- Email: [email protected]
Multifunctional electro-chemically exfoliated graphene with γ-alumina composite by spray-coating for energy efficient glass
Solar Energy Materials & Solar Cells 203 (2019) 110199
Contents lists available at ScienceDirect
Solar Energy Materials and Solar Cells journal homepage: http://www.elsevier.com/locate/solmat
Multifunctional electro-chemically exfoliated graphene with γ-alumina composite by spray-coating for energy efficient glass Lung-Hao Hu a, *, Sheng-Hui Kao b a b
Department of Mechanical and Electro-Mechanical Engineering, National Sun Yat-sen University, Kaohsiung, 804, Taiwan Department of Mechanical Engineering, Southern Taiwan University of Science and Technology, Tainan, 710, Taiwan
A R T I C L E I N F O
A B S T R A C T
Keywords: Electro-chemically exfoliated graphene Energy efficient glass Carbon composite ink Graphene/γ-alumina Transmittance Absorbance
Massive production of graphene is synthesized by a low-cost electro-chemically exfoliated approach and then incorporated with γ-alumina for developing a novel carbon composite ink, graphene/γ-alumina ink. It is coated onto the glass by spraying with an adiabatic closed system as the thermal and optical barriers for the energy efficient glass of the business buildings. The coating thickness of graphene/γ-alumina layer is around 2~3 μm. The temperature in an adiabatic closed system is measured with a heat source by lamp radiation. The temper ature difference between inside and outside of the adiabatic closed system can achieve around 20 � C with the coating of 0.3 wt% of electro-chemically exfoliated graphene incorporated with 1 wt% of γ-alumina in N-Methyl2-Pyrrolidone after thermal radiation test for 15 h. The transmittance is adjustable from 5.57%–81% at 550 nm wavelength, depending on the composition and content of graphene and γ-alumina. The coating of the carbon composite ink, electro-chemically exfoliated graphene with γ-alumina are also good absorbents for UV absorbance.
1. Introduction Extremely climate change is a crucial issue for the global environ ment influenced by, such as global warming and the emission of oxo carbon produced by fossil fuel usage. During summer, the sunlight and heat flows are highly possible to penetrate through the buildings win dow. If the business buildings are filled with air-conditioning during summer time, the heat flow outside the buildings will cause the heat consumption to increase the energy loss and lower the efficiency of those air-conditioners; moreover, the global warming effect would be raised. Therefore, energy conservation is one of the key points to overcome those issues. Heat insulation and sunlight shielding have been paid for much attention for optimizing energy consumption in the operation of space cooling, heating systems and building materials [1–3]. Adjustable and customized functions of the glass window with low cost are becoming more significant for enhancing energy efficiency of green buildings, such as heat insulation for lowering room temperature, transparency and UV-sunlight light shielding. Standard glass with a high emissivity makes solar heat move freely in and out; however, low-emission (low-e) coatings with glass window provide a low emis sivity and act like a filter to control heat transfer that certain energy
wavelengths are able to pass through, while others are not. Therefore, low-emission coatings are able to be used for reducing energy usage in both opaque and transparent areas of buildings. The opaque and transparent materials of low-e coatings or foils have different areas of application. Even though their functions are generally similar, they can be applied to some specific areas by different manners and have to fulfill some different criteria. The comparison between opaque and transparent materials is directly against each other. Opaque materials for low-e coatings should be practical to apply and perform well to blocking out radiant heat. They are usually only to block out radiant heat from the outside, as it has to be shown its effectiveness is in the cooling dominated regions, and the buildings have generally been installed thicker layers of conventional thermal insulation for climates dominated by heating. Low-e foils perform better results on energy savings when installed properly. However, low-e paints have the advantage of applying to the flexible substrates or curved window and can be added to materials or composites before installation in areas where the installation of foils is not practical, such as the inside of hollow bricks and on the exterior surface of buildings and buildings’ windows [4]. Hydrogel is one of the temperature-driven systems to modify the
* Corresponding author. E-mail addresses: [email protected] (L.-H. Hu), [email protected] (S.-H. Kao). https://doi.org/10.1016/j.solmat.2019.110199 Received 16 June 2019; Received in revised form 16 September 2019; Accepted 22 September 2019 Available online 25 September 2019 0927-0248/© 2019 Elsevier B.V. All rights reserved.
L.-H. Hu and S.-H. Kao
Solar Energy Materials and Solar Cells 203 (2019) 110199
Fig. 1. SEM images of (a) the starting material, artificial graphite (b) the artificial graphite electrochemically exfoliated (c) the Photograph of the large scale transferring of ECG on PET.
the increased temperature of the hydrogel heated by solar light, as a result, reducing the incident intensity of solar energy and room tem perature. The transmittance of 0.2% GO incorporated with PNIPAM dramatically reduced when the temperature reached 28 � C and when the smart glass changed opaque, the temperature reached 32 � C. That the issue should be concerned is whether the transparency changing with temperatures is suitable for the majority. S. S. Ghosh et al. [11] syn thesized different transparent conducting oxide with various dopants by sol-gel process. Two types of sols, host metal ion sol and doped metal sol were prepared; the final precursor sol was prepared by mixing these two types of sols incorporated with wettable agent. The glass was dip-coated in the final precursor sol bath and then pyrolyzed at 500 � C under mixture gas(Ar þ H2). The transparent conducting coating by magne tron sputtering has been predominantly used for solar cells. Current researches focusing on the conducting coating are trying to discover new materials to replace conventional indium tin oxide (ITO). J. Kim et al. [12] fabricated silver nanowire thin film as transparent conductive electrode with a thin layer of sputtered ZnO to increase the contact area with low-lying surface but also improve the conductivity due to the connection of silver nanowires at the junction. Carbon nanotube, gra phene, metal grid and metal nanowire network have been paid high attention for replacing ITO; however, owing to some optical criteria and cost issues those materials are still being investigated [13–17]. If the optical criteria are not being concerned, graphene or graphene oxide is one of the outstanding candidates for heat dissipation. Aluminum incorporated with graphene or graphene oxide had been fabricated with different processes to enhance the thermal conductivity that the higher graphene contains, the better performance of heat dissipation performs [18–25]. Many researchers have also indicated that graphene oxide is
Fig. 2. Schematic illustration of the spraying process.
transparency [5–7]; thus the external energy supply is able to be mini mized by the passive tunable process. Therefore, the dual or multi-mode thermotropic material with high performance is desirable to replace the monotonous tunable system. Conducting layer coatings for thermal insulation had been widely applied to low-e glass. With this regards, the hybrid system composed of indium tin oxide or other conducting ma terial coated glass and thermotropic materials is to be focusing on; it can be heated through a voltage applied [8,9]. H.T. Chou [10] used hydrogel method with poly(N-isopropylacrylamide) (PNIPAM) incorporated with graphene oxide (GO) to effectively convert the photo-energy into ther mal energy and cause the transparency decrease of the smart glass with 2
L.-H. Hu and S.-H. Kao
Solar Energy Materials and Solar Cells 203 (2019) 110199
Fig. 3. The Photographs of (a) different ECG content coated glass and 1 and 5 wt% γ-alumina with 0.03 wt% ECG coated glass (b) 1 and 5 wt% γ-alumina with 0.3 wt % ECG coated glass.
Fig. 4. The set-up of the lamp radiation-adiabatic closed system (a) front view (b) side view with heat source.
able to directly reduce by active metals like aluminum without any additional reducing agents [26,27]. Therefore, graphene oxide would be the optimal starting coating material. L. Zhang et al. [28] found that in-situ reduction of graphene oxide can make reduced graphene well-disperse onto the surfaces of aluminum particles. The powder metallurgy technique was further applied to fabricate the Al/G (alumi num/graphene) composites to enhance heat-dissipation with very low graphene content. Here, we developed a massive producible composite ink of electro-chemically exfoliated graphene (ECG) incorporated with nano γ-alumina by spraying approach that can be employed to various shapes of windows and flexible substrates for heat dissipation and op tical criteria of business buildings.
the residual electrolyte. The purified flake was partially dried at 100 � C for 10 min on hot plate. The sample has to be maintained under a partially wet status; otherwise, the graphene layers will tightly stack to cause the difficulty to uniformly disperse in further process. To obtain well-dispersed samples, one-minute sonication was carried out for the products in N-Methyl-2-pyrrolidone (NMP). Finally, the dispersion was filtered again to remove the unreacted starting materials. 10 ml of different weight percent of the electro-chemically exfoliated graphene (ECG) in NMP (0.03 wt%, 0.1 wt% and 0.3 wt% in 10 ml NMP) is ho mogenized with 10 ml of γ-Al2O3 (AEROSIL~100 nm) in NMP (1 wt% and 5 wt% of γ-Al2O3 in 10 ml NMP) to form the liquid composite, graphene/γ-alumina ink. The morphologies of ECG were characterized by optical microscope (OM), atomic force microscope (AFM, Veeco Dimension-Icon system), and scanning electron microscope (SEM, JEOLJSM-6500F). Fig. 1 displays the scanning electron microscope (SEM) images of the artificial graphite (starting material, Fig. 1(a)), ECG (Fig. 1(b)) and the sprayed ECG on the flexible substrate, PET (Fig. 1(c)), respectively.
