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.