Enhancement in thermal property and mechanical property of phase change microcapsule with modified carbon nanotube

Enhancement in thermal property and mechanical property of phase change microcapsule with modified carbon nanotube

Applied Energy 127 (2014) 166–171 Contents lists available at ScienceDirect Applied Energy journal homepage: www.elsevier.com/locate/apenergy Enhan...

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Applied Energy 127 (2014) 166–171

Contents lists available at ScienceDirect

Applied Energy journal homepage: www.elsevier.com/locate/apenergy

Enhancement in thermal property and mechanical property of phase change microcapsule with modified carbon nanotube Min Li a,b,⇑, Meirong Chen a, Zhishen Wu b a b

Jiangsu Key Laboratory for Construction Materials, Southeast University, Nanjing 211189, China International Institute for Urban System Engineering, Southeast University, Nanjing 210096, China

h i g h l i g h t s  Carbon nanotubes was grafted and used to enhance the thermal conductivities of the microcapsules.  The average particle size of the prepared MicroPCMs/CNTs-SA is 0.1 lm.  The thermal conductivity of MicroPCMs/CNTs-SA with 4% of CNTs increased by 79.2% compared with MicroPCMs.  MicroPCMs/CNTs-SA has better durability and thermal stability compared to the original MicroPCMs.

a r t i c l e

i n f o

Article history: Received 4 September 2013 Received in revised form 13 March 2014 Accepted 10 April 2014 Available online 4 May 2014 Keywords: Phase change materials Microcapsules Carbon nanotubes Thermal properties

a b s t r a c t Carbon nanotubes grafted with stearyl alcohol (CNTs-SA) was used in synthesizing phase change microcapsules (MicroPCMs) in order to enhance the thermal conductivities of the microcapsules. Urea–formaldehyde resin (UFR) was used as wall material. Scanning Electron Microscope (SEM), laser particle size analyzer, Fourier Transform Infrared Spectroscopy (FTIR), Differential Scanning Calorimeter (DSC) are employed to characterize the prepared MicroPCMs containing the grafted CNTs (MicroPCMs/CNTsSA). The results indicated that CNTs improved the performance of microcapsules. The average particle diameter of MicroPCMs/CNTs-SA is much smaller than that of MicroPCMs. There was no chemical reaction among paraffin, CNTs and UFR. The phase change temperature and latent heat of MicroPCMs/CNTsSA was 26.2 °C and 47.7 J/g, respectively. The thermal conductivity of MicroPCMs/CNTs-SA with 4% of CNTs increased by 79.2% compared with MicroPCMs. The initial decomposition temperature of MicroPCMs/CNTs-SA is 38 °C higher than that of MicroPCMs. After 100 heating and cooling cycles, MicroPCMs/CNTs-SA still has good durability and thermal stability. Ó 2014 Elsevier Ltd. All rights reserved.

1. Introduction Research on microencapsulated phase change materials (PCMs) began in the 1970s. PCMs are encapsulated by natural or synthetic macromolecular to form spherical particles with diameter from 1 to 1000 lm [1]. Phase change occurs within microcapsules and heat are absorbed or released. Consequently, the purpose of adjusting temperature or storing thermal energy was achieved [2]. Microencapsulation can not only solve the volume change in solid–liquid phase transition, but also prevent PCMs from being exposed to the external environment [3]. Phase change microcapsules (MicroPCMs) have drawn greater attention than traditional PCMs. Ahmet Sari used methyl meth⇑ Corresponding author at: Jiangsu Key Laboratory for Construction Materials, Southeast University, Nanjing 211189, China. Tel.: +86 2583790903; fax: +86 2583793232. E-mail address: [email protected] (M. Li). http://dx.doi.org/10.1016/j.apenergy.2014.04.029 0306-2619/Ó 2014 Elsevier Ltd. All rights reserved.

acrylate as wall material to prepare microcapsules with latent heat of 86.4 J/g and coating rate of 43% [4]. Zhang found that microcapsules using styrene maleic anhydride copolymer (SMA) as emulsifier exhibited good phase change characteristics, high encapsulation efficiency and excellent stability [5]. Wei used melamine–formaldehyde resin as wall material to prepare microcapsules through in situ polymerization. The microcapsules had an average diameter of 2.2 lm, the latent heat of 144 J/g and the encapsulation efficiency of 59% [6]. Ma et al. [7] prepared a series of MicroPCMs with butyl stearate and paraffin as a binary core material and poly(methyl methacrylateco-divinylbenzene) copolymer as shell material. The binary core material content in MicroPCMs was in the range between 50% and 85%. The MicroPCMs decomposed above 200 °C. Li et al. [8] fabricated a series of MicroPCMs with polyurethane shell. The fusion heat of polyurethane MicroPCMs was less than 60 J/g.

