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New approach for sol–gel synthesis of microencapsulated n-octadecane phase change material with silica wall using sodium silicate precursor
Energy 67 (2014) 223e233
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New approach for solegel synthesis of microencapsulated noctadecane phase change material with silica wall using sodium silicate precursor Fang He, Xiaodong Wang*, Dezhen Wu State Key Laboratory of OrganiceInorganic Composites, School of Materials Science and Engineering, 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 11 August 2013 Received in revised form 24 November 2013 Accepted 26 November 2013 Available online 30 January 2014
A new silica encapsulation technique toward n-octadecane PCM (phase change material) was developed through solegel synthesis using sodium silicate as a silica precursor. Fourier transform infrared spectra confirm the chemical composition of the synthesized microcapsules, and wide-angle X-ray scattering patterns indicate good crystallinity for the n-octadecane inside silica microcapsules. Scanning electric micrographs demonstrate that the microencapsulated n-octadecane obtained at pH 2.95w3.05 presents a perfect spherical morphology and a well-defined coreeshell microstructure. Because the pH value of reaction solution determines the silica condensation rate and, thus, influences the balance between the self-assembly and polycondensation of silica precursors on the surface of n-octadecane droplets, the microcapsules could achieve a smooth and compact surface at pH 2.95w3.05. The microencapsulated noctadecane also exhibits good phase change performance and achieves a high encapsulation rate and high encapsulation efficiency in this synthetic condition. The encapsulation of n-octadecane with compact and thick silica wall can impart a high thermal conductivity and a good anti-osmosis property to the microcapsules, and can also improve the thermal stability of the microcapsules by preventing inside n-octadecane from thermally evaporating. Owing to the easy availability and low cost of sodium silicate, this synthetic technique indicates a high feasibility in industrial manufacture for the microencapsulated PCMs with inorganic walls. Ó 2014 Elsevier Ltd. All rights reserved.
Keywords: Microencapsulated n-octadecane Silica wall Phase change performance Microstructure Thermal conductivity Phase change performance
1. Introduction The prompt development of global economics has brought increasing energy demands. It inevitably results in shortages of fossil fuels due to the limitation of conventional fossil energy sources. These issues have provided impetus to the development in highly efficient utilization of energy as well as to the discovery of new energy storage materials [1]. It is well understandable that the highly efficient utilization of energy can reduce the energy consumption so as to economize fossil fuels. Therefore, heat energy storage and thermal regulation have attracted increasing interests in the case of the impending shortage and increasing cost of energy resources nowadays [2,3]. PCM (phase change material) is a sort of latent heat storage material allowing the cycle of heat storagerelease through the solideliquid phase transition, and it has already exhibited a broad applicable prospect in the solar and industrial waste energy utilization as well as the thermal regulation * Corresponding author. Tel.: þ86 10 6441 0145; fax: þ86 10 6442 1693. E-mail addresses: [email protected], [email protected] (X. Wang). 0360-5442/$ e see front matter Ó 2014 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.energy.2013.11.088
due to its large heat storage capacity and isothermal behavior during the charging and discharging processes [4,5]. Nowadays, organic PCMs like paraffins and fatty acids have attracted great attention due to their excellent phase change performances, good thermal characteristics, and good compatibility with various materials or fibers [6,7]. Paraffin waxes (n-alkanes) have been considered as one of the most promising organic PCMs, which can be universally applied in the area of garments and home furnishing products. They have different melting temperatures (Tm) and crystallization temperatures (Tc), which depend on the number of carbon atoms in their chains [8]. These linear hydrocarbons have a Tm around 18e36 C and can change phases in this temperature range making human feel comfortable, so that paraffins could be used to make protective all-season outfits and garments in abruptly changing climatic conditions [9,10]. However, the conventional paraffin PCMs presented some drawbacks, particularly during the occurrence of phase change. These bulk PCMs are difficult to be handled as a liquid when melting. If the PCMs are directly incorporated into the other basematerials to form a phase change system, the liquid PCMs cannot
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maintain a stable shape and easily leak out [11]. On the other hand, the solidification of paraffins can cause a poor heat-transfer rate due to their low thermal conductivity, because it weakens the natural convection of paraffins in liquid state [12,13]. In addition, the paraffin PCMs also have an interfacial combination problem with the surrounding materials. Many attempts have been made to overcome these problems such as the formation of PCM emulsions or PCM slurries [7,14], and microencapsulation of PCMs is considered as a successful solution to hold the liquid and/or solid phases of PCMs within microcapsule containers. The microcapsule wall can keep PCMs isolated from the surrounding materials and can act as a barrier to protect them from harmful interaction with the environment. Such an attractive technique not only provides a stable structure and sufficient surface of PCMs for heat transfer, but also makes liquid PCMs easy to handle during the phase-change process [15,16]. The conventional microencapsulated PCMs consist of some coreeshell structured particles, which have a liquid core surrounded by a polymer shell that prevents the interior PCMs from leaking during the solideliquid phase change. These PCMcontaining microcapsules can also supply the large heat transfer areas and controllable changes in storage material volume when the phase change occurs [17,18]. In most cases, the cores of the conventional microencapsulated PCMs are always enwrapped with organic polymers as wall materials. A survey of publications regarding microencapsulated PCMs indicates that melaminee formaldehyde resin, ureaeformaldehyde resin, and polyurea are usually employed as wall materials to protect the PCM cores. Li et al. [19] reported the microencapsulation of PCMs into a formaldehyde shell with the low remnant formaldehyde content. Wang and Zhang [20] also developed the microencapsulated n-octadecane PCM with polyurea shells containing different soft segments through interfacial polymerization. Although the organic polymeric wall materials play an important role in improving the structure, permeability, controlled release, and thermal stability toward the microencapsulated PCMs, there are some limitations such as flammability, poor thermal and chemical stabilities, and low thermal conductivity for these microcapsules [21,22]. Additionally, there is a requirement of prompt heat transfer for PCMs to give a prompt response during heat energy storage and thermal regulation process. It is a well-known fact that the thermal conductivity of inorganic materials is always significantly higher than that of organic materials, and moreover, the chemical and thermal stabilities, mechanical strength, and flame retardancy of inorganic materials are also higher than those of organic materials. Owing to these attractive features, choosing an appropriate inorganic wall material for microencapsulated PCMs is a promising idea to enhance their phase-change performance. Recently, some attempts have been made to encapsulate the paraffin PCMs with inorganic silica materials based on the survey of literature. Wang et al. [23] first reported the preparation method of inorganic microcapsules by encapsulating PCMs with silica in O/W (oil-in-water) emulsion and also investigated the formation mechanism. Shi et al. [24] synthesized the microencapsulated polyacrylic acid hybrid hydrogel with silica wall as thermal energy storage materials via a solegel process and observed a significant improvement in thermal conductivity. Li et al. [25] also prepared the form-stable phase change composites of paraffin with silica and expanded graphite by solegel method and found that these phasechange composites achieved much higher thermal conductivity than pure paraffin. Moreover, Li et al. [26] and Fang et al. [22] reported the preparation and phase-change properties of the PCM microcapsules based on paraffin core and silica wall through in-situ polycondensation. In our previous study, we also successfully synthesized the well-defined PCM microcapsules with n-octadecane core and silica wall and observed a significant enhancement in
their thermal conductivity and phase-change performance [27,28]. It is noteworthy that these reported synthetic methods unexceptionally employed TEOS (tetraethoxysilane) as a silica precursor, and their two step wall formation process involved the hydrolysis of TEOS to deposit hydrolysis products, i.e. alkylsilanols and silanols, on the oil droplet surface followed by interfacial polymerization of these hydrolysis products to build an intact silica wall with much of the understanding of the chemistry concerned derived from solegel process of TEOS. However, the silica precursor, TEOS, is not economical for the industrial manufacture of microencapsulated PCMs due to its high cost as a raw material, which may restrict the applications of such a new encapsulation technology in industrial and civil areas. In the present work, we attempted to microencapsulate the noctadecane PCM with a silica wall using sodium silicate instead of TEOS as a precursor. Considering of the easy availability and low cost of sodium silicate, this synthetic technique indicates a high feasibility in industrial manufacture for the microencapsulated PCMs with inorganic walls. We employed n-octadecane as a paraffin PCM, because this paraffin is a desirable PCM for its good performances of heat energy storage and thermal regulation in an appropriate phase change temperature range of 20e29 C with high latent heat of around 210 J/g [29]. It is evident that such a phase change temperature is comfortable for the human body, while the latent heat is higher than that of other PCMs with the similar phase change temperature range [30]. Owing to such a certain range of phase change temperature, the heat produced by human body during vigorous motion makes n-octadecane melt, thus absorb in a great deal of heat energy. In this case, the local temperature surrounding the human body will decrease, and consequently the cooling local circumstance can comfort the hot human body. It is anticipated that the synthetic technology obtained in this study can be employed to encapsulate the other paraffin PCMs for applications in the outdoor apparel products, such as ski wear, hunting clothing, and smart garments with temperature regulating microclimate [31,32]. The aim of this work is to achieve a feasible inorganic encapsulation technology for paraffin PCMs and to investigate the effects of the synthetic methods on the microstructures, crystallization behaviors and phase change performance of the obtained products. Moreover, the formation mechanisms for a silica wall at the oilewater interface from the sodium silicate precursor were also clarified. 2. Experimental 2.1. Materials The n-octadecane with a purity of 90 wt% was purchased from Tianjin Alfa Aesar Company, China. Sodium silicate containing 19.86 wt% of SiO2 and 21.05 wt% of Na2O was commercially obtained from Beijing Chemical Reagents Company, China. Poly(ethylene oxideebepropylene oxideebeethylene oxide) triblock copolymer (EO27ePO61eEO27, PluronicÒ P104) was commercially supplied by BASF Corporation, Germany. Hydrochloric acid (HCl, 37.5 wt%) was also purchased from Beijing Chemical Reagents Company, China. All chemicals were of reagent quality and used as received without further purification. 2.2. Synthesis of microencapsulated n-octadecane with silica wall A series of microencapsulated n-octadecane with silica wall were synthesized through the hydrolysis and condensation reactions of sodium silicate in an O/W emulsion system via a solegel process. An amphipathic EO27ePO61eEO27 copolymer was used as the nonionic surfactant, and a solution of HCl was used as a catalyst.
