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silica composite phase change materials fabricated by sodium silicate precursor
Renewable Energy 74 (2015) 689e698
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Renewable Energy journal homepage: www.elsevier.com/locate/renene
Phase-change characteristics and thermal performance of form-stable n-alkanes/silica composite phase change materials fabricated by sodium silicate precursor Fang He, Xiaodong Wang*, Dezhen Wu State Key Laboratory of OrganiceInorganic Composites, Beijing University of Chemical Technology, Beijing 100029, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 25 March 2014 Accepted 28 August 2014 Available online
A series of n-alkanes/silica composites as form-stable phase change materials (PCMs) were synthesized in a solegel process using sodium silicate precursor. The chemical compositions and structures of the synthesized composites were characterized by Fourier transform infrared spectroscopy. Scanning electric micrographs show an irregularly spherical morphology of the n-alkanes/silica composites, and transmission electric micrographs confirm that the n-alkanes have been well encapsulated by silica. These nalkanes/silica composites keep a good sharp stability due to the support of silica wall even if the n-alkanes are in molten state. The differential scanning calorimetric analysis indicates that the phase change behaviors and characteristics of the n-alkanes/silica composites strongly depend on the carbon atom number in n-alkanes, and meanwhile, the encapsulated n-alkanes have a high thermal storage capability. The investigation on thermal performance demonstrated that the n-alkanes/silica composites achieved a high thermal conductivity, low supercooling, and good work reliability as a result of the encapsulation of n-alkanes with highly thermal conductive inorganic silica. Moreover, the thermal stability of the composites was also improved due to the protection of silica wall toward the encapsulated n-alkanes. It is anticipative that, owing to the easy availability and low cost of sodium silicate, the synthetic technology developed by this work has a high feasibility in the industrial manufacture of the form-stable PCMs. © 2014 Elsevier Ltd. All rights reserved.
Keywords: Form-stable phase change composite materials Solegel synthesis n-Alkaline Silica Sodium silicate precursor
1. Introduction With the prompt development of global economics, the consumption of fossil energy sources grows rapidly, thus resulting in a shortage of fossil fuels as well as an increase in greenhouse gas emission. The effective utilization of energy has given a potent impetus to develop new energy storage materials. Phase change materials (PCMs) are considered as a class of renewed energy materials with the highly efficient utilization of energy. They can perform the cycle of storage-release for latent heat through the solideliquid phase transition and can regulate the locally environmental temperatures accordingly [1,2]. Nowadays, PCMs exhibit a prospective application in the recovery and utilization of solar and industrial waste energy and have been widely used for energy conserving in buildings, thermal insulation, and thermal regulation, etc. The organic n-alkanes (paraffin waxes),
* Corresponding author. Tel.: þ86 10 6441 0145; fax: þ86 10 6442 1693. E-mail addresses: [email protected], [email protected] (X. Wang). http://dx.doi.org/10.1016/j.renene.2014.08.079 0960-1481/© 2014 Elsevier Ltd. All rights reserved.
neopentyl glycol, fatty alcohols, fatty acids and eutectic mixtures, and some inorganic substances like salt hydrates are the most usually used PCMs, which have high phase change enthalpies between 150 and 240 J/g [3]. However, one of the vital disadvantages of PCMs is a difficulty to handle with them in a mobile state when phase change occurs from solid to liquid. Furthermore, the salt hydrates are floating when combined with the other materials like building materials or textile materials and may sweat out at the surface or wash out in moist climate. Their crystal water content may change due to the variety of humidity [4]. The n-alkanes are also mobile when melt to low viscous liquids and may diffuse throughout the other materials. They may also evaporate into the ambient air and, accordingly, increase the volatile organic content (VOC) of the air [5]. Owing to the legal regulations in many countries to reduce the VOC of ambient air in civil fields, there has been a requirement to develop the new materials with low- or non-VOC characteristics. Therefore, the use of PCMs without encapsulation is commonly not recommended [6]. The problems rising in the application of bulk PCMs demand a packaging technology to make PCMs a form-stable feature either in liquid or in solid states, and
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consequently, these PCMs can be isolated with the surrounding materials [7]. Encapsulation of PCMs into inert polymeric or inorganic materials is the most potential and actually useful method for the preparation of the form-stable PCMs. This method can engulf small solid or liquid particles with a solid wall and thus will prevent the leakage of PCMs from their location, reduce the interference toward phase-change behaviors from the outside environment, increase the heat-transfer area, and make liquid PCMs easy to handle when the phase change occurs [8]. Consequently, the resultant encapsulated PCMs can achieve a form-stable structure and facilitate themselves a wide variety of applications in textiles, fabrics, fibres, coatings, insulation panels, physiotherapy devices, and building materials [9]. A literature survey indicates that there are numerous investigations on the encapsulation technologies for the development of form-stable composite PCMs, and most of them are involved in the preparation of microencapsulated PCMs with polymeric walls. These PCM microcapsules were fabricated with a wall material of melamine-formaldehyde resin [10], ureaformaldehyde resin [11], polyurethane [12], polystyrene, styrenebutadiene-styrene copolymer [13], or polymethyl methacrylate (PMMA) through in-situ polymerization or interfacial polymerization in an emulsion system [14,15], and generally, they exhibited a well-defined coreeshell structure. However, there are some drawbacks such as flammability, poor thermal and chemical stabilities, and low thermal conductivity for these microencapsulated PCMs due to the polymeric wall materials [16,17]. Furthermore, there is a requirement of prompt heat transfer for PCMs to give a prompt response during the heat energy storage and thermal regulation processes. Recently, some new attempts have been made to enhance the thermal conductivity and mechanical strength of the form-stable composite PCMs by encapsulating organic PCMs into inorganic materials. There are several publications concerning the inorganic silica as a wall material for encapsulation of PCMs. Wang et al. [18] first reported the encapsulation method and the mechanism of PCMs into silica through in-situ polycondensation in an oil-inwater emulsion. Chang et al. [19] investigated the synthetic technology of microencapsulated n-octadecane PCMs with PMMA network-silica hybrid shell via a solegel process. Jin et al. [20] developed a one-step synthetic technique for the microencapsulated PCMs with the silica wall under a surfactant- or dispersant-free condition. Li et al. [21] also studied the synthesis of the form-stable paraffin/SiO2 phase-change composites through insitu emulsion interfacial hydrolysis and polycondensation of tetraethyl orthosilicate (TEOS) and found that these phase-change composites achieved a very high heat storage capability. In our previous study, we also successfully synthesized the silicamicroencapsulated n-octadecane through interfacial polycondensation and in situ self-assembly in a solegel process and found that these microcapsules exhibited a well-defined coreeshell structure and a uniform particle size under a moderately acidic condition [22,23]. However, the aforementioned silicaencapsulated form-stable composite PCMs were unexceptionally synthesized through the solegel method using TEOS or organic siloxane derivatives as silica precursors. It is no doubt that the TEOS is an expensive silica precursor, and it is not economical for the form-stable composite PCMs to be put into mass production by using this raw material. Especially in recent years, the improvement in energy efficiency of buildings has become a prime objective for energy policy at regional, national and international levels. The incorporation of PCMs can compensate the conventional building materials for their small storage capacity through the latent heat storage, thus reducing the energy consumption for auxiliary cooling/heating as well as the indoor temperature
fluctuation. The form-stable PCMs trend to achieve a large-scale application in building materials in order to reduce the energy consumption of buildings [24]; however, the TEOS precursor is evidently hard to be employed in this field. Therefore, this gives an impetus to the development of new form-stable composite PCMs at a much lower cost. Sodium silicate is also a silica precursor with the features of easy availability and low cost; hence it shows a high feasibility in industrial manufacture for the form-stable composite PCMs with a silica wall. In our previous work, we have successfully synthesized the well-defined spherical microcapsules based on the n-octadecane core and silica shell by using sodium silicate precursor [25]. In the current work, we extend to the investigation on the phasechange characteristics and thermal performance of the formstable n-alkanes/silica composite phase change materials synthesized with sodium silicate precursor. The normal alkanes (n-alkanes) are considered as a type of the most promising organic PCMs. They have different melting temperatures (Tm) and crystallization temperatures (Tc) depending on the number of carbon atoms in their molecular chains [26,27]. These linear hydrocarbons have phase-change temperatures around 18e36 C, and this temperature range can make the human body feel comfortable when the phase change occurs. Therefore, the form-stable n-alkanesbased PCMs show a potential application in building materials for meeting the requirement in energy conservation and efficiency of buildings [28,29]. The goal of this work is to develop a feasible fabrication technique for the form-stable n-alkanes/silica composite PCMs and to understand the effect of the carbon number of nalkanes on the microstructures, phase-change behaviors, and thermal performance of the obtained composites. The resultant form-stable composite PCMs will also have a good potential in the application for the energy conservation and efficiency of buildings. 