Molecular dynamics study on the interaction between metal-organic frameworks and phase change core materials
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摘要: 金屬有機骨架材料（metal-organic frameworks, MOFs）由于具有規整的孔道結構，較高的孔隙率十分適合作為相變材料的載體，從而實現對相變芯材的有效封裝。本文采用分子動力學方法，對Cr-MIL-101負載十八烷，十八酸，十八胺和十八醇等不同芯材而構筑的復合相變材料的結構特性進行了研究，主要包括相變芯材和金屬有機骨架基材之間的相互作用，芯材在金屬有機骨架材料孔道內的擴散特性以及空間分布特性等。研究表明：十八酸和金屬有機骨架基體之間的相互作用最強，十八醇和十八胺次之，十八烷最弱，具體體現在相變芯材分子與金屬有機骨架材料之間的相互作用能，回轉半徑，分子動能，自擴散系數以及熱容等眾多方面，此外，當芯材分子間相互作用和金屬有機骨架材料與芯材之間的相互作用達到平衡時，芯材分子在孔道內處于較為自由的狀態，有利于擴散的進行，進而有利于芯材的結晶。Abstract: Advanced phase change energy storage materials are the core and key to promoting the development of energy storage technology. As the core of phase change energy storage technology, the development of phase change materials (PCMs) has attracted more and more attention. At present, solid?liquid PCMs are widely used. The main problem in the development of PCMs is that they are prone to liquid leakage in the process of phase change and need to be encapsulated before use. This not only increases the thermal resistance between the PCMs and heat source equipment, reducing the heat transfer efficiency, but also increases the weight of the energy storage device, which greatly limits its practical application. As a result, the development of PCMs with excellent comprehensive performance is of great significance for the field of thermal energy storage and utilization. Due to the regular channel structure and the high porosity, metal-organic frameworks (MOFs) are very suitable to serve as the PCMs carrier to realize effective packaging of phase change core materials. In this work, the structural properties of Cr-MIL-101 loaded with different core materials, namely, the octadecane, octadecanoic acid, octadecylamine, and octadecanol molecules, were investigated by molecular dynamics simulation method, which mainly considers the interaction between the phase change core and the MOFs substrate, the diffusion characteristics, and the spatial distribution characteristics of the core in the MOFs channel. The study indicates that the interaction between octadecanoic acid and MOFs substrate is the strongest, followed by the interaction between the substrate and octadecanol and octadecamine, while the interaction between the substrate and octadecane is the weakest. This result is also reflected in many aspects, such as the interaction energy between the molecules and MOFs, the radius of rotation, the molecular kinetic energy, the self-diffusion coefficient, and the heat capacity. In addition, when the interaction between the core material molecules and the interaction between MOFs and the core material reach an equilibrium, the core material molecules are in a relatively free state in the pore, which is conducive to diffusion, and then to the crystallization of the core materials.
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圖 2 Cr-MIL-101負載不同相變芯材的模型質量負載率為70%。（a） 十八酸；（b） 十八胺；（c） 十八醇；（d） 十八烷
Figure 2. Structure model of Cr-MIL-101 loaded with different phase change core materials: (a) octadecanoic acid; (b) octadecylamine; (c) octadecanol; (d) octodecane
圖 3 不同復合相變材料回轉半徑的概率分布。（a） Cr-MIL-101@十八酸；（b） Cr-MIL-101@十八胺；（c） Cr-MIL-101@十八醇；（d） Cr-MIL-101@十八烷
Figure 3. Probability distribution of radius of rotation: (a) Cr-MIL-101@octadecanoic acid; (b) Cr-MIL-101@octadecylamine; (c) Cr-MIL-101@octadecanol; (d) Cr-MIL-101@octodecane
圖 4 均方位移隨時間的演化曲線。（a） Cr-MIL-101@十八酸；（b） Cr-MIL-101@十八胺；（c） Cr-MIL-101@十八醇；（d） Cr-MIL-101@十八烷
Figure 4. Evolution curve of mean-square displacement with different times: (a) Cr-MIL-101@octadecanoic acid; (b) Cr-MIL-101@octadecylamine; (c) Cr-MIL-101@octadecanol; (d) Cr-MIL-101@octodecane
圖 5 芯材運動動能隨時間的演化曲線。（a） Cr-MIL-101@十八酸；（b） Cr-MIL-101@十八胺；（c） Cr-MIL-101@十八醇；（d） Cr-MIL-101@十八烷
Figure 5. Evolution curve of the kinetic energy of core materials with different time: (a) Cr-MIL-101@octadecanoic acid; (b) Cr-MIL-101@octadecylamine; (c) Cr-MIL-101@octadecanol; (d) Cr-MIL-101@octodecane
表 1 Cr-MIL-101與不同相變芯材之間的相互作用能
Table 1. Interaction energy between Cr-MIL-101 and different phase change core materials
相變芯材 十八酸 十八胺 十八醇 十八烷 相互作用能/(103 kJ·mol?1) ?7.62 ?6.10 ?6.50 ?5.25 
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表 2 Cr-MIL-101與不同相變芯材之間的靜電作用能
Table 2. Electrostatic energy between Cr-MIL-101 and different phase change core materials
相變芯材 十八酸 十八胺 十八醇 十八烷 相互作用能/(103 kJ·mol?1) ?3.78 ?3.39 ?3.43 ?2.72
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表 3 不同相變芯材在金屬有機骨架材料孔道中的自擴散系數
Table 3. Self-diffusion coefficients of different phase change core materials in MOFs channel
相變芯材 十八酸 十八胺 十八醇 十八烷 自擴散系數/(10?4 nm2·ps?1) 8.6 10.12 7.43 6.35
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表 4 金屬有機骨架材料負載不同相變芯材的熱容
Table 4. Heat capacity of MOFs loaded with different core materials
相變芯材 十八酸 十八胺 十八醇 十八烷 熱容/(103 kJ·mol?1) 5.49 ?6.27 7.99 5.08
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