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Paraffin is a promising phase change material due to its large latent heat, little supercooling, property stability, non-corrosiveness and low price. The leakage problem during phase change process can be solved by microencapsulation method. However, paraffin has low thermal conductivity and microencapsulation hinders heat transfer from paraffin to the outer of wall material of microcapsules. As a result, the thermal conductivity of MicroPCMs is very low and the applications are limited. In order to improve the thermal conductivity of PCMs, metals and carbon materials have been used. Wang et al. [9] used mechanical method to shorten carbon nanotubes (CNTs) for improving dispersion of CNTs in epoxy. The result suggested that the thermal conductivity of shortened CNTs/epoxy composites increased by 40% in comparison with epoxy resin. Cui et al. [10] studied PCMs filled with carbon nanofibers (CNFs) and CNTs. The experimental results show that the thermal conductivity of PCMs increased with loading content of CNFs or CNTs. Mills et al. [11] found that the thermal conductivity of paraffin wax was increased by two orders of magnitude by impregnating porous graphite matrices with paraffin. Xiao et al. [12] found that the thermal conductivity of paraffin/copper foam composite is nearly 15 times larger than that of pure paraffin. Oya et al. [13] developed new PCMs using graphite and nickel particles as highly thermal conductive fillers. The effective thermal conductivity became two orders of magnitude larger than that of the original PCMs. The study of Vitorino et al. [14] showed that

2000×

167

the gelled graphite suspensions had enhanced the thermal conductivity of phase change materials. However, little work has been focused on improving heat transfer of MicroPCMs with carbon materials. CNTs have light weight and high thermal conductivity (about 6000 W/m K for single walled nanotubes [15] and about 3000 W/ m K for multi-walled nanotubes [16]). This characteristic is beneficial to enhance the heat transfer performance of PCMs in heat storage application [17]. This study aims to prepare MicroPCMs with excellent thermal property, mechanical property and long-term behavior. The CNTs were modified to improve the dispersity and therefore improve properties of the original MicroPCMs. The influence of CNTs on morphology and properties of MicroPCMs was investigated by comparing the property differences between MicroPCMs and MicroPCMs/CNTs-SA.

2. Experiments 2.1. Materials Paraffin was provided by Rubitherm PCM Co., Ltd., China. The phase change temperature and latent heat of paraffin is 28.1 °C and 206.1 J/g, respectively. CNTs (multi-wall carbon nanotubes) were supplied by Beijing Dk Nano technology Co., Ltd. The purity

10000× (a) MicroPCMs

50000×

50000× (b) MicroPCMs/CNTs-SA

Fig. 1. SEM micrographs of (a) MicroPCMs (2000, 10,000) and (b) MicroPCMs/CNTs-SA (50,000, 50,000).

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of CNTs was 95%. The average diameter, average length, and specific surface area of the CNTs were 30–50 nm, 10–20 lm, and 60 m2/g, respectively. Nitric acid (HNO3) with a purity of 65% and stearyl alcohol (C18H37OH) were supplied by Shanghai Jiuyi Co., Ltd., China. Thionyl Chloride (SOCl2) was purchased from Shanghai Lingfeng Chemical Reagent Co., Ltd., China. Urea (AR) and formaldehyde (37–40 wt.%) were purchased from Xilong Chemical Co., Ltd., and Shanghai Jiuyi Co., Ltd., respectively. SMA was supplied by Shanghai Jiuyi Co., Ltd., China. Sulfuric acid (H2SO4, 95–98%) and sodium hydroxide (NaOH, AR) were provided by Sinopharm Chemical Reagent Co., Ltd. The above chemicals were analytical reagent. 2.2. Synthesis 2.2.1. Preparation of CNTs-SA 3 g of CNTs were mixed with 300 ml of HNO3 and refluxed for 6 h at 120 °C. The mixture was washed to neutral and vacuum-filtered after cooled to the room temperature. Then the mixture was mixed with 60 ml of SOCl2, stirred at 70 °C for 24 h and distilled. CNTs-SA was obtained after the production was mixed with 60 ml of stearyl alcohol at 70 °C for 48 h. 2.2.2. Synthesis of MicroPCMs/CNTs-SA MicroPCMs/CNTs-SA was synthesized through in situ polymerization. 6.0 g urea and 9.0 g formaldehyde was mixed to prepare prepolymer solution. The pH value of the solution was adjusted to 8.5–9.0 by dropping 1 wt.% of NaOH. The solution was stirred at 70 °C for 1 h with the speed of 450 rpm. 10 g paraffin, 0.1 g SMA, 0.1 g CNTs-SA and 100 ml distilled water were added into the solution and mixed at 1500 rpm for 2 h. Then the stirring rate was changed to 400 rpm. The pH value was adjusted to 2.0 within 30 min by dropping 2 wt.% of sulfuric acid solution. The solution was stirred at 70 °C for 3 h and was cured at 85 °C for 1 h. Finally, the production was filtered and washed successively with distilled water and ethanol for 3 times. MicroPCMs/CNTs-SA were obtained.

