**1. Introduction**

Since the 20th century, scientists have been working with considerable effort to develop fluids that can surpass those conventionally known by the scientific society and industry in terms of thermal and physical performance. The idea of dispersing solid particles of millimeter (mm) and micrometer (µm) in size is the milestone, which was physically initiated by Ahuja [1,2] in 1975, Liu et al. [3] in 1988, and other researchers at Argonne National Laboratory (ANL) [4–6] in 1992 on the bases of Maxwell theoretical work [7]. Such suspensions have shown tremendous improvements in heat transfer characteristics compared to their base fluids. This is due to the dispersed solid particles' significantly

**Citation:** Ali, N.; Bahman, A.M.; Aljuwayhel, N.F.; Ebrahim, S.A.; Mukherjee, S.; Alsayegh, A. Carbon-Based Nanofluids and Their Advances towards Heat Transfer Applications—A Review. *Nanomaterials* **2021**, *11*, 1628. https://doi.org/10.3390/ nano11061628

Academic Editor: S M Sohel Murshed

Received: 26 May 2021 Accepted: 17 June 2021 Published: 21 June 2021

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higher thermal conductivity compared to their hosting fluid, which would enhance the effective thermal conductivity of the colloidal. The term 'effective' is generally used when referring to the net property of a solid–liquid suspension [8]. However, it was found that in flow areas of low velocities, the particles hosted by the suspension tended to deposit from its carrier liquid. Additionally, hence the fluid starts to lose its tuned properties. Furthermore, clogging of small passages was also experienced due to the significant level of agglomeration between the dispersed particles, and therefore making it extremely challenging to employ in heat transfer devices containing small channels. This is when, in 1993, Masuda et al. [9] conceived the idea of fabricating suspensions with ultrafine particles of silica, alumina, and titanium dioxide, where these dispersions were afterward given the name 'Nanofluids' by Choi and Eastman [10], in 1995, as a result of their extensive research work at ANL. According to the founders, a nanofluid can be generally defined as an advanced category of fluid that is produced by homogeneously dispersing low concentrations (preferably ≤1 vol. %) of particles of less than 100 nanometers (nm) in size within a non-dissolving base fluid [11]. Both Masuda et al.'s [9] and Choi and Eastman's [10] primary motivation at that time was to overcome the limitations associated with suspensions made by their counterparts (i.e., colloidal containing millimeter or micrometer sized particles). In addition, Choi and Eastman [10] have theoretically known beforehand that reducing the size of the dispersed particles to the nanoscale would greatly enlarge the particle exposed surface area to the surrounding, and thus increasing the suspension overall thermal conductivity [12]. The significant variation in thermal conductivity between solid particles and liquids can be clearly seen in Figure 1 for some of the most commonly used particles and base fluids, at room temperature and atmospheric pressure, for fabricating nanofluids [13–17]. It is worth noticing that CuO, MgO, Al2O3, ZnO, TiO2, Fe2O3, SiO2, Ag, Cu, Au, Al, Fe, carbon nanotubes (CNTs), and multiwalled carbon nanotubes (MWCNTs) stands for cupric oxide, magnesium oxide, aluminum oxide, zinc oxide, titanium dioxide, iron(III) oxide, silicon dioxide, silver, copper, gold, aluminum, iron, carbon nanotubes, and multiwalled carbon nanotubes, respectively. Furthermore, the thermal conductivity of some of the materials shown in Figure 1 was seen to have a significant scatter of data across the literature, which can be linked to several factors such as the purity, crystallinity, particle size, and the determination approach used to find this thermal property. In addition, the thermal conductivity of graphene after being subjected to oxidization (i.e., having the form of graphene oxide) gets highly reduced, where it can reach values between 1000 and 2 W/m·K [18–20].

