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graphene aerogel supported phase change composite with good thermal conductivity
Thermochimica Acta 680 (2019) 178351
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Styrene-acrylic emulsion/graphene aerogel supported phase change composite with good thermal conductivity Liu Cao, Dong Zhang
T
⁎
Key laboratory of Advanced Civil Engineering Materials, Ministry of Education, School of Materials Science and Engineering, Tongji University, Shanghai, People’s Republic of China
ARTICLE INFO
ABSTRACT
Keywords: Thermal conductivity Shape stability Graphene aerogel Porous network
To overcome the application bottleneck of the low thermal conductivity and poor shape stability of organic phase-change materials, a styrene-acrylic emulsion/graphene aerogel supported microencapsulated phasechange composite (SGM composite) was fabricated by hydrothermal, vacuum-assisted impregnation and freezedrying methods. Graphene aerogel (GA), with an oriented porous network, provided a connected heat transfer network for the SGM composite. Compared with pure paraffin, the as-prepared SGM composite exhibited good thermal conductivity and stability. With a GA content of 14.7 wt%, the thermal conductivity of SGM reached 0.92 W/m∙K, which was 265% higher than that of pure paraffin. The styrene-acrylic emulsion imparted elastic properties to the SGM composite while reinforcing the graphene skeleton, which improved the thermal stability of SGM. Scanning electron microscopy results indicated that the heat transfer skeleton has different pore structure features in different parts. Therefore, a simple steady-state method for measuring the thermal conductivity of an SGM composite was introduced in this paper.
1. Introduction
leakage of PCM still persisted when the GO content was less than 6 wt %. Due to their excellent thermal conductivity and chemical stability, carbon-based materials such as carbon nanotubes [25], graphene nanoplatelets [26,27], graphite particles [28], and exfoliated graphite sheets [29] have been employed as conductive fillers to improve the thermal conductivity of PCMs. However, macroscopic three-dimensional (3D) graphene structures with continuous porous networks are rarely used in phase-change composites (PCC). In fact, 3D graphene can form a uniform heat conduction skeleton and allow graphene sheets to evenly fill PCMs, thereby avoiding sedimentation of the graphene sheets. In addition, the continuous porous network of 3D graphene can form a good coating effect on PCMs and improve the dimensional stability of PCC [30,31]. At present, several methods including templateassisted chemical vapor deposition (CVD) [32–34], hydrothermal reduction [35–37], direct freeze-drying [38,39] and electrochemical deposition [40–42] have been developed to produce 3D graphene structures. The hydrothermal method can be directly completed in a water system without any reducing aid. In this method, reduction and gelation can occur simultaneously, and their rate can be regulated by the hydrothermal temperature and pH [43–45]. It is a relatively common method for preparing 3D graphene. Thermal conductivity is one of the main factors affecting the ability
With the consumption of nonrenewable fossil fuels such as coal, oil and natural gas, energy and environmental issues have become serious challenges for future human development [1–5]. Based on this, phasechange energy storage materials have attracted attention from scientists owing to their energy conversion applications [6–8]. Among them, traditional organic phase-change materials (PCMs) can be widely used in solar energy storage and conversion [9–12], industrial waste heat recovery and utilization [13–15], and constant temperature energysaving materials because of their high phase transition enthalpy and nontoxic nature [6,16,17]. However, organic PCMs still have limitations: 1) there is a large thermal gradient in PCMs due to its low thermal conductivity, and 2) in the molten state, PCMs has poor shape stability and is easy to leak out [18–20]. To improve the thermal stability of PCMs, shape-stabilized PCMs have been prepared from nano-encapsulation and microencapsulation of PCMs using polymer shells [21–23]. However, a large mass ratio of polymer is usually needed to achieve effective encapsulation, which in turn leads to a reduction in the phase change enthalpy of the composite. Shang et al. [24] synthesized microencapsulated hexadecanol with graphene oxide (GO) as the shell material. With 6 wt% GO, the phasechange microcapsule exhibited excellent shape stability. However, the ⁎
Corresponding author. E-mail address: [email protected] (D. Zhang).
https://doi.org/10.1016/j.tca.2019.178351 Received 19 March 2019; Received in revised form 29 July 2019; Accepted 30 July 2019 Available online 31 July 2019 0040-6031/ © 2019 Elsevier B.V. All rights reserved.
Thermochimica Acta 680 (2019) 178351
L. Cao and D. Zhang
of PCMs to store and release heat [46–48] and is an important thermophysical property of PCMs. The thermal conductivity test methods for PCMs can generally be divided into two categories: steady-state methods and non-steady-state methods (transient methods). During a non-steady-state test of PCMs, the temperature distribution of the sample changes with time, and the thermal conductivity is generally estimated by measuring the change in the temperature [49,50]. Nonsteady-state methods have the advantages of a short measurement time, high precision and low environmental requirements, but there are strict limits on the size of the test piece. For heterogeneous anisotropic samples, the measurement accuracy of non-steady-state methods is limited [50–52]. Finding a simple method for testing the thermal conductivity of composites as a whole is highly desired. In this work, we prepared a porous 3D graphene aerogel (GA) skeleton by hydrothermal reduction and strengthened the skeleton by vacuum impregnation of a styrene-acrylic emulsion (SAE). For the PCMs, paraffin was coated with GO by Pickering emulsion templating to further improve its thermal stability. Then, the GO microencapsulated paraffin (MeP) was vacuum impregnated in the skeleton to obtain the SAE/GA/MeP (SGM) functional composite. Our results demonstrate that SGM composites do not have the defects of low thermal conductivity and poor dimensional stability. At the same time, SGM composites exhibit good elastic properties and have certain application prospects in special fields, such as electronic devices and the military industry. Considering the representativeness of sampling when testing an overall sample with uneven structure, we introduced a simple steady-state method to measure the thermal conductivity of SGM composite as a whole.
