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Enhancement in thermal property of phase change microcapsules with modified silicon nitride for solar energy
Solar Energy Materials & Solar Cells 151 (2016) 89–95
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Solar Energy Materials & Solar Cells journal homepage: www.elsevier.com/locate/solmat
Enhancement in thermal property of phase change microcapsules with modified silicon nitride for solar energy Yanyang Yang a,b, Jie Kuang a,b, Hao Wang a, Guolin Song a,n, Yuan Liu a, Guoyi Tang a,b,nn a b
Institute of Advanced Materials, Graduate School at Shenzhen, Tsinghua University, Shenzhen 518055, China Key Laboratory of Advanced Materials, School of Materials Science and Engineering, Tsinghua University, Haidian District, Beijing 100084, China
art ic l e i nf o
a b s t r a c t
Article history: Received 23 July 2015 Received in revised form 22 February 2016 Accepted 28 February 2016
Thermal behavior is one of the most important properties for phase change microcapsules in solar energy storage. Here, a new type of phase change microcapsules was synthesized based on n-octadecane core and polymethylmethacrylate shell supplemented with modified silicon nitride powders, aiming to achieve improvement of thermal property in the phase change materials. SEM micrographs showed that the as-prepared microcapsules have a regular spherical shape with a well-defined core-shell structure. FTIR curves and EDS spectrogram demonstrated that silicon nitride can be well cross-linked with microcapsules after surface modification. In addition, TGA, forward looking infra-red system and DSC (before and after 500 heating and cooling cycles) analyses were performed to investigate the thermal property of the as-prepared microcapsules. The results indicated that the microcapsules have high thermal storage capability, enhanced thermal reliability and stability, and increased thermal conductivity. Especially, the thermal conductivity of microcapsules is enhanced by 56.8% compared with that of the microcapsules without the addition of silicon nitride. & 2016 Elsevier B.V. All rights reserved.
Keywords: Phase change materials Silicon nitride Microcapsule N-octadecane Energy storage
1. Introduction With the rapidly increasing energy consumption and excessive exploitation of fossil fuels in the past several decades, energy crisis has become one of the most crucial problems worldwide. Therefore, great efforts have been made to develop new and sustainable energy. Solar energy is considered as one of the most promising green energy for large-scale application; however, the uneven distribution in space and time has severely restricted its practical application. One solution to the problem is the development of phase change materials (PCMs), which are capable of absorbing/ releasing a large amount of energy at a defined range of temperatures [1–5]. To prevent erosion and leakage of PCMs, many researches have been focused on the development and use of phase change microcapsules (micro-PCMs), which are composed of a shell of macromolecules and a core of phase change materials [6–10]. Micro-PCMs can prevent interior phase change materials from leaking as well as increase the heat transfer area and promote crystallization [11,12]. n
Corresponding author. Corresponding author at: Graduate School at Shenzhen, Tsinghua University, Shenzhen 518055, China. Tel.: þ 86 75526036752; fax: þ 86 75526036752. E-mail addresses: [email protected] (G. Song), [email protected] (G. Tang). nn
http://dx.doi.org/10.1016/j.solmat.2016.02.020 0927-0248/& 2016 Elsevier B.V. All rights reserved.
Polymer is one of the most popular materials for preparing micro-PCMs. Due to its appropriate plasticity, the polymeric shell is able to tolerate the volume changes caused by the phase transformation and thus can safely isolate the PCMs from the environment [13–15]. Excellent thermal properties have been achieved by different researchers using various shell materials. For instance, Ma et al. [16] prepared polyuria/polyurethane microcapsules with butyl stearate and paraffin as binary core via interfacial polymerization method, and the content of binary core in the micro-PCMs reached 45–60%. Zhang et al. [17] successfully prepared stearic acid/polycarbonate(SA/PC) microcapsules. The melting temperature, freezing temperature and latent heat of the microcapsule were reported to be 60 °C, 51.2 °C and 91.4 J g 1, respectively. Chen et al. [18] synthesized polyurea microcapsules using the method of interfacial polycondensation. The latent heat of fusion was about 80 J g 1, and the phase change temperature and enthalpy of encapsulated butyl stearate kept nearly constant over 400 heating and cooling cycles. The above researches undoubtedly mark the great progress in the field of PCMs. However, although excellent thermal properties have been achieved, the poisonous formaldehyde released by the shell materials, which is hazardous to environment and health, greatly hinders the further application of the technique. In an attempt to overcome such drawbacks, Qiu et al. [19], Sari et al. [20] and Huang et al. [21] attempted to employ a non-toxic and commercially available
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acrylic resin (polymethylmethacrylate, PMMA) as the shell material, which is proved to be an effective material. Thermal conductivity is another important property of microPCMs [22–25]. Many researches have been focused on the preparation of microcapsules with high thermal conductivity. Zhang et al. [26] prepared microcapsules based on n-eicosane core and ZrO2 shell with the dual-functional characteristics of thermal energy and photoluminescence. Li et al. [27] used carbon nanotubes grafted with stearyl alcohol (CNTs-SA) in synthesizing phase change microcapsules to enhance the thermal conductivities of the microcapsules. The thermal conductivity of microPCMs/CNTs-SA with 4% of CNTs was increased by 79.2% compared with that of the original microPCMs. Jiang et al. [28] synthesized phase change microcapsules based on paraffin wax core and poly(methyl methacrylate-co-methyl acrylate) shell with nanotubes alumina (nano-Al2O3) inlay. The optimized percentage of nano-Al2O3 introduced into the microcapsules is 16% of the monomer mass, which results in a phase change temperature of 23.75 °C, an enthalpy of 93.41 Jg 1 and a thermal conductivity of 0.3104 W m 1K 1. In our previous work, we successfully developed the phase change microcapsules supplemented with modified silicon nitride, which showed high thermal performance [29]. As an important ceramic powder [30–32], silicon nitride has various advantages such as economy, stable abradability, non-oxidizability, thermal vibration resistance and high thermal conductivity. It has been demonstrated to be an efficient supplementary material for thermal performance improvement of phase change microcapsules. However, nano-silicon nitride is easily attached to each other and forms a large cluster. In order to separate these nano-particles, a new desiccation method was proposed in this study. The thermal behavior of the microcapsules prepared with such silicon nitride nano-particles was systematically evaluated using DSC, TGA, forward looking infra-red system and heating-cooling cyclic oven.
