Halloysite clay nanotubes based phase change material composites with excellent thermal stability for energy saving and storage

Abstract

Superhydrophobic PDMS–HNTs (halloysite clay nanotubes) were prepared by the surface modification of purified HNTs (p-HNTs) with polydimethylsiloxane (PDMS). The strong hydrophobic properties and distinctive structure of PDMS–HNTs, resulting from the hollow and tubular structure of p-HNTs, have led to their use as supporting materials in the preparation of form-stable phase change material (PCM) composites, into which PCMs can be easily adsorbed and remain stable without leakage even at temperatures greater than the melting points of the PCMs. The PCMs were loaded at the outer and inner surfaces of PDMS–HNTs, and as a result of their incorporation, the latent heats of the PDMS–HNTs/PCMs composites were measured to be in the range of 44.7–72.1 kJ kg⁻¹. Taking advantage of the simple process, low cost and excellent thermal stability, the PDMS–HNTs/PCM composites may have great potential for practical applications as solar energy storage systems.

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Introduction

The problem of the effective utilization of energy needs to be settled to address environmental issues caused by the fast changing modern industry and the rapid dwindling fossil fuels, as well as for sustainable development.¹ It has been reported that the use of renewable energy might be a promising path to solve the deficiencies between energy generation and energy demand.² However, most of the renewable energy sources, including solar energy, wind energy, tidal energy and geothermal energy, are intermittent. Hence, it is necessary to develop safe and efficient energy storage devices and systems for utilization of these mentioned energy sources.

Phase change materials (PCMs) can be used to adjust the ambient temperature or indoor temperature by collecting and storing the temporarily unused or waste heat and releasing latent heat when their phase change occurs.^3,4 They exchange latent heat with the air outside and maintain a steady temperature during the phase transition process. PCMs show great potential for a wide variety of applications, such as saving solar energy, energy saving building coatings,^5,6 industrial waste heat recovery, air-conditioning energy conservation, medical treatment, aerospace science and technology, and functionally thermal fluid. PCMs involving microcapsules acting as the shell and phase change materials acting as the core have been well studied.⁷ However, the main deficiencies of microcapsules are the high cost and complicated processes. One of the methods for overcoming this issue is to store the PCMs in supporting materials. Recently, it was found that utilization of porous materials, such as carbon nanotubes,⁸ kaolinite,^9,10 perlite,¹¹ diatomite,¹² sandstone, gypsum and porous silica,⁵ for the direct absorption of PCMs to prepare PCM composites has the advantages of low cost and large latent heat. However, in most cases, the liquid phase of PCMs is prone to leakage when the temperature is higher than their melting point, which is the major obstacle hindering their large-scale practical applications. Therefore, the development of efficient technologies or new approaches that can enhance the thermal durability of PCMs is of significant importance and urgently demanded.

Natural halloysite clay nanotubes (HNTs), which are two-layered aluminosilicate clay minerals, are formed in multiple rolled layers consisting of hollow cylinders with sub-micrometer dimensions. The HNTs are natural nanomaterials that are abundantly available all over the world^13–15 and are considered to be “green” materials because they are natural and environmentally friendly products.¹⁶ Moreover, HNTs have opposite chemistry between the inner surface and the outer surface, because they are composed of gibbsite octahedral sheet (Al–OH) groups on the internal surface and siloxane groups (Si–O–Si) on the external surface.^17–23 These characteristics make the HNTs promising candidates as nanomaterials for the preparation of PCM composites. To date, however, the use of HNTs as a supporting material to prepare form-stable PCM composites has rarely been reported.^24,25

In this study, we developed a new approach using HNTs as supporting materials, and paraffin wax and n-carboxylic acids as phase change materials, to prepare novel PCMs composites. Our primary design involves changing the wettability of the HNTs from hydrophilic to hydrophobic by coating the HNTs with PDMS via the chemical vapor deposition (CVD) method, followed by incorporation of PCMs into PDMS-treated HNTs (PDMS–HNTs) by the vacuum melt-impregnation method, to prepare PCMs composites. Such resulting superhydrophobic PDMS–HNTs have strong affinity for organic PCMs, which is expected to enhance the thermal stability of the resulting PCM composites, and the effects of the hollow, tubular structure on the thermal properties are investigated herein. Taking advantage of the simple process, low cost and good stability, the resulting PCM composites are of great technological significance and may have great potential as solar energy storage systems for various applications such as wall coatings for buildings that can be constructed by only a simple painting process.

