Preparation and thermal properties of phase change materials based on paraffin with expanded graphite and carbon foams prepared from sucroses

Abstract

Carbon foam/expanded graphite composite (CEC) was prepared from a sucrose-expandable graphite resin using a thermal foaming method. This CEC was impregnated through its pores with paraffin to obtain a paraffin/carbon foam/expanded graphite composite (PCEC). In the case of CECs, when the amount of added expandable graphite reached 10 wt% to 15 wt%, the microstructure of the CEC was damaged because of the expansion in volume of the expandable graphite. Fourier transform infra-red spectroscopy and X-ray diffraction analysis of PCECs showed that there was no chemical interaction between the paraffin and CECs. With an increase in the amount of expandable graphite in CECs, the adsorption capacity of paraffin and the latent heat first showed an increase and then decreased. The heat transfer capability of the paraffin was truly improved by the CECs. The processes for the preparation of the CECs and PCECs were environmentally friendly, convenient, and inexpensive. The PCECs, with good thermal properties and chemical stabilities, are suitable for low temperature (40–50 °C) thermal energy storage applications.

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Introduction

Shortage of fossil fuels has led to the investigation of new and renewable energy sources.^1,2 Storage of thermal energy in solid–liquid phase change materials (PCMs) is a useful method and is also one of the most interesting technologies used to overcome the limitations, by also being cost-effective and more reliable when compared to the unreliability of other energy storage methods.^3,4 Selection of a suitable latent heat storage material and enhancement of the heat transfer between the storage material and the surrounding medium are two important aspects for designing cost-effective phase change thermal storage systems.^3,5–7 Paraffin is one of the most outstanding latent heat storage materials amongst the PCMs.^8,9 Paraffin has many advantages, such as low price, optimum melting point and a large latent heat. Additionally, it is non-toxic, environmentally friendly and also safe for public health.^10,11 However, applications of paraffin are limited because of its low thermal conductivity.^12,13 Therefore much research has been done to improve the thermal conductivity of paraffin.

Zhang et al.¹⁴ used copper foam to enhance the thermal conductivity of paraffin and there was a good agreement between the experimental and numerical results that were obtained. Metal fillers and metal fins have also been used to achieve this goal,⁷ but the weight and the cost are an impediment to their future use. It has been proved that a low cost and efficient way to enhance the heat transfer of paraffin is to impregnate paraffin with a high thermal conductivity material having a porous structure, such as expandable graphite, graphite foams, and carbon foams.^15–17

The high thermal stability and conductivity of carbon foam is a result of its open cell structure, and most of the macropores (cells) in carbon foams are interconnected.¹⁸ The thermal conductivity of carbon foams is not as high as that of graphite foams. However, carbon foams are inexpensive and the processing of carbon foams requires lower carbonization temperatures, which saves energy.^19–21 Expanded graphite is also considered to be an excellent promoter of heat transfer,^22–24 which has high thermal conductivity, high stability, and low density. Both paraffin/carbon foams and paraffin/expanded graphite have been reported extensively.^25–28 However, the use of carbon foam/expanded graphite composite (CEC) to improve the thermal conductivity of paraffin has not yet been systematically investigated.

In this study, different amounts of expandable graphite were added to aqueous sucrose resin to prepare CEC, and the expandable graphite changes to expanded graphite during the preparation, which further enhances the thermal conductivity of CEC. More recently, because of the depletion in the sources of pitch, the preparation of carbon foams from renewable biomolecules from plant origin is becoming more and more important.^29–31 So in this study, with the use of sucrose as the raw material, the preparation method is environmentally friendly and lower in energy consumption. Furthermore, most research on carbon foam prepared from sucrose mainly focuses on its preparation methods. However, its application in the field of phase change energy storage is rarely reported. This paper describes the preparation of CEC, its microstructure, and also the thermal properties of paraffin/carbon foam/expanded graphite composite (PCEC). The light and high heat transfer efficiency PCEC materials are suitable for thermal energy storage applications.

