Invention Journal of Research Technology in Engineering & Management (IJRTEM) ISSN: 2455-3689 www.ijrtem.com Volume 1 Issue 12 ǁ September. 2017 ǁ PP 18-24
Grinding graphene characteristics after physical process 1,
Ji-Hye Kim , 2,Gwi-Nam Kim , 3,Jung-Pil Noh , 4,HyoMin Jeong , 5,Sun-Chul Huh
1,2,3,4,5,
(Department of Energy and Mechanical Engineering, Gyeongsang National University, Republic of Korea)
ABSTRACT : Graphene features higher thermal conductivity than copper. However, despite its superior property, the research on its applicable technology was limited since the van der Waals’ forces between graphene. As a solution to such problem, research on making graphene distributed evenly in solvent is being actively conducted via physical and chemical method. Because the chemical method is likely to have harmful effect on the environment, we used the environmental-friendly process that does not consume toxic chemicals, and suitable for application. In this study, ball milling process controllable a range of experiment conditions more easily and conveniently than other physical methods was conducted so as to disperse graphene evenly in solvent and improve the thermal conductivity. Therefore, the effect of milling process was confirmed in TEM image and Raman ratio, and the shearing force makes the edge of graphene piece defective. When graphene is evenly dispersed, the wide specific surface area absorbs a great deal of light, improving absorbance. We confirmed the absorbance of pristine graphene was showed below milling graphene and considerable thermal conductivity increase compared to pristine graphene.
Keywords: Ball milling, Graphene, Nano-fluid, Physical process, Thermal conductivity I. INTRODUCTION Theoretical background: Emerging as a topic of nano scientific technology in 21st century, a range of nano carbon materials including graphene, carbon nano tube and carbon nano fiber became familiar terms even to the general public and symbol of scientific technology in the future. Graphene features having wider specific surface area, 200 times higher mechanical strength than steel but as much flexibility, 100 times higher electrical conductivity than copper, more superior thermal conductivity (around 5000W/mk) than diamond, optical characteristic of transmitting 97.7% of incident-light and high stability in structural and chemical way. Graphene is also recognized as the highest potential core material that can replace the existing electronic device based on silicon as the mobility of its electron in room temperature is 100 times as high as silicon, exceeding 200,000cm2/V᛫s. Since diverse chemical functionalization of graphene is possible, the possibility of controlling its superior property was presented in a range of methodology, resulting in active research on its application across industries such as electronic device, optical device, electrically conducting composite material, sensor, clear electrode, transistor, organic solar cell, and composite material with ultra-light ᛫ high intensity. [1-5] However, despite its superior property, the research on its applicable technology was very limited since the van der Waals’ forces between graphene and its very stable chemical structure prevent graphene from being evenly distributed in solvent, making it difficult to manufacture graphene nano-fluid with uniform feature. As a solution to such problem, research on making graphene distributed evenly in solvent is being actively conducted via physical and chemical method. The chemical method is separating graphene in solution state chemically via oxidant. Graphene oxide oxidized with strong acid and oxidizer assumes strong hydrophilic property and is separated with ease via ultrasonic waves. The graphene oxide then undergoes the reduction process additionally, turning into reduction graphene with structure and property equivalent to almost original graphene though in not perfect state. The chemical method is also likely to have harmful effect on the environment in that the research on applicable fields of graphene material will increase, and is unreasonable in terms of cost, thus generating active research on environmental-friendly, inexpensive and novel stripping method than can replace the chemical method. [6-8] There is a manufacture method of directly distributing graphene via surfactant without going through oxidation process. The method, however, has no substantial effect due to the inter-layer resistance of nano-scale graphene pieces, and has fatal disadvantage that the dispersion in nano-fluid is not improved than the chemical method and others. [9] Meanwhile, the dispersed graphene in solution made by the chemical method has the advantage of being employed in various ways by forming compound with other substances. Though reduced graphene assumes less electrical property than that made by mechanical method, it is differentiated from other manufacture processes in its applicability such as solution process with low production cost, the film thin and easy for mass production, and complex formation with other substances. The recent several studies revealed that the layers of carbon allotropes are comparatively easily separated in the environment with shearing force acted, and if proper surface stabilizer is employed at this stage in order to prevent re-cohesion, carbon allotropes are separated without chemical process to the extent of at least three layers of graphene being stuck together.