Experimental section

Chemicals

Natural halloysites were supplied by Hebei Lingshou Yanbo Mineral Processing Factory. Myristic acid (MA), palmitic acid (PA), stearic acid (SA) and sodium hexametaphosphate were purchased from BASF Chemical Co., Ltd.

Preparation of p-HNTs and PDMS–HNTs supporting materials

Natural halloysites were purified before use. The natural halloysites and sodium hexametaphosphate, with mass ratio of 50 : 1, were dispersed into distilled water with stirring for 1 h followed by filtration and drying. The resulting mixture (0.1 g) was added to a HCl solution (1 M, 500 mL) with stirring for 24 h, followed by washing, drying and milling to obtain purified halloysite that was named as p-HNTs. The PDMS modified p-HNTs (PDMS–HNTs) were prepared by placing a certain amount of p-HNTs powder and a PDMS (polydimethylsiloxane) stamp (Sylgard 184, Dow Corning) in a sealed glass container and heating at 240 °C for 2.5 h.

Preparation of form-stable p-HNTs/m-PCM composites and PDMS–HNTs/m-PCM composites

The p-HNTs/m-PCM composites were prepared by a melt-impregnation method, using paraffin wax, MA, PA and SA as the PCM, and HNTs-based samples as the supporting materials. In a typical procedure, the p-HNTs powder was filled into a glass dropper with its minor caliber tube tightly stuffed with cotton wool. Then, the abovementioned glass dropper with a rubber plug was inserted into the PCM (such as paraffin wax). The system was heated to 90 °C and allowed to stand for 3 h. The system was vacuumized every 0.5 h to make the hot PCM into HNTs nanotubes, and then cooled to room temperature. This heating and cooling process was repeated three times. The PDMS–HNTs/m-paraffin wax composite, p-HNTs/m-n-carboxylic acids composite and PDMS–HNTs/m-n-carboxylic acids composite (m-n-carboxylic acids were MA, PA, and SA) were fabricated by the same method mentioned above. Here, it should be noted that the PDMS–HNTs/m-paraffin wax composite was washed three times with benzene in a CNC ultrasonic cleaner to remove paraffin wax on the outer surface of the PDMS–HNTs tubes.

Preparation of form-stable p-HNTs/v-PCM composites by CVD and PDMS–HNTs/v-PCM composites by CVD

The p-HNTs/v-PCM composites and PDMS–HNTs/v-PCM composites were prepared by a chemical vapor deposition (CVD) method using MA and PA as the PCM, and p-HNTs and PDMS–HNTs as the supporting materials. Taking the p-HNTs/v-MA composite as an example, a certain amount of p-HNTs powder was filled into a glass dropper connected to a vacuum pump. The glass dropper was then placed in the MA vapor (200 °C) and was vacuumized every 0.5 h to convert the MA vapor into the HNTs nanotubes; subsequently, the system was cooled to room temperature. This heating and cooling process was repeated three times. The p-HNTs/v-PA composite, PDMS–HNTs/v-MA composite and PDMS–HNTs/v-PA composite were prepared using the same method mentioned above.

Characterization

Water contact angle (CA) measurements for the samples were performed on a contact angle meter (DSA100, Kruss). X-ray diffraction (XRD) measurements were performed on a Rigaku D/Max-2400 diffractometer with a Cu tube source, and 2θ scans were obtained from 5° to 80°. The Brunauer–Emmett–Teller (BET) surface area and the porous volume of the PDMS–HNTs and PDMS–HNTs/m-MA composite material were detected with a nitrogen adsorption/desorption isotherm measured at 77 K, using a physisorption analyzer (ASAP 2020, Micromeritics). The morphologies of the materials were investigated using field emission scanning electron microscopy (FE-SEM, JSM-6701F, JEOL, Ltd.) and transmission electron microscopy (TEM, JEM-1200EX, FEI, Co.). The thermodynamic properties of paraffin wax, MA, PA, SA and PCM composites were determined by differential scanning calorimetry (TGA/DSC1, METTLER TOLEDO) at a heating and cooling rate of 5 °C min⁻¹ in the range of 20–100 °C and under nitrogen atmosphere. The thermal diffusivities and the specific heats of the PDMS–HNTs/PCM composites were measured at 70 °C by a flash thermal conductivity testing instrument (LFA 447 Nanoflash, NETZSCH).

