Integration of Pore Confinement and Hydrogen-Bond Influence on the

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Integration of pore confinement and hydrogen-bond influence on the crystallization behavior of C18 PCMs in mesoporous silica for form-stable phase change materials Tingting Qian, Jinhong Li, Xin Min, and Bin Fan ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b03267 • Publication Date (Web): 06 Nov 2017 Downloaded from http://pubs.acs.org on November 8, 2017

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ACS Sustainable Chemistry & Engineering

Integration of pore confinement and hydrogen-bond influence on the crystallization behavior of C18 PCMs in mesoporous silica for form-stable phase change materials Tingting Qian1, 2, Jinhong Li3, Xin Min3, Bin Fan1, 2 1 State Key Laboratory of Environmental Aquatic Chemistry, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, China 2 Laboratory of Water Pollution Control, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, China 3 Beijing Key Laboratory of Materials Utilization of Nonmetallic Minerals and Solid Wastes, National Laboratory of Mineral Materials, School of Materials Science and Technology, China University of Geosciences (Beijing), Beijing 100083, P.R. China

*

Corresponding author. Tel./Fax: +86-010-62849354.

E-mail: [email protected] (Tingting Qian) *

Corresponding author. Tel./Fax: +86-010-62849354.

E-mail: [email protected] (Bin Fan) 1

ACS Paragon Plus Environment


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Abstract We report herein the integration of pore confinement and hydrogen-bond influence on the crystallization behavior of C18 PCMs PCM in mesoporous silica. Mesoporous silica nanoparticles with 2.74 nm pores are employed as supporting material. To evaluate the effect of the internal/external surfaces of silica on the crystallization behavior of C18 PCM, three kinds of PCMs with various functional terminals including stearic acid (SA), octodecane (OCC), and octadecanol (OCO) were employed, and the effects of various mass fractions of PCMs were comprehensively investigated as well. It is remarkable that the complete filling of the available nano-sized pore volume and newly formed hydrogen bonds (H–O···O) are bound to result in the formation of the mesomorphic or amorphous phase of PCM, thus no enthalpy can be evidenced by the DSC data. In addition, it turns out that the composite PCMs obtain at least 2-fold increase over neat PCM in the thermal conductivity, due to the introduction of silica supporting material. The resulting three stabilized composites all exhibit favorable chemical compatibility, high thermal stability, improved thermal conductivity and excellent thermal reliability, which are a prerequisite for the storage and release of latent heat in PCMs. Keywords: Form-stable composite; Nano-sized confinement; Hydrogen-bond influence; Crystallization.

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Introduction Phase change materials (PCMs) have attracted substantial academic and industrial attention for their extensive applications in the thermal energy storage (TES) systems. The study of PCMs was pioneered by Telkes and Raymond in the 1940s [1]. Nevertheless, it did not deserve its due attention until the energy crisis of the late 1970s and early 1980s when it was extensively studied for energy storage and management systems. A typical PCM can absorb, retain, and release large amounts of latent heat while undergoing a phase change. Usually, latent heat storage technique accomplished by means of PCMs, particularly solid–liquid PCMs, has been extensively practiced and has undergone rapid development in the past several decades [2]. However, they have low thermal conductivity, and always require extra encapsulation to hold back the leakage of the melted PCMs during the phase transition process. Now, these problems could be solved by introducing form-stable composite PCMs [3]. Because PCMs melt and crystallize repeatedly during use, it is necessary to stabilize them in a solid matrix. Form-stable composite PCM, as an alternative to the traditional PCM can be obtained through impregnation of high porosity matrices or through encapsulation into inert materials, which is becoming an intriguing hotspot of thermal storage research for its satisfactory form-stable effect during phase transition [4]. Inorganic porous containers for PCMs in particular have allured enormous interest in this field. Firstly, porous carriers equipped with high porosity and large BET areas tend to store large amounts of PCMs and maintain the thermal reliability 3



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even when subjected to numerous of melt-freeze cycles due to the sufficient capillary effect. Secondly, porous container can be considered as individual energy storage unit, which can improve the heat transfer efficiency of PCMs during phase change processes [5]. Commonly employed inorganic supporting materials include minerals, silica, and carbon fibers, among others [6]. Mesoporous silica nanoparticle (MSN)-based phase change composite with its favorable thermal performance along with improved thermal conductivity and incessant value-added application potential makes it an ideal candidate for novel solar energy storage material. PCM/MSN nanocomposites have received substantial academic and industrial interest in recent years [7]. Common hydrocarbon PCMs have been incorporated into MSN matrices such as shape-stabilized fatty acids, alcohols, paraffin waxes and unsaturated polyester [8]. It is reasonable to believe that the phase change behavior of PCM confined in pores is complicated and quite differs from that of the pristine PCM. Previous studies have already noted that the supporting carrier will somehow affect the phase change behaviors of the incorporated PCM because of the physical interactions between the two independent components [9]. Firstly, the most noticeable result is the loss of energy storage capacity resulting from the inclusion of the inert supporting carrier. It is expected that the large BET area and low density of porous carrier can enhance the shape stabilization capability and thus minimize the loss of heat storage density. On the other hand, the pore structure of the supporting carrier including pore size distribution, geometrical shape, network inter-connection, and functional groups on 4