(Pyris Diamond, Perkin–Elmer Company, America) at the heating rate of 5 °C/min from 0 to 60 °C under nitrogen atmosphere. The thermal stability of the PCM microcapsules was characterized with TG (Netzsch STA449 F3, Germany). The heating rates were 0.5 °C/min in a nitrogen atmosphere from 50 to 600 °C. A particle size analyzer (S3500, Microgram, America) was used to determine the size of the PCM microcapsules. The PCM microcapsules were dispersed in distilled water. The test range of the particles was from 0.01 to 2000 lm. 2.4. Performance test 2.4.1. Mechanical property 0.5 g MicroPCMs were mixed with alcohol and centrifuged. Then the precipitate was washed by distilled water and dried. The mechanical strength of MicroPCMs was characterized with breakage rate which is calculated as the following equation.

Breakage rate ð%Þ ¼

Initial mass  Mass after centrifugation Initial mass  100% ð1Þ

2.4.2. Thermal conductivity Hot Disk Thermal Constant Analyser (TPS2500, Hot Disk AB Company, Sweden) was used to test thermal conductivity of paraffin and MicroPCMs. The specimen was a cylinder with height of 20 mm and diameter of 30 mm. 2.4.3. Heat storage/release properties The MicroPCMs were heated and cooled alternatively. The temperature range was from 5 to 60 °C. The temperature-time curves of MicroPCMs during the heat storage and release process were recorded by a multi-channel temperature recorder (TP0008U, ShangHai Ruiqin Electronic Co., Ltd.). 3. Results and discussion

2.3. Characterization

3.1. Morphology

SEM (Sirion 200, FEI Company, Netherlands) was used to observe the surface morphology of PCM microcapsules. The accelerating rate was 20.0 KV. FTIR (Nicolet 5700, America) is used to analyze the component of PCM microcapsules. The samples were mixed with KBr and pressed into pellet. FTIR spectra in absorbance mode were recorded among the range of 450–4000 cm1. Thermal storage properties of PCM microcapsules were measured by DSC

Fig. 1 shows micrographs of MicroPCMs and MicroPCMs/CNTsSA. As can be seen from Fig. 1(a) that the particles of MicroPCMs are close to spherical and MicroPCMs are well dispersed. Few particles agglomerate together and the particle size distributes uniformly. Fig. 1(b) shows that particles of MicroPCMs/CNTs-SA are finer than those of MicroPCMs. However, the particles of MicroPCMs/CNTs-SA agglomerate together easily because of the large

)

)

)

)

Fig. 2. Particle size distribution of the microcapsules: (a) MicroPCMs and (b) MicroPCMs/CNTs-SA.

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Parameters

D10

D50

D90

addition of CNTs. The result is in agreement with reference [19] that CNTs make the particle size of MicroPCMs/CNTs-SA reduced and distribution narrower.

MicroPCMs MicroPCMs/CNTs-SA

3.89 0.06

6.54 0.10

13.08 0.15

3.3. FTIR spectra

Table 1 Particle size parameters of MicroPCMs and MicroPCMs/CNTs-SA (lm).

FTIR spectra of MicroPCMs, hollow microcapsules and MicroPCMs/CNTs-SA are presented in Fig. 3. Fig. 3 shows that MicroPCMs, hollow microcapsules and MicroPCMs/CNTs-SA have similar chemical structure. As can be seen from Fig. 3(c), the stretching vibration peak in 3363.6 cm1 is generated by NAH and OAH. The in-plane bending vibration peak of NAH and outof-plane bending vibration peak of OAH are located in 1558.4 and 643.6 cm1. Peaks around 2922.4 and 2845.7 cm1 are caused by stretching vibration of CAH. The bending vibration of CAH locates in 1385.8 cm1. Peak around 1646.1 cm1 belongs to stretching vibration peak of [email protected] Peaks in 1032.5 and 1136.6 cm1 are stretching vibration of CAO. These peaks agree with the characteristic peaks of UFR, which means that UFR has been formed in hollow microcapsules, MicroPCMs and MicroPCMs/CNTs-SA. Moreover, peaks representing CAH for MicroPCMs and MicroPCMs/CNTs-SA are stronger than those for hollow microcapsules. CAH comes from paraffin, UFR and CNTs-SA. This can be explained that paraffin has been encapsulated in MicroPCMs and MicroPCMs/CNTs-SA.