Results and discussion

In this study, two methods were employed for the preparation of PCM composites by the incorporation of PCMs into HNTs. During the melt-impregnation method, the liquid PCMs were in contact with the HNTs supporting materials through cotton wool when the temperature was above the melting point of the PCMs. Due to the strong affinity of the PDMS–HNTs for the PCMs, the liquid PCMs spontaneously permeated inside the PDMS–HNTs tubes. During this procedure, air trapped in the system was removed by vacuum. It is worth noting that the PCMs were also trapped on the outer surface of the HNTs; however, they were removed by washing with organic solvents. The schematic procedure is shown in Scheme 1. During the CVD, the PCMs vapor was incorporated into the HNTs, followed by washing with organic solvents.

Scheme 1 The production process of the PDMS–HNTs/m-paraffin wax composite.

We used HNTs as porous media for the preparation of PCM composites. Due to their unique hollow cylinders with morphologies of submicrometer dimensions, HNTs have excellent adsorption capacity for oil or organic compounds.²⁶ However, because of the hydrophilic nature of HNTs, the affinity for common and hydrophobic PCM (such as paraffin wax) is poor. Therefore, to enhance the interface affinity between PCM and HNTs, the hydrophobic modification of HNTs is necessary. To this end, polydimethylsiloxane (PDMS), a low surface free energy material, was used to improve the surface wettability of p-HNTs, according to our previous study.²⁷ As expected, the hydrophilic HNTs were changed into hydrophobic surfaces after the modification. The water contact angle (CA) was detected to be 157° (superhydrophobic surface) and the diesel oil/organic CA for the PDMS–HNTs was found to be very near to 0°. Due to the strong superoleophilicity of the PDMS–HNTs, paraffin wax and many n-carboxylic acids can be easily incorporated into the PDMS–HNTs.²⁸ The water CA and the oil CA of PDMS–HNTs can be seen in Fig. S1.†

The SEM images of p-HNTs and PDMS–HNTs are shown in Fig. 1a and b. In Fig. 1a, it can be seen that the p-HNTs consist of rod-like nanocrystals that together form aggregates. After modification with PDMS, the morphology of the p-HNTs remains unchanged (Fig. 1b), indicating that the surface modification has no obvious influence on the topology of the HNTs. Fig. 1c–f show the TEM images of PDMS–HNTs, PDMS–HNTs/m-paraffin wax composite, PDMS–HNTs/m-MA composite and PDMS–HNTs/v-MA composite, respectively. It can obviously be seen that the HNTs possess a hollow nanotubular structure and clean internal surface (Fig. 1c). The length of the halloysite tubules is in the range of 0.5–1.5 μm, with an inner diameter about 20 nm and an external diameter of about 50 nm. Fig. 1d–f show TEM images of the intercalated HNTs, which indicate that the PCMs were densely filled inside. A careful examination of the image in Fig. 1d (PDMS–HNTs filled with paraffin wax) reveals that some ordered striations (marked by arrows, several of which are highlighted in Fig. 1e and f) are visible beside the internal surface. As shown in Fig. 1e (MA filled PDMS–HNTs by the melt-impregnation method), the inner surfaces of these PDMS–HNTs are rather rough,²⁹ and MA was irregularly filled inside the nanotubes, which was probably caused by a non-uniform deposition of MA inside the HNTs tubes.^30,31 Fig. 1f is the TEM image of PDMS–HNTs filled with MA by the CVD method, which shows a similar morphology to that of MA filled PDMS–HNTs by the melt-impregnation method. A slight difference among Fig. 1e–f and d is that the presence of PCMs on both the outer and inner surfaces of the PDMS–HNTs is observed because the PDMS–HNTs/m-MA composite and PDMS–HNTs/v-MA composite were not washed with benzene.³²

Fig. 1 SEM images of (a) p-HNTs and (b) PDMS–HNTs. TEM images of (c) PDMS–HNTs, (d) PDMS–HNTs/m-paraffin wax composite, (e) PDMS–HNTs/m-MA composite and (f) PDMS–HNTs/v-MA composite. Scale bar: (a, b, d) 100 nm and (c, e, f) 50 nm.

