Form-stable phase change composites with high thermal conductivity and adjustable thermal management capacity
Introduction
In recent years, energy crisis has aroused worldwide concerns due to the continuous increase in energy consumption and the limited reserves of fossil energy. Improving the energy utilization efficiency is widely perceived as an effective route to realize the sustainable development, because it can not only save the available energy sources but also reduce CO2 emissions [[1], [2], [3]]. The significant development of phase change materials (PCMs) in recent years opens up new opportunities for realizing efficient energy use [4,5]. PCMs, especially organic PCMs, which can storage and release substantial latent heat by melting and solidifying over a narrow temperature range, possess many exceptional properties including large latent heat, appropriate phase transition temperature and good thermal reliability, making them as prospective candidates as thermal energy storage and thermal management materials [[6], [7], [8]]. In spite of these commendable features, the practical applicability of liquid-solid PCMs is severely limited due to the liquid leaking problem [[9], [10], [11]].
The microencapsulation technology has been demonstrated to be an efficient route in addressing the leaking issue of PCMs [12]. In this case, PCMs are capsuled with solid shell materials, thus realizing the leakage proof and control of the volume change during the phase transition [[13], [14], [15]]. In general, the final products of the microencapsulation technology are powdered microcapsules which are light weight and easy to be blew off, making them difficult to be handled when used [16]. Besides, the powder products can’t meet the requirement of some special application scenarios which demand for large dimensional parts. For the aim of broadening the application scopes of PCMs, the powdered microcapsules need to be further compounded with other host matrices to fabricate large phase change composites (PCCs) [16,17]. However, to the best of our knowledge, there are few works have been reported on the fabrication of PCCs with large dimensions.
Polymers are promising matrix materials due to the good processability and low cost. Nevertheless, the poor thermal conduction of polymers (0.2–0.5 Wm-1K-1) needs to be overcome in order to fabricate high-performance PCCs [18]. Boron nitride (BN), as a typical thermal conductive filler, has been widely utilized to improve the thermal conductivity of polymer materials due to its many desirable features including high thermal conductivity (250–300 Wm-1K-1), excellent electrical insulation, low dielectric constant and outstanding chemical inertness [19,20]. However, the inorganic BN exhibits poor interfacial compatibility with organic polymers and thereby lead to serious aggregation and large interfacial thermal resistance, making it difficult to obtain satisfactory thermal conductivity [21,22]. Surface modification of BN platelets, therefore, is necessary to improve the matrix-filler adhesion and thus remove the blocks in enhancing the thermal conductivity.
In the present study, steric acid (SA) is selected as the phase change material due to its low cost, nontoxicity, large phase transition enthalpy (about 170 J/g) and moderate phase transition temperature (approximately 45 °C–65 °C) [23,24]. Inorganic silicone dioxide (SiO2) is chosen as the shell material for SA due to its easy synthesis and good thermal stability. Different from the traditional sol-gel method, the core-shell structural SiO2@SA microcapsules are synthesized by means of interfacial polycondensation.
To enhance the interfacial affinity of BN platelets with polymer matrix, pristine BN is first treated with plasma to generate a hydroxylated surface and then grafted with a silane coupling agent. The plasma process is adopted due to its high efficiency and no generation of liquid waste which is a big problem in conventional chemical treatment [25,26]. Then the SiO2@SA microcapsules and m-BN are added into silicone rubber (SR) to achieve large-dimensional SR/SiO2@SA/m-BN phase change composites (PCCs). The microcapsules play the role as phase change filler while the surface-modified BN (m-BN) contributes to enhance the thermal conduction behavior. Here, the synthesis of microcapsules, surface modification of BN platelets and properties of PCCs are investigated in depth, thus providing a scientific basis for the design and development of high-performance PCMs.
Section snippets
Materials
Steric acid (SA), emulsifier OP-10, acetic acid (36 wt %), tetraethoxysilane (TEOS) and silane coupling agent (KH560) were supplied by Chengdu Kelong Chemical Reagent Co., Ltd. (China). Boron nitride (BN) was purchased from Aladdin Chemical Reagent Co., Ltd. The hydroxyl-terminated polysiloxane prepolymers with density of 0.98 g/cm3 and viscosity of 1.5 Pa s (25 °C) was provided by Zhongshan Sancheng Silicone Co., Ltd. (China).
Preparation of SiO2@SA microcapsules
The phase change microcapsules with a SA core and SiO2 shell were
Synthesis of SiO2@SA microcapsules
The SiO2@SA microcapsules with a core-shell structure are prepared via interfacial polycondensation, as shown in Fig. 1a. The preparation mechanism is elaborated in the Supplementary material. The prepared microcapsules are white powders and the shape can’t be identified by naked eyes (Fig. S1). As shown in the SEM images (Fig. 2), the synthesized microcapsules exhibit a near-spherical profile and rough surface with some protrusions, which should be ascribed to the SiO2 shell. Such spherical
Conclusion
The phase change microcapsules with SA core and SiO2 shell are synthesized though the interfacial polycondensation technology. The graft ratio of silane agent is measured as 7.5% and the enhancement of interfacial compatibility is realized. Large dimensional PCCs are prepared through compounding the microcapsule and m-BN into the host matrix. The PCCs attain high thermal conductivity up to 0.506 Wm-1K-1. The phase change enthalpies of PCCs can be facilely tailored from 3.74 J/g to 18.52 J/g by
Author statement
Honghui Liao: Methodology, Data curation, Roles/Writing - original draft, Software, Formal analysis, Shengwei Guo: Resources, Validation, Funding acquisition, Conceptualization, Yuan Liu: Project administration, Methodology, Writing - review & editing, Supervision, Qi Wang: Conceptualization.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement
This work is supported by National Key Research and Development Program of China (Project No. 2017YFB0309001), NSAF Fund (U183010085), NSFC Fund (51720105012), Sichuan Science and Technology Project (No. 2019YFSY0011), Key projects of Guangzhou Science and technology plan (201904020019) and the Fundamental Research Funds for the Central Universities.
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