Enhanced Thermal Conductivity and Durability of a Paraffin Wax

May 31, 2017 - However, paraffin wax is limited because of its poor thermal ... we present a novel paraffin wax nanocomposite filling with carbon-coat...
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Enhanced Thermal Conductivity and Durability of a Paraffin Wax Nanocomposite Based on Carbon-Coated Aluminum Nanoparticles Yiming Chen,†,‡ Wen Luo,† Jie Wang,† and Jin Huang*,† †

School of Material and Energy, Guangdong University of Technology, Guangzhou 510006, P. R. China Guangdong Provincial Key Laboratory of Functional Soft Condensed Matter, Guangzhou 510006, P. R. China



ABSTRACT: Phase change materials (PCMs) based on latent heat energy storage techniques over a nearly isothermal temperature range have been regarded as a promising strategy to meet the highly efficient thermal management system for electric vehicles (EVs). Ordinarily, paraffin wax is applied as a most widespread PCM especially in medium- and lowtemperature storage, owing to a large latent heat that is nontoxic and noncorrosive. However, paraffin wax is limited because of its poor thermal conductivity, resulting in a significant decrease of the heat transfer rates. Here, we present a novel paraffin wax nanocomposite filling with carbon-coated aluminum nanoparticles (Al−C/PW), in which carbon-coated aluminum nanoparticles (Al−C) can form a network, resulting in a remarkable increase of the thermal conductivity. The results show that the thermal conductivity is 0.189W/(m·K) with 4 wt % Al−C under 25 °C, which is about 206.5% compared to the pure paraffin, simultaneously keeping a good thermal durability. Our work paves the way for developing a paraffin wax nanocomposite filled with Al−C for enhanced PCM.



INTRODUCTION In consideration of environmental issues and energy shortage, electric vehicles (EVs) that rely on high specific power density batteries have received much more attentions in recent years. Among those batteries, the Li-ion batteries are regarded as the most promising and substituted energy due to a high power density, stable charge/discharge cycle, relatively long lifespan, and low cost.1,2 However, the practical application of Li-ion batteries is limited by a crucial parameter of the operating temperature,3,4 resulting in a distinct degradation of the life and capability.5,6 Moreover, the Li-ion batteries generate a significant amount of heat during high power discharge that can cause a capacity fade or even a safety hazard.7,8 Therefore, a thermal management system for Li-ion batteries is really necessary to regulate operating temperature in the desirable range and avoid a thermal runaway situation with the optimum performance. Phase change materials (PCMs) can store or release a huge amount of heat during its phase change transition, which is widely applied in the thermal management system to improve cooling efficiency and save energy.9−12 Among the PCMs, paraffin wax is one of the most common materials because of the following advantages: (1) a desirable phase change temperature range; (2) a large latent heat capability; (3) nonsupercooling during freezing; (4) a low cost; and (5) a fast rate by storing or releasing energy.13 However, paraffin wax (PW) still suffers from a serious problem of unsatisfactory thermal conductivity. More recently, scientists have made a lot of efforts to improve the thermal conductivity of paraffin wax including: (1) © 2017 American Chemical Society

adding thermal conductive additives; (2) inducing porous materials to form composites; and (3) adding metal screens/ spheres or metal fins.14−20 Especially, the carbon nanomaterials have been applied in PCM application and show a promising development owing to a high thermal conductivity. Fan et al. reported various carbon nanofillers including short and long multiwall carbon nanotubes, carbon nanofibers, and graphene nanoplatelets to enhance the thermal conductivity and energy storage properties of paraffin-based composites. The results indicated that these nanofillers can enhance the thermal conductivity and slightly decrease the phase change enthalpies.21 Li Min dispersed the nanographite (NG) in paraffin and found that the distributed NG could enhance the heat transmission and improve the performance of energy storage.22 Shi Jia-Nan et al. investigated the effects of adding graphene or exfoliated graphite nanoplatelets in paraffin. The results showed that the thermal conductivity of the composite is increased with the loading contents in both graphene and exfoliated graphite nanoplatelets.23 Silakhori et al. presented a form-stable PCM composed of palmitic acid/polypyrrole/graphene nanoplatelets (PA/PPy/GNPs). By doping 1.6% of GNPs, the thermal conductivity and thermal capacity of the PCMs could reach up to 0.43 W/m K and 151 J/g, and the thermal conductivity increased by 34.3% in comparison with PA/PPy PCMs.24 Metselaar et al. reported a kind of new form-stable composite PCM prepared by vacuum impregnation of paraffin within Received: March 20, 2017 Revised: May 18, 2017 Published: May 31, 2017 12603