2. Experimental 2.1. Materials Four types of representative n-alkanes, n-heptadecane, n-octadecane, n-nonadecane, and n-eicosane were employed as PCMs and were purchased from J & K Chemical Co., Ltd., China. These n-alkanes had a purity of 95% and were directly used as received without further purification. Sodium silicate containing 19.86 wt % of SiO2 and 21.05 wt % of Na2O used as a silica source was commercially obtained from Beijing Chemical Reagents Company. Poly(ethylene oxideebepropylene oxideebeethylene oxide) (EO27ePO61eEO27, Pluronic® P104) used as a nonionic surfactant was kindly supplied by BASF Corporation, Germany. Hydrochloric acid (HCl, 37.5 wt %) was also purchased from Beijing Chemical Reagents Company, China. Petroleum ether was acquired from Tianjin Chemical Regents Inc. 2.2. Synthesis of form-stable n-alkanes/silica composites A series of form-stable n-alkanes/silica composite PCMs were synthesized through the hydrolysis and condensation reactions of sodium silicate in an O/W emulsion templating system. An amphipathic EO27ePO61eEO27 copolymer was used as a nonionic surfactant, and an aqueous solution of HCl was used as a catalyst. A typical synthetic procedure includes three steps and is described as follows: As the first step, a beaker was charged with 20.0 g of nalkanes and 500 mL of aqueous solution of EO27ePO61eEO27 copolymer (2.0 g/L) followed by an agitation at 70 C for 1 h to form a stable emulsion. The second step was the preparation of silica sol solution. 20.0 g of sodium silicate was mixed with 100 ml of deionized water, and then the acidity of the mixture was adjusted
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to pH 2.9e3.1 by the aqueous solution of HCl. The mixture was continually stirred at 35 C until it became clearly transparent, indicating that the hydrolysis of sodium silicate was completed, and the silica sol solution was formed as the hydrolyzed silica monomers and oligomers. At the third step, the prepared silica sol solution was dropwise added into the emulsion for 1 h under a vigorous agitation, and then the reaction solution was heated to 70 C with stirring for 24 h to complete the polycondensation of silica species. Then, the suspension reactant solution was heated to 80 C and aged for 5 h without agitation. The form-stable n-alkanes/silica composites were obtained as some white powders by filtration. The collected products were washed with petroleum ether and deionized water several times and then were dried at 80 C overnight for further characterizations and measurements. 2.3. Characterization 2.3.1. Scanning electron microscopy (SEM) The morphology of the n-alkanes/silica composites was investigated by SEM on a Hitachi Se4700 scanning electron microscope. The specimens were prepared electrically conductive by sputter coating with a thin layer of goldepalladium alloy. The SEM micrographs were obtained in a high vacuum mode at an acceleration voltage of 20 kV and with a high-resolution image quality. 2.3.2. Transmission electron microscopy (TEM) The microstructures of the n-alkanes/silica composites were detected by TEM on a Hitachi H-800 transmission electron microscope operating at an acceleration voltage of 80 kV. The specimen was dispersed in ethanol by ultrasonicator, and some pieces were collected on carbon-coated 300-mesh copper grids for the TEM measurements. 2.3.3. Fourier transform infrared (FTeIR) spectroscopy FTeIR spectroscopy was performed to characterize the chemical structures of the n-alkanes/silica composites on a Nicolet iS5 FTeIR spectrophotometer at a scanning number of 32 using a KBr sampling sheet. 2.3.4. Differential scanning calorimetry (DSC) Dynamic DSC scans were performed to analyze the phasechange characteristics of the n-alkanes/silica composites on a TA Instruments Q100 differential scanning calorimeter equipped with a thermal analysis data station. All of the measurements were carried out under a nitrogen atmosphere at a heating or cooling rate of 10 C/min, and the mass for each specimen is about 6 mg. The first heating scan was run from 0 to 60 C, and the specimen was held at this temperature for 5 min to diminish the thermal history before the formal measurement. 2.3.5. Thermogravimetric analysis (TGA) TGA for the n-alkanes/silica composites was performed on a TA Instruments Q50 thermal gravimetric analyzer under a nitrogen atmosphere. The specimen with a mass of about 10 mg was placed in an aluminum crucible and then was ramped from room temperature up to about 700 C at a heating rate of 10 C/min. 2.3.6. Measurement of thermal conductivity The thermal conductivity of the n-alkanes/silica composites was measured using an EKO HCe110 thermal conductivity instrument according to ASTM C-518 standard.
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3. Results and discussion 3.1. Synthesis and characterizations The synthesis of the form-stable n-alkanes/silica composite PCMs adopted an O/W emulsion templating means through the hydrolysis and condensation reactions of sodium silicate in a solegel process. The solegel process includes the first preparation of the silica sol (i.e. colloidal silanol and its oligomers) commonly using sodium silicate as a precursor and the subsequent gelation of the silica sol accomplished through the hydrolysis and condensation reactions initiated by water. The condensation step results in the formation of an extended silicon oxide network. The encapsulation of n-alkanes by silica wall should be involved in this solegel process through the self-assembly of the silica sol on the surface of n-alkane domains by the emulsion templating. Compared to the easy synthesis based on the TEOS precursor, the fabrication of the form-stable n-alkanes/silica composites using the sodium silicate precursor seems to be a tough work. The difficulty is due to the instability of silica sol solution prepared by sodium silicate, because such a silica sol is very sensitive to the acidity of the reactant solution. This makes a difficulty for the silica species to be selfassembled on the surface of n-alkane domains and thus greatly influences the formation of silica wall encapsulating the n-alkanes. In the current work, we attempted to use the different types of surfactants as templates for the fabrication of the form-stable nalkanes/silica composites, but the syntheses failed in the use of cationic and anionic surfactants. Finally, the synthesis succeeded in the adoption of the nonionic surfactant, i.e. EO27ePO61eEO27 copolymer. A typical formation procedure of the form-stable n-alkanes/silica composites could be described as follows: The oily nalkane was first dispersed in an aqueous solution of EO27ePO61eEO27 copolymer to form a stable oil-in-water emulsion templating system. In this system, the hydrophilic part of the copolymer alternatively arranges along its hydrophobic one and consequently are associated with the water molecules and are trimly covered the surfaces of the n-alkane droplets with hydrophobic part oriented to the droplets. Moreover, the silica sol was prepared by dissolving sodium silicate in water in a weak acidic condition. The hydrolysis of sodium silicate followed with the oxolation could produce the water-soluble silica monomers and oligomers as starting substances for the further formation of silica wall. When the transparent silica sol was incorporated into the emulsion containing the n-alkane micelles, the silica monomers and oligomers were attracted onto the surfaces of the micelles through a hydrogen-bonding interaction between the hydrolyzed silica species and the hydrophilic part of the surfactant. These silica species further polycondensated in the presence of an acidic catalyst, leading to a polymeric network of siloxane bonds with hydroxy side groups surrounding the n-alkane micelles. With a long-term silica condensation, the n-alkane domain was well encapsulated by a massive silica wall. Such a fabrication process can be well depicted by a scheme shown in Fig. 1. The chemical structures of the synthesized form-stable n-alkanes/silica composites were characterized by FTeIR spectroscopy, and the corresponding infrared spectra were demonstrated in Fig. 2. Meanwhile, the infrared spectra of pure n-alkanes as controls are also presented in Fig. 2. It should be mentioned that there is a discrepancy of one methylene group in the molecular chains for the four types of n-alkanes, so their spectrum profiles are quite similar. Two intensive absorption peaks at 2925 and 2854 cm1 can be found in the infrared spectra of the form-stable n-alkanes/silica composites, which are attributed to the alkyl CeH stretching vibrations of methyl and methylene groups, respectively. The infrared spectra also show the absorption peaks of methylene deformation
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Fig. 1. Schematic fabrication of the n-alkanes/silica composites using sodium silicate as silica precursor.
vibration at 1464 and 1378 cm1. Moreover, an absorption band at 721 cm1 is assigned to the in-plane methylene rocking vibration. The above characteristic absorption peaks are in good agreement with the ones appearing in the infrared spectra of pure n-alkanes shown by Fig. 2. On the other hand, two absorption bands could be
observed at 1079 and 808 cm1 corresponding to the asymmetry stretching and the symmetric stretching vibrations of SieOeSi, respectively. An absorption peak appearing at 463 cm1 is attributed to the SieOeSi bending vibration. Furthermore, a broad band at 3442 cm1 is due to the SieOH stretching vibration while a peak at 954 cm1 represents the SieOH bending vibration. The above characteristic absorption peaks indicated the presence of silica wall in the form-stable composite PCMs. These infrared spectroscopy results confirmed the successful encapsulation of n-alkanes with a silica wall. 3.2. Morphology and form stability
Fig. 2. FTeIR spectra of pure n-alkanes and the n-alkanes/silica composites; (a) nheptadecane, (b) n-heptadecane/silica composite, (c) n-octadecane, (d) n-octadecane/ silica composite, (e) n-nonadecane, (f) n-nonadecane/silica composite, (g) n-eicosane, and (h) n-eicosane/silica composite.