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Grinding graphene characteristics after‌ The typical ways to separate graphene physically via shearing force include dry milling method using hard particle, wet milling method using both particle and solvent, and the method using supercritical state inducing explosive pressure change. Such methods make it possible to embody environmental-friendly process that does not consume toxic chemicals, and are very suitable for application using a great deal of graphene. As it is reported that the properties of graphene known theoretically can be exhibited in reality, many scientists are actively studying on its applications across industries, pouring out performances ready for commercialization. However, since the pure graphene is a hydrophobic material insoluble to most solvent, the reasonable and stable process suitable for mass production should be employed in dispersing and floating graphene evenly in solvent in order to apply its superb properties to industrial settings. [10, 11] Purpose of study: In this study, ball milling process controllable a range of experiment conditions more easily and conveniently than other physical methods was conducted so as to disperse graphene evenly in solvent and improve the resulting thermal conductivity. With each ball diameter(1mm and 3mm), milling speed (200rpm, 400rpm, and 600rpm) and milling time (30min, 60 min) applied, reducing the cohesive phenomenon of graphene was attempted, and features under each condition such as thermal conductivity of nano-fluid containing graphene were compared and their correlation was evaluated.
II. EXPERIMENTAL RESULTS TEM result: After ball milling process, whether graphene was milled was conformed via TEM prior to evaluating the effect of milling. Graphene was observable via optical microscope due to the effect of light interference arising from multiple reflection of graphene composed of 1 layer of carbon atom. In the Fig. 1 showing TEM images of pristine graphene and typical graphene gone through ball milling process, the palest part corresponds to single layer. In the Fig. 1 (a) showing pristine graphene, the thin layers of graphene were stuck together, and in the images of milled graphene of Fig. 1 (c), each layer of graphene was milled and dispersed via ball milling. It was also revealed through the 1,000,000 times of image magnification that the pattern of edge of a piece of graphene of Fig. 1 (b) was defective, which changed the pattern of the graphene particle as in Fig. 1 (d) via ball milling process. Therefore, the effect of ball milling process on the milling of graphene was conformed in TEM image, and the shearing force by ball milling makes the edge of graphene piece defective. [12, 13]
(a)
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Grinding graphene characteristics after‌
(c)
(d)
Figure 1. TEM images: (a) A mass of the pristine graphene; (b) An edge of the pristine graphene; (c) A mass of the milling graphene; (d) An edge of the milling graphene. Thermal conductivity result: In this study, thermal conductivity of graphene nano-fluid based on distilled water was assessed in the 20~40°C of room temperature via thermal conductivity measuring equipment of Transient Hot water Technique. Prior to measuring thermal conductivity, the measuring equipment was calibrated by comparing the thermal conductivity of distilled water with the standard value. [14, 15] The result of thermal conductivity of the 10 samples depending on ball diameter, milling speed and milling time, and pristine graphene before conducting ball milling process was presented in Fig. 2. The thermal conductivity of pristine graphene without experiencing any process increased very subtly by 0.01% than that of distilled water used in nano-fluid. Given the superior thermal property of graphene, the result was not satisfactory. It was revealed that when the pristine graphene alone was dispersed in solvent, it was not possible to utilize the thermal, electrical and mechanical properties of graphene. In the condition of applying ball milling method, the thermal conductivity increased the most at 60 min. of milling time and 200rpm of milling speed regardless of the diameter of zirconia ball. When using 1mm size, it generally showed higher resulting thermal conductivity than 3mm size. The extent of increase was 57E+6W/m*k and 46E+6W/m*k respectively compared to distilled water, showing considerable increase compared to pristine graphene. The thermal conductivity under the other conditions also increased compared to pristine graphene.
