# Fewer Dimensions for Higher Thermal Performance: A Review on 2D Nanofluids
## 2 Types of 2D Nanofluids  
### Hexagonal Boron Nitride  
Other 2D ordered nanocrystals, different from those in the carbon nanomaterials family, are of great research and practical interest. These include h-BN nanosheets with a very high TC. This thermophysical property of h-BN is approximately 3400 W\(\cdot\)m\({}^{-1}\cdot\)k\({}^{-1}\). The performant thermal characteristics and the low synthesis cost of the h-BN nanofluids compared to the Gr ones means that they can potentially be used in heat transfer and thermal energy storage. The h-BN innovative 2D material also has other noticeable characteristics, including mechanical stability and electrical resistivity. Furthermore, it is a solid lubricant with high efficiency in different areas, such as metal-working processes and wear-sealing of aerospace engines under high-temperature conditions. There exist many forms of BN, including carbon nanostructures possessing a variety of characteristics and functionalities. For instance, the honeycomb layered BN, also known as white graphite because of its  
Figure 3: Main practical applications of 2D nanofluids. structural similarity to that of graphite, is a common form of boron nitride that has a configuration similar to graphite with hexagonal ring layers separated by 3.33 A, in which every boron atom is covalently bonded to three nitrogen atoms and vice versa. Between the layers, every boron interacts by van der Waals forces with a nitrogen atom. Thus, the strong B-N bond gives strength to the h-BN atomic layer and the individual BN layers could be isolated from the bulk h-BN crystals. Since it possesses an improved TC, the BN can overcome other fillers and is suitable for thermal performant hybrids. When synthesized through wet exfoliation, h-BN can offer a maximum exposure to the (002) lattice planes. It should be noted that h-BN is especially useful in thermal management situations where it can play the role of the thermal conductor and the electrical insulator. The application of h-BN in mineral oils, natural esters, and other fluids has already been examined. For instance, Taha-Tijerina et al. [20] demonstrated that h-BN changed the TC of mineral oil and acted as an electric insulator rather than a conductor. This is caused by the ability of the h-BN nanoparticles to trap moving electrons. Additionally, Salehirad and Nikje [21] incorporated h-BN nanostructures in a mineral oil and observed an enhanced TC, decreased viscosity, and improved insulation capability compared with those characteristics of the mineral oil alone. Moreover, Li et al. [22] synthesized ethylene glycol/BN nanofluids with different sized nanoparticles. The investigation team demonstrated that the nanofluids with larger nanoparticles exhibited greater TC enhancements when compared with those with smaller nanoparticles. Bhunia et al. [23] developed a 2D nanofluid with h-BN nanosheets and conventional transformer oil as the base fluid. The prepared nanofluids had stable dispersions with an enhanced dielectric breakdown strength and TC at 35 \({}^{\circ}\)C, 45 \({}^{\circ}\)C, and 55 \({}^{\circ}\)C. The synthesis method of the nanofluids is schematically described in Figure 4.  
The dispersed layered 2D h-BN has high specific area, enabling it to create a larger interfacial region. Due to its (002) plane dominance of the hexagonal phase, the BN possesses an improved TC above 600 W-m-1.K-1. The very thin and high specific surface h-BN nanosheets are lipophilic, allowing the existence of a perfect contact with the transformer oil base fluid. The intra-sheet propagation of heat introduces a free path to the phonon, which faces lower Kapitsa resistance and smaller thermal loss. Additionally, the clustering caused by a shorter inter-particle distance leads to the formation of a percolation network that acts as a conductive path for heat propagation.