Fabrication and Performances of Microencapsulated Palmitic Acid with

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Fabrication and Performances of Microencapsulated Palmitic Acid with Enhanced Thermal Properties Sara Tahan Latibari,*,† Mohammad Mehrali,*,† Mehdi Mehrali,† Teuku Meurah Indra Mahlia,‡ and Hendrik Simon Cornelis Metselaar*,† †

Department of Mechanical Engineering and Advanced Material Research Center, University of Malaya, 50603 Kuala Lumpur, Malaysia ‡ Department of Mechanical Engineering, Universiti Tenaga Nasional, 43009 Kajang, Selangor, Malaysia ABSTRACT: This study focuses on the synthesis of microencapsulated phase change materials (MEPCMs), consisting of a palmitic acid (PA) core within an aluminum hydroxide oxide (Al2O3·xH2O) shell, using a sol−gel method. Aluminum isopropoxide (AIP) was used as a precursor for the aluminum hydroxide oxide shell. The MEPCMs were synthesized using four different weight ratios of PA/AIP. The effects of the PA/AIP weight ratio on the encapsulation characteristics and thermal properties of the MEPCMs have been investigated. The microcapsules were spherically shaped with an average diameter of 1.689−3.730 μm. Encapsulated PA confirmed the outstanding phase-change performance with specific heat and thermal stability enhancement. The final results suggested that the weight ratio of PA/AIP has an important effect on the morphology, encapsulation efficiency, and durability of the MEPCMs. A higher weight ratio of AIP/PA led to a smaller diameter size with enhanced thermal conductivity, thermal effusivity, and thermal stability of the MEPCMs. The thermal conductivity of PA microcapsules was considerably increased because of the fabrication of a thermally conductive aluminum hydroxide oxide shell. It can be concluded that the prepared MEPCMs employ an excellent energy storage potential because of their ideal latent heat, high thermal conductivity, and thermal stability. occurs.12 The traditional microencapsulated PCMs were fabricated using polymeric wall materials, such as melamine− formaldehyde resin, urea−formaldehyde resin, polyurethane, polystyrene, styrene−butadiene−styrene copolymer, or poly(methyl methacrylate) (PMMA), through in situ polymerization or interfacial polymerization in an emulsion system,10,13 and generally, these reported PCM microcapsules exhibited a well-defined core−shell structure. However, there are several defects, such as flammability, poor thermal and chemical stabilities, and low thermal conductivity, for these microencapsulated PCMs because of the polymeric shells.14,15 The thermal conductivity of inorganic materials is usually higher than organic materials. Moreover, the fire resistance and also the chemical and thermal stability of inorganic materials are superior to those of organic materials.16 Therefore, some attempts have recently been taken to enhance the thermal conductivity and mechanical strength of the microcapsules by encapsulating PCMs into inorganic silica. The preparation method of inorganic microcapsules of encapsulating PCMs with silica in oil-in-water emulsion was reported by Wang et al.17 Fang et al. and Zhang et al. also successfully synthesized and characterized the microencapsulated paraffin with a SiO2 shell via a sol−gel method. They have found that the high thermal conductivity of the silica shell enhances the thermal properties of the PCM.18 In our previous work, we also developed nanoencapsulated PA with a SiO2 shell using a sol−gel method. We investigated the influence of the synthesizing condition on

1-. INTRODUCTION The energy crisis and global warming have become serious concerns; thus, outstanding attempts have been made for the efficient utilization of alternative energy, such as solar energy. Nevertheless, the fluctuation of solar radiation makes latent heat thermal energy storage (LHTES) indispensable within the solar thermal energy applications. Phase change materials (PCMs) are recognized to become critical for LHTES because they can store and release considerable amounts of latent heat during their phase transition for efficient utilization of thermal energy.1 Organic PCMs, such as paraffin, fatty acids, and low melting polymers, have attracted extensive attention because of their high latent heat density, suitable phase-transition temperature, and stable physical and chemical properties.2−4 Palmitic acid (PA) is one of the fatty acids that offers exceptional properties, such as ideal melting temperature, high latent heat of fusion, outstanding thermal and chemical stability, and nontoxicity. PA possesses some shortcomings that hinder their application in practice, including low thermal conductivity, high volume change, and liquid seepage during phase transition.5,6 At present, there are several solutions, including employment of supporting materials, such as polymers or porous materials, for shape stabilizing the composite PCMs7−9 or encapsulation of PCMs.10,11 Encapsulation of PCMs into inert materials is the most potential and actually useful method for the preparation of the form-stable PCMs. This method can engulf small solid or liquid particles with a solid wall and, thus, will prevent the leakage of PCMs from their location, reduce the interference toward phase-change behaviors from the outside environment, increase the heat-transfer area, and make liquid PCMs easy to be handled when the phase change © XXXX American Chemical Society