Fig. 3 shows the SEM micrographs of the four n-alkanes/silica composite samples. It is observed that these four composites show some irregularly spherical particles with a diameter of 1.5e2 mm. However, the n-alkanes/silica composite was found to present serious agglomerations, and almost all of the particles were connected with each other by solid silica as a result of the further polycondensation of sodium silicate hydrolyzates. In general, the sodium silicate has much fast hydrolysis and condensation rates than the other silica precursors. It seems that, apart from the most of the hydrolyzates encapsulating the n-alkanes, some of them might polycondensate to form the free silica colloidal nanoparticles. These free colloidal nanoparticles were attached to the surface of the composite particles. This not only leads to the aggregation of the particles but also makes a coarse surface for the composites. It is noteworthy that the four composites seem to have a similar morphology with one another, indicating that the type of n-alkanes did not influence the morphology of their form-stable composites. Furthermore, the TEM micrographs shown by Fig. 3 confirm that the n-alkane cores as dark parts were well encapsulated by silica (gray parts). Although all of the n-alkanes/silica composites demonstrate irregular morphologies due to the agglomeration of small silica microcapsules, their macroscopical feature presents some white powders as shown by Fig. 3. We also evaluated the form stability of the synthesized composites by investigating the temperature dependence of shape from a representative n-eicosane/silica composite. Fig. 4 shows the digital photographs of the n-eicosane/
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Fig. 3. SEM micrographs of (a, b) the n-heptadecane/silica, (c) n-octadecane/silica, (d) n- nonadecane/silica, and (e) n-eicosane/silica composites; TEM micrographs of (f) the nnonadecane/silica, (g) n-eicosane/silica, and (g) n-heptadecane/silica composites.
Fig. 4. Digital photographs of (a) pure n-eicosane and (b) the n-eicosane/silica composites heated at hot stage with different temperatures.
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Fig. 5. DSC cooling (a) and heating (b) thermograms of pure n-alkanes and the n-alkanes/silica composites.
silica composite sample and pure n-eicosane when heating at 60 C on a hot stage. These photographs were taken at different melting stages so as to record the change of shape for two samples during melting process. It is anticipative that pure n-eicosane gradually lost its original shape due to a transformation from solid to liquid when melting, and it could be recovered to its original shape when cooling. However, the n-eicosane/silica composite sample always kept a triangle in shape during long-term heating at a temperature above the melting point of n-eicosane. There is no difference in shape between the original sample and the recovered one, and it seems that no liquid n-eicosane is found to leak out of the composite. These results indicate that the silica material well encapsulated the n-eicosane core and isolated it from the outside environment. In this case, the silica layer could not only maintain the original shape of PCMs but also could provide a good protection for the encapsulated PCMs, preventing the molten n-eicosane from leakage accordingly. 3.3. Phase change behaviors and properties The dynamic DSC scans were carried out to investigate the phase change behaviors and properties of the form-stable n-alkanes/silica composites. Fig. 5 shows the resulted DSC thermograms, in which the thermograms of four pure n-alkanes are also presented as controls, and the phase change parameters obtained from the DSC scans are summarized in Table 1. It is interestingly observed noted that these four pure n-alkanes exhibit different phase-change behaviors from one another. The pure n-heptadecane and n-nonadecane exhibit two crystallization peaks and two melting peaks, corresponding to the phase changes of solidification and fusion, respectively, while the pure n-octadecane and n-eicosane show a single crystallization peak as well as a single melting peak. It was reported that the n-alkanes usually present a rotator
phase above the bulk crystallization temperature [30,31]. The odd n-alkanes like n-heptadecane and n-nonadecane exhibit an orthorhombic rotator phase with molecules untitled with respect to the layers, and thus, they present two phase transitions between the isotropic liquid phase and the stable orthorhombic phase [32]. The higher and the lower crystallization peak temperatures, i.e. Tc,1 and Tc,2, are attributed to the transitions from the homogeneously nucleated liquid to a rotator phase and from the heterogeneously nucleated rotator phase to the crystalline phase, respectively. This feature results in a bimodal melting behavior due to twice phase transitions. However, the phase change behaviors of cooling process for the even n-alkanes are quite different. The even n-octadecane and n-eicosane directly undergo a phase transition from the liquid phase to the stable triclinic phase, and no rotator phase appears during the cooling process, leading to a single melting peak in the heating process accordingly [33]. It is noteworthy that the phase change temperatures are also dependent on the carbon numbers of n-alkanes, and the greater the carbon numbers, the higher the phase change temperatures. As shown by Fig. 5 and Table 1, the encapsulation of n-alkanes significantly influences their phase-change behaviors. For the odd n-alkanes/silica composites, their phase change behaviors in the cool and heating processes are similar to the pure ones. It should be noted that the two crystallization peaks show an obvious shift toward a lower temperature while the melting peak temperatures increase slightly. This phenomenon is attributed to the confinement effect of silica wall on the crystallization of encapsulated nalkanes. As the n-alkanes were encapsulated within a micrometric space, the motion of their molecules was restricted in confined geometry. This leads to an increase in crystallization activity energy, thus resulting in a decrease in crystallization temperature [34]. In the case of the even n-alkanes/silica composites, the phase change behavior in the heating process is similar to that of the pure
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Table 1 The phase-change characteristic parameters of the form-stable n-alkanes/silica composites. Sample
Pure n-heptadecane n-Heptadecane/silica Pure n-octadecane n-Octadecane/silica Pure n-nonadecane n-Nonadecane/silica Pure n-eicosane n-Eicosane/silica Pure silica
Crystallization process
Melting process
Tc,1 ( C)
Tc,2 ( C)
DHc (J/g)
Tm,1 ( C)
Tm,2 ( C)
DHm (J/g)
15.29 16.15 23.76 24.27 29.42 26.24 33.35 31.86 e
4.81 0.46 e 21.58 20.47 17.13 e 26.52 e
204.29 61.37 207.7 72.18 200.97 80.79 245.4 78.63 e
21.57 21.90 30.45 32.56 34.86 36.89 40.98 40.48 e
9.34 10.45 e e 24.93 25.8 e e e
194.71 60.25 206.4 73.52 181.86 74.78 246.3 81.21 e
ones, which is related to the phase transition between the triclinic phase and the melt. However, the phase change behaviors of cooling process are quite different, in which double exothermic peaks appear. It is reasonable to believe that the confined crystallization environment may induce a metastable rotator phase for the encapsulated even n-alkanes [35]. However, the metastable rotator phase lacks the long-range order in the rotational freedom degree of molecules and, therefore, promptly completes the rotator-tocrystal transition. As a result, a single melting peak and the bimodal crystallization peaks could be observed in Fig. 5. Furthermore, it should be notable that there is a small peak observed below the main crystallization peak temperature by 20 C in the crystallization process of n-eicosane/silica composite. This is assigned to the solidesolid phase transition resulting from the heterogeneous nucleation of the inner silica wall [36]. The phase change enthalpies for the crystallization and melting are also important parameters determining the thermal storage compatibility of a PCM. The crystallization enthalpy (DHc) and melting enthalpy (DHm) of both the n-alkanes/silica composites and the pure one were derived from the DSC scans and were collected in Table 1. All of pure n-alkanes show considerably high phase change enthalpies over 180 J/g according to the data in Table 1. This suggests that the four n-alkanes have a high capability of latent-heat storage and can fully release it when phase change occurs. However, the encapsulation of them with silica evidently reduced the absolute phase change enthalpies of the form-stable composite samples. Evidently, only the n-alkanes domain in the composite can perform phase changes to store the latent heat, the silica domain is just an inert material. Therefore, the phase change enthalpies of the n-alkanes/silica composites are strongly dependent on the n-alkanes loading. There are some simple equations used to evaluate the encapsulation and thermal storage performance of the composites [37]. The theoretical n-alkane loading (Lt) could be calculated from the weight ratio of the charged n-alkane (Walk) to the total amount of the charged n-alkane and the silica wall (Wsilica) converted from the charged sodium silicate using Eq. (1).
Lt ¼
Walk 100% Walk þ Wsilica
(1)
Meanwhile, the encapsulation efficiency (Een) of the n-alkanes/ silica composites could be calculated from the melting enthalpy obtained from DSC scans in terms of Eq. (2) [23]:
Een ¼
DHm;com 100% DHm;alk
Een (%)
Ees (%)
Ces (%)
0 67.03 e 67.03 e 67.03 e 67.03 100
e 30.94 e 35.62 e 41.12 e 32.97 e
e 30.48 e 35.18 e 40.64 e 32.51 e
e 98.52 e 98.78 e 98.82 e 98.60 e
efficiency (Ees) of the n-alkanes/silica composites could be deduced from Eq. (3) on the basis of the phase-change enthalpies [38].
