Preparation of phase change material–montmorillonite composites suitable for thermal energy storage

Thermochimica Acta 524 (2011) 39–46

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Preparation of phase change material–montmorillonite composites suitable for thermal energy storage Nihal Sarier a,∗ , Emel Onder b , Serap Ozay a , Yilmaz Ozkilic a a b

Faculty of Engineering and Architecture, Istanbul Kultur University, Atakoy Campus, Bakirkoy 34156, Istanbul, Turkey Faculty of Textile Technologies and Design, Istanbul Technical University, Inonu Street, Gumussuyu 34437, Istanbul, Turkey

a r t i c l e

i n f o

Article history: Received 11 April 2011 Accepted 13 June 2011 Available online 12 July 2011 Keywords: PCM Thermal property DSC XRD Intercalation Montmorillonite

a b s t r a c t A new type of organoclay composite (PMMT) with good heat storage and release capacity was produced by intercalating n-hexadecane with Na–montmorillonite (Na–MMT) in a surfactant-assisted medium. The expansion of Na–MMT galleries by a Na salt of 4-dodecylbenzene sulfonic acid (SDS) solution prior to the intercalation of n-hexadecane enhanced the process. In the XRD analysis, expansions of the d spacings in the (0 0 1) plane were observed in all PMMT samples, indicating that the intercalation of n-hexadecane in the galleries of Na–MMT was successfully achieved. The most successful PMMT sample, prepared by intercalating 9 mmol n-hexadecane per gram of Na–MMT, displayed a high heat capacity (126 J g−1 ) and good thermal durability. The sample also had a higher thermal conductivity compared to pure nhexadecane samples. The manufacture of the PCM–clay composites by the method of surfactant-mediated intercalation of n-hexadecane into Na–MMT is recommended to enhance the thermal performance of different thermal storage materials. © 2011 Elsevier B.V. All rights reserved.

1. Introduction A phase change material (PCM) possesses the ability to absorb and release large quantity of latent heat during a phase change process over a certain temperature range. The use of PCMs in energy storage and thermal insulation has been tested scientifically and industrially in many applications, such as in energy-efficient building materials [1–4], transportation packaging of temperature sensitive materials [5,6], engines and hydraulic machines [6,7], air conditioning applications [8], solar energy storage systems [9–12], greenhouses [13–15], temperature regulating textiles [16–20], heat management of electronics [21,22], and in biomedical systems [23,24]. A wide variety of PCMs are available with different heat storage capacities and phase change temperature intervals. Among them, paraffin waxes or n-alkanes (Cn H2n+2 ) are most popular because of their outstanding properties, such as high energy storage densities, little super-cooling trend, low vapor pressure in the liquid phase, chemical inertness and stability, non-toxicity and commercial availability at a relatively low cost [25–31]. Various processes have been used to incorporate PCMs into different composites. Adding PCMs to a coating blend [32], microencapsulation [19,33–37], shape-stabilization by melt mixing [38,39], foam manufacturing [40], impregnation into porous

∗ Corresponding author. Tel.: +90 2124984488; fax: +90 2124658308. E-mail addresses: [email protected] (N. Sarier), [email protected] (E. Onder), [email protected] (S. Ozay), [email protected] (Y. Ozkilic). 0040-6031/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.tca.2011.06.009

structures [41] or finned tubes [21] are some convenient processes to incorporate PCMs into a material; each method has advantages and disadvantages in terms of thermal, mechanical and physical properties of the final products. Currently, developing new energy-saving composites with improved thermal performances and durability has been the research focus for promoting the applicability of PCMs at the industrial scale [42–44]. Montmorillonite (MMT) is composed of a single octahedral sheet of alumina sandwiched between two tetrahedral sheets of silica, with the octahedral sheet sharing the apical oxygens of the tetrahedral sheets. The isomorphic substitution of Al3+ or Fe3+ for Si4+ in the tetrahedral layer and Mg2+ , Fe2+ , or Mn2+ for Al3+ in the octahedral layer results in a net negative charge on the clay surfaces that is neutralized by exchangeable cations such as Na+ or Ca2+ located in the interlayer spacing (gallery) and on the surface. Primarily due to the hydration of the cations, the environments on the clay surface and galleries are hydrophilic in nature. As a result, MMTs show a poor affinity to nonpolar organic molecules. However, the interlayer cations can be exchanged with different cationic or anionic surfactants, making the hydrophilic silicate surfaces organophilic and allow the fabrication of organoclays that are compatible with other organics [45]. Therefore, it is revealing that MMTs may adsorb and intercalate organic PCMs with high efficiencies if they are modified with appropriate surfactants beforehand. Thus far, little attention has been paid to MMTs and other types of clay minerals in the production of PCM–clay composites [46–48]. However, PCM–clay composites are promising materials that simultaneously provide enhanced thermal storage