Received: September 8, 2014 Revised: January 28, 2015

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emulsion templating system. In this instance, the surfactant molecules could totally cover the surfaces of the oil droplets of PA with hydrophobic chains oriented into the oil droplets, and at the same time, the hydrophilic groups additionally arranged along the hydrophobic chains of the surfactant molecules and, thus, were linked to the water molecules. With the formation of a surfactant layer covering the surfaces of the PA droplets, the templating system is ready for the self-assembly of the aluminum hydroxide oxide shell material. Second, AIP was integrated into the emulsion-templating system that contains the PA micelles. In this case, these alumina precursors could be drawn onto the surfaces of the PA micelles through an electrostatic interaction between the alumina precursors and the hydrophilic part of the surfactant. In the meantime, these alumina precursors could conduct further polycondensation to create an extended aluminum hydroxide oxide network around the PA micelles. Finally, using the long-term polycondensation of AIP hydrolysates as well as the following phase transformation, the aluminum hydroxide oxide shell was well-fabricated onto the surfaces of the PA micelles using the sol−gel process. 2.3. Characterization and Determination of MEPCMs. The chemical structure characterization of the microcapsules was performed by a Fourier transform infrared (FTIR, PerkinElmer Spectrum 100 model) spectrometer for the wave range of 450−4000 cm−1. The crystallography was investigated by an X-ray diffractometer (XRD) by Empyrean PANalytical. The surface morphology and surface elemental analysis of the PA microcapsules were analyzed using high-resolution field emission scanning electron microscopy (FESEM, FEI Quanta 200F). The melting and freezing temperatures, latent heat, and specific heat capacity of MEPCMs were obtained from differential scanning calorimetry (Mettler Toledo 820C, with an error of ±0.25−1 °C) at a heating rate of 5 °C/min in a purified nitrogen atmosphere. Also, thermogravimetric analysis (Mettler Toledo SDTA851) was used to obtain the weight loss and thermal stability of MEPCMs at the heating rate of 10 °C/min and temperature of 30−600 °C in a purified nitrogen atmosphere. To measure the thermal diffusivity and thermal conductivity of the microcapsules with the laser flash technique (Netzsch LFA 447 NanoFlash) at 35 °C, the product powders were compressed into pellets by a hydraulic press powder pelletizer in a steel mold with a diameter size of 12.7 mm. An accelerated thermal cycling test was performed up to 1500 cycles using an in-house-developed thermal cycling system design (University of Malaya),25 to investigate the thermal reliability of the microcapsules during the melting and freezing processes (Figure 2). The thermal cycling system was designed to operate heating and cooling cycles between 30 and 80 °C.

the morphology and thermal properties of the nanoencapsulated phase change material (NEPCMs).19 Al2O3 as an inorganic material has desirable properties, such as high-temperature stability, high thermal conductivity, high chemical durability, and environmental resistance.20 Al2O3 has premier properties compared to SiO2, such as thermal conductivity, thermal diffusivity, and thermal stability.21 Fang et al. prepared shape-stable PA/active aluminum oxide composites and investigated the thermal properties and thermal stability.22 Bugaje also observed that adding aluminum additives into the paraffin can decrease the phase change time in cooling and heating operations.15 Ho and Gao found that adding Al2O3 nanoparticles enhanced the thermal conductivity of paraffin.23 Nevertheless, there were no works about preparing MEPCMs of PA with an aluminum hydroxide oxide (Al2O3·xH2O) shell by the sol−gel method. The sol−gel method, as one of the various synthesis methods, is the most promising technique because of its facile procedure and the highest purity materials, which can be achieved through this method.24 Therefore, in this study, microcapsules of PA/(Al2O3·xH2O) were synthesized via a sol−gel method and the thermal properties and morphologies of four different weight ratios of PA/aluminum isopropoxide (AIP) have been investigated. Finally, thermal cycling tests confirmed the applicability of the MEPCMs.