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A typical synthetic procedure is described as follows: In one beaker, EO27ePO61eEO27 copolymer (1.0 g) was dissolved in 500 mL of deionized water at 70 C. Then, n-octadecane (20.0 g) was added into this solution and was continually stirred for 1 h to form a stable emulsion. In another beaker, sodium silicate (10.0 g) was dissolved in 100 mL of deionized water at 35 C, and the HCl aqueous solution was added to obtain a desired pH value under a vigorous stirring until a homogeneous solution was obtained. This indicated that the hydrolysis of sodium silicate was completed, and the silica sol solution was formed as silica monomers and oligomers. Subsequently, this silica sol solution was added dropwise into the prepared noctadecane emulsion for 1 h under a vigorous stirring. This mixture was heated to 70 C with agitation for 24 h to complete the silicate condensation. Then, the suspension solution obtained was continuously heated up to 80 C and aged for 5 h without agitation. Finally, the microencapsulated n-octadecane with silica wall was obtained as some white powders by filtration. The collected product was washed with petroleum ether and deionized water several times and, then, was dried at 40 C overnight for further characterizations and testing.
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measurements were carried out under a nitrogen atmosphere at a heating or cooling rate of 10 C/min with the weight of the specimen being about 6 mg. The first heating scan was carried out from 20 to 50 C, and the sample was held at this temperature for 5 min to diminish the thermal history before the formal measurement. TGA (thermogravimetric analysis) for the microcapsules was carried out on a TA Instruments Q50 thermal gravimetric analyzer under a nitrogen atmosphere. The specimen with a mass of about 5 mg was placed in an aluminum crucible and then was ramped from room temperature to 700 C at a heating rate of 10 C/min. Thermal conductivity measurement was performed on an EKO HC-110 thermal conductivity tester. Anti-osmosis measurement was carried out on a 723PC spectrophotometer. The releasing rate (weight percent of the released substance) of n-octadecane was measured using 10 g of the specimen dispersed in 50 mL acetone as an extraction solvent with slightly stirring (200 rpm), and all the data were conversed through transmittance of the spectrophotometer. 3. Results and discussion 3.1. Synthesis and microstructure
2.3. Characterization The status of the microcapsules during the synthetic process was monitored by an Olympus BX51 optical microscope equipped with a Sony CCD-IRIS digital camera. The microstructures of the prepared microcapsules were observed using a Hitachi S-4700 SEM (scanning electron microscopy). The specimens were made electrically conductive by sputter coating with a thin layer of golde palladium alloy. The micrographs were taken in high vacuum mode with 20 kV acceleration voltage and a medium spot size. The FTIR (Fourier transform infrared) spectra of specimens were obtained using a Nicole Magna-750 FTIR spectrophotometer on the KBr sampling sheet with a scanning number of 30. WAXS (wide angle X-ray scattering) measurement was carried out by a Rigaku D/maxr C diffractometer (40 kV, 50 mA) with Cu-Ka radiation (l ¼ 0.154 nm), and the diffraction patterns were collected in the 2q ranging from 5 to 60 at a scanning rate of 1 /min. DSC (differential scanning calorimetry) was performed using a TA Instruments Q100 differential scanning calorimeter, and all
The microencapsulated n-octadecane with a silica wall was synthesized through the in-situ polycondensation in a solegel process. Fig. 1 shows the schematic route of this solegel synthesis. The optical microscope recorded a typical formation process of microencapsulate n-octadecane as shown by Fig. 2. As illustrated by Figs. 1 and 2, the oily n-octadecane was first dispersed in an aqueous solution containing PEOePPOePEO triblock copolymer as a nonionic surfactant, which resulted in a stable oil-in-water emulsion. In this case, the hydrophilic segments of the surfactant alternatively arrange along its hydrophobic segments and, consequently, are associated with the water molecules and trimly cover the surfaces of the n-octadecane droplets with hydrophobic segments oriented to the droplets. On the other hand, the silica sol was prepared by dissolving sodium silicate in water under an acidic or weakly alkaline condition. The hydrolysis of sodium silicate followed with the oxolation could produce the water-soluble silanol monomers and silica oligomers as silica precursors. When the transparent silica sol was added dropwise into the emulsion
Fig. 1. Scheme of formation mechanism for the microencapsulated n-octadecane with silica wall using sodium silicate precursor.
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Fig. 2. Optical microscope images of the reactants at different stages during solegel synthesis: (a) n-octadecane droplets, (b) n-octadecane micelles, (c) the initial microcapsules formed after adding silica sol, (d) the microcapsules formed after 6 h reaction, (e) the microcapsules formed after 12 h reaction, and (f) the microcapsules formed after 24 h reaction.
containing the n-octadecane micelles, these silica precursors were attracted onto the surfaces of the micelles through a hydrogenbonding interaction between the silica precursors and the hydrophilic segments of the surfactant. Meanwhile, the polycondensation of the silanol monomers and silica oligomers occurred via the acid or base catalysis, leading to the formation of silica gel surrounding the n-octadecane micelles. As a result of the long-term silica condensation, the silica wall was successfully fabricated onto the surface of the n-octadecane droplets though this solegel process. Fig. 2def indicates that the sufficient time has allowed the polycondensation reaction to complete so that the structurally compact microcapsules are produced. Although this encapsulation strategy is quite straightforward, there is an elaborate balance between the aggregation and polycondensation of silica precursors on the surface of the n-octadecane micelles. Therefore, in order to ensure a perfect coreeshell structure for the microencapsulated n-octadecane, the synthetic conditions such as temperature, agitation speed, and pH value should be elaborately controlled. Fig. 3 shows the morphologies of the microcapsules synthesized at different conditions. It is observed from these SEM micrographs that the microcapsules obtained at pH 2.95w3.05 demonstrate some regular spheres with a diameter of about 8 mm, and these well-defined microcapsules have very compact and smooth surfaces without any disfigurements (see
Fig. 3a and b). However, when the acidity of the reaction solution was reduced to pH 2.40w2.60, the surfaces of the synthesized microcapsules became coarse, and some tiny silica particles could be found on them. It should also be mentioned that the further decrease in the pH value led to a more rough and loose wall of the microcapsules, and these microcapsules were easily damaged under a vigorous agitation, resulting a failure in synthesis. On the other hand, the synthesis was not very successful when carried out in a weakly alkaline condition. The obtained microcapsules present some irregular particles with a very loose wall structure (see Fig. 3e), indicating that the well-defined microcapsules cannot be achieved under the alkaline condition. To confirm the coreeshell structure of the n-octadecane microcapsules synthesized in the present work, we deliberately broke some microcapsules and then observed them using SEM. As shown by Fig. 4, a typical coreeshell structure for the microcapsules synthesized in acidic conditions could be distinguished from the SEM micrographs. Fig. 4d also displays that n-octadecane has been encapsulated by silica wall, though the coreeshell structure seems to be imperfect. On the basis of the above results, the pH value was found to play a critical role in the formation of the well-defined microcapsules, and it was also deduced that the intact microcapsules could be obtained only at the pH value around 3, equal to the isoelectric point of silica. It is well known that the acidity of the reaction
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Fig. 3. SEM micrographs of the microcapsules synthesized at different n-octadecane/sodium silicate mass ratios and pH values: (a, b) 50/50 and pH 2.95w3.05, (c, d) 50/50 and pH 2.40w2.60, (e, f) 50/50 and pH 7.90w8.10, (g) 60/40 and pH 2.95w3.05, and (h) 40/60 and pH 2.95w3.05.
solution dominated the condensation rate of the silica precursors during the solegel synthesis [33]. The condensation rate of silica precursors is slow enough to keep a stable silica sol at the isoelectric point of silica. In this case, the silica precursors could adequately aggregate on the surfaces of the n-octadecane micelles through the hydrogen-bonding interaction, and then, the polycondensation reaction mostly performs upon these surfaceaggregated silica precursors to form a well-defined silica wall. Any rapid silica condensation may disturb such an elaborate balance and, consequently, result in the crosslinking of silica precursors ahead of aggregation on the surfaces of the n-octadecane micelles. These silica particles subsequently accumulate on the surface of the microcapsules, thus making the microcapsule walls coarse and loose. It is also notable in Figs. 3 and 4 that the
microcapsules with different weight ratios of sodium silicate/noctadecane all present a well-defined spherical morphology and a perfect coreeshell structure when synthesized at the pH value around 3. 3.2. Chemical composition and crystallography The chemical compositions of the synthesized n-octadecane microcapsules were first examined by FTIR, and the corresponding spectra were presented in Fig. 5. It is notable that the profiles of FTIR spectra for these microcapsules samples obtained in different conditions are quite similar. The absorption bands appear at 1083 and 798 cm1 corresponding to the asymmetry stretching vibration and the symmetric stretching vibration of SieOeSi, respectively,
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Fig. 4. SEM micrographs of the damaged microcapsules synthesized at different n-octadecane/sodium silicate mass ratios and pH values: (a) 50/50 and pH 2.95w3.05, (b) 40/60 and pH 2.95w3.05, (c) 50/50 and pH 2.40w2.60, and (d) 50/50 and pH 7.90w8.10.
