Preparation and characterization of an expanded perlite/paraffin/graphene oxide composite with enhanced thermal conductivity and leakage-bearing properties

Abstract

A novel phase change material (PCMs) of expanded perlite/paraffin/graphene oxide (EP/PA/GO) with enhanced thermal conductivity and leakage-bearing properties was fabricated by depositing GO films on the surface of the EP/PA composite. The as-prepared EP/PA/GO composite was characterized by using differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM) techniques. The experimental results indicated that due to the small loading of the GO incorporated, the EP/PA/GO composite showed a small latent heat capacity and weight loss, and the thermal conductivity significantly increased with increasing the content of GO up to 0.5 wt%. The heat storage/release performance test results demonstrated that the EP/PA composite with 0.5 wt% GO had 2 times faster heat storage/release rate compared to the EP/PA composite because of the enhanced thermal conductivity. In addition, the FTIR and TGA results indicated that the EP/PA/GO composite had good chemical compatibility and thermal stability. More importantly, the GO films covering the surface of the EP/PA composite can greatly prevent the leakage problems of molten paraffin. No leakage of paraffin occurred even after thermal cycling 3000 times. Therefore, the EP/PA/GO composite has a great potential for thermal energy storage applications due to its enhanced thermal properties, good leakage-bearing properties and excellent chemical compatibility.

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1. Introduction

Latent thermal energy storage (LTES) is a promising approach to improve solar energy utilization and minimize energy dissipation in buildings due to its high energy storage density and capacity at constant temperature.^1,2 Phase change materials (PCMs) are widely used in the LTES system and have been receiving great attention for various applications in solar heating systems, building energy conservation and air-conditioning systems.^1,3–5 PCMs mainly include inorganic PCMs such as alloys, melted salts and crystalline hydrated salts, and organic PCMs such as paraffin, fatty acids/esters and lauric acid.^6,7 However, the leakage problem after melting and the lower thermal conductivity of PCMs are the two main drawbacks limiting wide application. One way to prevent the leakage problem is by using micro- and macro-encapsulation methods, including interfacial polymerisation, emulsion polymerization and situ polymerization.^8,9 PCMs form the core with a polymer shell to maintain the shape and protect the PCMs from leakage. However, the aging, cost and lower thermal conductivity of encapsulation shells have been the biggest concern for this method. The other way is to impregnate PCMs into porous materials, such as diatomite^10,11 and expanded perlite.^7,12 Lu et al.^13,14 have fabricated a form-stable expanded perlite/paraffin (EP/PA) composite by absorbing paraffin into porous networks of expanded perlite, resulting in good thermal energy storage, thermal stability and thermal reliability. The leakage problem can be resolved to some extent by the capillary and surface tension forces of porous materials.^15,16 However, the lower thermal conductivity of porous materials reduces the heat transfer rate, which has a negative effect on the energy storage efficiency of PCMs composites. Thus far, much research has been done to mix PCMs composites with graphite-based materials, due to excellent thermal conductivity, such as graphite, expanded graphite and carbon nanotubes (CNTs). Tian et al.¹⁷ investigated shaped-stabilized PCMs composites of ternary eutectic chloride/expanded graphite (EG), and revealed that the thermal conductivities of the composites were 1.35–1.78 times higher than that of the pure ternary eutectic chloride. Mhike et al.¹⁸ attempted to improve the thermal conductivity of PCMs composites by adding graphite and EG. It was found that 10 wt% graphite and EG showed increases of 60% and 200% in thermal conductivity of the PCMs composites, respectively. However, the higher content of graphite reduces the phase change enthalpy of PCMs composites, and the particle-like graphite has no contribution to preventing the leakage problems of the PCMs composites. In addition, Xu et al.¹⁹ have shown that the use of 0.26% MWCNTs can bring clear beneficial effects in improving the thermal conductivity and heat storage/release rates of the PCMs composites without influencing the chemical compatibility and thermal stability. However, it is still difficult to disperse CNTs uniformly in PCMs composites due to the high surface energy and hydrophobic properties. Therefore, it is very important and urgent to develop an improved method, not only encapsulating the PCMs composites, but also improving the thermal conductivity of the PCMs composites.