2. EXPERIMENTAL SECTION 2.1. Materials. PA with a purity of 98 wt % was used as a PCM and was commercially supplied by Fisher Scientific, Inc. Sodium dodecyl sulfate (SDS) as the surfactant and AIP (chemically pure) as an alkoxide for synthesizing Al2O3 were provided by Fisher Scientific, Inc. Other chemical reagents were of chemical pure grade and were used as received without further purification. 2.2. Fabrication of MEPCMs. PA was dissolved in preheated distilled water at 80 °C on a magnetic stirrer at 1000 rpm, and SDS was added to obtain a clear emulsion. The solution was stirred continuously for 2 h (Table 1).

Table 1. Compositions of the O/W Emulsion and the Solution of the AIP PCM solution

solution of AIP

sample code

PA/AIP weight ratio (w/w)

PA (g)

SDS (g)

distilled water (mL)

AIP (g)

distilled water (mL)

S1 S2 S3 S4

50:50 60:40 70:30 80:20

1 1.5 2.3 4

0.4 0.6 0.9 1.6

300 450 600 750

1 1 1 1

10 10 10 10

3. RESULTS AND DISCUSSION 3.1. Chemical Composition of MEPCMs. The FTIR spectra of the prepared Al2O3 sol, pure PA, and prepared MEPCMs are displayed in Figure 3. Spectrum a of Figure 3 shows broad absorption peaks at 3100−3700 cm−1 because of the O−H stretching of the absorbed water. The peaks at 450− 700 cm−1 are allocated to the Al−O vibrations. The stretching modes of Al−O−Al linkages are observed at 624 and 700 cm−1. The peak at 1080 cm−1 is due to the Al−O−H vibration modes of aluminum hydroxide oxide. Spectrum b of Figure 3 exhibits the spectrum of pure PA. The peaks at 2912 and 2846 cm−1 corresponded to the symmetrical stretching vibration peaks of −CH3 and −CH2 in PA. The absorption peak at 1691 cm−1 is associated with the C−O stretching vibration. The peak at 1293 cm−1 refers to the in-plane bending vibration of the −OH group of PA; the peak at 937 cm−1 corresponds to the out-of-plane bending vibration of the −OH functional group; and the peak at 720 cm−1 demonstrates the in-plane swinging vibration of the −OH functional group.19

A total of 4 g of AIP was separately dispersed in 40 mL of preheated distilled water at 75 °C and at 750 rpm on a magnetic stirrer. Afterward, ammonia solution was added to obtain the desired pH value under stirring at 85 °C. On the basis of preliminary experiments, the pH value was controlled around 10.5 to obtain properly shaped microcapsules. The sol was stirred for an additional 10 min to complete the hydrolysis and release the formed alcohol. Subsequently, 10 mL of the AIP/water homogeneous sol was added dropwise to the oil-in-water (O/W) emulsion throughout 3 h at 80 °C and 500 rpm. Finally, white precipitates were collected and washed several times with distilled water and ethanol. The white powder was dried at 55 °C for 24 h. Figure 1 schematically displays the formation of MEPCMs. The synthetical method for the microcapsules with the aluminum hydroxide oxide shell is very straightforward, as shown by Figure 1. A regular formation technique of such a kind of microcapsule can be explained as follows: First, oily PA was mixed with a preheated solution containing the anionic surfactant SDS to make a stable O/W B

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Figure 1. Schematic formation of microencapsulation of PA/(Al2O3·xH2O).

Figure 2. Image and schematic of the accelerated thermal cycler.