prepared by adjusting the concentration of the GO aqueous dispersion. 2.3. Characterization The morphologies of the MeP and SGM composites were characterized by scanning electron microscopy (SEM) (Quanta 200 FEG, FEI Company). An atomic force microscope (AFM) (SPI3800 N, Seiko Instruments, Japan) was used to obtain the morphology and height profile of GO after ultrasonication. Fourier transform infrared (FTIR) spectra of GO and GA were recorded on a SPI3800 N FTIR spectrometer (Seiko Instruments, Japan). The thermal analysis was carried out using an MDSC-Q100 instrument at a heating rate of 5 °C/min in a purified nitrogen atmosphere, and the sample mass was controlled at 3 ± 0.1 mg. The weight loss and thermal stability of MeP were measured by thermogravimetric analysis (TGA, STA 449 C Jupiter) at a heating rate of 5 ℃/min in the range of 30–500 ℃. Thermal images of the SGM composite were taken using a FLIR ETS320 camera to monitor the thermal stability during the heating process. A dynamic mechanical analyzer (DMA-Q800 TA Instruments) was applied to perform uniaxial ratcheting tests under controlled stress (tensile-tensile cyclic stress: 0.1 ± 0.05 MPa, peak stress holding for 5 s, at stressing rate of 0.1 MPa/min). The thickness of the specimens was 0.3 mm. The thermal conductivity of the SGM composites was determined by a simple steadystate method according to previous studies [53,54]. Samples with a diameter of 25 mm and a thickness of 5 mm were prepared for thermal conductivity testing. A schematic diagram of the experimental device is shown in Figure S1. A thermal property analyzer (KD-2 Pro Model) was used to compare and confirm the thermal conductivity test results. When there is a temperature gradient in an object, heat flows from high temperature to low temperature, which is called heat conduction or heat transfer. The heat transfer rate is proportional to the temperature gradient. The proportional coefficient is the thermal conductivity:
2. Experimental 2.1. Materials
dQ = dt
The PCM used in this study was paraffin (Product No. 69018961) supplied from Sinopharm Chemical Reagent Co., Ltd. The melting temperature was measured to be 55.36 ± 0.04 ℃ by a differential scanning calorimeter (DSC, MDSC-Q100). The peak temperature of the melting DSC curve was determined to be the melting temperature of paraffin. Pristine graphite (average size: 80 μm) was purchased from Shanghai Yifan Graphite Co., Ltd. SAE (Product No. ps608) was purchased from BASF (China) Co. Ltd., Shanghai.
dT ds dx
(1)
where dQ is the amount of heat passing through the ds area during dt time, dQ is the heat transfer rate, λ is the thermal conductivity and dT is dx dt the temperature gradient. As shown in Figure S1, the upper and lower surfaces of the sample are in close contact with the pure copper plate (heat sink, HS) and the positive temperature coefficient (PTC) heating device (HD), respectively. The heat is transferred from the HD to the HS through the thin sample being tested. After a steady state is reached, the upper and lower surface temperatures of the sample are T1 and T2, respectively. Since the sample is very thin, the amount of heat dissipated to the surrounding environment through the side of the sample is negligible, and the heat transfer rate of the sample can be considered equal to the rate at which HS dissipates heat to the surrounding environment. Considering that the heat dissipation rate of an object is proportional to the heat dissipation area, the following are true:
2.2. Preparation of SGM composites Graphite oxide was initially prepared by the modified Hummers method. Then, the as-prepared graphite oxide suspension was diluted with water and sonicated to prepare a homogeneous GO aqueous dispersion. To carry out the hydrothermal treatment, 60 ml of the GO aqueous dispersion was sealed in a Teflon-lined autoclave at 180 ℃ for 18 h. After subsequent freeze-drying of the hydrogel for over 24 h, we obtained GA. MeP, the phase-change component, was prepared by Pickering emulsion templating [24]. In a typical synthesis, graphite oxide (0.2 g) was dispersed in deionized water (100 ml) by sonication at room temperature for 60 min to form a uniform GO aqueous solution (2 mg/ml). Then, 19.8 g paraffin was melted at 80 ℃ and added under vigorous stirring to obtain a stable Pickering emulsion. After cooling to room temperature, the emulsion was converted to a suspension of GO-coated paraffin particles. Subsequently, MeP was washed with deionized water several times and dried in a vacuum oven at 35 ℃. Then, 50 g MeP was mixed with 100 g SAE (solid content of 50%). After low-speed stirring for 20 min, a SAE/MeP suspension with a mass ratio of 1:1 was prepared. By vacuum-assisted impregnation of SAE/ MeP into GA and freeze-drying for over 24 h, SGMs were fabricated. The process for the preparation of GA, MeP and SGMs is illustrated in Fig. 1. As shown in Table 1, SGMs with different GA contents were
dQs T T1 = 2 Ss dt L
(2)
dQ HS dT = mc |T = T1 dt dt
(3)
dQs dQHS / = dt dt 2
()+ ()+ D 2 2 D 2 2
D D
(4) 2
where is the heat transfer rate of the sample; L and Ss =D π/4 are the thickness and the upper surface area of the sample, respectively; dQHS is the heat dissipation rate of the HS when it is naturally cooled; dQs dt
dt dT | dt T = T1
is the natural cooling rate of the HS at T1; and m, D, δ and c = 0.3709 J/g∙K are the mass, diameter, thickness and specific heat capacity of the HS, respectively. 2
Thermochimica Acta 680 (2019) 178351
L. Cao and D. Zhang
Fig. 1. Schematic of the SGM composite synthesis process.
By combining Eqs. (2)–(5) for calculating the thermal conductivity of the sample can be obtained. The exact values of m, L, D, δ, T1, and T2 can be directly measured. To determine the natural cooling rate of the HS at T1, the HS was heated to over T1 on the PTC heating device, the PTC heating device was removed, and the cooling curve of the HS was recorded. The slope of the cooling curve at T1 is the natural cooling rate of the HS, K.
Table 1 GA contents of different SGMs.
SGM1 SGM2 SGM3 SGM4 SGM5 SGM6 SGM7
Concentration of GO solution (mg/ml)
GA (g)
SGM (g)
GA content (wt %)
4 6 8 10 12 15 18
0.2016 0.2739 0.3478 0.4523 0.5412 0.6241 0.7095
4.2316 4.5939 4.4578 4.7123 4.8512 4.8341 4.8446
4.8 6.0 7.8 9.6 11.2 12.9 14.7
=
cmKL (D + 4 ) D2 (T2 T1)(D + 2 )/2
where K =
dT | dt T = T1
is the natural cooling rate of HS at T1.
