**2. Introduction to nanofluids**

Among diverse techniques to cool down or maintain certain temperature in these systems, vanes, fins, or radiators as well as forced air/fluids thru cooling channels are being used, even though they are costly. Some equipment and devices use inexpensive conventional heat transfer fluids to intensify heat dissipation, such as water (DiW), ethylene-glycol (EG), oils, and other lubricants. However, the inherent limitation of these fluids is the relatively low thermal conductivity; water, for instance, is roughly three orders of magnitude less conductive than copper or aluminum (**Table 1**). What these conventional fluids lack in thermal conduc‐ tivity, however, is compensated by their ability to flow.

2D-Based Nanofluids: Materials Evaluation and Performance http://dx.doi.org/10.5772/63806 155


**1. General overview**

154 Two-dimensional Materials - Synthesis, Characterization and Potential Applications

devices and components.

mankind.

materials.

**2. Introduction to nanofluids**

tivity, however, is compensated by their ability to flow.

Energy, the input to impulse the world's future, has been pointed out as the most important issue facing humanity in the next 50 years and crucial to solve issues like mitigation of pollu‐ tion, global warming, among others [1]. For instance, among diverse forms of energy used, over 70% is produced in or through the form of heat [2]. Heat is transferred either to input energy into a system or to remove the energy produced in a system. Considering the rapid increase in energy demand worldwide, intensifying heat transfer process and thereby reducing energy loss are becoming increasingly important tasks. That is why thermal management plays a crucial factor concerning apparatuses or machines performance; thermal dissipation role has been subjected to many investigations and is under the scope of the operational useful life of these

Nanotechnology is a science that deals with diverse phenomenon's, properties', and materials' characteristics at the nanometric level (1 nanometer, nm = 10−9 m). Important and interesting discoveries have been realized in this field over the past 30 years. Among them, the discovery of Buckyball (C60) [3], carbon nanotubes (CNTs) [4], and graphene isolation and identification [5] are just to mention some of them. During the last couple of decades, diverse techniques, equipment, and instrumentations have been devised, as well as various relevant and interest‐ ing characteristics and properties of these materials were sort out for the betterment of

Energy management becomes crucial for meeting the rising needs of mankind [2]. Nowadays, with increasing pressure of globalized markets and companies' profit race, a dramatic search is carried out for obtaining proper material performance; optimizing components and devices designs; improving efficiencies; reducing tools wear, materials consumption, and pollution; and obtaining the most possible revenue. In addition to issues regarding scrap materials, maintenance, and components wear among others, a hot topic in industry is the heat dissipa‐ tion. Avoiding or reducing the use of resources for cooling equipment, or simply reflected in wear/friction issues among metal-mechanic processes, there is a high demand for successful heat management and energy-efficient fluid-based heat transfer systems, with aid of reinforced

Among diverse techniques to cool down or maintain certain temperature in these systems, vanes, fins, or radiators as well as forced air/fluids thru cooling channels are being used, even though they are costly. Some equipment and devices use inexpensive conventional heat transfer fluids to intensify heat dissipation, such as water (DiW), ethylene-glycol (EG), oils, and other lubricants. However, the inherent limitation of these fluids is the relatively low thermal conductivity; water, for instance, is roughly three orders of magnitude less conductive than copper or aluminum (**Table 1**). What these conventional fluids lack in thermal conduc‐

**Table 1.** Typical thermal conductivities for diverse conventional fluids and solid materials.

The primary mechanism for heat transfer in fluids is convection; its efficacy mostly depends on the thermo-physical properties of conventional fluids. Still, if the thermal conductivity of conventional fluids were enhanced, it would be much more effective. Hence, since the solid materials possess several orders higher thermal conductivity compared with that of conven‐ tional fluids, an idea to introduce conducting particles to fluids was consider. This idea occurred more than a century ago for Maxwell [6], where millimeter- or micrometer-sized solid particles were dispersed in conventional fluids to increase cooling rates, also a formulation of a method to calculate the effective conductivity of such suspensions was developed. Later, this technique was investigated and adapted for particle shape and composition by Hamilton and Crosser [7], but neither can predict the enhanced thermal conductivities of nanofluids (NFs) because their models do not include any dependence on particle size [8, 9]. Among diverse particle geometry, different particle shapes occur naturally or are engineered for specific applications (see **Figure 1**).

