**2. Potential thermal reinforcements**

Thermal reinforcements are defined as the external additives being mingled with paraffin waxes to fabricate PWTCs. Thermal reinforcements need to possess certain traits encompassing high/low thermal conductivity, good mechanical strength, porous surface geometry, adaptability and the uniform minglingcompatibility with the paraffin waxes. Depending on the chemistry of thermal reinforcements, they are majorly categorized into inorganic sorbents and organic encapsulants, as depicted in **Figure 2**. Inorganic sorbents are defined as thermal reinforcements providing the physical scaffolds wherein paraffin wax is infused through capillary action. These scaffolds can also be said as oleophilic (be noted all paraffinic PCMs have oiliness though the percentage may differ) that all paraffin waxes based on the wetting characteristics induced by micro/nano-porous structures. Inorganic sorbents are further classified into carbonic, silicate and metallic

#### **Figure 2.**

*Classification of thermal reinforcements and their role as thermal conductivity enhancer (TCE) or shape-stabilizer (SS). A few contents of this figure are partially restructured from references [6].*

thermal reinforcements. Organic encapsulants are defined as thermal reinforcements that build a shell (a boundary encircling the entity) wherein paraffin wax resides as the core material. The next section sheds a light on the preparation methods, morphologies and thermo-physical properties of PWTCs made of these thermal reinforcements.

In the light of application-dependent scenarios, there are two functions that PWTCs should endow to the paraffin wax: temperature-control capacity and shape-stability, but the existence of both functions may not be found in single PWTC. Temperature-control capacity is a coupled function of latent heat and thermal conductivity of PWTCs, imparting a suitable isothermal zone that is highly desired in thermal management applications. Thermal reinforcements that help enhances thermal conductivity of the paraffin waxes are called thermal conductivity enhancers (TCEs). As for the shape-stability of PWTCs, it is defined as the state in which the liquid drainage of the melted paraffin wax is minimized or eventually stopped, and such thermal reinforcements are termed as shape-stabilizers (SSs). In brief, thermal reinforcements can potentially act as the TCSs, SSs or both, and by incorporating the most appropriate thermal reinforcements; duplex or triplex PWTCs can be fabricated.

#### **2.1 Inorganic sorbents**

Inorganic sorbents and the corresponding PWTCs are discussed in the coming subsections.