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the internal surface, may greatly affect the crystallization behavior of the PCM, which is a prerequisite for the latent heat storage/release [10-11]. As described by Clapeyron equation, the phase change behavior of a liquid PCM phase is vulnerable to be affected by the surrounding pressure and temperature confined in a narrow porous space, which is quite different from those of the same phase in free condition [12]. Up to now, the effect of the confinement of pore size and geometry on the PCM phase change behavior has been a major concern. Although the research on form-stable PCMs starts from 1990s, however, experimental results on the effect of the surface properties of the supporting carrier on the phase change behavior remains scant. It is corroborated by the results obtained in previous studies that the mean pore size of the supporting carrier is one such structure characteristic which can substantially affect the final shape stability and crystallization behavior of the confined PCMs [10]. If the pores are too small, the movement of PCM molecules is hindered, causing the PCM chains not to aggregate and crystallize thereby losing thermal storage capacity. Conversely, if the pores are too large, there will be not sufficient capillary force to preserve the liquid PCM, as a result, additional encapsulation are required to prevent the leakage of PCMs. It is revealed that mesoporous carriers are the most prospective candidates to confine and support PCMs, which can be directly used as an actual thermal storage device without containers. The influence of confinement caused by mesopores alone cannot illustrate the obvious difference in the phase change behaviors including latent heats and phase change temperatures. Mesoporous silica nanoparticles have aroused widespread 5



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concern both in the theoretical researches of nanoconfinement of small molecules and in the effect of interactions between the functional groups of the PCM molecule and supporting carrier [13]. Usually, the crystallization behavior of PCMs can be regulated by engineering the surface properties of the supporting carrier [14-15]. However, it is noteworthy that the reported surface functionalization process of the supporting carrier is not economical for the industrial manufacture due to its tedious operation and high cost. Besides, the integration of different functional groups is typically complicated, presenting poor control over the resulting modification effect and always requiring relatively strict control of reactant concentration and reaction time and temperature. Thus, it is necessary to develop a green, facile and cheaper alternative route to study the crystallization behavior affected by the surface properties. PCMs employed must have suitable melting temperature for the practical operation, large thermal storage density, high thermal conductivity, excellent chemical stability and durability, and should be non-corrosive, low-cost and nontoxic as well. The most common PCMs studied during the past 40 years are mainly n-alkanes, fatty acids, alcohol, paraffin waxes and their mixtures [16]. Among this family, octodecane (– CH3), octadecanol (–OH) and stearic acid (–COOH) are medium-sized carbon chains (C18) with different functional terminals. We report herein the effect of the interactions between PCM molecules and internal/external surface of silica support on crystallization behavior of the PCM confined. Three types of PCMs compounded with various functional terminal groups, 6


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including –OH, –COOH, and –CH3 were applied, which stand distinct in the class of organic PCM with reference to the available literatures. Then, PCM/silica form-stable composites with high PCM loadings, large thermal storage density and excellent thermal stability and durability were obtained. The surface properties and mesoporous feature of the silica carrier were found to have different effects on the thermal behaviors of various PCMs. The present study will pave the way for the further investigation of polymer/inorganic heat storage composites and provide insight into the fabrication of stable polymer based high performance phase change systems for thermal storage application. Experimental Materials The chemicals employed in the present study, including cetyltriethylammonium bromide (CTAB) and Tetraethyl orthosilicate (TEOS, >99%) were purchased from Beijing Chemical Reagent Ltd., China and used as received without further purification. Three chemically pure PCMs including octodecane (OCC), octadecanol (OCO) and stearic acid (SA) were supplied by Xilong Chemical Reagent Beijing Co., Ltd., China, which were premelted and degassed under vacuum at 100 °C overnight before use. Preparation of mesoporous silica (MS) As schematically demonstrated in Fig. 1, the procedure of final sample preparation consisted of two steps. Mesoporous silica (MS) was synthesized through the 7



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surfactant templating method as reported elsewhere with little modification [15] as shown in Fig. 1(a). Typically, 1 g CTAB was firstly dissolved in the distilled water, and 4 mL of 2 M NaOH aqueous solution was added and stirred magnetically for 30 min at 80 ℃. Then 5 mL TEOS was added dropwise under constant stirring for another 3 h. Afterward, mesoporous silica submicrospheres with CTAB as templates (SiO2@CTAB) were received and further calcined at 550 ℃ for 4 h with a heating rate of 3 ℃/min to remove the organic template. Preparation of form-stable composite PCMs Three kinds of form-stable composite PCMs (fs-PCMs) were prepared by vacuum impregnation method as reported in our previous work [17]. In addition, the physical blending and impregnating method as a reference was also applied as demonstrated in Fig. 1(b). In a typical synthesis, OCC PCM was firstly dissolved in absolute ethanol. The composite PCM was prepared by dispersing the MS powder of desired amount into the melted OCC while subjected to rigorous stirring on a hot-plate magnetic stirrer at 80 °C for 30 min, followed by intensive sonication for another 40 min. Finally, the mixture was dried at 80 °C overnight so that the ethanol solvent would completely evaporate. Other PCMs, such as OCO and SA, were stabilized in as-synthesized MS, which was similar to the preparation of OCC/MS composite. Both methods have been compared and analyzed. The results indicated that the vacuum impregnation method is preferred. The composite PCM showed a solid content of PCM of more than 70 wt%. The leakage behavior of the tailor-made composite PCMs was evaluated as shown 8