Following their success, many researchers started to explore and develop this class of engineered fluid via modifying their production route, enhancing the suspension stability, and improving the colloidal thermal conductivity [13,21,22]. As of today, nanofluids are seen to have potential usage in a wide range of areas, including the energy sector, construction and building, transportation, oil and gas, medical sector, etc. [23–34]. Figure 2a shows the increasing trend in scientific publications in the field of nanofluids from 1995 to 2020, while Figure 2b illustrates the different types of these published documents that are available in the same database. It is worth mentioning that the data in Figure 2 was obtained from Elsevier's abstract and citation database, Scopus, via searching through the word 'Nanofluid' [35].

Despite the promising achievements that nanofluids could deliver to the scientific community, there are still some obstacles that need to be overcome before this category of fluids can be industrially accepted. For example, the colloidal preparation phase is still considered one of the most significant challenges, as this stage can strongly influence the fluid physical stability and effective thermophysical properties [13,36]. Meaning that if the fabrication process used was not well structured before being executed, the chances of an unstable nanofluid being produced is likely to occur. As a result, some of the suspension's thermophysical properties will gradually degrade with time due to the separation of particles from the hosting base fluid. Almurtaji et al. [37] have illustrated in their published work the relationship between the effective thermal conductivity and the physical stability

of suspensions. They showed that the effective thermal conductivity of a nanofluid could reach its optimum possible value when the dispersion is physically stable, and vice versa. In addition, the commonly employed two-step fabrication method that relies on an ultrasonic bath type device, was reported to raise the as-prepared nanofluid temperature and that the surrounding atmospheric conditions govern this increase in temperature along with the sonicator working power. Thus, it is highly unlikely that similar nanofluids can be produced through the conventional two-step route without simultaneously fabricating the products at the same preparation conditions. A more convenient two-step method employed for nanofluid production would be the two-step controlled sonicator bath temperature approach, as was reported by Ali et al. [8,11] and Song et al. [38]. The aforementioned approach would eliminate the rise in bath temperature obstacle, and hence will ensure an optimum level of nanofluids reproducibility to the manufacturer at any surrounding atmospheric conditions, and even when using different types of bath sonicators. Furthermore, as the thermal properties of a nanofluid are influenced mainly by the dispersed particles compared to its base fluid, researchers have been focusing more on carbon-based materials. This is because some of these materials, in the nanoscale, have exceptional thermophysical properties compared to other commonly used materials (e.g., metals and oxides) [39–41]. For instance, CNTs and graphene have significantly elevated thermal conductivity [42,43], large aspect ratio [44], lower density [45,46], lower erosion and corrosion surface effects [47], higher stability [43], and lower pressure drop and pumping power requirement in comparison to other types of nanomaterials [48,49]. Figure 3 demonstrates common allotropes of carbon nanomaterials. ity of suspensions. They showed that the effective thermal conductivity of a nanofluid could reach its optimum possible value when the dispersion is physically stable, and vice versa. In addition, the commonly employed two-step fabrication method that relies on an ultrasonic bath type device, was reported to raise the as-prepared nanofluid temperature and that the surrounding atmospheric conditions govern this increase in temperature along with the sonicator working power. Thus, it is highly unlikely that similar nanofluids can be produced through the conventional two-step route without simultaneously fabricating the products at the same preparation conditions. A more convenient two-step method employed for nanofluid production would be the two-step controlled sonicator bath temperature approach, as was reported by Ali et al. [8,11] and Song et al. [38]. The aforementioned approach would eliminate the rise in bath temperature obstacle, and hence will ensure an optimum level of nanofluids reproducibility to the manufacturer at any surrounding atmospheric conditions, and even when using different types of bath sonicators. Furthermore, as the thermal properties of a nanofluid are influenced mainly by the dispersed particles compared to its base fluid, researchers have been focusing more on carbon-based materials. This is because some of these materials, in the nanoscale, have exceptional thermophysical properties compared to other commonly used materials (e.g., metals and oxides) [39–41]. For instance, CNTs and graphene have significantly elevated thermal conductivity [42,43], large aspect ratio [44], lower density [45,46], lower erosion and corrosion surface effects [47], higher stability [43], and lower pressure drop and pumping power requirement in comparison to other types of nanomaterials [48,49]. Figure 3 demonstrates common allotropes of carbon nanomaterials.

thermophysical properties will gradually degrade with time due to the separation of particles from the hosting base fluid. Almurtaji et al. [37] have illustrated in their published work the relationship between the effective thermal conductivity and the physical stabil-

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**Figure 1.** Thermal conductivity of commonly used particles and base fluids for fabricating nanofluids showing an order of magnitude higher in the thermal property for some of the carbon-based materials.