Experimental

Preparation of carbon foam/expanded graphite composite (CEC)

The flowchart of the process for the preparation of CEC is shown in Fig. 1. Sucrose (analytically pure grade, Beijing Chemical Factory) and expandable graphite (50 mesh) were purchased from Beijing Chemical Reagent Ltd. Aqueous sucrose resin was prepared by dissolving 100 g of sucrose in 100 mL distilled water, followed by heating to obtain a dark brown resin. The weight ratios of expandable graphite to sucrose were 0, 0.05, 0.10, and 0.15, which were mixed with the resin by stirring mechanically for 2 h to form a dispersion, and were designated as CEC0, CEC1, CEC2, and CEC3, respectively. The dispersions were heated at 120 °C for 48 h to form a solid which was then cut into regular rectangular specimens. These organic specimens were dehydrated by heating at 250 °C in an air oven for 3 h. The dehydrated samples were carbonized in a highly pure argon atmosphere in a tubular furnace at 900 °C for 2 h at a heating rate of 2 °C min⁻¹. After cooling to room temperature, the CEC samples obtained were unloaded. The masses of the dehydrated samples and the CECs were recorded.

Fig. 1 Flowchart of the process for the preparation of CEC.

Preparation of paraffin/CEC composite (PCEC)

PCECs were prepared using a vacuum impregnation method, as shown in Fig. 2. Paraffin was purchased from the Beijing Chemical Reagent Ltd. and was produced by the Sinopharm Chemical Reagent Co. Ltd. CEC and solid paraffin were mixed together with CEC in a glass beaker, which was then placed in a vacuum drying oven at a temperature of 80 °C. The air in the oven was evacuated using a vacuum pump to a pressure below 0.01 MPa. After one hour the pores of the CEC were impregnated with the paraffin and the intermediate PCECs were obtained. The intermediate PCECs were then placed on a filter paper followed and were then placing together in an oven at a temperature of 80 °C in order to remove the liquid paraffin. The filter paper was replaced until the paraffin no longer leaked and the PCECs that were obtained were designated as PCEC0, PCEC1, PCEC2, and PCEC3 corresponding to CEC0, CEC1, CEC2, and CEC3. The masses of the PCECs were recorded before and after impregnation.

Fig. 2 The process for preparation of PCEC by vacuum impregnation method.

Determination of the suitable CEC to use to prepare a composite with paraffin

In this research, a suitable CEC to be used to prepare a composite with paraffin was considered overall and determined by two values: the experimental carbon yield (ξ) of the CEC and the mass fraction (η) of the paraffin in PCEC. The carbon yield (ξ) was calculated using eqn (1):where the value of ξ was the experimental carbon yield of the CEC; m_d and m_CEC were the masses of the CEC before and after the heat treatment (900 °C), respectively.

The mass fraction (η) was calculated using eqn (1):^28–30

where the value of η was the mass fraction of the paraffin; m_CEC was the mass of CEC before impregnation; m_PCEC was the mass of PCEC after impregnation.

Characterization

The chemical compositions of PCECs and their chemical compatibilities were studied using X-ray diffraction analysis (XRD, Xi'an Yima Opto-Electrical Technology Model XD-3) and Fourier transform infrared spectroscopy (FT-IR, PerkinElmer Frontier). The morphologies of CECs and PCECs were studied using scanning electron microscopy (SEM, Hitachi S-4800). Thermal properties of paraffin and PCECs, such as melting points, phase change temperatures, and the heats of fusion, were determined using differential scanning calorimetry (DSC, TA Instruments Q2000). Paraffin and the PCECs were heated from 20 °C to 100 °C, at a heating rate of 10 °C min⁻¹, and were then were cooled to 20 °C. The thermal stabilities of PCECs were investigated using thermogravimetric analysis (TGA, TA Instruments Q50). Paraffin and the PCECs were heated from 20 °C to 600 °C, at a heating rate of 10 °C min⁻¹ in a highly pure nitrogen atmosphere. The thermal diffusivity was measured using a Netzsch LFA 427 Laser Flash thermal analyzer.

