Design and Preparation of the Phase Change Materials Paraffin

Research Article pubs.acs.org/journal/ascecg

Design and Preparation of the Phase Change Materials Paraffin/ Porous Al2O3@Graphite Foams with Enhanced Heat Storage Capacity and Thermal Conductivity Yali Li,† Jinhong Li,* Wuwei Feng,*,† Xiang Wang,† and Hongen Nian‡ †

Beijing Key Laboratory of Materials Utilization of Nonmetallic Minerals and Solid Wastes, National Laboratory of Mineral Materials, School of Materials Science and Technology, China University of Geosciences (Beijing), Beijing, 100083, P. R. China ‡ Key Laboratory of Comprehensive and Highly Efficient Utilization of Salt Lake, Resources, Qinghai Institute of Salt Lakes, Chinese Academy of Sciences, Xining, 810008, P. R. China ABSTRACT: Porous Al2O3@graphite foams (PAGFs) were directly prepared by a particle-stabilized foaming method, with 40 vol % Al2O3 particles and different proportions of sucrose. The as-prepared PAGFs demonstrate three-dimensional interpenetrating structures and high porosities according to SEM images, with the porous morphology being markedly influenced by the concentration percentage of sucrose. Additionally, the PAGFs could be successfully impregnated with paraffin, reaching a maximum enclosed ratio (φ) of 66 wt % without any leakage. Differential scanning calorimetry measurement showed that the latent heat of the composites of paraffin/PAGF (PAGFPs) reach maxima of 105.76 and 105.98 J/g after 200 cycles of melting/freezing. Thermogravimetric analysis, Fourier transform infrared spectroscopy, and thermal cyclic tests demonstrated good thermal and chemical stability and good thermal reliability for the as-prepared form-stable PAGFPs. Our results also confirmed that a layer of ordered graphite film is formed on the surface of Al2O3 particles after sintering at 1600 °C. As a result, the specific surface area of PAGF is 13 times greater than that of the foams without coating graphite. Meanwhile, the thermal conductivities of the PAGFPs reached a maximum of 0.76W/m·K, which was 3.62 times that of pristine paraffin. In conclusion, we demonstrated here the design and preparation of form-stable composite phase change materials with controllable porous structures and superior thermal and chemical stabilities and reliabilities for heat energy storage applications. KEYWORDS: Thermal energy storage, Particle-stabilized, Direct foaming, Al2O3@graphite, Paraffin, Form-stable composite PCM

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INTRODUCTION In last 3 decades, with urbanization and industrialization rapidly developing and the human population shooting up, the reserves of fossil fuels have been drastically reduced.1 Moreover, the use of fossil resources has worsened the environmental pollution, causing acid rain and an increase of green house gas emission and damaging the environment. As such, renewable and new energy sources are badly needed.2 Thermal energy storage, especially latent heat storage, has received great attention and proved to be a promising, low-cost, sustainable, and renewable technique for energy conservation. For heat storage, energy is stored during melting and released during solidifying in phase change materials (PCMs). In recent years, PCMs have been widely researched for air-conditioning, waste heat recovery, heat transportation,3−5 solar heating, etc. due to their high energy storage density, nearly isothermal operating characteristics, and extremely small temperature variation during the phase change process. According to the different types of phase changes, PCMs can be divided into solid−solid PCMs, solid−liquid PCMs, solid− © 2017 American Chemical Society

gas PCMs, and liquid−gas PCMs, and solid−liquid PCM is the most preferred one because of its high energy density and good latent heat storage capacities.6,7 However, liquid leakage during the phase transition and the low thermal conductivity are the major drawbacks for the solid−liquid PCMs. Liquid leakage is harmful to the surroundings and may lead to potential dangers, and the low thermal conductivity decreases the heat transfer rates during phase change processes, thus in turn limiting their applications.8 Therefore, a new form-stable or shape-stabilized PCM has been developed, which consists of thermal energy storage materials and a supporting substance. The new type of composite PCMs still can maintain their original shape after the melting of the thermal energy storage materials because the liquid phase can be sustained by the porous material through capillary and surface tension forces. That is to say, the solid− liquid PCMs could prevent liquid leakage during its solid-toReceived: March 23, 2017 Revised: July 31, 2017 Published: August 9, 2017 7594