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because of the formation of microcapsules. The XRD analysis further proved that PA in the core of aluminum hydroxide oxide microcapsules became a short-range order compared to the high-melting-point pure PA crystals; therefore, some changes in the thermal properties of the MEPCMs might be expected. 3.3. Surface Morphology and Surface Element Analysis of MEPCMs. Panels a−d of Figure 5 and panels a and b of Figure 6 present the morphologies of the microcapsules synthesized with four different weight ratios of PA/AIP, and it can be seen that the microcapsules were spherical. It can be seen in panels a−d of Figure 5 that the weight ratio of PA/AIP influences the diameter and surface morphology of the MEPCMs. The images indicated that S1 and S2 have a smooth and compact surface without any porosity on the shell. Additionally, panels a and b of Figure 6 confirmed that the size distribution of the microcapsules in S4 was not homogeneous and uniform compared to the size distribution of the microcapsules in S1. The surface elemental composition of the MEPCMs was characterized by energy-dispersive X-ray spectroscopy (EDS), and typical results are shown in Figure 5e. Figure 5e indicates that aluminum and oxygen were the primary elements in Al2O3· xH2O. In the case of encapsulation of PA, peaks of carbon emerged clearly, which further confirmed that PA was encapsulated successfully. Furthermore, the mole ratio of O/ Al is calculated to be 4.56:1, indicating an oxygen-rich aluminum hydroxide oxide shell with a lot of hydroxyl groups. Panels c and d of Figure 6 illustrate the particle size distribution (PSD) of the MEPCMs. The mean diameter of the microcapsules changed from 1.689 to 3.730 μm according to S1−S4, because of the large size of the PA droplets in O/W emulsion. The broad PSD curve in S4 suggested that, at higher weight ratios of PA/AIP, the distribution of the MEPCMs is not uniform. 3.4. Thermal Energy Storage Properties. The heating and cooling DSC scans were carried out at the scanning rate of 5 °C/min to investigate the effects of the aluminum hydroxide oxide shell on the phase-change characteristics of the PA microcapsules with different weight ratios of PA/AIP. The DSC results are exhibited in Figure 7, and the phase change characterizations obtained via DSC analysis are listed in Table 2. All DSC curves in Figure 7a exhibit one endothermic peak with Tpeak located in the temperature range of 62−66 °C, which is ascribed to the melting of PA. The melting points of the MEPCMs were lower than the melting temperature of PA because of the confinement effect by shell−PA interactions. As shown in Figure 7, the melting and solidifying temperatures of the MEPCMs were close to each other; thus, there is little subcooling. It is found that the theoretical core loadings for all of the MEPCMs were different because of the different PA/AIP weight ratio. Samples S1−S4 with theoretical core loading of 50−80% show lower actual core loading compared to their theoretical core loading, which is noted in Table 2. This indicates that not all of the reactants could be converted into the shell materials. Some Al2O3·xH2O was not assembled on the surfaces of PA micelles but directly precipitated as solid particles during the synthesis process, and also, not all of the PA particles can be encapsulated within the shell materials, which were removed during the washing process, accordingly resulting in a decrease in actual core loading. However, the latent heat of

Figure 3. FTIR spectra of (a) Al2O3 sol, (b) pure PA, (c) S4, (d) S3, (e) S2, and (f) S1.

Figure 4. XRD patterns of (a) Al2O3 sol, (b) pure PA, (c) S4, (d) S3, (e) S2, and (f) S1.

Spectra c−f of Figure 3 reveal that the absorption peaks in PA at 2912, 2846, 1293, 937, and 720 cm−1 are essentially unchanged in the microcapsules. The effect of the absorption peaks of the Al2O3 sol at 480, 624, and 1080 cm−1 can also be found within the microcapsule spectra. The interaction between PA and aluminum hydroxide oxide crystals has been confirmed through the reduction of the intensity as well as the shifting of C−O stretching band of PA initially at 1691 cm−1 to the lower frequency of 1595 cm−1 in the FTIR spectra of microcapsules. This shift proved that there were interface interactions between the COOH group of PA and alkaline area in Al2O3·xH2O, which can change the thermophysical properties of encapsulated PA. The FTIR results confirmed that the aluminum hydroxide oxide shell was formed on the surface of the PA droplet by considering that the shell material can only be shaped around the interface of PA in O/W emulsion.20 3.2. Crystallography of the MEPCMs. Figure 4 shows the XRD patterns of the Al2O3 sol, pure PA, and MEPCMs, which reveal the crystalline structures. The broad peaks in Figure 4a at 28°, 38°, and 49° could be assigned to the formation of the boehmite phase, while the weak and broad diffraction lines indicate the poor crystallinity. Spectrum b of Figure 4 shows that the peaks at 21.8° and 24.3° belong to the normal crystallization of PA, and spectra c− f of Figure 4 show that the XRD peaks of PA in microcapsules also contain the Al2O3 sol smooth peaks. Spectra c−f of Figure 4 indicated that the strong and sharp diffraction peaks of pure PA become weak and broad peaks D

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Figure 5. SEM micrographs of the microcapsules of samples (a) S1, (b) S2, (c) S3, and (d) S4 and (e) EDS analysis of MEPCMs.