DOI: 10.1021/acs.jpcc.7b02651 J. Phys. Chem. C 2017, 121, 12603−12609

Article

The Journal of Physical Chemistry C

Figure 1. SEM images of (a) pure paraffin, (b) carbon-coated aluminum nanoparticle, (c) Al−C/PW composites with a low filling content (0.4%), and (d) Al−C/PW composites with a high filling content (4%).

Synthesis of Al−C. A mixture of 8 g of graphite powder (∼100 μm, 99% purity), 12 g of aluminum powder (∼200 μm, purity 99%), and 250 mL of ethanol (99% purity) was stirred for 1 h under ultrasonic vibration. Then the mixture was dried at 80 °C for 2 days after filtering out the ethanol, followed by pressing under 5 MPa to form a cylindrical disk of 24 mm diameter and 50 mm length that served as the anode. A cylindrical graphite (99% purity) rod of 10 mm diameter and 200 mm length was applied here as the cathode. The Al−C was prepared using a direct current (DC) arc discharge method. The chamber was evacuated, and Ar with pressure of 1 Pa was introduced after the anode and cathode were placed in the chamber. DC power was obtained from an alternating current (AC)/DC inverter shielded metal arc welding power source and then was used to produce the arc between the cathode and the anode disk at a current of 125 A and voltage of 25 V. The cathode was manually advanced toward the anode, resulting in an electric arc between them. The anode was consumed and deposited in the form of soot on the chamber inner wall which was water-cooled while maintaining a 3−4 mm gap between the two electrodes, and the deposited soot was collected by scraping the chamber inner wall with a metal scraper, 24 h after the reaction. Preparation of Al−C/PW. The paraffin was processed by premelting and degassing in a vacuum oven at 60 °C for 1 h. The Al−C nanoparticles of various loading content were first added into melted paraffin to form mixtures under rigorous stirring with a magnetic stirrer kept at 60 °C for 1 h, followed by intensive probe ultrasonication for another 30 min. In this work, the loading content of Al−C nanoparticles ranged from 0.2 to 10 wt % (mass fraction) compared with a reference sample of pure paraffin (0 wt %). Characterization and Measurement. The microscopic morphologies and structures of Al−C nanoparticles and Al−C/ PW were examined by a scanning electron microscopy (SEM; S- 4800, Hitachi, Japan) and transmission electron microscopy (TEM; JEM-2100F, Jeol, Japan). The dispersion state of Al−C nanoparticles in the composites with different loading contents

