1. Introduction
Graphene has better electrical conductivity [1, 2], thermal conductivity [3, 4], and mechanical strength than such metals as copper and aluminum [5, 6], and exhibits optically transparent nature [7, 8]. Graphene has a structure of stacking honeycomb layers. In order to maintain ideally the characteristics, it should be formed with single layer and have large area. Graphene has such good characteristics, so it is suitable for semiconductors [9], nuclear fusion reactors [10], base material of secondary batteries [11], base material of clear electrodes for displays and cellphones [12], and it is expected that applications of graphene will be broadened [13, 14]. In particular, since graphene has good electrical conductivity and transparency, it is an ideal material for transparent electrodes of display devices.
Chemical vapor deposition is a general method for manufacturing large-area graphene [15, 16]. However, there have been difficulties in commercializing the manufacturing method because it requires high-cost despite of low productivity [17, 18]. Nevertheless, research for applying graphene to transparent electrodes is widely progressed [19, 20], because graphene manufactured by chemical vapor deposition is suitable for transparent electrodes of displays and cellphones which require high conductivity and visible ray transparency [21, 22]. On the other hand, graphene having single layer and large area has not only good electrical conductivity but also good thermal conductivity [23, 24]. However, it has low thermal capacity because it is thin. Therefore, graphene is difficult to use as a heat radiator for radiating high heat.
Graphite having a graphene structure is suitably applicable to heat radiators [25, 26]. Graphite has a structure of stacking honeycomb layers and is abundant in nature. In recent years, graphite is widely applied to heat radiators for electrical devices [27, 28]. However, there are many technical difficulties in acquiring the same characteristics of graphene from the natural graphite [29, 30]. Graphite must be exfoliated to be thin and large-area, in order to have characteristic of thermal radiation similar to graphene [31, 32]. More specifically, for the sake of using graphite as a heat radiator, it is required to make the graphite powder as thin and wide as possible and manufacture the heat radiator using the graphite powder in such a large volume as to appropriately maintain the temperature of the heat radiator [33, 34]. A conventional method of manufacturing graphite powder is mechanically grinding graphite into fine powder [35, 36]. Another method is a chemical method that involves oxidizing graphite powder at high temperature to obtain an expanded graphite structure [37, 38]. However, there are technical limitations in obtaining graphite powder as thin and wide as graphene according to the mechanical grinding method or the chemical method.
Since graphite powder for heat radiator as manufactured by the mechanical grinding method depends on the particle size of the graphite, the shape of particle is generally irregular and similar to stone fragments. Generally, the graphite powder manufactured by the mechanical grinding method is used as its particle size is 40 microns or less [39, 40]. As such, the graphite powder mechanically ground has various advantages that high density is guaranteed when it is spread or molded and that the size of particle can be controlled according to its use purpose [41, 42]. However, it is difficult to control the thickness of the particle, that is, the thickness of a honeycomb layer, which is stacked layer by layer [43, 44]. Also, the method has the advantage that proportion of pores is low owing to high density [45, 46]. On the other hand, the method has the disadvantage that electrical conductivity and thermal conductivity decrease with an increase in the contact resistance between powder particles.
The chemical method of manufacturing graphite powder for heat radiator using oxidation-reduction reaction has advantage in terms of high productivity [47, 48]. However, it has such a disadvantage that it may cause environmental pollution [37, 38]. Further, it is difficult to produce high purity graphite because of poor manufacturing environment [39, 40]. Graphite oxidized under high temperature condition by the chemical method become powder that is bulky and has very low density in the case of expansion. This powder form of graphite is used as a sheet for heat radiation of electronic devices. The expanded graphite powder of which particle size is 300 microns or less is mainly used for heat radiation purpose [41, 42]. The expanded graphite powder is microscopically similar to bellows in shape. Macroscopically, the expanded graphite is cotton-shaped and light-weighted [33, 34]. However, when the graphite powder expands in volume by the oxidization process under high temperature condition, the gaps between layers crack like bellows to generate many pores [35, 36]. Therefore, the particles of the expanded graphite powder are bigger and thinner than those of the graphite powder manufactured by grinding natural graphite. Consequently, the expanded graphite powder has high electronic conductivity and thermal conductivity [49, 50] but contains many pores between particles to cause deterioration in the thermal conductivity.