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capacity and biodegradability to meet the requirements of many energy-saving systems in an environmentally friendly manner, which may be of interest in the current market. In the present study, the PCM–clay composites were prepared by pre-modifying Na–MMT with an anionic surfactant (a sodium salt of 4-dodecylbenzene sulfonic acid, SDS) to make it organophilic in character, followed by a surfactant-mediated intercalation of n-hexadecane into the galleries of organophilic MMT. The resultant PCM–clay composites were characterized using X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy and scanning electron microscopy (SEM). Thermal properties and thermal stabilities of PMMT samples were determined by thermogravimetry-differential thermogravimetry (TG-DTG) and differential scanning calorimetry (DSC) analysis. Particle size distributions of the samples were obtained with a nanosizer. 2. Experimental 2.1. Materials In this study, a linear chain alkane [n-hexadecane CH3 (CH2 )14 CH3 with a molar mass of 226.0 gmol−1 ] was chosen as a PCM. All technical grade chemicals were obtained from Merck Co. A sodium salt of 4-dodecylbenzene sulfonic acid (SDS) [CH3 (CH2 )11 C6 H4 SO3 − Na+ with a molar mass of 348.5 gmol−1 ] was used as an anionic surfactant to disperse the clay particles in the colloidal mixture and to emulsify the n-hexadecane. The Na–MMT named Nanocor® was purchased from AMCOL Int. Corp. A specific gravity of 2.6, an aspect ratio of 200–400, a pH (5% dispersion) of 9.5–10.5 and a cation exchange capacity (CEC) of 145 cmol kg−1 were listed as its physical properties. 2.2. Methods 2.2.1. Preparation of PCM–clay composite samples First, 100 mL of 3 mM aqueous SDS was blended with 9 g of dried Na–MMT (corresponding to 1.305 cmol of CEC) at 2400 rpm for 15 min with an IKA T25 agitator. Then the mixture was blended at 1600 rpm and 50 ◦ C for 60 min to modify Na–MMT into an organ-

Table 1 The PCM–clay composite samples prepared by intercalating n-hexadecane with Na–MMT. Sample name

Mass of Na–MMT/mass of n-hexadecane

n-Hexadecane/ clay (mmol g−1 )

C of n-hexadecane in the mixture (mM)

PMMT1 PMMT2 PMMT3

9.0 g/9.0 g (1:1) 9.0 g/13.7 g (2:3) 9.0 g/18.3 g (1:2)

4.50 6.75 9.0

400 600 800

oclay by intercalation with SDS and to expand the galleries of Na–MMT with the aqueous SDS. Subsequently, 400, 600 or 800 mM of n-hexadecane was added into the suspension, and the mixing process was continued at 1600 rpm and 50 ◦ C for 60 min. In each experiment, the pH of the mixture was kept constant at 6.6. After cooling to ambient temperature, each colloidal sample was sonicated with a Hielscher 400S ultrasound processor at an acoustic power density of 460 W cm−2 for 20 min. The amplitude was 210 ␮m, and the period was 1.0 s. Finally, the solid phase was separated from the liquid phase by centrifugation followed by washing with distilled water three times at room temperature. The resulting filter cake was dried at 25 ◦ C in a rotary evaporator under vacuum overnight and ground by ultra-sonication for 10 min under the same conditions described above. The final PCM–clay composite (PMMT) powders were stored in closed containers in a desiccator. The PMMT sample names and their mixing ratios of Na–MMT and n-hexadecane are shown in Table 1, and the scheme for surfactant-mediated intercalation of n-hexadecane to the galleries of Na–MMT is represented in Fig. 1. 2.2.2. Characterization of PCM–clay composite samples For characterization, all PMMT samples were first examined with a SHIMADZU XRD-6000 X-ray diffractometer, using Cu K␣ ˚ The applied voltage and X-rays with a wavelength of 1.5405 A. current values were 40.0 kV and 30.0 mA, respectively. X-ray diffractograms were obtained at the scanning rate of 2.0000◦ min−1 between a 2 range of 2.0000–70.0000◦ , with a step scanning speed of 0.0200◦ s−1 . The sample amount was held constant at 250 mg. The FTIR spectra of the samples were recorded between 4000 and 650 cm−1 at a resolution of 4 cm−1 by a Perkin Elmer Spectrum