DOI: 10.1021/acssuschemeng.7b00889 ACS Sustainable Chem. Eng. 2017, 5, 7594−7603

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ACS Sustainable Chemistry & Engineering

Sinopharm Chemical Reagent Co. Ltd.), consisting of short amphiphilic molecules, was used as a surfactant, which leads to high solubility and critical micelle concentration (cmc) in the aqueous phase and could contribute to surface modification of the Al2O3 particles in situ. Paraffin (CP, mp 50−52 °C; Xilong Chemical Reagent Beijing Co., Ltd.) was used as the PCM. Preparation of the Porous Al2O3@Grapite Foams. PAGFs were prepared using a modified particle-stabilized direct foaming method. As shown in the schematic in Figure 1, a suspension was first

liquid phase change with the aid of supporting materials. In this regard, multifarious form-stable PCM composites were prepared by inserting PCMs into supporting materials. For the moment, most of the supporting materials were polymers and inorganic porous materials. The encapsulation of PCMs with polymer materials is always difficult, and the products are usually costly, toxic, inflammable, and have poor thermal conductivities and stabilities.9−12 Therefore, the use of inorganic supporting materials has attracted more and more attention in recent years due to its low cost, good chemical stability, nontoxicity, high mechanical strength, fire resistance, and easy availability. The inorganic porous supporting materials could be synthetic porous materials or natural minerals, including vermiculite, perlite, silica, diatomite, bentonite, attapulgite, montmorillonite, expanded graphite, etc.13−17 Compared with the natural porous structure of minerals, the synthetic supporting materials have a controllable porous structure and high thermal conductivity, which can cater to the specific application demands. Thereby, synthetic porous materials, such as metallic foams, carbon foams, ceramic foams, have gradually been hot supporting materials in the research field of latent heat storage.18−22 Porous alumina (Al2O3) foam as a supporting material was seldom studied in the past, but it has numerous advantages over other supporting materials, such as low cost, fire resistance, high specific surface area, high thermal stability, high catalytic activity, high chemical stability, low density, and nontoxicity. Therefore, porous Al2O3 foams are very valuable and suitable for the synthesis of form-stable composite PCMs for energy storage.23,24 Up to now, lots of inorganic and organic PCMs and their mixtures have been widely studied as latent heat storage materials.25 Among the low-temperature PCMs, paraffin has been widely used for latent heat storage applications due to its high latent heat storage capacity, selfnucleating behavior, good thermal characteristics, and chemical stability;26 therefore, it is feasible to use porous Al2O3 foam as supporting material and paraffin as PCM material to prepare form-stable composite PCMs. In this paper, porous Al2O3@grapite foam (PAGF) with hierarchical porosity was prepared from particle-stabilized emulsions and then served as good supporting material of form-stable composite PCMs due to its three-dimensionally interpenetrating gradient pore structure, controllable cell sizes ranging from 80 to 250 nm, high porosity of 78−90%, and large mechanical strength. PAGF could also meet the requirements for certain applications with specific channel structure and porosity. In order to prepare the form-stable composite PCMs with high heat storage capacity and high thermal conductivity, Al2O3 particles were covered with a layer of uniform sucrose coating in the suspended state and then sintered at 1000−1600 °C. By combining paraffin, a low-temperature PCM, with PAGF, a supporting material, the form-stable composite PCMs (PAGFPs) with enhanced thermal and physical properties were successfully obtained in our work, which can serve as promising thermal energy storage materials in many thermal energy storage systems.