In order to solve such a problem as deterioration in the characteristics of the expanded graphite powder caused by the abundance of pores, there have been applied improvement methods such as compressing the expanded graphite powder with high pressure or using the expanded graphite powder in combination with a graphite powder of small particle size or a conductive metal powder [39, 40]. Those methods, however, have an insignificant effect of improving the characteristics [39, 40]. For the sake of making efficient heat radiators using a graphite material, it is required to exfoliate the graphite material as thin and wide as possible to secure good characteristics in terms of electrical conductivity and thermal conductivity as similar to graphene and also to adopt a method of minimizing generation of pores that greatly deteriorate electrical conductivity and thermal conductivity [27, 28]. In particular, the conventional methods such as the mechanical grinding method and the chemical method have technical difficulties in obtaining a graphite powder thin and, wide like graphene [27, 28] and limitations in solving the problem with the abundance of pores between particles.
Currently, the application thickness of the graphite powder is normally 25 microns or greater for radiant sheets used to radiate heat from cellphones and one millimeter or greater for radiant sheets used to radiate heat from electronic devices [41, 42]. In order to reduce the thickness of the radiant sheets used for those electronic devices and enhance the efficiency of heat radiation, it is necessary to make the graphite powder thin and wide and minimize the existence of pores between the particles of the graphite powder [41, 42]. For this purpose, there is a demand for a technique to exfoliate the graphite powder thin and wide like graphene [51, 52]. Contrived in consideration of the problems, it is necessary to provide a method for manufacturing graphene and a method for manufacturing a conductor that solve the problems regarding insufficient exfoliation of the graphite powder and deterioration in the electrical conductivity and thermal conductivity caused by the small particle size [53, 54]. It is also necessary to provide a method for manufacturing graphene and a method for manufacturing a conductor that solve the problem regarding deterioration in the electrical conductivity and thermal conductivity caused by the pores existing between the particles and the environmental problem [55, 56]. It is therefore of great importance to provide a method for manufacturing graphene and a method for manufacturing a conductor that improve the characteristics of the graphite material, in comparison with the conventional methods such as the mechanical grinding method and the chemical method.
Conventional heat transfer fluids play an important role in many industries including power generation, chemical production, air conditioning, transportation, and microelectronics. However, their inherently low thermal conductivities have hampered the development of energy-efficient heat transfer fluids that are required in a plethora of heat transfer applications. However, the heat transfer properties of these conventional fluids can be significantly enhanced by dispersing nanometer-sized solid particle and fibers, namely nanoparticles, in fluids. This new type of heat transfer suspensions is known as nanofluids. Such nanofluids may be engineered by dispersing metallic or nonmetallic nanoparticles in traditional base fluids. The thermal conductivity of heat transfer fluids can be enhanced by adding highly conductive particles. Depending on the application, nanofluids are generally classified such as heat transfer nanofluids, anti-wear nanofluids, metalworking nanofluids, coating nanofluids, and chemical nanofluids. Nanofluids can significantly improve heat transfer characteristics compared with the base fluids. Nanofluids have the potential to improve heat transfer and energy efficiency in thermal systems in applications such as microelectronics, power electronics, transportation, nuclear engineering, heat pipes, refrigeration, air-conditioning, and heat pump systems. The use of nanofluids in a wide variety of applications is promising, but poor suspension stability of nanoparticles in the solution hinders the further development of nanofluids applications. The present study aims to provide a fundamental understanding of the thermal and viscosity properties of inhomogeneous fluids with suspended graphene nanoparticles. Particular emphasis is placed upon the effect of nanoparticle volume fraction on the material properties of inhomogeneous fluids with suspended graphene nanoparticles.