Graphene oxide (GO) is a two-dimensional single layer material with sp²-bonded carbon atoms, decorated by a large number of covalent oxygen-containing groups-hydroxyl, carbonyl and carboxyl.²⁰ The exceptional mechanical and thermal properties of GO make it a good candidate for a wide range of applications, such as polymer composite materials, energy storage, biomedical applications and catalysis.^21,22 It is well known that graphene is a perfect membrane that can completely block the penetration of molecules.²³ Although defects are introduced into the GO structures due to the strong oxidation process, the permeability and thermal conductivity of the GO greatly depends on the oxidation degree and the numbers of layers of the GO. To the best of our knowledge, the well-controlled GO is generally thought to be a perfect material that can prevent the leakage of viscous PCMs and improve the thermal conductivity of the PCMs composites.

In this study, paraffin and expanded perlite were selected to prepare a EP/PA composite with a high latent heat and a suitable melting temperature for use in buildings, and the GO coating was deposited on the surface of the EP/PA composite. The as-prepared EP/PA/GO composite was composed of paraffin as the core, expanded perlite as the supporting material, and the GO films as the exterior shell. The oxidation degree and the thickness of the GO films deposited on the surface of the EP/PA/GO composite were investigated by X-ray photoelectron spectroscopy (XPS) and typical optical microscopy. The thermal performance of the EP/PA/GO composite was characterized by differential scanning calorimeter (DSC) analysis, thermogravimetric (TGA) analysis and heat storage/release performance tests. The morphology of the EP/PA/GO composite was observed by using a scanning electron microscope (SEM). Chemical compatibility between the paraffin, expanded perlite and GO, was measured by using Fourier transform infrared spectroscopy (FTIR) analysis. The leakage-bearing properties of the EP/PA/GO composite were investigated by a traditional and a newly designed leakage test.

2. Experimental

2.1 Materials

Paraffin in technical grade was supplied by the Ke Qitai Chemical Company, Guangzhou, China. The density and thermal conductivity of the paraffin were 0.81 g cm⁻³ and 0.21 W m⁻¹ °C⁻¹, respectively. Expanded perlite was obtained from the Zhongde Perlite Factory, Liaoning, China. The density and mean grain size of the expanded perlite used were 0.31 g cm⁻³ and 2–3 mm, respectively. Graphite powder (200 mesh, 99.9995%, metal basis) was purchased from Alfa Aesar Inc, Tianjin, China.

2.2 Preparation of the GO

GO was prepared from graphite powder (Alfa-Aesar, 200 mesh) according to the modified Hummers' method.²⁴ Graphite powder (3 g) was added to a solution containing K₂S₂O₈ (2 g), P₂O₅ (2 g) and concentrated H₂SO₄ (40 mL, 98 wt%) for 6 h mixing at 80 °C. The resulting mixture was then diluted with distilled water, filtered and washed until the pH value of the rinse water became neutral. The dried graphite oxide was re-dispersed into concentrated H₂SO₄ (100 mL, 98 wt%) in an ice bath. KMnO₄ (15 g) was gradually added and stirred for 2 h. The mixture was then stirred and mixed at 35 °C for another 2 h, followed by the addition of 230 mL of distilled water. The resultant bright yellow solution was terminated by adding 700 mL of distilled water and 15 mL 30% H₂O₂, and subjected to centrifugation and careful washing by 37% HCl and distilled water. After immersing the as-prepared suspension in dialysis tubing cellulose membranes for 7 days, it was finally centrifuged and collected for preparing different concentrations of graphene oxide solution. In this study, the concentration of the GO solution was 2.0 mg mL⁻¹.