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Figure 1. Schematic diagram of the preparation of PAGFs by a particle-stabilized, direct foaming method. prepared by mixing 40 vol % Al2O3 particles, 20−40 wt % sucrose, an appropriate amount of dispersing agent and 60 mL of deionized (DI) water in polyethylene bottles; it was then ball-milled for 20 h with agate milling balls with a diameter of 10 mm to achieve a homogeneously deagglomerated state. Afterward, a certain amount of propyl gallate dissolved in ethanol was added dropwise to the above suspension under magnetic stirring. After 5 min of magnetic stirring, the suspension was removed and stood for 30 min, and then was foamed using a hand-held mixer (Philips, HR1613). The stirring rate was kept at the 6000 rpm first and then at the full speed of 16 000 rpm for 3 min to introduce air into the suspension properly until enough air mixed with the suspension, and then the foams formed. The prepared emulsions were then transferred into a culture vessel and airdried at room temperature for 1 week. Finally, in order to achieve an oxygen-free atomosphere, the completely dried emulsions were first buried in carbon and then were sintered at a temperature of 1000− 1600 °C for 4 h. Preparation of Form-Stable Composite PCMs. Figure 2 schematically illustrates the procedure of the preparation of the

Figure 2. Schematic diagram of the vacuum impregnation method for preparing composite PAGFPs. form-stable PCMs by the vacuum impregnation method. PAGF and solid paraffin were encapsulated into a vacuumed glass container. Then the container was heated in a water bath at 80 °C during the impregnation process in order to melt the paraffin and facilitate its absorption into the PAGF. After 3 h of vacuum impregnating, the prepared paraffin/PAGF (PAGFP) was taken out from the liquid paraffin reservoir and then placed in an oven at a temperature of 80 °C to remove the liquid paraffin on the surface. At the moment when the paraffin stopped leaking from the body of the PAGFP, the maximum absorption ratio of PAGF for paraffin was determined. The mass of the initial PAGF is recorded as M0, and the mass of the final form-stable

EXPERIMENTAL SECTION

Materials. Commercial Al2O3 particles (99.99%, d50 = 0.2 μm, Lianyungang Lianlian Chemicals Co., Ltd.) and sucrose (AR, Xilong Chemical Reagent Beijing Co., Ltd.) were used as starting materials. The ammonium salt of polyacrylic acid (Adamas Reagent., Ltd.) served as a dispersing agent. Propyl gallate (C10H12O5,AR grade, 7595


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ACS Sustainable Chemistry & Engineering composite PAGFP is recorded as M. Then the paraffin storage capacity (φ) can be calculated by eq 1:

φ = (M − M 0)/M

(1)

The φ determined by eq 1 is consistent with that calculated from eq 2 φ = Htheo/HPCM

(2)

where the value of φ is the mass fraction of paraffin, HPCM denotes the latent heat of the pristine paraffin, and Htheo represents the theoretical phase change enthalpy of PAGFP. The φ of PAGF with different solid contents of sucrose is shown in Table 1. It is clear that paraffin could be maintained in the PAGF without leakage, and the final composite PAGFP was a form-stable PCM.

Table 1. Porosity, Pore Volume, Surface Area, and Enclosed Ratio Values of the Prepared Samples φ

samples

porosity (%)

pore volume (mL/g)

surface area (m2/g)

(Htheo/HPCM)

(M − M0)/M

PAGF0 PAGF20 PAGF30 PAGF40

71 90 84 78

0.8136 1.4095 1.0508 1.0478

2.7 − 35.3 −

42 51 58 47

46 57 66 55

Figure 3. SEM images of (a) PAGF20, (b) PAGF30, and (c) PAGF40 and the corresponding results [(e) PAGFP20, (f) PAGF30, and (g) PAGFP40] after vacuum impregnation of paraffin.