**Figure 1.** Diverse particle shapes and geometries.

Heat transfer using fluids is a very complicated phenomenon, and various factors such as fluid stability, composition, viscosity, surface charge, interface, and morphology of the dispersed particles influence the observed results [43–45]. Optimization and high efficiency of compo‐ nents and devices have gained great importance since these factors play a paramount role in diverse fields such as microelectronics, engine motors, fuel cells, air conditioning, power transmission systems, solar cells, medical therapy and diagnosis, biopharmaceuticals, com‐ ponents´ and tools´ wear, and nuclear reactors cooling, among others [15, 27, 46–49]. Solid materials such as metals, CNTs, oxide/nitride/carbide ceramics, semiconductors, and compo‐ site materials having higher thermal conductivity can be suspended within conventional fluids, resulting in better thermal transport performance composite fluids. Nevertheless, improvement in thermal conductivity cannot be achieved by just increasing the solid filler concentration because beyond a certain limit, increasing the filler fraction will also increase the viscosity, which will adversely affect the fluid properties and performance.

Most early studies used suspensions of millimeter- or micrometer-sized particles, which led to countless problems, such as a tendency to settle too rapidly, unless flow rate is increased; not only losing the enhancement in thermal conductivity but also forming sludge sediment, increasing the thermal resistance, and impairing the heat transfer capacity of the conventional fluids. In addition, increasing flow rate also increases erosion of pipelines or channels by the coarse and hard particles; outstanding thermal conductivity enhancement is based on high particle concentration, which leads to apparent increase in viscosity. Furthermore, fluids of this scale size could have considerably larger pressure drops, thus making flow through small channels much more difficult since diverse parameters are critical for device performance, such as morphology and stability of dispersed particles or structures within these fluids, fluids composition, viscosity, fast sedimentation, channels clogging, wear or erosion, among others, which are often very serious for systems consisting of small channels [48–53]. Therefore, the search for nanofillers which can obtain high thermal conductivities at lower concentrations is desirable. Hence, extensive research has been done upon these conventional fluids, in order to develop improved materials to sublimely achieve all critical needs.

The primary mechanism for heat transfer in fluids is convection; its efficacy mostly depends on the thermo-physical properties of conventional fluids. Still, if the thermal conductivity of conventional fluids were enhanced, it would be much more effective. Hence, since the solid materials possess several orders higher thermal conductivity compared with that of conven‐ tional fluids, an idea to introduce conducting particles to fluids was consider. This idea occurred more than a century ago for Maxwell [6], where millimeter- or micrometer-sized solid particles were dispersed in conventional fluids to increase cooling rates, also a formulation of a method to calculate the effective conductivity of such suspensions was developed. Later, this technique was investigated and adapted for particle shape and composition by Hamilton and Crosser [7], but neither can predict the enhanced thermal conductivities of nanofluids (NFs) because their models do not include any dependence on particle size [8, 9]. Among diverse particle geometry, different particle shapes occur naturally or are engineered for specific

156 Two-dimensional Materials - Synthesis, Characterization and Potential Applications

Heat transfer using fluids is a very complicated phenomenon, and various factors such as fluid stability, composition, viscosity, surface charge, interface, and morphology of the dispersed particles influence the observed results [43–45]. Optimization and high efficiency of compo‐ nents and devices have gained great importance since these factors play a paramount role in diverse fields such as microelectronics, engine motors, fuel cells, air conditioning, power transmission systems, solar cells, medical therapy and diagnosis, biopharmaceuticals, com‐ ponents´ and tools´ wear, and nuclear reactors cooling, among others [15, 27, 46–49]. Solid materials such as metals, CNTs, oxide/nitride/carbide ceramics, semiconductors, and compo‐ site materials having higher thermal conductivity can be suspended within conventional fluids, resulting in better thermal transport performance composite fluids. Nevertheless, improvement in thermal conductivity cannot be achieved by just increasing the solid filler concentration because beyond a certain limit, increasing the filler fraction will also increase

the viscosity, which will adversely affect the fluid properties and performance.