2. Materials and experimental
anhydride polymer (Shanghai Leather Chemical Works) was used as dispersant. 2, 2-azobisisobutyronitrile (AIBN, 98 wt%, Shanghai Jingchun Chemical Co., Ltd.) was employed as initiator. All chemicals were used as received without further purification. 2.2. Synthesis 2.2.1. Surface modification of silicon nitride In order to eliminate the negative interfacial effect between inorganic and organic matters, pretreatment was conducted for silicon nitride prior to polymerization. The surface modification process is as follows: 10 g silicon nitride was dissolved in 10 g KH570 with ultrasonic vibration for 15 min. Meanwhile, 3 g deionized water was first dissolved in 50 g C2H5OH and then oxalic acid was added to adjust the pH value to a range between 4.5 and 5.5. The mixture was then poured into a flask bathed in 80 °C water and stirred at 1000 r min 1 for 2 h. Finally, the modified silicon nitride was obtained by freeze drying after filtering and washing with deionized water for several times. 2.2.2. Synthesis of micro-PCMs/silicon nitride Fig. 1 is the schematic illustration of synthetic strategy for the microcapsule with modified silicon nitride. The method of preparing phase change microcapsule with modified silicon nitride was as same as that described in the previous research [29]. Recipes for various types of polymerization monomers are given in Table 1. 7 g sodium salt of styrene-maleic anhydride copolymer was dissolved in 100 g distilled water for 15 min under vigorous agitation of 1000 rpm. Meanwhile, 3 g modified silicon nitride and 7 g Methylmethacrylate, 3 g Pentaerythritol tetraacrylate and 10 g n-octadecane were ultrasonically dispersed separately for 15 min. Afterwards, all the materials prepared were mixed into a 250 mL three-neck round bottomed flask and kept in a 35 °C water bath for 15 min under vigorous agitation of 1000 rpm to form a stable oil-in-water emulsion. After the addition of 2, 2-azobisisobutyronitrile, the flask was placed in 80 °C water bath and subjected to a moderate agitation of 540 rpm for 5 h. Finally, the obtained microcapsules were purified by centrifugation and dried in an oven at 45 °C for 24 h.
2.1. Materials 2.3. Characterization Silicon nitride was provided by Tianjin Nitride Advanced Materials co., Ltd, China. Methacryloxy propyl trimethoxyl silane (Silane coupling agent KH-570) was purchased from Sinopharm Chemical Reagent Co., Ltd. Methylmathacrylate (MMA, A.R.,) was supplied by Tianjin Damao Chemistry Regent Co., Ltd. as a monomer of shell material. N-Octadecane(99 wt%, Alfalfa) was used as the core material. Pentaerythritol triacrylate (PETRA, 80 wt%) was provided by Nanjing Shoulashou Co., Ltd. and was used as crosslinking agents. Sodium salt of styrene-maleic
Field emission scanning electron microscope (FESEM, S4800, HITACHI) was used to observe the surface morphology of the samples. The chemical structures of the modified silicon nitride and the phase change microcapsules were identified using fourier transformed infrared spectrophotometer (FTIR, VERTEX 70, BRUKER). Differential scanning calorimeter (DSC, 823E METTLER TOLEDO) was used to measure the thermal properties of the materials at the temperatures
Fig. 1. Schematic of synthetic strategy for microcapsules with modified silicon nitride.
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Table 1 Recipes for various types of polymerization monomers. Sample
n-octadecane (g)
MMA (g)
PETRA (g)
TA (g)
AIBN (g)
Modified Si3N4/Mass percentage (g)
Micro-PCMs Sample 1 Sample 2 Sample 3 Sample 4 Sample 5
10 10 10 10 10 10
7 7 7 7 7 7
3 3 3 3 3 3
7 7 7 7 7 7
0.3 0.3 0.3 0.3 0.3 0.3
0/0% 0.5/5% 1/10% 1.5/15% 2/20% 3/30%
Fig. 2. SEM micrographs of the silicon nitride and microcapsule samples: a, silicon nitride; b, sample 1; c, sample 2; d sample 3; e, sample 4; f, sample 5.
ranging from 0–50 °C in argon atmosphere with a heating/cooling rate of 5 °C min 1 and a flow rate of argon atmosphere of 60 mL min 1. The thermal stability of the microcapsules was characterized by a thermal gravimetric analyzer (TGA, TGA/DSC1 METTLER TOLEDO) at the temperatures ranging from 0–500 °C in argon atmosphere with a heating rate of 10 °C min 1. In order to investigate the heat dissipation and thermal characteristics, composite discs with a thickness of 3 mm and a diameter of 20 mm were constructed using the microcapsules and epoxide resins. The thermal behavior of the disc was then studied using forward looking infra-red system (FLIR SC6000-series, FLIR Systems
Inc.) at a constant temperature of 60 °C. The test of thermal vibration resistance was conducted in a heating-cooling cyclic oven (BPH060A, Bluepard Experimental Equipment Co. Ltd., China) in which the micro-PCMs were subjected to 500 cycles of alternative heating and cooling in the temperature range of 0–50 °C. The heating and cooling rates were both 1 °C min 1. Each sample was kept for 5 min at 0 °C or 50 °C. Thermal conductivity was measured at room temperature by a thermal conductivity measuring apparatus (αPhase, China). For each testing point, at least five measurement were conducted and the average value was calculated and recorded.