tion of PA and the working effectiveness of the PCMs within microcapsules, respectively. Normally, the higher the actual core loading, the higher the encapsulation ratio, which was confirmed by the results. The small internal space of some produced nanosized microcapsules may lead to a confinement effect on the crystallization of PA. In this case, a small amount of encapsulated PA cannot change phase and does not contribute to heat storage. As shown in Table 2, the thermal storage capability of the MEPCMs synthesized at different concentrations of AIP exhibits the thermal storage capabilities higher than 98%, which indicates that PA in suitable sizes was encapsulated by the aluminum hydroxide oxide shell, and thus, most encapsulated PA could effectively store and release the latent heat through the phase change process. Latent heat is particularly attractive because of its ability to provide high-energy storage density in a quasi-isothermal process. However, the energy storage density can be further increased using PCM having a large sensible heat within the temperature range of the storage, while the total heat stored in a PCM can be calculated as follows:

the prepared microcapsules is still sufficient for being relevant in thermal energy storage. The other important properties of the MEPCMs, such as encapsulation ratio (R), encapsulation efficiency (E), and thermal energy capability (φ), have been determined by DSC outcomes through the subsequent equations19 R=

E=

φ=

ΔHm,MEPCM ΔHm,PCM

× 100%

ΔHm,MEPCM + ΔHc,MEPCM ΔHm,PCM + ΔHc,PCM

E × 100% R

(1)

× 100% (2)

(3)

where ΔHm,MEPCM and ΔHm,PCM are the melting enthalpies and ΔHc,MEPCM and ΔHc,PCM are the crystallization enthalpies of the MEPCMs and pure PA, respectively. The encapsulation ratio and encapsulation efficiency illustrate the efficient encapsulaE

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Figure 6. SEM images of sample numbers (a) S1 and (b) S4 and size distribution graphs of microcapsules in samples (c) S1 and (d) S4.

the heat of fusion at the phase change temperature Tm. It can be seen that the specific heat also affects the total heat storage. The specific heat of pure PA and encapsulated PA was measured by the multiple curve method for temperatures between 35 and 45 °C, and the results are shown in Figure 8. The results show that the specific heat of the capsules was higher than that of pure PA. It can be seen clearly that, at a higher weight amount of shell material, the specific heat values increase, which partly makes up for the lower latent heat of these capsules. 3.5. Thermal Conductivity. It is well-known that the low thermal conductivity of PA increases its thermal response time for storage and release of latent heat. Thus, enhancing the thermal conductivity of PCMs is a major aim in designing a microencapsulated material. The thermal diffusivity was measured by the laser flash method, and the thermal conductivity was calculated as follows:17

K = αρCp

where K is the thermal conductivity (W m−1 K−1), α is the thermal diffusivity coefficient (m2/s), ρ is the bulk density (kg/ m3), which was measured using the Archimedes method, and Cp is the specific heat capacity (J kg−1 K−1), which was obtained from the multiple curve method. The calculated thermal conductivities of the microcapsules and pure PA in solid state (35 °C) are given in Table 3. The outcomes indicated that the thermal conductivity of encapsulated PCMs in all samples was higher than that of PA, because of the high thermal conductivity of Al2O3·xH2O as the shell material. Additionally, it is notable that the thermal conductivities of microencapsulated PA samples are considerably related to the encapsulation ratio, with the lower thermal conductivity enhancements achieved at higher encapsulation ratios. This suggests that the aluminum hydroxide oxide shell

Figure 7. DSC thermograms of the (a) heating and (b) cooling of pure PA and MEPCMs.

Qt =

∫T

Tm

1

Cp,s dT + ΔHL +

∫T

T2

m

Cp,l dT

(5)

(4)

where Qt is the total heat stored per mass, Cp,s and Cp,l are the specific heats of solid and liquid PCM, respectively, and ΔHL is F

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solidifying

sample code

Tonset (°C)

Tpeak (°C)

Tend (°C)

Tonset (°C)

Tpeak (°C)

Tend (°C)

ΔHm (kJ/kg)

ΔHc (kJ/kg)

encapsulation ratio (%)

encapsulation efficiency (%)

thermal storage capability (%)