Figure 2. The result of the thermal conductivity Raman Spectrometer result: The analysis on the optical property of graphene plays a pivotal role in the study on graphene. The data that is not normally accessible optically due to its unique properties can be obtained via spectroscopy method, so the research on its optical property has been conducted from the beginning of research. The Raman spectroscopy among others is the most essential analysis method in the study of graphene as it is believed to be the surest way of identifying the single-layer graphene. Rayleigh scatter means that when light travels through material, the light is scattered by the interaction of light and material, bringing along
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Grinding graphene characteristics after‌ incident energy with it, whereas Raman scatter means that phonon is released or absorbed by lattice vibration, being scattered by losing or getting as much energy. In particular, stokes Raman scatter meaning that light is scattered in as low energy as phonon energy when phonon is released is used in most of the Raman spectroscope experiment. The information on crystal structure of material can be obtained typically via Raman spectroscope analysis. [16, 17] The graphitic materials such as graphite, carbon nanotubes, fullerene and graphene are based on hexagonal lattice, so Raman spectrum is considerably similar. Fig 3 indicates Raman spectrum graph of pristine graphene, Sample 3 and Sample 8 which has the high thermal conductivity according to diameter. The Fig. 3 shows that the most prominent Raman spectrum appeared in graphene is G peak around 1580 and 2D peak around 2700. [18, 19] These peaks are observed in carbon nanotubes, and graphitic materials have them in common. In particular, D peak, which can appear around 1340, is shown when the hexagonal structure of carbon atom is defective or when the edge of graphene is measured. D peak is almost invisible but at the edge of graphene via dynamic stripping process, whereas D peak is intense in graphene synthesized with thermal chemical vapor deposition method. Therefore, how much graphene is defective is evaluated with D peak. In case of graphene with much defect within crystal, D peak is only observed at the edge of the material or when sample contains much defect. The small D peak of pristine graphene arises from the defect happened during the process of stripping graphene from graphite. [20-22] Sample 8 shows the relatively high peak when the shearing force acted to 3mm of zirconia during the ball milling process concentrated on the edge of single layer of graphene. The 2D peak in Fig. 3 is used for identifying the single-layer graphene. While G peak show little change in its shape and location in the Raman spectrum of double-layer graphene, 2D changes into complicated shape and becomes wider, and whose relative intensity to G peak becomes weak. The 2D peak assessed in this study all represents the peak features of single-layer of graphene. Raman spectroscope shows more obvious effect of ball milling process.
Figure 3. The result of the raman spectrum The ratio of ID/IG indicates the consequence of external chemical or physical force. As the ratio of ID/IG is closer to 0, the law material is closer to 100% of purity. [23] In the resulting Raman assessment, the ratio of ID/IG of Sample 3 was 0.36, the highest ratio, that of Sample 8 was 0.33, and that of pristine graphene showed 0.21, the least ratio.1. In the ball milling process, the condition of 60 min. and 200rpm of speed was the most optimal.
ID/IG
Table 1. The ratio of ID/IG. Sample 11 Sample 3 0.21 0.36
Sample 8 0.33
UV-Spectrophotometer result: In order to be used as nano-fluid in heat exchanger employed in industrial settings, superior thermal conductivity and the even dispersion and floating in fluid are very crucial factors. The thermal conductivity of nano-fluid manufactured via ball milling process, the peak value of Raman spectroscope and the assessed result of ID/IG ratio were compared. Along with that, the dispersibility of graphene was evaluated via UV experiment.