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Fig. 3. Particle size distributions of silicon nitride and microcapusles: a, silicon nitride; b, sample 1; c, sample 2; d, sample 3; e, sample 4; f, sample 5.
3. Results and discussion 3.1. Morphology of the microcapsules SEM micrographs and particle size distribution of silicon nitride and the as-prepared microcapsules are presented in Figs. 2 and 3, respectively. It can be seen that the diameter of silicon nitride particles is approximately 100 nm. The phase change microcapsules, however, show a regular spherical shape with much smaller diameters ranging from 4 to 11 μm. The surface of the microcapsules is coarse with many pimples. Chemical composition of the microcapsules was determined by EDS. The spectra of sample 5 (Fig. 4) display peaks at 0.277, 0.392, 0.523, 1.740 keV, which correspond to the characteristic binding energy of C, N, O, Si respectively, suggesting the existence of these elements. The atomic ratio of Si to N was determined as 0.544:1, which is close to the ideal stoichiometric ratio of silicon nitride and therefore indicates that silicon nitride is the part of major components for the shell.
Fig. 4. EDS spectra of the microcapsules of sample 5.
3.2. Chemical characterization The FTIR spectra of silicon nitride, n-octadecane, PMMA and microcapsule samples are presented in Fig. 5. Peaks at 1045 cm 1,
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944 cm 1, 574 cm 1, 439 cm 1 and 377 cm 1 can be observed in the spectra of all microcapsule samples. By comparison with the standard spectra given in Fig. 5a [33], it could be found that these peaks correspond to silicon nitride, indicating the successful polymerization of the silicon nitride into the microcapsules. Besides, there are still some small peaks at 2925 cm 1 and 2857 cm 1, which might be the radical stretching vibration bands of methyl and methylene and should originate from the silane coupling agent KH570.
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3.3. Thermal properties of the microcapsules The DSC curves of the phase change microcapsules are shown in Fig. 6. It can be seen that the latent heat values of the microcapsules with different additions of silicon nitride are 159.36 J g 1, 153.92 J g 1, 134.97 J g 1, 119.95 J g 1 and 110.6 J g 1, respectively (Table 2). Obviously, with the increase of silicon nitride addition, the latent heat value of the microcapsules decreases. Furthermore, the addition of silicon nitride results in the shift of the exothermal peaks to the high temperature side during cooling and the shift of endothermal peaks towards the low temperature side while heating, implying a reduction in the supercooling and thus an improvement of cycling efficiency of the microcapsules. The influence of silicon nitride on the capability in storing thermal energy (represented by latent heat) is shown in Fig. 7. The temperature of D-values, which indicates the degree of overcooling/over-heating between exothermal and endothermal peak temperatures, was decreased by the addition of silicon nitride. The degree of supercooling was reduced to 1 °C when 3 g silicon nitride was added, which might be related to the increase of thermal conductivity in the microcapsules resulted from the addition of silicon nitride, 3.4. Thermal stability of the microcapsules
Fig. 5. FTIR spectra of the modified silicon nitride, n-octadecane, PMMA and microcapsule samples: a, modified silicon nitride; b, n-octadecane; c, PMMA; d, sample 1; e, sample 2; f, sample 3; g, sample 4; h, sample 5.
TGA curves of n-octadecane and microcapsules samples are shown in Fig. 8. It is obvious that for the microcapsules, the weight losing process could be divided into two stage, while for noctadecane only one stage is observed. The n-octadecane starts to lose its weight at 160 °C. When the temperature is increased to 270 °C, there is almost no substance left. This phenomenon could arise from the long chain alkene, which has a low decomposition temperature. For the microcapsules, the decomposition starts at 230 °C, which could be the result of the leakage of noctadecane from the microcapsules and its further decomposition. The weight decrease loses its momentum and the weight stays almost constant in the temperature span from 270 °C to 370 °C, indicating the general decomposition of n-octadecane. When the temperature reaches 370 °C, however, the weight of the microcapsules again starts to decrease dramatically, which could be explained by the decomposition of PMMA. When the temperature is raised up to 490 °C, the weight of the samples approaches to a constant value. The weight of the residue depends on the addition of silicon nitride. 3.5. Thermal cycling stabilization The thermal stability of the samples was also measured. After 500 heating and cooling cycles, DSC curves are shown in Fig. 9 and summarized in Table 3. It seems that the melting temperature and freezing temperature are not much influenced by the heating and
Fig. 6. DSC curves of microcapsules.
Table 2 Thermal properties of the microcapsules. Samples
ΔHc (Jg 1)
Toc (°C)
Tpc (°C)
Tec (°C)
ΔHm (Jg 1)
Tom (°C)
Tpm (°C)
Tem (°C)
Micro-PCMs Sample 1 Sample 2 Sample 3 Sample 4 Sample 5 n-ocatadecane
153.68 159.36 153.92 134.97 119.95 110.60 185.90
20.56 21.41 20.95 20.49 20.99 20.92 21.34
13.20 14.63 14.98 14.72 15.63 15.91 19.79
5.57 7.47 8.54 5.14 9.37 6.93 16.07
132.05 158.87 147.90 133.20 119.39 114.35 180.54
19.81 20.15 20.54 19.95 21.36 21.02 22.28
24.92 25.61 25.72 24.95 25.82 25.48 26.94
31.74 31.72 32.37 32.40 32.37 30.88 29.29
Note: Tom, Onset temperature on DSC heating curve; Tpm, Peak temperature on DSC heating curve; Tem, Endset temperature on DSC heating curve; ΔHm, Enthalpy on DSC heating curve; Toc, Onset temperature on DSC cooling curve; Tpc, Peak temperature on DSC cooling curve; Tec, Endset temperature on DSC cooling curve; ΔHc, Enthalpy on DSC cooling curve.