Transmittance(%)

(c)

(b)

0

1000

3363.6

2845.7 2922.4 1646.1

1032.5 1136.6 1240.7 1385.8 1558.4

643.6

(a)

2000

3000

4000

-1

Wavenumbers(cm ) 3.4. Mechanical property Fig. 3. FTIR spectra of the microcapsules (a) hollow microcapsules, (b) MicroPCMs and (c) MicroPCMs/CNTs-SA.

The samples are centrifuged at 2000, 4000, 6000 and 8000 rpm, respectively. The centrifugation time is 2, 5, 10 and 20 min, respectively. The breakage rates of MicroPCMs and MicroPCMs/CNTs-SA are calculated and listed in Tables 2 and 3. Table 2 shows that the breakage rate of MicroPCMs at 6000 rpm for 20 min is 1.15%. The breakage rate of MicroPCMs 8000 rpm for 10 min is 1.56%. However, the breakage rate of MicroPCMs/CNTs-SA at the same speed for the same time is 0%. This means that the mechanical property of MicroPCMs/CNTs-SA is better than that of MicroPCMs. The breakage rate at 8000 rpm for 20 min is 3.98% for MicroPCMs and 2.15% for MicroPCMs/CNTs-SA. The breakage rate of MicroPCMs/CNTs-SA decreases 46% comparing to MicroPCMs. This is attributed to the modified CNTs which show good dispersity and high mechanical property compared with polymer. CNTs can absorb energy under the action of mechanical force. Moreover, microcracks that appear between CNTs and the matrix can absorb energy.

specific area. A small amount of CNTs are dispersed disorderly around the particles. This is favorable to improve the heat transfer among the microcapsules.

3.2. Particle size distribution The particle size distribution of the samples is shown in Fig. 2 and the particle size parameters are presented in Table 1. Particle size distribution was determined by using D10, D50 and D90, which are the equivalent volume diameters at 10%, 50% and 90% of cumulative volume, respectively [18]. Table 1 indicates that the average particle diameter of MicroPCMs/CNTs-SA is much smaller than that of MicroPCMs. Moreover, the distribution of particle size of MicroPCMs/CNTs-SA is very concentrated due to the 0.5 Onset Temperature:25.9

0.0

0.0

Onset Temperature:26.2

-0.1 Heat Flow (W/g)

Heat Flow (W/g)

-0.5 -1.0 Enthalpy:77.05J/g

-1.5 -2.0

-0.2 Enthalpy:47.7J/g

-0.3

-0.4

-2.5 Peak:28.4

Peak:28.6

-3.0 0

10

20

30

Temperature (

(a)

40

)

50

60

-0.5 0

10

20

30

Temperature (

40

)

(b)

Fig. 4. DSC thermograms of microcapsules: (a) MicroPCMs and (b) MicroPCMs/CNTs-SA.

50

60

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The phase change latent heat of MicroPCMs and MicroPCMs/CNTsSA are given by the area under the exothermic peak. The encapsulate efficiency of paraffin in the microcapsules can be calculated as:

Table 2 Breakage rate of MicroPCMs. Time (min)

Centrifugal speed (rpm) 2000

4000

6000

8000

2 5 10 20

0 0 0 0

0 0 0 0

0 0 0.23 1.15

0 0.89 1.56 3.98

Table 3 Breakage rate of MicroPCMs/CNTs-SA. Time (min)

