**3. Microencapsulation shell materials**

In recent years, microencapsulation of PCMs has been widely used to avoid the leakage and reaction of the PCMs with the surrounding environment during the solid–liquid phase transition.

Microencapsulation of PCMs can also be responsible for relatively constant volume, high thermal cycling stability and large heat transfer area for PCM-based thermal storage [19]. Shell/wall materials play a vital role in controlling various physical properties like morphology, mechanical and thermal properties of the produced microcapsules [7]. On the basis chemical nature shell material can be divided into three groups: (a) organic, (b) inorganic and (c) organic–inorganic hybrid materials [20].

#### **3.1 Organic shells**

Organic shell materials include synthetic and natural polymeric materials, which have excellent structural flexibility, good sealing properties and high resistance to the volume change associated with repeated phase transformations of PCMs [21]. The organic shell materials most frequently used consist of urea-formaldehyde (UF) resin [22], melamine-formaldehyde resin [23], and acrylic resin [24]. Many workers around the world used MF resin as the wall material due to its good chemical compatibility, low cost and thermal stability [25]. Mohaddes et al. effectively utilized MF as the shell material for encapsulation of *n*-eicosane for application to textiles [26]. Fabrics doped with this type of microcapsules have higher thermoregulation capacity and low thermal delay efficiency. Among the group of acrylic resins, the copolymers of methacrylate have significant thermal stability, chemical resistance, nontoxic nature and easy preparation. Alkan et al. has shown that *n*-eicosane microencapsulation with polymethylmethacrylate (PMMA) shell had good thermal stability [27]. Ma et al. successfully encapsulated binary core materials, butyl stearate and paraffin using poly(methylmethacrylate-*co*divinylbenzene) (P(MMA-*co*-DVB)) copolymer as the shell material [28]. The microcapsules so obtained possess a uniform size of 5–10 *μ*m with a uniform spherical shape and dense surface. Moreover, the phase transition temperature of these microcapsules can be adjusted by adjusting the butyl stearate to paraffin ratio. Wang et al. studied the effect of GO on the thermal properties of capric acid@UF microcapsules by adding various contents of graphene oxide (GO) [29]. It was found that the microcapsules with 0.6% GO had the highest enthalpy of 109.60 J/g and encapsulation ratio of 60.7%. The microcapsules with GO presented smoother surfaces and good thermal conductivity.

### **3.2 Inorganic shells**

Microcapsules prepared by using organic polymeric shell materials are usually not suitable for application in some situations due to the low thermal conductivity, *Design and Fabrication of Microencapsulated Phase Change Materials for Energy/Thermal… DOI: http://dx.doi.org/10.5772/intechopen.102806*

flammable nature and poor mechanical strength of the organic shell materials [30]. In recent years, inorganic shells due to their good thermal conductivity, high rigidity and high mechanical strength, have been progressively employed as an alternative shell material for microcapsule preparation [21]. The commonly used inorganic shell materials include Silica (SiO2) [31], zinc oxide (ZnO) [32], titanium dioxide (TiO2) [33] and calcium carbonate (CaCO3) [34].

Silica because of its fire resistance nature, high thermal conductivity and ease of preparation are one the most commonly used shell materials for encapsulation of fatty acids [35], paraffin waxes [34] and inorganic hydrated salts [36]. Liang et al. prepared nanocapsules by encapsulating *n-*octadecane core material using silica as the shell material via interfacial hydrolysis and polycondensation of tetraethoxysilane (TEOS) in miniemulsion [37]. The thermal conductivity of nanocapsules so obtained was observed to be higher than 0.4 Wm−1 K−1 with melting enthalpy and encapsulation ratio of 109.5 J/g and 51.5%, respectively. The enthalpy of the nanocapsules was not altered and no leakage was observed after 500 thermal cycles. However, the hydrolysis and polycondensation of TEOS, used as a silica precursor, could cause a reduction in the compactness of the silica shell and have a relatively weak mechanical strength. CaCO3 shells have higher rigidity and better compactness compared to silica. Yu et al. employed CaCO3 as shells material for encapsulating *n*-octadecane through the self-assembly method [12]. The microcapsules obtained were of spherical morphology with a uniform diameter (5 *μ*m) and had good thermal stability, thermal conductivity, anti-osmosis properties and serving durability.

Metal oxides, like ZnO and TiO2, owing to their multifunctional properties including photochemical, catalytic and antibacterial characteristics are frequently used as shell materials to obtain PCM microcapsules with some remarkable characteristics. Li et al. synthesized multifunctional microcapsules with latent heat storage and photocatalytic and antibacterial properties by using ZnO as the shell material and *n*-eicosane as the core material [38]. Similarly, Liu et al. utilized TiO2 as shells material for encapsulating *n*-eicosane through interfacial polycondensation followed by ZnO impregnation [39]. The prepared microcapsules have both thermal storage and photocatalytic capacities with a melting temperature of 41.76°C and latent heat of 188.27 J/g.

#### **3.3 Organic: inorganic hybrid shells**

Organic–inorganic hybrid shells materials are used to overcome the shortcomings related to the individual organic or inorganic materials for encapsulating PCMs. In hybrid shells, organic materials offer structural flexibility while inorganic materials can improve thermal conductivity, thermal stability and mechanical rigidity [21]. Polymers (such as PMMA and PMF) based shells doped with SiO2 or TiO2 are extensively used to encapsulate PCMs [40]. Wang et al. prepared *n*-octadecane microcapsules using PMMA-silica hybrid shells via photocurable Pickering emulsion polymerization with good morphology and particles size of 5–15 *μ*m [41]. The highest encapsulation efficiency (62.55%) was achieved with the weight ratio of MMA to *n*-octadecane of 1:1. Zhao et al. successfully synthesized bifunctional microcapsules by using PMMA doped with TiO2 as the hybrid shell *and n*-octadecane as the core material [42]. TiO2 was observed to improve microcapsules' thermal conductivity but reduce encapsulation efficiency and enthalpy. The initial degradation temperature of microcapsules with 6% TiO2 reached 228.4°C, confirming good thermal stability of the microcapsules. Wang et al. prepared multifunctional microcapsules with

regular-spherical morphology by using poly(melamine-formaldehyde)/silicon carbide (PMF/SiC) hybrid shells and *n*-octadecane as cores material [43]. The thermal conductivity of microcapsules with 7% SiC had improved by 60.34% compared to those microcapsules with no SiC, which is also accompanied by a significant increase in heat transfer rate.