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cooling cycle. Regarding the heat storability, the as-prepared sample 1, 2, 3, 4 and 5 show latent heat values of 159.21 J g 1, 146.95 J g 1, 134.9 J g 1, 113.57 J g 1 and 108.1 J g 1, respectively. After 500 heat
and cooling cycles, the values, instead of dropping drastically, only decrease slightly by no more than 5%, suggesting good thermal stability of the materials prepared in the present study. 3.6. Thermal performance of the microcapsule composites
Fig. 7. DSC data analysis of microcapsule samples: 1, PCMs; 2, Sample 1; 3, Sample 2; 4, Sample 3; 5, Sample 4; 6, Sample 5; 7, n-octadecane.
Thermal behavior is one of the most important properties of phase change microcapsules for in application. Forward looking infra-red system was employed to detect the temperature change of the microcapsules at a constant environment temperature (Fig. 10). It is obvious that silicon nitride has the highest heating rate in all the samples: it only takes 89 s to reach 65 °C. The micro-PCMs without silicon nitride, however, exhibit the lowest heating rate in all the samples: it reaches only 40 °C at 89 s. For the microcapsules modified by silicon nitride, the heating rate is positively correlated with the amount of silicon nitride added, indicating the effect of silicon nitride on improving the thermal conductivity of the microcapsules. The thermal conductivity of the microcapsules is shown in Table 4. It can be found that the addition of silicon nitride particles results in the increase of thermal conductivity. As the amount of silicon nitride was raised up to 10 g, the thermal conductivity was increased by 56.8% and reached a value of 0.3630 W m 1 K 1, which is comparable with that reported by Xiang Jiang et al. [28]. In their work, a good thermal conductivity (0.3816 W m 1 K 1) was obtained in the microcapsules through adding Al2O3. The difference
Fig. 8. TGA curves of n-octadecane and microcapsule samples.
Fig. 10. Phase change microcapsule composite materials.
Table 4 Average thermal conductivities. Samples Sample Sample Sample Sample Sample Fig. 9. DSC curves after 500 heating and cooling cycles.
Thermal conductivity (W m 1 K 1)
1 0.23157 0.0037 2 0.2753 7 0.0026 3 0.29187 0.0017 4 0.29977 0.0051 5 0.3630 7 0.0021
Table 3 Thermal properties of the microcapsules after 500 cycles. Samples
ΔHc (Jg 1)
Toc (°C)
Tpc (°C)
Tec (°C)
ΔHm (Jg 1)
Tom (°C)
Tpm (°C)
Tem (°C)
Sample Sample Sample Sample Sample
159.21 146.95 134.90 113.57 108.10
20.82 21.53 20.73 21.34 22.72
15.11 15.31 14.76 14.95 16.29
9.16 8.56 5.78 8.19 5.53
153.21 146.20 131.11 113.16 107.08
20.68 22.05 19.06 21.66 20.47
26.00 26.67 24.17 26.26 24.86
31.80 34.97 30.72 34.02 31.61
1 2 3 4 5
Note: Tom, Onset temperature on DSC heating curve; Tpm, Peak temperature on DSC heating curve; Tem, Endset temperature on DSC heating curve; ΔHm, Enthalpy on DSC heating curve; Toc, Onset temperature on DSC cooling curve; Tpc, Peak temperature on DSC cooling curve; Tec, Endset temperature on DSC cooling curve; ΔHc, Enthalpy on DSC cooling curve.
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between their work and our study lies in the variation of latent heat. While in their work the increase of thermal conductivity resulted in drastic decrease of the latent heat, in this work we show that the impact of thermal conductivity on latent heat is negligible. Admittedly, the increase of thermal conductivity in the present research is not as pronounced as that resulted from the use of carbon materials (0.536 W m 1 K 1 in ref. [34], 0.463 W m 1 K 1 in ref. [35], 8.28 Wm 1 K 1 in ref. [36]), however, it is still more effective than the addition of other materials, such as Fe3O4 [37]. Considering its economic availability, silicon nitride is therefore believed to be a promising material to be added for improving thermal conductivity in phase change materials.
4. Conclusions We designed phase change microcapsules with high thermal performance based on n-octadecane core and PMMA shell cross-linked with modified silicon nitride. SEM micrographs show a well-defined core-shell structure and regular spherical morphology of the microcapsules. FTIR curves and EDS spectrogram indicate that silicon nitride is well cross-linked with the microcapsules. The results obtained from TGA, forward looking infra-red system and DSC indicate that the microcapsules with silicon nitride have a high thermal storage capability, enhanced thermal reliability and stability, and increased thermal conductivity. Additionally, our results suggest a new way of synthesizing phase change microcapsules with high thermal performance at low cost, which thus is beneficial to promote the application of phase change microcapsules in large scale in the future.
Acknowledgements The authors gratefully acknowledge the financial supports from Basic Research Project in Shenzhen (No. JCYJ20140417115840245) and International S&T cooperation program of Shenzhen (No. GJHZ20150316160614843).
