- Email: [email protected]
Morphological control and thermal properties of nanoencapsulated n-octadecane phase change material with organosilica shell materials
Energy Conversion and Management 119 (2016) 151–162
Contents lists available at ScienceDirect
Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman
Morphological control and thermal properties of nanoencapsulated n-octadecane phase change material with organosilica shell materials Yalin Zhu a,b, Shuen Liang a,c,⇑, Hui Wang a, Ke Zhang c, Xiaorong Jia a, Chunrong Tian a, Yuanlin Zhou b, Jianhua Wang a,⇑ a b c
Institute of Chemical Materials, China Academy of Engineering Physics (CAEP), Mianyang 621900, PR China College of Materials Science and Engineering, Southwest University of Science and Technology of China, Mianyang 621000, PR China CAS Key Laboratory of Soft Matter Chemistry, Department of Polymer Science and Engineering, University of Science and Technology of China, Hefei 230026, PR China
a r t i c l e
i n f o
Article history: Received 12 January 2016 Received in revised form 17 March 2016 Accepted 13 April 2016
Keywords: Phase change material n-Octadecane Nanoencapsulation Organosilica Morphology Supercooling
a b s t r a c t Morphological control was attempted on organosilica nanoencapsulated n-octadecane phase change material by adjusting various synthetic conditions, and the relationship between morphology and phase change property was investigated. The chemical structure and thermal stability of the nanocapsules were characterized by FT-IR spectroscopy and thermogravimetric analysis, respectively. The morphology and microstructure of the nanocapsules were observed by SEM and TEM, and the phase change property was determined by DSC and temperature-dependent XRD methods. With decreasing water-to-ethanol ratio, increasing cetyltrimethylammonium bromide (CTAB) concentration, or increasing NH3H2O concentration, the morphologies of the NanoPCMs can be regulated from thin-shelled nanocapsules with bowl like, hemispherical, or spherical geometries to thick-shelled spherical nanocapsules or mesoporous particles. Meanwhile, the average diameter of the nanocapsules also increases obviously. It was demonstrated that the phase change properties of these nanocapsules are intimately related to their morphologies: thicker organosilica shells induce heterogeneous nucleation better and result in less supercooling, compared with the thinner ones. The methods and mechanisms proposed herein might be helpful to prepare various micro/nano encapsulated phase change materials through interfacial hydrolysis– condensation method, and optimize their morphologies and thermal properties. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction Due to limited conventional fossil energy, growing population and increasing greenhouse gas emission, highly efficient utilization of energy as well as new energy storage materials become more and more important. Phase change materials (PCMs) have attracted much attention in the last decades because they can provide high energy storage density and near isothermal heat storage [1]. In order to prevent the leakage of PCMs in melt state, improve the heat storage/release efficiency, and control the volume changes during phase change process, micro/nano encapsulation of PCMs have been extensively explored [2–4]. As a result, they are applied in fields of solar energy storage, refrigeration system, energy efficient buildings, smart textiles and so on [5–13]. Compared to microencapsulated phase change materials (MicroPCMs), nanoencapsulated phase change materials (NanoPCMs) with much smaller ⇑ Corresponding author at: Institute of Chemical Materials, China Academy of Engineering Physics (CAEP), Mianyang 621900, PR China (S. Liang). E-mail addresses: [email protected] (S. Liang), [email protected] (J. Wang). http://dx.doi.org/10.1016/j.enconman.2016.04.049 0196-8904/Ó 2016 Elsevier Ltd. All rights reserved.
sizes and higher surface-area-to-volume ratios provide a stronger driving force to speed up thermodynamic processes [14]. In addition, when NanoPCMs are used in latent functionally thermal fluids (LFTF), they do not fracture easily in course of flow [15]. Micro/nano encapsulated PCMs typically consist of core materials (i.e., PCMs) and shell materials with core–shell structure. Traditionally, various organic polymeric materials are used as shell materials, such as melamine–formaldehyde (M–F) resin [16,17], urea–formaldehyde resin [18], polystyrene [8], and polymethylmethacrylate [19]. In recent years, there are growing interests on encapsulation of PCMs with inorganic shell materials, including silica [15,20–29], boehmite [30,31], CaCO3 [32,33], TiO2 [34] and so on, because they are flame retardant, free of poisonous gases release, more thermally stable and conductive, compared with polymeric materials. Especially, most attention has been paid to encapsulation of PCMs with silica shell materials, which are nontoxic, structurally stable and have well-defined surface properties [35,36]. However, the MicroPCMs with silica shell are often weak in mechanical properties and can be easily damaged [32,34]. Moreover, in many applications, encapsulated PCMs need
152
Y. Zhu et al. / Energy Conversion and Management 119 (2016) 151–162
to be dispersed in various polymeric matrices, to produce heat storage or thermoregulated composites [7,9,10,12]. Therefore, development of NanoPCMs with organically modified silica (organosilica) shell materials is highly desired. In previous work [37], we successfully nanoencapsulated n-octadecane with organosilica shells through co-hydrolysis and co-condensation of silane precursors in miniemulsion. By changing the volume ratio of different silane precursors, NanoPCMs with tunable chemical structure, thermal property, and hydrophobicity were obtained. The morphologies of micro/nano encapsulated PCMs are intimately related to their thermal properties. Zhang et al. [16] reported that capsule size has significant effects on the crystallizing behavior of microcapsules containing n-octadecane with M–F shell, and smaller capsule size results in larger super cooling. Wang’s group [23,24] investigated the influence of morphology on the phase change properties for microencapsulated n-octadecane with silica shell. Pan et al. [30,31] and Cao et al. [34] observed that nanoencapsulation of PCMs with AlOOH and TiO2 shell materials can lead to a remarkable decrease of phase change temperatures due to the confinement of capsule shells, which is known as Gibbs–Thomson effect. Generally, Gibbs–Thomson effect restricts and delays the crystallization of PCMs. On the contrary, it was also found that capsule shells can play an important role as nucleating sites for crystallization of encapsulated PCMs, which induces and promotes the crystallization process [29,38]. The interplay of Gibbs–Thomson effect and nucleation effect resulted from the capsule shells, and diverse morphologies of PCM capsules (spherical or non-spherical [39]; core–shell or matrix type [40]; compact or mesoporous [28]) make the morphology–thermal property relationship rather complicated, and currently the knowledge and understanding about this topic are not adequate and systematic. In order to acquire encapsulated PCMs with different morphologies, limited approaches were explored, mainly including variation of stirring speed [16], type and concentration of emulsifier [20,32,33], core/shell precursor ratio [23], pH value [15], etc. For micro/nano encapsulated PCMs with silica shell materials prepared through sol–gel (i.e., hydrolysis–condensation) process, it was disclosed that the pH value is critical to obtain compact and smooth shell [23,24] and different capsule sizes [15]. However, the influences of other synthetic conditions were rarely studied. Considering that the reaction system for encapsulating PCMs generally consists of multiple components, such as oil phase, water phase, emulsifier, and catalyst/initiator, morphological control may be realized by tuning various synthetic conditions during the preparation process. Therefore, it is very meaningful to investigate the influence of various synthetic conditions on the morphologies of the novel kind of nanoencapsulated PCMs with organosilica shell, and clarify their morphology–thermal property relationship.
In present study, we focused on the morphological control of nanoencapsulated n-octadecane with organosilica shell materials by changing synthetic conditions including water-to-ethanol ratio, emulsifier concentration, and NH3H2O concentrations. The mechanism for the variation of morphology was proposed, and the relationship between the morphology and phase change property of the NanoPCMs was clarified. This work provided new insights on morphological control and morphology–thermal property relationship of organosilica nanoencapsulated PCMs. The findings might be extended to prepare various micro/nano encapsulated PCMs through interfacial hydrolysis–condensation method and optimize their morphologies and thermal properties. 2. Materials and methods 2.1. Materials Tetraethyl orthosilicate (TEOS) (98%), anhydrous ethanol and ammonium hydroxide (NH3H2O, 25 wt%) were purchased from Sinopharm Chemical Reagents Company, China. Cetyltrimethylammonium bromide (CTAB) was provided by Tianjin Kermal Chemical Reagents Company, China. c-Methacryloxypropyltrimethoxysilane (MPS), methyltrimethoxysilane (MTMS) and n-octadecane (90 wt%) were purchased from Tianjin Alfa Aesar Company, China. All chemicals were of reagent quality and used as received without further purification. 2.2. Preparation of NanoPCMs with organosilica shell materials Nanoencapsulated n-octadecane with organosilica shell was prepared through interfacial co-hydrolysis and co-condensation of TEOS, MPS and MTMS in miniemulsion, as described in our previous study [37]. Typically, n-octadecane (1.0 g), MPS (or MTMS) (0.5 mL) and TEOS (1.0 mL) were mixed in a round bottomed flask (100 mL), to form a clear solution. Then, CTAB (0.328 g), deionized H2O (28.5 mL) and anhydrous ethanol (14.2 mL) were added into the flask in turn. The mixture was stirred at the rate of 1500 rpm for 30 min, and then sonicated (KQ-400KDB, 100% amplitude) for 10 min at 35 °C, to form a stable miniemulsion. Aqueous ammonia (25 wt%, 1.04 mL) was added to the flask to initiate the interfacial co-hydrolysis and co-condensation reaction of MPS (or MTMS) and TEOS. The reaction proceeded with gentle stirring (300 rpm) for 16 h. Finally, white powder-like products (around 1.60 g) were collected after filtration, washing with deionized water, and freeze drying. The mixture of MPS and TEOS was used to introduce c-methacryloxypropyl (CH2C(CH3)COOCH2CH2CH2-) groups into silica shell material, while the mixture of MTMS and TEOS was
Table 1 Synthetic conditions of the nanoencapsulated n-octadecane with organosilica shells. Samples
Silane precursors
Water-to-ethanol ratio (v/v)
CTAB concentration (mM)
NH3H2O concentration (wt%)
P1 P2 P3 P4 P5 P6 P7
MPS (0.5 mL) + TEOS (1.0 mL)
2.5/1 2/1 1.7/1 2/1 2/1 2/1 2/1
20 20 20 15 30 20 20
0.6 0.6 0.6 0.6 0.6 0.3 0.9
M1 M2 M3 M4 M5 M6 M7
MTMS (0.5 mL) + TEOS (1.0 mL)
2.5/1 2/1 1.7/1 2/1 2/1 2/1 2/1
20 20 20 15 35 20 20
0.6 0.6 0.6 0.6 0.6 0.3 0.9
Y. Zhu et al. / Energy Conversion and Management 119 (2016) 151–162
used to incorporate methyl groups into silica shell material. To obtain NanoPCMs with different morphologies, varied water-toethanol ratios (1.7/1–2.5/1), CTAB concentration (15–35 mM), and NH3H2O concentration (0.3–0.9 wt%) were adopted, as listed in Table 1.