Fig. 1. The suggested scheme for surfactant-mediated intercalation of n-hexadecane to the galleries of Na–MMT.

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Fig. 2. XRD patterns of PCM–clay composites in comparison with that of Na–MMT for 2 = 2.00–70.00◦ .

100 FTIR spectrometer equipped with a universal attenuated total reflection (ATR) accessory. Thermogravimetric analysis of the samples were performed from 20 to 900 ◦ C at 10 ◦ C min−1 under a dry nitrogen atmosphere purged at a rate 20 mL min−1 by a SEIKO 6200 TG/DTA instrument. The TG/DTA instrument was calibrated with indium, and platinum pans were used as sample holders. The approximate sample masses were 10 mg. Thermal properties of the samples were also examined by a Perkin Elmer DSC 4000 differential scanning calorimeter with an accuracy of ±0.001. It was calibrated with indium (melting point: 156.6 ◦ C, Hf = 28.1 J g−1 ), and an empty aluminum pan was used as a reference. A nitrogen flux (20 mL min−1 ) was used as a purge gas for the furnace. Temperature scans were run on samples that were placed in an open pan and weighed between 5 and 15 mg. All samples were brought to thermal equilibrium using the following process: the temperature was first equilibrated at −5 ◦ C, held for 1 min, followed by heating the samples to 65 ◦ C and holding for 1 min. Finally, the samples were cooled to −5 ◦ C at a 10 ◦ C min−1 cooling rate. The thermal durability of the samples for ten successive melting and freezing cycles between 16 and 30 ◦ C was measured using DSC at a rate of 2 ◦ C min−1 . Dynamic light scattering (DLS) measurements were performed using a Malvern ZS-3600 nanosizer equipped with a He–Ne solidstate laser operating at  = 633 nm to determine the average particle size in 0.01% dilute suspensions at 25 ◦ C. The mean particle diameter was calculated by using the backscattered light detected at 90◦ and from the quadratic fitting of the correlation function over 50 runs in 70 s. SEM analysis of the selected samples was performed on a JEOL JXA 840A type SEM. To prepare the SEM samples, approximately 0.01 g of dried, powdered clay samples were placed on standard mounts, 15 mm in diameter and 2 mm in depth, under vacuum and coated with a 1–2 nm thick conductive layer of gold to prevent charging during imaging. 3. Results and discussion 3.1. XRD analysis of the samples Fig. 2 shows the XRD patterns of PMMT1, 2 and 3 in comparison with that of Na–MMT in the 2 range of 2.00–70.00◦ .