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in Fig. 1(c). The macroscopic aspects of the specimen including color change, leakage and

mass

loss

were

visually

assessed

when

subjected

to

consecutive

melting/re-solidification cycles. It is reasonable to believe that all of the final composite PCMs were of desirable shape stability and negligible mass loss up to 400 cycles. Characterization Scanning electron microscopy (SEM, S-4800, Hitachi, Japan) and Transmission electron microscopy (TEM, JEOL JEM-2010, Japan) were performed to observe the microstructures of mesoporous silica (MS) before and after PCM impregnation. MS powder was dispersed in ethanol and ultrasonicated for 5 min. Then a drop of sample suspending liquid was placed on copper grid and dried before TEM characterization. BET surface area, average pore size and total pore volume of mesoporous silica were determined employing the nitrogen adsorption method using a Quantachrome Autosorb-IQ surface area analyzer at liquid nitrogen temperature. The crystal structures of composite PCMs were recorded by X-ray diffraction (XRD, D’Max-Ra 12 kW, Ouyatu, Japan) using Ni-filtered CuKα radiation (λ=0.1541 nm) and operating at 40 kV and 100 mA. The XRD patterns of pure PCM and the composite PCMs were obtained at a scanning rate of 4°/min in the 2θ range 4~80°. The chemical compatibility of the prepared composite PCM was obtained via Fourier transform infrared spectroscopy (FT-IR, Model Frontier) using KBr pellet in the wave range of 400~4000 cm−1 at room temperature. The phase change behavior of composite PCMs was determined on a differential 9



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scanning calorimetry (DSC, Q2000) in nitrogen atmosphere. The DSC measurement in the present study was performed at the very slow increasing temperature rate of 1 ℃/min from room temperature to 80 °C. The reason to choose the very slow increasing temperature rate is to diminish the disturbance of the thermal resistance of porous materials for the phase change behavior. It is noted that the initial temperature of OCC PCM was 15 °C. The thermal stability and weight loss of composite PCMs were measured by thermos-gravimetric Instrument (TGA, Q50) from 25 °C to 500 °C at a heating rate of 10 °C/min under a nitrogen atmosphere. Thermal conductivities of pristine PCM and also composite PCMs were measured by a hot disk thermal constant analyzer (Hot Disk TPS2500, Hot Disk AB Company, Sweden) at room temperature and repeated five times. The error bars associated with the measurements are also calculated. To evaluate the thermal reliability, a thermal cycle test of the repeated melting-freezing process was performed up to 400 cycles. The test was alternated between room temperature (15 °C for OCC) and 80 ℃. Then FT-IR and DSC analyses were repeated to assess the chemical and thermal changes after the heat treatment. Results and discussion Structure observations and pore structure of silica and prepared stabilized PCMs SEM and TEM were performed to evaluate the morphology and microstructure of as-synthesized silica nanoparticles and the micrographs are presented in Fig. 2. As seen from SEM micrographs in Fig. 2(a), the particles exhibit ellipsoid shape with

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radius of ca. 100~200 nm. The detailed structural and morphological features of the silica particles were further evaluated by TEM as displayed in Fig. 2(b). The well-defined channels of silica nanoparticles can be easily visualized from the contrast in the TEM image. Fig. 2(c) demonstrates the adsorption/desorption isotherm of the prepared nanosilica. The shape of the isotherm is of a typical IV, with a pronounced hysteresis loop at P/P0 0.4~1.0, which is an indication for a mesoporous material. Applying the Barret–Joyner–Halenda (BJH) method to calculate the pore size distribution (inset of Fig. 2c), the mean pore diameter of nanosilica was 2.74 nm with a specific surface area of 164.5 m2/g determined by BET method. SEM images of SA/MS, OCC/MS and OCO/MS stabilized samples are presented in Figs. 2(d), (e) and (f), respectively. These images exhibited that all the PCMs were supported by/or absorbed in the porous framework of silica, and the diameters of silica spheres appeared to increase upon the absorption of PCMs. As known, organic polymers including SA, OCC and OCO, tend to locally melt during the focus of hot spot made by the electric beam of SEM and experience the melting-crystallization process. No leakage of any PCM was observed from the surface of the composites. Higher PCM mass fraction resulted in higher fraction of the material in a crystalline phase and thus larger enthalpy. Therefore, the maximum mass fraction of PCM in the composite was determined to be 70 wt%, and no leakage happens while it melts. Chemical compatibility analysis The chemical compositions and structures of the as-synthesized samples were first evaluated by FT-IR spectroscopy as exhibited in Fig. 3. In Fig. 3(a), in Curve (1), the 11



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peak positioned at 1085 cm−1 can be attributed to the stretching vibration of functional group of siloxane (–Si–O–Si–); the peak at 802 cm−1 belongs to the vibration of functional group of SiO–H; the peak at 464 cm−1 presents the bending vibration of functional group of Si–O. In addition, the peak that appears at 1650 cm-1 can be assigned to the –OH bending vibration of the adsorptive water and its corresponding stretching vibration is located at 3409 cm-1. In the spectrum of pure SA of Curve (2), the band at 1411 cm-1 is caused by the bending vibration of functional group of –CH2– COOH. Bands at 2850 and 2918 cm-1 belong to the symmetrical vibration and asymmetrical vibration of –CH2 group, respectively. Besides, bands caused by the in-plane swinging vibration and out-of-plane bending vibration of –OH group are observed at 722 and 933 cm-1, respectively. In the spectrum of composite PCMs in Curve (3) and (4), it is noteworthy that those typical characteristic vibration bands of SA can be distinguished in the spectra of the composite PCMs, such as –CH2–COOH bending vibration (1467 cm−1), –CH2 vibration (2919 and 2852 cm−1), and –OH vibration (702 cm−1). The primary SiO2 functional groups are also found in the spectrum of composite PCM. There is no significant new peak appearing, indicating that there is no chemical interaction between SA and SiO2. As can be seen in the spectrum of OCC in Curve (2) of Fig. 3(b), three intensive absorption bands at 2952, 2917, and 2846 cm−1 can be assigned to the alkyl C–H stretching vibrations of methylene and methyl groups. Besides, two C–H stretching vibrations at 1471 and 1367 cm-1 are caused by methylene bridges. The infrared spectrum also presents a characteristic absorption peak at 715 cm–1 corresponding to 12