153
testing temperature, the samples were equilibrated for 2 min before measurements. 3. Results and discussion 3.1. Chemical structure
2.3. Characterization The FT-IR spectra were obtained using a Nicolet 6700 IRspectrophotometer on the KBr sampling sheet with a scanning number of 32. Thermogravimetric analysis (TGA) was performed on a TA Instruments Q500 thermal gravimetric analyzer under a nitrogen atmosphere. The specimen with a mass of about 5 mg was placed in an aluminum crucible and then ramped from room temperature up to 550 °C at a heating rate of 10 °C/min. Thermal conductivity measurements were performed using an EKO HC074-200 thermal conductivity tester through a heat flux method at 25 °C. The morphologies of the synthesized nanocapsules were observed using a CamScan Apollo 300 field emission scanning electron microscope (SEM). The specimen was made electrically conductive by sputter coating with a thin layer of gold–palladium alloy. The micrographs were taken in high vacuum mode with 10 kV acceleration voltage and a medium spot size. The microstructures of the synthesized nanocapsules were determined by using a Hitachi H-800 transmission electron microscope (TEM) operated at an accelerating voltage of 100 kV. The specimen was dispersed in ethanol under sonication, and some pieces were collected on carbon-coated 300-mesh copper grids for TEM observation. High resolution TEM (HRTEM) characterization of the NanoPCMs was performed on a JEOL-2010 transmission electron microscope. DSC scans were performed using a TA Instruments Q2000 differential scanning calorimeter, and all measurements were carried out in a nitrogen atmosphere at a heating or cooling rate of 10 °C/min with the weight of the specimen being about 2 mg. For each NanoPCMs sample, DSC measurements were conducted on three individual specimens. Temperature-dependent XRD measurements were performed on a Bruker D8 Advance diffractometer (40 kV, 50 mA) with Cu Ka radiation (k = 0.154 nm), and the diffraction patterns were collected in the 2h ranging from 5° to 40° at a scanning rate of 10°/min. Samples were first heated from ambient temperature to 28 °C, and kept for 2 min, followed by cooling to 0 °C. The cooling rates were 6 °C/min, and at every
FT-IR spectra of the as-prepared NanoPCMs were presented in Fig. 1. For NanoPCMs with c-methacryloxypropyl functionalized silica shells (P1–P7), their FT-IR spectra are very similar. The absorption peaks at 2922, 2852, 1469 and 719 cm 1 are ascribed to core material (n-octadecane), while the peaks at 3427, 1718, 1635, 1082 and 463 cm 1 are attributed to shell material. In particular, the peaks at 1718 and 1635 cm 1 are characteristic for c-methacryloxypropyl groups. For NanoPCMs with methyl functionalized silica shells (M1–M7), their FT-IR spectra are also similar. The absorption peaks at 2922, 2852, 1469 and 719 cm 1 are ascribed to n-octadecane, while the peaks at 3427, 1270, 1082, and 463 cm 1 are attributed to organosilica shell material. The peak at 1270 cm 1 is characteristic for Si–CH3 groups [37]. These results demonstrated that synthetic conditions (water-to-ethanol ratio, emulsifier concentration, and NH3H2O concentrations) have minor influences on the chemical structure of NanoPCMs. 3.2. Thermal stability and thermal conductivity The thermal stability of the as-prepared NanoPCMs was first tested by heating them in an oven (50 °C). As shown in Fig. S1, the appearance of the NanoPCMs does not change after being heated for 15 min, proving that they are shape-stabilized PCMs and n-octadecane is encapsulated by shell materials indeed. On the contrary, under the same condition, pristine n-octadecane melts into clear liquid completely. The thermal stability of the NanoPCMs was further characterized by TGA method, as shown in Fig. 2. For all the NanoPCMs, n-octadecane evaporates during the temperature range of 100–250 °C, from which the contents of n-octadecane in the NanoPCMs can be estimated. For samples P1–P7, the second weight loss at around 419 °C is attributed to the decomposition of c-methacryloxypropyl functionalized silica shell materials [37]. It can be observed that the NanoPCMs prepared under different synthetic conditions exhibit similar thermal stability. However, the contents of n-octadecane in the NanoPCMs are different, displaying some dependence on the synthetic conditions.
Fig. 1. FT-IR spectra of the as-prepared NanoPCMs with organosilica shells: (a) P1–P7; (b) M1–M7.
154
Y. Zhu et al. / Energy Conversion and Management 119 (2016) 151–162
Fig. 2. TGA curves of the as-prepared NanoPCMs with organosilica shells: (a) P1–P7; (b) M1–M7.
Thermal conductivity measurements were performed on representative samples P2 and M2, and their thermal conductivities are as large as 0.575 and 0.414 W m 1 K 1, respectively. These values are quite higher than that of pristine n-octadecane (0.153 W m 1 K 1 [32]), and also comparable with that of NanoPCMs with unmodified silica shell (0.42–0.45 W m 1 K 1 [29]). Therefore, NanoPCMs with organosilica shell materials can provide not only advantageous surface hydrophobicity, but also high thermal conductivity, which are beneficial to accelerate thermal storage/release processes. 3.3. Effects of water-to-ethanol ratio on the morphology and phase change property of NanoPCMs In preparation of silica micro/nano particles through sol–gel process, water/alcohol mixed solvents are often used to enhance the regularity and homogeneity of the products [41,42]. However, the effects of water/alcohol mixed solvents on the morphologies and properties of micro/nano encapsulated PCMs were rarely studied. The morphology and microstructure of the NanoPCMs prepared at different water-to-ethanol ratios were characterized by SEM and TEM, and the images were presented in Fig. 3. Detailed data about the geometry, average diameter and shell thickness of the NanoPCMs were obtained from the SEM and TEM images, and summarized in Table S1. It can be found that, with decreasing water-to-ethanol ratio, the NanoPCMs transform from bowl like (P1, P2 and M1) to spherical (P3, M2 and M3) geometry, and the average diameter and shell thickness increase obviously. Thus, a morphology transition from bowl-like thin-shelled nanocapsules to spherical thick-shelled nanocapsules is evident. Particularly, in sample P3, the particles possess very thick shells, or even do not exhibit clear core–shell structure. In that case, n-octadecane is possibly embedded in the mesopores of organosilica material, called a matrix type of encapsulated PCMs [4,40,43]. It is known that silica capsules or particles obtained through hydrolysis– condensation of alkoxy silanes are usually amorphous and mesoporous [28,41,42]. In order to verify this point, we conducted HRTEM characterization on representative sample P2 and P3, and the results were shown is Fig. 4. It can be observed that the organosilica shells of the former are much denser than the latter case, so the encapsulation of n-octadecane in the mesoporous particles is highly feasible for sample P3. DSC curves of the NanoPCMs prepared at different water-toethanol ratios were shown in Fig. 5. There is only one endothermic peak during the melting process of these NanoPCMs, due to the
phase transition from triclinic crystal to liquid. However, 2–3 exothermic peaks are observed during the solidifying process, and labeled as a, b, and c peaks from high to low temperature, respectively. According to references [38,44], the peaks a and b are ascribed to the liquid–rotator and rotator–triclinic phase transitions based on heterogeneous nucleation, and peak c is attributed to direct liquid–triclinic phase transition based on homogeneous nucleation, respectively. The results revealed that sample P2, P3, M2 and M3 crystallize fully based on heterogeneous nucleation, while sample P1 and M1 crystallize based on both heterogeneous and homogeneous nucleation. The reason may be that nucleation sites (impurities) may be lacking in the small [16] and thinshelled NanoPCMs, and n-octadecane partially undergoes direct liquid–triclinic phase transition at a large supercooling. To further confirm the crystallizing behavior of the NanoPCMs prepared at different water-to-ethanol ratios, temperature dependent XRD measurements were carried out on representative sample M1 and M2, and the results were shown in Figs. 6 and S2. For sample M1 with thin organosilica shell, when temperature is 22 °C or higher, there is only a broad diffraction band (15–30°) on the XRD pattern (Fig. 6a), suggesting that n-octadecane core material is in isotropic liquid state. As temperature decreases to 20 °C, characteristic diffraction peaks of rotator phase are observed at 2h = 21.1° and 22.9°. At 18 °C, the diffractive intensities of the rotator phase weaken, meanwhile characteristic diffraction peaks of triclinic phase emerge, demonstrating the coexistence of both crystal phases. At 6 °C, rotator phase almost disappears, and only triclinic phase exists. We assigned the intensity of diffraction peak at 21.1° as a representation of the degree of crystallinity for rotator phase, and that at 23.4° as a representation of the degree of crystallinity for triclinic phase. Consequently, the intensities at 21.1° and 23.4° as functions of temperature were shown in Fig. 6b, from which the phase transition temperatures can be determined. At around 20 °C, the degree of crystallinity for rotator phase increases rapidly, indicating the liquid–rotator phase transition. Then, at around 12 °C, the degree of crystallinity for rotator phase declines, meanwhile that for triclinic phase increases promptly, demonstrating the rotator–triclinic phase transition. Finally, at around 6 °C, the degree of crystallinity for triclinic phase increases further, but that for rotator phase keeps constant, confirming the occurrence of direct liquid–triclinic phase transition. Similarly, for sample M2, the liquid–rotator phase transition occurs at around 24 °C, the rotator–triclinic phase transition occurs at around 21 °C, and the liquid–triclinic phase transition does not exist (Fig. S2). It is worth noting that the phase transition temperatures determined by the temperature-dependent XRD method only roughly agree
Y. Zhu et al. / Energy Conversion and Management 119 (2016) 151–162
155
Fig. 3. (a–c, g–i) SEM and (d–f, j–l) TEM images of the NanoPCMs prepared at different water-to-ethanol ratios: (a, d) P1; (b, e) P2; (c, f) P3; (g, j) M1; (h, k) M2; (i, l) M3. Inserts of (a) and (g) are images at higher magnification.
Fig. 4. HRTEM images of the NanoPCMs prepared at different water-to-ethanol ratios: (a) P2; (b) P3.
156
Y. Zhu et al. / Energy Conversion and Management 119 (2016) 151–162
Fig. 5. DSC curves of the NanoPCMs prepared at different water-to-ethanol ratios: (a) P1–P3; (b) M1–M3.
Fig. 6. Temperature-dependent XRD patterns of sample M1 during cooling process: (a) diffraction intensity as a function of diffraction angle (2h) at various temperatures; (b) intensities of diffraction peaks at 21.1° and 23.4° as functions of temperature. L: liquid phase. R: rotator crystal phase. T: triclinic crystal phase.