Fig. 2. AFM image and height profile of GO obtained by ultrasonication (a). SEM images (b, d) and particle size distribution (c) of MeP. 3
(5)
Thermochimica Acta 680 (2019) 178351
L. Cao and D. Zhang
single-layered GO with a thickness of approximately 1.0 nm with the aid of ultrasonication (Fig. 2a). Coated with GO, MeP possessed a smooth surface and a particle size distributed between 8 and 15 μm (Fig. 2b–d). Due to the excellent thermal stability of GO, the thermal properties of the PCM paraffin could be enhanced. Fig. 3 shows the FTIR spectra of GO and GA obtained by hydrothermal reduction. The designations CeO, CeOeC, C]C, C]O, COOH and CeOH are used to generalize the derivatives of ethers, epoxides, sp2 carbon, ketones, carboxyl and hydroxyl groups, respectively. In the spectrum of GA, the vibrational peaks of COOH (∼1724 cm−1), CeOeC (∼1230 cm−1) and C]O (∼1624 cm−1) are significantly weakened relative to that of GO, and a new peak of sp2-hybridized C]C appears at ∼1524 cm−1 [55,56]. That is, during the hydrothermal treatment of the GO aqueous dispersion, the oxygen-containing functional groups on the GO sheets can be substantially eliminated, and the conjugated structure of graphene can be restored. Accompanied by changes in the interaction force, such as the increased hydrophobicity of the graphene sheet and broken electrostatic equilibrium, GO sheets are simultaneously self-assembled to form a 3D network and converted to reduced GO. Fig. 4 shows the general morphologies of GA at different magnifications. It is worth noting that different parts of the GA exhibit different pore structures (Fig. 4b, c). The center portion of the GA is formed by wavy graphene sheets that are smooth and oriented. The graphene sheets are connected through edges, leaving gaps between them. For the outside edge of GA, it is apparent that the voids between the graphene sheets are narrower. Graphene sheets tend to agglomerate and crack. The difference in pore structure is due to the difference in the pressure between the inside and the outside of the hydrogel during the hydrothermal processes. Therefore, as a heat transfer skeleton, GA has different thermal conductive networks in different parts.
Fig. 3. FTIR spectra of the as-prepared GO and GA. Vibrational modes are shown for epoxides (CeOeC, asymmetric stretching, ∼1230 cm−1), sp2-hybridized C]C (in-plane stretching, ∼1500–1600 cm−1), ketones (C]O, ∼1600–1650 cm−1), carboxyl groups (COOH, ∼1650–1750 cm−1) and hydroxyl groups (possible COOH and H2O contribution) (CeOH, 3000–3700 cm−1).
3. Results and discussion 3.1. Structural characterization and morphology of GO, MeP, GA and SGM As shown in Fig. 2, graphite oxide was completely exfoliated into
Fig. 4. Photographs of the cross section of GA (a). SEM images of the center portion (b) and rim portion (c) of GA. 4
Thermochimica Acta 680 (2019) 178351
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Fig. 5. (a, c, d) SEM images of SGM prepared by one-time impregnation at different magnification; (b) SEM image of SGM prepared by 3 impregnation cycles. Table 2 Thermal conductivities of paraffin, SAE, and SGM composite measured by different methods. Sample
Paraffin Dry SAE SGM1
Thermal conductivity (W/m∙K) Simple steady-state method
Transient short-hot-wire method
Reference [57]
0.252 0.27 0.48
0.261 0.274 0.381 0.495
0.242 – –
To more clearly observe the dispersion of SAE and MeP among the graphene layers, the SGM obtained by one-time impregnation is shown in Fig. 5a, c, d. The emulsion is coated on the graphene sheets while MeP is evenly distributed in the SAE (Fig. 5c, d). The interlayer space of the graphene sheets is occupied by the SAE and MeP. As the number of impregnation cycles increases, the spacing of the graphene sheets decreases and the SGM becomes denser (Fig. 5b). The SEM images of MeP at larger magnification show that the size of MeP is uniform (˜15 μm), which is slightly larger than that in the above test results due to the coating of SAE (Fig. 5c, d).
Fig. 6. Thermal conductivities of SGMs with different GA contents (mean average of 5 replicate samples, error bars show the standard deviation among repeats).
(Table 2). It can be seen that the thermal conductivities of the paraffin and the dry SAE measured by the steady-state method are very close to the results measured by the transient method, and are also in good agreement with a previous study [57], indicating that the test results from the steady-state method are credible. However, due to the heterogeneity of the SGM, the thermal conductivity of the SGM composite measured using the transient short-hot-wire method is variable, which means that it is difficult to characterize the thermal conductivity of the whole composite using the transient method. The steady-state method described above has great advantages in testing the thermal
3.2. Thermal conductivity of SGM measured by the simple steady-state method Through the steady-state method mentioned above, we measured the thermal conductivities of paraffin, SAE and SGM composite and compared them with the results measured by a thermal property analyzer (KD-2 Pro Model) using a transient short-hot-wire method 5
Thermochimica Acta 680 (2019) 178351
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of paraffin and SGM composites obtained by the MDSC-Q100 instrument are summarized in Table 3. The melting temperature decreases as the GA content increases, which means that the thermal conductivity of the composite increases as the GA content increases. Ignoring the GO content of MeP, Table 3 lists the theoretical paraffin content of the SGM composites. Paraffin is the only PCM in SGM composites, and thus, only paraffin absorbs thermal energy during the melting process. The paraffin contents can be calculated from Eq. (6) and are 47.95, 47.28, 46.70, 46.12, 45.40, 44.78 and 44.15 wt%, which are slightly higher than the theoretical contents of paraffin in the SGM composites (reproducibility of the paraffin content is shown in Table S1). This may be because the higher thermal conductivity of SGM composite increases the degree of phase transition of paraffin.
Paraffin (wt%) =
HSGM × 100 % Hparaffin
(6)
where Paraffin (wt%) represents the mass percentage of paraffin in the SGM composites, ΔHSGM is the melting latent heat of the SGM composites, and ΔHparaffin is the melting latent heat of paraffin.