while the peak at 461 cm1 is observed due to the SieOeSi bending vibration. A broad band at 3436 cm1 is attributed to the SieOH stretching vibration, and meanwhile, the peak at 952 cm1 corresponds to the SieOH bending vibration. These characteristic absorption peaks indicated the existence of silica wall in the noctadecane microcapsules. On the other hand, the FTIR spectra of the microcapsule samples also reveal two intensive absorption peaks at 2925 and 2854 cm1, which are associated with the alkyl CeH stretching vibrations of methyl and methylene groups, respectively. The absorption peaks of methylene deformation vibration are found to appear at 1463 cm1 and 1376 cm1, and the peak at 721 cm1 is attributed the in-plane methylene rocking vibration. These characteristic bands are identical with those appearing in the spectrum of pristine n-octadecane (see Fig. 5). The
above results confirmed the successful encapsulation of n-octadecane with silica. Fig. 6 shows the WAXS patterns of the microcapsule samples, and the diffraction patterns of pristine n-octadecane and hollow microcapsules are also presented as references. The diffraction pattern of the hollow microcapsules shows no reflex peak, indicating an amorphous silica wall for the microcapsules synthesized in the current study. Therefore, it is facile to confirm the existence of n-octadecane within the microcapsules from the WAXS patterns of the synthesized microcapsules due to the crystallinity of noctadecane when freezing. As is shown by Fig. 6, all of the microcapsule samples exhibit the similar WAXS profiles with that of pristine n-octadecane. These patterns show a set of well-resolved diffraction peaks at 2q of 19.24 , 19.81, 23.31, 24.71, and 24.84 , which can be indexed as (010), (011), (105), (101), and (110) reflections of the b-form crystal of n-octadecane, respectively. Meanwhile, the reflections at 2q of 7.74 , 11.59 , 15.45 , 39.67, and 44.58 are assigned to the plane (002), (003), (004), (0e22), and (207) of the a-crystal of n-octadecane, respectively [34]. These characteristic reflections not only confirmed the encapsulation of n-octadecane within the silica microcapsules but also identified the good crystallinity of n-octadecane inside the microcapsules [35,36]. However, it is noteworthy that there is a considerable decrease in the intensity of diffraction peaks for the microcapsule sample synthesized at pH 7.90w8.10, indicating the less amount of noctadecane is loaded in the inside of microcapsules. 3.3. Phase change characteristics
Fig. 5. FTIR spectra of the microencapsulated n-octadecane synthesized in different conditions; the curve number is corresponding to the sample numbered with the same code in Table 1.
The phase change behaviors of the microencapsulated n-octadecane synthesized in different conditions were investigated by DSC. The resulted thermograms are shown in Fig. 7, and the phase change parameters obtained from the DSC evaluation are summarized in Table 1. It is noted from Fig. 7 that the pristine n-octadecane presents a single crystallization peak at 21.16 C as well as a single melting peak at 28.47 C, which are associated with the phase changes of solidification and fusion. However, the microencapsulated n-octadecane presents a bimodal crystallization
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Fig. 6. WAXS patterns of the microencapsulated n-octadecane synthesized in different conditions; the curve number is corresponding to the sample numbered with the same code in Table 1.
behavior on its cooling thermogram but still exhibits a single melting peak during heating process. Such bimodal exothermic peaks at around 20 and 23 C are attributed to the crystallization of the a- and b-crystals of n-octadecane, respectively. It has been reported that the a- and b-crystals of n-octadecane are induced by heterogeneous and homogeneous nucleation, respectively [36]. It is reasonable to deduce that the a-crystal formation of the microencapsulated n-octadecane is due to the heterogeneous nucleation effect caused by inner silica wall, inducing a crystalline transformation from b-form to the a-one, Furthermore, the formation of more stable a-crystal also resulted in a slightly shift of the a-form crystallization peak toward a high temperature. Moreover, for the microcapsule sample synthesized at pH 7.90w8.10, the poor encapsulation reduced the crystallinity of n-octadecane, thus resulting in a significant decrease in Tm. The nonisothermal crystallization behaviors of the pristine n-octadecane and the
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microencapsulated one were investigated by running dynamic DSC scans, and their thermograms at different cooling rates were demonstrated in Fig. 8. It is clearly observed that the crystallization peak shifts toward a lower temperature with increasing the cooling rate in the cases of both pristine n-octadecane and the microencapsulated one. This is due to the fact that the faster cooling rate leads to a higher degree of supercooling, and thus, the shorter time scale compels n-octadecane to crystallize at a lower temperature. Moreover, it is noteworthy that bimodal crystallization of the microencapsulated n-octadecane is increasingly enhanced with decreasing the cooling rate, while the pristine n-octadecane mainly forms the crystalline phase in b-form at any cooling rates. This indicates that the a-crystal of n-octadecane resulting from the heterogeneous nucleation effect of inner silica wall is induced only at a long time scale. On the other hand, the pristine n-octadecane is found to have considerably high phase change enthalpies according to the data listed in Table 1. This implies that the n-octadecane PCM has a high latent heat of storage energy and can fully release it when phase change occurs. However, the encapsulation of n-octadecane with silica evidently reduced the absolute phase change enthalpies of the microcapsule samples. Since the silica wall is an inert material, the latent heat can be only storied by the phase changeable noctadecane core. So it is believable that the phase change enthalpies of microcapsule samples are strongly dependant on the loading of n-octadecane within the microcapsules. There are two important parameters, i.e. the encapsulation ratio (R) and encapsulation efficiency (E) that can be adopted to characterize the phase change performance of the microencapsulated PCMs [20,27]. These two parameters could be calculated by DSC results through the following equations:
R ¼
DHm;microPCM 100% DHm;PCM
(1)
E ¼
DHm;microPCM þ DHc;microPCM 100% DHm;PCM þ DHc;PCM
(2)
where DHm,PCM and DHm,micro-PCM are the melting enthalpies of pristine n-octadecane and the microencapsulated one, respectively;
Fig. 7. DSC cooling (a) and heating (b) thermograms of the microencapsulated n-octadecane synthesized in different conditions; the curve number is corresponding to the sample numbered with the same code in Table 1.
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Table 1 The phase-change characteristics of the microencapsulated n-octadecane synthesized in different conditions. Sample code
Mass ratio of n-octadecane/sodium silica (wt./wt.)
Solution acidity (pH)
Crystallization process Tc,a ( C)
Tc,b ( C)
DHc (J g1)
Tm ( C)
DHm (J g1)
1 2 3 4 5 6 7
Pristine n-octadecane 50/50 60/40 40/60 50/50 50/50 0/100
e 2.95w3.05 2.95w3.05 2.95w3.05 2.40w2.60 7.90w8.10 2.95w3.05
e 23.33 23.72 23.16 23.73 23.89 e
21.16 19.86 21.84 19.45 20.55 19.11 e
206.75 68.83 84.89 52.83 37.96 19.55 e
28.74 27.88 27.96 27.97 28.43 23.34 e
209.10 71.14 87.46 55.12 40.35 22.03 e
DHc,PCM and DHc,micro-PCM are the crystallization enthalpies of pristine n-octadecane and the microencapsulated one, respectively. Table 1 reveals the encapsulation ratio and the encapsulation efficiency of the microcapsule samples obtained in different conditions. In fact, the encapsulation efficiency represents an effective performance of the n-octadecane inside the microcapsules for latent heat storage, while the encapsulation ratio describes the effective encapsulation of n-octadecane within the microcapsules. It should be mentioned that the sodium moiety is excluded from the wall material during the hydrolysis and polycondensation of silica precursor, and therefore, the actual mass ratio of core/wall is much lower than that of n-octadecane/sodium silicate. The microcapsule
Melting process
Encapsulation ratio (%)
Encapsulation efficiency (%)
Thermal storage capability (%)
Thermal conductivity (W m1 K1)
e 34.15 41.83 26.36 19.30 10.54 e
e 33.66 41.45 25.60 18.83 10.02 e
e 98.56 99.10 98.48 97.57 94.87 e
0.153 0.904 0.891 0.981 0.874 0.875 1.296
sample achieved a high encapsulation ratio of 34.15% and a high encapsulation efficiency of 33.66% when synthesized with 50/50 mass ratio of n-octadecane/sodium silicate at pH 2.95w3.05. This may be ascribed to the perfect self-assemble polycondensation forming a compact silica wall. In this case, most of the silica precursors were converted into the silica wall by polycondensation, and n-octadecane can be efficiently encapsulated by silica wall without any leakage. However, the samples obtained at pH 2.50w2.60 and pH 7.90w8.10 achieved the lower encapsulation ratio and efficiency, indicating a less amount of n-octadecane loaded in microcapsules. It is deduced that when the silica condensation rate was accelerated at above pH values, some of the silica precursors were not assembled on the surface of n-octadecane droplets and directly polycondensated in the form of solid particles. Accordingly, some of noctadecane was not encapsulated by silica wall, consequently resulting in a decrease in the loading of PCM in microcapsules. Furthermore, the fast silica polycondensation may also lead to a loose silica wall, which is easily damaged. These problems resulted in a reduction of the encapsulation ratio and efficiency. Moreover, the thermal storage capability (4) of the microencapsulated noctadecane can also be determined by the results from DSC measurements using Equation (3) [26]: DHm;microPCM þDHc;microPCM
4 ¼
R
DHm;PCM þ DHc;PCM
100%
(3)
where R is the encapsulation ratio calculated by Equation (1). The calculated results of thermal storage capability are listed in Table 1. It is interestingly found that the three microcapsule samples synthesized at pH 2.95w3.05 present a thermal storage capability higher than 98%, indicating that almost all of the encapsulated noctadecane can effectively store the latent heat through phase change. As for the sample obtained at pH 7.90w8.10, some of the encapsulated n-octadecane cannot perform thermal storage by phase change. For example, the phase change usually does not occur upon the excessively micronized n-octadecane in microcapsules due to the confinement effect on the molecular motion. These results reduce the thermal storage capability of the microencapsulated n-octadecane. 3.4. Thermal stability
Fig. 8. Nonisothermal crystallization DSC thermograms of (a) pristine n-octadecane and (b) the microencapsulated n-octadecane synthesized with 50/50 mass ratio of noctadecane/sodium silicate at pH 2.95w3.05.