Characterization. The microstructures of the prepared samples were examined by scanning electron microscopy (SEM) (S-4800, Hitachi). Mercury intrusion porosimetry (MIP) was used to measure the pore size distribution of the sample (AUTOPORE IV 9510). X-ray photoelectron spectroscopy (XPS) (ESCALAB 250Xi) and transmission electron microscopy (TEM) (Tecnai G2 F30 S-TWIN) were carried out to confirm the presence of the carbon membrane loaded on the Al2O3 surface. The specific surface area of the prepared sample and the pore size distribution of the mesopores on surface of the prepared sample were tested by the Brunner−Emmet−Teller (BET) method (AUTOSORD STATION 6). Thermal properties of the prepared form-stable composite PCMs were measured at a temperature ranging from 25 to 80 °C by differential scanning calorimetery (DSC) (Q100). The thermal stability of the foam-stable PCMs was explored by thermogravimetric analysis (TGA) (Q50) with a heating rate of 10 °C/min. The thermal reliability of the form-stable composite PCMs was analyzed by Fourier transform infrared spectroscopy (FTIR) (SHIMADZU FTIR-8400S) and thermal cycling tests. The thermal conductivity of the samples was measured by the transient plane source method at room temperature using a laser thermal conductivity analyzer (LFA-427).

Figure 4. Pore size distribution of the windows in the PAGF30 were tested by MIP.

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RESULTS AND DISCUSSION Characterization of the Form-Stable Composite PCMs. Figure 3a−c shows the SEM patterns of the prepared PAGFs samples (40 wt % of solids of the Al2O3 particles) coated with 20−40 wt % of sucrose (PAGF20, PAGF30, PAGF40) that were sintered at 1600 °C. The PAGFs showed a bubblelike macroporous structures, which was due to the dispersion of the air in the suspension during the direct foaming process. There are a lot of windows on the cells, forming a three-dimensionally connected porous structure, which make the impregnation of paraffin feasible and avoid leakage. It was obviously seen that the concentration of sucrose significantly influenced the porous morphology of the PAGFs. The average cell diameters increase from around 80 to 250 μm with sucrose concentrations decreasing from 40 to 20 wt %. For higher concentrations of sucrose, the suspension viscosity became higher, which impeded air from being incorporated

Figure 5. Partial, enlarged SEM images of PAGF30. 7596


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Figure 6. EDS results of PAGF30 with different testing positions.

The C 1s deconvoluted scan displays three representative domains with binding energies of 284.7, 285.7, and 289.3 eV, which can be attributed to the C−C, C−O, and CO functional groups, respectively. Obviously, the strong intensity at 284.7 eV of the C−C group is dominant over that of the C− O and CO groups due to their test out peak area percentage respectively being 60.5%, 24.5%, and 15%, which confirmed that the coating layer corresponded exactly to graphite. Meanwhile, substantially more C atomic signals (58.3 atom %) and less Al and O atomic signals (14.91 and 26.79 atom %) are detected in XPS measurement, which indicated that the Al2O3 surface is well covered by a graphite layer with a thickness comparable to the XPS sampling depth of 5−10 nm. As shown in the TEM in Figure 8, an approximately 8 nm thick layer of graphite is coated on the surfaces of Al2O3 particles. The average size of Al2O3@graphite particles is 150− 200 nm, which is suitable for preparing particle-stabilized foams by a direct foaming method. We expect that the heat storage capacity of the supporting materials would be highly enhanced due to the pyrolysis of sucrose producing a large amount of mesopores on the surface of the Al2O3 particles. In order to confirm this conjecture, we prepared a porous Al2O3 foam (PAGF0) without adding sucrose and compared its specific surface area with that of PAGF30. As shown in Table 1, the specific surface area of PAGF30 is 35.3 m2/g, which is 13 times greater than that of PAGF0. Meanwhile, the BET result in Figure 9 shows that most of the mesopores on the surface of