The basal plane (0 0 1) reflection between the clay platelets of the Na–MMT sample appeared at 2 = 6.38◦ , corresponding to d001 = 1.38 nm. In the X-ray diffractogram of PMMT1, the reflection shifted to 2 = 4.62◦ , corresponding to d001 = 1.91 nm, as a result of the expansion of the interlayer spacing. Similarly, in the Xray diffractogram of PMMT2, the same reflection was observed at 2 = 4.59◦ and d001 = 1.92 nm. In the XRD pattern of PMMT3, two prominent peaks appeared at 2 = 2.90◦ and 4.46◦ , corresponding to an interlayer spacing of d001 = 3.04 nm and 1.98 nm, respectively. The multiple reflections observed in the PMMT3 pattern show that different molecular arrangements of n-hexadecane molecules within the clay layers were possible. Therefore, particles in PMMT had varying gallery distances [45]. Additionally, the new reflections appeared at 2 = 2.76–2.98◦ for PMMT3, indicating that more than one molecular layer of alkyl chains was present in the interlamellar space of the PMMT3 sample [49]. The expansions of the d001 spacing of all PMMT samples indicate that the intercalation of nhexadecane in the galleries of Na–MMT was successfully achieved. The reflections at 2 = 19.92◦ , 20.82◦ , 26.66◦ , 28.72◦ , 34.92◦ , 50.15◦ , 54.08◦ and 61.97◦ in the diffractogram of Na–MMT were related to the structural properties of MMT. However, the diffractograms of the PMMT samples show slightly shifted reflections with significantly decreased intensities relative to the reflections of the Na–MMT sample. The shifts and reduced intensities in the higher order reflections of all PMMT samples compared to those of the Na–MMT sample were attributed to the delamination and disorientation of clay platelets [50]. 3.2. FTIR analysis of the PMMT samples To obtain complementary evidence to the XRD data for the intercalation of n-hexadecane into the galleries of Na–MMT, the full FTIR spectra of n-hexadecane, Na–MMT and PMMT samples were recorded in the range of 4000–650 cm−1 as illustrated in Fig. 3. The asymmetric stretching vibration (3 ) of the structural –OH groups of Na–MMT at 3637 cm−1 shifted slightly to 3641–3643 cm−1 in the PMMT samples. This slight shift towards the smaller frequencies and the decreased intensities imply that some structural hydroxyl groups were removed from the Si–OH and Al–OH sites as a result of n-hexadecane intercalation. The broad band of Na–MMT observed at 3410 cm−1 corresponded to the over-

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Fig. 3. The FTIR transmission spectra of the PMMT samples in comparison with those of Na–MMT and n-hexadecane.

lapping stretching vibrations of both the structural and the free –OH groups. This band shifted to 3437–3375 cm−1 and became broader, smaller, and almost disappeared in the IR spectra of all PMMT samples. This phenomenon indicates the successful hydrophilic to organophilic conversion for the PMMT samples [51,52]. The bending-in-plane vibrations of the –OH groups (2 ) are characterized by a broad band at 1651 cm−1 . Similar to the stretching vibrations discussed earlier, this band shifted slightly to 1661–1669 cm−1 and had lower intensity in the PMMT samples compared to the Na–MMT sample, confirming their hydrophilic to hydrophobic conversion. The FTIR spectrum of pure n-hexadecane had four major transmission bands. The bands at 2921 cm−1 and 2853 cm−1 , which corresponded to the asymmetric stretching vibration (asym ) and the symmetric stretching vibration (sym ) of the CH2 groups, respectively, were sensitive to the conformational changes of the chains [43,48]. In the IR spectra of PMMTs, the position of the asym (CH2 ) transmission peak at 2921 cm−1 was slightly shifted to 2922–2923 cm−1 and the sym (CH2 ) peak at 2853 cm−1 was also slightly shifted to 2854 cm−1 compared to those in the IR spectra of pure n-hexadecane. The shifts suggest limited conformational disorder of the intercalated alkyl chains, and the confined hexadecane chains essentially adopted all-trans conformation following pure n-hexadecane molecules [49]. The in-plane scissoring motion of the CH2 group at 1471 cm−1 and out-of-plane bending motion of the –CH group at 721 cm−1 observed in n-hexadecane almost kept their positions in the PMMT samples, which also indicates an ordered configuration of intercalated hexadecane chains. The FTIR spectrum of Na–MMT showed a sharp peak at 1000 cm−1 , which corresponded to the stretching vibrations of Si–O–Si bonds of the tetrahedral silica layers. The bands observed at 913 cm−1 and 846 cm−1 were associated with the bending-in-plane vibrations of the Si–O and Al–O bonds of the octahedral and tetrahedral silica–alumina layers of Na–MMT, respectively. Furthermore, the locations of these peaks were sensitive to the intercalation of Na–MMT [49,53]. The peak at 1000 cm−1 shifted to 1014 cm−1 in the IR spectra of PMMT3, whereas the peak at 913 cm−1 shifted to 915 cm−1 and the peak at 846 cm−1 shifted to 850 cm−1 in the IR spectra of all PMMTs. The physical attractions between Si–O or Al–O groups to the alkyl chains of n-hexadecane present in the environment caused these slight shifts to the higher wave numbers and the decrease in their intensities observed in the IR spectra [49].