Thermal stability is considered as an important parameter assessing the performance of new designed PCM microcapsules when used for heat energy storage or thermal regulation. The thermal degradation behaviors of the microcapsule samples synthesized in different conditions were investigated by TGA, and their TGA and DTG (derivative thermogravimetry) thermograms are shown in Fig. 9. The pristine n-octadecane exhibits a typical onestep weight loss at 202.5 C almost without any char remained at 650 C, suggesting that the pristine n-octadecane experienced a simple evaporation. However, the microcapsule samples show a
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two-stage degradation behavior in the temperature range of 130e 510 C. The first stage of weight loss is ascribed to the evaporation of n-octadecane, while the second one is related to silica condensation among the silanols of silica wall. It is reasonable to believe that there are a lot of silanol groups remaining on the silica wall during hydrothermal synthesis. The encapsulated n-octadecane broke throughout of the microcapsules during the first stage and gave a sharp weight loss starting at around 140 C and ending at around 250 C. It is interestingly noted in Fig. 9(b) that the microcapsule samples all show a maximum weight loss at higher temperatures compared to the pristine n-octadecane. This indicates that the silica wall can prevent the encapsulated n-octadecane from evaluating at the normal boil point, thus improving the degradation temperature of the microcapsules. Therefore, the thick and compact silica wall may favor the improvement in thermal stability of the microcapsules. On the other hand, the occurrence of the second weight loss only caused a loss of less than 5 wt% in weight as a result of the elimination of the water produced by further silica condensation at around 430 C. It is also observed from Fig. 9(a) that the microcapsule samples exhibit different char yields as the silica wall. These char yields seem to strongly depend both on the encapsulation rate and on the mass ratio of n-octadecane/silica sodium. For three samples obtained at pH 2.95w3.05, the char yield is proportional to the mass ratio of n-octadecane/silica sodium, which determined the exact loading of n-octadecane in
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microcapsules. Moreover, the microcapsule sample obtained at pH 7.90w8.10 achieved a high char yield of 58 wt% due to its low encapsulation ratio of 10%. 3.5. Thermal conductivity It is well known that the pristine n-octadecane as an organic PCM has a low thermal conductivity despite its other desirable properties. This may increase its thermal response time for the storage and release of latent heat. Therefore, the techniques of thermally conductive enhancement are absolutely required when designing a microencapsulated PCM [37,38]. It is expected that the encapsulation of n-octadecane with higher thermally conductive silica wall can enhance the thermal conductivities of the resulting microcapsules. The thermal conductivities of the microcapsule samples are listed in Table 1. As predicted, the pristine n-octadecane in solid state shows a thermal conductivity as low as 0.153 W m1 K1. Nevertheless, it is surprisingly found from the data in Table 1 that all of the microcapsule samples achieved the thermal conductivities higher than 0.85 W m1 K1. The silica wall material was obtained by removing the n-octadecane core from the synthesized microcapsules, and then, its thermal conductivity was measured to give a value of 1.296 W m1 K1. Such a value is much higher than those of polymeric wall materials, which are only around 0.20 W m1 K1 [39]. These results confirmed that the encapsulation of n-octadecane with highly thermally conductive silica actually impart a high thermal conductivity to the microcapsules. It is also noteworthy that the thermal conductivities of the microencapsulated n-octadecane samples are somewhat associated with the encapsulation rate, and the higher the encapsulation rate, the lower the thermal conductivity. It is understandable that the high encapsulation rate means the low percentage of silica wall in microcapsules as well as the high n-octadecane loading. This suggests that the silica wall play a critical role in enhancing the heat transfer of microencapsulated n-octadecane. In addition, the microcapsules synthesized with 40/60 mass ratio of n-octadecane/ sodium silicate at pH 2.95w3.05 are found to have a much higher thermal conductivity than the other samples, which may be attributed to the more compact and thicker silica wall. It seems that the silica wall forms a continuous phase when n-octadecane is encapsulated by silica material. Such a continuous inorganic phase can be considered as a virtual heat transfer network and, therefore, can enhance the heat transfer rate over the whole microcapsules. As a result, the microencapsulated n-octadecane with a more compact and thicker silica wall achieved a much higher thermal conductivity. 3.6. Reliability and durability
Fig. 9. TGA (a) and DTG (b) thermograms of the microencapsulated n-octadecane synthesized in different conditions; the curve number is corresponding to the sample numbered with the same code in Table 1.
To evaluate the reliability of PCMs during phase-change process, one hundred loops of heatingecooling dynamic scans were carried out by DSC to investigate the multicycle phase change performance of the microencapsulated n-octadecane, and the corresponding thermograms are demonstrated in Fig. 10. It is observed that the aform crystallization peak slightly shifts toward a lower temperature at the initial several loops and then, its position becomes stable till the hundredth loop. These results indicate that the a-form crystallization is considerably influenced by the heat history due to its heterogeneous nucleation in nature. However, the b-form crystallization peak is almost invariable because of the homogeneous nucleation effect of inner silica wall. In this case, the melting peak also shows a slight shift toward a lower temperature during the initial several DSC scans. On the basis of the overall phase change behaviors shown by Fig. 10, the microencapsulated n-octadecane obtained in the current work maintains a normal multicycle phase
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the loose and porous silica wall of the microcapsules, which cannot well prevent n-octadecane from permeating. The sample obtained at pH 2.95w3.05 shows the best anti-osmosis property among three samples synthesized with 50/50 mass ratio of n-octadecane/ sodium silicate. This indicates that a compact silica wall can effectively prevent the n-octadecane core from penetrating through and provide good anti-osmosis performance for the microcapsules. As shown by Fig. 11, for three samples obtained at pH 2.95w3.05, the microcapsules synthesized with 40/60 mass ratio of n-octadecane/sodium silicate have the lowest release rate, indicating that the thicker silica wall of microcapsules can lead to the better antiosmosis performance. These results suggest that the anti-osmosis property of the microencapsulated n-octadecane strongly depends on the compaction of silica wall as well as its thickness, and the microcapsules can achieve a good anti-osmosis performance as long as they are synthesized with an optimal core/wall mass ratio in an appropriate condition. 4. Conclusion Fig. 10. DSC thermograms of the microencapsulated n-octadecane synthesized with 50/50 mass ratio of n-octadecane/sodium silicate at pH 2.95w3.05 under one hundred loops of heatingecooling cycle.
transformation and can be repetitiously used for the storage and release of latent heat at an almost stable temperature. For any microencapsulated PCMs, the PCM cores will be eventually released from the inside of microcapsules due to the permeation of wall materials. Therefore, the anti-osmosis performance is an important factor determining the durability of the microcapsules. The durability of the microencapsulated n-octadecane was evaluated by measuring the weight loss of the extracted microcapsules in terms of the duration date. Fig. 11 displays the release curves obtained from the above measurements upon the microcapsule samples, in which all of the reported data are the average values from five specimens. It is believed that these release behaviors can reflect the anti-osmosis capability of silica wall preventing the n-octadecane core from permeation. It is clearly observed that the microcapsule sample synthesized at pH 7.90w8.10 exhibits a poorest anti-osmosis behavior because of its high release rate as a function of duration date. This result is due to
The new silica encapsulation technique upon n-octadecane PCM was successfully developed through self-assemble synthesis using sodium silicate as a silica precursor. The FTIR spectra and the WAXS patterns confirmed the chemical composition and crystallinity of the synthesized microcapsule samples. The microstructure of the obtained microcapsules strongly depends on the pH value of reaction solution, which determines the silica condensation rate and thus, influences the balance between the self-assembly and polycondensation of silica precursors on the surface of n-octadecane droplets. The microencapsulated n-octadecane shows a welldefined coreeshell structure as well as a perfect spherical morphology when synthesized at pH 2.95w3.05. The microencapsulated n-octadecane obtained at pH 2.95w3.05 exhibited good phase change performance and also achieved a high encapsulation rate and a high encapsulation efficiency. The encapsulation of n-octadecane with compact and thick silica wall can impart a high thermal conductivity and a good anti-osmosis property to the microcapsules, and can also improve the thermal stability of the microcapsules by preventing inside n-octadecane from thermally evaporating. The synthetic technique exploited by this work indicates a high feasibility in industrial manufacture for the microencapsulated PCMs with inorganic walls due to the easy availability and low cost of sodium silicate. And the resulting PCM microcapsules will be a potential candidate for the application in the fields of building air conditioning, electronics cooling systems, waste heat recovery, 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
Fig. 11. Release curves of the microencapsulated n-octadecane synthesized in different conditions.