into the Al2O3 suspension; it was difficult for air bubbles to cohere inside the body of the suspension during the foaming process; thus, the average cell diameters became smaller and uniformly distributed. As seen in Figure 3c, the diameters of the cells of the macroporous PAGF40 sample are clearly smaller than 100 μm and distributed uniformly. In addition, a large number of windows were uniformly distributed on the cell walls from the SEM image, which was also certified by MIP. Meanwhile, the size and distribution of the windows are shown in Figure 4, where the estimation of the average windows pore size is 0.5 μm, which is beneficial to the impregnation and the leaching of phase change materials. Figure 3e−g represents the SEM patterns of the prepared paraffin/PAGF samples, corresponding to Figure 3a−c. It could be seen that paraffin was completely enclosed in the pores of PAGFs after 200 thermal cycles. In Figure 5, it is seen that the surfaces of Al2O3 are coated with a layer of carbon, which indicated that a layer of sucrose deposited on the surface of each Al2O3 particle can be carbonized after sintering treatment. According to the composition analysis in Figure 6, the mass percentage of C element on the Al2O3 particle surface ranges from ∼13 to ∼33 wt %. In order to determine the specific composition of the carbonaceous material, the PAGF40 sample was characterized by XPS and TEM. As shown in Figure 7, the presence of the graphite coating on the surface of Al2O3 is further confirmed by the XPS peak position at 284.11 eV, which corresponds to C 1s. 7597


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Figure 10. DSC curves of paraffin and the prepared form-stable composite PAGFPs, after 200 thermal cycles.

clearly demonstrate that the specific surface area of the supporting materials is greatly increased by loading a layer of graphite. Thus, we think the introduction of a graphite layer has a great effect on the improvement of heat storage capacity, which was also proved by the calculation of the enclosed ratio of paraffin. Furthermore, due to the high thermal conductivity with graphite, we can conclude that the prepared PAGFP has high thermal conductivity. Therefore, the prepared particlestabilized PAGFs, with hierarchical porous structures, enhanced heat storage capacity, and probably high thermal conductivity, were suitable supporting materials in the preparation of composite PCMs. Thermal Properties of the Form-Stable Composite PCMs. As shown in Figure 10, DSC analysis was carried out to study the thermal properties of the paraffin and composite PCMs, including phase change temperatures and the latent heat of melting and crystallization. The heating and cooling rate of measurements was 5 °C/min. The DSC curves of all PAGFPs are clearly consistent with the pure paraffin because they all demonstrate two phase change peaks which are ascribed to the solid−liquid and solid−solid phase transition. Thus, the pure paraffin and the composite PAGFP both show four peaks in the melting and crystallization curves. The phase change parameters of paraffin and PAGFP20−PAGF40 samples evaluated from DSC curves are presented in Table 2, wherein Tm and Tc are the melting peak temperatures and cooling peak temperatures during the melting and cooling processes. The supercooling (the difference between Tm and Tc) of paraffin and PAGFPs is also shown in Table 2. The Tm of the composite PAGFPs increased significantly to the higher temperature, whereas the Tc decreased to the lower temperature compared to that of paraffin. That is to say, the phase change temperature of paraffin was largely influenced by the confinement within the porous structures of PAGFs. The supercooling of paraffin decreased from 12.1 to 6.92 °C, because pores in the PAGFs with large specific surface areas could act as heterogeneous nucleation centers to promote the crystallization of the enclosed paraffin. The heterogeneous nucleation effect of PAGFs could lower the grain size of paraffin, which led to the decrease of Tm and the increase of Tc. In addition, it also clearly shows that the latent heats of melting and cooling processes of the pure paraffin were 192.65 and 196.02 J/g, but the latent heats of melting and cooling processes of the composites

Figure 7. XPS spectra of the prepared composite Al2O3@graphite particles.

Figure 8. TEM images of the prepared composite PTFC particles.