Fig. 7. Melting DSC curves of pure paraffin and SGM composites. Table 3 DSC results for paraffin and SGM composites.
3.4. Thermal stability of SGM composites
Sample
Theoretical paraffin content (wt%)
Melting temperature (℃)
Melting latent heat ΔHm (kJ/kg)
Paraffin SGM1 SGM2 SGM3 SGM4 SGM5 SGM6 SGM7
100.0 47.6 47.0 46.1 45.2 44.4 43.5 42.7
55.3 54.5 54.4 54.3 54.2 54.2 54.1 53.8
224.0 107.4 105.9 104.6 103.3 101.7 100.3 98.9
The core-shell ratio of MeP was determined by TGA. Fig. 8 shows the weight loss curves of MeP, paraffin (core material) and GO (shell material) at 30 − 500 ℃. The TGA curve of GO shows two major weight losses. The first rapid weight loss (˜15%) is between 100 and 150 ℃, mainly due to the dehydration of GO and the slight decomposition of some oxygen-containing groups. The second weight loss (˜25%) at approximately 200 ℃ indicates major decomposition of the oxygen-containing groups on GO. As the temperature further increases, GO gradually loses weight. Paraffin has a weight loss (˜100%) between 150 ℃ and 350 ℃ and completely decomposes at 350 ℃. Due to the presence of the shell material, the TGA curve of MeP shifts to a higher temperature compared to that of paraffin, indicating that the thermal decomposition temperature increases. The main heat loss of MeP (˜96.5%) is complete at approximately 400 ℃. When the temperature is higher than the temperature at which paraffin completely decomposes, the mass percentage of GO (shell material) can be calculated according to Eq. (7) as follows: GO (wt%)=(100-WLMeP)/(100-WLGO)×100%
(7)
where GO (wt%) is the mass percentage of GO in the SGM composite, and WLGO and WLMeP represent the weight loss of GO and MeP, respectively. At 500 ℃, WLGO and WLMeP are 58.5 and 96.9, respectively. Hence, GO (wt%) is calculated to be 7.5, which is higher than the design value of 1%. This might be because the shell material increases the stability of the PCM, resulting in a less weight loss of MeP and a larger GO (wt%). Fig. 9 records the morphology and temperature changes of pure paraffin and SGM composites during the heating process. On the PTC heating device at 85 ℃, the paraffin wax began to melt quickly and melted completely at 20 s. However, due to the microencapsulation process and the GA skeleton stabilization, the SGMs did not undergo paraffin leakage for up to 50 min at an ambient temperature of 85 °C. The SGMs showed excellent thermal stability.
Fig. 8. TGA curves of paraffin, GO, and MeP.
conductivity of the composite as a whole. In addition, it can be seen that the thermal conductivity of the SGM composite can reach 0.92 W/ m∙K, which is 2.65 times higher than that of pure paraffin (Fig. 6). The enhancement is ascribed to the directional network structure of GA and the high thermal conductivity of graphene.
3.5. Mechanical properties of SGM composites Some energetic materials (such as explosives and rocket propellant) require temperature stability and avoidance of mechanical shock during storage. Therefore, SAE was vacuum-assisted impregnated with MeP in the GA to improve the flexibility of the SGM. As shown in Fig. 10, the SGMs are compressible and elastic. The columnar sample was compressed when applying a pressure and recovered its initial shape after releasing the pressure. The sample without SAE (MeP
3.3. Thermal storage properties of SGM composites The DSC analysis results of pure paraffin and the SGMs are shown in Fig. 7. There are two phase transition peaks during the melting process. The lower temperature peak can be attributed to the solid-solid phase transition of paraffin, and the higher temperature peak is associated with the solid-liquid phase change of paraffin. The thermal properties 6
Thermochimica Acta 680 (2019) 178351
L. Cao and D. Zhang
Fig. 9. Infrared thermography images during the heating process of paraffin (a, b) and the SGMs (c, d).
Fig. 10. Photographs of the SGM (a) and the sample without SAE (b) when applying and releasing pressure.
7
Thermochimica Acta 680 (2019) 178351
L. Cao and D. Zhang
obtained by the hydrothermal reduction method provides a connected directional network for the PCM, resulting in good thermal conductivities for the SGMs. The thermal conductivity of the SGM with 14.7 wt% GA content increased to 0.92 W/m∙K, which is 2.65 times higher than that of pure paraffin. We have highlighted the potential of a simple steady-state method for measuring the thermal conductivity of PCMs as a whole. Compared to the commonly used testing technology, this method is simpler, more cost effective, and more accurate in some ways. Moreover, SGMs retain the elasticity of the SAE while enhancing its deformation resistance, broadening the application prospects of PCMs in specific fields. This novel SGM composite retains the same shape with no paraffin leaks after heating at 85℃ for 50 min, exhibiting good thermal stability. Acknowledgements This work was supported by the NSAF Foundation of National Natural Science Foundation of China and Chinese Academy of Engineering Physics under Grant [No. U1730117].
Fig. 11. Typical stress-strain hysteresis loops in the uniaxial ratcheting tests.
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Fig. 12. Ratcheting behavior of SAE and SGMs.