2. Experimental method 2.1. Synthesis of electro-chemically exfoliated graphene (ECG)with γ-Al2O3 Our samples were prepared by electrochemical exfoliation method. A piece of artificial graphite was as the working electrode and a plat inum wire was served as a reference electrode for the experimental setup. The electrolyte was a mixture of H2SO4 and KOH (composition: 2.4 g of H2SO4 and 11 ml 30% KOH in 100 ml DI water). A DC bias, �20 V was applied to the working electrode for the electrochemical exfoliation reaction. The following steps show the voltage program: 1) þ2.5 V for 60 s; 2) þ20 V for 5 s; 3) 20 V for 5 s. Steps 2) and 3) are repeated alternatively until the amount of the product reaches our requirement [29]. The products were then vacuum-filtrated by a filter membrane (pore size � 0.2 μm) and rinsed with DI water and ethanol for removing
2.2. Spray-coating process of ECG and graphene/γ-alumina ink For the present investigation, 9 ml of the mixed solution, graphene/ γ-alumina ink and pure ECG ink were extracted for spray-coating on the standard glass slide with the dimension of 100 by 300 by 1 mm thick on the hotplate as shown in Fig. 2. The temperature of the hotplate is 180 � C for glass substrate. The transparency is adjustable, depending on the requirement controlled by the ink’s concentrations and spraying time as shown in Fig. 3. Morphologies of each ink sprayed onto the glass are characterized by scanning electron microscope (SEM) and transmission 3
L.-H. Hu and S.-H. Kao
Solar Energy Materials and Solar Cells 203 (2019) 110199
Fig. 5. (a) the picture of electrochemically exfoliated graphene dispersed in NMP (b) the OM image of graphene thin sheets (c) Raman spectra of starting material and the product after reaction (d) AFM scan of single ECG transferred on SiO2/Si.
electron microscope (TEM).
Methyl-2-pyrrolidone (NMP) for 1 min as shown in Fig. 5(a). The products deposited on 300 nm SiO2/Si substrate by dip-coating is shown in Fig. 5(b). The lateral size of these sheets is over ten μm. The confocal Raman spectra (excited by 473 nm laser; power: 0.5 mW; spot size: � 0.5 μm) for starting material and exfoliated graphene sheets is shown in Fig. 5(c), respectively. AFM was used to measure the thickness of ECG transferred on the silicon substrate as shown in Fig. 5(d). The strong Gband (at 1578.4 cm 1) and tiny D-band (near 1360 cm 1) from the artificial graphite indicate a feature of well-crystallized bulk graphite [29–33]. After the starting material exfoliated, the D-band greatly in creases and a shift of G-band (at 1591.8 cm 1) appears. It is known that the I2D/IG ratio of bi-layer graphene is lower than that of monolayer one [29–34]. The ID/IG ratio is 1.21, suggesting that graphene surface is partially oxidized because of sulfuric acid intercalation. The appearance of disorder-induced D-band can be attributed to the formation of nanodomains in graphene lattice during exfoliation. Note that the 2D-band (at � 2700 cm 1) is composed of four components (2D1B, 2D1A, 2D2A, 2D2B) and is an asymmetric peak, except the case of monolayer graphene. In the inset of Fig. 5(c), the sample shows an asymmetric 2D-band, but the intensity of the higher frequency 2D2 peaks is not dominant, suggesting that our product is mainly bi-layer graphene. Additionally, the larger I2D/IG ratio (the integrated peak ratio between 2D- and G-bands) represents higher degree of crystallization for sp2 C¼C bonds in graphene structure (Krauss.,2009). Although our sample is as-prepared and without any reduction treatment, its I2D/IG ratio (0.36) is still higher than that of GO, even which is reduced by hydrazine and 800oC-annealing (Su et al., 2009) [35,36]. Size of nano γ-alumina is around 10–20 nm and the cluster is about 100 nm as shown in Fig. 6 (a) and (b). Graphene sheet is uniformly covered by nano γ-alumina as shown in Fig. 6 (c) and (d). Fig. 6(e) shows that the thickness of gra phene/γ-alumina layer sprayed on the glass is about 2–3 μm. These
2.3. Lamp radiation with an adiabatic closed system The set-up of the lamp radiation-adiabatic closed system is shown in Fig. 4 (a,b). The standard glass slide with the dimension, 100 by 3” by 1 mm was placed in the center of a rectangular box (70 mm by 150 mm by 100 mm) shielded with thermal insulation layers. The heat source for lamp radiation is a 100 W bulb place in front of the glass with a fixed distance, 10 cm. The temperatures of T1, T2 and T3 are measured in transient as shown in Fig. 4 (b). 3. Results and discussion 3.1. Characterization of electro-chemically exfoliated graphene and graphene/γ-alumina The working electrode for the electrochemical exfoliation process is artificial graphite, reacting at the programmed voltage (�20 V) in a mixed electrolyte solution (H2SO4 þ KOH). The intercalation into graphite layers relies on the use of sulfate ions (SO24 ) by applying a positive bias to the working electrode. The addition of hydroxyl ions (OH ) is to partially neutralize the electrolyte for avoiding oxidation of the products during the reaction. The optimization of applied voltage is needed depending on the kind of starting materials. The exfoliation process cannot occur, if the bias is too low, whereas the electrode could be destroyed under a very high bias due to strong intercalation behavior. After reaction, the residual electrolyte was removed from the products by vacuum filtration and rinsing with DI water and ethanol for several times until the pH is close to neutral. The uniform stable dispersion of the graphene ink is achieved by sonicating the purified samples in N4
L.-H. Hu and S.-H. Kao
Solar Energy Materials and Solar Cells 203 (2019) 110199
Fig. 6. (a) TEM images with lower magnification of graphene/γ-alumina (b) (a) TEM images with higher magnification of graphene/γ-alumina (c) SEM images with lower magnification of graphene/γ-alumina (d) SEM images with higher magnification of graphene/γ-alumina (e) the thickness of coating graphene/γ-alumina layer on glass.
images indicate that the graphene/γ-alumina ink is able to be coated uniformly by simple spraying process.
3.2. Thermal behavior of lamp radiation 3.2.1. Lamp radiation of ECG coating and graphene/γ-alumina Short term measurement of lamp radiation with an adiabatic closed system is shown in Fig. 7 and the result is shown in Table 1. Fig. 7(a–f) 5
L.-H. Hu and S.-H. Kao
Solar Energy Materials and Solar Cells 203 (2019) 110199
Fig. 7. Short term radiation test of temperature measurement of T1, T2 and T3 with (a) 0.03 wt% ECG coating (c) 0.1 wt% ECG coating and (e) 0.3 wt% ECG coating; short term radiation test of temperature difference of (b) 0.03 wt% ECG coating (d) 0.1 wt% ECG coating and (f) 0.3 wt% ECG coating.
carbon composite ink, representing as T2 (the temperature of the coating side). The rest of heat is accumulated on the glass surface, which is represented as T1. Fig. 7 (a) and (b) display the temperature change of 0.03 wt% ECG coating with lamp radiation. Low content of ECG coated on glass is not able to provide significant function of heat distribution and absorption. Heat accumulated on the glass surface makes T1 reach 49.4 � C close to T2, 50.1 � C and the heat passing through makes T3 reach 36.8 � C. The temperature difference between T1 and T3 is 12.8 � C; therefore, even thin layer of ECG is only coated, the temperature in the adiabatic closed system can be certainly reduced because the partial heat is absorbed by thin ECG layer and glass. Fig. 7 (c) and (d) display the temperature change of 0.1 wt% ECG coating that makes the heat pass-through be absorbed and distributed. Once the flux-in heat passing through is absorbed and distributed by 0.1 wt% ECG coating, heat accumulated on the glass surface is extremely reduced; therefore, T1 reaches 45.5 � C that is much lower than that of T1 with 0.03 wt% of ECG coating. After 0.1 wt% of ECG coating absorbs some heat to cause heat dissipation regarded as reflected heat partially accumulated on ECG coating, the temperature, T2 slightly increases to achieve 51.7 � C and heat absorption is not enough; therefore, the reflection is not able to reach the glass surface, being stopped at the heat reflected boundary.
Table 1 The result of short term measurement of lamp radiation. wt.% of ECG coating
Short term lamp radiation (� C) T1
T2
T3
T1- T3
T2- T3
0.03 0.1 0.3
49.4 45.5 50.8
50.1 51.7 51.6
36.8 32.5 36.4
12.6 13.0 14.4
13.3 19.2 15.2
display the temperature measurement of 0.3, 0.1 and 0.03 wt% ECG spray-coated glass, respectively. The thermal conductivity of graphene is excellent in plane but poor along out of plane; therefore, heat flux perpendicular to the graphene basal plane is blocked. Its in-plane ther mal conductivity (2000–5300 Wm 1K 1) and intrinsic carrier mobility (2 � 105 cm2 V 1s 1) are extremely high, depending on the size of gra phene flake and number of graphene layers, as well as its quality [37–40]. Therefore, graphene in plane is excellent for heat distribution and out of plane is poor for conducting heat but good for absorbing and blocking heat. Some heat generated from the heat source passes through the glass, representing as T3 (the temperature inside adiabatic closed system) and some heat is partially absorbed by the glass and coating of 6
L.-H. Hu and S.-H. Kao
Solar Energy Materials and Solar Cells 203 (2019) 110199
Fig. 9. (a) the comparison of T3 with different concentrations of ECG coating for short term radiation test (b) the comparison of T3 with different concen trations of graphene/γ-alumina coating for short term radiation test.
Fig. 10. The schematic mechanism of thermal trapped by graphene/ γ-alumina coating.