Figure 9. BET results of the pore size distribution of the mesopores on the PAGF30 surface.

the PAGF30, which were produced by the pyrolysis of sucrose, are concentrated around 3.83 nm. Therefore, the test results 7598


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ACS Sustainable Chemistry & Engineering Table 2. Thermal Characteristics of Paraffin and Composite PAGFPs before and after Thermal Cycling melting process Tm (°C)

crystallization process Hm (J/g)

Tc (°C)

Hc (J/g)

samples

before cycling

after cycling

before cycling

after cycling

before cycling

after cycling

before cycling

after cycling

supercooling

PAGFP20 PAGFP30 PAGFP40 paraffin

49.30 49.10 49.17 52.70

49.57 49.55 49.73 −

115.92 113.98 93.56 192.65

93.19 105.76 85.82 −

41.60 42.01 41.60 40.60

41.91 42.63 41.92 −

114.39 114.26 94.11 196.02

95.15 105.98 84.94 −

7.66 6.92 7.81 12.1

Table 3. Comparison of the Thermal Properties of Some Recently Reported Composite PCMs and the Samples Prepared in This Study composite PCMs

Tm (°C)

Hm (J/g)

Tc (°C)

Hf (J/g)

refs

paraffin/CaCO3/30% EG fatty acid/Ag-coated polyurethane fibers membrane paraffin/expanded vermiculite polyethylene glycol/expanded graphite polyethylene glycol/expanded vermiculite paraffin/SiO2 paraffin/macroporous poly(ethylene dimethacrylate) paraffin/expanded perlite paraffin@CaCO3 microcapsules (1:1) paraffin/diatomite paraffin/expanded perlite paraffin/graphite/porous Al2O3 foams

48.46 16.99 27.0 61.46 59.97 58.17 − 42.3 47.45 41.84 21.6 49.55

95.48 97.93 77.6 161.2 99.10 98.00 132.6 87.4 105.5 53.15 56.3 105.76

− 10.07 25.1 46.91 43.51 58.39 − 40.8 47.45 36.78 −− 42.63

− 97.24 71.5 146.9 96.40 88.30 133.4 90.3 103.1 58.83 −− 105.98

27 28 29 30 31 32 33 34 35 36 37 this study

Figure 11. Digital photographs of PAGF and the corresponding results after vacuum impregnation of paraffin (PAGFP).

Figure 13. FT-IR spectra of PAGF, PAGFP30, and paraffin, after 200 thermal cycles.

Table 1, the porosity and cell diameter of the prepared PAGFs declined with the rising of the sucrose content, which means that the valid volume of PAGFs for the encapsulation of paraffin decreased, that is to say, the storage volume of PCMs will decrease, which was demonstrated by the pore volume in Table 1, calculated by the total intrusion volume of mercury by MIP. However, the porous structure with smaller cell diameter has stronger surface tension and capillary forces and thus makes it difficult for the encapsulated PCMs to leak compared to pores with a larger cell diameter. Therefore, both its cell diameter and porosity determined the storage capacity of the prepared PAGFs together. This view is identical with the φ exhibition in Table 1, in which the sample PAGFP30 has the highest φ value, signifying the highest latent heat storage capacity. The sample PAGF20 has a pore structure with the biggest cell diameter, which decreases its adsorption capacity.

Figure 12. TGA curves of paraffin and composite PCMs PAGFP20− PAGFP40, after 200 thermal cycles.

PAGFP20−PAGFP40 were 93.19 and 95.15, 105.76 and 105.98, and 85.82 and 84.94 J/g, respectively. As shown in 7599


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Figure 14. Raman spectra of Al2O3@graphite particles sintered at different temperature (1000, 1200, 1400, and 1600 °C).

Figure 16. TEM images of the carbonaceous layers on the surface of Al2O3 particles with the sinter temperature increasing from 1000 to 1600 °C.

Figure 15. Sintered temperature dependence of the full width at halfmaximum of D peaks (fwhmD) and the ratio of the intensities of the D and G peaks (ID/IG).