without SAE was vacuum-assisted impregnated in GA) is rigid and remains unchanged after the same pressure is applied. Thus, SAE gives the SGM composite elasticity and expands the range of PCM use in special areas where the material is required to be compressible and flexible. Ratcheting is a phenomenon of the progressive accumulation of deformation when materials are subjected to cyclic loading. Uniaxial ratcheting tests were conducted with cyclic tensile stress. As shown in Fig. 11, the typical stress-strain hysteresis loops are not closed at the beginning, resulting in ratcheting deformation of the SGM composite. However, as the number of cycles increases to 100, closed hysteresis loops gradually emerge. The ratcheting behavior of different samples for 100 cycles is shown in Fig. 12. The ratcheting strain of SGM composites tends to be balanced earlier, and the final strain value is smaller than that of the pure SAE. As the GA content increased, the resistance of the SGM composites to ratcheting deformation increased. This may be because the interaction between a small portion of the oxygen-containing functional groups remaining on the graphene skeleton and the polymer molecules enhances the polymer's resistance to deformation. In addition, the graphene sheets are interspersed in the SAE, which effectively hinders the motion of the polymer molecule chains. 4. Conclusions In summary, a facile route to prepare shape-stabilized phase change composites supported by SAE/GA has been demonstrated. The GA 8
Thermochimica Acta 680 (2019) 178351
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9
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Styrene-acrylic emulsion/graphene aerogel supported phase change composite with good thermal conductivity Liu Cao, Dong Zhang
T
⁎
Key laboratory of Advanced Civil Engineering Materials, Ministry of Education, School of Materials Science and Engineering, Tongji University, Shanghai, People’s Republic of China
ARTICLE INFO
ABSTRACT
Keywords: Thermal conductivity Shape stability Graphene aerogel Porous network
To overcome the application bottleneck of the low thermal conductivity and poor shape stability of organic phase-change materials, a styrene-acrylic emulsion/graphene aerogel supported microencapsulated phasechange composite (SGM composite) was fabricated by hydrothermal, vacuum-assisted impregnation and freezedrying methods. Graphene aerogel (GA), with an oriented porous network, provided a connected heat transfer network for the SGM composite. Compared with pure paraffin, the as-prepared SGM composite exhibited good thermal conductivity and stability. With a GA content of 14.7 wt%, the thermal conductivity of SGM reached 0.92 W/m∙K, which was 265% higher than that of pure paraffin. The styrene-acrylic emulsion imparted elastic properties to the SGM composite while reinforcing the graphene skeleton, which improved the thermal stability of SGM. Scanning electron microscopy results indicated that the heat transfer skeleton has different pore structure features in different parts. Therefore, a simple steady-state method for measuring the thermal conductivity of an SGM composite was introduced in this paper.
1. Introduction
leakage of PCM still persisted when the GO content was less than 6 wt %. Due to their excellent thermal conductivity and chemical stability, carbon-based materials such as carbon nanotubes [25], graphene nanoplatelets [26,27], graphite particles [28], and exfoliated graphite sheets [29] have been employed as conductive fillers to improve the thermal conductivity of PCMs. However, macroscopic three-dimensional (3D) graphene structures with continuous porous networks are rarely used in phase-change composites (PCC). In fact, 3D graphene can form a uniform heat conduction skeleton and allow graphene sheets to evenly fill PCMs, thereby avoiding sedimentation of the graphene sheets. In addition, the continuous porous network of 3D graphene can form a good coating effect on PCMs and improve the dimensional stability of PCC [30,31]. At present, several methods including templateassisted chemical vapor deposition (CVD) [32–34], hydrothermal reduction [35–37], direct freeze-drying [38,39] and electrochemical deposition [40–42] have been developed to produce 3D graphene structures. The hydrothermal method can be directly completed in a water system without any reducing aid. In this method, reduction and gelation can occur simultaneously, and their rate can be regulated by the hydrothermal temperature and pH [43–45]. It is a relatively common method for preparing 3D graphene. Thermal conductivity is one of the main factors affecting the ability
With the consumption of nonrenewable fossil fuels such as coal, oil and natural gas, energy and environmental issues have become serious challenges for future human development [1–5]. Based on this, phasechange energy storage materials have attracted attention from scientists owing to their energy conversion applications [6–8]. Among them, traditional organic phase-change materials (PCMs) can be widely used in solar energy storage and conversion [9–12], industrial waste heat recovery and utilization [13–15], and constant temperature energysaving materials because of their high phase transition enthalpy and nontoxic nature [6,16,17]. However, organic PCMs still have limitations: 1) there is a large thermal gradient in PCMs due to its low thermal conductivity, and 2) in the molten state, PCMs has poor shape stability and is easy to leak out [18–20]. To improve the thermal stability of PCMs, shape-stabilized PCMs have been prepared from nano-encapsulation and microencapsulation of PCMs using polymer shells [21–23]. However, a large mass ratio of polymer is usually needed to achieve effective encapsulation, which in turn leads to a reduction in the phase change enthalpy of the composite. Shang et al. [24] synthesized microencapsulated hexadecanol with graphene oxide (GO) as the shell material. With 6 wt% GO, the phasechange microcapsule exhibited excellent shape stability. However, the ⁎
Corresponding author. E-mail address: [email protected] (D. Zhang).
https://doi.org/10.1016/j.tca.2019.178351 Received 19 March 2019; Received in revised form 29 July 2019; Accepted 30 July 2019 Available online 31 July 2019 0040-6031/ © 2019 Elsevier B.V. All rights reserved.
Thermochimica Acta 680 (2019) 178351
L. Cao and D. Zhang
of PCMs to store and release heat [46–48] and is an important thermophysical property of PCMs. The thermal conductivity test methods for PCMs can generally be divided into two categories: steady-state methods and non-steady-state methods (transient methods). During a non-steady-state test of PCMs, the temperature distribution of the sample changes with time, and the thermal conductivity is generally estimated by measuring the change in the temperature [49,50]. Nonsteady-state methods have the advantages of a short measurement time, high precision and low environmental requirements, but there are strict limits on the size of the test piece. For heterogeneous anisotropic samples, the measurement accuracy of non-steady-state methods is limited [50–52]. Finding a simple method for testing the thermal conductivity of composites as a whole is highly desired. In this work, we prepared a porous 3D graphene aerogel (GA) skeleton by hydrothermal reduction and strengthened the skeleton by vacuum impregnation of a styrene-acrylic emulsion (SAE). For the PCMs, paraffin was coated with GO by Pickering emulsion templating to further improve its thermal stability. Then, the GO microencapsulated paraffin (MeP) was vacuum impregnated in the skeleton to obtain the SAE/GA/MeP (SGM) functional composite. Our results demonstrate that SGM composites do not have the defects of low thermal conductivity and poor dimensional stability. At the same time, SGM composites exhibit good elastic properties and have certain application prospects in special fields, such as electronic devices and the military industry. Considering the representativeness of sampling when testing an overall sample with uneven structure, we introduced a simple steady-state method to measure the thermal conductivity of SGM composite as a whole.