2.3 Preparation of the EP/PA/GO composite

The form-stable EP/PA composite was prepared in the following steps. The mass fraction of paraffin in the EP/PA composite was 0.55 because the homogenous EP/PA composite can be obtained by the direct impregnation method based on our previous research.¹³ Firstly, paraffin was melted at a temperature of 50 °C ± 5 °C for 1 h in an oven, and then immediately mixed with expanded perlite at ambient temperature and put back in the oven. The mixture was mixed every hour until the paraffin was uniformly dispersed in the expanded perlite. Finally, the form-stable EP/PA composite was formed after cooling down at room temperature. Then, the as-prepared EP/PA composites were added into the GO solution (2 mg mL⁻¹) and magnetically stirred for 30 min to ensure a uniform dispersion. The resultant mixture was poured into a glass beaker and put into an oven at a temperature of 80 °C ± 5 °C for 3 hours. The EP/PA/GO composites were finally fabricated after completing GO solvent evaporation. The color of the as-prepared EP/PA/GO composite changed from white to black due to the successful GO deposition onto the surface of the EP/PA composite, as shown in Fig. 1. The mass fractions of GO in the EP/PA/GO composite were 0.25 wt% and 0.5 wt%, and referred to as EP/PA/GO0.25 and EP/PA/GO0.5, respectively.

Fig. 1 Image of the EP/PA composite and the EP/PA/GO0.5 composite.

2.4 Characterization

The oxidation degree and the thickness of the GO films were determined by XPS analysis (5600 multi-technique system, Physical Electronics) and typical optical microscopy (BX51, Olympus). The thermal performances of the EP/PA/GO composite were characterized by DSC analysis (TAQ 1000) with a heating and cooling rate of 5 °C min⁻¹ under a nitrogen atmosphere. The thermal conductivity the EP/PA/GO composite was verified by comparing the heat storage/release rates of the EP/PA composite with that of the EP/PA/GO composite. The thermal stability of the EP/PA/GO composite was studied by the TGA method (Perkin Elmer) at 10 °C min⁻¹ under a nitrogen environment. The morphology of the EP/PA/GO composite was observed by using SEM (JEOL, JEM-6390). Chemical compatibility between paraffin, expanded perlite and GO, was measured by FTIR analysis (Bio-Rad FTS 6000). The leakage-bearing properties of the EP/PA/GO composite were investigated by a traditional and a newly designed leakage test.

3. Results and discussion

3.1 DSC analysis of the EP/PA/GO composite

Fig. 2 shows the DSC curves of the EP/PA composite with different GO contents. The detailed thermal results of the measured latent heat (H_M) and the peak phase change temperature (T_peak) are listed in Table 1. As presented in Fig. 2 and Table 1, the value of T_peak in the melting and freezing processes were determined as 50.72 °C and 42.54 °C for the EP/PA composite, and 50.86 °C and 42.22 °C for the EP/PA/GO0.25 composite, and 50.96 °C and 42.14 °C for the EP/PA/GO0.5 composite. The results indicate that the GO incorporation does not cause a shift of the peak phase change temperature of the EP/PA composite. Moreover, the value of H_M in the melting and freezing processes were 80.92 J g⁻¹ and 80.48 J g⁻¹ for the EP/PA composite, 80.16 J g⁻¹ and 80.21 J g⁻¹ for the EP/PA/GO0.25 composite and 79.75 J g⁻¹ and 79.36 J g⁻¹ for the EP/PA/GO0.5 composite. It clearly can be seen that the EP/PA composite, with and without the GO, shows an almost equivalent latent heat capacity and low latent heat capacity loss resulting from the small loading of GO incorporation, within 0.5 wt%. Therefore, it can be concluded that 0.5 wt% GO leads to a trivial change of the values of H_M and T_peak of the EP/PA composite, and that the EP/PA/GO composite can be effectively used as an energy storage material for exterior walls in building applications.

Fig. 2 DSC curves of the EP/PA composite with different GO contents.

