3RD INTERNATIONAL CONGRESS ON TECHNOLOGY - ENGINEERING & SCIENCE - Kuala Lumpur - Malaysia (2017-02-09)

The Study Of An Improvement Of Thermal Conductivity And Dispersibility Of Nanofluids Containing Graphene.

With a stability of chemical structure and low cost, graphite, carbon nanotube and graphene are essential for nanofluids applications. [1] Above all, graphene has high yield properties and thermal conductivity. Therefore graphene nanofluids (GN) is compatible material.[2] Over the few decades, nanofluids have remarkably improved thermal conductuvuty and heat-tansfer coefficient, as well as reduced power consumption and costs.[3-8] However graphene is a flat monolayer of sp2 bonded carbon atoms tightly packed into a honeycomb lattice.[9] High performance of graphene isn’t effectively revealed with a water-based fluid in the event that graphene doesn’t completely diffuse.[10] The variance method of graphene have presented around academic world until today. For better floating and dispersion, a surfactant is added like SDBS (sodium dodecylbenzene sulfonate) and SDS (sodium dodecyl sulfate). But these additives degrade thermal and electrical conductivity. And many methods have been applied to prepare graphene, such as sonication in various solvents, solvothermal synthesis, micromechanical methods and CVD (chemical vapour deposition). [11-13] Because there are many advanced research about chemical method such as functionization, this paper proposes ball milling method for an improvement of equational dispersibility. In this study, graphene was grinded by planetary ball mill as an experimental parameter. It is the simple and convenient means to produce particles arbitrarily. [14] The particle size was measured by PSD equipment for comparing characteristic of grinded graphene following ball mill conditions. Then thermal conductivity was measured using Transient Hot-wire method and absorbance was measured by UV equipment in order to measure dispersions of graphene particles. Test parameters were determined as rotation speed and time. This parameters affect graphene particle during ball mill process. Rotation speed is 200, 400, 600 rpm and working time is 30, 60, 90 min, respectively. Then, the grinded graphene in nanofluids was dispersed by ultrasonic excitation (1510E-DTH, Branson Ultrasonic Corporation 41, Danbury, CT 06813, USA). The absorbnce was measured to confirm dispersibility of particle by UV-visible (uv-vis, x-ma 3000, human corporation, South Korea). The thermal conductivity of the graphene nanofluids was measured by using the measurement device (LAMBDA, F5 Technologie GmbH, Willingshausen, Germany) employing the transient hot-wire method. Temperature were ranged from 20℃ to 40℃ by the spacing of 4℃. The PSD (Zetasizer, Nano-S90, Malvern Instruments Ltd, UK) was also used to compare the size of each graphene samples. In the planetary ball mill measurement, it can make strong impact energy at low rotation speed because containers and disk were rotated in the opposite direction at same time. (Figure.1) Therefore, the low rpm with optimal grinding time is the most efficient. In result of after grinding the graphene, we found the particle size decreasing surely comparing with raw graphene. Due to this result, we expect the higher thermal conductivity by increasing the specific surface area. In the absorbance result, we confirm that the absorbance of the processing of graphene is higher than the absorbance of the raw graphene. When the rpm is 200, the absorbance of 60 minutes is the highest in the graph of figure.2 (a). The higher absorbance, the higher thermal conductivity in both graph of Figure.2 (a) and 2(b). Therefore, by decreasing the graphene size, the thermal conductivity and the absorbance increase in the graphs. Stability and thermal properties of graphene nanofluids have been studied systematically, and the following conclusions is revealed. When the revolution is 200rpm, the outcome of the thermal conductivity and of the absorbance is the highest. These results are consistently consistent with the results of the particle size. The measurement of absorbance implement to check the dispersibility of the grinding graphene. Overall the lower the rpm the bigger absorbance. The processing of graphene decreased the smallest also the thermal conductivity is lower than the original graphene, from the experimental condition of the duration of 90 minutes and with 200rpm of rotation. Thus the high number of revolution and the long experimental time are not suitable for the grinding conditions of the graphene. Therefore the high thermal conductivity and absorbance are relatively obtained in the both conditions of the smooth disintegration with long experimental time and the high speed with short measurement duration.
Ji-Hye Kim, Gwi-Nam Kim, Joun-Sung Park, Sun-chul Huh