**Figure 1.** Thermal conductivity of commonly used particles and base fluids for fabricating nanofluids showing an order

of magnitude higher in the thermal property for some of the carbon-based materials.

**Figure 2.** Search result obtained from Scopus database on nanofluids, where (**a**) illustrates the number of published works per year and (**b**) shows the percentage of each type of these documents [35]. **Figure 2.** Search result obtained from Scopus database on nanofluids, where (**a**) illustrates the number of published works per year and (**b**) shows the percentage of each type of these documents [35].

**Figure 3.** Common allotropes of carbon nanomaterials that grant distinctive thermophysical properties [50]. **Figure 3.** Common allotropes of carbon nanomaterials that grant distinctive thermophysical properties [50].

Many published numerical and experimental studies on nanofluids fabricated with particles of carbon-based materials were found in the literature, which show the continued growth of interest in such materials [35,51–54]. Figure 4 classifies these documents in terms of the number of available publications at the Scopus database for each type of carbon-based material used in nanofluids production. The single-walled carbon nanotube (SWCNT) and double-walled carbon nanotube (DWCNT) abbreviations in Figure 4 refer to the single-walled carbon nanotube and double-walled carbon nanotube, respectively. During the reviewing process, which led to the formation of Figure 4, the authors remarkably recognized that the researchers had used different sonication duration and intensities to fabricate their nanofluids. However, some of the suspensions had the same particles type, size, and hosting base fluid. This shows that, up to today, there is no standard fabrication method for the production of the colloidal. The authors have also found that dispersing carbon-based materials, such as walled carbon nanotubes (MCNTs) and graphene, can tremendously enhance the quality of biofuels blends, in specific biodiesel [55,56]. This includes lowering the brake specific fuel consumption, stabilizing the fuel consumption rate and brake thermal efficiency, and improving the diesel engine perfor-Many published numerical and experimental studies on nanofluids fabricated with particles of carbon-based materials were found in the literature, which show the continued growth of interest in such materials [35,51–54]. Figure 4 classifies these documents in terms of the number of available publications at the Scopus database for each type of carbon-based material used in nanofluids production. The single-walled carbon nanotube (SWCNT) and double-walled carbon nanotube (DWCNT) abbreviations in Figure 4 refer to the single-walled carbon nanotube and double-walled carbon nanotube, respectively. During the reviewing process, which led to the formation of Figure 4, the authors remarkably recognized that the researchers had used different sonication duration and intensities to fabricate their nanofluids. However, some of the suspensions had the same particles type, size, and hosting base fluid. This shows that, up to today, there is no standard fabrication method for the production of the colloidal. The authors have also found that dispersing carbon-based materials, such as walled carbon nanotubes (MCNTs) and graphene, can tremendously enhance the quality of biofuels blends, in specific biodiesel [55,56]. This includes lowering the brake specific fuel consumption, stabilizing the fuel consumption rate and brake thermal efficiency, and improving the diesel engine performance and the resulting emissions from the combustion process.