Table 1 Thermal properties of the EP/PA composite with different content of the GO

Composite	Melting process		Freezing process
	Tpeak (°C)	HM (J g^−1)	Tpeak (°C)	HM (J g^−1)
EP/PA	50.72	80.92	42.54	80.84
EP/PA/GO0.25	50.86	80.16	42.22	80.21
EP/PA/GO0.5	50.96	79.75	42.14	79.36

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3.2 Thermal stability of the EP/PA/GO composite

The thermal stability of the EP/PA composite with different GO contents was evaluated by TGA analysis. Fig. 3 shows the measured TGA curves of the EP/PA composite, the EP/PA/GO0.25 composite and the EP/PA/GO0.5 composite. As shown in Fig. 3, the EP/PA composite starts to lose weight at approximately 105 °C. After rapid weight loss due to paraffin evaporation, there is a loss of 55.24% of the paraffin weight at 360 °C, which is almost consistent with the design mix proportion of the expanded perlite and paraffin, indicating that the EP/PA composite is homogeneous. With the incorporation of the GO, there are no obvious changes in the starting decomposition temperature and the weight loss rate the EP/PA/GO composite. The only differences, by comparing the three curves in Fig. 3, are the final weight loss of the different composites. Compared with the weight loss of the EP/PA composite (55.24%), it slightly increases to 55.50% and 55.82% for the EP/PA/GO0.25 composite and the EP/PA/GO0.5 composite, respectively. The negligible change is attributed to the small GO loading, up to 0.5 wt%. In addition, no decomposition is seen for all the composites over their phase change temperature ranges, showing the good thermal stability of the EP/PA/GO composite.

Fig. 3 TGA curves of the EP/PA composite with different GO contents.

3.3 Thermal conductivity of the EP/PA/GO composite

Thermal conductivity is a key parameter to determine the thermal storage performance of PCMs composites. The heat storage/release performance of the EP/PA/GO composite was investigated by using the test setup schematically shown in Fig. 4.¹⁹ In this test, 5 g of the PCMs composite was stored in a sealed tube. Two water baths, at temperatures of 60 °C and 30 °C, were adopted during the heating storage/release processes. The temperature change of the PCMs composite during the heating storage/release test was measured by thermocouples located in the middle of the PCMs composite stack in the sealed tube and recorded through a data-logger and a computer.

Fig. 4 Schematic diagram of heat storage/release performance test.

Fig. 5 shows the temperature curves for melting and freezing of the EP/PA composite with different GO mass fractions in the heat storage/release performance tests. Table 2 lists the time taken for temperatures increasing from 30 °C to 60 °C and decreasing from 60 °C to 30 °C. The shorter the time, the faster the heat storage and release rate, and the higher the thermal conductivity of the composite. As shown in Fig. 5 and Table 2, all the composites have obvious temperature plateaus in the heating and cooling processes due to the occurrence of phase changes. In detail, when temperature increases from 30 °C to 60 °C, it takes 8.8 min, 6.2 min and 4.1 min for the EP/PA composite, the EP/PA/GO0.25 composite and the EP/PA/GO0.5, respectively. Similarly, it takes 14.2 min, 11.8 min and 7.5 min for each composite when the temperature decreases from 60 °C to 30 °C. It clearly shows that the time used for both the melting and freezing processes decreases with the increasing content of GO within 0.5 wt%, and the EP/PA/GO0.25 and EP/PA/GO0.5 composites show clearly faster heat storage and release rates than the EP/PA composite. The experimental results indicate that the EP/PA/GO0.5 composite has 2 times faster heat storage/release rates compared to that of the EP/PA composite due to the enhanced thermal conductivity. The significant improvement is much better than many other studies reported. For example, Li et al.¹⁹ demonstrated that 0.26 wt% CNTs led to 1.25 times faster heat storage/release rates for paraffin/diatomite composites; however, uniform dispersion of CNTs in the PCMs composite is difficult to accomplish. Liu et al.²⁵ have shown that the time required in heat storage/release tests was reduced by 3 times with the addition of 1 wt% graphite; however, the latent heat loss of the composite decreased significantly due to the higher loading of the graphite, and the leakage problem still could not be avoided by the addition of graphite. In this study, 0.5 wt% GO incorporation not only reduces the time required in heat storage/release tests by a factor of 2, but also keeps the latent heat capacity of the PCMs composite stable. Therefore, GO has a great influence on improving the thermal properties of the EP/PA composite.