prepared by adjusting the concentration of the GO aqueous dispersion. 2.3. Characterization The morphologies of the MeP and SGM composites were characterized by scanning electron microscopy (SEM) (Quanta 200 FEG, FEI Company). An atomic force microscope (AFM) (SPI3800 N, Seiko Instruments, Japan) was used to obtain the morphology and height profile of GO after ultrasonication. Fourier transform infrared (FTIR) spectra of GO and GA were recorded on a SPI3800 N FTIR spectrometer (Seiko Instruments, Japan). The thermal analysis was carried out using an MDSC-Q100 instrument at a heating rate of 5 °C/min in a purified nitrogen atmosphere, and the sample mass was controlled at 3 ± 0.1 mg. The weight loss and thermal stability of MeP were measured by thermogravimetric analysis (TGA, STA 449 C Jupiter) at a heating rate of 5 ℃/min in the range of 30–500 ℃. Thermal images of the SGM composite were taken using a FLIR ETS320 camera to monitor the thermal stability during the heating process. A dynamic mechanical analyzer (DMA-Q800 TA Instruments) was applied to perform uniaxial ratcheting tests under controlled stress (tensile-tensile cyclic stress: 0.1 ± 0.05 MPa, peak stress holding for 5 s, at stressing rate of 0.1 MPa/min). The thickness of the specimens was 0.3 mm. The thermal conductivity of the SGM composites was determined by a simple steadystate method according to previous studies [53,54]. Samples with a diameter of 25 mm and a thickness of 5 mm were prepared for thermal conductivity testing. A schematic diagram of the experimental device is shown in Figure S1. A thermal property analyzer (KD-2 Pro Model) was used to compare and confirm the thermal conductivity test results. When there is a temperature gradient in an object, heat flows from high temperature to low temperature, which is called heat conduction or heat transfer. The heat transfer rate is proportional to the temperature gradient. The proportional coefficient is the thermal conductivity:
2. Experimental 2.1. Materials
dQ = dt
The PCM used in this study was paraffin (Product No. 69018961) supplied from Sinopharm Chemical Reagent Co., Ltd. The melting temperature was measured to be 55.36 ± 0.04 ℃ by a differential scanning calorimeter (DSC, MDSC-Q100). The peak temperature of the melting DSC curve was determined to be the melting temperature of paraffin. Pristine graphite (average size: 80 μm) was purchased from Shanghai Yifan Graphite Co., Ltd. SAE (Product No. ps608) was purchased from BASF (China) Co. Ltd., Shanghai.
dT ds dx
(1)
where dQ is the amount of heat passing through the ds area during dt time, dQ is the heat transfer rate, λ is the thermal conductivity and dT is dx dt the temperature gradient. As shown in Figure S1, the upper and lower surfaces of the sample are in close contact with the pure copper plate (heat sink, HS) and the positive temperature coefficient (PTC) heating device (HD), respectively. The heat is transferred from the HD to the HS through the thin sample being tested. After a steady state is reached, the upper and lower surface temperatures of the sample are T1 and T2, respectively. Since the sample is very thin, the amount of heat dissipated to the surrounding environment through the side of the sample is negligible, and the heat transfer rate of the sample can be considered equal to the rate at which HS dissipates heat to the surrounding environment. Considering that the heat dissipation rate of an object is proportional to the heat dissipation area, the following are true:
2.2. Preparation of SGM composites Graphite oxide was initially prepared by the modified Hummers method. Then, the as-prepared graphite oxide suspension was diluted with water and sonicated to prepare a homogeneous GO aqueous dispersion. To carry out the hydrothermal treatment, 60 ml of the GO aqueous dispersion was sealed in a Teflon-lined autoclave at 180 ℃ for 18 h. After subsequent freeze-drying of the hydrogel for over 24 h, we obtained GA. MeP, the phase-change component, was prepared by Pickering emulsion templating [24]. In a typical synthesis, graphite oxide (0.2 g) was dispersed in deionized water (100 ml) by sonication at room temperature for 60 min to form a uniform GO aqueous solution (2 mg/ml). Then, 19.8 g paraffin was melted at 80 ℃ and added under vigorous stirring to obtain a stable Pickering emulsion. After cooling to room temperature, the emulsion was converted to a suspension of GO-coated paraffin particles. Subsequently, MeP was washed with deionized water several times and dried in a vacuum oven at 35 ℃. Then, 50 g MeP was mixed with 100 g SAE (solid content of 50%). After low-speed stirring for 20 min, a SAE/MeP suspension with a mass ratio of 1:1 was prepared. By vacuum-assisted impregnation of SAE/ MeP into GA and freeze-drying for over 24 h, SGMs were fabricated. The process for the preparation of GA, MeP and SGMs is illustrated in Fig. 1. As shown in Table 1, SGMs with different GA contents were
dQs T T1 = 2 Ss dt L
(2)
dQ HS dT = mc |T = T1 dt dt
(3)
dQs dQHS / = dt dt 2
()+ ()+ D 2 2 D 2 2
D D
(4) 2
where is the heat transfer rate of the sample; L and Ss =D π/4 are the thickness and the upper surface area of the sample, respectively; dQHS is the heat dissipation rate of the HS when it is naturally cooled; dQs dt
dt dT | dt T = T1
is the natural cooling rate of the HS at T1; and m, D, δ and c = 0.3709 J/g∙K are the mass, diameter, thickness and specific heat capacity of the HS, respectively. 2
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Fig. 1. Schematic of the SGM composite synthesis process.
By combining Eqs. (2)–(5) for calculating the thermal conductivity of the sample can be obtained. The exact values of m, L, D, δ, T1, and T2 can be directly measured. To determine the natural cooling rate of the HS at T1, the HS was heated to over T1 on the PTC heating device, the PTC heating device was removed, and the cooling curve of the HS was recorded. The slope of the cooling curve at T1 is the natural cooling rate of the HS, K.
Table 1 GA contents of different SGMs.
SGM1 SGM2 SGM3 SGM4 SGM5 SGM6 SGM7
Concentration of GO solution (mg/ml)
GA (g)
SGM (g)
GA content (wt %)
4 6 8 10 12 15 18
0.2016 0.2739 0.3478 0.4523 0.5412 0.6241 0.7095
4.2316 4.5939 4.4578 4.7123 4.8512 4.8341 4.8446
4.8 6.0 7.8 9.6 11.2 12.9 14.7
=
cmKL (D + 4 ) D2 (T2 T1)(D + 2 )/2
where K =
dT | dt T = T1
is the natural cooling rate of HS at T1.