Fig. 8. The schematic mechanism of thermal behavior of ECG coating (arrow length represents the heat magnitude).
is too thin to distribute and absorb the heat pass-through as the heat flux-out shown in Fig. 8 (a) to make T3 increase. 0.1 wt% of ECG coating compared to the other two ECG coatings provides the optimal condition as shown in Fig. 8 (b) to avoid heat accumulation/pass-through on the glass surface and thermal superposition behavior to maintain the tem perature and ramping rate of T3 as the lowest. The thermal superposition of the heat flux-in/out and reflected heat as shown in Fig. 8 (c) of 0.3 wt % of ECG coating as the second heat source causes T3 increase. If the temperature of T3 for pure ECG coating is only concerned as shown in Fig. 9 (a), 0.1 wt% of ECG coating gives the lowest T3 compared to that of 0.03 and 0.3 wt% of ECG coating because of the lowest heat accu mulation of heat flux-out and reflected from the thermal superposition; the reflected heat stops passing through the heat reflected boundary. The composite of graphene/γ-alumina coating on the glass for lamp
This makes the temperature difference between T2 and T3 is much higher than that between T1 and T3. Owing to some heat absorbed and distributed by 0.1 wt% ECG coating, the temperature of T3 dramatically reduces to 32.5 � C. For 0.3 wt% of ECG coating most of the flux-in heat is absorbed and distributed by the thick ECG coating that is regarded as a second heat source for dissipating huge amount of the heat absorbed. Because of most heat absorbed, the reflected heat is able to reach the double side, the glass surface and coating side, so called thermal su perposition to make the temperature of T1 increase to 50.8 � C and T2 be close to that of 0.1 wt% of ECG coating and T3 raises up to 36.4 � C as shown in Fig. 7 (e) and (f). The schematic mechanism of thermal behavior of ECG coating is displayed in Fig. 8. 0.03 wt% of ECG coating 7
L.-H. Hu and S.-H. Kao
Solar Energy Materials and Solar Cells 203 (2019) 110199
Fig. 11. (a) long term radiation test of temperature measurement of T3 between the pure ECG coating and glass (b) long term radiation test of temperature dif ference, T1-T3 between the pure ECG coating and glass (c) long term radiation test of temperature measurement of T3 of graphene/γ-alumina coating (d) long term radiation test of temperature difference, T1-T3 of graphene/γ-alumina coating.
radiation in Fig. 9 (b) shows that the temperature, T3 of the coating of 0.03 wt% ECG incorporated with 1 wt% γ-alumina reaches 33.7 � C that is lower than that of 0.03 wt% ECG coating, 36.8 � C. Same situation as T3 of the coating of 0.3 wt% ECG incorporated with 1 wt% γ-alumina can reach 31.3 � C that is lower than that of 0.3 wt% ECG coating, 36.4 � C. The content of γ-alumina influences the behavior of thermal radiation for absorbing the radiation heat to cause T3 decrease [41]. However, higher content of γ-alumina doesn’t enhance the thermal performance that 1 wt% of γ-alumina absorbs higher heat than that of 5 wt% of γ-alumina. It is possibly because the higher content of γ-alumina possibly has the same thermal behavior as that of higher content of ECG to be regarded as the third heat source to raise T3 from radiation. After absorbing heat, the heat radiation from less content of γ-alumina is trapped by ECG that the thermal fluctuation is within the graphene layers as shown in Fig. 10(a). For higher content of γ-alumina ECG is not able to trap the heat radiated from γ-alumina; moreover, the heat es capes from graphene layers to raise the temperature of T3 as shown in Fig. 10(b). Long term measurement of lamp radiation with an adiabatic closed system is shown in Fig. 11. The average temperature of T3 with 0.1 wt% ECG coating is 33.2 � C, which is lower than that of 0.3 wt% ECG coating and the pure glass, 34.5 and 36.4 � C, respectively as shown in Fig. 11(a). Due to thermal superposition of thick graphene coating the temperature difference of 0.3 wt% of ECG coating between T1 and T3 is 23.6 � C that is the highest compared to that of 0.1 wt% of ECG coating and the pure glass, 13.1 � C and 6.0 � C as shown in Fig. 11(b). The measurement of long term radiation has the same trend as the result from short term radiation. Fig. 11(c) displays the long term radiation of different content of ECG incorporated with 1 wt% of γ-alumina, reducing about 1 � C of T3 compared to that of pure ECG coating. The temperature, T3 of the coating of 0.1 wt% ECG with 1 wt%γ-alumina is still slightly lower than that of the coating 0.3 wt% of ECG with 1 wt% of γ-alumina. Due to the uniformity of γ-alumina dispersion, thermal fluctuation still exists within the graphene layers to cause the fluctuation of the temperature difference as shown in Fig. 11(d). The higher content of ECG provides higher thermal fluctuation. The uniformity of γ-alumina dispersion is shown in Fig. 6(a) and (b) that the γ-alumina clusters
aggregate on or in the graphene layers, causing thermal fluctuation. 3.2.2. Cyclic lamp radiation of graphene/γ-alumina The cyclic lamp radiation of 0.1 wt% ECG with 1 wt% γ-alumina is shown in Fig. 12 (a). Each thermal cycle is tested with lamp on and off in every 2 h. The result indicates that T1, T2 and T3 are recoverable during thermal cycling; however, T3 is not able to return to its initial temper ature before testing, indicating that heat cannot fully dissipate from the adiabatic closed system. The energy radiation emitted and stored can be estimated as following, the total heat flow density, Qtot. that equals to the summation of the heat flow density of conduction, Qcon. and radiation, Qrad..Therefore, the net heat flow rate can be represented as eq. (1). ε is the emissivity, for graphene with single and multi-layers in a range of 0.02–0.06 [42]; the alumina with rough surface is 0.06. σ: the Stefan constant equals to 5.670 � 10 8 Wm 2K 4; A is the surface of the object emitting or absorbing thermal radiation. T1 is absolute temperature of the uncoated glass surface. T2 is absolute temperature of the object emitting or absorbing thermal radiation. T3 is absolute temperature of the environment in the adiabatic closed system before and after thermal radiation. Kg and KECG are the thermal conductivity of the glass and graphite out-off plane, respectively. dg and dECG are the thickness of the glass and ECG, respectively. ΔQtot: ΔQrad: ΔQcon: Kg AðT1 ¼ þ ¼ εσðT2 T3 Þ4 A þ dg Δt Δt Δt
T2 Þ
þ
KECG AðT1 dECG
T2 Þ (1)
Fig. 12 (b) displays the result of net heat flow of radiation and con duction that heat conduction significantly dominates the thermal behavior during light-on period. And the radiation heat flow is linearly related to the emissivity shown in Fig. 12 (c) indicating that the multilayers of graphene provide higher radiation heat flow; however, the heat flow rate of radiation is about the 11th power of 10 smaller than that of heat conduction; therefore, heat radiation can be neglected. This result represents that the coating layer of graphene/γ-alumina is effec tively to absorb and distribute the heat within the graphene layers and γ-alumina to reduce the thermal radiation effect. 8
L.-H. Hu and S.-H. Kao
Solar Energy Materials and Solar Cells 203 (2019) 110199
Fig. 12. (a) the cyclic lamp radiation test (b) Heat flow rate during lamp on and off (c) radiation heat flow rate with different emissivity of graphene.
3.3. UV–vis spectrum of electro-chemically exfoliated graphene and graphene/γ-alumina
compared to that of thin film reduces its transmittance and ECG coating is the other factor to lower its transmittance. The result shown in Fig. 13 (a) represents that the transmittance is highly dominated by the gra phene content. Fig. 13(b) shows the absorbance of each coating. Gra phene has strong absorbance with its layers [44,45]. 0.3 wt% of ECG coating gives higher UV absorbance than that of 0.03 wt% of ECG coating. All the coating with 0.03 wt% of ECG shows low UV absorbance with any content of γ-alumina. 1 wt% of γ-alumina incorporated coating enhances UV absorbance of 0.3 wt% ECG coating.
Fig. 13 displays the transmittance and absorbance of the electrochemically exfoliated graphene and graphene/γ-alumina coated glass by uv–vis spectrum. For only graphene coated glass the transmittance is highly proportional to the content of ECG that the higher ECG contains, the higher transmittance achieves. The transmittance of 0.03 wt% of ECG coated glass can reach 70–75% within the wavelength of visible light. The coating of 0.3 wt% ECG with 5 wt% γ-aluminagives the lowest transmittance, 3.26–7.74% at 400–700 nm wavelength. The coating of 0.03 wt% ECG with 1 wt% γ-alumina makes the transmittance achieve 81–83% within the wavelength of visible light and the graphene/ γ-alumina coating fully covers the glass as shown in Fig. 6, indicating that the content of γ-alumina doesn’t influence the transmittance as shown in Fig. 13(a). For thin film γ-alumina coating its transmittance can reach 96.8% at wavelength 550 nm [43]; however, the coating of γ-alumina nanoparticle with higher specific surface area (SSA)
4. Conclusion Graphene/γ-alumina ink is developed for energy efficient glass by simple spray coating. The poor thermal conduction of graphene out of plane is the advantage for thermal barrier. The thermal behavior of graphene/γ-alumina coated glass shows that the temperature difference between inside the adiabatic closed system and the glass surface can achieve 20 � C after lamp radiation testing for 15 h. Radiation heat is 9
L.-H. Hu and S.-H. Kao
Solar Energy Materials and Solar Cells 203 (2019) 110199
Fig. 13. (a) the transmittance measurement of graphene/γ-alumina and pure ECG coatings (b) the absorbance of graphene/γ-alumina and pure ECG coatings.
retarded by graphene/γ-alumina coating. The content of electrochemically exfoliated graphene (ECG) is the key factor to adjust the transmittance and absorbance and the γ-alumina substantially enhances the thermal behavior. The transmittance is adjustable from 5.57%~81% at 550 nm wavelength, depending on the composition and content of graphene/γ-alumina and they are good UV absorbents. Graphene/ γ-alumina coated glass provides good thermal barrier and optical properties. Consequently, it is a potential green material for energy coating with low cost and simple coating process to replace the current low-E glass in near future.