3.3. TG analysis of the PMMT samples To determine the thermal stability and the decomposition behavior of PCM–clay composites, TG curves and the corresponding derivative curves (DTG) of PMMT3 and Na–MMT are compared in Fig. 4. As described in our previous study [50], the thermal analysis of Na–MMT demonstrated the typical behavior of MMT [54]. Thermal degradation of Na–MMT proceeded in two steps: first, the sample was dehydrated when the temperature increased from room temperature to 270 ◦ C as the water molecules adsorbed in pores and clay galleries were removed. Then, the sample underwent dehydroxylation of structural –OH groups, which occurred between 500 ◦ C and 700 ◦ C. The residue at 850 ◦ C was found at 83% of the original sample. The thermal degradation process of PMMT3 from room temperature to 850 ◦ C differed notably from that of Na–MMT: the incline of the TG curve was less steep compared to that of Na–MMT, up to 145 ◦ C, and the corresponding DTG peak became smaller and shifted to a higher temperature because the water molecules were replaced by n-hexadecane in the pores and in the MMT galleries. As seen in Fig. 4a and b, minor thermal degradation of n-hexadecane in the PMMT3 structure occurred between 200 ◦ C and 300 ◦ C, and the maximum rate that signified the thermal stability of the PCM–clay composite was attained at 250 ◦ C. From 300 ◦ C to 700 ◦ C, the dehydroxylation peak in the DTG curve of PMMT3 and the corresponding mass loss in its TG curve completely disappeared when the curves were compared to those of the Na–MMT sample. These findings imply that the structure of PMMT3 was converted from hydrophilic to organophilic. PMMT3 residue at 850 ◦ C was found at 88% of the original sample, which was greater than the residue of Na–MMT at the same temperature. The presence of higher percent of residue was possibly due to the charred remains of n-hexadecane entrapped in the galleries and pores of Na–MMT, which in turn slowed the escape of the volatile products generated during thermal degradation and suggested that the composite structure of PMMT3 is thermally stable [55].

3.4. DSC analysis of the PMMT samples The temperature range of phase transition and the corresponding enthalpy changes of PMMT samples and n-hexadecane obtained from DSC measurements are summarized in Table 2. The relevant

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Fig. 5. The DSC curves obtained during the heating and subsequent cooling cycles of the PMMT samples at the rate of 10 ◦ C min−1 in comparison with n-hexadecane between −5 ◦ C and 65 ◦ C with a sample weight of 65.00 mg. 80

Heat Flow Endo Up (mW)

70

Fig. 4. (a) TG graphs of Na–MMT and PMMT3 samples and (b) the corresponding DTG graphs between room temperature and 900 ◦ C.

60 50 40 30 20 t

(PMMT)

t

(n-hexadecane)

t

(PMMT)

t

(n-hexadecane)

10 0

DSC curves of these samples during heating and subsequent cooling are shown in Fig. 5. As seen from Table 2 and Fig. 5, the phase transition of nhexadecane occurred between 17 ◦ C and 25 ◦ C during heating, and the corresponding heat capacity (Hfusion ) was 209 J g−1 . While cooling from 65 ◦ C to −5 ◦ C, the heat released (Hfreezing ) for nhexadecane was 209 J g−1 . However, the temperature range of solidification was lowered to between 14 ◦ C and 9 ◦ C due to the super-cooled n-hexadecane chains in their liquid state that arose from the considerable cooling rate during DSC analysis [19]. Consequently, the phase transition temperature range of the PMMT samples greatly overlapped with that of n-hexadecane, which was intercalated in the galleries of Na–MMT. This observation implies that the chemical structure of n-hexadecane was not adversely impacted during the manufacturing process of the PCM–clay composites. As seen from the solid-to-liquid phase transition temperature range in Table 2, the end temperatures for the PMMT samples were higher (28–39 ◦ C) compared to those of nhexadecane and imply a stronger physical attractions between