graphene oxide (GO) sheets. The thermal conductivity of the composite PCM was highly improved from 0.305 to 0.985 (W/ mk) and with a latent heat of 64.89 J/g with a good thermal reliability and chemical stability by 2500 thermal cycling tests. However, the high thermal conductivity of the PCM composites by adding nanoscale carbon materials can only be exhibited in the horizontal direction (not in the axial direction) because the heat can transfer through the interface. As a result, core−shell-structured carbon nanoparticles with large specific surface area may be a desirable and alternative option for the thermal conductivity enhancement. Recently, core−shell-structured carbon-coated metal nanoparticles have become a potential PCM with considerably high thermal conductivity of the metal core and oxidative stability of the carbon layer. Among the metals, aluminum is found to be a great heat conductor with many advantages including a high thermal conductivity about 236W/(m·k), a small density, a good corrosion resistance, and a favorable ductility. Although the synthesis of carbon-coated aluminum nanoparticles has been reported by different methods, until now few reports consider carbon-coated aluminum nanoparticles as a potential and feasible thermal conductivity filler. Here, we designed and fabricated a novel PCM based on the paraffin wax nanocomposite filling with carbon-coated aluminum nanoparticles (Al−C/PW), in which the Al−C was obtained by the direct-current arc discharge method. The thermal conductivity, thermal durability, and heat storage properties of the composites were improved due to the nano network formed by the by the Al−C. The Al−C/PW shows a good thermal durability and a thermal conductivity of 0.189W/ (m·K) with 4 wt % Al−C loading at 25 °C, which is about 2 times compared to that of the pure paraffin.



EXPERIMENTAL SECTION Materials. The raw materials and reagents to prepare the Al−C and Al−C/PW are all supported by Sinopharm Chemical Reagent Co., Ltd., China. 12604

DOI: 10.1021/acs.jpcc.7b02651 J. Phys. Chem. C 2017, 121, 12603−12609

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The Journal of Physical Chemistry C was observed by photography images. The crystal structure of the Al−C nanoparticles was investigated using X-ray diffraction (XRD; Rigaku Corporation, Japan) with scanning angle 2θ from 5° to 80°. The thermal properties of Al−C and Al−C/PW phase change material were determined by thermal gravity analysis (TGA; SDT 2960, TA Instruments, American) and differential scanning calorimetry (DSC; Diamond DSC, PerkinElmer, USA). All the DSC measurements were performed at 20 °C/min heating rate from 25 °C to 80 °C in a nitrogen gas atmosphere. For TGA measurements, all the samples were heated at a ramping rate of 20 °C/min from 350 to 900 °C in dry air environment. The thermal conductivity (λ) of the Al−C/PW composites was tested using a LFA447 NanoFlash apparatus (Netzsch Instruments N.A. LLC.) based on the laser flash method. According to the laser flash method, the laser is generated by a xenon lamp to produce heat pulse, resulting in hitting the front side of the sample and causing a temperature rise on the rear surface, which can be detected by the infrared sensor. The thermal conductivity coefficients were calculated based on eq 1 λ = α ·ρ ·Cp

Figure 2. TEM image of Al−C (a) and HRTEM image of Al−C (b).

The X-ray diffraction pattern of the Al−C nanoparticles is shown in Figure 3. The strong peaks at 2θ values of 38.5°,

(1)

where λ represents the thermal conductivity of specimen, and α, ρ, and Cp stand for the thermal diffusivity, density, and specific heat capacity of the specimen, respectively. Then the thermal diffusivity was computed based on the collected data from the infrared sensor. The samples were shaped as disks with a diameter of 12.7 mm and a thickness of 2 mm, and the final density of the samples with different Al−C filling ratio was determined by an average of three measurements under 25 °C. The specific heat capacity was calculated by comparing with the reference sample at 25 and 45 °C.

Figure 3. XRD pattern of Al−C nanoparticles.



44.7°, 65.1°, 78.2°, and 82.5° can correspond to the (111), (200), (220), (311), and (222) crystal planes of alumina, respectively, which signify a face centered cubic (fcc) crystal structure. The sharp peak at 26.5° belongs to the C(002) reflection of the hexagonal graphite structure. From the XRD pattern, the strong intensity and narrow width of Al diffraction peaks show that resulting products are highly crystalline in nature without the additional Al2O3 by this method. Combined with the TEM images, we can conclude that the spherical aluminum nanocrystal surface is coated with a thin graphitic layer of the Al−C nanoparticle. The TGA and DSC results of Al−C nanoparticles are shown in Figure 4. From the TGA curve, we can find that the mass