[4] H.T. Chou, Y.C. Chen, C. YLeea, H.You Chang, N.H. Tai, Switchable transparency of dual-controlled smart glass prepared with hydrogel-containing graphene oxide for energy efficiency, Sol. Energy Mater. Sol. Cells 166 (2017) 45–51. [5] A. Seeboth, R. Ruhmann, O. Muhling, Thermotropic and thermochromic polymer based materials for adaptive solar control, Materials 3 (2010) 5143–5168. [6] H. Watanabe, Intelligent window using a hydrogel layer for energy efficiency, Sol. Energy Mater. Sol. Cells 54 (1998) 203–211. [7] X.Q. Gong, J.X. Li, S.Y. Chen, W.J. Wen, Copolymer solution-based smart window, Appl. Phys. Lett. 95 (2009) 1907. [8] T. Gyenes, A. Szilagyi, T. Lohonyai, M. Zrinyi, Electrically adjustable thermotropic windows based on polymer gels, Polym. Adv. Technol. 14 (2003) 757–762. [9] T. Fischer, R. Lange, A. Seeboth, Hybrid solar and electrically controlled transmission of light filters, Sol. Energy Mater. Sol. Cells 64 (2000) 321–331. [10] B. Petter, J. Simen, E. Kalnæs, T. Gao, Low-emissivity materials for building applications: a state-of-the-art review and future research perspectives, Energy Build. 96 (2015) 329–356. [11] S.S. Ghosh, P.K. Biswas, S. Neogi, Thermal performance of solar cooker with special cover glass of low-e antimony doped indium oxide (IAO) coating, Appl. Therm. Eng. 113 (25) (2017) 103–111. [12] M. Singh, T.R. Rana, S. Kim, K. Kim, J. Yun, J. Kim, Silver nanowires binding with sputtered ZnO to fabricate highly conductive and thermally stable transparent electrode for solar cell applications, ACS Appl. Mater. Interfaces 8 (20) (2016) 12764–12771. [13] G. Gruner, Carbon nanotube films for transparent and plastic electronics, J. Mater. Chem. 16 (2006) 3533–3539. [14] H.-Z. Geng, K.K. Kim, K.P. So, Y.S. Lee, Y. Chang, Y.H. Lee, Effect of acid treatment on carbon nanotube-based flexible transparent conducting films, J. Am. Chem. Soc. 129 (2007) 7758–7759. [15] L. Gomez De Arco, Y. Zhang, C.W. Schlenker, K. Ryu, M.E. Thompson, C. Zhou, Continuous, highly flexible, and transparent graphene films by chemical vapor deposition for organic photovoltaics, ACS Nano 4 (2010) 2865–2873. [16] S. Bae, H. Kim, Y. Lee, X. Xu, J.-S. Park, Y. Zheng, J. Balakrishnan, T. Lei, H.R. Kim, Y. Il Song, Y.-J. Kim, K.S. Kim, B. Ozyilmaz, J.-H. Ahn, B.H. Hong, S. Iijima, Rolltoroll production of 30-inch graphene films for transparent electrodes, Nat. Nanotechnol. 5 (2010) 574–578. [17] J. Zou, H.L. Yip, S.K. Hau, A.K.Y. Jen, Metal grid/conducting polymer hybrid transparent electrode for inverted polymer solar cells, Appl. Phys. Lett. 96 (2010) 203301. [18] C.H. Jeon, Y.H. Jeong, J.J. Seo, H.N. Tien, S.T. Hong, Y.J. Yum, S.H. Hur, K.J. Lee, Material properties of graphene/aluminum metal matrix composites fabricated by friction stir processing, Int. J. Precis. Eng. Manuf. 15 (2014) 1235–1239. [19] F.H. Latief, E.S.M. Sherif, Effects of sintering temperature and graphite addition on the mechanical properties of aluminum, J. Ind. Eng. Chem. 18 (2012) 2129–2134. [20] J. Liu, U. Khan, J. Coleman, B. Fernandez, P. Rodriguez, S. Naher, D. Brabazon, Graphene oxide and graphene nanosheet reinforced aluminium matrix composites:
Acknowledgments This research project is funded by the Ministry of Science and Technology, Taiwan with the project number: MOST 108-2221-E-110075 and 107-2221-E-218-002. The technical support of electrochemically exfoliated graphene is from Nitronix Nanotechnology cor poration, Taiwan. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.solmat.2019.110199. References [1] A.C. Rotzetter, C.M. Schumacher, S.B. Bubenhofer, R.N. Grass, L.C. Gerber, M. Zeltner, W.J. Stark, Thermoresponsive polymer induced sweating surfaces as an efficient way to passively cool buildings, Adv. Mater. 24 (2012) 5352–5353. [2] D.H.W. Li, J.C. Lam, C.C.S. Lau, T.W. Huan, Lighting and energy performance of solar film coating in air-conditioned cellular offices, Renew. Energy 29 (2004) 921–937. [3] R. Baetens, B.P. Jelle, A. Gustavsen, Properties, requirements and possibilities of smart windows for dynamic daylight and solar energy control in buildings: a stateof- the-art review, Sol. Energy Mater. Sol. Cells 97 (2010) 87–105.
10
L.-H. Hu and S.-H. Kao
[21] [22] [23] [24]
[25]
[26] [27] [28]
[29] [30] [31]
Solar Energy Materials and Solar Cells 203 (2019) 110199 [32] X.S. Li, Y.W. Zhu, W.W. Cai, M. Borysiak, B.Y. Han, D. Chen, R.D. Piner, L. Colombo, R.S. Ruoff, Transfer of large-area graphene films for high-performance transparent conductive electrodes, Nano Lett. 9 12 (2009) 4359–4363. [33] C. Lee, X.D. Wei, J. Kysar, W.J. Hone, Measurement of the elastic properties and intrinsic strength of monolayer graphene, Science 321 5887 (2008) 385–388. [34] Z.H. Ni, H.M. Wang, J. Kasim, H. Fan, M.T. Yu, Y.H. Wu, Y.P. Feng, Z.X. Shen, Graphene thickness determination using reflection and contrast spectroscopy, Nano Lett. 7 9 (2007) 2758–2763. [35] X.X. Ni, G.C. Liang, J.S. Wang, B.W. Li, Disorder enhanceds thermoelectrics figure of merit in armchair graphene nanoribbons, Appl. Phys. Lett. 95 (2009) 192114. [36] C.Y. Su, Y.P. Xu, W.J. Zhang, C.H. Tasi, J.W. Zhao, X.H. Tang, L.J. Li, Electrical and spectroscopic characterizations of ultra-large reduced graphene oxide monolayers, Chem. Mater. 21 23 (2009) 5674–5680. [37] J.H. Chen, C. Jang, S.D. Xiao, M. Ishigami, M.S. Fuhrer, Intrinsic and extrinsic performance limits of graphene devices on SiO2, Nat. Nanotechnol. 3 (2008) 206–209. [38] J.G. Checkelsky, N.P. Ong, The thermopower and nerst effect in graphene in a magnetic field, Phys. Rev. B 80 (2009), 081413(R). [39] A.C. Ferrari, J.C. Meyer, V. Scardaci, C. Casiraghi, M. Lazzeri, F. Mauri, S. Piscanec, D. Jiang, K.S. Novoselov, S. Roth, A.K. Geim, Raman spectrum of graphene and graphene layers, Phys. Rev. Lett. 97 (18) (2006) 187401. [40] A.K. Geim, K.S. Novoselov, The rise of graphene, Nat. Mater. 6 (3) (2007) 183–191. [41] J. Kim, J. Kim, D. Jerng, E. Kim, H. Ahn, Effect of aluminum oxide and reduced graphene oxide mixtures on critical heat flux enhancement, Int. J. Heat Mass Transf. 116 (2018) 858–870. [42] L.M. Lawton, N.H. Mahlmeister, I.J. Luxmoore, G.R. Nash, Prospective for graphene based thermal mid-infrared light emitting devices, AIP Adv. 4 (2014), 087139. [43] J. Park, K. Park, D. Spoor, B. Hall, Y. Song, Hand-manageable graphene sticker for ultrafast mode-locked fiber lasers, Opt. Express 23 (2015) 7940–7945. [44] R.R. Nair, P. Blake, A.N. Grigorenko, K.S. Novoselov, T.J. Booth, T. Stauber, N.M. R. Peres, A.K. Geim, Fine structure constant defines visual transparency of graphene, Science 320 (2008) 1308. [45] J.R. Piper, S. Fan, Total Absorption in a Graphene Monolayer in the Optical Regime by Critical Coupling with a Photonic Crystal Guided Resonance ACS Photonics vol. 1, 2014, pp. 347–353.
powder synthesis and prepared composite characteristics, Mater. Des. 94 (2016) 87–94. J. Wang, Z. Li, G. Fan, H. Pan, Z. Chen, D. Zhang, Reinforcement with graphene nanosheets in aluminum matrix composites, Scr. Mater. 66 (2012) 594–597. M. Rashad, F. Pan, A. Tang, M. Asif, Effect of Graphene Nanoplatelets addition on mechanical properties of pure aluminum using a semi-powder method, Prog. Nat. Sci.: Mater. Int. 24 (2014) 101–108. X. Gao, H. Yue, E. Guo, H. Zhang, X. Lin, L. Yao, B. Wang, Preparation and tensile properties of homogeneously dispersed graphene reinforced aluminum matrix composites, Mater. Des. 94 (2016) 54–60. Z. Li, Q. Guo, Z. Li, G. Fan, D.B. Xiong, Y. Su, J. Zhang, D. Zhang, Enhanced mechanical properties of graphene (reduced graphene oxide)/aluminum composites with a bioinspired nanolaminated structure, Nano Lett. 15 (2015) 8077–8083. Z. Li, G. Fan, Z. Tan, Q. Guo, D. Xiong, Y. Su, Z. Li, D. Zhang, Uniform dispersion of graphene oxide in aluminum powder by direct electrostatic adsorption for fabrication of graphene/aluminum composites, Nanotechnology 25 (2014) 325601. C. Hu, X. Zhai, L. Liu, Y. Zhao, L. Jiang, L. Qu, Spontaneous reduction and assembly of graphene oxide into three-dimensional graphene network on arbitrary conductive substrates, Sci. Rep. 3 (2013) 2065. Z. Fan, K. Wang, T. Wei, J. Yan, L. Song, B. Shao, An environmentally friendly and efficient route for the reduction of graphene oxide by aluminum powder, Carbon 48 (2010) 1686–1689. L. Zhang, G. Hou, W. Zhai, Q. Ai, J. Feng, L. Zhang, P. Si, L. Ci, Aluminum/ graphene composites with enhanced heat-dissipation properties by in-situ reduction of graphene oxide on aluminum particles, J. Alloy. Comp. 748 (5) (2018) 854–860. C.Y. Su, A.Y. Lu, Y. Xu, F.R. Chen, A.N. Khlobystov, L.J. Li, High-quality thin graphene films from fast electrochemical exfoliation, ACS Nano 5 (3) (2011) 2332–2339. A. Gupta, G. Chen, P. Joshi, S. Tadigadapa, P.C. Eklund, Raman scattering from high-frequency phonons in supported n-graphene layer films, Nano Lett. 6 (12) (2006) 2667–2673. B. Krauss, T. Lohmann, D.H. Chae, M. Haluska, K. von Klitzing, J.H. Smet, Laserinduced disassembly of a graphene single crystal into a nanocrystalline network, Phys. Rev. B 79 16 (2009) 165428.
