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Synthesis and Performances of Phase Change Materials Microcapsules with a Polymer/BN/TiO2 Hybrid Shell for Thermal Energy Storage Na Sun, and Zhenggang Xiao Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b01271 • Publication Date (Web): 11 Aug 2017 Downloaded from http://pubs.acs.org on August 15, 2017
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Synthesis and Performances of Phase Change Materials Microcapsules with a Polymer/BN/TiO2 Hybrid Shell for Thermal Energy Storage Na Sun, Zhenggang Xiao*
School of Chemical Engineering, Nanjing University of Science and Technology, Nanjing 210094, P. R. China
Z. Xiao* Tel: +86-25-84315138; E-mail:
[email protected]. ORCID: 0000-0002-7836-1397
N. Sun E-mail:
[email protected] ORCID: 0000-0001-5944-9281
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Abstract: Paraffin was encapsulated in a poly (methyl methacrylate) (PMMA)/BN/TiO2 hybrid shell to obtain a novel phase change material (PCM) microcapsule for latent-heat storage. Scanning electron microscope (SEM) micrographs showed that the resultant composite PCMs exhibited a nearly spherical morphology with a size of 10-20 µm and BN/TiO2 nanoparticles were tightly embedded on the microcapsule surface. The results of thermogravimetric analysis (TGA) and thermal conductivity test demonstrated that the PMMA/BN/TiO2 composite shell gave the microencapsulated paraffin an excellent thermal performance. Compared to pure paraffin, the thermal conductivity of microcapsules could significantly to be enhanced by 117.0% and the maximum decomposing temperature has an increase of 25.4°C. The analysis of differential scanning calorimetry (DSC) and thermal reliability test indicated that the composite PCMs also presented good heat storage capacity and high thermal reliability for latent-heat storage and release. After a heating-cooling cycle test for 100 times, the composite microcapsule still showed a similar phase change performance as before, which can be attribute to the protective effect of PMMA/BN/TiO2 hybrid shell. With increased demands for economical and high-performance renewed energy storage materials, the prepared composite PCMs show great potential applications in thermal energy storage. Keywords: Phase change materials; Hybrid shell; Composite microcapsules; Boron nitride; Thermal energy storage; Pickering emulsion 1. INTRODUCTION
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In recent decades, owing to the shortage of energy resources and growing consumption of the fossil fuels, phase change materials (PCMs) have been recognized as good candidates for renewed energy storage. Currently, they have been applied in various areas such as energy-saving building materials, solar energy systems, battery thermal energy management and thermal-regulated textiles
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. In general, the direct utilization of bulk
PCMs for thermal energy storage medium requires some containers or packages. However, the inherent low thermal conductivity of organic PCMs and liable leakage of liquid state PCMs have greatly restricted their wide applications 6,7. Microencapsulation is an effective way to give PCMs with a shapely characteristic either in solid or in liquid states and increase the heat transfer area 8-10. The conventional encapsulated PCMs were constructed by using organics as shells such as polyurea, ethyl cellulose, polystyrene, cellulose and poly (methyl methacrylate) (PMMA)
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. While the encapsulation of PCMs with organics exhibited a desired
microcapsule structure and good shape stability over phase change without flowing, the microencapsulated PCMs with organics shells have some shortages such as poor thermal and chemical stabilities, flammability, low mechanical strength, and low thermal conductivity. Recently, many studies have been focused on using inorganic materials to cover the disadvantages of polymeric shells 17-23. For example, Chai et al. 20 designed a sort of novel microencapsulated PCM by encapsulating n-eicosane into a crystalline titanium dioxide shell through in-situ polycondensation. Yu et al.
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exploited a self-assembly
method for the encapsulating of PCMs with a calcium carbonate shell. Zhang et al. 3
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successfully prepared microencapsulated PCMs with a compact ZrO2 shell. As a result, the obtained microcapsules using these inorganic shells as the supporting material presented excellent thermal conductivity and thermal stability. However, owing to the poor compatibility between inorganics and polymeric matrix, the use-pattern of these microcapsules is limited. It is well known that organic-inorganic composite has received particular interests because it can achieve a synergetic combination of unique properties that cannot be obtained from individual components
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. Therefore, when using organic-inorganic
composite material as hybrid shell to encapsulate PCMs, the prepared composite PCMs microcapsules will possess excellent interface compatibilities in polymer matrix, good thermal stability, high thermal conductivity and other characteristic functions. In our previous work, we used nano-Si3N4 to modify paraffin based microcapsules via an eco-friendly Pickering emulsion polymerization. The thermal conductivity of PCMs microcapsule modified with Si3N4 was significantly improved
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. However, it needs a
tedious grafting and modification process before using nano-Si3N4 since it is very fluffy and easily attached to each other. Moreover, to improve the stability of Pickering emulsion system, some sodium dodecylbenzene sulfonate (SDBS) as co-surfactant was added in reaction system, resulting the hybrid shell was not compact and strong enough to protect PCM cores from penetration. In this study, to avoid complicated pretreatment process for nano-Si3N4 and remove the surfactant which has negative effects on products performance from the reaction system, 4
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boron nitride nanoparticles (nano-BN) were employed as Pickering stabilizer instead of Si3N4/SDBS to prepare a more stable Pickering emulsion based on PCMs. Then, a series of composite PCMs microcapsules with a compact PMMA/BN/TiO2 hybrid shell were high effectively fabricated. As an important ceramic material, BN has outstanding thermal resistance, excellent mechanical strength, high corrosion resistance and thermal conductivity. The thermal conductivity of perfect single crystal hexagonal BN (h-BN) platelet is about 600 W/m·K in plane direction, which is far greater than that of Si3N4 (16.7 W/m·K)
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. However, considering the cost and benefit, part of nano-BN particles were
replaced by TiO2 nanoparticles which also possess excellent thermal characteristics. The prepared PCMs microcapsules in this paper with a rigid PMMA/BN/TiO2 hybrid shell provide a better mechanical protection to the PCM core and give the composite PCMs high shape and thermal stability as well as good thermal conductivity. 2. EXPERIMENTAL 2.1. Materials Solid paraffin (SP, 52-54 °C), hexagonal boron nitride nanoparticles (nano-BN, S2>S3>S4 and S5