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Grinding graphene characteristics after‌ Fig. 4 indicates the absorbance graph of manufactured graphene nano-fluid. In the graph, peak is shown at 260nm of the inherent wavelength of graphene, and the dispersibility of graphene can be discerned by the result of absorbance. Basically, when graphene is evenly dispersed and floated in nano-fluid, the wide specific surface area absorbs a great deal of light, improving absorbance. The result of absorbance is proportional to that of thermal conductivity in Fig. 2. The pristine graphene shows the absorbance below 1.5, cohering and precipitating faster than other samples. Consequently, graphene was dispersed and floated evenly in nano-fluid via ball milling process, and the high thermal conductivity of nano-fluid was conformed. [24] In the conditions of 1mm ball, the absorbance of sample2 is the highest and next is sample 3 and sample 1. As a result of 3mm ball, the absorbance increases sample 10, 9, 6, 7 and 8, in sequence. Comparing with the resulting thermal conductivity, it is corresponded exactly.
Figure 4. The result of the UV spectrum
III. MATERIALS AND METHODS This study used graphene with 100m2/g of specific surface area, 99.9% of purity, 8nm of average thickness of a piece, 550nm of average particle size (150~3,000nm). In the milling process using planetary ball mill equipment, number of revolution, milling time, ball size and ball charge have a great impact on milling, and dry or wet condition also leads to varied milling effects. Each condition was applied to evaluate the effect of milling speed and milling time on the change of the properties of graphene in ball milling. To prevent the inter-layer cohesive force of graphene, mixing it with distilled water was established as wet condition. Also, 1mm and 3mm of zirconia balls were used in order to assess the effect of diameter of balls. Table 2 represents the conditions of ball milling, and nano-fluid was manufactured by using the identical graphene weight under each condition. Fig. 5 indicates the mimetic diagram of planetary ball mill equipment used in this study. This equipment with two ports is designed in the way the circular main rotary valve is rotated clockwise, and at the same time the two ports are rotated independently in the opposite direction of main rotary valve, anti-clockwise. [25-28] Table 2. The experimental conditions. Sample 1 2 3 4 5 6 7 8 9 10 11
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Ball diameter 1mm 1mm 1mm 1mm 1mm 3mm 3mm 3mm 3mm 3mm
Milling time Milling speed 30 minutes 400 rpm 30 minutes 600 rpm 60 minutes 200 rpm 60 minutes 400 rpm 60 minutes 600 rpm 30 minutes 400 rpm 30 minutes 600 rpm 60 minutes 200 rpm 60 minutes 400 rpm 60 minutes 600 rpm No milling(Pure Graphene)
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Grinding graphene characteristics after…
Figure 5. Schematic diagram of ball milling
IV. MATERIALS AND METHODS In this study, the nano-fluid used in heat exchanger was manufactured for application to industrial settings. Since the mass production of nano-fluid in chemical way using complex oxidation and reduction process generates environmental pollution, the experiment was conducted via physical way using zirconia ball with variables of ball size, milling speed and milling time in order to find out the optimal condition of nano-fluid. The results of conducting the experiment with 11 samples are as follows. ① ②
③
The effect of ball milling on the graphene milling was confirmed via the comparison of TEM images. Going through ball milling process, the layers of graphene stuck together due to Vander Waals’ force were milled and separated. The fact that ball milling process has an impact on nano-fluid was confirmed via Raman spectroscope, and the result of thermal conductivity was compared with that of UV absorbance. When the resulting thermal conductivity increases, both the absorbance and Raman ratio increase. The thermal conductivity was assessed in order to evaluate the performance of graphene as nano-fluid of heat exchanger. Though the increase of thermal conductivity in the experiment seems insignificant, there is significance in that the improvement of thermal conductivity was confirmed via simple and inexpensive process. In order to manufacture more efficient nano-fluid later on, the synergy effect of the improvement of thermal conductivity can be created by dispersing and milling graphene via relatively reasonable and safe ball milling process, and then by applying chemical functional group to the opened tip of each edge of graphene via shearing force of ball milling.
V. ACKNOWLEDGEMENTS This work was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and future planning (2015R1A2A2A01004579).
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