Fig. 2. AFM image and height profile of GO obtained by ultrasonication (a). SEM images (b, d) and particle size distribution (c) of MeP. 3
(5)
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L. Cao and D. Zhang
single-layered GO with a thickness of approximately 1.0 nm with the aid of ultrasonication (Fig. 2a). Coated with GO, MeP possessed a smooth surface and a particle size distributed between 8 and 15 μm (Fig. 2b–d). Due to the excellent thermal stability of GO, the thermal properties of the PCM paraffin could be enhanced. Fig. 3 shows the FTIR spectra of GO and GA obtained by hydrothermal reduction. The designations CeO, CeOeC, C]C, C]O, COOH and CeOH are used to generalize the derivatives of ethers, epoxides, sp2 carbon, ketones, carboxyl and hydroxyl groups, respectively. In the spectrum of GA, the vibrational peaks of COOH (∼1724 cm−1), CeOeC (∼1230 cm−1) and C]O (∼1624 cm−1) are significantly weakened relative to that of GO, and a new peak of sp2-hybridized C]C appears at ∼1524 cm−1 [55,56]. That is, during the hydrothermal treatment of the GO aqueous dispersion, the oxygen-containing functional groups on the GO sheets can be substantially eliminated, and the conjugated structure of graphene can be restored. Accompanied by changes in the interaction force, such as the increased hydrophobicity of the graphene sheet and broken electrostatic equilibrium, GO sheets are simultaneously self-assembled to form a 3D network and converted to reduced GO. Fig. 4 shows the general morphologies of GA at different magnifications. It is worth noting that different parts of the GA exhibit different pore structures (Fig. 4b, c). The center portion of the GA is formed by wavy graphene sheets that are smooth and oriented. The graphene sheets are connected through edges, leaving gaps between them. For the outside edge of GA, it is apparent that the voids between the graphene sheets are narrower. Graphene sheets tend to agglomerate and crack. The difference in pore structure is due to the difference in the pressure between the inside and the outside of the hydrogel during the hydrothermal processes. Therefore, as a heat transfer skeleton, GA has different thermal conductive networks in different parts.
Fig. 3. FTIR spectra of the as-prepared GO and GA. Vibrational modes are shown for epoxides (CeOeC, asymmetric stretching, ∼1230 cm−1), sp2-hybridized C]C (in-plane stretching, ∼1500–1600 cm−1), ketones (C]O, ∼1600–1650 cm−1), carboxyl groups (COOH, ∼1650–1750 cm−1) and hydroxyl groups (possible COOH and H2O contribution) (CeOH, 3000–3700 cm−1).
3. Results and discussion 3.1. Structural characterization and morphology of GO, MeP, GA and SGM As shown in Fig. 2, graphite oxide was completely exfoliated into
Fig. 4. Photographs of the cross section of GA (a). SEM images of the center portion (b) and rim portion (c) of GA. 4
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Fig. 5. (a, c, d) SEM images of SGM prepared by one-time impregnation at different magnification; (b) SEM image of SGM prepared by 3 impregnation cycles. Table 2 Thermal conductivities of paraffin, SAE, and SGM composite measured by different methods. Sample
Paraffin Dry SAE SGM1
Thermal conductivity (W/m∙K) Simple steady-state method
Transient short-hot-wire method
Reference [57]
0.252 0.27 0.48
0.261 0.274 0.381 0.495
0.242 – –
To more clearly observe the dispersion of SAE and MeP among the graphene layers, the SGM obtained by one-time impregnation is shown in Fig. 5a, c, d. The emulsion is coated on the graphene sheets while MeP is evenly distributed in the SAE (Fig. 5c, d). The interlayer space of the graphene sheets is occupied by the SAE and MeP. As the number of impregnation cycles increases, the spacing of the graphene sheets decreases and the SGM becomes denser (Fig. 5b). The SEM images of MeP at larger magnification show that the size of MeP is uniform (˜15 μm), which is slightly larger than that in the above test results due to the coating of SAE (Fig. 5c, d).
Fig. 6. Thermal conductivities of SGMs with different GA contents (mean average of 5 replicate samples, error bars show the standard deviation among repeats).
(Table 2). It can be seen that the thermal conductivities of the paraffin and the dry SAE measured by the steady-state method are very close to the results measured by the transient method, and are also in good agreement with a previous study [57], indicating that the test results from the steady-state method are credible. However, due to the heterogeneity of the SGM, the thermal conductivity of the SGM composite measured using the transient short-hot-wire method is variable, which means that it is difficult to characterize the thermal conductivity of the whole composite using the transient method. The steady-state method described above has great advantages in testing the thermal
3.2. Thermal conductivity of SGM measured by the simple steady-state method Through the steady-state method mentioned above, we measured the thermal conductivities of paraffin, SAE and SGM composite and compared them with the results measured by a thermal property analyzer (KD-2 Pro Model) using a transient short-hot-wire method 5
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of paraffin and SGM composites obtained by the MDSC-Q100 instrument are summarized in Table 3. The melting temperature decreases as the GA content increases, which means that the thermal conductivity of the composite increases as the GA content increases. Ignoring the GO content of MeP, Table 3 lists the theoretical paraffin content of the SGM composites. Paraffin is the only PCM in SGM composites, and thus, only paraffin absorbs thermal energy during the melting process. The paraffin contents can be calculated from Eq. (6) and are 47.95, 47.28, 46.70, 46.12, 45.40, 44.78 and 44.15 wt%, which are slightly higher than the theoretical contents of paraffin in the SGM composites (reproducibility of the paraffin content is shown in Table S1). This may be because the higher thermal conductivity of SGM composite increases the degree of phase transition of paraffin.
Paraffin (wt%) =
HSGM × 100 % Hparaffin
(6)
where Paraffin (wt%) represents the mass percentage of paraffin in the SGM composites, ΔHSGM is the melting latent heat of the SGM composites, and ΔHparaffin is the melting latent heat of paraffin.