Table 2 Phase change characteristics of n-octadecane and the NanoPCMs prepared under different water-to-ethanol ratios. Samples
n-Octadecane P1 P2 P3 M1 M2 M3 a b c d
Melting
R (%)c
Solidifying
Tm (°C)
DHm (J/g)a
Tc, a (°C)
Tc,
28.52 29.36 27.92 28.58 26.27 28.19 27.40
204.4 ± 11.9 102.6 ± 9.9 109.4 ± 10.4 96.3 ± 7.4 96.7 ± 7.4 109.9 ± 16.1 107.2 ± 6.2
23.92 25.15 24.94 24.76 20.82 24.57 24.01
–d 21.53 20.55 22.84 11.48 19.44 22.26
b
(°C)
Tc, c (°C)
DHc (J/g)b
– 6.68 – – 5.46 – –
202.4 ± 13.9 100.0 ± 8.9 99.4 ± 15.4 96.2 ± 7.9 83.4 ± 8.2 103.4 ± 19.6 104.0 ± 7.7
– 50.2 53.5 47.1 47.3 53.8 52.4
Melting enthalpy with 95% confidence interval. Crystallizing enthalpy with 95% confidence interval. Encapsulation Ratio (R) = DHm, NanoPCMs/DHm, n-octadecane 100%. Not exists.
with that obtained by the DSC method. The deviations are possibly due to the different temperature controlling style of the two characterization methods [38]. The phase change temperatures, phase change enthalpies, and encapsulation ratios of the NanoPCMs prepared at different water-to-ethanol ratios were summarized in Table 2. The melting and crystallizing enthalpies are similar and relatively high (around
100 J/g). It should be mentioned that the phase change enthalpy or encapsulation ratio cannot be reliably estimated from the diameter and shell thickness of the nanocapsules, because the interior of the nanocapsules may not be completely filled with PCMs, as observed by Wang et al. [20], and the organosilica shell materials are mesoporous (Fig. 4). Moreover, the encapsulation ratios obtained from DSC results are lower than the n-octadecane contents estimated
Y. Zhu et al. / Energy Conversion and Management 119 (2016) 151–162
157
Fig. 7. (a, b, e, f) SEM and (c, d, g, h) TEM images of the NanoPCMs prepared at different CTAB concentrations: (a, c) P4; (b, d) P5; (e, g) M4; (f, h) M5.
by TGA method. The reason may be that some water is absorbed by the NanoPCMs and evaporates in the same temperature range with the evaporation of n-octadecane. 3.4. Effects of CTAB concentrations on the morphologies and phase change properties of NanoPCMs CTAB plays important roles in the miniemulsion system, as both emulsifiers and templates for deposition of organosilica oligomers. SEM and TEM images of the NanoPCMs prepared at different CTAB concentrations were shown in Fig. 7. Detailed information about the geometry, average diameter and shell thickness of the NanoPCMs prepared at different CTAB concentration were listed in Table S1. At low CTAB concentration (15 mM), bowl like (P4)
or spherical (M4) nanocapsules with well-defined core–shell structure are obtained, and the shell thickness is relatively low. However, at higher CTAB concentration (30 or 35 mM), spherical nanocapsules with thicker shells are acquired. In addition, the average diameter of the NanoPCMs also increases with increasing CTAB concentration, which seems contradictory to the common knowledge that higher surfactant concentration leads to smaller dispersed droplets or colloidal particles, but the mechanism is currently not clear. From HRTEM images (Fig. S3) of sample M4 and M5, it is evident that higher CTAB concentration results in thicker but looser capsule shells. Similar phenomenon was also reported by Cao and Yang [44] for microencapsulated n-octadecane with M–F resin shell when using sodium dodecyl sulfate (SDS) as an emulsifier at high concentration.
158
Y. Zhu et al. / Energy Conversion and Management 119 (2016) 151–162
Fig. 8. DSC curves of the NanoPCMs prepared at different CTAB concentrations: (a) P4, P5; (b) M4, M5.
Table 3 Phase change characteristics of the NanoPCMs prepared under different CTAB concentrations. Samples
P4 P5 M4 M5
Melting
Solidifying
R (%)
Tm (°C)
DHm (J/g)
Tc, a (°C)
Tc,
28.44 27.76 27.14 28.83
113.3 ± 16.1 75.4 ± 24.3 105.6 ± 9.4 121.0 ± 6.0
23.76 25.82 23.80 24.19
20.26 20.61 17.78 20.73
DSC curves of the NanoPCMs prepared at different CTAB concentrations were shown in Fig. 8. Based on the DSC curves, the phase change temperatures, phase change enthalpies, and encapsulation ratios were calculated and summarized in Table 3. These NanoPCMs exhibit mainly two exothermic peaks (a, b) during the solidifying processes, showing that they crystallize via shell induced heterogeneous nucleation. However, it is also observed that, with increasing CTAB concentration, the crystallizing peaks a and b become narrower, suggesting that thicker nanocapsule shell induces the heterogeneous nucleation better. In addition, the phase change enthalpies of P5 are lower than that of P4, due to reduced stability of the emulsion system at 30 mM, which was observed experimentally. On the contrary, the phase change enthalpies of M5 are higher than that of M4, at higher CTAB concentration (35 mM). These results demonstrated that optimal synthetic conditions for nanoencapsulated n-octadecane by using different silane precursors are not uniform, possibly ascribed to their different chemical or physical nature. 3.5. Effects of NH3H2O concentration on the morphologies and phase change properties of NanoPCMs It is an important approach to control the morphologies and sizes of micro/nano encapsulated PCMs with silica shell materials by tuning the pH value (i.e., acid/base concentration) in the sol– gel system, because the hydrolysis/condensation kinetics of silane precursors is greatly influenced by acid/base as catalyst [20,23,24]. SEM and TEM micrographs of the NanoPCMs prepared at different NH3H2O concentrations were presented in Fig. 9. The geometry, average diameter and shell thickness of the NanoPCMs prepared at different NH3H2O concentration were also summarized in Table S1. With increasing NH3H2O concentration, the geometries of the NanoPCMs transform from bowl like (sample P6) or hemispherical (sample M6) nanocapsules to regular
b
(°C)
Tc, c (°C)
DHc (J/g)
– – – –
108.8 ± 16.6 74.1 ± 17.4 95.0 ± 11.9 115.7 ± 8.4
55.4 36.9 51.7 59.2
spherical nanocapsules (samples P7 and M7). All the NanoPCMs exhibit well-defined core–shell structure, and the shell thickness increases obviously with the increase of NH3H2O concentration. In addition, the average diameter of the NanoPCMs also increases with increasing NH3H2O concentration, which is consistent with the results reported by Tahan Latibari et al. [15]. DSC curves of the NanoPCMs prepared at different NH3H2O concentrations were presented in Fig. 10. Based on the DSC curves, the phase change temperatures, phase change enthalpies, and encapsulation ratios were calculated and summarized in Table 4. For sample P6 and P7, peaks a and b are predominant during the solidifying process, whereas peak c is very weak. However, from sample P6 to P7, crystallizing peaks a and b become narrower. For samples M6 and M7, three and two exothermic peaks are observed during the solidifying process, respectively. These results confirm that thicker organosilica shell induces the heterogeneous nucleation of n-octadecane better. In addition, the phase change enthalpies of the NanoPCMs increase obviously with increasing NH3H2O concentration from 0.3 to 0.9 wt%, which is indicative that relative thicker shell is preferred for obtaining higher encapsulation ratio. 3.6. Mechanism for morphology variation of NanoPCMs In summary, similar morphology variation of NanoPCMs is observed by changing water-to-ethanol ratio, CTAB concentration, or NH3H2O concentration, as illustrated in Scheme 1. At high water-to-ethanol ratio, or low CTAB and NH3H2O concentration, nonspherical nanocapsules with clear core–shell structure are obtained, and the shells are very thin (Scheme 1a–c). With decreasing water-to-ethanol ratio, or increasing CTAB and NH3H2O concentration, spherical nanocapsules with well-defined core– shell structure are also acquired, but the shells get thicker (Scheme 1d). Finally, at low water-to-ethanol ratio, or high CTAB
Y. Zhu et al. / Energy Conversion and Management 119 (2016) 151–162
159
Fig. 9. (a, b, e, f) SEM and (c, d, g, h) TEM images of the NanoPCMs prepared at different NH3H2O concentrations: (a, c) P6; (b, d) P7; (e, g) M6; (f, h) M7.
and NH3H2O concentration, thick-shelled nanocapsules or mesoporous particles tend to be produced (Scheme 1e). A tentative mechanism is proposed for the formation of NanoPCMs with varied morphologies at different water-toethanol ratios. At a relatively high water-to-ethanol ratio, the polarity difference between water phase and oil phase is high, so the oil/water miscibility is very poor. As a result, the hydrolysis and condensation reaction is confined at the oil/water interface, and the formed organosilica oligomers self-assemble therein immediately to produce a thin and dense organosilica shell. Because of the pressure difference between inside and outside of the nanocapsules, thin-shelled organosilica nanocapsules are easily deformed from spherical to nonspherical geometries. With decreasing water-to-ethanol ratio, the oil/water miscibility is
enhanced, causing that the hydrolysis and condensation reaction occur both at the oil/water interface and inside the oil droplet. Therefore, a portion of organosilica oligomers form inside the oil droplets and migrate to the oil/water interface, resulting in spherical nanocapsules with thicker but looser shells, which are not easily distorted. Further decreasing the water-to-ethanol ratio leads to more organosilica oligomers formed in the oil droplets. Meanwhile, the migration of organosilica oligomers gets slower due to the increase of oil/water miscibility, so mesoporous particles (matrix type NanoPCMs) will be fabricated. The variation of capsule size at different water-to-ethanol ratios may be explained as follows. With increasing water-to-ethanol ratio, the relative dielectric constant of water/ethanol mixed solvent increases, resulting in stronger electrostatic repulsion
160
Y. Zhu et al. / Energy Conversion and Management 119 (2016) 151–162
Fig. 10. DSC curves of the NanoPCMs prepared at different NH3H2O concentrations: (a) P6, P7; (b) M6, M7.
Table 4 Phase change characteristics of the NanoPCMs prepared under different NH3H2O concentrations. Samples
P6 P7 M6 M7
Melting
Solidifying
R (%)
Tm (°C)
DHm (J/g)
Tc, a (°C)
Tc,
26.72 28.57 27.62 28.50
91.2 ± 16.4 109.7 ± 6.0 105.8 ± 21.1 116.7 ± 22.6
23.49 24.69 23.49 24.92
17.88 21.44 16.76 20.28
b
(°C)
Tc, c (°C)
DHc (J/g)
– – 4.62 –
77.7 ± 12.2 98.1 ± 12.7 100.3 ± 15.9 111.2 ± 25.3
44.6 53.7 51.8 57.1
Scheme 1. Morphology variation of the NanoPCMs by changing water-to-ethanol ratios, CTAB concentrations, and NH3H2O concentrations: (a) hemispherical nanocapsules; (b) bowl like nanocapsules; (c) thin-shelled spherical nanocapsules; (d) thick-shelled spherical nanocapsules; (e) mesoporous nanoparticles.
between the charged surfactants (CTAB) [45]. Consequently, the number of CTAB molecules on per surface area of oil droplets is lowered, so more oil droplets will be produced and smaller nanocapsule size will be achieved.