Results and discussion

Determination of the most suitable CEC and the microstructures of the CECs

The theoretical carbon yield and experimental carbon yield of CECs are shown in Fig. 3. The theoretical carbon yields calculated for the prepared CECs, using expandable graphite to sucrose weight ratios of 0–15 wt%, were in the range of 42.11–49.65%, respectively. The theoretical carbon yields were calculated using eqn (3):where the value of ξ_theoretical was the theoretical carbon yield of the CEC; a was the mass of sucrose and b was the mass of expandable graphite.

Fig. 3 The theoretical carbon yield and the experimental carbon yield of CECs.

The experimentally determined carbon yield was much lower than the theoretical carbon yield, especially for CEC2 and CEC3. There were two main reasons. One reason was that because of the formation of carbon monoxide and carbon dioxide during carbonization, the carbon yield was reduced.³² Another reason was that during the experiments, when the weight ratios of expandable graphite to sucrose reached 5 wt%, a certain amount of residue was left behind for CEC2 and CEC3, which were 14.59 wt% and 29.61 wt% of the masses of the CEC2 and CEC3, respectively. The SEM micrograph of the residue of CEC3 [Fig. 4(b)] showed the twisted wormlike appearance of expanded graphite, which was formed from the expandable graphite [Fig. 4(a)], after thermal treatment.

Fig. 4 SEM micrographs of (a) expandable graphite and (b) the residue from CEC3.

Fig. 5 shows the microstructures of CEC0, CEC1 and CEC3. CEC0 had a cellular structure, the cells of which were interconnected through windows and a thin membrane.³⁰ The average size of the cells was between 200–500 μm. This type of interconnected structure speeded up the process of the heat transfer of paraffin. CEC0 and CEC1 showed a relatively complete structure compared to CEC3. The microstructure of CEC1 was not damaged, and the expanded graphite embedded into carbon foam. The structure of CEC3 was greatly destroyed in the process of the expansion of the expandable graphite. The volume of the expandable graphite increased. The expanded graphite together with the foam fragment came off from CEC2 and CEC3. So the carbon yields of CEC2 and CEC3 were much lower than their theoretical carbon yields and in fact even lower than that of CEC1. From the perspective of the preparation of carbonaceous materials, carbon foam added with 5 wt% of the expandable graphite could give relatively higher yields.

Fig. 5 SEM micrographs of (a) CEC0, (b) CEC1, (c) CEC3 and (d) magnification of one segment of CEC3.

The mass fractions of paraffin in PCEC0, PCEC1, PCEC2, and PCEC3 were 71.47 wt%, 81.66 wt%, 66.38 wt%, and 62.76 wt%, respectively. The reason for the maximum mass fraction of paraffin in PCEC1 was that the expanded graphite could load a certain amount of paraffin to form a shape stabilized composite. But because the expanded graphite fell off it led to the content of the expanded graphite in CEC2 and CEC3 to be close to that of CEC1. Furthermore, the structures of CEC2 and CEC3 were broken and could not be loaded with much paraffin. So the mass fraction of paraffin in PCEC1 was much higher than for the others. The changes in the trends of mass fractions of paraffin in PCECs were in accordance with the carbon yields of CECs. They increased first and then decreased, and when the amount of the expandable graphite added was 5 wt%, the ξ and η reached a maximum value.

XRD patterns and FT-IR analysis

Fig. 6(1) shows the FT-IR spectra of CEC1 (a), paraffin (b), and PCEC1 (c). Based on the interactions between the components of the composite, the chemical compatibility was determined. Two peaks appeared in the spectrum of paraffin at 2919 cm⁻¹ and 2848 cm⁻¹, which were because of the asymmetric and symmetric stretching vibrations of C–H. The peaks at 1475 cm⁻¹ and 1373 cm⁻¹ were because of the bending vibrations of C–H and the peak at 719 cm⁻¹ was because of the rocking vibrations of CH₂ in the spectrum of the paraffin. There was no prominent peak in the spectrum of CEC1. Comparing CEC1 (a) and PCEC1 (c) of Fig. 6(1) to that of the paraffin (b), the spectrum of PCEC1 showed all main absorption peaks at 2919 cm⁻¹, 2848 cm⁻¹, 1475 cm⁻¹, 1373 cm⁻¹, and 719 cm⁻¹, without the appearance of any new peaks. CECs and paraffin are chemically inert to each other, as the FT-IR results indicate, because only physical absorption was involved without any chemical interactions between the CECs and paraffin.