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Energy journal homepage: www.elsevier.com/locate/energy
New approach for solegel synthesis of microencapsulated noctadecane phase change material with silica wall using sodium silicate precursor Fang He, Xiaodong Wang*, Dezhen Wu State Key Laboratory of OrganiceInorganic Composites, School of Materials Science and Engineering, 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 11 August 2013 Received in revised form 24 November 2013 Accepted 26 November 2013 Available online 30 January 2014
A new silica encapsulation technique toward n-octadecane PCM (phase change material) was developed through solegel synthesis using sodium silicate as a silica precursor. Fourier transform infrared spectra confirm the chemical composition of the synthesized microcapsules, and wide-angle X-ray scattering patterns indicate good crystallinity for the n-octadecane inside silica microcapsules. Scanning electric micrographs demonstrate that the microencapsulated n-octadecane obtained at pH 2.95w3.05 presents a perfect spherical morphology and a well-defined coreeshell microstructure. Because the pH value of reaction solution determines the silica condensation rate and, thus, influences the balance between the self-assembly and polycondensation of silica precursors on the surface of n-octadecane droplets, the microcapsules could achieve a smooth and compact surface at pH 2.95w3.05. The microencapsulated noctadecane also exhibits good phase change performance and achieves a high encapsulation rate and high encapsulation efficiency in this synthetic condition. The encapsulation of n-octadecane with compact and thick silica wall can impart a high thermal conductivity and a good anti-osmosis property to the microcapsules, and can also improve the thermal stability of the microcapsules by preventing inside n-octadecane from thermally evaporating. Owing to the easy availability and low cost of sodium silicate, this synthetic technique indicates a high feasibility in industrial manufacture for the microencapsulated PCMs with inorganic walls. Ó 2014 Elsevier Ltd. All rights reserved.
Keywords: Microencapsulated n-octadecane Silica wall Phase change performance Microstructure Thermal conductivity Phase change performance
1. Introduction The prompt development of global economics has brought increasing energy demands. It inevitably results in shortages of fossil fuels due to the limitation of conventional fossil energy sources. These issues have provided impetus to the development in highly efficient utilization of energy as well as to the discovery of new energy storage materials [1]. It is well understandable that the highly efficient utilization of energy can reduce the energy consumption so as to economize fossil fuels. Therefore, heat energy storage and thermal regulation have attracted increasing interests in the case of the impending shortage and increasing cost of energy resources nowadays [2,3]. PCM (phase change material) is a sort of latent heat storage material allowing the cycle of heat storagerelease through the solideliquid phase transition, and it has already exhibited a broad applicable prospect in the solar and industrial waste energy utilization as well as the thermal regulation * Corresponding author. Tel.: þ86 10 6441 0145; fax: þ86 10 6442 1693. E-mail addresses: [email protected], [email protected] (X. Wang). 0360-5442/$ e see front matter Ó 2014 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.energy.2013.11.088
due to its large heat storage capacity and isothermal behavior during the charging and discharging processes [4,5]. Nowadays, organic PCMs like paraffins and fatty acids have attracted great attention due to their excellent phase change performances, good thermal characteristics, and good compatibility with various materials or fibers [6,7]. Paraffin waxes (n-alkanes) have been considered as one of the most promising organic PCMs, which can be universally applied in the area of garments and home furnishing products. They have different melting temperatures (Tm) and crystallization temperatures (Tc), which depend on the number of carbon atoms in their chains [8]. These linear hydrocarbons have a Tm around 18e36 C and can change phases in this temperature range making human feel comfortable, so that paraffins could be used to make protective all-season outfits and garments in abruptly changing climatic conditions [9,10]. However, the conventional paraffin PCMs presented some drawbacks, particularly during the occurrence of phase change. These bulk PCMs are difficult to be handled as a liquid when melting. If the PCMs are directly incorporated into the other basematerials to form a phase change system, the liquid PCMs cannot
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maintain a stable shape and easily leak out [11]. On the other hand, the solidification of paraffins can cause a poor heat-transfer rate due to their low thermal conductivity, because it weakens the natural convection of paraffins in liquid state [12,13]. In addition, the paraffin PCMs also have an interfacial combination problem with the surrounding materials. Many attempts have been made to overcome these problems such as the formation of PCM emulsions or PCM slurries [7,14], and microencapsulation of PCMs is considered as a successful solution to hold the liquid and/or solid phases of PCMs within microcapsule containers. The microcapsule wall can keep PCMs isolated from the surrounding materials and can act as a barrier to protect them from harmful interaction with the environment. Such an attractive technique not only provides a stable structure and sufficient surface of PCMs for heat transfer, but also makes liquid PCMs easy to handle during the phase-change process [15,16]. The conventional microencapsulated PCMs consist of some coreeshell structured particles, which have a liquid core surrounded by a polymer shell that prevents the interior PCMs from leaking during the solideliquid phase change. These PCMcontaining microcapsules can also supply the large heat transfer areas and controllable changes in storage material volume when the phase change occurs [17,18]. In most cases, the cores of the conventional microencapsulated PCMs are always enwrapped with organic polymers as wall materials. A survey of publications regarding microencapsulated PCMs indicates that melaminee formaldehyde resin, ureaeformaldehyde resin, and polyurea are usually employed as wall materials to protect the PCM cores. Li et al. [19] reported the microencapsulation of PCMs into a formaldehyde shell with the low remnant formaldehyde content. Wang and Zhang [20] also developed the microencapsulated n-octadecane PCM with polyurea shells containing different soft segments through interfacial polymerization. Although the organic polymeric wall materials play an important role in improving the structure, permeability, controlled release, and thermal stability toward the microencapsulated PCMs, there are some limitations such as flammability, poor thermal and chemical stabilities, and low thermal conductivity for these microcapsules [21,22]. Additionally, there is a requirement of prompt heat transfer for PCMs to give a prompt response during heat energy storage and thermal regulation process. It is a well-known fact that the thermal conductivity of inorganic materials is always significantly higher than that of organic materials, and moreover, the chemical and thermal stabilities, mechanical strength, and flame retardancy of inorganic materials are also higher than those of organic materials. Owing to these attractive features, choosing an appropriate inorganic wall material for microencapsulated PCMs is a promising idea to enhance their phase-change performance. Recently, some attempts have been made to encapsulate the paraffin PCMs with inorganic silica materials based on the survey of literature. Wang et al. [23] first reported the preparation method of inorganic microcapsules by encapsulating PCMs with silica in O/W (oil-in-water) emulsion and also investigated the formation mechanism. Shi et al. [24] synthesized the microencapsulated polyacrylic acid hybrid hydrogel with silica wall as thermal energy storage materials via a solegel process and observed a significant improvement in thermal conductivity. Li et al. [25] also prepared the form-stable phase change composites of paraffin with silica and expanded graphite by solegel method and found that these phasechange composites achieved much higher thermal conductivity than pure paraffin. Moreover, Li et al. [26] and Fang et al. [22] reported the preparation and phase-change properties of the PCM microcapsules based on paraffin core and silica wall through in-situ polycondensation. In our previous study, we also successfully synthesized the well-defined PCM microcapsules with n-octadecane core and silica wall and observed a significant enhancement in
their thermal conductivity and phase-change performance [27,28]. It is noteworthy that these reported synthetic methods unexceptionally employed TEOS (tetraethoxysilane) as a silica precursor, and their two step wall formation process involved the hydrolysis of TEOS to deposit hydrolysis products, i.e. alkylsilanols and silanols, on the oil droplet surface followed by interfacial polymerization of these hydrolysis products to build an intact silica wall with much of the understanding of the chemistry concerned derived from solegel process of TEOS. However, the silica precursor, TEOS, is not economical for the industrial manufacture of microencapsulated PCMs due to its high cost as a raw material, which may restrict the applications of such a new encapsulation technology in industrial and civil areas. In the present work, we attempted to microencapsulate the noctadecane PCM with a silica wall using sodium silicate instead of TEOS as a precursor. Considering of the easy availability and low cost of sodium silicate, this synthetic technique indicates a high feasibility in industrial manufacture for the microencapsulated PCMs with inorganic walls. We employed n-octadecane as a paraffin PCM, because this paraffin is a desirable PCM for its good performances of heat energy storage and thermal regulation in an appropriate phase change temperature range of 20e29 C with high latent heat of around 210 J/g [29]. It is evident that such a phase change temperature is comfortable for the human body, while the latent heat is higher than that of other PCMs with the similar phase change temperature range [30]. Owing to such a certain range of phase change temperature, the heat produced by human body during vigorous motion makes n-octadecane melt, thus absorb in a great deal of heat energy. In this case, the local temperature surrounding the human body will decrease, and consequently the cooling local circumstance can comfort the hot human body. It is anticipated that the synthetic technology obtained in this study can be employed to encapsulate the other paraffin PCMs for applications in the outdoor apparel products, such as ski wear, hunting clothing, and smart garments with temperature regulating microclimate [31,32]. The aim of this work is to achieve a feasible inorganic encapsulation technology for paraffin PCMs and to investigate the effects of the synthetic methods on the microstructures, crystallization behaviors and phase change performance of the obtained products. Moreover, the formation mechanisms for a silica wall at the oilewater interface from the sodium silicate precursor were also clarified. 2. Experimental 2.1. Materials The n-octadecane with a purity of 90 wt% was purchased from Tianjin Alfa Aesar Company, China. Sodium silicate containing 19.86 wt% of SiO2 and 21.05 wt% of Na2O was commercially obtained from Beijing Chemical Reagents Company, China. Poly(ethylene oxideebepropylene oxideebeethylene oxide) triblock copolymer (EO27ePO61eEO27, PluronicÒ P104) was commercially supplied by BASF Corporation, Germany. Hydrochloric acid (HCl, 37.5 wt%) was also purchased from Beijing Chemical Reagents Company, China. All chemicals were of reagent quality and used as received without further purification. 2.2. Synthesis of microencapsulated n-octadecane with silica wall A series of microencapsulated n-octadecane with silica wall were synthesized through the hydrolysis and condensation reactions of sodium silicate in an O/W emulsion system via a solegel process. An amphipathic EO27ePO61eEO27 copolymer was used as the nonionic surfactant, and a solution of HCl was used as a catalyst.