In terms of the morphology variation of the NanoPCMs at different CTAB and NH3H2O concentration, a tentative mechanism was also proposed from a viewpoint of kinetics. With increasing CTAB or NH3H2O concentration, the hydrolysis and condensation
Y. Zhu et al. / Energy Conversion and Management 119 (2016) 151–162
reaction of alkoxy silanes is accelerated [41], resulting in faster formation of organosilica oligomers, but the migration and selfassembly kinetics of organosilica oligomers remains unchanged. When the formation of organosilica oligomers gets too fast to match the self-assembly of organosilica oligomers at the oil/water interface, nanocapsules with thicker but looser shells will be produced. Moreover, compared to sample M1–M7, sample P1–P7 have a stronger tendency to form nonspherical morphologies, which may be attributed to that the c-methacryloxypropyl groups are much longer than the methyl groups and result in more flexible organosilica shells. 4. Conclusions In summary, organosilica nanoencapsulated n-octadecane with different morphologies and microstructures was obtained by adjusting various synthetic conditions, including water-toethanol ratio, CTAB concentration, and NH3H2O concentration. These NanoPCMs are shape-stabilized and possess high thermal conductivity. Higher water-to-ethanol ratio, lower CTAB concentration, or lower NH3H2O concentration generally lead to thinshelled nanocapsules. On the contrary, lower water-to-ethanol ratio, higher CTAB concentration, or higher NH3H2O concentration will result in thick-shelled nanocapsules or mesoporous particles which might contain n-octadecane in their matrices. It was found that supercooling behavior exists in the thin-shelled nanocapsules, but does not occur in the thick-shelled nanocapsules or mesoporous particles containing n-octadecane. This work can offer useful approaches for design and synthesis of micro/nano encapsulated PCMs with high thermal conductivity, high thermal energy storage capability, and low supercooling. Acknowledgements The financial support from National Natural Science Foundation of China (No. 51273183) is gratefully acknowledged. We thank Dr. Yu Liu, Dr. Wei Sang, Mr. Jinjiang Xu, Ms. Lin Wang and Ms. Tianli Liu for their kind help on characterizations. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.enconman.2016. 04.049. References [1] Sharma RK, Ganesan P, Tyagi VV, Metselaar HSC, Sandaran SC. Developments in organic solid–liquid phase change materials and their applications in thermal energy storage. Energy Convers Manage 2015;95:193–228. [2] Salunkhe PB, Shembekar PS. A review on effect of phase change material encapsulation on the thermal performance of a system. Renew Sustain Energy Rev 2012;16:5603–16. [3] Rathod MK, Banerjee J. Thermal stability of phase change materials used in latent heat energy storage systems: a review. Renew Sustain Energy Rev 2013;18:246–58. [4] Jamekhorshid A, Sadrameli SM, Farid M. A review of microencapsulation methods of phase change materials (PCMs) as a thermal energy storage (TES) medium. Renew Sustain Energy Rev 2014;31:531–42. [5] Alay S, Göde F, Alkan C. Preparation and characterization of poly (methylmethacrylate-coglycidyl methacrylate)/n-hexadecane nanocapsules as a fiber additive for thermal energy storage. Fiber Polym 2010;11:1089–93. [6] Barreneche C, de Gracia A, Serrano S, Elena Navarro M, Borreguero AM, Inés Fernández A, et al. Comparison of three different devices available in Spain to test thermal properties of building materials including phase change materials. Appl Energy 2013;109:421–7. [7] Borreguero AM, Rodríguez JF, Valverde JL, Peijs T, Carmona M. Characterization of rigid polyurethane foams containing microencapsulted phase change materials: microcapsules type effect. J Appl Polym Sci 2013;109:582–90.
161
[8] Fang Y, Yu H, Wan W, Gao X, Zhang Z. Preparation and thermal performance of polystyrene/n-tetradecane composite nanoencapsulated cold energy storage phase change materials. Energy Convers Manage 2013;76:430–6. [9] Jeong SG, Jeon J, Seo J, Lee JH, Kim S. Performance evaluation of the microencapsulated PCM for wood-based flooring application. Energy Convers Manage 2012;64:516–21. [10] Marchi S, Pagliolico S, Sassi G. Characterization of panels containing microencapsulated phase change materials. Energy Convers Manage 2013;74:261–8. [11] Zheng S, Xu Y, Shen Q, Yang H. Preparation of thermochromic coatings and their energy saving analysis. Sol Energy 2015;112:263–71. [12] Li W, Ma YJ, Tang XF, Jiang N, Zhang R, Han N, et al. Composition and characterization of thermoregulated fiber containing acrylic-based copolymer microencapsulated phase-change materials (MicroPCMs). Ind Eng Chem Res 2014;53:5413–20. [13] Sirohi S, Singh D, Nain R, Parida D, Agrawal AK, Jassal M. Electrospun composite nanofibres of PVA loaded with nanoencapsulated n-octadecane. RSC Adv 2015;5:34377–82. [14] Zhang GH, Bon SAF, Zhao CY. Synthesis, characterization and thermal properties of novel nanoencapsulated phase change materials for thermal energy storage. Sol Energy 2012;86:1149–54. [15] Tahan Latibari S, Mehrali M, Mehrali M, Indra Mahlia TM, Cornelis Metselaar HS. Synthesis, characterization and thermal properties of nanoencapsulated phase change materials via sol–gel method. Energy 2013;61:664–72. [16] Zhang X, Fan Y, Tao X, Yick K. Fabrication and properties of microcapsules and nanocapsules containing n-octadecane. Mater Chem Phys 2004;88:300–7. [17] Wu B, Zheng G, Chen X. Effect of graphene on the thermophysical properties of melamine-urea-formaldehyde/N-hexadecane microcapsules. RSC Adv 2015;5:74024–31. [18] Konuklu Y, Paksoy HO, Unal M, Konuklu S. Microencapsulation of a fatty acid with poly(melamine–urea–formaldehyde). Energy Convers Manage 2014;80:382–90. [19] Al-Shannaq R, Farid M, Al-Muhtaseb S, Kurdi J. Emulsion stability and crosslinking of PMMA microcapsules containing phase change materials. Sol Energy Mater Sol Cells 2015;132:311–8. [20] 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;23:3–14. [21] Miao C, Lü G, Yao Y, Tang G, Weng D, Qiu J, et al. Preparation of silica microcapsules containing octadecane as temperature-adjusting powder. Chem Lett 2007;36:494–5. [22] Zhang H, Sun S, Wang X, Wu D. Fabrication of microencapsulated phase change materials based on n-octadecane core and silica shell through interfacial polycondensation. Colloid Surf A 2011;389:104–17. [23] Zhang H, Wang X, Wu D. Silica encapsulation of n-octadecane via sol–gel process: a novel microencapsulated phase-change material with enhanced thermal conductivity and performance. J Colloid Interface Sci 2010;343:246–55. [24] He F, Wang X, Wu D. New approach for sol–gel synthesis of microencapsulated n-octadecane phase change material with silica wall using sodium silicate precursor. Energy 2014;67:223–33. [25] Chen Z, Cao L, Fang G, Shan F. Synthesis and characterization of microencapsulated paraffin microcapsules as shape-stabilized thermal energy storage materials. Nanoscale Microscale Thermophys Eng 2013;17:112–23. [26] Chen Z, Cao L, Shan F, Fang G. Preparation and characteristics of microencapsulated stearic acid as composite thermal energy storage material in buildings. Energy Build 2013;62:469–74. [27] Fang G, Chen Z, Li H. Synthesis and properties of microencapsulated paraffin composites with SiO2 shell as thermal energy storage materials. Chem Eng J 2010;163:154–9. [28] Wang W, Wang C, Li W, Fan X, Wu Z, Zheng J, et al. Novel phase change behavior of n-eicosane in nanoporous silica: emulsion template preparation and structure characterization using small angle X-ray scattering. Phys Chem Chem Phys 2013;15:14390–5. [29] Liang S, Li Q, Zhu Y, Chen K, Tian C, Wang J, et al. Nanoencapsulation of noctadecane phase change material with silica shell through interfacial hydrolysis and polycondensation in miniemulsion. Energy 2015;93:1684–92. [30] Pan L, Ji Q, Qin Y, Jiang Y, Zhang Z, Zhang S, et al. Diverting phase transition of high-melting-point stearic acid to room temperature by microencapsulation in boehmite. RSC Adv 2013;3:22326. [31] Pan L, Tao Q, Zhang S, Wang S, Zhang J, Wang S, et al. Preparation, characterization and thermal properties of micro-encapsulated phase change materials. Sol Energy Mater Sol Cells 2012;98:66–70. [32] Yu S, Wang X, Wu D. Microencapsulation of n-octadecane phase change material with calcium carbonate shell for enhancement of thermal conductivity and serving durability: synthesis, microstructure, and performance evaluation. Appl Energy 2014;114:632–43. [33] Yu S, Wang X, Wu D. Self-assembly synthesis of microencapsulatedn-eicosane phase-change materials with crystalline-phase-controllable calcium carbonate shell. Energy Fuel 2014;28:3519–29. [34] Cao L, Tang F, Fang G. Preparation and characteristics of microencapsulated palmitic acid with TiO2 shell as shape-stabilized thermal energy storage materials. Sol Energy Mater Sol Cells 2014;123:183–8. [35] Ciriminna R, Sciortino M, Alonzo G, Schrijver AD, Pagliaro M. From molecules to systems: sol gel microencapsulation in silica-based materials. Chem Rev 2010;111:765–89.
162
Y. Zhu et al. / Energy Conversion and Management 119 (2016) 151–162
[36] Li Y, Shi J. Hollow-structured mesoporous materials: chemical synthesis, functionalization and applications. Adv Mater 2014;26:3176–205. [37] Zhu Y, Liang S, Chen K, Gao X, Chang P, Tian C, et al. Preparation and properties of nanoencapsulated n-octadecane phase change material with organosilica shell for thermal energy storage. Energy Convers Manage 2015;105:908–17. [38] Xie B, Liu G, Jiang S, Zhao Y, Wang D. Crystallization behaviors of n-octadecane in confined space: crossover of rotator phase from transient to metastable induced by surface freezing. J Phys Chem B 2008;112:13310–5. [39] Zhang J, Wang S, Zhang S, Tao Q, Pan L, Wang Z, et al. In situ synthesis and phase change properties of Na2SO410H2O@SiO2 solid nanobowls toward smart heat storage. J Phys Chem C 2011;115:20061–6. [40] Su W, Darkwa J, Kokogiannakis G. Review of solid–liquid phase change materials and their encapsulation technologies. Renew Sustain Energy Rev 2015;48:373–91.
[41] Teng Z, Han Y, Li J, Yan F, Yang W. Preparation of hollow mesoporous silica spheres by a sol–gel/emulsion approach. Microporous Mesoporous Mater 2010;127:67–72. [42] Zhang H, Wu J, Zhou L, Zhang D, Qi L. Facile synthesis of monodisperse microspheres and gigantic hollow shells of mesoporous silica in mixed water– ethanol solvents. Langmuir 2007;23:1107–13. [43] Zhao CY, Zhang GH. Review on microencapsulated phase change materials (MEPCMs): fabrication, characterization and applications. Renew Sustain Energy Rev 2011;15:3813–32. [44] Cao F, Yang B. Supercooling suppression of microencapsulated phase change materials by optimizing shell composition and structure. Appl Energy 2014;113:1512–8. [45] Jiang Y, Xu S, Zhao Z, Zheng L, Wang Z, Wang C, et al. Water–ethanol solvent mixtures: a promising liquid environment for high quality positively-charged CdTe nanocrystal preparation. RSC Adv 2015;5:18379–83.