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Contents lists available at ScienceDirect
Solar Energy Materials & Solar Cells journal homepage: www.elsevier.com/locate/solmat
Enhancement in thermal property of phase change microcapsules with modified silicon nitride for solar energy Yanyang Yang a,b, Jie Kuang a,b, Hao Wang a, Guolin Song a,n, Yuan Liu a, Guoyi Tang a,b,nn a b
Institute of Advanced Materials, Graduate School at Shenzhen, Tsinghua University, Shenzhen 518055, China Key Laboratory of Advanced Materials, School of Materials Science and Engineering, Tsinghua University, Haidian District, Beijing 100084, China
art ic l e i nf o
a b s t r a c t
Article history: Received 23 July 2015 Received in revised form 22 February 2016 Accepted 28 February 2016
Thermal behavior is one of the most important properties for phase change microcapsules in solar energy storage. Here, a new type of phase change microcapsules was synthesized based on n-octadecane core and polymethylmethacrylate shell supplemented with modified silicon nitride powders, aiming to achieve improvement of thermal property in the phase change materials. SEM micrographs showed that the as-prepared microcapsules have a regular spherical shape with a well-defined core-shell structure. FTIR curves and EDS spectrogram demonstrated that silicon nitride can be well cross-linked with microcapsules after surface modification. In addition, TGA, forward looking infra-red system and DSC (before and after 500 heating and cooling cycles) analyses were performed to investigate the thermal property of the as-prepared microcapsules. The results indicated that the microcapsules have high thermal storage capability, enhanced thermal reliability and stability, and increased thermal conductivity. Especially, the thermal conductivity of microcapsules is enhanced by 56.8% compared with that of the microcapsules without the addition of silicon nitride. & 2016 Elsevier B.V. All rights reserved.
Keywords: Phase change materials Silicon nitride Microcapsule N-octadecane Energy storage
1. Introduction With the rapidly increasing energy consumption and excessive exploitation of fossil fuels in the past several decades, energy crisis has become one of the most crucial problems worldwide. Therefore, great efforts have been made to develop new and sustainable energy. Solar energy is considered as one of the most promising green energy for large-scale application; however, the uneven distribution in space and time has severely restricted its practical application. One solution to the problem is the development of phase change materials (PCMs), which are capable of absorbing/ releasing a large amount of energy at a defined range of temperatures [1–5]. To prevent erosion and leakage of PCMs, many researches have been focused on the development and use of phase change microcapsules (micro-PCMs), which are composed of a shell of macromolecules and a core of phase change materials [6–10]. Micro-PCMs can prevent interior phase change materials from leaking as well as increase the heat transfer area and promote crystallization [11,12]. n
Corresponding author. Corresponding author at: Graduate School at Shenzhen, Tsinghua University, Shenzhen 518055, China. Tel.: þ 86 75526036752; fax: þ 86 75526036752. E-mail addresses: [email protected] (G. Song), [email protected] (G. Tang). nn
http://dx.doi.org/10.1016/j.solmat.2016.02.020 0927-0248/& 2016 Elsevier B.V. All rights reserved.
Polymer is one of the most popular materials for preparing micro-PCMs. Due to its appropriate plasticity, the polymeric shell is able to tolerate the volume changes caused by the phase transformation and thus can safely isolate the PCMs from the environment [13–15]. Excellent thermal properties have been achieved by different researchers using various shell materials. For instance, Ma et al. [16] prepared polyuria/polyurethane microcapsules with butyl stearate and paraffin as binary core via interfacial polymerization method, and the content of binary core in the micro-PCMs reached 45–60%. Zhang et al. [17] successfully prepared stearic acid/polycarbonate(SA/PC) microcapsules. The melting temperature, freezing temperature and latent heat of the microcapsule were reported to be 60 °C, 51.2 °C and 91.4 J g 1, respectively. Chen et al. [18] synthesized polyurea microcapsules using the method of interfacial polycondensation. The latent heat of fusion was about 80 J g 1, and the phase change temperature and enthalpy of encapsulated butyl stearate kept nearly constant over 400 heating and cooling cycles. The above researches undoubtedly mark the great progress in the field of PCMs. However, although excellent thermal properties have been achieved, the poisonous formaldehyde released by the shell materials, which is hazardous to environment and health, greatly hinders the further application of the technique. In an attempt to overcome such drawbacks, Qiu et al. [19], Sari et al. [20] and Huang et al. [21] attempted to employ a non-toxic and commercially available
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acrylic resin (polymethylmethacrylate, PMMA) as the shell material, which is proved to be an effective material. Thermal conductivity is another important property of microPCMs [22–25]. Many researches have been focused on the preparation of microcapsules with high thermal conductivity. Zhang et al. [26] prepared microcapsules based on n-eicosane core and ZrO2 shell with the dual-functional characteristics of thermal energy and photoluminescence. Li et al. [27] used carbon nanotubes grafted with stearyl alcohol (CNTs-SA) in synthesizing phase change microcapsules to enhance the thermal conductivities of the microcapsules. The thermal conductivity of microPCMs/CNTs-SA with 4% of CNTs was increased by 79.2% compared with that of the original microPCMs. Jiang et al. [28] synthesized phase change microcapsules based on paraffin wax core and poly(methyl methacrylate-co-methyl acrylate) shell with nanotubes alumina (nano-Al2O3) inlay. The optimized percentage of nano-Al2O3 introduced into the microcapsules is 16% of the monomer mass, which results in a phase change temperature of 23.75 °C, an enthalpy of 93.41 Jg 1 and a thermal conductivity of 0.3104 W m 1K 1. In our previous work, we successfully developed the phase change microcapsules supplemented with modified silicon nitride, which showed high thermal performance [29]. As an important ceramic powder [30–32], silicon nitride has various advantages such as economy, stable abradability, non-oxidizability, thermal vibration resistance and high thermal conductivity. It has been demonstrated to be an efficient supplementary material for thermal performance improvement of phase change microcapsules. However, nano-silicon nitride is easily attached to each other and forms a large cluster. In order to separate these nano-particles, a new desiccation method was proposed in this study. The thermal behavior of the microcapsules prepared with such silicon nitride nano-particles was systematically evaluated using DSC, TGA, forward looking infra-red system and heating-cooling cyclic oven.