As for the sample PAGF40, the smallest cell diameter gives rise to fewer valid voids for encapsulation of paraffin. In addition, the decrease of the porosity of PAGFs would increase the weight percentage of Al2O3 that did not undergo phase change within the testing temperature range. Moreover, the enthalpies of PAGFPs were generally lower than the calculated values because the PAGFs with a large specific surface area would restrict the crystallization of paraffin. Thus, we can draw the conclusion that the sample PAGF30 has the most proper cell diameter and porosity for the encapsulation of paraffin among the three samples. Furthermore, compared with other composite PCMs in Table 3,27−37 the prepared composite PAGFPs here have a relatively higher and reasonable enthalpy value and controllable hierarchical pore structure and, thus, could be promising candidates for thermal energy storage applications. Thermal Reliability and Chemical Stability of the Composite PCMs. Figure 11 shows the photographic images of PAGF and PAGFP. The morphology and structure of the PAGF showed no changes after the impregnation of paraffin and 200 thermal cycles. Table 2 shows the thermal properties of the composite PAGFPs after 200 melting and cooling cycles. The Tm and Tc values only changed slightly after the melting and cooling cycles. After 200 melting and cooling cycles, the

Figure 17. Comparison of thermal conductivities between paraffin and composite PAGFP30.

enthalpy value of the composite PAGFPs was still reasonable (only decreased by less than 20 J/g). Therefore, the prepared form-stable composite PAGFPs showed good thermal reliability after thermal cycles. Figure 12 shows the TGA curves of pure paraffin and the prepared PCM composites. It clearly illustrates that after the onset of degradation at about 160 °C, pure paraffin shows a maximum weight loss at about 250 °C and the weight loss ends at about 305 °C. However, the onset of degradation for the prepared composite PCMs was at approximately 180 °C, reaching the maximum degradation at approximately 275 °C and the weight loss ending at approximately 315 °C. Thus, it can be seen that the onset temperature of the degradation of composites was higher than that of the pristine paraffin. For the case of the PAGF, no decomposition was observed up until 600 °C. Therefore, the distinct weight loss in the composites PCMs was due to the decomposition of the paraffin component. Thus, 7600


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Table 4. Comparison of Thermal Conductivities of Composite PCMs Prepared in This Study with Some Recently Reported PCMs composite PCMs

thermal conductivity (W/m·K)

refs

paraffin/porous TiO2 foam (covered by carbon) graphite/paraffin paraffin/nickel foam paraffin/polymer/nano-Si3N4 (16 wt %) carbon nanofiber/paraffin wax paraffin/calcium carbonate microcapsules with 20 wt % EG 2 wt % nano-Cu−paraffin paraffin/expanded graphite paraffin/porous Al2O3@graphite foam

1.059 0.39−1.55 1.2 0.38 0.45 2.75 0.345 0.82 0.76

22 43 44 45 46 47 48 49 this study

the thermal conductivity of PAGF, due to its three-dimensionally connected structures. Thus, we can expect that the sample sintered at 1600 °C would have the highest thermal conductivity. As shown in Figure 17, our test results show that composite PAGFP has a higher thermal conductivity of 0.76 W/m·K, which is 0.55 W/m·K greater than that of pure paraffin. Meanwhile, as shown in Table 4, it also had a higher thermal conductivity, compared to the other PCMs.