2 5 10 20

Centrifugal speed (rpm) 2000

4000

6000

8000

0 0 0 0

0 0 0 0

0 0 0 0.77

0 0 1.14 2.15

0.16

DH  100% DH P

ð2Þ

where DH is the latent heat of the MicroPCMs, DHP is the latent heat of paraffin. Fig. 4 shows that the enthalpy of MicroPCMs is 77.05 J/g and the enthalpy of MicroPCMs-SA is 47.7 J/g. It can be calculated from Eq. (2) that the encapsulate efficiency of MicroPCMs and MicroPCMsSA is 37.38% and 23.14%, respectively. Compared to MicroPCMs, the latent heat of MicroPCMs/CNTsSA decreased. One reason is that the encapsulate efficiency of paraffin in MicroPCMs/CNTs-SA decreased. The other reason is that the interaction between CNTs-SA and paraffin hinders the phase change process. The phase change temperature of MicroPCMs/ CNTs-SA increases slightly. This result corresponds with the result of other studies. Radhakrishnan et al. [20] found that the melting point will increase if there is a strong interaction between PCMs and porous materials. 3.5.2. Thermal conductivity Fig. 5 shows thermal conductivities of MicroPCMs/CNTs-SA with different content of CNTs. The thermal conductivity of MicroPCMs/CNTs-SA increases with the content of CNTs. The thermal conductivity of MicroPCMs/CNTs-SA with 4% of CNTs increased by 79.2% compared with MicroPCMs. CNTs are excellent conductor of heat and the modified CNTs has good dispersity in microcapsules. As a result, heat can be transferred quickly into microcapsules and the thermal conductivity of MicroPCMs is improved effectively.

0.14

Thermal conductivity (W/m·K)

Encapsulate efficiency ¼

0.12 0.10 0.08 0.06 0.04 0.02 0.00 0%

1%

2%

3%

4%

Mass ratio (%) Fig. 5. Thermal conductivities of MicroPCMs/CNTs-SA.

3.5. Thermal properties 3.5.1. DSC thermograms The DSC curves of MicroPCMs and MicroPCMs/CNTs-SA are displayed in Fig. 4. The phase change temperatures of MicroPCMs and MicroPCMs/CNTs-SA are the onset temperature on the DSC curves.

3.5.3. Thermal stability Fig. 6 shows TG curves of MicroPCMs and MicroPCMs/CNTs-SA before and after 100 heating and cooling cycles. It can be seen from Fig. 6(a) that there are two steps of mass loss. The first mass loss that occurs between 150 and 250 °C results from thermal degradation of paraffin. The second mass loss that happens from 250 to 350 °C stands for the degradation of UFR. Before 100 cycles, the initial decomposition temperature of MicroPCMs/CNTs-SA is 38 °C higher than that of MicroPCMs. After 100 cycles, the initial decomposition temperature of MicroPCMs/ CNTs-SA is still 37 °C higher than that of MicroPCMs. It means that MicroPCMs/CNTs-SA have better thermal stability than MicroPCMs. This is attributed to the high thermal conductivity of CNTs and strong interaction between the CNTs and UFR. The interaction restricts the mobility of polymer chains [21]. In addition, CNTs can hinder the permeation and transfer of gas.

Fig. 6. TG curves of (a) MicroPCMs and (b) MicroPCMs/CNTs-SA before and after 100 cycles.

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After 100 cycles, the initial decomposition temperature of MicroPCMs/CNTs-SA is 205.5 °C, which decreases by 4.7%. This implies that MicroPCMs/CNTs-SA still has good thermal stability after 100 cycles.

4. Conclusions This paper presents an experimental study of the MicroPCMs. From the above discussion, the following conclusions can be reached: (1) Phase change microcapsules are prepared successfully through in situ polymerization using UFR as wall material, SMA as emulsifier and CNTs as thermal conductivity enhancer. The phase change temperature and latent heat of MicroPCMs/CNTs-SA was 26.2 °C and 47.7 J/g. (2) MicroPCMs/CNTs-SA is not inclined to agglomerate and has smaller average particle size than that of MicroPCMs. There is no chemical reaction among paraffin, UFR and CNTs. (3) MicroPCMs/CNTs-SA has higher thermal conductivity and mechanical strength compared with MicroPCMs/CNTs. The thermal conductivity of MicroPCMs/CNTs-SA with 4% of CNTs increases by 79.2% compared with that of MicroPCMs. (4) The thermal stability of the MicroPCMs is improved by CNTs. The initial decomposition temperature of MicroPCMs/CNTsSA is 38 °C higher than that of MicroPCMs. (5) After 100 heating and cooling cycles, MicroPCMs/CNTs-SA still has good durability and thermal stability. The initial decomposition temperature of MicroPCMs/CNTs-SA is decreases by 4.7%.

Acknowledgments The authors gratefully acknowledge the financial support for this research from the National Natural Science Foundation of China (51178102), Program for New Century Excellent Talents in University of Ministry of Education of China (NCET- 11 -0091), 12th Five Years Key Programs for Science and Technology Development of China (2011BAJ03B11-3) and Science and technology project of Ministry of Housing and Urban-Rural development of China (2011-k1-40).

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