1

2

3

4

5

6

Time (min) Na-MMT

n-hexadecane

PMMT1

PMMT3

Fig. 6. (a) Heat flow rate (mW) versus time (min) of the PMMT samples during the heating cycle of DSC experiments in comparison with Na–MMT and n-hexadecane. The samples were heated to 65 ◦ C at a rate of 10 ◦ C min−1 . All samples weighed 65.00 mg.

n-hexadecane molecules and the galleries and pores of the clay mineral [56]. Furthermore, a positive correlation was obtained between the enthalpies of fusion of PMMT samples and those of n-hexadecane. Depending on the amount of n-hexadecane in the oil–water–clay colloidal dispersions, the corresponding enthalpies of fusion were 82, 93 and 126 J g−1 for PMMT1, PMMT2 and PMMT3, respectively. All enthalpy values, especially those obtained for PMMT3, were notable compared to the heat capacity values of some composite PCMs and commercial PCM products reported in the literature in the temperature range of 10–50 ◦ C [11,42,47,48,57]. When the PMMT samples were cooled from 65 ◦ C to −5 ◦ C, the onset of the freezing temperatures in the DSC curves were similar

Table 2 Phase transition characteristics of n-hexadecane and PMMTs for the heating and subsequent cooling cycles at the rate of 10 ◦ C min−1 as measured by DSC. Sample

Heating from −5 to 65 ◦ C ◦

Tonset –Tend ( C) n-Hexadecane PMMT1 PMMT2 PMMT3 PMMT4

17–25 16–30 17–39 17–28 17–31

Cooling from 65 to −5 ◦ C −1

H (J g 209 82 93 126 98

) (>0)

(HPMMT /Hhexadecane ) (%)

Tonset –Tend (◦ C)

– 39 45 60 47

14–9 14–1 14–1 14–5 16–2

H (J g−1 ) (<0) 209 78 80 125 80

(HPMMT /Hhexadecane ) (%) – 37 38 60 38

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to those of n-hexadecane. However, the end temperatures of the phase transition shifted to even lower temperatures compared to n-hexadecane due to the physical bonding between the interlayers of n-hexadecane and clay. The enthalpy changes of PMMT samples during the cooling cycle, especially for PMMT3, were all comparable with those of the heating cycle and indicated the desegregation and stability of the composite structures. The ratios of HPMMT to Hhexadecane for the PMMT samples were between 39% and 60% for the heating cycle and between 37% and 60% for the cooling cycle, as given in Table 2. These values also pointed out a good extent of intercalation and adsorption of n-hexadecane in Na–MMT for all PMMT samples. The heating curve of n-hexadecane was narrow and elongated, whereas the heating curves of PMMT samples were broad and distorted, with larger widths at half height and lower peak heights and response ratios compared to the heating curve of n-hexadecane. These findings could be related to the porous and layered structure of Na–MMT and the physical attractions between the entrapped n-hexadecane and the surface of clay galleries characteristic of the composite structure of the PMMT samples [48,50,53]. Fig. 6a shows the heat transfer rate versus time (in min) of the samples PMMT1, PMMT3 and n-hexadecane during the heating step in DSC analysis. As seen from the figure, the heat absorption responses of the PMMT samples were faster than those of pure nhexadecane, suggesting that Na–MMT contributed to the thermal conductivity of the PCM–clay composites. The thermal response

Fig. 8. Comparison of the particle size distributions of the Na–MMT, PMMT2 and PMMT3 samples obtained from Malvern Nanosizer ZS-3600.