S1 S2 S3 S4 PA

56.75 55.7 55.16 55.37 60.58

62.20 62.2 62.09 62.03 65.22

66.25 65.25 65.98 66.25 72.08

55.44 55.4 55.12 55.05 61.94

49.78 49.76 49.68 51.33 57.47

45.83 45.73 45.52 45.22 51.44

55.58 77.09 97.31 121.97 182.42

55.37 77.13 97.71 123.93 185.96

30.46 42.26 53.34 66.86

30.11 41.86 52.9 66.75

98.78 99.06 99.24 99.83

Table 4. Decomposition Temperatures of PA and MEPCMs sample code

onset decomposition temperature (°C)

maximum weight loss temperature (°C)

pure PA S1 S2 S3 S4

183.25 212.48 206.69 203.45 194.80

277.33 355.68 323.48 306.81 286.16

Table 5. Decomposition Improvement of PA/SiO2 and PA/ Al2O3·xH2O sample code PA/Al2O3·xH2O (S1, 50:50) (this study) PA/SiO2 (S1, 50:50)19 PA/SiO2 (S2, 50:50)19 PA/SiO2 (S3, 50:50)19

Figure 8. Specific heat curves of PA and encapsulated PCMs.

Table 3. Thermal Conductivity of Encapsulated PCMs and Pure PA sample code

thermal conductivity (W m−1 K−1)

thermal conductivity enhancement (%)

pure PA S1 S2 S3 S4

0.282 0.530 0.449 0.410 0.352

87.94 59.21 45.39 24.82

decomposition temperature improvement compared to pure PA (%) 15.95 0.00 1.16 2.90

Table 6. Thermal Effusivity of PA and MEPCMs sample code

thermal effusivity (kJ K−1 m−2 s−0.5)

pure PA S1 S2 S3 S4

0.640 0.877 0.803 0.770 0.711

considered as a virtual heat-transfer network. Therefore, this can enhance the heat-transfer rate over the entire microcapsules and, thus, attain a significantly improved thermal conductivity. Furthermore, the thermal conductivity of the produced capsules of PA/Al2O3·xH2O is higher in comparison to the thermal conductivity of the capsules of PA/SiO2 in our previous work.19 3.6. Thermal Stability of the MEPCMs. Thermal stability is a significant factor in evaluating the microencapsulated PCMs for the applications of heat energy storage or thermal regulation. The thermal degradation behaviors of the microcapsule samples synthesized in different weight ratios were investigated by TGA. The TGA thermograms in Figure 9 show the weight loss of the microcapsules (S1−S4) and that of pure PA. Figure 9 demonstrates that pure PA starts to lose weight at approximately 200−520 °C with only one step, which corresponds to the volatilization of PA. According to Figure 9 microcapsules of the three samples (S1−S4) degrade in one step as well as pure PA. It is believed that the increase of AIP proportion could lead to a thicker shell for the microcapsules and, thus, generate a better barrier to prevent the encapsulated PA from decomposing. Moreover, the microencapsulated PA samples produced a considerable

Figure 9. TGA thermograms of pure PA and MEPCMs.

performs an important function in improving the heat transfer of microencapsulated PA. In samples S1 and S2, Al2O3·xH2O creates an entirely continuous and compact shell around PA; this can be G

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Figure 10. Latent heat values of the MEPCMs before and after thermal cycling.

The thermal effusivity of all MEPCMs is larger than that of pure PA, which is beneficial for the LHTES systems. 3.8. Thermal Reliability and Durability of MEPCMs. Thermal reliability of the MEPCMs is critical to evaluate the possibility of their long-term use of thermal energy storage (TES) systems. A PCM needs to maintain its TES attributes even when it is put through a prolonged thermal cycling process. In this research, the thermal reliability of the composite PCMs was examined after the exposure to 1500 melting− freezing cycles. The changes of latent heat in composite PCMs after thermal cycling were evaluated to determine whether they are thermally reliable. After 1500 cycles, the MEPCMs still showed a single-phase transition curve with onset temperature as before the cycling. In other words, there were no extra peaks related to chemical decomposition or phase segregation within the MEPCMs. Through comparing the latent heat values in Figure 10, the maximum change in latent heat capacity was determined as 3.84% for S4 and the minimum was 0.39% for S1. These results suggested that the changes in the latent heat capacity of the composites after thermal cycling were less than 1% for S1 and S2, demonstrating their good thermal reliability. The results indicate that S3 and S4 may have had some leakage during cycling that caused effects on the latent heat capacity of the MEPCMs.