Ees ¼
DHm;com þ DHc;com 100% DHm;alk þ DHc;alk
(3)
where, DHc,com and DHc,alk are the crystallization enthalpies of the n-alkanes/silica composites and pure n-alkanes, respectively. These encapsulation parameters calculated by Eqs. (1)e(3) are also summarized in Table 1. It is observed that the theoretical n-alkane loadings of the n-alkanes/silica composites were constantly identical to 67.03%, because the fixed weight ratio of n-alkanes/sodium silicate was charged for synthesis. The encapsulation efficiency and energy-storage efficiency are considered as two important characteristic parameters to describe the phase change properties of a PCM-based composite [22,38]. The former represents the effective encapsulation of PCMs by the wall materials, and the latter reflects the effective phase change performance of the n-alkanes encapsulated within silica wall for latent heat storage. It is noteworthy that the encapsulation efficiency of the n-alkanes/silica composites is much lower than the theoretical ones. This indicates that no all of the hydrolysates of sodium silicate were converted into the silica wall for encapsulating the n-alkanes. In fact, some of silica precursor was not assembled on the surfaces of n-alkane micelles but directly polycondensated as solid partials during the solegel synthesis, resulting in a decrease in exact n-alkane loading accordingly. Moreover, the four n-alkanes/silica composites are found to show a variety of encapsulation efficiency, which may be attributed to the influences from the fluctuation of synthetic conditions like agitation speed, dropping speed of reactants, temperature, etc. It should be mentioned that the encapsulation efficiency is only dependent on the latent-heat-release performance of the n-alkane loaded in composites. However, the energy-storage efficiency is derived from the phase change enthalpies involved in both crystallization and melting processes and, therefore, is more suitable to evaluate the energy storage and release performance of a form-stable PCM composite. It is observed that the energy-storage efficiency of the n-alkanes/silica composites is quite closed to their encapsulation efficiency, suggesting that almost hundred percent of stored latent heat could be released by the encapsulated n-alkanes during a phase-change cycle. In addition, the thermal storage capability (Ces) of the n-alkanes/silica composites could also be evaluated by the thermal analysis data from DSC scans using Eq. (4) [21]:
(2)
where, DHm,com is the recorded melting enthalpy of the n-alkanes/ silica composites from DSC scans, corresponding to the encapsulated n-alkanes, and DHm,alk is the melting enthalpy of pure n-alkanes derived from DSC scans. Furthermore, the energy-storage
Lt (%)
Ces ¼
DHm;com þ DHc;com 100% DHm;alk þ DHc;alk Een
(4)
where Een is the encapsulation efficiency obtained by Eq. (2). As shown by Table 1, the thermal storage capability of the n-alkanes/ silica composites exhibits the thermal storage capabilities higher
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0.154 W m1 K1 to 0.165 W m1 K1. However, it is surprisingly noted that all of the n-alkanes/silica composites present much higher thermal conductivities in the range of 0.68e0.97 W m1 K1. The pure silica material directly obtained from the solegel synthesis of sodium silicate was found to have a thermal conductivity of 1.296 W m1 K1. These data confirmed that the encapsulation of n-alkanes with highly thermally conductive silica actually imparted a high thermal conductivity to the n-alkanes/silica composites. The supercooling is considered as a major obstacle to the practical application of the traditional microencapsulated PCMs. The supercooling degree (DTs) is an important parameter reflecting the fusionefreezing hysteresis of a PCM, and it could be calculated by Eq (5) [41].
DTs ¼ Tm Tc
Fig. 6. Thermal conductivities of pure n-alkanes and the n-alkanes/silica composites.
than 98%, indicating that the n-alkanes were well encapsulated by silica wall in a proper size and form, and thus, most of the encapsulated n-alkanes could effectively store and release the latent heat through phase changes. It is evident that the thermal storage capability for the form-stable PCM composites obtained in the current study is very similar to that of the previously reported microencapsulated PCMs with a silica shell and an aluminum hydroxide shell [21,39]. 3.4. Thermal performance The pure n-alkanes belong to typical organic PCMs, whose low thermal conductivities may delay the thermal response toward the latent-heat storage and release. Therefore, one of the most advantageous features for the n-alkanes/silica composites is a significant enhancement in thermal conductivity as a result of the encapsulation using highly thermally conductive silica wall [40]. Fig. 6 shows the thermal conductivities of the n-alkanes/silica composites and pure n-alkanes. It is anticipative that four pure n-alkanes in solid state exhibit low thermal conductivities ranging from
Fig. 7. Supercooling degrees of pure n-alkanes and the n-alkanes/silica composites.
(5)
where Tm and Tc are the melting and crystallization peak temperatures, respectively. Fig. 7 demonstrates the supercooling degrees of the n-alkanes/silica composites and pure n-alkanes. It was reported that the supercooling degree of microencapsulated PCMs increased by 13 C when the microcapsule size was reduced from 100 mm to 5 mm [41]. The supercooling degrees of the n-alkanes/ silica composites generally show a slight increase by 0e3 C over the pure n-alkanes, whereas that of the n-heptadecane/silica composite even decreases by 0.22 C compared to pure n-heptadecane. It should be noted that such an increment was far less than that of the traditional microencapsulated PCMs, though the n-alkanes was encapsulated within a narrow space smaller than 1 mm as showed by TEM. This result indicates that the heat transfer capability of the n-alkanes/silica composites was greatly enhanced due to the encapsulation with a highly thermally conductive silica wall, and consequently, the supercooling degrees of the encapsulated n-alkanes were significantly reduced compared to the traditional microencapsulated PCMs [41]. The multicycle DSC scans were performed to investigate the working reliability of the n-alkanes/silica composites during a longterm phase change process. Fig. 8 shows the one-hundred-loop DSC thermograms of n-alkanes/silica composites. It is clearly observed that the thermogram profiles of these four composites keep a good coincidence from the first loop to the last one, and both the crystallization peak and the melting one also maintain a stable position till the hundredth loop. This indicates that the n-alkanes/silica composites could always maintain the stable phase-change temperatures and enthalpies over the multicycle phase transitions. On the basis of the overall phase change behaviors illustrated by Fig. 8, the form-stable n-alkanes/silica composites synthesized in this study have a high working reliability to perform the energy storageerelease repetitiously at an almost stable temperature. The thermal stabilities of n-alkanes/silica composites were evaluated by TGA, and the resulted thermograms were revealed in Fig. 9. The pure n-alkanes as controls are found to present a typical one-step thermal degradation behavior almost with no char remained. Their TGA thermograms demonstrate the maximum decomposition temperatures of 174e195 C, at which the weight loss of the n-alkanes occurred most rapidly due to the major degradation of the linear alkane chains. It is interesting to note that the maximum decomposition temperature is dependent on the carbon numbers of n-alkanes and shows an improvement with the increase of carbon number. However, the n-alkanes/silica composites exhibit a two-step degradation behavior in their TGA thermograms. The first stage of weight loss is a major degradation corresponding to the decomposition of the encapsulated n-alkanes. It seems that the maximum decomposition temperatures of n-alkanes/silica composites are slightly higher that those of pure nalkanes, indicating that the silica wall could prevent the
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Fig. 8. DSC thermograms of the n-alkanes/silica composites under one hundred loops of heatingecooling cycle.
encapsulated n-alkanes from decomposing and thus improve the thermal stabilities of n-alkanes/silica composites. Nevertheless, the second weight loss is due to the polycondensation of silanols on silica wall, because it is reasonable to believe that there are still lots of hydroxyl groups remaining on silica wall after the hydrothermal
Fig. 9. TGA thermograms of the n-alkanes/silica composites.
synthesis. Therefore, the further silica polycondensation only resulted in a weight loss less than 5 wt %. 4. Conclusion The form-stable n-alkanes/silica composite PCMs were synthesized in the solegel process using sodium silicate precursor. The chemical structures of the synthesized composites were confirmed by FTeIR spectra, and their microstructures were determined by SEM and TEM. Although all of the n-alkanes/silica composites exhibited an irregularly spherical morphology, the n-alkanes were well encapsulated by silica wall so that the composites still presented a macroscopical feature of white powders. The n-alkanes/ silica composites could keep a good form stability when the encapsulated n-alkanes were molten. The n-alkanes/silica composites showed characteristic phase-change behaviors with the variation of carbon numbers in n-alkanes while they obtained a high thermal storage capability. The n-alkanes/silica composites also achieved a high thermal conductivity, low supercooling, and good work reliability as a result of the encapsulation of n-alkanes with highly thermal conductive inorganic silica. Moreover, the silica wall could also prevent the encapsulated n-alkanes from leaking, and accordingly, thermal stability of the composites was improved. The synthetic technology developed by this work exhibits a high feasibility in industrial manufacture for the silicaencapsulated PCMs due to the easy availability and low cost of sodium silicate. The resulting form-stable composite PCMs will be a potential candidate for the application in the fields of building air conditioning, electronic cooling systems, waste heat recovery,
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intelligent textiles or fabrics, preservation of foods, and solar energy storage. Acknowledgments The financial support from the National Natural Science Foundation of China (Grant No.: 51173010) is gratefully acknowledged. References [1] Cabeza LF, Castell A, Barreneche C, Gracia AD, Fernandez AL. Materials used as PCM in thermal energy storage in buildings: a review. Renew Sustain Energy Rev 2011;15:1675e95. [2] Mujeebu MA, Abdullah MZ, Abu Bakar MZ, Mohamad AA, Abdullah MK. Applications of porous media combustion technologyea review. Appl Energy 2009;86:1365e75. [3] Sharma A, Tyagi VV, Chen CR, Buddhi D. Review on thermal energy storage with phase change materials and applications. Renew Sustain Energy Rev 2009;13:318e45. [4] Zhou D, Zhao CY, Tian Y. Review on thermal energy storage with phase change materials (PCMs) in building applications. Appl Energy 2012;92:593e605. [5] Himran S, Suwono A, Mansoori GA. Characterization of alkanes and paraffin waxes of application as phase change energy storage medium. Energy Sources 1994;16:117e28. [6] Cho K, Choi SH. Thermal characteristics of paraffin in a spherical capsule during freezing and melting processes. Int J Heat Mass Transf 2000;43: 3183e96. [7] Zhao CY, Zhang GH. Review on microencapsulated phase change materials (MEPCMs): fabrication, characterization and applications. Renew Sustain Energy Rev 2011;15:3813e32. [8] Sari A, Alkan A, Karaipekli A. Preparation, characterization and thermal properties of PMMA/n-heptadecane microcapsules as novel solid-liquid microPCM for thermal energy storage. Appl Energy 2010;87:1529e34. [9] Zalba B, Marin JM, Cabeza LF, Mehling H. Review on thermal energy storage with phase change: materials, heat transfer analysis and applications. Appl Therm Eng 2003;23:251e83. [10] Su JF, Wang LX, Ren L. Fabrication and thermal properties of microPCMs: used melamine-formaldehyde resin as shell material. J Appl Polym Sci 2006;101: 1522e8. [11] Brown EN, Kessler MR, Sottos NR, White SR. In situ poly(urea-formaldehyde) microencapsulation of dicyclopentadiene. J Microencapsul 2003;6:719e30. [12] Saihi D, Vroman I, Giraud S, Bourbigot S. Microencapsulation of ammonium phosphate with a polyurethane shell part I: coacervation technique. React Funct Polym 2005;3:127e38. [13] Xiao M, Feng B, Gong K. Preparation and performance of shape stabilized phase change thermal storage materials with high thermal conductivity. Energy Convers Manag 2002;43:103e8. [14] Park BJ, Lee JY, Sung JH, Choi HJ. Microcapsules containing electrophoretic suspension of TiO2, modified with poly(methyl methacrylate). Current Appl Phys 2006;6:632e5. [15] Alkan C, Sari A. Fatty acid/poly(methyl methacrylate) (PMMA) blends as formstable phase change materials for latent heat thermal energy storage. Sol Energy 2008;82:118e24. [16] Delgado M, Lazaro A, Mazo J, Zalba B. Review on phase change material emulsions and microencapsulated phase change material slurries: materials, heat transfer studies and applications. Renew Sustain Energy Rev 2012;16: 253e73. [17] Romero-Cano MS, Vincent B. Controlled release of 4-nitroanisole from poly(lactic acid) nanoparticles. J Control Release 2002;82:127e35. [18] Wang LY, Tsai PS, Yang YM. Preparation of silica microspheres encapsulating phase-change material by sol-gel method in O/W emulsion. J Microencapsul 2006;23:3e14. [19] Chang CC, Tsai YL, Chiu JJ, Chen H. Preparation of phase change materials microcapsules by using PMMA network-silica hybrid shell via sol-gel process. J Appl Polym Sci 2009;112:1850e7.