11
Contents lists available at ScienceDirect
Solar Energy Materials and Solar Cells journal homepage: http://www.elsevier.com/locate/solmat
Multifunctional electro-chemically exfoliated graphene with γ-alumina composite by spray-coating for energy efficient glass Lung-Hao Hu a, *, Sheng-Hui Kao b a b
Department of Mechanical and Electro-Mechanical Engineering, National Sun Yat-sen University, Kaohsiung, 804, Taiwan Department of Mechanical Engineering, Southern Taiwan University of Science and Technology, Tainan, 710, Taiwan
A R T I C L E I N F O
A B S T R A C T
Keywords: Electro-chemically exfoliated graphene Energy efficient glass Carbon composite ink Graphene/γ-alumina Transmittance Absorbance
Massive production of graphene is synthesized by a low-cost electro-chemically exfoliated approach and then incorporated with γ-alumina for developing a novel carbon composite ink, graphene/γ-alumina ink. It is coated onto the glass by spraying with an adiabatic closed system as the thermal and optical barriers for the energy efficient glass of the business buildings. The coating thickness of graphene/γ-alumina layer is around 2~3 μm. The temperature in an adiabatic closed system is measured with a heat source by lamp radiation. The temper ature difference between inside and outside of the adiabatic closed system can achieve around 20 � C with the coating of 0.3 wt% of electro-chemically exfoliated graphene incorporated with 1 wt% of γ-alumina in N-Methyl2-Pyrrolidone after thermal radiation test for 15 h. The transmittance is adjustable from 5.57%–81% at 550 nm wavelength, depending on the composition and content of graphene and γ-alumina. The coating of the carbon composite ink, electro-chemically exfoliated graphene with γ-alumina are also good absorbents for UV absorbance.
1. Introduction Extremely climate change is a crucial issue for the global environ ment influenced by, such as global warming and the emission of oxo carbon produced by fossil fuel usage. During summer, the sunlight and heat flows are highly possible to penetrate through the buildings win dow. If the business buildings are filled with air-conditioning during summer time, the heat flow outside the buildings will cause the heat consumption to increase the energy loss and lower the efficiency of those air-conditioners; moreover, the global warming effect would be raised. Therefore, energy conservation is one of the key points to overcome those issues. Heat insulation and sunlight shielding have been paid for much attention for optimizing energy consumption in the operation of space cooling, heating systems and building materials [1–3]. Adjustable and customized functions of the glass window with low cost are becoming more significant for enhancing energy efficiency of green buildings, such as heat insulation for lowering room temperature, transparency and UV-sunlight light shielding. Standard glass with a high emissivity makes solar heat move freely in and out; however, low-emission (low-e) coatings with glass window provide a low emis sivity and act like a filter to control heat transfer that certain energy
wavelengths are able to pass through, while others are not. Therefore, low-emission coatings are able to be used for reducing energy usage in both opaque and transparent areas of buildings. The opaque and transparent materials of low-e coatings or foils have different areas of application. Even though their functions are generally similar, they can be applied to some specific areas by different manners and have to fulfill some different criteria. The comparison between opaque and transparent materials is directly against each other. Opaque materials for low-e coatings should be practical to apply and perform well to blocking out radiant heat. They are usually only to block out radiant heat from the outside, as it has to be shown its effectiveness is in the cooling dominated regions, and the buildings have generally been installed thicker layers of conventional thermal insulation for climates dominated by heating. Low-e foils perform better results on energy savings when installed properly. However, low-e paints have the advantage of applying to the flexible substrates or curved window and can be added to materials or composites before installation in areas where the installation of foils is not practical, such as the inside of hollow bricks and on the exterior surface of buildings and buildings’ windows [4]. Hydrogel is one of the temperature-driven systems to modify the
* Corresponding author. E-mail addresses: [email protected] (L.-H. Hu), [email protected] (S.-H. Kao). https://doi.org/10.1016/j.solmat.2019.110199 Received 16 June 2019; Received in revised form 16 September 2019; Accepted 22 September 2019 Available online 25 September 2019 0927-0248/© 2019 Elsevier B.V. All rights reserved.
L.-H. Hu and S.-H. Kao
Solar Energy Materials and Solar Cells 203 (2019) 110199
Fig. 1. SEM images of (a) the starting material, artificial graphite (b) the artificial graphite electrochemically exfoliated (c) the Photograph of the large scale transferring of ECG on PET.
the increased temperature of the hydrogel heated by solar light, as a result, reducing the incident intensity of solar energy and room tem perature. The transmittance of 0.2% GO incorporated with PNIPAM dramatically reduced when the temperature reached 28 � C and when the smart glass changed opaque, the temperature reached 32 � C. That the issue should be concerned is whether the transparency changing with temperatures is suitable for the majority. S. S. Ghosh et al. [11] syn thesized different transparent conducting oxide with various dopants by sol-gel process. Two types of sols, host metal ion sol and doped metal sol were prepared; the final precursor sol was prepared by mixing these two types of sols incorporated with wettable agent. The glass was dip-coated in the final precursor sol bath and then pyrolyzed at 500 � C under mixture gas(Ar þ H2). The transparent conducting coating by magne tron sputtering has been predominantly used for solar cells. Current researches focusing on the conducting coating are trying to discover new materials to replace conventional indium tin oxide (ITO). J. Kim et al. [12] fabricated silver nanowire thin film as transparent conductive electrode with a thin layer of sputtered ZnO to increase the contact area with low-lying surface but also improve the conductivity due to the connection of silver nanowires at the junction. Carbon nanotube, gra phene, metal grid and metal nanowire network have been paid high attention for replacing ITO; however, owing to some optical criteria and cost issues those materials are still being investigated [13–17]. If the optical criteria are not being concerned, graphene or graphene oxide is one of the outstanding candidates for heat dissipation. Aluminum incorporated with graphene or graphene oxide had been fabricated with different processes to enhance the thermal conductivity that the higher graphene contains, the better performance of heat dissipation performs [18–25]. Many researchers have also indicated that graphene oxide is
Fig. 2. Schematic illustration of the spraying process.
transparency [5–7]; thus the external energy supply is able to be mini mized by the passive tunable process. Therefore, the dual or multi-mode thermotropic material with high performance is desirable to replace the monotonous tunable system. Conducting layer coatings for thermal insulation had been widely applied to low-e glass. With this regards, the hybrid system composed of indium tin oxide or other conducting ma terial coated glass and thermotropic materials is to be focusing on; it can be heated through a voltage applied [8,9]. H.T. Chou [10] used hydrogel method with poly(N-isopropylacrylamide) (PNIPAM) incorporated with graphene oxide (GO) to effectively convert the photo-energy into ther mal energy and cause the transparency decrease of the smart glass with 2
L.-H. Hu and S.-H. Kao
Solar Energy Materials and Solar Cells 203 (2019) 110199
Fig. 3. The Photographs of (a) different ECG content coated glass and 1 and 5 wt% γ-alumina with 0.03 wt% ECG coated glass (b) 1 and 5 wt% γ-alumina with 0.3 wt % ECG coated glass.
Fig. 4. The set-up of the lamp radiation-adiabatic closed system (a) front view (b) side view with heat source.
able to directly reduce by active metals like aluminum without any additional reducing agents [26,27]. Therefore, graphene oxide would be the optimal starting coating material. L. Zhang et al. [28] found that in-situ reduction of graphene oxide can make reduced graphene well-disperse onto the surfaces of aluminum particles. The powder metallurgy technique was further applied to fabricate the Al/G (alumi num/graphene) composites to enhance heat-dissipation with very low graphene content. Here, we developed a massive producible composite ink of electro-chemically exfoliated graphene (ECG) incorporated with nano γ-alumina by spraying approach that can be employed to various shapes of windows and flexible substrates for heat dissipation and op tical criteria of business buildings.
the residual electrolyte. The purified flake was partially dried at 100 � C for 10 min on hot plate. The sample has to be maintained under a partially wet status; otherwise, the graphene layers will tightly stack to cause the difficulty to uniformly disperse in further process. To obtain well-dispersed samples, one-minute sonication was carried out for the products in N-Methyl-2-pyrrolidone (NMP). Finally, the dispersion was filtered again to remove the unreacted starting materials. 10 ml of different weight percent of the electro-chemically exfoliated graphene (ECG) in NMP (0.03 wt%, 0.1 wt% and 0.3 wt% in 10 ml NMP) is ho mogenized with 10 ml of γ-Al2O3 (AEROSIL~100 nm) in NMP (1 wt% and 5 wt% of γ-Al2O3 in 10 ml NMP) to form the liquid composite, graphene/γ-alumina ink. The morphologies of ECG were characterized by optical microscope (OM), atomic force microscope (AFM, Veeco Dimension-Icon system), and scanning electron microscope (SEM, JEOLJSM-6500F). Fig. 1 displays the scanning electron microscope (SEM) images of the artificial graphite (starting material, Fig. 1(a)), ECG (Fig. 1(b)) and the sprayed ECG on the flexible substrate, PET (Fig. 1(c)), respectively.
2. Experimental method 2.1. Synthesis of electro-chemically exfoliated graphene (ECG)with γ-Al2O3 Our samples were prepared by electrochemical exfoliation method. A piece of artificial graphite was as the working electrode and a plat inum wire was served as a reference electrode for the experimental setup. The electrolyte was a mixture of H2SO4 and KOH (composition: 2.4 g of H2SO4 and 11 ml 30% KOH in 100 ml DI water). A DC bias, �20 V was applied to the working electrode for the electrochemical exfoliation reaction. The following steps show the voltage program: 1) þ2.5 V for 60 s; 2) þ20 V for 5 s; 3) 20 V for 5 s. Steps 2) and 3) are repeated alternatively until the amount of the product reaches our requirement [29]. The products were then vacuum-filtrated by a filter membrane (pore size � 0.2 μm) and rinsed with DI water and ethanol for removing
2.2. Spray-coating process of ECG and graphene/γ-alumina ink For the present investigation, 9 ml of the mixed solution, graphene/ γ-alumina ink and pure ECG ink were extracted for spray-coating on the standard glass slide with the dimension of 100 by 300 by 1 mm thick on the hotplate as shown in Fig. 2. The temperature of the hotplate is 180 � C for glass substrate. The transparency is adjustable, depending on the requirement controlled by the ink’s concentrations and spraying time as shown in Fig. 3. Morphologies of each ink sprayed onto the glass are characterized by scanning electron microscope (SEM) and transmission 3
L.-H. Hu and S.-H. Kao
Solar Energy Materials and Solar Cells 203 (2019) 110199
Fig. 5. (a) the picture of electrochemically exfoliated graphene dispersed in NMP (b) the OM image of graphene thin sheets (c) Raman spectra of starting material and the product after reaction (d) AFM scan of single ECG transferred on SiO2/Si.
electron microscope (TEM).
