Research paperThermophysical properties of three-dimensional palygorskite based composite phase change materials
Introduction
Improving current energy utilization and developing renewable energy are central to achieving the goal of energy sustainability, as they reduce the conventional fossil-based energy consumption and consequently the emission of greenhouse gases without jeopardising the development of global economy (Li and Shi, 2019; Wang et al., 2018; Rashidi et al., 2018; Peng et al., 2017). For the intermittent nature and contradictory characteristic between supply and demand of renewable or excessive energy sources, a suitable energy storage device is important and necessary (Leong et al., 2019; Zhang et al., 2017a; Wang et al., 2015). Among the multitude devices for thermal energy storage (TES) and retrieval, latent heat thermal energy storage (LHTES) embedded with phase change materials (PCMs) is considered to be the most effective and promising technique (Farid et al., 2004; Li and Shi, 2019). Phase change material, working as storage media or storage unit in LHTES system, is a sort of functional material allowing the cycle of heat storage and release from its phase change from solid state to liquid state or vice versa within a slight temperature variations (Wang et al., 2009; Wang et al., 2012; Qian et al., 2015; Pielichowska and Pielichowski, 2014). Although PCMs have prominent advantages of high latent heat capacity and small temperature variations, they also have insurmountable defects in traditional manners, such as leakage, low thermal conductivity and large supercooling degree of inorganic PCMs, which lead to the loss of PCM, contamination of equipment, hysteresis of thermal response, and consequently restriction of the final applications (Guo et al., 2016; Su et al., 2015; Zhang et al., 2019).
The above problems are expected to be addressed by developing shape-stabilized PCMs (SSPCMs) with incorporating or macro/microencapsulating techniques (Yang et al., 2016a; Qian et al., 2015). Among these, mineral-based SSPCMs have received significant attention. This is because most of mineral supporting materials have prominent pore structure, considerable specific surface area, abundant natural resources and environmentally friendly nature (Li and Shi, 2019). Due to the capillary force, surface tension, hydrogen bond, Van der Waals' force and the interfacial adsorption effect of supporting medium, the PCM can be breezily compounded with mineral materials and the leakage issue can be effectively solved (Lv et al., 2017; Li and Shi, 2019). Moreover, the heat charging and discharging transfer rates of mineral-based SSPCMs are greatly improved due to the melioration effect of mineral matrix on thermal conductivity (Li et al., 2019a; Rashidi et al., 2018).
The common minerals used for SSPCMs supporting substrates mainly include kaolin (Memon et al., 2015; Lv et al., 2016), diatomite (Qian et al., 2015; Sari and Bicer, 2012a), sepiolite (Konuklu and Ersoy, 2016), expanded perlite (Li et al., 2013), vermiculite (Sari and Bicer, 2012b), palygorskite (Song et al., 2014; Li et al., 2011; Yang et al., 2016a; Wang et al., 2011; Liang et al., 2014), fly ash (Ying et al., 2014), etc. Nevertheless, as noted above, current researches only focused on the utilization of natural and inherent features of minerals. Despite various methods are employed to optimize the structure and capacity of mineral carrier, such as acid activation, thermal treatment, surface modification and surface functionalization (Song et al., 2014; Zhang et al., 2019), the locally improved microstructure and partially modified surface cannot change the characteristic fundamentally. The specific surface area of mineral units cannot be fully utilized because of large free energy of nanoparticles and its porous structure cannot be regulated. Due to the impregnated PCM amounts depending on the porous features, the fabricated SSPCMs, traditionally prepared via vacuum impregnation method, melt embedding technique and melt adsorption method, has limited thermal storage capacity (Peng et al., 2017). Aiming at it, the promising route seems to be dissociated the mineral aggregates and then constructed 3D interconnected network which is bonded with covalent bond.