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the in-plane rocking vibration of methylene group. Moreover, a broad absorption peak centered at 3409 cm–1 can be assigned to the stretching vibration of the O–H bond of adsorptive water. In Curve (3) and (4), the primary functional groups of SiO2 and OCC are all seen in the spectra of composite PCMs. There is no significant new peak appearing, indicating that no chemical interaction occurs. In the spectrum of OCO, the typical stretching vibrations of C–H at around 2917, 2850 and 1473 cm−1 correspond to –CH2 of OCO. The stretching vibration of C–O appears at 1062 cm−1. A broad band of 3600~3000 cm-1 was assigned to stretching vibration of –OH group. These characteristic absorption bands are in good agreement with the characteristic peaks of pure OCO revealed in Curve (2) of Fig. 3(c). FT-IR spectroscopy highlights the presence of the OCO and silica matrix in the composite PCMs. No significant peak shifts from the spectra can be observed, indicating physical interactions during the composite formation. Importantly, the peak at 3694 cm-1 indicates the presence of hydrogen bonds that resulted from interaction between alcohol group (CH2-OH) at the end of octadecanol and the silanol group (Si-OH) located at the surface of the nanosilica material. It is noted that independent of the elevated PCM loadings, the prepared composite PCMs all exhibit combined spectra with the intrinsic appearance of both silica and PCM. Effect of PCM loadings on the thermal behavior of the resulting composites Fig. 4(a) shows the DSC curves of the SA in mesoporous silica and bulk SA. The thermal energy storage data obtained from the DSC curves, such as onset melting/solidification temperature (TMO/TSO), peak melting/ solidification temperature 13



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(TMP/TSP),

end

melting/solidification

temperature

(TME/TSE),

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and

melting/

solidification enthalpy of SA/MS composite PCMs (ΔHM/ΔHS), are listed in Table 1. In Fig. 4(a), the two sharp peaks representing the solid–liquid phase change of SA can be clearly found. No melting and freezing peaks were detected in those composite PCMs when the SA mass fraction was less than 30%. Obviously, the other composites all present similar curve shapes with that of the SA, indicating that there was no transformation of SA occurred during the impregnation process. As known, the phase change enthalpy is a critical factor in PCMs, which could be used as a measure to evaluate the thermal energy storage capacity of the PCM. Fig. 4(b) illustrates the change of the enthalpy with increasing the fraction of SA. It is observed that the phase change enthalpy increases monotonously along with the SA concentration increment from 40% to 70%. MS-based composite PCM with SA content up to 70% has maintained both the macroscopic shape and white color upon heating at 80 °C. However, the enthalpy value maintained even zero when the SA content was much lower than 30%. The resulting DSC thermograms of the OCC/MS at different mass ratios of OCC are shown in Fig. 4(c), and the phase change parameters obtained from DSC evaluation are summarized in Table 2. As shown in both Fig. 4(c) and Table 2, the solid to liquid phase transition of OCC occurred between 28.21 °C and 36.7 °C with a peak temperature at 30.4 °C, and the heat of fusion was 232.5 J/g. In the cooling process, the phase change temperature ranged from 28.7 °C to 22.3 °C with a peak temperature of 26.12 °C and the released heat was 230.4 J/g. Fig. 4(d) illustrates that 14


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the influence of the OCC mass fraction on phase change enthalpy, which follows the similar trend as SA. The enthalpy value keeps zero with increasing OCC content up to 30%, and then increases at higher content. Fig. 4(e) displays the typical DSC curves of the OCO/MS composites with various OCO mass fractions, and the corresponding data collected are summarized in Table 3. It is interestingly found that pure OCO presents a bimodal crystallization behavior during the freezing process, two exothermic peaks at 50.49 and 55.54 ℃, respectively. However, only a single endothermic peak at 60.4 ℃ is found in the melting process. When the mass fraction of OCO was 60%, the melting and freezing enthalpy were 22.35 and 12.94 J/g, respectively, whereas below this percentage, for example, 50%, the enthalpy was zero. It is noteworthy that the “special mass fraction” of OCO/MS was substantially higher than the SA/MS and OCC/MS. As can be seen, the phase change temperatures of the PCMs confined in mesoporous silica were more or less affected when compared with bulk PCM. Tables 1, 2 and 3 present the changes of melting and freezing temperatures with different PCM loadings. It is noted that the melting temperature decreases, whereas the freezing temperature increases with increasing the amount of MS in the resulting PCM composites due to the thermal conductivity enhancement by SiO2. In this connection, increasing the mass fraction of SiO2 will further enhance the heat transfer performance.

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Comparison with theoretical enthalpy Theoretically, the latent heat enthalpy of the composite PCMs could be determined by multiplying latent heat of bulk PCM with its mass percentage in composite PCMs. Here, the experimental phase change latent heats of the composites are compared with their calculated values in Fig. 5. It is observed in Fig. 5 that both the melting and freezing enthalpies of the composite PCMs were far lower than their theoretical values, and were even zero when the content of PCM in the composite PCMs decreased to a certain value. It is reasonable to believe that if PCM content was far below 30%, almost all of PCM molecules were completely immersed into the nano-sized pores in the silica framework. During the impregnation process, however, once the liquid phase of PCM molecules were confined, the thickening of thin lamellar polymer crystallites into stable crystal would be inevitably restricted. In this case, there was hardly any latent heat of phase change can be detected by DSC technique. It is due to the fact that most PCM existed in the form of mesomorphic phase or disordered amorphous phase. The abundant pores of silica framework impede free movement of the incorporated PCM, but stabilize the composite PCMs above their melting point. Furthermore, when the mass percentage of PCM increased to a certain degree (approximate to 30 wt%), part of PCM molecules were out of the nano-sized pores and became free from the confinement, which favors the crystallization of PCM, thereby latent heat of phase change can be distinguished in DSC test. Although higher mass fraction of PCM in the composites resultes in higher fractions of the material in a crystalline phase and 16