RESULTS AND DISCUSSION The morphologies and structures of pure paraffin, Al−C nanoparticles, and Al−C/PW composites were observed by SEM. Figure 1a shows the SEM image of pure paraffin with a smooth surface, and Figure 1b shows Al−C nanoparticles as a regular sphere with the measurement of 50−100 nm. We point out that the uniform spheres with different size may be more favorable to diffuse in the paraffin. The SEM images of Al−C/ PW composites with different filling contents were shown in Figure 1c and Figure 1d. Compared with that of pure paraffin, a low filling content (0.4%) and a high filling content (4%) both show obvious fillers in the paraffin. With the increasing filling content, the Al−C nanoparticles could easily be connected together with a relatively good dispersion even at a high filling content. This microstructure is different from that of only carbon nanomaterials as fillers,21,22 resulting in a thermal conductive network at a high filling content (about 4%) which is very important for the improvement of the thermal conductivity of the Al−C/PW composites. From the TEM images (Figure 2a) of Al−C nanoparticles, the sphere structure can be also demonstrated by the HRTEM image (Figure 2b) similar to the result of SEM (Figure 1b). We can find that the metallic aluminum core is covered by the carbon shell completely with about 4 nm thickness of outer layer. Compared with other carbon nanomaterials (such as graphene, carbon nanotubes, and carbon fibers),21,25 the carbon-coated aluminum nanoparticles more easily form shell−core structure, which can result in a considerably high thermal conductivity and oxidative stability.

Figure 4. DSC and TGA curve of the Al−C nanoparticles. 12605

DOI: 10.1021/acs.jpcc.7b02651 J. Phys. Chem. C 2017, 121, 12603−12609

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The Journal of Physical Chemistry C

Figure 5. (a) DSC curves of the composites and (b) latent heat compacity of PW and Al−C/PW composites.

addition of Al−C particles reduces the component ratio of PW in the Al−C/PW composites, affecting the ability to absorb or release heat during the phase change process. This result indicates that the high latent heat capacity needs a probe filling concentration of Al−C particles. Figure 6 shows the TGA curves of Al−C/PW composites with different loading contents. Compared with that of pure

gain can be observed with a gradual weight increase of about 20% from the initial 400 to 660 °C (melting point of Al) due to the formation of aluminum oxide. It is quite different from the TGA of the pure aluminum particles which have a rapid weight increase of more than 20% from 600 to 660 °C.26Another less intensive Al oxidation with a mass gain of about 7% from 660 to 850 °C can also be observed. The similar mass gain of the pure aluminum particles is about 40% from 660 to 1100 °C.27 The results indicated that the outer carbon layer could effectively increase the thermal stability of the Al−C nanoparticles to a certain degree. Moreover, there are two exothermic peaks on the curves of DSC, which respond to the above two mass gains of TGA analysis. The first peak shows a much higher enthalpy change of 3115 J/g compared with that of the lower peak (227 J/g), indicating that the oxidation of the Al core occurs in two steps. The gradual rise of mass gain in the first process by the TGA curve and the greater enthalpy change by the DSC curve below 660 °C imply the tardy oxidation of the surface of the Al core. The mass gain of the second process of Al−C nanoparticles increases quite slowly because of the compact oxide shell formed during the first oxidation which limits the further oxidation. When the temperature exceeds 660 °C, the cracking and oxidation process continues at the same time until the oxide alumina shell ruptures by the tension due to the different volume expansion coefficients. These above results of the mass gain during the oxidation process with the increasing temperature indicate that the Al core is well protected from the oxidation by the outer carbon layer. The DSC curves of the Al−C/PW composites with different Al−C loading contents are displayed in Figure 5a. It can be seen that there are a low endothermic peak between 30 and 40 °C and a sharper endothermic peak between 50 and 60 °C, which can be attributed to the solid−solid phase change process and solid−liquid phase change process, respectively. The DSC results show that the phase change temperature and thermal behavior of the composites is unchanged under the existence of Al−C nanoparticles compared with that of pure paraffin. Figure 5b depicts the latent heat capacity change of the Al−C/PW composites with different Al−C loading contents. It can find that the latent heat capacity reaches the peak (195.4 W/g) at 1 wt % Al−C loading content. However, the latent heat capacity suffers a great drop with the increasing Al−C loading content of more than 1 wt % due to the following two elements: (1) The interactions between the PW molecules and Al−C particles increase the latent heat capacity, and (2) the