The surface areas and pore diameter distributions of PDMS–HNTs and the PDMS–HNTs/m-MA composite can be evaluated by nitrogen adsorption and desorption analyses at 77 K. Fig. S2a† shows that the N₂ adsorption and desorption isotherms of PDMS–HNTs and the PDMS–HNTs/m-MA composite were type III, with narrow H3 type hysteresis loops.³³ The Brunauer–Emmett–Teller (BET) surface area was measured to be 26.1 m² g⁻¹ for PDMS–HNTs and 4.3 m² g⁻¹ for the PDMS–HNTs/m-MA composite. The BJH adsorption cumulative surface area of pores (radius range 0.85 nm to 150 nm) was evaluated separately as 30.2 m² g⁻¹ for PDMS–HNTs and 4.7 m² g⁻¹ for the PDMS–HNTs/m-MA composite. The pore volume achieved from the amount of gas adsorption at P/P₀ = 0.99 was found to be 0.15 cm³ g⁻¹ for PDMS–HNTs and 0.03 cm³ g⁻¹ for the PDMS–HNTs/m-MA composite. Obviously, the BET surface area, the BJH adsorption cumulative surface area of pores (radius range 0.85 nm to 150 nm) and the pore volume of the PDMS–HNTs/m-MA composite are much smaller than those of PDMS–HNTs, which could be attributed to the filling of PDMS–HNTs with MA. From the pore size distribution curves (Fig. S2b†), PDMS–HNTs and the PDMS–HNTs/m-MA composite consist of mesopores (pore size larger than 2 nm)³⁴ and micropores (pore size less than 2 nm), respectively.³⁵

Fig. 2 shows the XRD patterns of HNTs and the composites. As shown in Fig. 2a, the peak positions of PDMS–HNTs and paraffin wax in the PDMS–HNTs/m-paraffin wax composite remain nearly unchanged compared to PDMS–HNTs and pure paraffin wax. However, the peak intensity of the PDMS–HNTs/m-paraffin wax composite is much weaker than that of pure paraffin wax, suggesting that the crystallinity of paraffin wax in the PDMS–HNTs/m-paraffin wax composite was reduced. A similar observation was found in the conjugated microporous polymer based PCMs materials and PDMS–HNTs/m-MA composite (Fig. 2b). The peaks of the PDMS–HNTs/m-MA composite and PDMS–HNTs/v-MA composite are weaker than that of pure MA, suggesting that the crystal size of MA in the PDMS–HNTs/m-MA composite and PDMS–HNTs/v-MA composite are lower than that of pure MA.

Fig. 2 (a) XRD patterns of p-HNTs, PDMS–HNTs, paraffin wax and the PDMS–HNTs/m-paraffin wax composite. (b) XRD patterns of p-HNTs, PDMS–HNTs, MA, the PDMS–HNTs/m-MA composite and the PDMS–HNTs/v-MA composite.

The prepared PCM composites have excellent thermal properties. The DSC curves of paraffin wax, three different n-carboxylic acids and their relevant PCM composites are shown in Fig. 3a–f. The results obtained from the DSC measurement are presented in Table 1. The latent heat of the composites depends on the amount of PCM loaded in the supporting materials. The mass fraction of PCM in the composites can be calculated by the following equation:¹