mance and the resulting emissions from the combustion process. This review paper provides an overview of three types of carbon-based nanofluids: CNT, nanodiamond (ND), and graphene. The selection reason for these three carbonbased particles is due to their outstanding thermal properties compared to any other sort of nanoscaled solids. Hence, they can be considered promising candidates for fabricating nanofluids targeted towards heat transfer applications. The main contribution of the present review study is that this work starts from the synthesis stage of these three carbonbased materials, followed by their dispersed form, and up to their employment in selected energy applications. Furthermore, recommendations on the different nanofluids production methods used are shown along with the colloidal stability and its effect on the thermophysical properties. Moreover, the experimental measuring devices and theoretical equations used to determine and predict the thermophysical properties are provided. In addition, the research work done on utilizing these carbon-based suspensions are presented for three thermal applications, namely, parabolic trough solar collectors (PTSCs), This review paper provides an overview of three types of carbon-based nanofluids: CNT, nanodiamond (ND), and graphene. The selection reason for these three carbonbased particles is due to their outstanding thermal properties compared to any other sort of nanoscaled solids. Hence, they can be considered promising candidates for fabricating nanofluids targeted towards heat transfer applications. The main contribution of the present review study is that this work starts from the synthesis stage of these three carbon-based materials, followed by their dispersed form, and up to their employment in selected energy applications. Furthermore, recommendations on the different nanofluids production methods used are shown along with the colloidal stability and its effect on the thermophysical properties. Moreover, the experimental measuring devices and theoretical equations used to determine and predict the thermophysical properties are provided. In addition, the research work done on utilizing these carbon-based suspensions are presented for three thermal applications, namely, parabolic trough solar collectors (PTSCs), nuclear reactors, and air conditioning and refrigeration (AC&R) systems, with a comparison to

those of conventional working fluids. Finally, the gaps in present scientific knowledge that scientists need to tackle are highlighted in order to promote these advanced types of heat transfer fluids commercially. ison to those of conventional working fluids. Finally, the gaps in present scientific knowledge that scientists need to tackle are highlighted in order to promote these advanced types of heat transfer fluids commercially.

nuclear reactors, and air conditioning and refrigeration (AC&R) systems, with a compar-

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**Figure 4.** The number of publications available at the Scopus database for common carbon-based material used in nanofluids fabrication [35]. **Figure 4.** The number of publications available at the Scopus database for common carbon-based material used in nanofluids fabrication [35].

### **2. Synthesis of Nanoscaled Carbon-Based Materials 2. Synthesis of Nanoscaled Carbon-Based Materials**

Carbon ranks as the 4th most common element after hydrogen, helium, and oxygen in our solar system, and the 17th in the crust of our planet [57]. Remarkably, this element is distinctive so that when the crystal structure of carbon atoms is changed into deferent arrangements, the material properties significantly differ [58–63]. For example, both ND and graphene are made of carbon but of different atomic bounds arrangement. While the first is an electrical isolator and transparent towards visible light waves, the second has excellent electrical conductivity with complete visible light blockage. Such materials that contain various arrangements of carbon atoms are known as 'allotropes of carbon', which means that the material has chemically identical elements but with different atomic arrangements, and hence different physical properties. Due to this fact, many allotropes of carbon exist or have been discovered by scientists, e.g., diamond, graphene, and CNTs. The following Sections 2.1–2.3. will provide a short overview of the fabrication of three allotropes of carbon in the nanoscale, namely ND, graphene, and CNTs. Knowing the pro-Carbon ranks as the 4th most common element after hydrogen, helium, and oxygen in our solar system, and the 17th in the crust of our planet [57]. Remarkably, this element is distinctive so that when the crystal structure of carbon atoms is changed into deferent arrangements, the material properties significantly differ [58–63]. For example, both ND and graphene are made of carbon but of different atomic bounds arrangement. While the first is an electrical isolator and transparent towards visible light waves, the second has excellent electrical conductivity with complete visible light blockage. Such materials that contain various arrangements of carbon atoms are known as 'allotropes of carbon', which means that the material has chemically identical elements but with different atomic arrangements, and hence different physical properties. Due to this fact, many allotropes of carbon exist or have been discovered by scientists, e.g., diamond, graphene, and CNTs. The following Sections 2.1–2.3. will provide a short overview of the fabrication of three allotropes of carbon in the nanoscale, namely ND, graphene, and CNTs. Knowing the

duction methods of these materials is essential and will, later on, help us understand

which nanofluid fabrication route is suitable to conduct.

production methods of these materials is essential and will, later on, help us understand which nanofluid fabrication route is suitable to conduct.