2. Materials and experimental
anhydride polymer (Shanghai Leather Chemical Works) was used as dispersant. 2, 2-azobisisobutyronitrile (AIBN, 98 wt%, Shanghai Jingchun Chemical Co., Ltd.) was employed as initiator. All chemicals were used as received without further purification. 2.2. Synthesis 2.2.1. Surface modification of silicon nitride In order to eliminate the negative interfacial effect between inorganic and organic matters, pretreatment was conducted for silicon nitride prior to polymerization. The surface modification process is as follows: 10 g silicon nitride was dissolved in 10 g KH570 with ultrasonic vibration for 15 min. Meanwhile, 3 g deionized water was first dissolved in 50 g C2H5OH and then oxalic acid was added to adjust the pH value to a range between 4.5 and 5.5. The mixture was then poured into a flask bathed in 80 °C water and stirred at 1000 r min 1 for 2 h. Finally, the modified silicon nitride was obtained by freeze drying after filtering and washing with deionized water for several times. 2.2.2. Synthesis of micro-PCMs/silicon nitride Fig. 1 is the schematic illustration of synthetic strategy for the microcapsule with modified silicon nitride. The method of preparing phase change microcapsule with modified silicon nitride was as same as that described in the previous research [29]. Recipes for various types of polymerization monomers are given in Table 1. 7 g sodium salt of styrene-maleic anhydride copolymer was dissolved in 100 g distilled water for 15 min under vigorous agitation of 1000 rpm. Meanwhile, 3 g modified silicon nitride and 7 g Methylmethacrylate, 3 g Pentaerythritol tetraacrylate and 10 g n-octadecane were ultrasonically dispersed separately for 15 min. Afterwards, all the materials prepared were mixed into a 250 mL three-neck round bottomed flask and kept in a 35 °C water bath for 15 min under vigorous agitation of 1000 rpm to form a stable oil-in-water emulsion. After the addition of 2, 2-azobisisobutyronitrile, the flask was placed in 80 °C water bath and subjected to a moderate agitation of 540 rpm for 5 h. Finally, the obtained microcapsules were purified by centrifugation and dried in an oven at 45 °C for 24 h.
2.1. Materials 2.3. Characterization Silicon nitride was provided by Tianjin Nitride Advanced Materials co., Ltd, China. Methacryloxy propyl trimethoxyl silane (Silane coupling agent KH-570) was purchased from Sinopharm Chemical Reagent Co., Ltd. Methylmathacrylate (MMA, A.R.,) was supplied by Tianjin Damao Chemistry Regent Co., Ltd. as a monomer of shell material. N-Octadecane(99 wt%, Alfalfa) was used as the core material. Pentaerythritol triacrylate (PETRA, 80 wt%) was provided by Nanjing Shoulashou Co., Ltd. and was used as crosslinking agents. Sodium salt of styrene-maleic
Field emission scanning electron microscope (FESEM, S4800, HITACHI) was used to observe the surface morphology of the samples. The chemical structures of the modified silicon nitride and the phase change microcapsules were identified using fourier transformed infrared spectrophotometer (FTIR, VERTEX 70, BRUKER). Differential scanning calorimeter (DSC, 823E METTLER TOLEDO) was used to measure the thermal properties of the materials at the temperatures
Fig. 1. Schematic of synthetic strategy for microcapsules with modified silicon nitride.
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Table 1 Recipes for various types of polymerization monomers. Sample
n-octadecane (g)
MMA (g)
PETRA (g)
TA (g)
AIBN (g)
Modified Si3N4/Mass percentage (g)
Micro-PCMs Sample 1 Sample 2 Sample 3 Sample 4 Sample 5
10 10 10 10 10 10
7 7 7 7 7 7
3 3 3 3 3 3
7 7 7 7 7 7
0.3 0.3 0.3 0.3 0.3 0.3
0/0% 0.5/5% 1/10% 1.5/15% 2/20% 3/30%
Fig. 2. SEM micrographs of the silicon nitride and microcapsule samples: a, silicon nitride; b, sample 1; c, sample 2; d sample 3; e, sample 4; f, sample 5.
ranging from 0–50 °C in argon atmosphere with a heating/cooling rate of 5 °C min 1 and a flow rate of argon atmosphere of 60 mL min 1. The thermal stability of the microcapsules was characterized by a thermal gravimetric analyzer (TGA, TGA/DSC1 METTLER TOLEDO) at the temperatures ranging from 0–500 °C in argon atmosphere with a heating rate of 10 °C min 1. In order to investigate the heat dissipation and thermal characteristics, composite discs with a thickness of 3 mm and a diameter of 20 mm were constructed using the microcapsules and epoxide resins. The thermal behavior of the disc was then studied using forward looking infra-red system (FLIR SC6000-series, FLIR Systems
Inc.) at a constant temperature of 60 °C. The test of thermal vibration resistance was conducted in a heating-cooling cyclic oven (BPH060A, Bluepard Experimental Equipment Co. Ltd., China) in which the micro-PCMs were subjected to 500 cycles of alternative heating and cooling in the temperature range of 0–50 °C. The heating and cooling rates were both 1 °C min 1. Each sample was kept for 5 min at 0 °C or 50 °C. Thermal conductivity was measured at room temperature by a thermal conductivity measuring apparatus (αPhase, China). For each testing point, at least five measurement were conducted and the average value was calculated and recorded.