Methyl-2-pyrrolidone (NMP) for 1 min as shown in Fig. 5(a). The products deposited on 300 nm SiO2/Si substrate by dip-coating is shown in Fig. 5(b). The lateral size of these sheets is over ten μm. The confocal Raman spectra (excited by 473 nm laser; power: 0.5 mW; spot size: � 0.5 μm) for starting material and exfoliated graphene sheets is shown in Fig. 5(c), respectively. AFM was used to measure the thickness of ECG transferred on the silicon substrate as shown in Fig. 5(d). The strong Gband (at 1578.4 cm 1) and tiny D-band (near 1360 cm 1) from the artificial graphite indicate a feature of well-crystallized bulk graphite [29–33]. After the starting material exfoliated, the D-band greatly in creases and a shift of G-band (at 1591.8 cm 1) appears. It is known that the I2D/IG ratio of bi-layer graphene is lower than that of monolayer one [29–34]. The ID/IG ratio is 1.21, suggesting that graphene surface is partially oxidized because of sulfuric acid intercalation. The appearance of disorder-induced D-band can be attributed to the formation of nanodomains in graphene lattice during exfoliation. Note that the 2D-band (at � 2700 cm 1) is composed of four components (2D1B, 2D1A, 2D2A, 2D2B) and is an asymmetric peak, except the case of monolayer graphene. In the inset of Fig. 5(c), the sample shows an asymmetric 2D-band, but the intensity of the higher frequency 2D2 peaks is not dominant, suggesting that our product is mainly bi-layer graphene. Additionally, the larger I2D/IG ratio (the integrated peak ratio between 2D- and G-bands) represents higher degree of crystallization for sp2 C¼C bonds in graphene structure (Krauss.,2009). Although our sample is as-prepared and without any reduction treatment, its I2D/IG ratio (0.36) is still higher than that of GO, even which is reduced by hydrazine and 800oC-annealing (Su et al., 2009) [35,36]. Size of nano γ-alumina is around 10–20 nm and the cluster is about 100 nm as shown in Fig. 6 (a) and (b). Graphene sheet is uniformly covered by nano γ-alumina as shown in Fig. 6 (c) and (d). Fig. 6(e) shows that the thickness of gra phene/γ-alumina layer sprayed on the glass is about 2–3 μm. These
2.3. Lamp radiation with an adiabatic closed system The set-up of the lamp radiation-adiabatic closed system is shown in Fig. 4 (a,b). The standard glass slide with the dimension, 100 by 3” by 1 mm was placed in the center of a rectangular box (70 mm by 150 mm by 100 mm) shielded with thermal insulation layers. The heat source for lamp radiation is a 100 W bulb place in front of the glass with a fixed distance, 10 cm. The temperatures of T1, T2 and T3 are measured in transient as shown in Fig. 4 (b). 3. Results and discussion 3.1. Characterization of electro-chemically exfoliated graphene and graphene/γ-alumina The working electrode for the electrochemical exfoliation process is artificial graphite, reacting at the programmed voltage (�20 V) in a mixed electrolyte solution (H2SO4 þ KOH). The intercalation into graphite layers relies on the use of sulfate ions (SO24 ) by applying a positive bias to the working electrode. The addition of hydroxyl ions (OH ) is to partially neutralize the electrolyte for avoiding oxidation of the products during the reaction. The optimization of applied voltage is needed depending on the kind of starting materials. The exfoliation process cannot occur, if the bias is too low, whereas the electrode could be destroyed under a very high bias due to strong intercalation behavior. After reaction, the residual electrolyte was removed from the products by vacuum filtration and rinsing with DI water and ethanol for several times until the pH is close to neutral. The uniform stable dispersion of the graphene ink is achieved by sonicating the purified samples in N4
L.-H. Hu and S.-H. Kao
Solar Energy Materials and Solar Cells 203 (2019) 110199
Fig. 6. (a) TEM images with lower magnification of graphene/γ-alumina (b) (a) TEM images with higher magnification of graphene/γ-alumina (c) SEM images with lower magnification of graphene/γ-alumina (d) SEM images with higher magnification of graphene/γ-alumina (e) the thickness of coating graphene/γ-alumina layer on glass.
images indicate that the graphene/γ-alumina ink is able to be coated uniformly by simple spraying process.
3.2. Thermal behavior of lamp radiation 3.2.1. Lamp radiation of ECG coating and graphene/γ-alumina Short term measurement of lamp radiation with an adiabatic closed system is shown in Fig. 7 and the result is shown in Table 1. Fig. 7(a–f) 5
L.-H. Hu and S.-H. Kao
Solar Energy Materials and Solar Cells 203 (2019) 110199
Fig. 7. Short term radiation test of temperature measurement of T1, T2 and T3 with (a) 0.03 wt% ECG coating (c) 0.1 wt% ECG coating and (e) 0.3 wt% ECG coating; short term radiation test of temperature difference of (b) 0.03 wt% ECG coating (d) 0.1 wt% ECG coating and (f) 0.3 wt% ECG coating.
carbon composite ink, representing as T2 (the temperature of the coating side). The rest of heat is accumulated on the glass surface, which is represented as T1. Fig. 7 (a) and (b) display the temperature change of 0.03 wt% ECG coating with lamp radiation. Low content of ECG coated on glass is not able to provide significant function of heat distribution and absorption. Heat accumulated on the glass surface makes T1 reach 49.4 � C close to T2, 50.1 � C and the heat passing through makes T3 reach 36.8 � C. The temperature difference between T1 and T3 is 12.8 � C; therefore, even thin layer of ECG is only coated, the temperature in the adiabatic closed system can be certainly reduced because the partial heat is absorbed by thin ECG layer and glass. Fig. 7 (c) and (d) display the temperature change of 0.1 wt% ECG coating that makes the heat pass-through be absorbed and distributed. Once the flux-in heat passing through is absorbed and distributed by 0.1 wt% ECG coating, heat accumulated on the glass surface is extremely reduced; therefore, T1 reaches 45.5 � C that is much lower than that of T1 with 0.03 wt% of ECG coating. After 0.1 wt% of ECG coating absorbs some heat to cause heat dissipation regarded as reflected heat partially accumulated on ECG coating, the temperature, T2 slightly increases to achieve 51.7 � C and heat absorption is not enough; therefore, the reflection is not able to reach the glass surface, being stopped at the heat reflected boundary.
Table 1 The result of short term measurement of lamp radiation. wt.% of ECG coating
Short term lamp radiation (� C) T1
T2
T3
T1- T3
T2- T3
0.03 0.1 0.3
49.4 45.5 50.8
50.1 51.7 51.6
36.8 32.5 36.4
12.6 13.0 14.4
13.3 19.2 15.2
display the temperature measurement of 0.3, 0.1 and 0.03 wt% ECG spray-coated glass, respectively. The thermal conductivity of graphene is excellent in plane but poor along out of plane; therefore, heat flux perpendicular to the graphene basal plane is blocked. Its in-plane ther mal conductivity (2000–5300 Wm 1K 1) and intrinsic carrier mobility (2 � 105 cm2 V 1s 1) are extremely high, depending on the size of gra phene flake and number of graphene layers, as well as its quality [37–40]. Therefore, graphene in plane is excellent for heat distribution and out of plane is poor for conducting heat but good for absorbing and blocking heat. Some heat generated from the heat source passes through the glass, representing as T3 (the temperature inside adiabatic closed system) and some heat is partially absorbed by the glass and coating of 6
L.-H. Hu and S.-H. Kao
Solar Energy Materials and Solar Cells 203 (2019) 110199
Fig. 9. (a) the comparison of T3 with different concentrations of ECG coating for short term radiation test (b) the comparison of T3 with different concen trations of graphene/γ-alumina coating for short term radiation test.
Fig. 10. The schematic mechanism of thermal trapped by graphene/ γ-alumina coating.