where ΔM (pure PCM) and ΔM (PCM composite) are the mass of pure PCM and the mass of its relevant composites. Moreover, the PCM latent heat percent of pure PCM and PCM composites can be calculated by the following equation:⁷where ΔH (PCM composite) refers to the latent heat of the PCM composites and ΔH (pure PCM) refers to the latent heat of the relevant pure PCM. The PCM percentage latent heat in the PCM composites and the mass fraction of crystallized PCM in the PCM composites are listed in Table 1. The latent heat storage values of the PDMS–HNTs/m-paraffin wax composite, PDMS–HNTs/m-MA composite, PDMS–HNTs/m-PA composite and PDMS–HNTs/m-SA composite range from 44.7 J g⁻¹ to 72.1 J g⁻¹, accounting for the latent heat storage values of 41.9% for pure paraffin wax, 41.82% for MA, 31.2% for PA and 42.3% for SA. The latent heat storage values of the PDMS–HNTs/m-paraffin wax composite (44.7 J g⁻¹), PDMS–HNTs/m-MA composite (72.1 J g⁻¹), PDMS–HNTs/m-PA composite (58.4 J g⁻¹) and PDMS–HNTs/m-SA composite (70.3 J g⁻¹) are lower than those of pure paraffin wax (106.7 J g⁻¹), MA (172.4 J g⁻¹), PA (187.3 J g⁻¹) and SA (166.0 J g⁻¹); however, these values are increased compared to those of the p-HNTs/m-paraffin wax composite (21.4 J g⁻¹), p-HNTs/m-MA composite (59.2 J g⁻¹), p-HNTs/m-PA composite (54.8 J g⁻¹) and p-HNTs/m-SA composite (57.6 J g⁻¹). The PCM latent heat percentages of the PDMS–HNTs/PCM composites are higher than that of the p-HNTs/PCM composites, improving by 21.8%, 7.5%, 1.9% and 7.6%, respectively. In addition, PCM composites synthesized by the CVD method have similar results. As shown in Table 2, the latent heat storage values of the PDMS–HNTs/v-MA composite and PDMS–HNTs/v-PA composite range from 54.7 J g⁻¹ to 59.9 J g⁻¹, accounting for the latent heat storage values of 34.74% for pure MA and 29.2% for of pure PA. The latent heat percentages of both the PDMS–HNTs/v-MA composite and PDMS–HNTs/v-PA composite presented are higher than that of the p-HNTs/v-MA composite and p-HNTs/v-PA composite, increasing by 34.4% and 25.8%. We used the parameter F_c to evaluate the preservation of the crystalline phase in the composites:³⁶where ΔH_PCMs and ΔH_pure are the latent heats of the composite and pure MA, β is the mass fraction of PCMs in the composites. A higher F_c value means higher preservation of the crystalline phase and thus means weaker interaction. F_c is 91.3% for PDMS–HNTs/m-MA and 68.8% for PDMS–HNTs/v-MA; both values are lower than that of pure MA, which means lower preservation of the crystalline phase in the composites.

Fig. 3 DSC curves for heating and cooling of different phase change materials and their corresponding PCM composites.

Table 1 Thermal properties of PCMs and PCM composites by the melt-impregnation method

Samples	Heating				Cooling
	PCM percentage (%)	Onset T/(°C)	Latent heat (J g^−1)	PCM latent heat percentage (%)	Onset T/(°C)	Latent heat (J g^−1)
Pure paraffin wax	100.0	54.7	106.7	100.0	58.1	120.9
p-HNTs/m-paraffin wax	51.5	52.5	21.4	20.1	59.0	27.4
PDMS–HNTs/m-paraffin wax	62.9	52.3	44.7	41.9	57.6	50.1
PDMS–HNTs/m-paraffin wax 200 cycles	62.9	57.4	39.4	36.9	66.4	46.3
Pure MA	100.0	54.1	172.4	100.0	51.0	172.6
p-HNTs/m-MA	41.8	51.6	59.2	34.3	50.8	48.2
PDMS–HNTs/m-MA	45.6	48.1	72.1	41.8	47.8	53.2
Pure PA	100.0	62.4	187.3	100.0	58.1	187.3
p-HNTs/m-PA	49.7	60.6	54.8	29.3	59.9	57.8
PDMS–HNTs/m-PA	51.3	46.2	58.4	31.2	47.6	48.4
Pure SA	100.0	55.2	166.0	100.0	53.1	166.5
p-HNTs/m-SA	52.3	53.0	57.6	34.7	53.1	53.0
PDMS–HNTs/m-SA	58.4	52.7	70.3	42.3	52.5	62.4

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Table 2 Thermal properties of PCMs and PCM composites by CVD