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Fig. 3. Particle size distributions of silicon nitride and microcapusles: a, silicon nitride; b, sample 1; c, sample 2; d, sample 3; e, sample 4; f, sample 5.
3. Results and discussion 3.1. Morphology of the microcapsules SEM micrographs and particle size distribution of silicon nitride and the as-prepared microcapsules are presented in Figs. 2 and 3, respectively. It can be seen that the diameter of silicon nitride particles is approximately 100 nm. The phase change microcapsules, however, show a regular spherical shape with much smaller diameters ranging from 4 to 11 μm. The surface of the microcapsules is coarse with many pimples. Chemical composition of the microcapsules was determined by EDS. The spectra of sample 5 (Fig. 4) display peaks at 0.277, 0.392, 0.523, 1.740 keV, which correspond to the characteristic binding energy of C, N, O, Si respectively, suggesting the existence of these elements. The atomic ratio of Si to N was determined as 0.544:1, which is close to the ideal stoichiometric ratio of silicon nitride and therefore indicates that silicon nitride is the part of major components for the shell.
Fig. 4. EDS spectra of the microcapsules of sample 5.
3.2. Chemical characterization The FTIR spectra of silicon nitride, n-octadecane, PMMA and microcapsule samples are presented in Fig. 5. Peaks at 1045 cm 1,
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944 cm 1, 574 cm 1, 439 cm 1 and 377 cm 1 can be observed in the spectra of all microcapsule samples. By comparison with the standard spectra given in Fig. 5a [33], it could be found that these peaks correspond to silicon nitride, indicating the successful polymerization of the silicon nitride into the microcapsules. Besides, there are still some small peaks at 2925 cm 1 and 2857 cm 1, which might be the radical stretching vibration bands of methyl and methylene and should originate from the silane coupling agent KH570.
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3.3. Thermal properties of the microcapsules The DSC curves of the phase change microcapsules are shown in Fig. 6. It can be seen that the latent heat values of the microcapsules with different additions of silicon nitride are 159.36 J g 1, 153.92 J g 1, 134.97 J g 1, 119.95 J g 1 and 110.6 J g 1, respectively (Table 2). Obviously, with the increase of silicon nitride addition, the latent heat value of the microcapsules decreases. Furthermore, the addition of silicon nitride results in the shift of the exothermal peaks to the high temperature side during cooling and the shift of endothermal peaks towards the low temperature side while heating, implying a reduction in the supercooling and thus an improvement of cycling efficiency of the microcapsules. The influence of silicon nitride on the capability in storing thermal energy (represented by latent heat) is shown in Fig. 7. The temperature of D-values, which indicates the degree of overcooling/over-heating between exothermal and endothermal peak temperatures, was decreased by the addition of silicon nitride. The degree of supercooling was reduced to 1 °C when 3 g silicon nitride was added, which might be related to the increase of thermal conductivity in the microcapsules resulted from the addition of silicon nitride, 3.4. Thermal stability of the microcapsules
Fig. 5. FTIR spectra of the modified silicon nitride, n-octadecane, PMMA and microcapsule samples: a, modified silicon nitride; b, n-octadecane; c, PMMA; d, sample 1; e, sample 2; f, sample 3; g, sample 4; h, sample 5.
TGA curves of n-octadecane and microcapsules samples are shown in Fig. 8. It is obvious that for the microcapsules, the weight losing process could be divided into two stage, while for noctadecane only one stage is observed. The n-octadecane starts to lose its weight at 160 °C. When the temperature is increased to 270 °C, there is almost no substance left. This phenomenon could arise from the long chain alkene, which has a low decomposition temperature. For the microcapsules, the decomposition starts at 230 °C, which could be the result of the leakage of noctadecane from the microcapsules and its further decomposition. The weight decrease loses its momentum and the weight stays almost constant in the temperature span from 270 °C to 370 °C, indicating the general decomposition of n-octadecane. When the temperature reaches 370 °C, however, the weight of the microcapsules again starts to decrease dramatically, which could be explained by the decomposition of PMMA. When the temperature is raised up to 490 °C, the weight of the samples approaches to a constant value. The weight of the residue depends on the addition of silicon nitride. 3.5. Thermal cycling stabilization The thermal stability of the samples was also measured. After 500 heating and cooling cycles, DSC curves are shown in Fig. 9 and summarized in Table 3. It seems that the melting temperature and freezing temperature are not much influenced by the heating and
Fig. 6. DSC curves of microcapsules.
Table 2 Thermal properties of the microcapsules. Samples
ΔHc (Jg 1)
Toc (°C)
Tpc (°C)
Tec (°C)
ΔHm (Jg 1)
Tom (°C)
Tpm (°C)
Tem (°C)
Micro-PCMs Sample 1 Sample 2 Sample 3 Sample 4 Sample 5 n-ocatadecane
153.68 159.36 153.92 134.97 119.95 110.60 185.90
20.56 21.41 20.95 20.49 20.99 20.92 21.34
13.20 14.63 14.98 14.72 15.63 15.91 19.79
5.57 7.47 8.54 5.14 9.37 6.93 16.07
132.05 158.87 147.90 133.20 119.39 114.35 180.54
19.81 20.15 20.54 19.95 21.36 21.02 22.28
24.92 25.61 25.72 24.95 25.82 25.48 26.94
31.74 31.72 32.37 32.40 32.37 30.88 29.29
Note: Tom, Onset temperature on DSC heating curve; Tpm, Peak temperature on DSC heating curve; Tem, Endset temperature on DSC heating curve; ΔHm, Enthalpy on DSC heating curve; Toc, Onset temperature on DSC cooling curve; Tpc, Peak temperature on DSC cooling curve; Tec, Endset temperature on DSC cooling curve; ΔHc, Enthalpy on DSC cooling curve.