Contents lists available at ScienceDirect
Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman
Morphological control and thermal properties of nanoencapsulated n-octadecane phase change material with organosilica shell materials Yalin Zhu a,b, Shuen Liang a,c,⇑, Hui Wang a, Ke Zhang c, Xiaorong Jia a, Chunrong Tian a, Yuanlin Zhou b, Jianhua Wang a,⇑ a b c
Institute of Chemical Materials, China Academy of Engineering Physics (CAEP), Mianyang 621900, PR China College of Materials Science and Engineering, Southwest University of Science and Technology of China, Mianyang 621000, PR China CAS Key Laboratory of Soft Matter Chemistry, Department of Polymer Science and Engineering, University of Science and Technology of China, Hefei 230026, PR China
a r t i c l e
i n f o
Article history: Received 12 January 2016 Received in revised form 17 March 2016 Accepted 13 April 2016
Keywords: Phase change material n-Octadecane Nanoencapsulation Organosilica Morphology Supercooling
a b s t r a c t Morphological control was attempted on organosilica nanoencapsulated n-octadecane phase change material by adjusting various synthetic conditions, and the relationship between morphology and phase change property was investigated. The chemical structure and thermal stability of the nanocapsules were characterized by FT-IR spectroscopy and thermogravimetric analysis, respectively. The morphology and microstructure of the nanocapsules were observed by SEM and TEM, and the phase change property was determined by DSC and temperature-dependent XRD methods. With decreasing water-to-ethanol ratio, increasing cetyltrimethylammonium bromide (CTAB) concentration, or increasing NH3H2O concentration, the morphologies of the NanoPCMs can be regulated from thin-shelled nanocapsules with bowl like, hemispherical, or spherical geometries to thick-shelled spherical nanocapsules or mesoporous particles. Meanwhile, the average diameter of the nanocapsules also increases obviously. It was demonstrated that the phase change properties of these nanocapsules are intimately related to their morphologies: thicker organosilica shells induce heterogeneous nucleation better and result in less supercooling, compared with the thinner ones. The methods and mechanisms proposed herein might be helpful to prepare various micro/nano encapsulated phase change materials through interfacial hydrolysis– condensation method, and optimize their morphologies and thermal properties. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction Due to limited conventional fossil energy, growing population and increasing greenhouse gas emission, highly efficient utilization of energy as well as new energy storage materials become more and more important. Phase change materials (PCMs) have attracted much attention in the last decades because they can provide high energy storage density and near isothermal heat storage [1]. In order to prevent the leakage of PCMs in melt state, improve the heat storage/release efficiency, and control the volume changes during phase change process, micro/nano encapsulation of PCMs have been extensively explored [2–4]. As a result, they are applied in fields of solar energy storage, refrigeration system, energy efficient buildings, smart textiles and so on [5–13]. Compared to microencapsulated phase change materials (MicroPCMs), nanoencapsulated phase change materials (NanoPCMs) with much smaller ⇑ Corresponding author at: Institute of Chemical Materials, China Academy of Engineering Physics (CAEP), Mianyang 621900, PR China (S. Liang). E-mail addresses: [email protected] (S. Liang), [email protected] (J. Wang). http://dx.doi.org/10.1016/j.enconman.2016.04.049 0196-8904/Ó 2016 Elsevier Ltd. All rights reserved.
sizes and higher surface-area-to-volume ratios provide a stronger driving force to speed up thermodynamic processes [14]. In addition, when NanoPCMs are used in latent functionally thermal fluids (LFTF), they do not fracture easily in course of flow [15]. Micro/nano encapsulated PCMs typically consist of core materials (i.e., PCMs) and shell materials with core–shell structure. Traditionally, various organic polymeric materials are used as shell materials, such as melamine–formaldehyde (M–F) resin [16,17], urea–formaldehyde resin [18], polystyrene [8], and polymethylmethacrylate [19]. In recent years, there are growing interests on encapsulation of PCMs with inorganic shell materials, including silica [15,20–29], boehmite [30,31], CaCO3 [32,33], TiO2 [34] and so on, because they are flame retardant, free of poisonous gases release, more thermally stable and conductive, compared with polymeric materials. Especially, most attention has been paid to encapsulation of PCMs with silica shell materials, which are nontoxic, structurally stable and have well-defined surface properties [35,36]. However, the MicroPCMs with silica shell are often weak in mechanical properties and can be easily damaged [32,34]. Moreover, in many applications, encapsulated PCMs need
152
Y. Zhu et al. / Energy Conversion and Management 119 (2016) 151–162
to be dispersed in various polymeric matrices, to produce heat storage or thermoregulated composites [7,9,10,12]. Therefore, development of NanoPCMs with organically modified silica (organosilica) shell materials is highly desired. In previous work [37], we successfully nanoencapsulated n-octadecane with organosilica shells through co-hydrolysis and co-condensation of silane precursors in miniemulsion. By changing the volume ratio of different silane precursors, NanoPCMs with tunable chemical structure, thermal property, and hydrophobicity were obtained. The morphologies of micro/nano encapsulated PCMs are intimately related to their thermal properties. Zhang et al. [16] reported that capsule size has significant effects on the crystallizing behavior of microcapsules containing n-octadecane with M–F shell, and smaller capsule size results in larger super cooling. Wang’s group [23,24] investigated the influence of morphology on the phase change properties for microencapsulated n-octadecane with silica shell. Pan et al. [30,31] and Cao et al. [34] observed that nanoencapsulation of PCMs with AlOOH and TiO2 shell materials can lead to a remarkable decrease of phase change temperatures due to the confinement of capsule shells, which is known as Gibbs–Thomson effect. Generally, Gibbs–Thomson effect restricts and delays the crystallization of PCMs. On the contrary, it was also found that capsule shells can play an important role as nucleating sites for crystallization of encapsulated PCMs, which induces and promotes the crystallization process [29,38]. The interplay of Gibbs–Thomson effect and nucleation effect resulted from the capsule shells, and diverse morphologies of PCM capsules (spherical or non-spherical [39]; core–shell or matrix type [40]; compact or mesoporous [28]) make the morphology–thermal property relationship rather complicated, and currently the knowledge and understanding about this topic are not adequate and systematic. In order to acquire encapsulated PCMs with different morphologies, limited approaches were explored, mainly including variation of stirring speed [16], type and concentration of emulsifier [20,32,33], core/shell precursor ratio [23], pH value [15], etc. For micro/nano encapsulated PCMs with silica shell materials prepared through sol–gel (i.e., hydrolysis–condensation) process, it was disclosed that the pH value is critical to obtain compact and smooth shell [23,24] and different capsule sizes [15]. However, the influences of other synthetic conditions were rarely studied. Considering that the reaction system for encapsulating PCMs generally consists of multiple components, such as oil phase, water phase, emulsifier, and catalyst/initiator, morphological control may be realized by tuning various synthetic conditions during the preparation process. Therefore, it is very meaningful to investigate the influence of various synthetic conditions on the morphologies of the novel kind of nanoencapsulated PCMs with organosilica shell, and clarify their morphology–thermal property relationship.
In present study, we focused on the morphological control of nanoencapsulated n-octadecane with organosilica shell materials by changing synthetic conditions including water-to-ethanol ratio, emulsifier concentration, and NH3H2O concentrations. The mechanism for the variation of morphology was proposed, and the relationship between the morphology and phase change property of the NanoPCMs was clarified. This work provided new insights on morphological control and morphology–thermal property relationship of organosilica nanoencapsulated PCMs. The findings might be extended to prepare various micro/nano encapsulated PCMs through interfacial hydrolysis–condensation method and optimize their morphologies and thermal properties. 2. Materials and methods 2.1. Materials Tetraethyl orthosilicate (TEOS) (98%), anhydrous ethanol and ammonium hydroxide (NH3H2O, 25 wt%) were purchased from Sinopharm Chemical Reagents Company, China. Cetyltrimethylammonium bromide (CTAB) was provided by Tianjin Kermal Chemical Reagents Company, China. c-Methacryloxypropyltrimethoxysilane (MPS), methyltrimethoxysilane (MTMS) and n-octadecane (90 wt%) were purchased from Tianjin Alfa Aesar Company, China. All chemicals were of reagent quality and used as received without further purification. 2.2. Preparation of NanoPCMs with organosilica shell materials Nanoencapsulated n-octadecane with organosilica shell was prepared through interfacial co-hydrolysis and co-condensation of TEOS, MPS and MTMS in miniemulsion, as described in our previous study [37]. Typically, n-octadecane (1.0 g), MPS (or MTMS) (0.5 mL) and TEOS (1.0 mL) were mixed in a round bottomed flask (100 mL), to form a clear solution. Then, CTAB (0.328 g), deionized H2O (28.5 mL) and anhydrous ethanol (14.2 mL) were added into the flask in turn. The mixture was stirred at the rate of 1500 rpm for 30 min, and then sonicated (KQ-400KDB, 100% amplitude) for 10 min at 35 °C, to form a stable miniemulsion. Aqueous ammonia (25 wt%, 1.04 mL) was added to the flask to initiate the interfacial co-hydrolysis and co-condensation reaction of MPS (or MTMS) and TEOS. The reaction proceeded with gentle stirring (300 rpm) for 16 h. Finally, white powder-like products (around 1.60 g) were collected after filtration, washing with deionized water, and freeze drying. The mixture of MPS and TEOS was used to introduce c-methacryloxypropyl (CH2C(CH3)COOCH2CH2CH2-) groups into silica shell material, while the mixture of MTMS and TEOS was
Table 1 Synthetic conditions of the nanoencapsulated n-octadecane with organosilica shells. Samples
Silane precursors
Water-to-ethanol ratio (v/v)
CTAB concentration (mM)
NH3H2O concentration (wt%)
P1 P2 P3 P4 P5 P6 P7
MPS (0.5 mL) + TEOS (1.0 mL)
2.5/1 2/1 1.7/1 2/1 2/1 2/1 2/1
20 20 20 15 30 20 20
0.6 0.6 0.6 0.6 0.6 0.3 0.9
M1 M2 M3 M4 M5 M6 M7
MTMS (0.5 mL) + TEOS (1.0 mL)
2.5/1 2/1 1.7/1 2/1 2/1 2/1 2/1
20 20 20 15 35 20 20
0.6 0.6 0.6 0.6 0.6 0.3 0.9
Y. Zhu et al. / Energy Conversion and Management 119 (2016) 151–162
used to incorporate methyl groups into silica shell material. To obtain NanoPCMs with different morphologies, varied water-toethanol ratios (1.7/1–2.5/1), CTAB concentration (15–35 mM), and NH3H2O concentration (0.3–0.9 wt%) were adopted, as listed in Table 1.
