Figure 4. Raman spectra of the graphene that solves the problems regarding insufficient exfoliation of the graphite powder and deterioration in the electrical conductivity and thermal conductivity.
The effect of temperature on thermal conductivity of the graphene in purified water is illustrated in Figure 5 for which the material consists of stacks of graphene sheets having a platelet shape. The quest for thermal systems that are more efficient in removing heat is never ending, with design improvements of heat exchangers being one of the main areas of focus. Water is the working fluid of choice for years for several reasons; it is clean, readily available and has fairly good thermal properties for heat removal. Over a century ago, micro-sized particles with high thermal conductivity are used as a way to increase the thermal characteristics of working fluids [57, 58]. However, micro-sized particles can be abrasive and can precipitate out due to their higher density. Once the nanoparticles settle, the nanofluid loses its property enhancement. Nano-sized particles introduced into a base liquid are also used and called nanofluids. This concept of using copper, aluminum, or carbon nanoparticles to create colloidal suspension fluids has been accepted as a new avenue for enhancing the thermal characteristics [59, 60]. Since heat transfer can be enhanced due to particle size and dispersion isometry, the key technical challenge in implementing this technology is to understand the fundamental physics responsible for enhancing the transport of heat, which can lead to the knowledge-based development of a stable nanofluid with maximum thermal conductivity. Due to the necessity of compact thermal management systems, many researchers have begun to investigate the benefits of the nanofluids on the heat transfer in the thermal management system [61, 62]. Scientists have reported varying degrees of increase in thermal performance with the addition of the nanoparticles to the thermal fluid [63, 64]. The earlier studies are primarily on the enhancements of the thermal conductivity [65, 66]. Most studies report very good enhancements of the thermal conductivity even with small volume percent concentrations [67, 68]. Researchers have investigated the addition of nanoparticles made of highly conductive materials such as aluminum, carbon, diamond, and copper with varying positive results [69, 70]. Many prior researchers have focused on the use of copper oxide nanoparticles to form the nanofluid due to the favorable thermal properties of copper oxide powders [71, 72]. However, nanofluids formed with copper oxide suffer from several drawbacks that can impede its commercial use in a thermal management system [73, 74]. For example, fluids containing copper oxide nanoparticles have a tendency to mix with and retain air and oxygen within the fluid, which adversely affects the thermal properties of the fluid and can create problems in the thermal system [75, 76]. Additionally, the copper oxide nanoparticles tend to agglomerate and stick to the container of the fluid in the thermal system, which can lead to impairment and fouling of fluid flow in the system [77, 78]. In the present study, the nanofluid includes graphene nanoparticles suspended in a base liquid at a nanoparticle concentration in the nanofluid of about 0.008 percent by volume. Graphene nanoparticles are an excellent method to improve thermal conductivity in water. Graphene-containing nanofluids provide several advantages over the conventional fluids, including thermal conductivities far above those of traditional solid-liquid suspensions, a nonlinear relationship between thermal conductivity and concentration, strongly temperature-dependent thermal conductivity, and a significant increase in critical heat flux. Each of these features is highly desirable for thermal systems and together makes nanofluids strong candidates for the next generation of heat transfer fluids. The observed substantial increases in the thermal conductivities of nanofluids can have broad industrial applications and can also potentially generate numerous economic and environmental benefits. Enhancement in the heat transfer ability could translate into high energy efficiency, better performance, and low operating costs. The need for maintenance and repair can also be minimized by developing a nanofluid with a better wear and load-carrying capacity. Consequently, classical heat dissipating systems widely used today can become smaller and lighter, thus resulting in better fuel efficiency, less emission, and a cleaner environment. Nanoparticles of various materials can be used to make heat transfer nanofluids, including copper, aluminum, copper oxide, alumina, titania, and graphene. Of these nanoparticles, graphene shows greatest promise due to their excellent chemical stability and extraordinary thermal conductivity.