Samples	Heating
	Onset T/(°C)	Latent heat (J g^−1)	PCM latent heat percentage (%)
Pure MA	54.1	172.4	100.0
p-HNTs/v-MA	50.0	0.5	0.3
PDMS–HNTs/v-MA	50.2	59.9	34.7
Pure PA	62.4	187.3	100.0
p-HNTs/v-PA	35.4	6.3	3.4
PDMS–HNTs/v-PA	50.7	54.7	29.2

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In addition, from the DSC curves, the onset melting temperatures of the PDMS–HNTs/m-paraffin wax composite, PDMS–HNTs/m-MA composite, PDMS–HNTs/m-PA composite and PDMS–HNTs/m-SA composite decreased compared with those of pure paraffin wax, pure MA, pure PA and pure SA, respectively. For example, the onset melting temperatures were measured to be 52.3 °C and 48.1 °C for the PDMS–HNTs/m-paraffin wax composite and PDMS–HNTs/m-MA composite, respectively, showing a decrease by 2.4 °C and 6.0 °C, compared with pure paraffin wax and pure MA (Table 1). In comparison with the pure paraffin wax and the pure MA, the endothermic process of the PCM composites exhibits a slight hysteresis due to the thermal transitions of HNTs, which results in the decrease in the onset melting temperature. We also found that the onset melting temperatures of the PDMS–HNTs/m-paraffin wax composite, PDMS–HNTs/m-MA composite, PDMS–HNTs/m-PA composite and PDMS–HNTs/m-SA composite are lower than those of the p-HNTs/m-paraffin wax composite, p-HNTs/m-MA composite, p-HNTs/m-PA composite and p-HNTs/m-SA composite, respectively. For example, the onset melting temperatures of PDMS–HNTs/m-paraffin wax composite and PDMS–HNTs/m-MA composite decreased by 0.2 °C and 3.5 °C, respectively, with respect to that of the p-HNTs/m-paraffin wax composite and the p-HNTs/m-MA composite (Table 1). Interestingly, the onset melting temperature of PDMS–HNTs/m-PA composite decreased by 16.2 °C, compared with that of pure PA, and was 14.4 °C less than that of the p-HNTs/m-PA composite. As shown in Table 2, the onset melting temperature of the PDMS–HNTs/v-MA composite decreased by 4.1 °C, compared with that of pure MA and decreased 0.2 °C lower than that of the p-HNTs/v-MA composite as well, but the PDMS–HNTs/v-PA composite decreased by 11.7 °C compared to that of pure PA. These observations can also be found in the lauric acid-expanded perlite PCM composite systems.⁴ Moreover, the onset cooling temperatures of PDMS–HNTs/m-paraffin wax composite, PDMS–HNTs/m-MA composite, PDMS–HNTs/m-PA composite and PDMS–HNTs/m-SA composite are all lower than those of pure paraffin wax, pure MA, pure PA and pure SA, respectively. However, the onset cooling temperatures of p-HNTs/m-paraffin wax composite, p-HNTs/m-MA composite, p-HNTs/m-PA composite and p-HNTs/m-SA-composite are very close to that of PDMS–HNTs/m-paraffin wax composite, PDMS–HNTs/m-MA composite, PDMS–HNTs/m-PA composite and PDMS–HNTs/m-SA composite, respectively.

Thermal durability and stability of PCM composites are very important for evaluating their performance, especially in their practical application.³⁷ These properties were measured by thermal cycling tests and TGA. After 200 heating and cooling cycles, the PDMS–HNTs/m-paraffin wax composite maintained excellent thermal stability, and the results are shown in the DSC curves presented in Fig. 3a and Table 1. The latent heat of the PDMS–HNTs/m-paraffin wax composite after 200 continuous cycles is 39.4 J g⁻¹, showing a decrease of only 5.3 J g⁻¹. The TGA curves of pure MA, PDMS–HNTs/m-MA composite and PDMS–HNTs/v-MA composite are presented in Fig. 4. As can be seen from the curves, rapid weight loss of the pure MA, PDMS–HNTs/m-MA composite and PDMS–HNTs/v-MA composite is observed between 150 °C and 250 °C, due to the evaporation of MA. The weight loss ratio of pure MA is a bit larger than that in the PDMS–HNTs/m-MA composite and PDMS–HNTs/v-MA composite, suggesting that pure MA evaporates easier than that in the PDMS–HNTs/MA composite. The weight loss ratio of the PDMS–HNTs/v-MA composite is smaller than that of the PDMS–HNTs/m-MA composite. The weight curves of the pure MA, PDMS–HNTs/m-MA composite and PDMS–HNTs/v-MA composite are almost parallel over 250 °C. The MA in the composites was removed by complete evaporation at 450 °C. It is important to note that the mass loss at about 500 °C is crystal water in PDMS–HNTs.³⁸ Moreover, no decomposition is observed for the PDMS–HNTs/m-MA composite and PDMS–HNTs/v-MA composite at 180 °C, suggesting their good thermal stability. As shown in Fig. 4, the total weight loss of MA in PDMS–HNTs/m-MA composite, PDMS–HNTs/v-MA composite and p-HNTs/m-MA composite are 45.6%, 39.5% and 41.2%, close to the initial weight fractions of 41.8%, 34.7% and 34.3%, respectively, measured in the DSC test.