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A typical synthetic procedure is described as follows: In one beaker, EO27ePO61eEO27 copolymer (1.0 g) was dissolved in 500 mL of deionized water at 70 C. Then, n-octadecane (20.0 g) was added into this solution and was continually stirred for 1 h to form a stable emulsion. In another beaker, sodium silicate (10.0 g) was dissolved in 100 mL of deionized water at 35 C, and the HCl aqueous solution was added to obtain a desired pH value under a vigorous stirring until a homogeneous solution was obtained. This indicated that the hydrolysis of sodium silicate was completed, and the silica sol solution was formed as silica monomers and oligomers. Subsequently, this silica sol solution was added dropwise into the prepared noctadecane emulsion for 1 h under a vigorous stirring. This mixture was heated to 70 C with agitation for 24 h to complete the silicate condensation. Then, the suspension solution obtained was continuously heated up to 80 C and aged for 5 h without agitation. Finally, the microencapsulated n-octadecane with silica wall was obtained as some white powders by filtration. The collected product was washed with petroleum ether and deionized water several times and, then, was dried at 40 C overnight for further characterizations and testing.
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measurements were carried out under a nitrogen atmosphere at a heating or cooling rate of 10 C/min with the weight of the specimen being about 6 mg. The first heating scan was carried out from 20 to 50 C, and the sample was held at this temperature for 5 min to diminish the thermal history before the formal measurement. TGA (thermogravimetric analysis) for the microcapsules was carried out on a TA Instruments Q50 thermal gravimetric analyzer under a nitrogen atmosphere. The specimen with a mass of about 5 mg was placed in an aluminum crucible and then was ramped from room temperature to 700 C at a heating rate of 10 C/min. Thermal conductivity measurement was performed on an EKO HC-110 thermal conductivity tester. Anti-osmosis measurement was carried out on a 723PC spectrophotometer. The releasing rate (weight percent of the released substance) of n-octadecane was measured using 10 g of the specimen dispersed in 50 mL acetone as an extraction solvent with slightly stirring (200 rpm), and all the data were conversed through transmittance of the spectrophotometer. 3. Results and discussion 3.1. Synthesis and microstructure
2.3. Characterization The status of the microcapsules during the synthetic process was monitored by an Olympus BX51 optical microscope equipped with a Sony CCD-IRIS digital camera. The microstructures of the prepared microcapsules were observed using a Hitachi S-4700 SEM (scanning electron microscopy). The specimens were made electrically conductive by sputter coating with a thin layer of golde palladium alloy. The micrographs were taken in high vacuum mode with 20 kV acceleration voltage and a medium spot size. The FTIR (Fourier transform infrared) spectra of specimens were obtained using a Nicole Magna-750 FTIR spectrophotometer on the KBr sampling sheet with a scanning number of 30. WAXS (wide angle X-ray scattering) measurement was carried out by a Rigaku D/maxr C diffractometer (40 kV, 50 mA) with Cu-Ka radiation (l ¼ 0.154 nm), and the diffraction patterns were collected in the 2q ranging from 5 to 60 at a scanning rate of 1 /min. DSC (differential scanning calorimetry) was performed using a TA Instruments Q100 differential scanning calorimeter, and all
The microencapsulated n-octadecane with a silica wall was synthesized through the in-situ polycondensation in a solegel process. Fig. 1 shows the schematic route of this solegel synthesis. The optical microscope recorded a typical formation process of microencapsulate n-octadecane as shown by Fig. 2. As illustrated by Figs. 1 and 2, the oily n-octadecane was first dispersed in an aqueous solution containing PEOePPOePEO triblock copolymer as a nonionic surfactant, which resulted in a stable oil-in-water emulsion. In this case, the hydrophilic segments of the surfactant alternatively arrange along its hydrophobic segments and, consequently, are associated with the water molecules and trimly cover the surfaces of the n-octadecane droplets with hydrophobic segments oriented to the droplets. On the other hand, the silica sol was prepared by dissolving sodium silicate in water under an acidic or weakly alkaline condition. The hydrolysis of sodium silicate followed with the oxolation could produce the water-soluble silanol monomers and silica oligomers as silica precursors. When the transparent silica sol was added dropwise into the emulsion
Fig. 1. Scheme of formation mechanism for the microencapsulated n-octadecane with silica wall using sodium silicate precursor.
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Fig. 2. Optical microscope images of the reactants at different stages during solegel synthesis: (a) n-octadecane droplets, (b) n-octadecane micelles, (c) the initial microcapsules formed after adding silica sol, (d) the microcapsules formed after 6 h reaction, (e) the microcapsules formed after 12 h reaction, and (f) the microcapsules formed after 24 h reaction.
containing the n-octadecane micelles, these silica precursors were attracted onto the surfaces of the micelles through a hydrogenbonding interaction between the silica precursors and the hydrophilic segments of the surfactant. Meanwhile, the polycondensation of the silanol monomers and silica oligomers occurred via the acid or base catalysis, leading to the formation of silica gel surrounding the n-octadecane micelles. As a result of the long-term silica condensation, the silica wall was successfully fabricated onto the surface of the n-octadecane droplets though this solegel process. Fig. 2def indicates that the sufficient time has allowed the polycondensation reaction to complete so that the structurally compact microcapsules are produced. Although this encapsulation strategy is quite straightforward, there is an elaborate balance between the aggregation and polycondensation of silica precursors on the surface of the n-octadecane micelles. Therefore, in order to ensure a perfect coreeshell structure for the microencapsulated n-octadecane, the synthetic conditions such as temperature, agitation speed, and pH value should be elaborately controlled. Fig. 3 shows the morphologies of the microcapsules synthesized at different conditions. It is observed from these SEM micrographs that the microcapsules obtained at pH 2.95w3.05 demonstrate some regular spheres with a diameter of about 8 mm, and these well-defined microcapsules have very compact and smooth surfaces without any disfigurements (see
Fig. 3a and b). However, when the acidity of the reaction solution was reduced to pH 2.40w2.60, the surfaces of the synthesized microcapsules became coarse, and some tiny silica particles could be found on them. It should also be mentioned that the further decrease in the pH value led to a more rough and loose wall of the microcapsules, and these microcapsules were easily damaged under a vigorous agitation, resulting a failure in synthesis. On the other hand, the synthesis was not very successful when carried out in a weakly alkaline condition. The obtained microcapsules present some irregular particles with a very loose wall structure (see Fig. 3e), indicating that the well-defined microcapsules cannot be achieved under the alkaline condition. To confirm the coreeshell structure of the n-octadecane microcapsules synthesized in the present work, we deliberately broke some microcapsules and then observed them using SEM. As shown by Fig. 4, a typical coreeshell structure for the microcapsules synthesized in acidic conditions could be distinguished from the SEM micrographs. Fig. 4d also displays that n-octadecane has been encapsulated by silica wall, though the coreeshell structure seems to be imperfect. On the basis of the above results, the pH value was found to play a critical role in the formation of the well-defined microcapsules, and it was also deduced that the intact microcapsules could be obtained only at the pH value around 3, equal to the isoelectric point of silica. It is well known that the acidity of the reaction
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Fig. 3. SEM micrographs of the microcapsules synthesized at different n-octadecane/sodium silicate mass ratios and pH values: (a, b) 50/50 and pH 2.95w3.05, (c, d) 50/50 and pH 2.40w2.60, (e, f) 50/50 and pH 7.90w8.10, (g) 60/40 and pH 2.95w3.05, and (h) 40/60 and pH 2.95w3.05.
solution dominated the condensation rate of the silica precursors during the solegel synthesis [33]. The condensation rate of silica precursors is slow enough to keep a stable silica sol at the isoelectric point of silica. In this case, the silica precursors could adequately aggregate on the surfaces of the n-octadecane micelles through the hydrogen-bonding interaction, and then, the polycondensation reaction mostly performs upon these surfaceaggregated silica precursors to form a well-defined silica wall. Any rapid silica condensation may disturb such an elaborate balance and, consequently, result in the crosslinking of silica precursors ahead of aggregation on the surfaces of the n-octadecane micelles. These silica particles subsequently accumulate on the surface of the microcapsules, thus making the microcapsule walls coarse and loose. It is also notable in Figs. 3 and 4 that the
microcapsules with different weight ratios of sodium silicate/noctadecane all present a well-defined spherical morphology and a perfect coreeshell structure when synthesized at the pH value around 3. 3.2. Chemical composition and crystallography The chemical compositions of the synthesized n-octadecane microcapsules were first examined by FTIR, and the corresponding spectra were presented in Fig. 5. It is notable that the profiles of FTIR spectra for these microcapsules samples obtained in different conditions are quite similar. The absorption bands appear at 1083 and 798 cm1 corresponding to the asymmetry stretching vibration and the symmetric stretching vibration of SieOeSi, respectively,
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Fig. 4. SEM micrographs of the damaged microcapsules synthesized at different n-octadecane/sodium silicate mass ratios and pH values: (a) 50/50 and pH 2.95w3.05, (b) 40/60 and pH 2.95w3.05, (c) 50/50 and pH 2.40w2.60, and (d) 50/50 and pH 7.90w8.10.