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thus larger enthalpy, the maximum mass percentage of PCM in the composite PCMs was determined to be 70% in the current study. Proposed mechanism of phase change behavior of OCO confined in MS Why do these three kinds of organic PCMs, OCO in particular, have a quite different phase change behavior when immersed into the MS? It is noteworthy that no crystallization of OCO occurred when the OCO mass fraction in the composite was much less than 50%. However, the SA and OCC molecules are free to crystalize when their mass fractions in the composites were higher than 30%. Such a different phase change behavior of OCO indicates that the interaction between OCO molecules and internal surface of silica support is vital for the crystallization of OCO. Fig. 6(a) shows the XRD patterns of the SA/MS stabilized composites with various SA contents of 10%~70%. In the patterns, the broad peak around 20~30°indicates a non-crystalline structure of silica derived from the hydrolysis of TEOS. Besides, the typical diffraction peaks are assigned to SA crystal. The peak positions of the composite PCMs are basically the same as those of bulk SA, which is consistent with the FT-IR results that no chemical reactions occurred. It is noted that when the SA mass fraction decreased to a certain value, no crystalline peaks of SA were observed. The lack of SA crystalline peaks can be explained by the complete confinement of the SA molecules into the nano-sized pores. Importantly, the intensity of the main diffraction peaks increased gradually along with SA content increment when the SA content was beyond 40%. Therefore, it is reasonable to believe that the crystallinity of SA decreased with the addition of the MS supporting material. 17



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Fig. 6(b) shows the XRD patterns of the OCC/MS composites with OCC contents ranging from 10% to 70%. It is remarkable that the XRD pattern of the OCC/MS composite is similar to that of SA/MS. Only an amorphous state was seen if the mass fraction of OCC was much lower than 30%. At this value, the OCC molecules are thoroughly confined by the supporting materials. Consequently, no crystalline peaks were available. When the mass fraction reached 40%, blunt diffraction peaks were observed, while the peaks became sharp if the content of OCC continued to increase to 70%. Fig. 6(c) displays the XRD patterns of the OCO/MS composites with different OCO contents of 10%~70%. Interestingly, the XRD patterns of the OCO/MS composites with 40%~50% of OCO exhibit no crystallization peak of OCO, whereas the SA/MS and OCC/MS composites have obvious XRD peaks of SA and OCC. Besides the confinement derived from nano-sized pores, the movement of the OCO molecules during the crystallization will be also restricted due to the effect of hydrogen bonding interactions. The available hydrogen bonds (H–O···O) are formed between neighboring oxygen atoms of OCO molecules as hydrogen acceptors and the surface silanol groups of the supporting silica, which was confirmed by the FT-IR data. As a result, the OCO was in amorphous state. The mass percentage for complete disappearance of the PCM diffraction peaks is 30% for the SA/MS and OCC/MS composites and 50% for the OCO/MS composites, respectively. The mechanism for phase change behavior of a typical PCM immersed in the MS is proposed and illustrated in Fig. 6(d). Based on the results above, if the 18


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mass fraction of OCO fell to a certain degree (e.g. 30%), the adsorbed OCO molecules in nano-sized pores will inevitably form a disordered layer, which behaves like a liquid and could not crystallize even at the low temperature. Similar phenomenon also occurred for SA and OCC. Upon increasing the PCM content higher than the pore volume, the free SA and OCC can crystallize and melting, irrespective of the confinement of silica framework. Nevertheless, the OCO stabilized composites also exhibit no XRD patterns associated with the presence of crystalline organic phase when the OCO mass fraction was far below 50%, which can be explained by the restriction of the newly formed hydrogen bonds (H–O···O). With the amount of OCO increasing up to 60%, the free OCO PCMs are finally able to melting and crystallize, as shown in DSC test and XRD analysis. It is remarkable that the crystallization behavior of a typical PCM could be controlled not only by adjusting the pore size of the supporting material, but by regulating the surface properties of the supporting material. Thermal stability analysis The thermal stabilities of the three PCMs and their correspondent stabilized composites are characterized with TGA (Fig. 7). It is anticipated that the weight-loss behavior of each PCM was not obviously affected by the silica supporter, indicating that no transformation of PCM occurred during the impregnation. As also seen in Fig. 7, the residual weight of the three bulk PCMs at 600 ◦C is close to zero, while the composites have higher residue of 30 wt% at 600 °C as calculated from their TGA curves. 19



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In Fig. 7(a), the SA/MS composite displayed a higher decomposition temperature than the neat SA, which can be attributed to the restriction powder of the nano-sized pores that impeded the spillover of SA chains from the silica pores, thereby promoting the stabilization of the composite PCM. The result indicated that once incorporated into the silica supporter, the thermal stability of the SA PCM is believed to be enhanced by the surrounding inorganic matrix while exposed to severe environmental conditions (i. e. high temperature). Similar results have been obtained in Fig. 7(b) for OCC and its corresponding stabilized composite. It is noteworthy that the pore-confined OCC must break through from the nano-sized pores firstly and then can evaporate out while heating. In this case, the OCC/silica composite was more stable and showed improved resistance to the high temperature than the bulk OCC. However, as seen in Fig. 7(c), the OCO/MS composite displayed a little lower thermal stability than that of the bulk OCC. The earlier occurrence of the rapid weight loss of OCO/silica composite can be explained by the difference in the physical behavior between neat OCC and pore-confined OCC. The similar polarities of the silanol groups and OCO molecules caused the easy spillover of OCO chains that adsorbed onto the external surface of silica supporter, even though pore-confined effect also existed in this composite. Determination of the thermal conductivity Thermal conductivity dominates the thermal transfer rate and is crucial to the applicability of phase change materials. Therefore, determination of thermal conductivity of the prepared composite PCMs appears to be a primary measure for evaluating their performance. Fig. 8(a) demonstrates the thermal conductivity of the 20