Fig. 4 The TGA curves of pure MA, and its relevant PCM composites between room temperature and 800 °C.

To investigate the effect of the PDMS–HNTs supporting materials on the thermal conductivities of n-carboxylic acids, the thermal conductivities of the samples were measured using the thermal conductivity testing instrument. The thermal conductivities of the PCMs (paraffin wax, MA, PA and SA) and PDMS–HNTs/PCM composites are listed in Table 3. The PDMS–HNTs/PCM composites have higher thermal conductivities than the PCMs. The different HNTs/m-PCM composites achieved an increase in thermal conductivity of 210.7%, 295.9%, 344.4% and 288.0%, compared to pure paraffin wax, MA, PA and SA, respectively. The different HNTs/v-PCM composites achieved an increase in thermal conductivity of 281.4% and 336.4%, compared with pure MA and PA, suggesting that the PDMS–HNT could enhance the thermal conductivities of n-carboxylic acids, which is of great importance for practical industrial applications.

Table 3 Thermal conductivity properties of the samples

Samples	Thermal diffusivity (mm^2 s^−1)	Specific heat (J g^−1 K^−1)	Density (g cm^−3)	Thermal conductivity (W m^−1 K^−1)
Pure paraffin wax	0.139	2.800	0.889	0.335
Pure MA	0.092	2.200	0.848	0.172
Pure PA	0.068	2.800	0.850	0.162
Pure SA	0.073	2.400	0.861	0.150
PDMS–HNTs/m-paraffin wax	0.208	1.435	1.459	0.706
PDMS–HNTs/m-MA	0.294	1.101	1.609	0.509
PDMS–HNTs/m-PA	0.269	1.306	1.588	0.558
PDMS–HNTs/m-SA	0.256	1.172	1.451	0.432
PDMS–HNTs/v-MA	0.261	1.128	1.646	0.484
PDMS–HNTs/v-PA	0.284	1.255	1.533	0.545

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Conclusions

A series of PCM composites were fabricated and characterized, having excellent superhydrophobic and thermal properties, low cost and a simple synthetic procedure. In addition, the PCMs can be spontaneously absorbed in PDMS–HNTs and there is no leakage after heating these PCM composites, even beyond their melting points. Results obtained from TEM indicate that the PCMs can be absorbed into halloysite clay nanotubes. XRD analysis confirmed that the decrease in the crystal size of the PCMs was as a result of their incorporation into both p-HNTs and PDMS–HNTs. The latent heats of PDMS–HNTs/PCM composites were measured to be in the range of 44.7–72.1 kJ kg⁻¹. Moreover, the PDMS–HNTs/PCM (m-paraffin wax) composites exhibit excellent thermal stability after 200 heating and cooling cycles and have great potential for renewable energy saving applications.

Acknowledgements

The authors are grateful to the National Natural Science Foundation of China (Grant No. 51263012, 51262019, 51462021 and 51403092), Gansu Provincial Science Fund for Distinguished Young Scholars (Grant No. 1308RJDA012), Support Program for Hongliu Young Teachers (Q201411), Hongliu Elitist Scholars of LUT (J201401), Support Program for Longyuan Youth and Fundamental Research Funds for the Universities of Gansu Province.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra27964j

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