while the peak at 461 cm1 is observed due to the SieOeSi bending vibration. A broad band at 3436 cm1 is attributed to the SieOH stretching vibration, and meanwhile, the peak at 952 cm1 corresponds to the SieOH bending vibration. These characteristic absorption peaks indicated the existence of silica wall in the noctadecane microcapsules. On the other hand, the FTIR spectra of the microcapsule samples also reveal two intensive absorption peaks at 2925 and 2854 cm1, which are associated with the alkyl CeH stretching vibrations of methyl and methylene groups, respectively. The absorption peaks of methylene deformation vibration are found to appear at 1463 cm1 and 1376 cm1, and the peak at 721 cm1 is attributed the in-plane methylene rocking vibration. These characteristic bands are identical with those appearing in the spectrum of pristine n-octadecane (see Fig. 5). The
above results confirmed the successful encapsulation of n-octadecane with silica. Fig. 6 shows the WAXS patterns of the microcapsule samples, and the diffraction patterns of pristine n-octadecane and hollow microcapsules are also presented as references. The diffraction pattern of the hollow microcapsules shows no reflex peak, indicating an amorphous silica wall for the microcapsules synthesized in the current study. Therefore, it is facile to confirm the existence of n-octadecane within the microcapsules from the WAXS patterns of the synthesized microcapsules due to the crystallinity of noctadecane when freezing. As is shown by Fig. 6, all of the microcapsule samples exhibit the similar WAXS profiles with that of pristine n-octadecane. These patterns show a set of well-resolved diffraction peaks at 2q of 19.24 , 19.81, 23.31, 24.71, and 24.84 , which can be indexed as (010), (011), (105), (101), and (110) reflections of the b-form crystal of n-octadecane, respectively. Meanwhile, the reflections at 2q of 7.74 , 11.59 , 15.45 , 39.67, and 44.58 are assigned to the plane (002), (003), (004), (0e22), and (207) of the a-crystal of n-octadecane, respectively [34]. These characteristic reflections not only confirmed the encapsulation of n-octadecane within the silica microcapsules but also identified the good crystallinity of n-octadecane inside the microcapsules [35,36]. However, it is noteworthy that there is a considerable decrease in the intensity of diffraction peaks for the microcapsule sample synthesized at pH 7.90w8.10, indicating the less amount of noctadecane is loaded in the inside of microcapsules. 3.3. Phase change characteristics
Fig. 5. FTIR spectra of the microencapsulated n-octadecane synthesized in different conditions; the curve number is corresponding to the sample numbered with the same code in Table 1.
The phase change behaviors of the microencapsulated n-octadecane synthesized in different conditions were investigated by DSC. The resulted thermograms are shown in Fig. 7, and the phase change parameters obtained from the DSC evaluation are summarized in Table 1. It is noted from Fig. 7 that the pristine n-octadecane presents a single crystallization peak at 21.16 C as well as a single melting peak at 28.47 C, which are associated with the phase changes of solidification and fusion. However, the microencapsulated n-octadecane presents a bimodal crystallization
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Fig. 6. WAXS patterns of the microencapsulated n-octadecane synthesized in different conditions; the curve number is corresponding to the sample numbered with the same code in Table 1.
behavior on its cooling thermogram but still exhibits a single melting peak during heating process. Such bimodal exothermic peaks at around 20 and 23 C are attributed to the crystallization of the a- and b-crystals of n-octadecane, respectively. It has been reported that the a- and b-crystals of n-octadecane are induced by heterogeneous and homogeneous nucleation, respectively [36]. It is reasonable to deduce that the a-crystal formation of the microencapsulated n-octadecane is due to the heterogeneous nucleation effect caused by inner silica wall, inducing a crystalline transformation from b-form to the a-one, Furthermore, the formation of more stable a-crystal also resulted in a slightly shift of the a-form crystallization peak toward a high temperature. Moreover, for the microcapsule sample synthesized at pH 7.90w8.10, the poor encapsulation reduced the crystallinity of n-octadecane, thus resulting in a significant decrease in Tm. The nonisothermal crystallization behaviors of the pristine n-octadecane and the
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microencapsulated one were investigated by running dynamic DSC scans, and their thermograms at different cooling rates were demonstrated in Fig. 8. It is clearly observed that the crystallization peak shifts toward a lower temperature with increasing the cooling rate in the cases of both pristine n-octadecane and the microencapsulated one. This is due to the fact that the faster cooling rate leads to a higher degree of supercooling, and thus, the shorter time scale compels n-octadecane to crystallize at a lower temperature. Moreover, it is noteworthy that bimodal crystallization of the microencapsulated n-octadecane is increasingly enhanced with decreasing the cooling rate, while the pristine n-octadecane mainly forms the crystalline phase in b-form at any cooling rates. This indicates that the a-crystal of n-octadecane resulting from the heterogeneous nucleation effect of inner silica wall is induced only at a long time scale. On the other hand, the pristine n-octadecane is found to have considerably high phase change enthalpies according to the data listed in Table 1. This implies that the n-octadecane PCM has a high latent heat of storage energy and can fully release it when phase change occurs. However, the encapsulation of n-octadecane with silica evidently reduced the absolute phase change enthalpies of the microcapsule samples. Since the silica wall is an inert material, the latent heat can be only storied by the phase changeable noctadecane core. So it is believable that the phase change enthalpies of microcapsule samples are strongly dependant on the loading of n-octadecane within the microcapsules. There are two important parameters, i.e. the encapsulation ratio (R) and encapsulation efficiency (E) that can be adopted to characterize the phase change performance of the microencapsulated PCMs [20,27]. These two parameters could be calculated by DSC results through the following equations:
R ¼
DHm;microPCM 100% DHm;PCM
(1)
E ¼
DHm;microPCM þ DHc;microPCM 100% DHm;PCM þ DHc;PCM
(2)
where DHm,PCM and DHm,micro-PCM are the melting enthalpies of pristine n-octadecane and the microencapsulated one, respectively;
Fig. 7. DSC cooling (a) and heating (b) thermograms of the microencapsulated n-octadecane synthesized in different conditions; the curve number is corresponding to the sample numbered with the same code in Table 1.
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Table 1 The phase-change characteristics of the microencapsulated n-octadecane synthesized in different conditions. Sample code
Mass ratio of n-octadecane/sodium silica (wt./wt.)
Solution acidity (pH)
Crystallization process Tc,a ( C)
Tc,b ( C)
DHc (J g1)
Tm ( C)
DHm (J g1)
1 2 3 4 5 6 7
Pristine n-octadecane 50/50 60/40 40/60 50/50 50/50 0/100
e 2.95w3.05 2.95w3.05 2.95w3.05 2.40w2.60 7.90w8.10 2.95w3.05
e 23.33 23.72 23.16 23.73 23.89 e
21.16 19.86 21.84 19.45 20.55 19.11 e
206.75 68.83 84.89 52.83 37.96 19.55 e
28.74 27.88 27.96 27.97 28.43 23.34 e
209.10 71.14 87.46 55.12 40.35 22.03 e
DHc,PCM and DHc,micro-PCM are the crystallization enthalpies of pristine n-octadecane and the microencapsulated one, respectively. Table 1 reveals the encapsulation ratio and the encapsulation efficiency of the microcapsule samples obtained in different conditions. In fact, the encapsulation efficiency represents an effective performance of the n-octadecane inside the microcapsules for latent heat storage, while the encapsulation ratio describes the effective encapsulation of n-octadecane within the microcapsules. It should be mentioned that the sodium moiety is excluded from the wall material during the hydrolysis and polycondensation of silica precursor, and therefore, the actual mass ratio of core/wall is much lower than that of n-octadecane/sodium silicate. The microcapsule
Melting process
Encapsulation ratio (%)
Encapsulation efficiency (%)
Thermal storage capability (%)
Thermal conductivity (W m1 K1)
e 34.15 41.83 26.36 19.30 10.54 e
e 33.66 41.45 25.60 18.83 10.02 e
e 98.56 99.10 98.48 97.57 94.87 e
0.153 0.904 0.891 0.981 0.874 0.875 1.296
sample achieved a high encapsulation ratio of 34.15% and a high encapsulation efficiency of 33.66% when synthesized with 50/50 mass ratio of n-octadecane/sodium silicate at pH 2.95w3.05. This may be ascribed to the perfect self-assemble polycondensation forming a compact silica wall. In this case, most of the silica precursors were converted into the silica wall by polycondensation, and n-octadecane can be efficiently encapsulated by silica wall without any leakage. However, the samples obtained at pH 2.50w2.60 and pH 7.90w8.10 achieved the lower encapsulation ratio and efficiency, indicating a less amount of n-octadecane loaded in microcapsules. It is deduced that when the silica condensation rate was accelerated at above pH values, some of the silica precursors were not assembled on the surface of n-octadecane droplets and directly polycondensated in the form of solid particles. Accordingly, some of noctadecane was not encapsulated by silica wall, consequently resulting in a decrease in the loading of PCM in microcapsules. Furthermore, the fast silica polycondensation may also lead to a loose silica wall, which is easily damaged. These problems resulted in a reduction of the encapsulation ratio and efficiency. Moreover, the thermal storage capability (4) of the microencapsulated noctadecane can also be determined by the results from DSC measurements using Equation (3) [26]: DHm;microPCM þDHc;microPCM
4 ¼
R
DHm;PCM þ DHc;PCM
100%
(3)
where R is the encapsulation ratio calculated by Equation (1). The calculated results of thermal storage capability are listed in Table 1. It is interestingly found that the three microcapsule samples synthesized at pH 2.95w3.05 present a thermal storage capability higher than 98%, indicating that almost all of the encapsulated noctadecane can effectively store the latent heat through phase change. As for the sample obtained at pH 7.90w8.10, some of the encapsulated n-octadecane cannot perform thermal storage by phase change. For example, the phase change usually does not occur upon the excessively micronized n-octadecane in microcapsules due to the confinement effect on the molecular motion. These results reduce the thermal storage capability of the microencapsulated n-octadecane. 3.4. Thermal stability
Fig. 8. Nonisothermal crystallization DSC thermograms of (a) pristine n-octadecane and (b) the microencapsulated n-octadecane synthesized with 50/50 mass ratio of noctadecane/sodium silicate at pH 2.95w3.05.