153
testing temperature, the samples were equilibrated for 2 min before measurements. 3. Results and discussion 3.1. Chemical structure
2.3. Characterization The FT-IR spectra were obtained using a Nicolet 6700 IRspectrophotometer on the KBr sampling sheet with a scanning number of 32. Thermogravimetric analysis (TGA) was performed on a TA Instruments Q500 thermal gravimetric analyzer under a nitrogen atmosphere. The specimen with a mass of about 5 mg was placed in an aluminum crucible and then ramped from room temperature up to 550 °C at a heating rate of 10 °C/min. Thermal conductivity measurements were performed using an EKO HC074-200 thermal conductivity tester through a heat flux method at 25 °C. The morphologies of the synthesized nanocapsules were observed using a CamScan Apollo 300 field emission scanning electron microscope (SEM). The specimen was made electrically conductive by sputter coating with a thin layer of gold–palladium alloy. The micrographs were taken in high vacuum mode with 10 kV acceleration voltage and a medium spot size. The microstructures of the synthesized nanocapsules were determined by using a Hitachi H-800 transmission electron microscope (TEM) operated at an accelerating voltage of 100 kV. The specimen was dispersed in ethanol under sonication, and some pieces were collected on carbon-coated 300-mesh copper grids for TEM observation. High resolution TEM (HRTEM) characterization of the NanoPCMs was performed on a JEOL-2010 transmission electron microscope. DSC scans were performed using a TA Instruments Q2000 differential scanning calorimeter, and all measurements were carried out in a nitrogen atmosphere at a heating or cooling rate of 10 °C/min with the weight of the specimen being about 2 mg. For each NanoPCMs sample, DSC measurements were conducted on three individual specimens. Temperature-dependent XRD measurements were performed on a Bruker D8 Advance diffractometer (40 kV, 50 mA) with Cu Ka radiation (k = 0.154 nm), and the diffraction patterns were collected in the 2h ranging from 5° to 40° at a scanning rate of 10°/min. Samples were first heated from ambient temperature to 28 °C, and kept for 2 min, followed by cooling to 0 °C. The cooling rates were 6 °C/min, and at every
FT-IR spectra of the as-prepared NanoPCMs were presented in Fig. 1. For NanoPCMs with c-methacryloxypropyl functionalized silica shells (P1–P7), their FT-IR spectra are very similar. The absorption peaks at 2922, 2852, 1469 and 719 cm 1 are ascribed to core material (n-octadecane), while the peaks at 3427, 1718, 1635, 1082 and 463 cm 1 are attributed to shell material. In particular, the peaks at 1718 and 1635 cm 1 are characteristic for c-methacryloxypropyl groups. For NanoPCMs with methyl functionalized silica shells (M1–M7), their FT-IR spectra are also similar. The absorption peaks at 2922, 2852, 1469 and 719 cm 1 are ascribed to n-octadecane, while the peaks at 3427, 1270, 1082, and 463 cm 1 are attributed to organosilica shell material. The peak at 1270 cm 1 is characteristic for Si–CH3 groups [37]. These results demonstrated that synthetic conditions (water-to-ethanol ratio, emulsifier concentration, and NH3H2O concentrations) have minor influences on the chemical structure of NanoPCMs. 3.2. Thermal stability and thermal conductivity The thermal stability of the as-prepared NanoPCMs was first tested by heating them in an oven (50 °C). As shown in Fig. S1, the appearance of the NanoPCMs does not change after being heated for 15 min, proving that they are shape-stabilized PCMs and n-octadecane is encapsulated by shell materials indeed. On the contrary, under the same condition, pristine n-octadecane melts into clear liquid completely. The thermal stability of the NanoPCMs was further characterized by TGA method, as shown in Fig. 2. For all the NanoPCMs, n-octadecane evaporates during the temperature range of 100–250 °C, from which the contents of n-octadecane in the NanoPCMs can be estimated. For samples P1–P7, the second weight loss at around 419 °C is attributed to the decomposition of c-methacryloxypropyl functionalized silica shell materials [37]. It can be observed that the NanoPCMs prepared under different synthetic conditions exhibit similar thermal stability. However, the contents of n-octadecane in the NanoPCMs are different, displaying some dependence on the synthetic conditions.
Fig. 1. FT-IR spectra of the as-prepared NanoPCMs with organosilica shells: (a) P1–P7; (b) M1–M7.
154
Y. Zhu et al. / Energy Conversion and Management 119 (2016) 151–162
Fig. 2. TGA curves of the as-prepared NanoPCMs with organosilica shells: (a) P1–P7; (b) M1–M7.
Thermal conductivity measurements were performed on representative samples P2 and M2, and their thermal conductivities are as large as 0.575 and 0.414 W m 1 K 1, respectively. These values are quite higher than that of pristine n-octadecane (0.153 W m 1 K 1 [32]), and also comparable with that of NanoPCMs with unmodified silica shell (0.42–0.45 W m 1 K 1 [29]). Therefore, NanoPCMs with organosilica shell materials can provide not only advantageous surface hydrophobicity, but also high thermal conductivity, which are beneficial to accelerate thermal storage/release processes. 3.3. Effects of water-to-ethanol ratio on the morphology and phase change property of NanoPCMs In preparation of silica micro/nano particles through sol–gel process, water/alcohol mixed solvents are often used to enhance the regularity and homogeneity of the products [41,42]. However, the effects of water/alcohol mixed solvents on the morphologies and properties of micro/nano encapsulated PCMs were rarely studied. The morphology and microstructure of the NanoPCMs prepared at different water-to-ethanol ratios were characterized by SEM and TEM, and the images were presented in Fig. 3. Detailed data about the geometry, average diameter and shell thickness of the NanoPCMs were obtained from the SEM and TEM images, and summarized in Table S1. It can be found that, with decreasing water-to-ethanol ratio, the NanoPCMs transform from bowl like (P1, P2 and M1) to spherical (P3, M2 and M3) geometry, and the average diameter and shell thickness increase obviously. Thus, a morphology transition from bowl-like thin-shelled nanocapsules to spherical thick-shelled nanocapsules is evident. Particularly, in sample P3, the particles possess very thick shells, or even do not exhibit clear core–shell structure. In that case, n-octadecane is possibly embedded in the mesopores of organosilica material, called a matrix type of encapsulated PCMs [4,40,43]. It is known that silica capsules or particles obtained through hydrolysis– condensation of alkoxy silanes are usually amorphous and mesoporous [28,41,42]. In order to verify this point, we conducted HRTEM characterization on representative sample P2 and P3, and the results were shown is Fig. 4. It can be observed that the organosilica shells of the former are much denser than the latter case, so the encapsulation of n-octadecane in the mesoporous particles is highly feasible for sample P3. DSC curves of the NanoPCMs prepared at different water-toethanol ratios were shown in Fig. 5. There is only one endothermic peak during the melting process of these NanoPCMs, due to the
phase transition from triclinic crystal to liquid. However, 2–3 exothermic peaks are observed during the solidifying process, and labeled as a, b, and c peaks from high to low temperature, respectively. According to references [38,44], the peaks a and b are ascribed to the liquid–rotator and rotator–triclinic phase transitions based on heterogeneous nucleation, and peak c is attributed to direct liquid–triclinic phase transition based on homogeneous nucleation, respectively. The results revealed that sample P2, P3, M2 and M3 crystallize fully based on heterogeneous nucleation, while sample P1 and M1 crystallize based on both heterogeneous and homogeneous nucleation. The reason may be that nucleation sites (impurities) may be lacking in the small [16] and thinshelled NanoPCMs, and n-octadecane partially undergoes direct liquid–triclinic phase transition at a large supercooling. To further confirm the crystallizing behavior of the NanoPCMs prepared at different water-to-ethanol ratios, temperature dependent XRD measurements were carried out on representative sample M1 and M2, and the results were shown in Figs. 6 and S2. For sample M1 with thin organosilica shell, when temperature is 22 °C or higher, there is only a broad diffraction band (15–30°) on the XRD pattern (Fig. 6a), suggesting that n-octadecane core material is in isotropic liquid state. As temperature decreases to 20 °C, characteristic diffraction peaks of rotator phase are observed at 2h = 21.1° and 22.9°. At 18 °C, the diffractive intensities of the rotator phase weaken, meanwhile characteristic diffraction peaks of triclinic phase emerge, demonstrating the coexistence of both crystal phases. At 6 °C, rotator phase almost disappears, and only triclinic phase exists. We assigned the intensity of diffraction peak at 21.1° as a representation of the degree of crystallinity for rotator phase, and that at 23.4° as a representation of the degree of crystallinity for triclinic phase. Consequently, the intensities at 21.1° and 23.4° as functions of temperature were shown in Fig. 6b, from which the phase transition temperatures can be determined. At around 20 °C, the degree of crystallinity for rotator phase increases rapidly, indicating the liquid–rotator phase transition. Then, at around 12 °C, the degree of crystallinity for rotator phase declines, meanwhile that for triclinic phase increases promptly, demonstrating the rotator–triclinic phase transition. Finally, at around 6 °C, the degree of crystallinity for triclinic phase increases further, but that for rotator phase keeps constant, confirming the occurrence of direct liquid–triclinic phase transition. Similarly, for sample M2, the liquid–rotator phase transition occurs at around 24 °C, the rotator–triclinic phase transition occurs at around 21 °C, and the liquid–triclinic phase transition does not exist (Fig. S2). It is worth noting that the phase transition temperatures determined by the temperature-dependent XRD method only roughly agree
Y. Zhu et al. / Energy Conversion and Management 119 (2016) 151–162
155
Fig. 3. (a–c, g–i) SEM and (d–f, j–l) TEM images of the NanoPCMs prepared at different water-to-ethanol ratios: (a, d) P1; (b, e) P2; (c, f) P3; (g, j) M1; (h, k) M2; (i, l) M3. Inserts of (a) and (g) are images at higher magnification.
Fig. 4. HRTEM images of the NanoPCMs prepared at different water-to-ethanol ratios: (a) P2; (b) P3.
156
Y. Zhu et al. / Energy Conversion and Management 119 (2016) 151–162
Fig. 5. DSC curves of the NanoPCMs prepared at different water-to-ethanol ratios: (a) P1–P3; (b) M1–M3.
Fig. 6. Temperature-dependent XRD patterns of sample M1 during cooling process: (a) diffraction intensity as a function of diffraction angle (2h) at various temperatures; (b) intensities of diffraction peaks at 21.1° and 23.4° as functions of temperature. L: liquid phase. R: rotator crystal phase. T: triclinic crystal phase.