Fig. 5 Melting (a) and freezing (b) curves of the EP/PA composite, the EP/PA/GO0.25 composite and the EP/PA/GO0.5 composite.

Table 2 Time for heating and cooling of EP/PA composite with different GO content

Temperature range	Composite
	EP/PA	EP/PA/GO0.25	EP/PA/GO0.5
30−60 °C	8.8 min	6.2 min	4.1 min
60−30 °C	14.2 min	11.8 min	7.5 min

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3.4 Chemical compatibility of the EP/PA/GO composite

Fig. 6 shows the FTIR spectrums of the EP/PA composite with different GO contents. As presented in Fig. 6, the characteristic peaks of the GO at 1723 cm⁻¹, 1621 cm⁻¹, 1222 cm⁻¹ and 1058 cm⁻¹ indicate carboxyl or carbonyl C=O stretching, H–O–H bending band of the absorbed H₂O molecules, phenolic C–OH stretching and alkoxy C–O stretching, respectively. Moreover, the EP/PA composite has six characteristic absorption bands: skeleton vibration of C–C at 455 cm⁻¹, rocking vibration of –CH₂ at 718 cm⁻¹, two deformation vibrations of –CH₂ and –CH₃ at 1368 cm⁻¹ and 1468 cm⁻¹, and two stretching vibrations of –CH₂ and –CH₃ at 2848 cm⁻¹ and 2917 cm⁻¹, respectively. It is clearly seen that all the above-mentioned characteristic absorption bands of the GO and the EP/PA composite are included in the FTIR spectrums of the EP/PA/GO0.5 composite. More importantly, no new characteristic absorption bands are generated in the FTIR spectrums of the EP/PA/GO0.5 composite, suggesting that there is no chemical reaction between the GO and the EP/PA composite. Therefore, it can be concluded that the EP/PA/GO composite has a good chemical compatibility.

Fig. 6 FTIR curves of the GO, EP/PA composite and EP/PA/GO0.5 composite.

3.5 Leakage-bearing properties of the EP/PA/GO composite

Fig. 7 shows the SEM images of the surface morphologies of the expanded perlite, the EP/PA composite and the EP/PA/GO composite. As seen from Fig. 7a, the expanded perlite has a highly porous structure, which makes it a good supporting material to absorb molten paraffin. Fig. 7b shows that the paraffin is uniformly absorbed into the pores of the expanded perlite, and hence the leakage of the molten paraffin from the composites is largely prevented. Fig. 7c shows the GO coating on the surface of the EP/PA composite. It is clear that the pores of the EP/PA composite are encapsulated with the GO coating, which acts like a shell to prevent the molten paraffin leaking out. However, whether the molten paraffin can permeate the GO coating makes a great influence on the leakage-bearing properties of the EP/PA/GO composite. In order to verify this point, leakage testing for the GO films was conducted, as shown in Fig. 8. The GO film was obtained from the GO solution (the dosage and concentration equivalent to that used for the fabrication of the EP/PA/GO0.5 composite) by an oven drying method, and then placed into the middle of the suction device (Fig. 8). The lamp was turned on to ensure that the paraffin was in a molten condition during the leakage test. The experimental results showed that there is no molten paraffin penetrated though the GO films to the bottom conical flask after 24 h heating, which confirms that the GO films are effective in preventing the leakage of the molten paraffin. Moreover, the traditional leakage test was carried under condition in which the EP/PA composite and the EP/PA/GO0.5 composite were put on a filter and placed under the lamp, as seen in Fig. 9. The distance between the lamp and the composite was 10 cm to ensure that all the paraffin could melt and solidify in the thermal cycling test with 3 min of lamp ‘on’ and 9 min of lamp ‘off’. The experimental results revealed that there was no paraffin trace on the filter for the EP/PA composite and the EP/PA/GO0.5 composite after 1500 thermal cycles, because the molten paraffin could be limited in the pores of the expanded perlite by the capillary and surface tension forces. However, after 3000 cycling, a small amount of paraffin was observed on the filter for the EP/PA composite, caused by the paraffin leakage, but there was no paraffin trace on the filter for the EP/PA/GO0.5 composite, as seen in Fig. 10c and f. The possible reason is that the GO coating on the surface of the EP/PA/GO0.5 composite contributes to preventing the molten paraffin leakage, which is consistent with the previous results as seen in Fig. 8. It also indicates that the leakage protection for the molten paraffin, caused by the capillary and surface tension forces of the expanded perlite, is good for short term use; however, the EP/PA composite covered with GO coating shows an excellent leakage-bearing property, which has great potential for use in energy efficient buildings for long term consideration.