Figure 6. TGA curves of the composites.

PW, the onset decomposition temperature of Al−C/PW composites increases to 230 °C, which is 80 °C higher than the original value, indicating that the thermal stability of Al−C/ PW composites is improved. It can be explained by the following reasons: (a) the onset decomposition temperature is related to the entire specific heat capacity (Cp) of Al−C/PW composites which can be elevated by the Al−C nanoparticles, and (b) the improved thermal conductivity of PCM can transfer heat faster and make the PCM more uniform. In addition, the remaining weights of the samples are increased with the increasing Al−C loading content. The thermal conductivity of the composites is shown in Figure 7. Under the temperature of 25 and 45 °C, the thermal conductivity curves show a similar variation trend with different loading contents. The thermal conductivity at 25 °C reaches the highest point at 0.189W/(m·K) with 4 wt % loading content, and at 45 °C the highest thermal conductivity is 0.178W/(m·K) with the 10 wt % loading content because the structure and thermal resistance of the Al−C/PW composites have been changed after the phase change transition resulting from the aggregated Al−C nanoparticles with different loading contents. Figure 8 displays thermal conductivity enhancement 12606

DOI: 10.1021/acs.jpcc.7b02651 J. Phys. Chem. C 2017, 121, 12603−12609

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The Journal of Physical Chemistry C

The mechanism of the Al−C nanoparticles work on the thermal conductivity of Al−C/PW composites is shown in Figure 9. At a low Al−C loading concentration of 0.2 wt % to

Figure 7. Thermal conductivity vs carbon-coated Al mass loading content.

Figure 9. Schematic analysis of the thermal conductive network in Al− C/PW composites with increasing Al−C loading content: (a) 0.2 wt % ∼ 0.4 wt %, (b) 0.4 wt % ∼ 1 wt %, (c) 1 wt % ∼ 2 wt %, and (d) 2 wt % ∼ 4 wt %.

0.4 wt % (Figure 9a), the Al−C particles independently and well disperse in the PW, which can reduce the thermal interfacial resistance and enhance the heat flow, resulting in a slight improvement of the thermal conductivity. When the Al− C loading content is about 0.4 wt % ∼ 1 wt %, a small part of Al−C is united as clusters due to the intermolecular interaction between the small particles. The clusters are in favor of forming segmental thermal conductive networks that can provide the heat transferring route (Figure 9b) and improve the thermal conductivity of Al−C/PW composites. When the Al−C loading content increases from 1 wt % to 4 wt %, the Al−C particles can easily be connected together to build up the complete thermal conductive networks as shown in Figure 9c and Figure 9d, leading to the significant rise of thermal conductivity. However, the Al−C particles were aggregated together to form big groups at high mass ratio (more than 4 wt %) with a bad distribution that hardly improves the thermal conductivity any more. As a result, the thermal conductivity of the composite tends to keep stable and saturated at high mass ratio of Al−C content. We will further discus the dispersion state of Al−C nanoparticles with different Al−C filling contents by the photographic images (Figure 10). The photographic images of Al−C/PW exhibit the practical dispersion states with different Al−C loading contents. It can be clearly seen that the Al−C nanoparticles are visible and distributed fairly uniformly with a slight connection under a low mass ratio (less than 1 wt %) and tend to aggregate together under a high mass ratio (more than 4 wt %). As discussed above, it is inferred that the Al−C particles could connect as clusters to form thermal conductive networks with increasing loading content at a quite low level (less than 4 wt %), which is beneficial for effective and rapid heat transfer through the PW matrix, exhibiting a remarkable improvement of thermal conductivity.