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bulk PCM and the prepared composite PCMs. The thermal conductivity of bulk OCC, SA and OCC is only 0.23, 0.27 and 0.24 W/mK at 25 °C, respectively. After impregnation, the thermal conductivity of the OCC/MS, SA/MS and OCC/MS composite more than doubled to 0.48, 0.56 and 0.51 W/mK, respectively, which can be attributed to the relatively high thermal conductivity of the supporting silica. The porous silica base in the PCM composites provides another quick thermal path for the PCM and consequently improves the phase transition speed of the whole composite. Usually, low thermal conductivity inevitably reduces the rate of heat storage and release, thus, the enhanced thermal conductivity is preferred for the efficient heat transfer in practical application. As reported by Bugaje [18] the phase change (melting or freezing) time is the most important design parameter in latent heat storage systems. Moreover, the improvement of the thermal transfer rate can also be evaluated by comparing the melting and freezing performance with that of the pristine PCM. Fig. 8(b) compares the time that required for the melting (18~80 ℃) and freezing (80~18 ℃) performance of pure PCM and composite PCMs. As can be seen, the melting and freezing time of OCC were respectively 67 and 73 min, while the melting and freezing time of OCC/MS composite were obviously shortened to 42 and 51 min, indicating the decreases of approximately 37.3% and 30.1%, respectively. Similar decreasing tendency were evaluated for SA/MS and SA, as well as OCO/MS and OCO. The results support the hypothesis that a higher thermal conductivity leads to shorter melting/freezing time, indicating that more intensive heat transfer occurred in composite PCM relatively in the pure PCM.

21



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Thermal reliability of the stabilized composite It is reasonable to believe that PCMs have a tendency to lose their thermal performances after repetitive melting and freezing cycles, which is known as thermal reliability. In the present study, a 400-thermal cycle test was performed to evaluate the changes in chemical structures and thermal properties for the prepared three stabilized composites. No trace of melted PCM was observed as anticipated. Figs. 9(a)~(c) demonstrate the FT-IR spectra of the SA/MS, OCC/MS and OCO/MS stabilized composites before and after thermal cycle, respectively. In each figure, no obvious difference in both the shape and frequency bands can be found. The almost overlapping curves indicated that the chemical structure of each stabilized composite was not destroyed during thermal cycling process. Figs. 9(d)~(f) compare the DSC curves of the SA/MS, OCC/MS and OCO/MS before and after thermal cycle, respectively. Summary of the phase change parameters are also given in Tables 1~3. After thermal cycles, the phase change temperatures of SA/MS, OCC/MS and OCO/MS are similar to those of the original one. Besides, their melting enthalpy and freezing enthalpy only changed by 2.04, 1.05, 1.51 J/g and 1.46, 2.63, 0.35 J/g, respectively, which are not in significant magnitude for thermal energy storage applications. Besides, the thermal conductivities of SA/MS, OCC/MS and OCO/MS composite PCMs after cycling were 0.48, 0.54 and 0.5 W/mK at 25 °C, respectively. Those thermal conductivities remained almost unchanged after the thermal cycle. The little changes in the phase change behavior indicate that the prepared form-stable composite PCMs have excellent thermal reliability and can be used as heat storage 22


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material in building applications. Conclusion In the present study, mesoporous silica-based phase change composites were prepared through the direct synthesis method using different phase change materials with various functional terminals, including stearic acid (SA), octodecane (OCC), and octadecanol

(OCO).

Besides,

the

effect

of

various

PCM

loadings

was

comprehensively investigated. The maximum load of PCM in the stabilized composite could reach 70 wt%. The main conclusions rise as follows: (1) Both the pore confinement and hydrogen-bond influence are significantly influential on the crystallization and phase change behavior of PCMs confined in silica pores. For SA/silica and OCC/silica composites, the crystallinty and thermodynamic behavior of the composites are strongly confined by SiO2 framework. If the SA (or OCC) content is higher than the pore volume (30 wt%), both the crystallinity and enthalpy of the composites can be detected by XRD and DSC results. (2) For OCO/silica composite, except for the pores confinement, the crystallization and phase change behavior were also affected by the newly formed hydrogen bonds (H–O···O) between PCM molecules and silica supporter. OCO crystallization inside the mesopores might also occur as a result of a higher amount of OCO of 60 wt%. (3) The thermal conductivity of composite has more than twice increases over the neat PCM due to the introduction of silica supporting material, while the phase 23



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change enthalpy are lower than the values of bulk PCM. Importantly, the prepared stabilized PCMs all have good chemical compatibility, improved thermal stability and excellent thermal reliability. Acknowledgments This project was supported by National Natural Science Foundation of China (No. U1607113). References [1] Huang, X.; Alva, G.; Jia, Y.T., et al. Morphological characterization and applications of phase change materials in thermal energy storage: A review. Renew. Sustain. Energy Rev. 2017; 72: 128–145. [2] Li, Y.L.; Li, J.H.; Feng, W.W., et al. Design and preparation of the phase change materials paraffin/porous Al2O3@graphite foams with enhanced heat storage capacity and thermal conductivity. ACS Sustainable Chem. Eng. 2017; 5: 7594–7603. [3] Li, B.X.; Liu, T.X.; Hu, L.Y. et al. Fabrication and properties of microencapsulated paraffin@SiO2 phase change composite for thermal energy storage. ACS Sustainable Chem. Eng. 2013; 1: 374–380. [4] Zhang, X.G.; Liu, H.T.; Huang, Z.H.; Yin, Z.Y.; Wen, R.L.; Min, X.; Huang, Y.T.; Fang, M.H.; Liu, Y.G.; Wu, X.W. Preparation and characterization of the properties of polyethylene glycol@ Si3N4 nanowires as phase-change materials. Chem. Eng. J. 2016; 301: 229–237. [5] Li, Y.; Fu, Z.Y.; Su, B.L. Hierarchically structured porous materials for energy 24