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Contents lists available at ScienceDirect
Renewable Energy journal homepage: www.elsevier.com/locate/renene
Phase-change characteristics and thermal performance of form-stable n-alkanes/silica composite phase change materials fabricated by sodium silicate precursor Fang He, Xiaodong Wang*, Dezhen Wu State Key Laboratory of OrganiceInorganic Composites, Beijing University of Chemical Technology, Beijing 100029, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 25 March 2014 Accepted 28 August 2014 Available online
A series of n-alkanes/silica composites as form-stable phase change materials (PCMs) were synthesized in a solegel process using sodium silicate precursor. The chemical compositions and structures of the synthesized composites were characterized by Fourier transform infrared spectroscopy. Scanning electric micrographs show an irregularly spherical morphology of the n-alkanes/silica composites, and transmission electric micrographs confirm that the n-alkanes have been well encapsulated by silica. These nalkanes/silica composites keep a good sharp stability due to the support of silica wall even if the n-alkanes are in molten state. The differential scanning calorimetric analysis indicates that the phase change behaviors and characteristics of the n-alkanes/silica composites strongly depend on the carbon atom number in n-alkanes, and meanwhile, the encapsulated n-alkanes have a high thermal storage capability. The investigation on thermal performance demonstrated that the n-alkanes/silica composites achieved a high thermal conductivity, low supercooling, and good work reliability as a result of the encapsulation of n-alkanes with highly thermal conductive inorganic silica. Moreover, the thermal stability of the composites was also improved due to the protection of silica wall toward the encapsulated n-alkanes. It is anticipative that, owing to the easy availability and low cost of sodium silicate, the synthetic technology developed by this work has a high feasibility in the industrial manufacture of the form-stable PCMs. © 2014 Elsevier Ltd. All rights reserved.
Keywords: Form-stable phase change composite materials Solegel synthesis n-Alkaline Silica Sodium silicate precursor
1. Introduction With the prompt development of global economics, the consumption of fossil energy sources grows rapidly, thus resulting in a shortage of fossil fuels as well as an increase in greenhouse gas emission. The effective utilization of energy has given a potent impetus to develop new energy storage materials. Phase change materials (PCMs) are considered as a class of renewed energy materials with the highly efficient utilization of energy. They can perform the cycle of storage-release for latent heat through the solideliquid phase transition and can regulate the locally environmental temperatures accordingly [1,2]. Nowadays, PCMs exhibit a prospective application in the recovery and utilization of solar and industrial waste energy and have been widely used for energy conserving in buildings, thermal insulation, and thermal regulation, etc. The organic n-alkanes (paraffin waxes),
* Corresponding author. Tel.: þ86 10 6441 0145; fax: þ86 10 6442 1693. E-mail addresses: [email protected], [email protected] (X. Wang). http://dx.doi.org/10.1016/j.renene.2014.08.079 0960-1481/© 2014 Elsevier Ltd. All rights reserved.
neopentyl glycol, fatty alcohols, fatty acids and eutectic mixtures, and some inorganic substances like salt hydrates are the most usually used PCMs, which have high phase change enthalpies between 150 and 240 J/g [3]. However, one of the vital disadvantages of PCMs is a difficulty to handle with them in a mobile state when phase change occurs from solid to liquid. Furthermore, the salt hydrates are floating when combined with the other materials like building materials or textile materials and may sweat out at the surface or wash out in moist climate. Their crystal water content may change due to the variety of humidity [4]. The n-alkanes are also mobile when melt to low viscous liquids and may diffuse throughout the other materials. They may also evaporate into the ambient air and, accordingly, increase the volatile organic content (VOC) of the air [5]. Owing to the legal regulations in many countries to reduce the VOC of ambient air in civil fields, there has been a requirement to develop the new materials with low- or non-VOC characteristics. Therefore, the use of PCMs without encapsulation is commonly not recommended [6]. The problems rising in the application of bulk PCMs demand a packaging technology to make PCMs a form-stable feature either in liquid or in solid states, and
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consequently, these PCMs can be isolated with the surrounding materials [7]. Encapsulation of PCMs into inert polymeric or inorganic materials is the most potential and actually useful method for the preparation of the form-stable PCMs. This method can engulf small solid or liquid particles with a solid wall and thus will prevent the leakage of PCMs from their location, reduce the interference toward phase-change behaviors from the outside environment, increase the heat-transfer area, and make liquid PCMs easy to handle when the phase change occurs [8]. Consequently, the resultant encapsulated PCMs can achieve a form-stable structure and facilitate themselves a wide variety of applications in textiles, fabrics, fibres, coatings, insulation panels, physiotherapy devices, and building materials [9]. A literature survey indicates that there are numerous investigations on the encapsulation technologies for the development of form-stable composite PCMs, and most of them are involved in the preparation of microencapsulated PCMs with polymeric walls. These PCM microcapsules were fabricated with a wall material of melamine-formaldehyde resin [10], ureaformaldehyde resin [11], polyurethane [12], polystyrene, styrenebutadiene-styrene copolymer [13], or polymethyl methacrylate (PMMA) through in-situ polymerization or interfacial polymerization in an emulsion system [14,15], and generally, they exhibited a well-defined coreeshell structure. However, there are some drawbacks such as flammability, poor thermal and chemical stabilities, and low thermal conductivity for these microencapsulated PCMs due to the polymeric wall materials [16,17]. Furthermore, there is a requirement of prompt heat transfer for PCMs to give a prompt response during the heat energy storage and thermal regulation processes. Recently, some new attempts have been made to enhance the thermal conductivity and mechanical strength of the form-stable composite PCMs by encapsulating organic PCMs into inorganic materials. There are several publications concerning the inorganic silica as a wall material for encapsulation of PCMs. Wang et al. [18] first reported the encapsulation method and the mechanism of PCMs into silica through in-situ polycondensation in an oil-inwater emulsion. Chang et al. [19] investigated the synthetic technology of microencapsulated n-octadecane PCMs with PMMA network-silica hybrid shell via a solegel process. Jin et al. [20] developed a one-step synthetic technique for the microencapsulated PCMs with the silica wall under a surfactant- or dispersant-free condition. Li et al. [21] also studied the synthesis of the form-stable paraffin/SiO2 phase-change composites through insitu emulsion interfacial hydrolysis and polycondensation of tetraethyl orthosilicate (TEOS) and found that these phase-change composites achieved a very high heat storage capability. In our previous study, we also successfully synthesized the silicamicroencapsulated n-octadecane through interfacial polycondensation and in situ self-assembly in a solegel process and found that these microcapsules exhibited a well-defined coreeshell structure and a uniform particle size under a moderately acidic condition [22,23]. However, the aforementioned silicaencapsulated form-stable composite PCMs were unexceptionally synthesized through the solegel method using TEOS or organic siloxane derivatives as silica precursors. It is no doubt that the TEOS is an expensive silica precursor, and it is not economical for the form-stable composite PCMs to be put into mass production by using this raw material. Especially in recent years, the improvement in energy efficiency of buildings has become a prime objective for energy policy at regional, national and international levels. The incorporation of PCMs can compensate the conventional building materials for their small storage capacity through the latent heat storage, thus reducing the energy consumption for auxiliary cooling/heating as well as the indoor temperature
fluctuation. The form-stable PCMs trend to achieve a large-scale application in building materials in order to reduce the energy consumption of buildings [24]; however, the TEOS precursor is evidently hard to be employed in this field. Therefore, this gives an impetus to the development of new form-stable composite PCMs at a much lower cost. Sodium silicate is also a silica precursor with the features of easy availability and low cost; hence it shows a high feasibility in industrial manufacture for the form-stable composite PCMs with a silica wall. In our previous work, we have successfully synthesized the well-defined spherical microcapsules based on the n-octadecane core and silica shell by using sodium silicate precursor [25]. In the current work, we extend to the investigation on the phasechange characteristics and thermal performance of the formstable n-alkanes/silica composite phase change materials synthesized with sodium silicate precursor. The normal alkanes (n-alkanes) are considered as a type of the most promising organic PCMs. They have different melting temperatures (Tm) and crystallization temperatures (Tc) depending on the number of carbon atoms in their molecular chains [26,27]. These linear hydrocarbons have phase-change temperatures around 18e36 C, and this temperature range can make the human body feel comfortable when the phase change occurs. Therefore, the form-stable n-alkanesbased PCMs show a potential application in building materials for meeting the requirement in energy conservation and efficiency of buildings [28,29]. The goal of this work is to develop a feasible fabrication technique for the form-stable n-alkanes/silica composite PCMs and to understand the effect of the carbon number of nalkanes on the microstructures, phase-change behaviors, and thermal performance of the obtained composites. The resultant form-stable composite PCMs will also have a good potential in the application for the energy conservation and efficiency of buildings. 