Fig. 6 FT-IR spectra (1) and XRD patterns (2) of (a) CEC1, (b) paraffin, and (c) PCEC1.

Fig. 6(2) shows the XRD patterns of the CEC1 (a), paraffin (b), and PCEC1 (c). Two peaks at 2θ values of 38.2° and 44.5° could be seen in the XRD, which were obviously because of the presence of aluminium. The intensity of the peak at approximately 38.2° was very high. This was because of the fact that the powder sample was loaded into a cylindrical aluminium sample holder during the diffraction scan.³³ The intensity of the peak at approximately 26.5° for expanded graphite was not very conspicuous, because of the very high intensity of the peak at approximately 38.2°. Comparing the XRD patterns of CEC1 and paraffin, it can be ascertained that paraffin was really impregnated into the CECs and that no new phase was formed, which indicated that there was no chemical reaction during the impregnation.

Fig. 7 shows the SEM images of CEC1 (a) and PCEC1 (b). Compared to CEC1, the pores in PCEC1 are less visible. The pores of CEC1 were filled with paraffin, and the cell walls were also covered by paraffin. CEC1 was buried in paraffin, and CEC1 seemed like a mesh which connected the paraffin and improved the heat transfer capacity of paraffin.

Fig. 7 SEM micrographs of (a) CEC1 and (b) PCEC1.

Thermal properties of PCECs

The DSC technique was used to investigate the phase change temperatures and latent heats of paraffin and PCECs. DSC curves of paraffin and PCEC1 are shown in Fig. 8, and their thermal properties are presented in Table 1.

Fig. 8 DSC curves of paraffin and PCEC1.

Table 1 Thermal properties of paraffin and PCECs

Samples	Tm (°C)	Hm (J g^−1)	Tf (°C)	Hf (J g^−1)
Paraffin	44.52	140.8	48.38	144.3
PCEC0	44.51	105.8	48.48	105.8
PCEC1	48.71	111.4	44.68	113.0
PCEC2	48.51	92.97	44.55	92.53
PCEC3	48.43	89.23	44.72	89.70

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The melting and solidification curves of paraffin and PCEC1 were almost the same. Both of them showed peaks for two phase changes. The first peak at 22.84 °C for the paraffin sample was ascribed to the solid–solid phase transition, whereas the second peak at 44.52 °C was ascribed to the solid–liquid phase change.²⁸ The melting temperature (T_m) and the freezing temperature (T_f) of paraffin were 44.52 °C and 48.38 °C, respectively. These two temperatures of PCECs were almost the same as those of the paraffin. The T_m and T_f of PCECs were close to 44.5 °C and 48.5 °C, respectively. The latent heats of paraffin for melting (H_m) and freezing (H_f) were 140.8 J g⁻¹ and 144.3 J g⁻¹, respectively. Although the composites of paraffin and CECs did not affect the melting temperature (T_m) and the freezing temperature, the latent heats showed a considerable change. The latent heats of melting and freezing for paraffin in PCEC1 were closest to those of the paraffin, which were 113.0 J g⁻¹ and 111.4 J g⁻¹, respectively. Compared to pure paraffin, the two values of the PCECs showed an increase at first and then decreased. The variations in the trends of the latent heats of PCECs were in agreement with the mass fractions of paraffin in PCECs. The porous structure of carbon foams (CEC0) could decrease the weight percentages of paraffin in PCECs and interfere with the solidification of paraffin. However, the addition of expanded graphite to carbon foams (CEC1) changed the situation. As mentioned previously, when the weight percentage of expanded graphite in CECs was above 5 wt%, the expanded graphite fell off from the carbon foams causing structural damages to the carbon foams. This explains why PCEC1 had the maximum value of the mass fraction of paraffin and latent heat of the PCECs. So in the process of preparation of CECs, the weight ratio of expandable graphite to sucrose was 5 wt%, which was a better choice.