Figure 8. Enhancement percentage of thermal conductivity: k0 represents the thermal conductivity of pure paraffin, while kc is the thermal conductivity of the composites.

percentages with different Al−C loading contents at 25 and 45 °C. It can be seen that the thermal conductivity of all the samples is greater than that of PW. The thermal conductivity clearly increases with increasing Al−C content at lower mass ratio and slowly changes when the Al−C loading content is higher than 2 wt %.The greatest enhancement percentage of the thermal conductivity is 106.5% with 4 wt % loading content and 173% with 10 wt % loading content for 25 and 45 °C, respectively. The enhancement of thermal conductivity at a low loading content is attributed to the addition of the Al−C nanoparticles. However, at extremely high loading content (more than 2 wt %), a lot of Al−C particles can attract together and form uniform Al−C clusters due to the attractive force between tiny particles, leading to the limit of the improvement of thermal conductivity. In addition, we also studied the thermal cycling test for assessing the thermal reliability of the Al−C/PW composites with different Al−C filling contents by analyzing in the repeated cyclic state based on the method according to ref 28. The Al− C/PW composites were tested through 200, 600, or 1000 cycles by repeated heating and cooling. We find that the Al−C/ PW composites have a good thermal reliability during the cycling test without supercooling or a significant change in volume and material degradation. The variation of the melting and freezing process is less than 5% after 200, 600, and 1000 cycles compared with that of 0 cycles. The above results (DSC, TGA, and cycling test) show that the Al−C/PW composites have a good durability for long-term application in the field of PCMs. 12607

DOI: 10.1021/acs.jpcc.7b02651 J. Phys. Chem. C 2017, 121, 12603−12609

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The Journal of Physical Chemistry C Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. These authors contributed equally. Notes

The authors declare no competing financial interest.



ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (No. 51476038), Frontier and Key Technology Innovation Special Funds Project of Guangdong Province (No. 2016A050503042), Science and Technology Plan Industry-University-Research Collaborative Innovation Major special projects of Guangzhou City (No. 2016201604030020), and Science and Technology Plan Industry-University-Research Cooperation Project of Nansha District (No. 2015CX009).



Figure 10. Photographic images of composites with different Al−C loading: (a) 0 wt %, (b) 0.2 wt %, (c) 0.4 wt %, (d) 1 wt %, (e) 2 wt %, and (f) 4 wt %.



CONCLUSIONS An enhanced latent heat thermal energy storage system adapted to medium- and low-temperature application has been designed and synthesized. This thermal energy storage system is based on the paraffin wax filling with Al−C nanoparticles, in which the Al−C nanoparticles exhibit a core−shell-structured morphology with a 50−100 nm aluminum core and 4 nm thin carbon shell. The Al−C nanoparticles can connect together to build up a thermal conductive network at a low loading content, resulting in a distinct improvement of the thermal performance of the Al−C/PW composites including latent heat storage property, thermal stability, thermal conductivity, and cycling reliability. The results show that the thermal conductivity is 0.189W/(m·K) with 4 wt % Al−C loading at 25 °C, which is about 206.5% compared with that of the pure paraffin, simultaneously keeping a good thermal reliability with an onset decomposition temperature of 230 °C. Our work provides a strategy to apply Al−C/PW composites as enhanced thermally conductive and durable PCMs.



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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Jin Huang: 0000-0002-8044-9865 12608

DOI: 10.1021/acs.jpcc.7b02651 J. Phys. Chem. C 2017, 121, 12603−12609

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DOI: 10.1021/acs.jpcc.7b02651 J. Phys. Chem. C 2017, 121, 12603−12609