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cooling cycle. Regarding the heat storability, the as-prepared sample 1, 2, 3, 4 and 5 show latent heat values of 159.21 J g 1, 146.95 J g 1, 134.9 J g 1, 113.57 J g 1 and 108.1 J g 1, respectively. After 500 heat
and cooling cycles, the values, instead of dropping drastically, only decrease slightly by no more than 5%, suggesting good thermal stability of the materials prepared in the present study. 3.6. Thermal performance of the microcapsule composites
Fig. 7. DSC data analysis of microcapsule samples: 1, PCMs; 2, Sample 1; 3, Sample 2; 4, Sample 3; 5, Sample 4; 6, Sample 5; 7, n-octadecane.
Thermal behavior is one of the most important properties of phase change microcapsules for in application. Forward looking infra-red system was employed to detect the temperature change of the microcapsules at a constant environment temperature (Fig. 10). It is obvious that silicon nitride has the highest heating rate in all the samples: it only takes 89 s to reach 65 °C. The micro-PCMs without silicon nitride, however, exhibit the lowest heating rate in all the samples: it reaches only 40 °C at 89 s. For the microcapsules modified by silicon nitride, the heating rate is positively correlated with the amount of silicon nitride added, indicating the effect of silicon nitride on improving the thermal conductivity of the microcapsules. The thermal conductivity of the microcapsules is shown in Table 4. It can be found that the addition of silicon nitride particles results in the increase of thermal conductivity. As the amount of silicon nitride was raised up to 10 g, the thermal conductivity was increased by 56.8% and reached a value of 0.3630 W m 1 K 1, which is comparable with that reported by Xiang Jiang et al. [28]. In their work, a good thermal conductivity (0.3816 W m 1 K 1) was obtained in the microcapsules through adding Al2O3. The difference
Fig. 8. TGA curves of n-octadecane and microcapsule samples.
Fig. 10. Phase change microcapsule composite materials.
Table 4 Average thermal conductivities. Samples Sample Sample Sample Sample Sample Fig. 9. DSC curves after 500 heating and cooling cycles.
Thermal conductivity (W m 1 K 1)
1 0.23157 0.0037 2 0.2753 7 0.0026 3 0.29187 0.0017 4 0.29977 0.0051 5 0.3630 7 0.0021
Table 3 Thermal properties of the microcapsules after 500 cycles. Samples
ΔHc (Jg 1)
Toc (°C)
Tpc (°C)
Tec (°C)
ΔHm (Jg 1)
Tom (°C)
Tpm (°C)
Tem (°C)
Sample Sample Sample Sample Sample
159.21 146.95 134.90 113.57 108.10
20.82 21.53 20.73 21.34 22.72
15.11 15.31 14.76 14.95 16.29
9.16 8.56 5.78 8.19 5.53
153.21 146.20 131.11 113.16 107.08
20.68 22.05 19.06 21.66 20.47
26.00 26.67 24.17 26.26 24.86
31.80 34.97 30.72 34.02 31.61
1 2 3 4 5
Note: Tom, Onset temperature on DSC heating curve; Tpm, Peak temperature on DSC heating curve; Tem, Endset temperature on DSC heating curve; ΔHm, Enthalpy on DSC heating curve; Toc, Onset temperature on DSC cooling curve; Tpc, Peak temperature on DSC cooling curve; Tec, Endset temperature on DSC cooling curve; ΔHc, Enthalpy on DSC cooling curve.
Y. Yang et al. / Solar Energy Materials & Solar Cells 151 (2016) 89–95
between their work and our study lies in the variation of latent heat. While in their work the increase of thermal conductivity resulted in drastic decrease of the latent heat, in this work we show that the impact of thermal conductivity on latent heat is negligible. Admittedly, the increase of thermal conductivity in the present research is not as pronounced as that resulted from the use of carbon materials (0.536 W m 1 K 1 in ref. [34], 0.463 W m 1 K 1 in ref. [35], 8.28 Wm 1 K 1 in ref. [36]), however, it is still more effective than the addition of other materials, such as Fe3O4 [37]. Considering its economic availability, silicon nitride is therefore believed to be a promising material to be added for improving thermal conductivity in phase change materials.
4. Conclusions We designed phase change microcapsules with high thermal performance based on n-octadecane core and PMMA shell cross-linked with modified silicon nitride. SEM micrographs show a well-defined core-shell structure and regular spherical morphology of the microcapsules. FTIR curves and EDS spectrogram indicate that silicon nitride is well cross-linked with the microcapsules. The results obtained from TGA, forward looking infra-red system and DSC indicate that the microcapsules with silicon nitride have a high thermal storage capability, enhanced thermal reliability and stability, and increased thermal conductivity. Additionally, our results suggest a new way of synthesizing phase change microcapsules with high thermal performance at low cost, which thus is beneficial to promote the application of phase change microcapsules in large scale in the future.
Acknowledgements The authors gratefully acknowledge the financial supports from Basic Research Project in Shenzhen (No. JCYJ20140417115840245) and International S&T cooperation program of Shenzhen (No. GJHZ20150316160614843).
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