TGA was used to investigate the thermal stabilities of PCECs. Fig. 9 shows TGA (a) and differential thermogravimetry (DTG) (b) curves of paraffin and PCECs. The onset temperatures, endpoint temperatures, and the residual masses of paraffin and PCECs are presented in Table 2. The onset temperatures of paraffin and the PCECs did not differ much. The decomposition temperature of paraffin was reduced to a small extent by PCECs, which indicated that the PCECs accelerated the heat transfer of paraffin. The residual mass of the paraffin was 0.01 wt%. The paraffin was almost completely decomposed at 283.9 °C. The residual masses of paraffin in PCEC0, PCE1, PCE2 and PCEC3 were 27.2 wt%, 16.6 wt%, 31.7 wt%, and 33.4 wt%, respectively. These were also in agreement with the mass fractions of the paraffin in PCECs. Compared to the DTG curves of paraffin and PCEC0, the DTG curves of PCEC1, PCEC2, and PCEC3 were no longer smooth, which indicated that the addition of expanded graphite improved the thermal stability of paraffin. Additionally, in the preparation of the carbon foams, the addition of expanded graphite broadened the applications of the carbon foams in the fields involving thermochemical energy storage.

Fig. 9 TGA (a) and DTG (b) curves of the paraffin and PCECs.

Table 2 TGA data of paraffin and PCECs

Samples	Onset temperature (°C)	Peak temperature (°C)	Endpoint temperature (°C)	Residual mass (%)
Paraffin	152.8	245.1	283.9	0.01
PCEC0	143.2	237.2	283.1	27.2
PCEC1	144.8	239.7	287.9	16.6
PCEC2	136.9	240.2	291.5	31.7
PCEC3	153.9	261.9	350.1	33.4

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The heat transfer capacity of the PCECs

Temperature curves for the melting and freezing processes of paraffin, PCEC0, and PCEC1 are shown in Fig. 10(a) and (b), respectively.

Fig. 10 Curves of melting (a) and freezing (b) processes of the paraffin, PCEC0, and PCEC1.

The melting and freezing temperatures were determined every 20 seconds. The masses of paraffin, PCEC0, and PCEC1 were the same (0.8 g). The melting temperature was measured in boiling water (99.8 °C) and the freezing temperature was measured in cold water (13.6 °C). In both the processes involving temperature rise or fall, PCEC1 was the fastest, and the PCEC0 was faster than the pure paraffin. This result was also consistent with the thermal diffusivities of the three samples. The thermal diffusivities of paraffin, PCEC0, and PCEC1 were 0.104 m s⁻¹, 0.151 m s⁻¹ and 0.163 m s⁻¹, respectively. The heat transfer capacity of the paraffin was improved by the carbon foams, and further enhanced by the expanded graphite.

Conclusions

Based on the results obtained and the subsequent discussions, the conclusions can be summarized as follows:

The CEC was prepared using a thermal foaming method. When the amount of the expandable graphite added was 5 wt%, the carbon yield of CEC reached a maximum value, and CEC had a relatively complete structure. The structure of carbon foams was destroyed with excessive amounts of expandable graphite. The PCEC was prepared using a vacuum impregnation method. There was no chemical reaction between the CEC and paraffin. When the amount of the expandable graphite added to CEC was 5 wt%, the loading amount of paraffin in CEC reached a maximum value, and the latent heats were 113.0 J g⁻¹ at the melting temperature of 44.68 °C and 111.4 J g⁻¹ at the freezing temperature of 48.71 °C. PCECs showed good thermal properties and stabilities and the initial decomposition temperature was above 140 °C. The heat transfer capability of the paraffin was improved by the carbon foams and further improved by the expanded graphite. Thus, the PCECs are suitable for low temperature thermal energy storage applications. The whole process of the preparation of CECs and PCECs is environmentally friendly and inexpensive. Thus there is a new and useful application for CEC in the field of thermal energy storage.

Acknowledgements

This present work was supported by the National Natural Science Foundation of China (Grant No. 51472222, 51372232), and the Research Fund for the Doctoral Program of Higher Education of China (Grant No. 20130022110006). Xiaowen Wu also thanks the Beijing Higher Education Young Elite Teacher Project (Grant no. YETP0636).

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