Thermal stability is considered as an important parameter assessing the performance of new designed PCM microcapsules when used for heat energy storage or thermal regulation. The thermal degradation behaviors of the microcapsule samples synthesized in different conditions were investigated by TGA, and their TGA and DTG (derivative thermogravimetry) thermograms are shown in Fig. 9. The pristine n-octadecane exhibits a typical onestep weight loss at 202.5 C almost without any char remained at 650 C, suggesting that the pristine n-octadecane experienced a simple evaporation. However, the microcapsule samples show a
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two-stage degradation behavior in the temperature range of 130e 510 C. The first stage of weight loss is ascribed to the evaporation of n-octadecane, while the second one is related to silica condensation among the silanols of silica wall. It is reasonable to believe that there are a lot of silanol groups remaining on the silica wall during hydrothermal synthesis. The encapsulated n-octadecane broke throughout of the microcapsules during the first stage and gave a sharp weight loss starting at around 140 C and ending at around 250 C. It is interestingly noted in Fig. 9(b) that the microcapsule samples all show a maximum weight loss at higher temperatures compared to the pristine n-octadecane. This indicates that the silica wall can prevent the encapsulated n-octadecane from evaluating at the normal boil point, thus improving the degradation temperature of the microcapsules. Therefore, the thick and compact silica wall may favor the improvement in thermal stability of the microcapsules. On the other hand, the occurrence of the second weight loss only caused a loss of less than 5 wt% in weight as a result of the elimination of the water produced by further silica condensation at around 430 C. It is also observed from Fig. 9(a) that the microcapsule samples exhibit different char yields as the silica wall. These char yields seem to strongly depend both on the encapsulation rate and on the mass ratio of n-octadecane/silica sodium. For three samples obtained at pH 2.95w3.05, the char yield is proportional to the mass ratio of n-octadecane/silica sodium, which determined the exact loading of n-octadecane in
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microcapsules. Moreover, the microcapsule sample obtained at pH 7.90w8.10 achieved a high char yield of 58 wt% due to its low encapsulation ratio of 10%. 3.5. Thermal conductivity It is well known that the pristine n-octadecane as an organic PCM has a low thermal conductivity despite its other desirable properties. This may increase its thermal response time for the storage and release of latent heat. Therefore, the techniques of thermally conductive enhancement are absolutely required when designing a microencapsulated PCM [37,38]. It is expected that the encapsulation of n-octadecane with higher thermally conductive silica wall can enhance the thermal conductivities of the resulting microcapsules. The thermal conductivities of the microcapsule samples are listed in Table 1. As predicted, the pristine n-octadecane in solid state shows a thermal conductivity as low as 0.153 W m1 K1. Nevertheless, it is surprisingly found from the data in Table 1 that all of the microcapsule samples achieved the thermal conductivities higher than 0.85 W m1 K1. The silica wall material was obtained by removing the n-octadecane core from the synthesized microcapsules, and then, its thermal conductivity was measured to give a value of 1.296 W m1 K1. Such a value is much higher than those of polymeric wall materials, which are only around 0.20 W m1 K1 [39]. These results confirmed that the encapsulation of n-octadecane with highly thermally conductive silica actually impart a high thermal conductivity to the microcapsules. It is also noteworthy that the thermal conductivities of the microencapsulated n-octadecane samples are somewhat associated with the encapsulation rate, and the higher the encapsulation rate, the lower the thermal conductivity. It is understandable that the high encapsulation rate means the low percentage of silica wall in microcapsules as well as the high n-octadecane loading. This suggests that the silica wall play a critical role in enhancing the heat transfer of microencapsulated n-octadecane. In addition, the microcapsules synthesized with 40/60 mass ratio of n-octadecane/ sodium silicate at pH 2.95w3.05 are found to have a much higher thermal conductivity than the other samples, which may be attributed to the more compact and thicker silica wall. It seems that the silica wall forms a continuous phase when n-octadecane is encapsulated by silica material. Such a continuous inorganic phase can be considered as a virtual heat transfer network and, therefore, can enhance the heat transfer rate over the whole microcapsules. As a result, the microencapsulated n-octadecane with a more compact and thicker silica wall achieved a much higher thermal conductivity. 3.6. Reliability and durability
Fig. 9. TGA (a) and DTG (b) thermograms of the microencapsulated n-octadecane synthesized in different conditions; the curve number is corresponding to the sample numbered with the same code in Table 1.
To evaluate the reliability of PCMs during phase-change process, one hundred loops of heatingecooling dynamic scans were carried out by DSC to investigate the multicycle phase change performance of the microencapsulated n-octadecane, and the corresponding thermograms are demonstrated in Fig. 10. It is observed that the aform crystallization peak slightly shifts toward a lower temperature at the initial several loops and then, its position becomes stable till the hundredth loop. These results indicate that the a-form crystallization is considerably influenced by the heat history due to its heterogeneous nucleation in nature. However, the b-form crystallization peak is almost invariable because of the homogeneous nucleation effect of inner silica wall. In this case, the melting peak also shows a slight shift toward a lower temperature during the initial several DSC scans. On the basis of the overall phase change behaviors shown by Fig. 10, the microencapsulated n-octadecane obtained in the current work maintains a normal multicycle phase
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the loose and porous silica wall of the microcapsules, which cannot well prevent n-octadecane from permeating. The sample obtained at pH 2.95w3.05 shows the best anti-osmosis property among three samples synthesized with 50/50 mass ratio of n-octadecane/ sodium silicate. This indicates that a compact silica wall can effectively prevent the n-octadecane core from penetrating through and provide good anti-osmosis performance for the microcapsules. As shown by Fig. 11, for three samples obtained at pH 2.95w3.05, the microcapsules synthesized with 40/60 mass ratio of n-octadecane/sodium silicate have the lowest release rate, indicating that the thicker silica wall of microcapsules can lead to the better antiosmosis performance. These results suggest that the anti-osmosis property of the microencapsulated n-octadecane strongly depends on the compaction of silica wall as well as its thickness, and the microcapsules can achieve a good anti-osmosis performance as long as they are synthesized with an optimal core/wall mass ratio in an appropriate condition. 4. Conclusion Fig. 10. DSC thermograms of the microencapsulated n-octadecane synthesized with 50/50 mass ratio of n-octadecane/sodium silicate at pH 2.95w3.05 under one hundred loops of heatingecooling cycle.
transformation and can be repetitiously used for the storage and release of latent heat at an almost stable temperature. For any microencapsulated PCMs, the PCM cores will be eventually released from the inside of microcapsules due to the permeation of wall materials. Therefore, the anti-osmosis performance is an important factor determining the durability of the microcapsules. The durability of the microencapsulated n-octadecane was evaluated by measuring the weight loss of the extracted microcapsules in terms of the duration date. Fig. 11 displays the release curves obtained from the above measurements upon the microcapsule samples, in which all of the reported data are the average values from five specimens. It is believed that these release behaviors can reflect the anti-osmosis capability of silica wall preventing the n-octadecane core from permeation. It is clearly observed that the microcapsule sample synthesized at pH 7.90w8.10 exhibits a poorest anti-osmosis behavior because of its high release rate as a function of duration date. This result is due to
The new silica encapsulation technique upon n-octadecane PCM was successfully developed through self-assemble synthesis using sodium silicate as a silica precursor. The FTIR spectra and the WAXS patterns confirmed the chemical composition and crystallinity of the synthesized microcapsule samples. The microstructure of the obtained microcapsules strongly depends on the pH value of reaction solution, which determines the silica condensation rate and thus, influences the balance between the self-assembly and polycondensation of silica precursors on the surface of n-octadecane droplets. The microencapsulated n-octadecane shows a welldefined coreeshell structure as well as a perfect spherical morphology when synthesized at pH 2.95w3.05. The microencapsulated n-octadecane obtained at pH 2.95w3.05 exhibited good phase change performance and also achieved a high encapsulation rate and a high encapsulation efficiency. The encapsulation of n-octadecane with compact and thick silica wall can impart a high thermal conductivity and a good anti-osmosis property to the microcapsules, and can also improve the thermal stability of the microcapsules by preventing inside n-octadecane from thermally evaporating. The synthetic technique exploited by this work indicates a high feasibility in industrial manufacture for the microencapsulated PCMs with inorganic walls due to the easy availability and low cost of sodium silicate. And the resulting PCM microcapsules will be a potential candidate for the application in the fields of building air conditioning, electronics cooling systems, waste heat recovery, 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
Fig. 11. Release curves of the microencapsulated n-octadecane synthesized in different conditions.
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