Table 2 Phase change characteristics of n-octadecane and the NanoPCMs prepared under different water-to-ethanol ratios. Samples
n-Octadecane P1 P2 P3 M1 M2 M3 a b c d
Melting
R (%)c
Solidifying
Tm (°C)
DHm (J/g)a
Tc, a (°C)
Tc,
28.52 29.36 27.92 28.58 26.27 28.19 27.40
204.4 ± 11.9 102.6 ± 9.9 109.4 ± 10.4 96.3 ± 7.4 96.7 ± 7.4 109.9 ± 16.1 107.2 ± 6.2
23.92 25.15 24.94 24.76 20.82 24.57 24.01
–d 21.53 20.55 22.84 11.48 19.44 22.26
b
(°C)
Tc, c (°C)
DHc (J/g)b
– 6.68 – – 5.46 – –
202.4 ± 13.9 100.0 ± 8.9 99.4 ± 15.4 96.2 ± 7.9 83.4 ± 8.2 103.4 ± 19.6 104.0 ± 7.7
– 50.2 53.5 47.1 47.3 53.8 52.4
Melting enthalpy with 95% confidence interval. Crystallizing enthalpy with 95% confidence interval. Encapsulation Ratio (R) = DHm, NanoPCMs/DHm, n-octadecane 100%. Not exists.
with that obtained by the DSC method. The deviations are possibly due to the different temperature controlling style of the two characterization methods [38]. The phase change temperatures, phase change enthalpies, and encapsulation ratios of the NanoPCMs prepared at different water-to-ethanol ratios were summarized in Table 2. The melting and crystallizing enthalpies are similar and relatively high (around
100 J/g). It should be mentioned that the phase change enthalpy or encapsulation ratio cannot be reliably estimated from the diameter and shell thickness of the nanocapsules, because the interior of the nanocapsules may not be completely filled with PCMs, as observed by Wang et al. [20], and the organosilica shell materials are mesoporous (Fig. 4). Moreover, the encapsulation ratios obtained from DSC results are lower than the n-octadecane contents estimated
Y. Zhu et al. / Energy Conversion and Management 119 (2016) 151–162
157
Fig. 7. (a, b, e, f) SEM and (c, d, g, h) TEM images of the NanoPCMs prepared at different CTAB concentrations: (a, c) P4; (b, d) P5; (e, g) M4; (f, h) M5.
by TGA method. The reason may be that some water is absorbed by the NanoPCMs and evaporates in the same temperature range with the evaporation of n-octadecane. 3.4. Effects of CTAB concentrations on the morphologies and phase change properties of NanoPCMs CTAB plays important roles in the miniemulsion system, as both emulsifiers and templates for deposition of organosilica oligomers. SEM and TEM images of the NanoPCMs prepared at different CTAB concentrations were shown in Fig. 7. Detailed information about the geometry, average diameter and shell thickness of the NanoPCMs prepared at different CTAB concentration were listed in Table S1. At low CTAB concentration (15 mM), bowl like (P4)
or spherical (M4) nanocapsules with well-defined core–shell structure are obtained, and the shell thickness is relatively low. However, at higher CTAB concentration (30 or 35 mM), spherical nanocapsules with thicker shells are acquired. In addition, the average diameter of the NanoPCMs also increases with increasing CTAB concentration, which seems contradictory to the common knowledge that higher surfactant concentration leads to smaller dispersed droplets or colloidal particles, but the mechanism is currently not clear. From HRTEM images (Fig. S3) of sample M4 and M5, it is evident that higher CTAB concentration results in thicker but looser capsule shells. Similar phenomenon was also reported by Cao and Yang [44] for microencapsulated n-octadecane with M–F resin shell when using sodium dodecyl sulfate (SDS) as an emulsifier at high concentration.
158
Y. Zhu et al. / Energy Conversion and Management 119 (2016) 151–162
Fig. 8. DSC curves of the NanoPCMs prepared at different CTAB concentrations: (a) P4, P5; (b) M4, M5.
Table 3 Phase change characteristics of the NanoPCMs prepared under different CTAB concentrations. Samples
P4 P5 M4 M5
Melting
Solidifying
R (%)
Tm (°C)
DHm (J/g)
Tc, a (°C)
Tc,
28.44 27.76 27.14 28.83
113.3 ± 16.1 75.4 ± 24.3 105.6 ± 9.4 121.0 ± 6.0
23.76 25.82 23.80 24.19
20.26 20.61 17.78 20.73
DSC curves of the NanoPCMs prepared at different CTAB concentrations were shown in Fig. 8. Based on the DSC curves, the phase change temperatures, phase change enthalpies, and encapsulation ratios were calculated and summarized in Table 3. These NanoPCMs exhibit mainly two exothermic peaks (a, b) during the solidifying processes, showing that they crystallize via shell induced heterogeneous nucleation. However, it is also observed that, with increasing CTAB concentration, the crystallizing peaks a and b become narrower, suggesting that thicker nanocapsule shell induces the heterogeneous nucleation better. In addition, the phase change enthalpies of P5 are lower than that of P4, due to reduced stability of the emulsion system at 30 mM, which was observed experimentally. On the contrary, the phase change enthalpies of M5 are higher than that of M4, at higher CTAB concentration (35 mM). These results demonstrated that optimal synthetic conditions for nanoencapsulated n-octadecane by using different silane precursors are not uniform, possibly ascribed to their different chemical or physical nature. 3.5. Effects of NH3H2O concentration on the morphologies and phase change properties of NanoPCMs It is an important approach to control the morphologies and sizes of micro/nano encapsulated PCMs with silica shell materials by tuning the pH value (i.e., acid/base concentration) in the sol– gel system, because the hydrolysis/condensation kinetics of silane precursors is greatly influenced by acid/base as catalyst [20,23,24]. SEM and TEM micrographs of the NanoPCMs prepared at different NH3H2O concentrations were presented in Fig. 9. The geometry, average diameter and shell thickness of the NanoPCMs prepared at different NH3H2O concentration were also summarized in Table S1. With increasing NH3H2O concentration, the geometries of the NanoPCMs transform from bowl like (sample P6) or hemispherical (sample M6) nanocapsules to regular
b
(°C)
Tc, c (°C)
DHc (J/g)
– – – –
108.8 ± 16.6 74.1 ± 17.4 95.0 ± 11.9 115.7 ± 8.4
55.4 36.9 51.7 59.2
spherical nanocapsules (samples P7 and M7). All the NanoPCMs exhibit well-defined core–shell structure, and the shell thickness increases obviously with the increase of NH3H2O concentration. In addition, the average diameter of the NanoPCMs also increases with increasing NH3H2O concentration, which is consistent with the results reported by Tahan Latibari et al. [15]. DSC curves of the NanoPCMs prepared at different NH3H2O concentrations were presented in Fig. 10. Based on the DSC curves, the phase change temperatures, phase change enthalpies, and encapsulation ratios were calculated and summarized in Table 4. For sample P6 and P7, peaks a and b are predominant during the solidifying process, whereas peak c is very weak. However, from sample P6 to P7, crystallizing peaks a and b become narrower. For samples M6 and M7, three and two exothermic peaks are observed during the solidifying process, respectively. These results confirm that thicker organosilica shell induces the heterogeneous nucleation of n-octadecane better. In addition, the phase change enthalpies of the NanoPCMs increase obviously with increasing NH3H2O concentration from 0.3 to 0.9 wt%, which is indicative that relative thicker shell is preferred for obtaining higher encapsulation ratio. 3.6. Mechanism for morphology variation of NanoPCMs In summary, similar morphology variation of NanoPCMs is observed by changing water-to-ethanol ratio, CTAB concentration, or NH3H2O concentration, as illustrated in Scheme 1. At high water-to-ethanol ratio, or low CTAB and NH3H2O concentration, nonspherical nanocapsules with clear core–shell structure are obtained, and the shells are very thin (Scheme 1a–c). With decreasing water-to-ethanol ratio, or increasing CTAB and NH3H2O concentration, spherical nanocapsules with well-defined core– shell structure are also acquired, but the shells get thicker (Scheme 1d). Finally, at low water-to-ethanol ratio, or high CTAB
Y. Zhu et al. / Energy Conversion and Management 119 (2016) 151–162
159
Fig. 9. (a, b, e, f) SEM and (c, d, g, h) TEM images of the NanoPCMs prepared at different NH3H2O concentrations: (a, c) P6; (b, d) P7; (e, g) M6; (f, h) M7.
and NH3H2O concentration, thick-shelled nanocapsules or mesoporous particles tend to be produced (Scheme 1e). A tentative mechanism is proposed for the formation of NanoPCMs with varied morphologies at different water-toethanol ratios. At a relatively high water-to-ethanol ratio, the polarity difference between water phase and oil phase is high, so the oil/water miscibility is very poor. As a result, the hydrolysis and condensation reaction is confined at the oil/water interface, and the formed organosilica oligomers self-assemble therein immediately to produce a thin and dense organosilica shell. Because of the pressure difference between inside and outside of the nanocapsules, thin-shelled organosilica nanocapsules are easily deformed from spherical to nonspherical geometries. With decreasing water-to-ethanol ratio, the oil/water miscibility is
enhanced, causing that the hydrolysis and condensation reaction occur both at the oil/water interface and inside the oil droplet. Therefore, a portion of organosilica oligomers form inside the oil droplets and migrate to the oil/water interface, resulting in spherical nanocapsules with thicker but looser shells, which are not easily distorted. Further decreasing the water-to-ethanol ratio leads to more organosilica oligomers formed in the oil droplets. Meanwhile, the migration of organosilica oligomers gets slower due to the increase of oil/water miscibility, so mesoporous particles (matrix type NanoPCMs) will be fabricated. The variation of capsule size at different water-to-ethanol ratios may be explained as follows. With increasing water-to-ethanol ratio, the relative dielectric constant of water/ethanol mixed solvent increases, resulting in stronger electrostatic repulsion
160
Y. Zhu et al. / Energy Conversion and Management 119 (2016) 151–162
Fig. 10. DSC curves of the NanoPCMs prepared at different NH3H2O concentrations: (a) P6, P7; (b) M6, M7.
Table 4 Phase change characteristics of the NanoPCMs prepared under different NH3H2O concentrations. Samples
P6 P7 M6 M7
Melting
Solidifying
R (%)
Tm (°C)
DHm (J/g)
Tc, a (°C)
Tc,
26.72 28.57 27.62 28.50
91.2 ± 16.4 109.7 ± 6.0 105.8 ± 21.1 116.7 ± 22.6
23.49 24.69 23.49 24.92
17.88 21.44 16.76 20.28
b
(°C)
Tc, c (°C)
DHc (J/g)
– – 4.62 –
77.7 ± 12.2 98.1 ± 12.7 100.3 ± 15.9 111.2 ± 25.3
44.6 53.7 51.8 57.1
Scheme 1. Morphology variation of the NanoPCMs by changing water-to-ethanol ratios, CTAB concentrations, and NH3H2O concentrations: (a) hemispherical nanocapsules; (b) bowl like nanocapsules; (c) thin-shelled spherical nanocapsules; (d) thick-shelled spherical nanocapsules; (e) mesoporous nanoparticles.
between the charged surfactants (CTAB) [45]. Consequently, the number of CTAB molecules on per surface area of oil droplets is lowered, so more oil droplets will be produced and smaller nanocapsule size will be achieved.