2. Experimental 2.1. Materials Four types of representative n-alkanes, n-heptadecane, n-octadecane, n-nonadecane, and n-eicosane were employed as PCMs and were purchased from J & K Chemical Co., Ltd., China. These n-alkanes had a purity of 95% and were directly used as received without further purification. Sodium silicate containing 19.86 wt % of SiO2 and 21.05 wt % of Na2O used as a silica source was commercially obtained from Beijing Chemical Reagents Company. Poly(ethylene oxideebepropylene oxideebeethylene oxide) (EO27ePO61eEO27, Pluronic® P104) used as a nonionic surfactant was kindly supplied by BASF Corporation, Germany. Hydrochloric acid (HCl, 37.5 wt %) was also purchased from Beijing Chemical Reagents Company, China. Petroleum ether was acquired from Tianjin Chemical Regents Inc. 2.2. Synthesis of form-stable n-alkanes/silica composites A series of form-stable n-alkanes/silica composite PCMs were synthesized through the hydrolysis and condensation reactions of sodium silicate in an O/W emulsion templating system. An amphipathic EO27ePO61eEO27 copolymer was used as a nonionic surfactant, and an aqueous solution of HCl was used as a catalyst. A typical synthetic procedure includes three steps and is described as follows: As the first step, a beaker was charged with 20.0 g of nalkanes and 500 mL of aqueous solution of EO27ePO61eEO27 copolymer (2.0 g/L) followed by an agitation at 70 C for 1 h to form a stable emulsion. The second step was the preparation of silica sol solution. 20.0 g of sodium silicate was mixed with 100 ml of deionized water, and then the acidity of the mixture was adjusted
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to pH 2.9e3.1 by the aqueous solution of HCl. The mixture was continually stirred at 35 C until it became clearly transparent, indicating that the hydrolysis of sodium silicate was completed, and the silica sol solution was formed as the hydrolyzed silica monomers and oligomers. At the third step, the prepared silica sol solution was dropwise added into the emulsion for 1 h under a vigorous agitation, and then the reaction solution was heated to 70 C with stirring for 24 h to complete the polycondensation of silica species. Then, the suspension reactant solution was heated to 80 C and aged for 5 h without agitation. The form-stable n-alkanes/silica composites were obtained as some white powders by filtration. The collected products were washed with petroleum ether and deionized water several times and then were dried at 80 C overnight for further characterizations and measurements. 2.3. Characterization 2.3.1. Scanning electron microscopy (SEM) The morphology of the n-alkanes/silica composites was investigated by SEM on a Hitachi Se4700 scanning electron microscope. The specimens were prepared electrically conductive by sputter coating with a thin layer of goldepalladium alloy. The SEM micrographs were obtained in a high vacuum mode at an acceleration voltage of 20 kV and with a high-resolution image quality. 2.3.2. Transmission electron microscopy (TEM) The microstructures of the n-alkanes/silica composites were detected by TEM on a Hitachi H-800 transmission electron microscope operating at an acceleration voltage of 80 kV. The specimen was dispersed in ethanol by ultrasonicator, and some pieces were collected on carbon-coated 300-mesh copper grids for the TEM measurements. 2.3.3. Fourier transform infrared (FTeIR) spectroscopy FTeIR spectroscopy was performed to characterize the chemical structures of the n-alkanes/silica composites on a Nicolet iS5 FTeIR spectrophotometer at a scanning number of 32 using a KBr sampling sheet. 2.3.4. Differential scanning calorimetry (DSC) Dynamic DSC scans were performed to analyze the phasechange characteristics of the n-alkanes/silica composites on a TA Instruments Q100 differential scanning calorimeter equipped with a thermal analysis data station. All of the measurements were carried out under a nitrogen atmosphere at a heating or cooling rate of 10 C/min, and the mass for each specimen is about 6 mg. The first heating scan was run from 0 to 60 C, and the specimen was held at this temperature for 5 min to diminish the thermal history before the formal measurement. 2.3.5. Thermogravimetric analysis (TGA) TGA for the n-alkanes/silica composites was performed on a TA Instruments Q50 thermal gravimetric analyzer under a nitrogen atmosphere. The specimen with a mass of about 10 mg was placed in an aluminum crucible and then was ramped from room temperature up to about 700 C at a heating rate of 10 C/min. 2.3.6. Measurement of thermal conductivity The thermal conductivity of the n-alkanes/silica composites was measured using an EKO HCe110 thermal conductivity instrument according to ASTM C-518 standard.
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3. Results and discussion 3.1. Synthesis and characterizations The synthesis of the form-stable n-alkanes/silica composite PCMs adopted an O/W emulsion templating means through the hydrolysis and condensation reactions of sodium silicate in a solegel process. The solegel process includes the first preparation of the silica sol (i.e. colloidal silanol and its oligomers) commonly using sodium silicate as a precursor and the subsequent gelation of the silica sol accomplished through the hydrolysis and condensation reactions initiated by water. The condensation step results in the formation of an extended silicon oxide network. The encapsulation of n-alkanes by silica wall should be involved in this solegel process through the self-assembly of the silica sol on the surface of n-alkane domains by the emulsion templating. Compared to the easy synthesis based on the TEOS precursor, the fabrication of the form-stable n-alkanes/silica composites using the sodium silicate precursor seems to be a tough work. The difficulty is due to the instability of silica sol solution prepared by sodium silicate, because such a silica sol is very sensitive to the acidity of the reactant solution. This makes a difficulty for the silica species to be selfassembled on the surface of n-alkane domains and thus greatly influences the formation of silica wall encapsulating the n-alkanes. In the current work, we attempted to use the different types of surfactants as templates for the fabrication of the form-stable nalkanes/silica composites, but the syntheses failed in the use of cationic and anionic surfactants. Finally, the synthesis succeeded in the adoption of the nonionic surfactant, i.e. EO27ePO61eEO27 copolymer. A typical formation procedure of the form-stable n-alkanes/silica composites could be described as follows: The oily nalkane was first dispersed in an aqueous solution of EO27ePO61eEO27 copolymer to form a stable oil-in-water emulsion templating system. In this system, the hydrophilic part of the copolymer alternatively arranges along its hydrophobic one and consequently are associated with the water molecules and are trimly covered the surfaces of the n-alkane droplets with hydrophobic part oriented to the droplets. Moreover, the silica sol was prepared by dissolving sodium silicate in water in a weak acidic condition. The hydrolysis of sodium silicate followed with the oxolation could produce the water-soluble silica monomers and oligomers as starting substances for the further formation of silica wall. When the transparent silica sol was incorporated into the emulsion containing the n-alkane micelles, the silica monomers and oligomers were attracted onto the surfaces of the micelles through a hydrogen-bonding interaction between the hydrolyzed silica species and the hydrophilic part of the surfactant. These silica species further polycondensated in the presence of an acidic catalyst, leading to a polymeric network of siloxane bonds with hydroxy side groups surrounding the n-alkane micelles. With a long-term silica condensation, the n-alkane domain was well encapsulated by a massive silica wall. Such a fabrication process can be well depicted by a scheme shown in Fig. 1. The chemical structures of the synthesized form-stable n-alkanes/silica composites were characterized by FTeIR spectroscopy, and the corresponding infrared spectra were demonstrated in Fig. 2. Meanwhile, the infrared spectra of pure n-alkanes as controls are also presented in Fig. 2. It should be mentioned that there is a discrepancy of one methylene group in the molecular chains for the four types of n-alkanes, so their spectrum profiles are quite similar. Two intensive absorption peaks at 2925 and 2854 cm1 can be found in the infrared spectra of the form-stable n-alkanes/silica composites, which are attributed to the alkyl CeH stretching vibrations of methyl and methylene groups, respectively. The infrared spectra also show the absorption peaks of methylene deformation
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Fig. 1. Schematic fabrication of the n-alkanes/silica composites using sodium silicate as silica precursor.
vibration at 1464 and 1378 cm1. Moreover, an absorption band at 721 cm1 is assigned to the in-plane methylene rocking vibration. The above characteristic absorption peaks are in good agreement with the ones appearing in the infrared spectra of pure n-alkanes shown by Fig. 2. On the other hand, two absorption bands could be
observed at 1079 and 808 cm1 corresponding to the asymmetry stretching and the symmetric stretching vibrations of SieOeSi, respectively. An absorption peak appearing at 463 cm1 is attributed to the SieOeSi bending vibration. Furthermore, a broad band at 3442 cm1 is due to the SieOH stretching vibration while a peak at 954 cm1 represents the SieOH bending vibration. The above characteristic absorption peaks indicated the presence of silica wall in the form-stable composite PCMs. These infrared spectroscopy results confirmed the successful encapsulation of n-alkanes with a silica wall. 3.2. Morphology and form stability
Fig. 2. FTeIR spectra of pure n-alkanes and the n-alkanes/silica composites; (a) nheptadecane, (b) n-heptadecane/silica composite, (c) n-octadecane, (d) n-octadecane/ silica composite, (e) n-nonadecane, (f) n-nonadecane/silica composite, (g) n-eicosane, and (h) n-eicosane/silica composite.