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conversion and storage. Adv. Funct. Mater. 2012; 22: 4634–4667. [6] Fang, G.Y.; Tang, F.; Cao, L. Preparation thermal properties and applications of shape-stabilized thermal energy storage materials. Renew. Sustain. Energy Rev. 2014; 40: 237–259. [7] Mitran, R.A.; Berger, D.; Munteanu, C. Evaluation of different mesoporous silica supports for energy storage in shape-stabilized phase change materials with dual thermal responses. J. Phys. Chem. C 2015; 27: 15177–15184. [8] Lv, P.Z.; Liu, C.Z.; Rao, Z.H. Review on clay mineral-based form-stable phase change materials: Preparation characterization and applications. Renew. Sustain. Energy Rev. 2017; 68: 707–726. [9] Wang, J.J.; Yang, M.; Lu, Y.F., et al. Surface functionalization engineering driven crystallization behavior of polyethylene glycol confined in mesoporous silica for shape-stabilized phase change materials. Nano Energy 2016; 19: 78–87. [10] Feng, L.L.; Zhao, W.; Zheng, J.; Frisco, S.; Song, P.; Li, X.G. The shape-stabilized phase change materials composed of polyethylene glycol and various mesoporous matrices (AC SBA-15 and MCM-41). Sol. Energy Mater. Sol. Cells 2011; 95: 3550–3556. [11] Zhang, D; Tian, S.L.; Xiao, D.Y. Experimental study on the phase change behavior of phase change material confined in pores. Sol. Energy 2007; 81: 653–660. [12] Hu, Q.; Li, J.J.; Hao, Z.P.; Li, L.D.; Qiao, S.Z. Dynamic adsorption of volatile organic compounds on organofunctionalized SBA-15 materials. Chem. Eng. J. 2009; 149: 281–288. 25



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[13] Colilla, M.; Gonzalez, B.; Vallet-Regi, M. Mesoporous silica nanoparticles for the design of smart delivery nanodevices. Biomater Sci. 2013; 1: 114−134. [14] Sutton, S.J.; Izumi, K.; Miyaji, H. The morphology of isotactic polystyrene crystals grown in thin films: the effect of substrate materia. J. Mater. Sci. 1997; 32: 5621–5627. [15] Wei, T.; Zheng, B.; Liu, J.; Gao, Y.F.; Guo, W.H. Structures and thermal properties of fatty acid/expanded perlite composites as form-stable phase change materials. Energy Build. 2014; 68: 587–592. [16] Su, W.G.; Darkwa, J.; Kokogiannakis, G. Review of solid–liquid phase change materials and their encapsulation technologies. Renew. Sustain. Energy Rev. 2015; 48: 373–391. [17] Qian, T.T.; Li, J.H.; Min, X.; Guan, W.M.; Deng, Y.; Ning, L. Enhanced thermal conductivity of PEG/diatomite form-stable phase change materials with Ag nanoparticles for thermal energy storage. J. Mater. Chem. A 2015; 3: 8526–8536. [18] Bugaje, M.I. Enhancing the thermal response of latent heat storage systems. Int. J. Energy Res. 1997; 21: 759–766.

26


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Figure captions Fig. 1. A scheme of the overall preparation procedure of: (a) Mesoporous silica (MS); (b) PCM/MS stabilized composites; and (c) Leakage test. Fig. 2. Mesoporous silica (MS) supporter: (a) SEM image; (b) TEM image; (c) N2 adsorption–desorption isotherm and pore size distribution; SEM images of: (d) SA/MS; (e) OCC/MS; (f) OCO/MS stabilized composites. Fig. 3. FT-IR spectra of (a) MS, SA, 30%SA/MS and 70%SA/MS; (b) MS, OCC, 30%OCC/MS and 70%OCC/MS; (a) MS, OCO, 30%OCO/MS and 70%OCO/MS. Fig. 4. DSC curves of stabilized composites with various PCM loadings of 10%~70%: (a) SA/MS; (c) OCC/MS; (e) OCO/MS; The relation between the PCM mass fraction and phase change enthalpie: (b) SA/MS; (d) OCC/MS; (f) OCO/MS. Fig. 5. Comparison of theoretical and actual enthalpies of stabilized composites Fig. 6. XRD patterns of stabilized composites with various PCM loadings of 10%~70%: (a) SA/MS; (b) OCC/MS; (c) OCO/MS; (d) Schematic formation mechanism of various stabilized composite. Fig. 7. TGA curves of (a) SA and SA/MS; (b) OCC and OCC/MS; (c) OCO and OCO/MS. Fig. 8. Various bulk PCM and stabilized composites: (a) Thermal conductivity measurement; (b) The comparison of melting and freezing time. Fig. 9. (a), (b), (c) FT-IR spectra and (d), (e), (f) Thermal properties of 70%SA/MS, 70%OCC/MS, and 70%OCO/MS after 400 thermal cycling.