Most early studies used suspensions of millimeter- or micrometer-sized particles, which led to countless problems, such as a tendency to settle too rapidly, unless flow rate is increased; not only losing the enhancement in thermal conductivity but also forming sludge sediment, increasing the thermal resistance, and impairing the heat transfer capacity of the conventional fluids. In addition, increasing flow rate also increases erosion of pipelines or channels by the coarse and hard particles; outstanding thermal conductivity enhancement is based on high particle concentration, which leads to apparent increase in viscosity. Furthermore, fluids of this scale size could have considerably larger pressure drops, thus making flow through small

applications (see **Figure 1**).

**Figure 1.** Diverse particle shapes and geometries.

A revolution in the field of heat transfer fluids arose with the advent of NFs, a term introduced by Choi's research group in the late 1990s at Argonne National Lab (ANL) [52]. First research was conducted by Masuda et al. [53] for γ-Al203 particles in water, and by Choi-Eastman group [52] for Cu in water as well. As depicted by **Figure 2**, nanofluids research has been increasing through time, reaching up to 1100 publications in 2015, according to scientific search engine "sciencedirect.com" and keyword *Nanofluid*. On the inset of **Figure 2**, two keywords were applied *Nanofluid* and *2D*, where nearly 150 publications in 2015. Nanofluids are a new class of stable heat transfer liquid suspensions which are engineered containing homogeneously dispersed solid nanofillers (ultra-fine particles, fibers rods, or tubes <100 nm). Compared to micro- or milli-fluids, nanofluids tend to be more stable, since nanofillers possess unique properties, such as large surface area to volume ratio, as well as dimension-dependent physical properties, which make nanostructures better and more stably dispersed within conventional fluids. Nevertheless, some limitations of the effective incorporation of nanoparticles within conventional fluids are dispersion and solubility because these tend to aggregate and settle. In some cases, additives or surfactants are used to stabilize the nanoparticles within the fluids, even though the surfactants can decrease the thermal conductivity of the nanofluids, since surfactants introduce defects at the interfaces [54].

**Figure 2.** Number of publications with "Nanofluid" as search topic, according to Sciencedirect.com; inset showing number of publications with keywords "Nanofluid" and "2D."

Therefore, one of the main advantages of nanofluids is that they can be specially engineered to optimally fulfill particular objectives, such as enhanced thermal conductivity, a higher thermal energy storage capacity, a higher heat transfer coefficients, a better temperature stabilization, and less pressure drop, among others. Moreover, nanofluids are promising for practical application without clogging, sediment or such. Nanofluids will keep the fluidic properties of the conventional fluids, behave almost like conventional fluids, and incur in little or no extra penalty of pressure drop (i.e., the viscosity increase is small) due to the fact that dispersed nanoparticles are extremely small, which are very stably suspended in fluids with or without the aid of additives or surfactants [55]. Hence, search for new nanofillers which can get high thermal conductivities at lower filler fractions is important [15, 56].

It has been demonstrated that nanofluids for heat transfer applications have provided better thermal performance than conventional fluids [12, 15, 48, 49, 56–58]. Therefore, the advent of nanofluid-based heat transfer systems can make compact designs with highly efficient thermal, physical, and electrical performance for instruments and devices.

Experiments on convection heat transfer of nanofluids were conducted by several research groups [57, 59–61]. The results showed significant improvements in heat transfer rates of nanofluids. Meanwhile, the thermal conductivity enhancement of nanofluids shows a tem‐ perature-dependent characteristic—increase of enhancement with rising temperature, which makes the nanofluids more suitable for applications at elevated temperatures [15, 49, 62–66]. Additionally, previous research has shown that nanofluids display better performance in their thermo-physical and tribology properties, such as thermal conductivity, thermal diffusivity, viscosity, friction, etc., compared to conventional fluids [15, 46–49, 67–71]. Hence, nanofluids could be used for aforesaid engineering applications. From all these, a great variety of nanocomposite materials have been developed, using diverse techniques and methodologies, obtaining significant performance.