Fig. 3 shows the SEM micrographs of the four n-alkanes/silica composite samples. It is observed that these four composites show some irregularly spherical particles with a diameter of 1.5e2 mm. However, the n-alkanes/silica composite was found to present serious agglomerations, and almost all of the particles were connected with each other by solid silica as a result of the further polycondensation of sodium silicate hydrolyzates. In general, the sodium silicate has much fast hydrolysis and condensation rates than the other silica precursors. It seems that, apart from the most of the hydrolyzates encapsulating the n-alkanes, some of them might polycondensate to form the free silica colloidal nanoparticles. These free colloidal nanoparticles were attached to the surface of the composite particles. This not only leads to the aggregation of the particles but also makes a coarse surface for the composites. It is noteworthy that the four composites seem to have a similar morphology with one another, indicating that the type of n-alkanes did not influence the morphology of their form-stable composites. Furthermore, the TEM micrographs shown by Fig. 3 confirm that the n-alkane cores as dark parts were well encapsulated by silica (gray parts). Although all of the n-alkanes/silica composites demonstrate irregular morphologies due to the agglomeration of small silica microcapsules, their macroscopical feature presents some white powders as shown by Fig. 3. We also evaluated the form stability of the synthesized composites by investigating the temperature dependence of shape from a representative n-eicosane/silica composite. Fig. 4 shows the digital photographs of the n-eicosane/
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Fig. 3. SEM micrographs of (a, b) the n-heptadecane/silica, (c) n-octadecane/silica, (d) n- nonadecane/silica, and (e) n-eicosane/silica composites; TEM micrographs of (f) the nnonadecane/silica, (g) n-eicosane/silica, and (g) n-heptadecane/silica composites.
Fig. 4. Digital photographs of (a) pure n-eicosane and (b) the n-eicosane/silica composites heated at hot stage with different temperatures.
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Fig. 5. DSC cooling (a) and heating (b) thermograms of pure n-alkanes and the n-alkanes/silica composites.
silica composite sample and pure n-eicosane when heating at 60 C on a hot stage. These photographs were taken at different melting stages so as to record the change of shape for two samples during melting process. It is anticipative that pure n-eicosane gradually lost its original shape due to a transformation from solid to liquid when melting, and it could be recovered to its original shape when cooling. However, the n-eicosane/silica composite sample always kept a triangle in shape during long-term heating at a temperature above the melting point of n-eicosane. There is no difference in shape between the original sample and the recovered one, and it seems that no liquid n-eicosane is found to leak out of the composite. These results indicate that the silica material well encapsulated the n-eicosane core and isolated it from the outside environment. In this case, the silica layer could not only maintain the original shape of PCMs but also could provide a good protection for the encapsulated PCMs, preventing the molten n-eicosane from leakage accordingly. 3.3. Phase change behaviors and properties The dynamic DSC scans were carried out to investigate the phase change behaviors and properties of the form-stable n-alkanes/silica composites. Fig. 5 shows the resulted DSC thermograms, in which the thermograms of four pure n-alkanes are also presented as controls, and the phase change parameters obtained from the DSC scans are summarized in Table 1. It is interestingly observed noted that these four pure n-alkanes exhibit different phase-change behaviors from one another. The pure n-heptadecane and n-nonadecane exhibit two crystallization peaks and two melting peaks, corresponding to the phase changes of solidification and fusion, respectively, while the pure n-octadecane and n-eicosane show a single crystallization peak as well as a single melting peak. It was reported that the n-alkanes usually present a rotator
phase above the bulk crystallization temperature [30,31]. The odd n-alkanes like n-heptadecane and n-nonadecane exhibit an orthorhombic rotator phase with molecules untitled with respect to the layers, and thus, they present two phase transitions between the isotropic liquid phase and the stable orthorhombic phase [32]. The higher and the lower crystallization peak temperatures, i.e. Tc,1 and Tc,2, are attributed to the transitions from the homogeneously nucleated liquid to a rotator phase and from the heterogeneously nucleated rotator phase to the crystalline phase, respectively. This feature results in a bimodal melting behavior due to twice phase transitions. However, the phase change behaviors of cooling process for the even n-alkanes are quite different. The even n-octadecane and n-eicosane directly undergo a phase transition from the liquid phase to the stable triclinic phase, and no rotator phase appears during the cooling process, leading to a single melting peak in the heating process accordingly [33]. It is noteworthy that the phase change temperatures are also dependent on the carbon numbers of n-alkanes, and the greater the carbon numbers, the higher the phase change temperatures. As shown by Fig. 5 and Table 1, the encapsulation of n-alkanes significantly influences their phase-change behaviors. For the odd n-alkanes/silica composites, their phase change behaviors in the cool and heating processes are similar to the pure ones. It should be noted that the two crystallization peaks show an obvious shift toward a lower temperature while the melting peak temperatures increase slightly. This phenomenon is attributed to the confinement effect of silica wall on the crystallization of encapsulated nalkanes. As the n-alkanes were encapsulated within a micrometric space, the motion of their molecules was restricted in confined geometry. This leads to an increase in crystallization activity energy, thus resulting in a decrease in crystallization temperature [34]. In the case of the even n-alkanes/silica composites, the phase change behavior in the heating process is similar to that of the pure
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Table 1 The phase-change characteristic parameters of the form-stable n-alkanes/silica composites. Sample
Pure n-heptadecane n-Heptadecane/silica Pure n-octadecane n-Octadecane/silica Pure n-nonadecane n-Nonadecane/silica Pure n-eicosane n-Eicosane/silica Pure silica
Crystallization process
Melting process
Tc,1 ( C)
Tc,2 ( C)
DHc (J/g)
Tm,1 ( C)
Tm,2 ( C)
DHm (J/g)
15.29 16.15 23.76 24.27 29.42 26.24 33.35 31.86 e
4.81 0.46 e 21.58 20.47 17.13 e 26.52 e
204.29 61.37 207.7 72.18 200.97 80.79 245.4 78.63 e
21.57 21.90 30.45 32.56 34.86 36.89 40.98 40.48 e
9.34 10.45 e e 24.93 25.8 e e e
194.71 60.25 206.4 73.52 181.86 74.78 246.3 81.21 e
ones, which is related to the phase transition between the triclinic phase and the melt. However, the phase change behaviors of cooling process are quite different, in which double exothermic peaks appear. It is reasonable to believe that the confined crystallization environment may induce a metastable rotator phase for the encapsulated even n-alkanes [35]. However, the metastable rotator phase lacks the long-range order in the rotational freedom degree of molecules and, therefore, promptly completes the rotator-tocrystal transition. As a result, a single melting peak and the bimodal crystallization peaks could be observed in Fig. 5. Furthermore, it should be notable that there is a small peak observed below the main crystallization peak temperature by 20 C in the crystallization process of n-eicosane/silica composite. This is assigned to the solidesolid phase transition resulting from the heterogeneous nucleation of the inner silica wall [36]. The phase change enthalpies for the crystallization and melting are also important parameters determining the thermal storage compatibility of a PCM. The crystallization enthalpy (DHc) and melting enthalpy (DHm) of both the n-alkanes/silica composites and the pure one were derived from the DSC scans and were collected in Table 1. All of pure n-alkanes show considerably high phase change enthalpies over 180 J/g according to the data in Table 1. This suggests that the four n-alkanes have a high capability of latent-heat storage and can fully release it when phase change occurs. However, the encapsulation of them with silica evidently reduced the absolute phase change enthalpies of the form-stable composite samples. Evidently, only the n-alkanes domain in the composite can perform phase changes to store the latent heat, the silica domain is just an inert material. Therefore, the phase change enthalpies of the n-alkanes/silica composites are strongly dependent on the n-alkanes loading. There are some simple equations used to evaluate the encapsulation and thermal storage performance of the composites [37]. The theoretical n-alkane loading (Lt) could be calculated from the weight ratio of the charged n-alkane (Walk) to the total amount of the charged n-alkane and the silica wall (Wsilica) converted from the charged sodium silicate using Eq. (1).