amount of residual char to be associated with the theoretical PA loading in the microcapsules. Additionally, the onset temperatures of degradation for all are higher than that of pure PA (Table 4), which indicates that the thermal stability of PCM increases after being encapsulated. Moreover, the microcapsule samples show a maximum weight loss at higher temperatures compared to pure PA, which indicates that the shell can prevent encapsulated PA from evaporating at the normal boiling point and improve the degradation temperature of the microcapsules. On the other hand, the degradation temperatures of samples S1 and S2 compared to the other two samples are higher, which proves that the thick and non-porous aluminum hydroxide oxide shell in the microcapsules of these two samples may favor the improvement in thermal stability of the microcapsules. It is significant that the weight loss of the microcapsules strongly depends upon the encapsulation ratios of PA to the microcapsules and the PA/AIP weight ratio; therefore, sample S4, which was synthesized with the weight ratio of 80:20, achieved the highest weight loss. The occurrence of a small weight loss before 130 °C is mainly due to the incomplete removal of water. Table 5 indicated that, at the 50:50 weight ratio of PA/metal oxide precursor, the decomposition temperature of the produced PA/Al2O3·xH2O microcapsules was improved more than that of PA/SiO2. This clarifies that aluminum hydroxide oxide has superior thermal stability in contrast with SiO2 in microcapsules of PCMs. 3.7. Thermal Effusivity of MEPCMs. The thermal effusivity is a measure of the ability of a material to exchange thermal energy with its surroundings. The thermal effusivity is considered to be a critical physical quantity to depict the unsteady-state heat transfer in a LHTES system. The effusivity of a material is the square root of the product of the thermal conductivity, density, and heat capacity e=

ρKCp

4. CONCLUSION A series of PA microcapsules with an aluminum hydroxide oxide shell were synthesized at different weight ratios of PA/ AIP via a sol−gel method. The effects of the weight ratio of PA/AIP on the phase transition properties of the PA microcapsules have been investigated. From the FESEM, FTIR, and XRD results, it is found that PA has been wellencapsulated in the aluminum hydroxide oxide shell. Important observations are listed as follows: The microcapsules have spherical shapes with the mean diameter varying from 1.689 to 3.730 μm for the 50:50 to 80:20 weight ratios. The higher weight ratio of PA/AIP caused nonuniformity of the size and porous shell in the microcapsules. The latent heat of the microcapsules was enhanced by increasing their size. When the quantity of the PA/AIP weight ratio was increased, the encapsulation ratio and efficiency of

(6)

where e (kJ K−1 m−2 s−0.5) is the thermal effusivity, K is the thermal conductivity (W m−1 K−1), ρ is the density (kg/m3), and Cp is the specific heat capacity (kJ kg−1 K−1). The thermal effusivity of PA and MEPCMs are evaluated at 35 °C and listed in Table 6. H

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Energy & Fuels

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microcapsules improved considerably. The results show that the specific heat of PA was increased after encapsulation and improved by increasing the weight ratio of PA/AIP. The thermal stability of PA increased after being encapsulated. Samples S1 and S2 show higher thermal stability than the other two samples, which indicated their thick, compact, and non-porous shell that was also observed in the SEM images. The thermal conductivities of encapsulated PCMs in all samples are improved significantly, while the lower encapsulation ratios caused higher thermal conductivity. The thermal effusivity of all MEPCMs is larger than that of pure PA, which is beneficial for the LHTES systems. Altogether, the improved thermal properties, thermal stability, and enhanced heat storage characteristics of the MEPCMs facilitate them to be considered as a potential candidate for thermal storage applications.

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AUTHOR INFORMATION

Corresponding Authors

*Telephone: +60-3-79674451. Fax: +60-3-79675317. E-mail: [email protected]. *Telephone: +60-3-79674451. Fax: +60-3-79675317. E-mail: [email protected]. *Telephone: +60-3-79674451. Fax: +60-3-79675317. E-mail: h. [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work has been financially supported by the Ministry of Higher Education (MOHE) of Malaysia [Grant UM.C/HIR/ MOHENG/21-(D000021-16001) “Phase Change Materials (PCM)” for Energy Storage System] and the University of Malaya Research Grant UMRG RP021-2012A.

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REFERENCES

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DOI: 10.1021/ef502840f Energy Fuels XXXX, XXX, XXX−XXX