percent contents of paraffin waxes is completely undesired. For example, 20% of paraffin wax/expanded perlite is declared to be the optimum in the main skeleton of cement mortar [17]. The reason is the non-stiffness of paraffin waxes, leading to decrease the flexural and compressive strength of building materials at percent contents. Therefore, the quantitative optimization of silicate/PWTCs is highly crucial. Based on this design principle, low latent heat of paraffin wax/diatomite/gypsum triplex composite (around 45 J/g) has been found quite reasonable [18]. Nonetheless, enhancing the thermal response of silicate/PWTCs for building applications is debatable on the standpoint: buildings need to reserve the heat inside the room in winter or outside of it in summer, but high thermal conductivity of silicate/PWTCs is expected to boost up the heat transfer rate which is almost similar to the cement-based walls. For example, a case-study is depicted in **Figure 7** [19]. In heating mode (**Figure 7a**), the paraffin wax/diatomite thermal composite a larger isothermal zone (blue curve in **Figure 7a**) compared with that of paraffin wax/diatomite/ CNT (red curve in **Figure 7a**), while in cooling mode (**Figure 7b**), the opposite trend holds true. This is ascribed to the high thermal conductivity of PWTCs achieved on behalf of CNTs. Suppose that this PWTC is applied in building walls in hot countries where the average temperature in summer is higher than 45; the high thermal conductivity of PWTC is deemed to allow heat transfer at fast rates, meaning that the time consumed in saturating the PWTC is less. With this trend, the overall time to keep the thermal management of buildings, both in heating and cooling modes, is expected to be decreased. In simple words, the isothermal zone should be long-lasting so that the more time can be ensured for thermal management. Therefore, there is a great need to decide whether silicate/PWTCs should have high or low thermal conductivity.

#### *2.1.3 Metallic thermal reinforcements*

Metallic thermal reinforcements are regarded as highly thermal conductive materials existing in three scaffolds, namely: fins that are extruded plate/tube-like thin structures, foams consisting of wire/fiber-based network with varying degree of number of pores, and powders that are composed of micro/nano-particles [20]. The micro/nano-porous scaffolds of metallic thermal reinforcements serve as the confinement sites wherein paraffin waxes reside, as demonstrated the surface morphologies in **Figure 8**.

**Figure 7.** *(a) Heating mode, and (b) cooling mode of PWTCs via transient thermal analysis [19].*

**Figure 8.** *Micro/nano-porous scaffolds of (a) copper foam [21], (b) nickel foam [22] and (c) graded aluminum foam [23].*

Metallic materials own a very high thermal conductivity owing to the freely available electronic carries and vibration-assisted modes of heat transfer. The same metallic material, for examples, copper, nickel and aluminum, can be either available in foam or powder, but their initial preparation methods differ a lot. Metallic thermal reinforcements can only act as TCEs, and shape-stability cannot be ensured since the interaction between their network and paraffin waxes is solely based on capillary forces lacking of the liquid-soaking capability.

This drawback may create additional challenges such as weakening the thermal interface between paraffin wax and metallic fibers. Nonetheless, the effective thermal conductivity of PWTCs via copper foams has been achieved very high, such as 16 Wm−1 K−1 which is based on the high inherent thermal conductivity of copper foam (i.e., 400 Wm−1 K−1) [24]. Overall, the work on improving the weak thermal interface is left as a research area of future.