Fig. 8. The schematic mechanism of thermal behavior of ECG coating (arrow length represents the heat magnitude).
is too thin to distribute and absorb the heat pass-through as the heat flux-out shown in Fig. 8 (a) to make T3 increase. 0.1 wt% of ECG coating compared to the other two ECG coatings provides the optimal condition as shown in Fig. 8 (b) to avoid heat accumulation/pass-through on the glass surface and thermal superposition behavior to maintain the tem perature and ramping rate of T3 as the lowest. The thermal superposition of the heat flux-in/out and reflected heat as shown in Fig. 8 (c) of 0.3 wt % of ECG coating as the second heat source causes T3 increase. If the temperature of T3 for pure ECG coating is only concerned as shown in Fig. 9 (a), 0.1 wt% of ECG coating gives the lowest T3 compared to that of 0.03 and 0.3 wt% of ECG coating because of the lowest heat accu mulation of heat flux-out and reflected from the thermal superposition; the reflected heat stops passing through the heat reflected boundary. The composite of graphene/γ-alumina coating on the glass for lamp
This makes the temperature difference between T2 and T3 is much higher than that between T1 and T3. Owing to some heat absorbed and distributed by 0.1 wt% ECG coating, the temperature of T3 dramatically reduces to 32.5 � C. For 0.3 wt% of ECG coating most of the flux-in heat is absorbed and distributed by the thick ECG coating that is regarded as a second heat source for dissipating huge amount of the heat absorbed. Because of most heat absorbed, the reflected heat is able to reach the double side, the glass surface and coating side, so called thermal su perposition to make the temperature of T1 increase to 50.8 � C and T2 be close to that of 0.1 wt% of ECG coating and T3 raises up to 36.4 � C as shown in Fig. 7 (e) and (f). The schematic mechanism of thermal behavior of ECG coating is displayed in Fig. 8. 0.03 wt% of ECG coating 7
L.-H. Hu and S.-H. Kao
Solar Energy Materials and Solar Cells 203 (2019) 110199
Fig. 11. (a) long term radiation test of temperature measurement of T3 between the pure ECG coating and glass (b) long term radiation test of temperature dif ference, T1-T3 between the pure ECG coating and glass (c) long term radiation test of temperature measurement of T3 of graphene/γ-alumina coating (d) long term radiation test of temperature difference, T1-T3 of graphene/γ-alumina coating.
radiation in Fig. 9 (b) shows that the temperature, T3 of the coating of 0.03 wt% ECG incorporated with 1 wt% γ-alumina reaches 33.7 � C that is lower than that of 0.03 wt% ECG coating, 36.8 � C. Same situation as T3 of the coating of 0.3 wt% ECG incorporated with 1 wt% γ-alumina can reach 31.3 � C that is lower than that of 0.3 wt% ECG coating, 36.4 � C. The content of γ-alumina influences the behavior of thermal radiation for absorbing the radiation heat to cause T3 decrease [41]. However, higher content of γ-alumina doesn’t enhance the thermal performance that 1 wt% of γ-alumina absorbs higher heat than that of 5 wt% of γ-alumina. It is possibly because the higher content of γ-alumina possibly has the same thermal behavior as that of higher content of ECG to be regarded as the third heat source to raise T3 from radiation. After absorbing heat, the heat radiation from less content of γ-alumina is trapped by ECG that the thermal fluctuation is within the graphene layers as shown in Fig. 10(a). For higher content of γ-alumina ECG is not able to trap the heat radiated from γ-alumina; moreover, the heat es capes from graphene layers to raise the temperature of T3 as shown in Fig. 10(b). Long term measurement of lamp radiation with an adiabatic closed system is shown in Fig. 11. The average temperature of T3 with 0.1 wt% ECG coating is 33.2 � C, which is lower than that of 0.3 wt% ECG coating and the pure glass, 34.5 and 36.4 � C, respectively as shown in Fig. 11(a). Due to thermal superposition of thick graphene coating the temperature difference of 0.3 wt% of ECG coating between T1 and T3 is 23.6 � C that is the highest compared to that of 0.1 wt% of ECG coating and the pure glass, 13.1 � C and 6.0 � C as shown in Fig. 11(b). The measurement of long term radiation has the same trend as the result from short term radiation. Fig. 11(c) displays the long term radiation of different content of ECG incorporated with 1 wt% of γ-alumina, reducing about 1 � C of T3 compared to that of pure ECG coating. The temperature, T3 of the coating of 0.1 wt% ECG with 1 wt%γ-alumina is still slightly lower than that of the coating 0.3 wt% of ECG with 1 wt% of γ-alumina. Due to the uniformity of γ-alumina dispersion, thermal fluctuation still exists within the graphene layers to cause the fluctuation of the temperature difference as shown in Fig. 11(d). The higher content of ECG provides higher thermal fluctuation. The uniformity of γ-alumina dispersion is shown in Fig. 6(a) and (b) that the γ-alumina clusters
aggregate on or in the graphene layers, causing thermal fluctuation. 3.2.2. Cyclic lamp radiation of graphene/γ-alumina The cyclic lamp radiation of 0.1 wt% ECG with 1 wt% γ-alumina is shown in Fig. 12 (a). Each thermal cycle is tested with lamp on and off in every 2 h. The result indicates that T1, T2 and T3 are recoverable during thermal cycling; however, T3 is not able to return to its initial temper ature before testing, indicating that heat cannot fully dissipate from the adiabatic closed system. The energy radiation emitted and stored can be estimated as following, the total heat flow density, Qtot. that equals to the summation of the heat flow density of conduction, Qcon. and radiation, Qrad..Therefore, the net heat flow rate can be represented as eq. (1). ε is the emissivity, for graphene with single and multi-layers in a range of 0.02–0.06 [42]; the alumina with rough surface is 0.06. σ: the Stefan constant equals to 5.670 � 10 8 Wm 2K 4; A is the surface of the object emitting or absorbing thermal radiation. T1 is absolute temperature of the uncoated glass surface. T2 is absolute temperature of the object emitting or absorbing thermal radiation. T3 is absolute temperature of the environment in the adiabatic closed system before and after thermal radiation. Kg and KECG are the thermal conductivity of the glass and graphite out-off plane, respectively. dg and dECG are the thickness of the glass and ECG, respectively. ΔQtot: ΔQrad: ΔQcon: Kg AðT1 ¼ þ ¼ εσðT2 T3 Þ4 A þ dg Δt Δt Δt
T2 Þ
þ
KECG AðT1 dECG
T2 Þ (1)
Fig. 12 (b) displays the result of net heat flow of radiation and con duction that heat conduction significantly dominates the thermal behavior during light-on period. And the radiation heat flow is linearly related to the emissivity shown in Fig. 12 (c) indicating that the multilayers of graphene provide higher radiation heat flow; however, the heat flow rate of radiation is about the 11th power of 10 smaller than that of heat conduction; therefore, heat radiation can be neglected. This result represents that the coating layer of graphene/γ-alumina is effec tively to absorb and distribute the heat within the graphene layers and γ-alumina to reduce the thermal radiation effect. 8
L.-H. Hu and S.-H. Kao
Solar Energy Materials and Solar Cells 203 (2019) 110199
Fig. 12. (a) the cyclic lamp radiation test (b) Heat flow rate during lamp on and off (c) radiation heat flow rate with different emissivity of graphene.
3.3. UV–vis spectrum of electro-chemically exfoliated graphene and graphene/γ-alumina
compared to that of thin film reduces its transmittance and ECG coating is the other factor to lower its transmittance. The result shown in Fig. 13 (a) represents that the transmittance is highly dominated by the gra phene content. Fig. 13(b) shows the absorbance of each coating. Gra phene has strong absorbance with its layers [44,45]. 0.3 wt% of ECG coating gives higher UV absorbance than that of 0.03 wt% of ECG coating. All the coating with 0.03 wt% of ECG shows low UV absorbance with any content of γ-alumina. 1 wt% of γ-alumina incorporated coating enhances UV absorbance of 0.3 wt% ECG coating.
Fig. 13 displays the transmittance and absorbance of the electrochemically exfoliated graphene and graphene/γ-alumina coated glass by uv–vis spectrum. For only graphene coated glass the transmittance is highly proportional to the content of ECG that the higher ECG contains, the higher transmittance achieves. The transmittance of 0.03 wt% of ECG coated glass can reach 70–75% within the wavelength of visible light. The coating of 0.3 wt% ECG with 5 wt% γ-aluminagives the lowest transmittance, 3.26–7.74% at 400–700 nm wavelength. The coating of 0.03 wt% ECG with 1 wt% γ-alumina makes the transmittance achieve 81–83% within the wavelength of visible light and the graphene/ γ-alumina coating fully covers the glass as shown in Fig. 6, indicating that the content of γ-alumina doesn’t influence the transmittance as shown in Fig. 13(a). For thin film γ-alumina coating its transmittance can reach 96.8% at wavelength 550 nm [43]; however, the coating of γ-alumina nanoparticle with higher specific surface area (SSA)
4. Conclusion Graphene/γ-alumina ink is developed for energy efficient glass by simple spray coating. The poor thermal conduction of graphene out of plane is the advantage for thermal barrier. The thermal behavior of graphene/γ-alumina coated glass shows that the temperature difference between inside the adiabatic closed system and the glass surface can achieve 20 � C after lamp radiation testing for 15 h. Radiation heat is 9
L.-H. Hu and S.-H. Kao
Solar Energy Materials and Solar Cells 203 (2019) 110199
Fig. 13. (a) the transmittance measurement of graphene/γ-alumina and pure ECG coatings (b) the absorbance of graphene/γ-alumina and pure ECG coatings.
retarded by graphene/γ-alumina coating. The content of electrochemically exfoliated graphene (ECG) is the key factor to adjust the transmittance and absorbance and the γ-alumina substantially enhances the thermal behavior. The transmittance is adjustable from 5.57%~81% at 550 nm wavelength, depending on the composition and content of graphene/γ-alumina and they are good UV absorbents. Graphene/ γ-alumina coated glass provides good thermal barrier and optical properties. Consequently, it is a potential green material for energy coating with low cost and simple coating process to replace the current low-E glass in near future.