Fig. 7 The surface morphologies of (a) the expanded perlite, (b) the EP/PA composite and (c) the EP/PA/GO0.5 composite.

Fig. 8 Leakage test for the GO films.

Fig. 9 Thermal cycling test.

Fig. 10 Leakage test for the EP/PA composite (a) before thermal cycling test (b) after 1500 cycling (c) after 3000 cycling; the EP/PA/GO0.5 composite (d) before thermal cycling test (e) after 1500 cycling (f) after 3000 cycling.

Since the permeability of the GO coating greatly depends on the thickness and the oxidation degree of the GO layers, the cross-section of the EP/PA/GO0.5 composite was examined by an optical microscopy to measure the thickness of the GO coating, and the XPS test was carried out to investigate the oxidation degree of the GO. Fig. 11a shows the cross-sectional image of the EP/PA/GO0.5 composite. It is clear that the surface of the composite is uniformly covered by the GO films, and the thickness of the GO films is about 50 μm. Fig. 11b shows the XPS spectra of the GO used in this study, and the deconvoluted C1s XPS spectra of the GO, clearly shows four types of carbon bonds, including the C–C at 284.5 eV, C–O at 286.4 eV, C=O 288.3 at eV and –COOH at 289.0 eV. The elemental analysis of the XPS results indicate that the C/O ratio and oxygen content of the GO in this study are 3.0 and 30.7%, respectively. Therefore, it can be concluded that the GO films with a C/O ratio of 3.0 and thickness of 50 μm, in this study, can effectively prevent the leakage problem of the EP/PA composite for long term use. However, it should be noted that the leakage-bearing properties of the EP/PA/GO composite greatly depends on the oxidation degree and the thickness of the GO films. A greater oxidation degree or less thickness of the GO films might cause negative effects on the leakage-bearing properties of the EP/PA/GO composite.

Fig. 11 (a) Cross-section image of the EP/PA/GO0.5 composite; (b) XPS spectra of the GO.

4. Conclusions

In this study, a novel phase change material of expanded perlite/paraffin/graphene oxide (EP/PA/GO) with enhanced thermal properties was fabricated by depositing GO films on the surface of the EP/PA composite. The DSC and TGA results showed that the EP/PA/GO composites had small latent heat capacity and weight losses due to the small loading of the GO incorporation, within 0.5 wt%. The FTIR results indicated that the EP/PA/GO composite had good chemical compatibility. In addition, due to the excellent thermal conductivity of the GO, the heat storage/release performance test results showed that the EP/PA/GO0.5 composite had 2 times faster heat storage/release rates compared to that of the EP/PA composite. More importantly, the GO films covering the surface of the EP/PA composite can greatly prevent the leakage problems of molten paraffin. No leakage of paraffin occurred after thermal cycling 3000 times. Therefore, the EP/PA/GO composites have shown a great potential for thermal energy storage applications due to the enhanced thermal properties, good leakage-bearing properties and excellent chemical compatibility.

Acknowledgements

The authors would like to acknowledge the financial support from the China Ministry of Science and Technology under the Grant 2015CB655100, Information Technology of Guangzhou under the Grant 2013J4500069 and the Natural Science Foundation of China under the Grant 51302104.

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