Fig. 7. Melting DSC curves of pure paraffin and SGM composites. Table 3 DSC results for paraffin and SGM composites.
3.4. Thermal stability of SGM composites
Sample
Theoretical paraffin content (wt%)
Melting temperature (℃)
Melting latent heat ΔHm (kJ/kg)
Paraffin SGM1 SGM2 SGM3 SGM4 SGM5 SGM6 SGM7
100.0 47.6 47.0 46.1 45.2 44.4 43.5 42.7
55.3 54.5 54.4 54.3 54.2 54.2 54.1 53.8
224.0 107.4 105.9 104.6 103.3 101.7 100.3 98.9
The core-shell ratio of MeP was determined by TGA. Fig. 8 shows the weight loss curves of MeP, paraffin (core material) and GO (shell material) at 30 − 500 ℃. The TGA curve of GO shows two major weight losses. The first rapid weight loss (˜15%) is between 100 and 150 ℃, mainly due to the dehydration of GO and the slight decomposition of some oxygen-containing groups. The second weight loss (˜25%) at approximately 200 ℃ indicates major decomposition of the oxygen-containing groups on GO. As the temperature further increases, GO gradually loses weight. Paraffin has a weight loss (˜100%) between 150 ℃ and 350 ℃ and completely decomposes at 350 ℃. Due to the presence of the shell material, the TGA curve of MeP shifts to a higher temperature compared to that of paraffin, indicating that the thermal decomposition temperature increases. The main heat loss of MeP (˜96.5%) is complete at approximately 400 ℃. When the temperature is higher than the temperature at which paraffin completely decomposes, the mass percentage of GO (shell material) can be calculated according to Eq. (7) as follows: GO (wt%)=(100-WLMeP)/(100-WLGO)×100%
(7)
where GO (wt%) is the mass percentage of GO in the SGM composite, and WLGO and WLMeP represent the weight loss of GO and MeP, respectively. At 500 ℃, WLGO and WLMeP are 58.5 and 96.9, respectively. Hence, GO (wt%) is calculated to be 7.5, which is higher than the design value of 1%. This might be because the shell material increases the stability of the PCM, resulting in a less weight loss of MeP and a larger GO (wt%). Fig. 9 records the morphology and temperature changes of pure paraffin and SGM composites during the heating process. On the PTC heating device at 85 ℃, the paraffin wax began to melt quickly and melted completely at 20 s. However, due to the microencapsulation process and the GA skeleton stabilization, the SGMs did not undergo paraffin leakage for up to 50 min at an ambient temperature of 85 °C. The SGMs showed excellent thermal stability.
Fig. 8. TGA curves of paraffin, GO, and MeP.
conductivity of the composite as a whole. In addition, it can be seen that the thermal conductivity of the SGM composite can reach 0.92 W/ m∙K, which is 2.65 times higher than that of pure paraffin (Fig. 6). The enhancement is ascribed to the directional network structure of GA and the high thermal conductivity of graphene.
3.5. Mechanical properties of SGM composites Some energetic materials (such as explosives and rocket propellant) require temperature stability and avoidance of mechanical shock during storage. Therefore, SAE was vacuum-assisted impregnated with MeP in the GA to improve the flexibility of the SGM. As shown in Fig. 10, the SGMs are compressible and elastic. The columnar sample was compressed when applying a pressure and recovered its initial shape after releasing the pressure. The sample without SAE (MeP
3.3. Thermal storage properties of SGM composites The DSC analysis results of pure paraffin and the SGMs are shown in Fig. 7. There are two phase transition peaks during the melting process. The lower temperature peak can be attributed to the solid-solid phase transition of paraffin, and the higher temperature peak is associated with the solid-liquid phase change of paraffin. The thermal properties 6
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Fig. 9. Infrared thermography images during the heating process of paraffin (a, b) and the SGMs (c, d).
Fig. 10. Photographs of the SGM (a) and the sample without SAE (b) when applying and releasing pressure.
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obtained by the hydrothermal reduction method provides a connected directional network for the PCM, resulting in good thermal conductivities for the SGMs. The thermal conductivity of the SGM with 14.7 wt% GA content increased to 0.92 W/m∙K, which is 2.65 times higher than that of pure paraffin. We have highlighted the potential of a simple steady-state method for measuring the thermal conductivity of PCMs as a whole. Compared to the commonly used testing technology, this method is simpler, more cost effective, and more accurate in some ways. Moreover, SGMs retain the elasticity of the SAE while enhancing its deformation resistance, broadening the application prospects of PCMs in specific fields. This novel SGM composite retains the same shape with no paraffin leaks after heating at 85℃ for 50 min, exhibiting good thermal stability. Acknowledgements This work was supported by the NSAF Foundation of National Natural Science Foundation of China and Chinese Academy of Engineering Physics under Grant [No. U1730117].
Fig. 11. Typical stress-strain hysteresis loops in the uniaxial ratcheting tests.
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Fig. 12. Ratcheting behavior of SAE and SGMs.
without SAE was vacuum-assisted impregnated in GA) is rigid and remains unchanged after the same pressure is applied. Thus, SAE gives the SGM composite elasticity and expands the range of PCM use in special areas where the material is required to be compressible and flexible. Ratcheting is a phenomenon of the progressive accumulation of deformation when materials are subjected to cyclic loading. Uniaxial ratcheting tests were conducted with cyclic tensile stress. As shown in Fig. 11, the typical stress-strain hysteresis loops are not closed at the beginning, resulting in ratcheting deformation of the SGM composite. However, as the number of cycles increases to 100, closed hysteresis loops gradually emerge. The ratcheting behavior of different samples for 100 cycles is shown in Fig. 12. The ratcheting strain of SGM composites tends to be balanced earlier, and the final strain value is smaller than that of the pure SAE. As the GA content increased, the resistance of the SGM composites to ratcheting deformation increased. This may be because the interaction between a small portion of the oxygen-containing functional groups remaining on the graphene skeleton and the polymer molecules enhances the polymer's resistance to deformation. In addition, the graphene sheets are interspersed in the SAE, which effectively hinders the motion of the polymer molecule chains. 4. Conclusions In summary, a facile route to prepare shape-stabilized phase change composites supported by SAE/GA has been demonstrated. The GA 8
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