[4] H.T. Chou, Y.C. Chen, C. YLeea, H.You Chang, N.H. Tai, Switchable transparency of dual-controlled smart glass prepared with hydrogel-containing graphene oxide for energy efficiency, Sol. Energy Mater. Sol. Cells 166 (2017) 45–51. [5] A. Seeboth, R. Ruhmann, O. Muhling, Thermotropic and thermochromic polymer based materials for adaptive solar control, Materials 3 (2010) 5143–5168. [6] H. Watanabe, Intelligent window using a hydrogel layer for energy efficiency, Sol. Energy Mater. Sol. Cells 54 (1998) 203–211. [7] X.Q. Gong, J.X. Li, S.Y. Chen, W.J. Wen, Copolymer solution-based smart window, Appl. Phys. Lett. 95 (2009) 1907. [8] T. Gyenes, A. Szilagyi, T. Lohonyai, M. Zrinyi, Electrically adjustable thermotropic windows based on polymer gels, Polym. Adv. Technol. 14 (2003) 757–762. [9] T. Fischer, R. Lange, A. Seeboth, Hybrid solar and electrically controlled transmission of light filters, Sol. Energy Mater. Sol. Cells 64 (2000) 321–331. [10] B. Petter, J. Simen, E. Kalnæs, T. Gao, Low-emissivity materials for building applications: a state-of-the-art review and future research perspectives, Energy Build. 96 (2015) 329–356. [11] S.S. Ghosh, P.K. Biswas, S. Neogi, Thermal performance of solar cooker with special cover glass of low-e antimony doped indium oxide (IAO) coating, Appl. Therm. Eng. 113 (25) (2017) 103–111. [12] M. Singh, T.R. Rana, S. Kim, K. Kim, J. Yun, J. Kim, Silver nanowires binding with sputtered ZnO to fabricate highly conductive and thermally stable transparent electrode for solar cell applications, ACS Appl. Mater. Interfaces 8 (20) (2016) 12764–12771. [13] G. Gruner, Carbon nanotube films for transparent and plastic electronics, J. Mater. Chem. 16 (2006) 3533–3539. [14] H.-Z. Geng, K.K. Kim, K.P. So, Y.S. Lee, Y. Chang, Y.H. Lee, Effect of acid treatment on carbon nanotube-based flexible transparent conducting films, J. Am. Chem. Soc. 129 (2007) 7758–7759. [15] L. Gomez De Arco, Y. Zhang, C.W. Schlenker, K. Ryu, M.E. Thompson, C. Zhou, Continuous, highly flexible, and transparent graphene films by chemical vapor deposition for organic photovoltaics, ACS Nano 4 (2010) 2865–2873. [16] S. Bae, H. Kim, Y. Lee, X. Xu, J.-S. Park, Y. Zheng, J. Balakrishnan, T. Lei, H.R. Kim, Y. Il Song, Y.-J. Kim, K.S. Kim, B. Ozyilmaz, J.-H. Ahn, B.H. Hong, S. Iijima, Rolltoroll production of 30-inch graphene films for transparent electrodes, Nat. Nanotechnol. 5 (2010) 574–578. [17] J. Zou, H.L. Yip, S.K. Hau, A.K.Y. Jen, Metal grid/conducting polymer hybrid transparent electrode for inverted polymer solar cells, Appl. Phys. Lett. 96 (2010) 203301. [18] C.H. Jeon, Y.H. Jeong, J.J. Seo, H.N. Tien, S.T. Hong, Y.J. Yum, S.H. Hur, K.J. Lee, Material properties of graphene/aluminum metal matrix composites fabricated by friction stir processing, Int. J. Precis. Eng. Manuf. 15 (2014) 1235–1239. [19] F.H. Latief, E.S.M. Sherif, Effects of sintering temperature and graphite addition on the mechanical properties of aluminum, J. Ind. Eng. Chem. 18 (2012) 2129–2134. [20] J. Liu, U. Khan, J. Coleman, B. Fernandez, P. Rodriguez, S. Naher, D. Brabazon, Graphene oxide and graphene nanosheet reinforced aluminium matrix composites:
Acknowledgments This research project is funded by the Ministry of Science and Technology, Taiwan with the project number: MOST 108-2221-E-110075 and 107-2221-E-218-002. The technical support of electrochemically exfoliated graphene is from Nitronix Nanotechnology cor poration, Taiwan. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.solmat.2019.110199. References [1] A.C. Rotzetter, C.M. Schumacher, S.B. Bubenhofer, R.N. Grass, L.C. Gerber, M. Zeltner, W.J. Stark, Thermoresponsive polymer induced sweating surfaces as an efficient way to passively cool buildings, Adv. Mater. 24 (2012) 5352–5353. [2] D.H.W. Li, J.C. Lam, C.C.S. Lau, T.W. Huan, Lighting and energy performance of solar film coating in air-conditioned cellular offices, Renew. Energy 29 (2004) 921–937. [3] R. Baetens, B.P. Jelle, A. Gustavsen, Properties, requirements and possibilities of smart windows for dynamic daylight and solar energy control in buildings: a stateof- the-art review, Sol. Energy Mater. Sol. Cells 97 (2010) 87–105.
10
L.-H. Hu and S.-H. Kao
[21] [22] [23] [24]
[25]
[26] [27] [28]
[29] [30] [31]
Solar Energy Materials and Solar Cells 203 (2019) 110199 [32] X.S. Li, Y.W. Zhu, W.W. Cai, M. Borysiak, B.Y. Han, D. Chen, R.D. Piner, L. Colombo, R.S. Ruoff, Transfer of large-area graphene films for high-performance transparent conductive electrodes, Nano Lett. 9 12 (2009) 4359–4363. [33] C. Lee, X.D. Wei, J. Kysar, W.J. Hone, Measurement of the elastic properties and intrinsic strength of monolayer graphene, Science 321 5887 (2008) 385–388. [34] Z.H. Ni, H.M. Wang, J. Kasim, H. Fan, M.T. Yu, Y.H. Wu, Y.P. Feng, Z.X. Shen, Graphene thickness determination using reflection and contrast spectroscopy, Nano Lett. 7 9 (2007) 2758–2763. [35] X.X. Ni, G.C. Liang, J.S. Wang, B.W. Li, Disorder enhanceds thermoelectrics figure of merit in armchair graphene nanoribbons, Appl. Phys. Lett. 95 (2009) 192114. [36] C.Y. Su, Y.P. Xu, W.J. Zhang, C.H. Tasi, J.W. Zhao, X.H. Tang, L.J. Li, Electrical and spectroscopic characterizations of ultra-large reduced graphene oxide monolayers, Chem. Mater. 21 23 (2009) 5674–5680. [37] J.H. Chen, C. Jang, S.D. Xiao, M. Ishigami, M.S. Fuhrer, Intrinsic and extrinsic performance limits of graphene devices on SiO2, Nat. Nanotechnol. 3 (2008) 206–209. [38] J.G. Checkelsky, N.P. Ong, The thermopower and nerst effect in graphene in a magnetic field, Phys. Rev. B 80 (2009), 081413(R). [39] A.C. Ferrari, J.C. Meyer, V. Scardaci, C. Casiraghi, M. Lazzeri, F. Mauri, S. Piscanec, D. Jiang, K.S. Novoselov, S. Roth, A.K. Geim, Raman spectrum of graphene and graphene layers, Phys. Rev. Lett. 97 (18) (2006) 187401. [40] A.K. Geim, K.S. Novoselov, The rise of graphene, Nat. Mater. 6 (3) (2007) 183–191. [41] J. Kim, J. Kim, D. Jerng, E. Kim, H. Ahn, Effect of aluminum oxide and reduced graphene oxide mixtures on critical heat flux enhancement, Int. J. Heat Mass Transf. 116 (2018) 858–870. [42] L.M. Lawton, N.H. Mahlmeister, I.J. Luxmoore, G.R. Nash, Prospective for graphene based thermal mid-infrared light emitting devices, AIP Adv. 4 (2014), 087139. [43] J. Park, K. Park, D. Spoor, B. Hall, Y. Song, Hand-manageable graphene sticker for ultrafast mode-locked fiber lasers, Opt. Express 23 (2015) 7940–7945. [44] R.R. Nair, P. Blake, A.N. Grigorenko, K.S. Novoselov, T.J. Booth, T. Stauber, N.M. R. Peres, A.K. Geim, Fine structure constant defines visual transparency of graphene, Science 320 (2008) 1308. [45] J.R. Piper, S. Fan, Total Absorption in a Graphene Monolayer in the Optical Regime by Critical Coupling with a Photonic Crystal Guided Resonance ACS Photonics vol. 1, 2014, pp. 347–353.
powder synthesis and prepared composite characteristics, Mater. Des. 94 (2016) 87–94. J. Wang, Z. Li, G. Fan, H. Pan, Z. Chen, D. Zhang, Reinforcement with graphene nanosheets in aluminum matrix composites, Scr. Mater. 66 (2012) 594–597. M. Rashad, F. Pan, A. Tang, M. Asif, Effect of Graphene Nanoplatelets addition on mechanical properties of pure aluminum using a semi-powder method, Prog. Nat. Sci.: Mater. Int. 24 (2014) 101–108. X. Gao, H. Yue, E. Guo, H. Zhang, X. Lin, L. Yao, B. Wang, Preparation and tensile properties of homogeneously dispersed graphene reinforced aluminum matrix composites, Mater. Des. 94 (2016) 54–60. Z. Li, Q. Guo, Z. Li, G. Fan, D.B. Xiong, Y. Su, J. Zhang, D. Zhang, Enhanced mechanical properties of graphene (reduced graphene oxide)/aluminum composites with a bioinspired nanolaminated structure, Nano Lett. 15 (2015) 8077–8083. Z. Li, G. Fan, Z. Tan, Q. Guo, D. Xiong, Y. Su, Z. Li, D. Zhang, Uniform dispersion of graphene oxide in aluminum powder by direct electrostatic adsorption for fabrication of graphene/aluminum composites, Nanotechnology 25 (2014) 325601. C. Hu, X. Zhai, L. Liu, Y. Zhao, L. Jiang, L. Qu, Spontaneous reduction and assembly of graphene oxide into three-dimensional graphene network on arbitrary conductive substrates, Sci. Rep. 3 (2013) 2065. Z. Fan, K. Wang, T. Wei, J. Yan, L. Song, B. Shao, An environmentally friendly and efficient route for the reduction of graphene oxide by aluminum powder, Carbon 48 (2010) 1686–1689. L. Zhang, G. Hou, W. Zhai, Q. Ai, J. Feng, L. Zhang, P. Si, L. Ci, Aluminum/ graphene composites with enhanced heat-dissipation properties by in-situ reduction of graphene oxide on aluminum particles, J. Alloy. Comp. 748 (5) (2018) 854–860. C.Y. Su, A.Y. Lu, Y. Xu, F.R. Chen, A.N. Khlobystov, L.J. Li, High-quality thin graphene films from fast electrochemical exfoliation, ACS Nano 5 (3) (2011) 2332–2339. A. Gupta, G. Chen, P. Joshi, S. Tadigadapa, P.C. Eklund, Raman scattering from high-frequency phonons in supported n-graphene layer films, Nano Lett. 6 (12) (2006) 2667–2673. B. Krauss, T. Lohmann, D.H. Chae, M. Haluska, K. von Klitzing, J.H. Smet, Laserinduced disassembly of a graphene single crystal into a nanocrystalline network, Phys. Rev. B 79 16 (2009) 165428.
11