27



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Page 28 of 46

Tables Table 1 Thermal characteristics of SA and SA/MS composite PCMs SA Mass Samples Ratio (wt%)

TMO (oC)

TMP (oC)

TME (oC)

HM (J/g)

TSO (oC)

TSP (oC)

TSE (oC)

HS (J/g)

SA

100

52.51

55.7

58.9

172.7

43.7

52.05

54.02

170.2

fs-SA1

10

0

0

0

0

0

0

0

0

fs-SA2

20

0

0

0

0

0

0

0

0

fs-SA3

30

0

0

0

0

0

0

0

0

fs-SA4

40

52.22

55.1

56.9

29.33

45.3

52.03

54.7

26.65

fs-SA5

50

52.00

54.6

56.3

38.97

47.1

52.42

54.6

36.93

fs-SA6

60

52.13

54.3

55.7

50.06

47.6

52.84

55.2

48.43

fs-SA7

70

52.12

53.8

55.2

77.57

47.9

52.62

55.6

75.28

fs-SA7 after cycle

70

52.05

53.3

55.4

75.53

47.7

52.51

55.3

73.82

28


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Table 2 Thermal characteristics of OCC and OCC/MS composite PCMs

Samples

OCC Mass Ratio (wt%)

TMO (oC)

TMP (oC)

TME (oC)

HM (J/g)

TSO (oC)

TSP (oC)

TSE (oC)

HS (J/g)

OCC

100

28.21

30.4

36.7

232.5

22.3

26.12

28.7

230.4

fs-OCC1

10

0

0

0

0

0

0

0

0

fs-OCC2

20

0

0

0

0

0

0

0

0

fs-OCC3

30

0

0

0

0

0

0

0

0

fs-OCC4

40

26.95

29.4

33.3

40.43

23.3

26.47

29.4

40.17

fs-OCC5

50

27.49

28.6

33.1

68.26

23.7

26.82

29.6

67.13

fs-OCC6

60

27.47

27.9

32.9

75.15

24.1

26.74

30.6

74.66

fs-OCC7

70

27.67

28.3

30.2

84.97

24.5

26.86

31.2

84.97

fs-OCC7 after cycle

70

27.58

28.1

30

83.92

24.3

27

31

82.34

29



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Table 3 Thermal characteristics of OCO and OCO/MS composite PCMs

Samples

OCO Mass Ratio (wt%)

TMO (oC)

TMP (oC)

TME (oC)

HM (J/g)

TSO (oC)

TSP (oC)

TSE (oC)

HS (J/g)

OCO

100

57.24

60.4

66.7

234.5

52.3

55.54

57.7

235.3

fs-OCO1

10

0

0

0

0

0

0

0

0

fs-OCO2

20

0

0

0

0

0

0

0

0

fs-OCO3

30

0

0

0

0

0

0

0

0

fs-OCO4

40

0

0

0

0

0

0

0

0

fs-OCO5

50

0

0

0

0

0

0

0

0

fs-OCO6

60

56.03

58.9

62.9

22.35

54.1

55.70

56.6

22.93

fs-OCO7

70

56.37

57.3

61.2

47.03

54.5

55.94

56.2

45.10

fs-OCO7 after cycle

70

55.97

57.1

60.3

45.52

54.3

56.03

56

44.75

30


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Figure 1

31



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Figure 2

Figure 3

32


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Figure 4

33



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Figure 5

34


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Figure 6

Figure 7

35



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Figure 8

Figure 9

36


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For Table of Contents Use Only

Nano-sized pores and hydrogen bonds cause the formation of the mesomorphic or amorphous PCM phase.

37



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Fig. 1. A scheme of the overall preparation procedure of: (a) Mesoporous silica (MS); (b) PCM/MS stabilized composites; and (c) Leakage test. 170x136mm (300 x 300 DPI)


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Fig. 2. Mesoporous silica (MS) supporter: (a) SEM image; (b) TEM image; (c) N2 adsorption–desorption isotherm and pore size distribution; SEM images of: (d) SA/MS; (e) OCC/MS; (f) OCO/MS stabilized composites. 170x175mm (300 x 300 DPI)



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Fig. 3. FT-IR spectra of (a) MS, SA, 30%SA/MS and 70%SA/MS; (b) MS, OCC, 30%OCC/MS and 70%OCC/MS; (a) MS, OCO, 30%OCO/MS and 70%OCO/MS. 170x47mm (300 x 300 DPI)


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Fig. 4. DSC curves of stabilized composites with various PCM loadings of 10%~70%: (a) SA/MS; (c) OCC/MS; (e) OCO/MS; The relation between the PCM mass fraction and phase change enthalpie: (b) SA/MS; (d) OCC/MS; (f) OCO/MS. 170x196mm (300 x 300 DPI)



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Fig. 5. Comparison of theoretical and actual enthalpies of stabilized composites 170x161mm (300 x 300 DPI)


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Fig. 6. XRD patterns of stabilized composites with various PCM loadings of 10%~70%: (a) SA/MS; (b) OCC/MS; (c) OCO/MS; (d) Schematic formation mechanism of various stabilized composite. 170x135mm (300 x 300 DPI)



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Fig. 7. Various bulk PCM and stabilized composites: (a) Thermal conductivity measurement; (b) The comparison of melting and freezing time. 170x45mm (300 x 300 DPI)


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Fig. 8. TGA curves of (a) SA and SA/MS; (b) OCC and OCC/MS; (c) OCO and OCO/MS. 170x62mm (300 x 300 DPI)



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Fig. 9. (a), (b), (c) FT-IR spectra and (d), (e), (f) Thermal properties of 70%SA/MS, 70%OCC/MS, and 70%OCO/MS after 400 thermal cycling. 170x91mm (300 x 300 DPI)


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Integration of Pore Confinement and Hydrogen-Bond Influence on the

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