Lt ¼
Walk 100% Walk þ Wsilica
(1)
Meanwhile, the encapsulation efficiency (Een) of the n-alkanes/ silica composites could be calculated from the melting enthalpy obtained from DSC scans in terms of Eq. (2) [23]:
Een ¼
DHm;com 100% DHm;alk
Een (%)
Ees (%)
Ces (%)
0 67.03 e 67.03 e 67.03 e 67.03 100
e 30.94 e 35.62 e 41.12 e 32.97 e
e 30.48 e 35.18 e 40.64 e 32.51 e
e 98.52 e 98.78 e 98.82 e 98.60 e
efficiency (Ees) of the n-alkanes/silica composites could be deduced from Eq. (3) on the basis of the phase-change enthalpies [38].
Ees ¼
DHm;com þ DHc;com 100% DHm;alk þ DHc;alk
(3)
where, DHc,com and DHc,alk are the crystallization enthalpies of the n-alkanes/silica composites and pure n-alkanes, respectively. These encapsulation parameters calculated by Eqs. (1)e(3) are also summarized in Table 1. It is observed that the theoretical n-alkane loadings of the n-alkanes/silica composites were constantly identical to 67.03%, because the fixed weight ratio of n-alkanes/sodium silicate was charged for synthesis. The encapsulation efficiency and energy-storage efficiency are considered as two important characteristic parameters to describe the phase change properties of a PCM-based composite [22,38]. The former represents the effective encapsulation of PCMs by the wall materials, and the latter reflects the effective phase change performance of the n-alkanes encapsulated within silica wall for latent heat storage. It is noteworthy that the encapsulation efficiency of the n-alkanes/silica composites is much lower than the theoretical ones. This indicates that no all of the hydrolysates of sodium silicate were converted into the silica wall for encapsulating the n-alkanes. In fact, some of silica precursor was not assembled on the surfaces of n-alkane micelles but directly polycondensated as solid partials during the solegel synthesis, resulting in a decrease in exact n-alkane loading accordingly. Moreover, the four n-alkanes/silica composites are found to show a variety of encapsulation efficiency, which may be attributed to the influences from the fluctuation of synthetic conditions like agitation speed, dropping speed of reactants, temperature, etc. It should be mentioned that the encapsulation efficiency is only dependent on the latent-heat-release performance of the n-alkane loaded in composites. However, the energy-storage efficiency is derived from the phase change enthalpies involved in both crystallization and melting processes and, therefore, is more suitable to evaluate the energy storage and release performance of a form-stable PCM composite. It is observed that the energy-storage efficiency of the n-alkanes/silica composites is quite closed to their encapsulation efficiency, suggesting that almost hundred percent of stored latent heat could be released by the encapsulated n-alkanes during a phase-change cycle. In addition, the thermal storage capability (Ces) of the n-alkanes/silica composites could also be evaluated by the thermal analysis data from DSC scans using Eq. (4) [21]:
(2)
where, DHm,com is the recorded melting enthalpy of the n-alkanes/ silica composites from DSC scans, corresponding to the encapsulated n-alkanes, and DHm,alk is the melting enthalpy of pure n-alkanes derived from DSC scans. Furthermore, the energy-storage
Lt (%)
Ces ¼
DHm;com þ DHc;com 100% DHm;alk þ DHc;alk Een
(4)
where Een is the encapsulation efficiency obtained by Eq. (2). As shown by Table 1, the thermal storage capability of the n-alkanes/ silica composites exhibits the thermal storage capabilities higher
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0.154 W m1 K1 to 0.165 W m1 K1. However, it is surprisingly noted that all of the n-alkanes/silica composites present much higher thermal conductivities in the range of 0.68e0.97 W m1 K1. The pure silica material directly obtained from the solegel synthesis of sodium silicate was found to have a thermal conductivity of 1.296 W m1 K1. These data confirmed that the encapsulation of n-alkanes with highly thermally conductive silica actually imparted a high thermal conductivity to the n-alkanes/silica composites. The supercooling is considered as a major obstacle to the practical application of the traditional microencapsulated PCMs. The supercooling degree (DTs) is an important parameter reflecting the fusionefreezing hysteresis of a PCM, and it could be calculated by Eq (5) [41].
DTs ¼ Tm Tc
Fig. 6. Thermal conductivities of pure n-alkanes and the n-alkanes/silica composites.
than 98%, indicating that the n-alkanes were well encapsulated by silica wall in a proper size and form, and thus, most of the encapsulated n-alkanes could effectively store and release the latent heat through phase changes. It is evident that the thermal storage capability for the form-stable PCM composites obtained in the current study is very similar to that of the previously reported microencapsulated PCMs with a silica shell and an aluminum hydroxide shell [21,39]. 3.4. Thermal performance The pure n-alkanes belong to typical organic PCMs, whose low thermal conductivities may delay the thermal response toward the latent-heat storage and release. Therefore, one of the most advantageous features for the n-alkanes/silica composites is a significant enhancement in thermal conductivity as a result of the encapsulation using highly thermally conductive silica wall [40]. Fig. 6 shows the thermal conductivities of the n-alkanes/silica composites and pure n-alkanes. It is anticipative that four pure n-alkanes in solid state exhibit low thermal conductivities ranging from
Fig. 7. Supercooling degrees of pure n-alkanes and the n-alkanes/silica composites.
(5)
where Tm and Tc are the melting and crystallization peak temperatures, respectively. Fig. 7 demonstrates the supercooling degrees of the n-alkanes/silica composites and pure n-alkanes. It was reported that the supercooling degree of microencapsulated PCMs increased by 13 C when the microcapsule size was reduced from 100 mm to 5 mm [41]. The supercooling degrees of the n-alkanes/ silica composites generally show a slight increase by 0e3 C over the pure n-alkanes, whereas that of the n-heptadecane/silica composite even decreases by 0.22 C compared to pure n-heptadecane. It should be noted that such an increment was far less than that of the traditional microencapsulated PCMs, though the n-alkanes was encapsulated within a narrow space smaller than 1 mm as showed by TEM. This result indicates that the heat transfer capability of the n-alkanes/silica composites was greatly enhanced due to the encapsulation with a highly thermally conductive silica wall, and consequently, the supercooling degrees of the encapsulated n-alkanes were significantly reduced compared to the traditional microencapsulated PCMs [41]. The multicycle DSC scans were performed to investigate the working reliability of the n-alkanes/silica composites during a longterm phase change process. Fig. 8 shows the one-hundred-loop DSC thermograms of n-alkanes/silica composites. It is clearly observed that the thermogram profiles of these four composites keep a good coincidence from the first loop to the last one, and both the crystallization peak and the melting one also maintain a stable position till the hundredth loop. This indicates that the n-alkanes/silica composites could always maintain the stable phase-change temperatures and enthalpies over the multicycle phase transitions. On the basis of the overall phase change behaviors illustrated by Fig. 8, the form-stable n-alkanes/silica composites synthesized in this study have a high working reliability to perform the energy storageerelease repetitiously at an almost stable temperature. The thermal stabilities of n-alkanes/silica composites were evaluated by TGA, and the resulted thermograms were revealed in Fig. 9. The pure n-alkanes as controls are found to present a typical one-step thermal degradation behavior almost with no char remained. Their TGA thermograms demonstrate the maximum decomposition temperatures of 174e195 C, at which the weight loss of the n-alkanes occurred most rapidly due to the major degradation of the linear alkane chains. It is interesting to note that the maximum decomposition temperature is dependent on the carbon numbers of n-alkanes and shows an improvement with the increase of carbon number. However, the n-alkanes/silica composites exhibit a two-step degradation behavior in their TGA thermograms. The first stage of weight loss is a major degradation corresponding to the decomposition of the encapsulated n-alkanes. It seems that the maximum decomposition temperatures of n-alkanes/silica composites are slightly higher that those of pure nalkanes, indicating that the silica wall could prevent the
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Fig. 8. DSC thermograms of the n-alkanes/silica composites under one hundred loops of heatingecooling cycle.
encapsulated n-alkanes from decomposing and thus improve the thermal stabilities of n-alkanes/silica composites. Nevertheless, the second weight loss is due to the polycondensation of silanols on silica wall, because it is reasonable to believe that there are still lots of hydroxyl groups remaining on silica wall after the hydrothermal
Fig. 9. TGA thermograms of the n-alkanes/silica composites.
synthesis. Therefore, the further silica polycondensation only resulted in a weight loss less than 5 wt %. 4. Conclusion The form-stable n-alkanes/silica composite PCMs were synthesized in the solegel process using sodium silicate precursor. The chemical structures of the synthesized composites were confirmed by FTeIR spectra, and their microstructures were determined by SEM and TEM. Although all of the n-alkanes/silica composites exhibited an irregularly spherical morphology, the n-alkanes were well encapsulated by silica wall so that the composites still presented a macroscopical feature of white powders. The n-alkanes/ silica composites could keep a good form stability when the encapsulated n-alkanes were molten. The n-alkanes/silica composites showed characteristic phase-change behaviors with the variation of carbon numbers in n-alkanes while they obtained a high thermal storage capability. The n-alkanes/silica composites also achieved a high thermal conductivity, low supercooling, and good work reliability as a result of the encapsulation of n-alkanes with highly thermal conductive inorganic silica. Moreover, the silica wall could also prevent the encapsulated n-alkanes from leaking, and accordingly, thermal stability of the composites was improved. The synthetic technology developed by this work exhibits a high feasibility in industrial manufacture for the silicaencapsulated PCMs due to the easy availability and low cost of sodium silicate. The resulting form-stable composite PCMs will be a potential candidate for the application in the fields of building air conditioning, electronic cooling systems, waste heat recovery,
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