High thermal response rate and super low supercooling degree microencapsulated phase change materials (MEPCM) developed by optimizing shell with various nanoparticles

Highlights

• MEPCM with graphene, CNT and nano-Ag doped into shell were prepared respectively.

• Morphology and thermal response rate of MEPCM were compared.

• MEPCM doped with 0.02% graphene exhibits the highest thermal response rate.

• Supercooling degree of MEPCM can be reduced to 0.5 °C.

Abstract

Low thermal conductivity and high supercooling degree are the two major drawbacks for microencapsulated phase change materials (MEPCM). To improve thermal performance, MEPCM have been prepared and characterized using paraffin as the core and various nanoparticles (graphene, CNT and nano-Ag) doped into melamine urea-formaldehyde as the shell. Surface morphology and thermal response rate were compared. The results showed that MEPCM with graphene and nono-Ag doped into shell had good morphology. Further comparison indicated that MEPCM with shell doped with 0.02% graphene exhibited the quickest thermal response rate. Based on that, MEPCM with various core/shell ratios have been prepared and DSC test was conducted. Results showed that supercooling degree reduces with the decrease of core/shell ratio and the value can be reduced to 0.5 °C at the core/shell ratio of 1/2, which is the lowest value in publications for MEPCM. The present study can provide significant guidance for nanoparticles selection and core/shell ratio determination for preparation of MEPCM with high thermal response rate and super low supercooling degree.

Introduction

To relieve the contradiction between energy supply and demand, it is urgent to develop efficient energy storage technique. Latent heat storage is one of the most efficient and useful method due to its advantages of high heat storage density, little temperature fluctuation and easily controllable utility [1], [2]. As a potential latent energy storage medium, phase change material (PCM) can change its phase from solid to liquid with heat storage during the melting process, and changes its phase from liquid to solid with heat release during the solidification process. Among PCM, microencapsulated phase change materials (MEPCM) have attracted more and more attention in recent years since it can increase the heat transfer area, prevent leakage and control volume variation during phase change [3], [4], [5], [6]. They can be extensively applied in solar thermal energy storage and photocatalysis [7], [8], [9], textiles [10], [11], buildings [12], [13], [14], [15], waste heat recovery and high-temperature thermal energy storage [16], [17], [18], etc.

There are various encapsulation techniques available to synthesize MEPCM, such as physico-mechanical, chemical, physical, and physicochemical [19]. For MEPCM, shell materials play an important role in heat transfer characteristics as well as mechanical strength [20]. Various organic polymers are generally used as shell material due to better compatibility and easier preparation, such as PMMA [21], [22], PUA [23], [24], PUF [25], [26] and PMF [27], [28], [29], etc.

One of the main drawbacks of the polymer-based MEPCM is the low thermal conductivity. To address this problem, many researchers have focused on loading of high thermal conductive materials into polymeric shells, such as graphene [30], graphene oxide (GO) [31], [32], [33], carbon nanotube [34], SiC [35], nano siliver [36], nano-Al₂O₃ [37] and so on. For example, Wang et al. [35] have found that thermal properties of the PMF/SiC MEPCM were improved greatly and the thermal conductivity increased by 60.34% at the addition of 7% nano-SiC. Nihal Sarier et al. [36] have concluded that the highest improvement of the MEPCM doped with Ag nanoparticle could be 121% higher than those of pure MEPCM. Jiang et al. [37] have synthesized a type of MEPCM based on paraffin core and poly(methyl methacrylate-co-methyl acrylate) shell doped with nano-Al₂O₃. Results showed that the thermal conductivity was nearly linear increased with the dosage of nano-Al₂O₃, but the latent heat would decrease with the increased amount of nano-Al₂O₃.

Another major drawback of MEPCM is the supercooling which leads to reduced crystallization temperatures and the latent heat will be released at a lower temperature [38]. Many attempts have been made to suppress the supercooling process [39], [40], [41]. Adding nucleating agents such as high-melting PCM or solid nanoparticles is the most common method to reduce the supercooling degree [42]. Al-Shannaq et al. [39] have found that the onset crystallization temperature of PCMMC was nearly 10 °C lower than that of bulk PCM due to supercooling. The supercooling degree has been reduced dramatically when either RT58 or 1-octadecanol was used. However, the defect of this method is that the latent heat of the MEPCM is reduced due to the loading of additives [42]. So, in recent years, a new method for supercooling suppression has been proposed by optimizing the composition and structure of the shell. Tang et al. [43] have developed a low supercooling microencapsulated n-octadecane (MicroC18) with ODMA-MAA copolymer as shell. Results showed that the onset crystallizing temperatures of MicroC18 are only 4 °C below that of n-octadecane as the unique copolymer shell has a significant impact on the low supercooling of MicroC18. Cao et al. [42] have developed a new method for supercooling suppression by optimizing the composition and structure of the MEPCM resin shell. It is discovered that the homogeneous nucleation can be mediated by shell-induced nucleation when the shell composition and structure are optimized without any nucleating additives.

From the literature reviewed above, many studies have conducted on loading high thermal conductive additives to improve the thermal conductivity of polymer. However, to the best of our knowledge, there are few studies comparing the effect of different types of nanoparticles on the morphology and thermal properties of MEPCM. Furthermore, considerable efforts have been made to reduce the supercooling of the MEPCM, but there is still a long way to eliminate the supercooling completely. In this study, graphene with sheet-like structure, carbon tube with tube-like structure and nano siliver with globe-like structure were selected as the additives. MEPCM using paraffin as the core and melamine urea–formaldehyde (MF) as the shell with various nanoparticles embedded in have been prepared via in situ polymerization. In comparison, the MEPCM with the best morphology and the fastest thermal response rate can be obtained. Based on that, MEPCM of various core/shell ratios have been prepared and supercooling is expected to eliminate by varying the core-shell structure. In a word, the aim of this work is to develop a high thermal response rate and super low supercooling degree MEPCM by comparing various nanoparticles doped into shell.