it can be seen that the prepared composite PCMs had a good thermal stability below 180 °C. As for the spectrum of Al2O3 and the composite PAGFP in Figure 13, the bands near 595 cm−1 are attributed to the Al−O bonds, and the bands at 661 cm−1 correspond to the absorption peaks of octahedral AlO6 species.22−24 In the spectrum of paraffin and the composite PAGFP, peaks at 2845 and 2915 cm−1 could be ascribed to the symmetric and asymmetric stretching vibrations of C−H. The peak at 719 cm −1 corresponds to the rocking vibration of CH2. The peaks at 1466 and 1377 cm−1 are ascribed to the deformation vibrations of the C−H bond. As shown in Figure 13, the characteristic absorption peaks of paraffin and PAGF all appeared in the spectrum of the composite PCMs, which indicates that no chemical reaction occurred in the prepared form-stable composite PCMs. Therefore, the prepared form-stable composite PCM had a good chemical compatibility and stability. Thermal Conductivity of the Composite PCM. The typical Raman spectra of Al2O3@graphite particles sintered at different temperatures (1000, 1200, 1400, and 1600 °C) are shown in Figure 14. The Raman characteristic sp3 (D) and sp2 (G) peaks at 1330 and 1585 cm−1 can be surely ascribed to the presence of carbonaceous layers. The D-band is derived from the breathing mode of sp2 carbon atoms in aromatic rings, whereas the G-band could be generated by any sp2 carbon atoms organized in chains or rings.37 The information on sp2/ sp3 is represented by the ratio of intensities of D and G peaks (ID/IG), which is often used to describe the sp2 “grain size”.38 Additionally, the variation of the full width at half-maximum of the D peak (fwhmD) will result in changes of the number and/ or size of the graphite structure.39−42 Figure 15 shows the variations of the ID/IG and fwhmD for samples sintered at 1000, 1200, 1400, and 1600 °C, respectively. ID/IG increases from 1.01 to 1.30 and fwhmD decreases linearly from 72 to 37 cm−1 with the synthesis temperature increasing from 1000 to 1600 °C, which indicates the growth of graphite structural units during carbonization. At the temperature of 1600 °C, the carbonaceous layers on the surface of Al2O3 particles probably achieve a high degree of graphitization, which can also be confirmed by the TEM images shown in Figure 16. The phase of graphite grew up and became more ordered with the sinter temperature increasing from 1000 to 1600 °C. Meanwhile, the thickness of the graphite layer decreased from about 15 to 8 nm due to the enhanced crystallinity of the graphite phase with increasing sinter temperature. According to TEM results, the graphite layer showed a very ordered crystalline structure at the sinter temperature of 1600 °C. When paraffin impregnated into PAGF, the heat conduction process significantly depends on

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CONCLUSION In this paper, PAGFPs were prepared with 40 vol % Al2O3 particles and 20−40 wt % sucrose using a novel particlestabilized direct foaming method. At the optimal sinter temperature of 1600 °C, an approximately 8-nm-thick graphite layer with ordered crystalline structure formed on the surface of Al2O3 particles, which makes the prepared composite PCMs not only have an increased heat storage capacity but also a high thermal conductivity. It was also demonstrated that the prepared PAGFs had a three-dimensionally interpenetrating porous structure with controllable cell diameters ranging from around 250 to 80 μm. In addition, the pores of the prepared PAGFs could be fully encapsulated with paraffin. The maximum φ of reserved paraffin reached 66 wt % in the composite PAGFP30, without any leakage. DSC results showed that the latent heat in the melting/cooling process of the composites PAGFPs reach the maxima 105.76 and 105.98 J/g. Our results also confirmed that the prepared form-stable composite PAGFPs had good chemical stability and thermal reliability. More importantly, the specific surface area of PAGF30 is 35.3 m2/g, which is 13 times more than that of PAGF0. Meanwhile, the thermal conductivity of the composite PAGFP30 sintered at 1600 °C was greatly enhanced by 3.6 times that of pristine paraffin. Therefore, the prepared formstable composite PAGFPs demonstrate good thermal properties/reliabilities and chemical stabilities and high thermal conductivities and thus are promising candidate materials for thermal energy storage applications.

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AUTHOR INFORMATION

Corresponding Authors

*J.L. e-mail: [email protected]. *W.F. e-mail: [email protected]. ORCID

Jinhong Li: 0000-0002-0368-8184 Notes

The authors declare no competing financial interest. 7601


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ACKNOWLEDGMENTS This project was supported by funds from the National Natural Science Foundation of China (Grant No. U1607113), Program of Qinghai Science and Technology Department (Grant No. 2017-HZ-805), and the Fundamental Research Funds for the Central Universities (Grant Nos. 35732016062).

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Design and Preparation of the Phase Change Materials Paraffin

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