and make full use of the performance characteristics of functional fillers, the introduction of 3D structural materials assembled from small building blocks into the phase change matrices has become a research hotspot.[80] The preconstructed 3D structural materials can improve the utilization rate of thermally conductive filler and endow the composites with optimized thermally conductive network. Also, the use of 3D architectures can avoid the unstable factors such as sedimentation during the long‐term recycle.
Table 2.3 Thermal conductivities of frequently used organic PCMs and thermally conductive fillers.
Sources: Based on [80–82].
| Materials | Thermal Conductivity (W m−1 K−1) | |
|---|---|---|
| Organic PCMs | PEG | 0.30 |
| PW | 0.25 | |
| n‐Octadecane | 0.15 | |
| Erythritol | 0.72 | |
| Octadecanoic acid | 0.18 | |
| Metals | Silver | 429 |
| Cupper | 380–400 | |
| Aluminum | 204 | |
| Nickel | 158 | |
| Carbon materials | Carbon fiber (CF) | 1000 |
| CNT | 2000–6000 | |
| Graphite | 100–400 | |
| Graphene | 5300 | |
| Ceramics | BN | 250–300 |
| Aluminum nitride (AlN) | 200 | |
| Silicon carbide (SiC) | 120 | |
| Aluminum oxide (Al2O3) | 30–40 |
2.3.1 Metals
Among metal materials, the outstanding thermal conductivity of silver and copper is favored in the preparation of high thermally conductive composites. Qian et al.[84] fabricated leakage‐proof PEG/diatomite phase change composites. When a large amount of diatomite was added, the thermal conductivity improvement of the composite was very limited. Furthermore, the thermal conductivity of PEG/diatomite/silver nanoparticle ternary phase change composites was greatly improved by depositing spherical silver nanoparticles with the diameter of 3–10 nm on diatomite and then compounding with PEG. Compared with PEG/diatomite composites, the thermal conductivity of the ternary composites with silver nanoparticle loading of 7.2 wt% was increased by 127%, up to 0.82 W m−1 k−1. Similarly, physical blending and impregnation were adopted to prepare PEG/EVM/silver nanowire composites, in which EVM with porous structure and silver nanowire with the length of 5–20 μm and the diameter of 50–100 nm acted as supporting skeleton and thermally conductive component to improve the shape stability and thermal conductivity of the phase change composites, respectively.[51] Apart from silver, thermal conductivity improvement of form‐stable PEG/SiO2 phase change composites has been achieved by in‐situ cupper doping.[85]
Three‐dimensional metal foams (copper foam, nickel foam, and aluminum foam) with continuous skeleton structure exhibit excellent mechanical properties, ultra‐low relative density, high porosity, and high thermal conductivity, but they are mainly used for the preparation of PW‐based phase change composites. In addition, it is worth noting that the pore size of commercial metal foams is generally large, which is not conducive to the improvement of shape stability of phase change composites. Meanwhile, copper nanowire aerogel[86] and silver/polyvinylpyrrolidone (PVP) nanowire aerogel[87] with tunable structures have been developed to yield phase change composites with enhanced thermal and electrical conductivities, exhibiting great potentials in metal‐based polymeric phase change composites for thermal energy conversion and storage.
2.3.2 Carbon Materials
Owing to the low density, excellent thermal/chemical stability, and high thermal conductivity, carbon materials can be considered as one of the most promising candidates for production of thermally conductive phase change composites. CNT,[88–90] CF,[91] EG,[54] and GNP[89, 92, 93] have been employed to develop PEG‐based phase change composites with enhanced thermal conductivity. Additionally, carbon materials [94] can be added into leakage‐proof composites containing supporting polymers to improve the thermal conductivity. Noteworthy, it is essential to conduct thermal annealing treatment at high temperature to repair the defects corresponding to phonon scattering when GO is employed as precursor to yield graphitized graphene with high intrinsic thermal conductivity for thermally conductive phase change composites.
In addition to direct introduction of carbon‐based fillers into phase change matrices, 3D carbon‐based structural materials have been widely used to prepare phase change composites in recent years. Three‐dimensional thermally conductive networks have been successfully constructed in polymeric PCMs by incorporating biological porous carbon,[16] nanofibrillated cellulose/CNT aerogel,[67] cellulose/graphene aerogel,[68] microcrystalline cellulose/GNP aerogel,[95] GO/GNP hybrid aerogel,[55] and cellulose nanofiber/GNP hybrid‐coated melamine foam[96]. Owing to that the nanostructured materials are physically overlapped through π–π interaction in common carbon‐based aerogel/foam prepared by self‐assembly strategy, there is pronounced thermal contact resistance at the joints. Covalent‐bond diamond and graphene foams prepared by chemical vapor deposition (CVD) are able to provide continuous phonon transmission pathway. Unfortunately, these hydrophobic foams are preferential for nonpolar PCMs like PW. Hydrophilic modification such as plasma treatment is considered as an effective solution to enrich the variety of phase change composites.
2.3.3 Ceramics
Compared with electrically conductive metal and carbon materials, ceramics represented by BN gain unprecedented popularity in the fabrication of thermally conductive yet electrically insulating phase change composites, expanding their application scenarios. Thermally conductive PEG/silicon nitride (Si3N4) nanowire phase change composites have been developed via a facile solution blending.[97] AlN[98] and Al2O3[99] have been used as functional fillers to improve thermal conductivity of shape‐stabilized PEG/SiO2 phase change composites. Similarly, SiC nanowires have been incorporated into leakage‐proof PEG/EVM composites to improve the thermal conductivity.[100] PEG and BN have compounded by melt blending,[101] solution blending,[17] and interfacial polyelectrolyte complex spinning[102] to fabricate polymeric phase change composites with high thermal conductivity.
In order to improve the utilization efficiency of functional fillers and construct thermally conductive pathways, ceramic‐based porous scaffolds have been proposed for polymeric phase change composites. Unfortunately, it is more difficult to exfoliate and functionalize BN than graphite owing to the higher electronegativity of N atoms originating from the partially ionic B‐N bonds, and thus the formation of 3D‐BN structural materials