On the other hand, thermo physical properties of mineral-based SSPCMs are substantially dependent on the morphology, crystallographic structure, microstructure (including strut, pore and cell sizes, specific surface area and particle size) of supports and coupling action between mineral and PCMs. In general, the employed supports with higher thermal conductivity will enhance the phase change rate, reduce the inside temperature gradient and make the internal temperature distribution more homogeneous (Li et al., 2019c; Abishek et al., 2018). According to phonon propagation theory, thermal conductivity is confined by phonon mean free path. The poorly ordered crystallographic structure (Li et al., 2019a), small particle size (Lv et al., 2016), random contact geometry and weak physical interaction (Qi et al., 2017; Li et al., 2019b) of supports caused an increasing number of phonon-defect scattering events, further limited the mean free path of phonons in matrix lattices, and thus resulted in the low conductivity. The stronger interfacial coupling of composite PCMs (CPCMs) formed by hydrogen bond or electrostatic attraction will result in low interface thermal resistance (Li et al., 2019b; Liu and Zhang, 2019). Moreover, the enthalpy and temperature of phase transition are also sensitive to the pore size, filling ratio and interfacial coupling. The composite with smaller pore diameter usually has lower melting point and smaller latent heat even in the same identical impregnation ratio (Zhang et al., 2015; Nomura et al., 2015). The friendly coupling behavior is conducive to eliminate the existence of nonfreezing layer (Schmidt et al., 1995), improve crystallinity of incorporated PCMs and finally enhance thermal energy storage ability of composite (Nomura et al., 2015). It is obvious that thermophysical properties of mineral-based SSPCMs can be optimized by tailoring the microstructure of supporting material.
Palygorskite (Pal), also called attapulgite, is a kind of crystalline hydrated magnesium aluminum silicate mineral with micro fibrous morphology (Zhang et al., 2019; Yang et al., 2016a, Yang et al., 2016b). The rod-like crystal has large specific surface area, moderate cationic exchange capacity, pronounced adsorption property and reactive silanol groups (Nomura et al., 2015). Due to its plentiful surface charges and reactive groups, the three-dimensional network with palygorskite rod-like crystal skeleton could be easily obtained via “grafting onto” or “grafting from” methods. The advantage of this new-type three-dimensional network is the microstructure and surface properties can be adjusted through the coupling or bonding agent. The coupling molecule also acted as upholder to prevent aggregation of rod crystals. It can be predicted that the SSPCMs based on this carrier will have ameliorative heat storage capacity and adjustable thermal-physical properties.
In light of the above discussion, series of three-dimensional palygorskite (3D Pal) carriers were fabricated by reacting the mixture of dihydric alcohol with modified palygorskite which pre-grafted by 2, 4-toluene diisocyanate (TDI). The pore size of 3D Pal was adjusted by the length of dihydric alcohol, acted as cross-linking agent. And then, stearic acid/three-dimensional palygorskite-based composites were prepared by vacuum impregnation method, and the structure, thermal properties were determined through various techniques to further understand the effects of supporting materials' microstructure on thermophysical properties of PCM incorporated into porous supports. This research is expected to provide a deep insight into design porous media based PCMs.
Section snippets
Materials
Palygorskite, from Dianjinshi technology Co. Ltd. in Jiangsu China and pretreated by distilled water and hydrochloric acid, was employed to prepare three-dimensional Pal. Stearic acid (SA, commercial grade), used as phase change material, was obtained from Shanghai Aladdin biological technology Co. Ltd. 2,4-toluene diisocyanate (TDI), hexanediol (HED) and polyethylene glycol (PEG) with molecular weights of 300, 600, 2000, 4000 and 6000 were selected as coupling reagent and cross-linking agent
The Structure and morphology of TDI-HO-Pal
The FTIR spectra of bare Pal, TDI-Pal and TDI-HO-Pal were shown in Fig. 2. From the FTIR spectra of Pal, the peaks at wavenumbers of 3400 cm−1 and 3550 cm−1 are respectively caused by the adsorbed water and crystal water in the Pal tunnel and the absorption peak at 1650 cm−1 is derived from the bending vibration of hydroxyl groups (-OH) in water molecules (Liu et al., 2018; Guan et al., 2015). The characteristic peaks located at 1200 cm−1, 1031 cm−1 and 985 cm−1 are attributed to the stretching
Conclusions
Series of three-dimensional palygorskite carriers (TDI-HO-Pal) with different pore size were fabricated by the reaction of isocyanate functionalized palygorskite with dihydric alcohol. The successful preparation of three-dimensional carrier provides a new way for surface grafting or modification of palygorskite and other mineral. Once the mineral is coupled by isocyanates, it can be grafted by substances containing active hydrogen atoms, or cross-bonded with dihydric alcohols or binary amines.
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.
The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
Acknowledgments
The authors gratefully acknowledge the financial support of the National Natural Science Foundation of China (Grant No. 51562023) and the Research Fund Program of Guangdong Provincial Key Laboratory of Emergency Test for Dangerous Chemicals (KF2018001).
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