In terms of the morphology variation of the NanoPCMs at different CTAB and NH3H2O concentration, a tentative mechanism was also proposed from a viewpoint of kinetics. With increasing CTAB or NH3H2O concentration, the hydrolysis and condensation
Y. Zhu et al. / Energy Conversion and Management 119 (2016) 151–162
reaction of alkoxy silanes is accelerated [41], resulting in faster formation of organosilica oligomers, but the migration and selfassembly kinetics of organosilica oligomers remains unchanged. When the formation of organosilica oligomers gets too fast to match the self-assembly of organosilica oligomers at the oil/water interface, nanocapsules with thicker but looser shells will be produced. Moreover, compared to sample M1–M7, sample P1–P7 have a stronger tendency to form nonspherical morphologies, which may be attributed to that the c-methacryloxypropyl groups are much longer than the methyl groups and result in more flexible organosilica shells. 4. Conclusions In summary, organosilica nanoencapsulated n-octadecane with different morphologies and microstructures was obtained by adjusting various synthetic conditions, including water-toethanol ratio, CTAB concentration, and NH3H2O concentration. These NanoPCMs are shape-stabilized and possess high thermal conductivity. Higher water-to-ethanol ratio, lower CTAB concentration, or lower NH3H2O concentration generally lead to thinshelled nanocapsules. On the contrary, lower water-to-ethanol ratio, higher CTAB concentration, or higher NH3H2O concentration will result in thick-shelled nanocapsules or mesoporous particles which might contain n-octadecane in their matrices. It was found that supercooling behavior exists in the thin-shelled nanocapsules, but does not occur in the thick-shelled nanocapsules or mesoporous particles containing n-octadecane. This work can offer useful approaches for design and synthesis of micro/nano encapsulated PCMs with high thermal conductivity, high thermal energy storage capability, and low supercooling. Acknowledgements The financial support from National Natural Science Foundation of China (No. 51273183) is gratefully acknowledged. We thank Dr. Yu Liu, Dr. Wei Sang, Mr. Jinjiang Xu, Ms. Lin Wang and Ms. Tianli Liu for their kind help on characterizations. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.enconman.2016. 04.049. References [1] Sharma RK, Ganesan P, Tyagi VV, Metselaar HSC, Sandaran SC. Developments in organic solid–liquid phase change materials and their applications in thermal energy storage. Energy Convers Manage 2015;95:193–228. [2] Salunkhe PB, Shembekar PS. A review on effect of phase change material encapsulation on the thermal performance of a system. Renew Sustain Energy Rev 2012;16:5603–16. [3] Rathod MK, Banerjee J. Thermal stability of phase change materials used in latent heat energy storage systems: a review. Renew Sustain Energy Rev 2013;18:246–58. [4] Jamekhorshid A, Sadrameli SM, Farid M. A review of microencapsulation methods of phase change materials (PCMs) as a thermal energy storage (TES) medium. Renew Sustain Energy Rev 2014;31:531–42. [5] Alay S, Göde F, Alkan C. Preparation and characterization of poly (methylmethacrylate-coglycidyl methacrylate)/n-hexadecane nanocapsules as a fiber additive for thermal energy storage. Fiber Polym 2010;11:1089–93. [6] Barreneche C, de Gracia A, Serrano S, Elena Navarro M, Borreguero AM, Inés Fernández A, et al. Comparison of three different devices available in Spain to test thermal properties of building materials including phase change materials. Appl Energy 2013;109:421–7. [7] Borreguero AM, Rodríguez JF, Valverde JL, Peijs T, Carmona M. Characterization of rigid polyurethane foams containing microencapsulted phase change materials: microcapsules type effect. J Appl Polym Sci 2013;109:582–90.
161
[8] Fang Y, Yu H, Wan W, Gao X, Zhang Z. Preparation and thermal performance of polystyrene/n-tetradecane composite nanoencapsulated cold energy storage phase change materials. Energy Convers Manage 2013;76:430–6. [9] Jeong SG, Jeon J, Seo J, Lee JH, Kim S. Performance evaluation of the microencapsulated PCM for wood-based flooring application. Energy Convers Manage 2012;64:516–21. [10] Marchi S, Pagliolico S, Sassi G. Characterization of panels containing microencapsulated phase change materials. Energy Convers Manage 2013;74:261–8. [11] Zheng S, Xu Y, Shen Q, Yang H. Preparation of thermochromic coatings and their energy saving analysis. Sol Energy 2015;112:263–71. [12] Li W, Ma YJ, Tang XF, Jiang N, Zhang R, Han N, et al. Composition and characterization of thermoregulated fiber containing acrylic-based copolymer microencapsulated phase-change materials (MicroPCMs). Ind Eng Chem Res 2014;53:5413–20. [13] Sirohi S, Singh D, Nain R, Parida D, Agrawal AK, Jassal M. Electrospun composite nanofibres of PVA loaded with nanoencapsulated n-octadecane. RSC Adv 2015;5:34377–82. [14] Zhang GH, Bon SAF, Zhao CY. Synthesis, characterization and thermal properties of novel nanoencapsulated phase change materials for thermal energy storage. Sol Energy 2012;86:1149–54. [15] Tahan Latibari S, Mehrali M, Mehrali M, Indra Mahlia TM, Cornelis Metselaar HS. Synthesis, characterization and thermal properties of nanoencapsulated phase change materials via sol–gel method. Energy 2013;61:664–72. [16] Zhang X, Fan Y, Tao X, Yick K. Fabrication and properties of microcapsules and nanocapsules containing n-octadecane. Mater Chem Phys 2004;88:300–7. [17] Wu B, Zheng G, Chen X. Effect of graphene on the thermophysical properties of melamine-urea-formaldehyde/N-hexadecane microcapsules. RSC Adv 2015;5:74024–31. [18] Konuklu Y, Paksoy HO, Unal M, Konuklu S. Microencapsulation of a fatty acid with poly(melamine–urea–formaldehyde). Energy Convers Manage 2014;80:382–90. [19] Al-Shannaq R, Farid M, Al-Muhtaseb S, Kurdi J. Emulsion stability and crosslinking of PMMA microcapsules containing phase change materials. Sol Energy Mater Sol Cells 2015;132:311–8. [20] 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;23:3–14. [21] Miao C, Lü G, Yao Y, Tang G, Weng D, Qiu J, et al. Preparation of silica microcapsules containing octadecane as temperature-adjusting powder. Chem Lett 2007;36:494–5. [22] Zhang H, Sun S, Wang X, Wu D. Fabrication of microencapsulated phase change materials based on n-octadecane core and silica shell through interfacial polycondensation. Colloid Surf A 2011;389:104–17. [23] Zhang H, Wang X, Wu D. Silica encapsulation of n-octadecane via sol–gel process: a novel microencapsulated phase-change material with enhanced thermal conductivity and performance. J Colloid Interface Sci 2010;343:246–55. [24] He F, Wang X, Wu D. New approach for sol–gel synthesis of microencapsulated n-octadecane phase change material with silica wall using sodium silicate precursor. Energy 2014;67:223–33. [25] Chen Z, Cao L, Fang G, Shan F. Synthesis and characterization of microencapsulated paraffin microcapsules as shape-stabilized thermal energy storage materials. Nanoscale Microscale Thermophys Eng 2013;17:112–23. [26] Chen Z, Cao L, Shan F, Fang G. Preparation and characteristics of microencapsulated stearic acid as composite thermal energy storage material in buildings. Energy Build 2013;62:469–74. [27] Fang G, Chen Z, Li H. Synthesis and properties of microencapsulated paraffin composites with SiO2 shell as thermal energy storage materials. Chem Eng J 2010;163:154–9. [28] Wang W, Wang C, Li W, Fan X, Wu Z, Zheng J, et al. Novel phase change behavior of n-eicosane in nanoporous silica: emulsion template preparation and structure characterization using small angle X-ray scattering. Phys Chem Chem Phys 2013;15:14390–5. [29] Liang S, Li Q, Zhu Y, Chen K, Tian C, Wang J, et al. Nanoencapsulation of noctadecane phase change material with silica shell through interfacial hydrolysis and polycondensation in miniemulsion. Energy 2015;93:1684–92. [30] Pan L, Ji Q, Qin Y, Jiang Y, Zhang Z, Zhang S, et al. Diverting phase transition of high-melting-point stearic acid to room temperature by microencapsulation in boehmite. RSC Adv 2013;3:22326. [31] Pan L, Tao Q, Zhang S, Wang S, Zhang J, Wang S, et al. Preparation, characterization and thermal properties of micro-encapsulated phase change materials. Sol Energy Mater Sol Cells 2012;98:66–70. [32] Yu S, Wang X, Wu D. Microencapsulation of n-octadecane phase change material with calcium carbonate shell for enhancement of thermal conductivity and serving durability: synthesis, microstructure, and performance evaluation. Appl Energy 2014;114:632–43. [33] Yu S, Wang X, Wu D. Self-assembly synthesis of microencapsulatedn-eicosane phase-change materials with crystalline-phase-controllable calcium carbonate shell. Energy Fuel 2014;28:3519–29. [34] Cao L, Tang F, Fang G. Preparation and characteristics of microencapsulated palmitic acid with TiO2 shell as shape-stabilized thermal energy storage materials. Sol Energy Mater Sol Cells 2014;123:183–8. [35] Ciriminna R, Sciortino M, Alonzo G, Schrijver AD, Pagliaro M. From molecules to systems: sol gel microencapsulation in silica-based materials. Chem Rev 2010;111:765–89.
162
Y. Zhu et al. / Energy Conversion and Management 119 (2016) 151–162
[36] Li Y, Shi J. Hollow-structured mesoporous materials: chemical synthesis, functionalization and applications. Adv Mater 2014;26:3176–205. [37] Zhu Y, Liang S, Chen K, Gao X, Chang P, Tian C, et al. Preparation and properties of nanoencapsulated n-octadecane phase change material with organosilica shell for thermal energy storage. Energy Convers Manage 2015;105:908–17. [38] Xie B, Liu G, Jiang S, Zhao Y, Wang D. Crystallization behaviors of n-octadecane in confined space: crossover of rotator phase from transient to metastable induced by surface freezing. J Phys Chem B 2008;112:13310–5. [39] Zhang J, Wang S, Zhang S, Tao Q, Pan L, Wang Z, et al. In situ synthesis and phase change properties of Na2SO410H2O@SiO2 solid nanobowls toward smart heat storage. J Phys Chem C 2011;115:20061–6. [40] Su W, Darkwa J, Kokogiannakis G. Review of solid–liquid phase change materials and their encapsulation technologies. Renew Sustain Energy Rev 2015;48:373–91.
[41] Teng Z, Han Y, Li J, Yan F, Yang W. Preparation of hollow mesoporous silica spheres by a sol–gel/emulsion approach. Microporous Mesoporous Mater 2010;127:67–72. [42] Zhang H, Wu J, Zhou L, Zhang D, Qi L. Facile synthesis of monodisperse microspheres and gigantic hollow shells of mesoporous silica in mixed water– ethanol solvents. Langmuir 2007;23:1107–13. [43] Zhao CY, Zhang GH. Review on microencapsulated phase change materials (MEPCMs): fabrication, characterization and applications. Renew Sustain Energy Rev 2011;15:3813–32. [44] Cao F, Yang B. Supercooling suppression of microencapsulated phase change materials by optimizing shell composition and structure. Appl Energy 2014;113:1512–8. [45] Jiang Y, Xu S, Zhao Z, Zheng L, Wang Z, Wang C, et al. Water–ethanol solvent mixtures: a promising liquid environment for high quality positively-charged CdTe nanocrystal preparation. RSC Adv 2015;5:18379–83.