observed in the PMMT samples could be utilized for future practical applications [47,48]. 3.5. Thermal cycling tests The thermal durability and segregation properties of the products were tested by heating and cooling the n-hexadecane and PMMT3 samples. During DSC analysis, the samples were heated and cooled for ten cycles between 16 ◦ C and 30 ◦ C at a heating and cooling rate of 2 ◦ C min−1 . Fig. 7a and b shows the heat flow (mW) versus temperature (◦ C) curves of n-hexadecane and PMMT3 after 1, 3, 5, 7 and 10 cycles. Almost no change in the enthalpy of n-hexadecane was observed, whereas the enthalpy change curves of PMMT3 through ten heating and cooling cycles overlapped. The results indicate that no chemical degradation and segregation of n-hexadecane and PMMT3 occurred during thermal cycling. The PMMT samples exhibited good thermal durability, in parallel with the findings of TG-DTG analysis, suggesting that PMMT is good for latent heat storage applications. 3.6. Particle size distribution of the PMMT samples Particle size distributions of the Na–MMT, PMMT2 and PMMT3 samples are given in Fig. 8. The Na–MMT particles had a unimodal distribution from 0.20 ␮m to 8.4 ␮m, where 50% of the particles were smaller than 1.26 ␮m. The particle size distribution of PMMT2 and PMMT3 samples generally matched that of Na–MMT. However, the curves shifted slightly towards lower particle sizes (up to 78 nm) and were narrower. The shift towards smaller particle sizes for the PMMT samples could be related to the partial degradation and lateral size reduction of clay particles during the intercalation and ultrasonication steps of the manufacturing process. On the other hand, some PMMT2 and PMMT3 particles were between 3.3 ␮m and 8.4 ␮m due to occasional agglomeration, which could be the result of the cohesive forces between clay particles intercalated with n-hexadecane in some parts of the specimen. 3.7. SEM examination of the PMMT samples

Fig. 7. The heat flow versus temperature curves of (a) n-hexadecane and (b) PMMT3 between 16 ◦ C and 26 ◦ C after 1, 3, 5, 7 and 10 heating/cooling cycles.

The surface morphologies of Na–MMT, PMMT2 and PMMT3 samples were examined based on SEM images. The physical appearance of Na–MMT grains shown in Fig. 9a were distinctly different from the PMMT grains as shown in Fig. 9b and c. The large and small sphere-like particles were replaced by highand low-aspect ratio flakes, respectively. The clay platelets stacked together in a disordered pattern to form agglomerates in certain regions, consistent with the particle size distribution analysis. In various parts of the SEM images of PMMT2 and PMMT3 seen in Fig. 9b and c, the composite structures of these samples were evi-

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with SDS to make them organophilic in character, followed by surfactant-mediated intercalation of n-hexadecane into the galleries of organophilic MMT. XRD, FTIR, TG-DTG, DSC and SEM results revealed that PMMT samples achieved the high heat capacities of n-hexadecane and retained the thermal stability, physical durability and thermal conductivity of Na–MMT. The XRD patterns and the IR spectra of the PMMT samples revealed that n-hexadecane was successfully intercalated in the galleries of Na–MMT in all of the samples, and the SEM image analysis also supported these findings. According to DSC analysis, the PMMT samples had remarkable thermal capacities and thermal conductivities owing to the combination of n-hexadecane and MMT. As the mass percentage of n-hexadecane in PMMT composites increased to 60% in the PMMT3 sample, the thermal storage and release capacities reached 126 J g−1 and 125 J g−1 , respectively, which corresponded to the phase change temperature range of 14–28 ◦ C. The thermal stability of PMMT observed in the TG-DTG analysis indicates that PCMs and their composites could be suitable for industrial applications. The advantages of PCM–clay composites prepared in this study include their thermal energy storage capacity, reduced PCM reactivity with the outside environment, increased heat-transfer area provided by the numerous clay particles, the potential of large scale processing, and their simple, environmentally friendly and low cost of production. In conclusion, this research recommends the method of surfactant-mediated intercalation of n-hexadecane into Na–MMT to enhance the thermal performance of different thermal storage materials. The incorporation of PMMT into thermal storage building materials such as wallboards is under investigation. Acknowledgements We would like to thank The Scientific & Technical Research Council of Turkey (TUBITAK) for the financial support of this study (project code: TUBITAK 107M126). We also acknowledge Istanbul Kultur University for their financial support. References

Fig. 9. SEM images of (a) Na–MMT, (b) PMMT2 and (c) PMMT3 grains.

denced by the disappearing grain boundaries and disperse flakes of clay minerals. These observations were consistent with the findings of the TG and DSC experiments. 4. Conclusion The fabrication method, characterization and thermal performance of PCM–clay composites are presented in this paper. The composites were prepared by the pre-modification of Na–MMTs

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