**7. Summary**

**6. Hexagonal boron nitride (h-BN)**

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

electrical insulator.

In our research, main focus is on a novel 2D material, hexagonal boron nitride (h-BN), which is a ceramic material that exhibits versatile properties such as outstanding mechanical stability, remarkable chemical inertness, anti-wear promotion, high electrical resistivity, and superb thermal conductivity [5, 46, 234–239]. On the other hand, h-BN is an effective solid lubricant, it is suitable for diverse applications such as metal-working processes where lubrication at high-temperatures is required and is widely used in high-temperature wear-sealing materials of aerospace engines [155, 239]. Boron and nitrogen atoms behave similarly to carbon when bonded with each other to form boron nitride, exhibiting many similar structures as carbon only with alternating B and N atoms instead of C atoms (see **Figure 8**). Because of this, there are many forms of boron nitride (BN) that coincide with carbon structures and have a variety of properties and functions. h-BN (so-called "white graphite" due to its structural similarity to graphite) is a common form of BN that has a similar configuration to graphite which has been recently studied [15, 49, 108, 133]. Like graphite, this honeycomb layered material has hexagonal ring layers separated by 3.33 Å, in which every boron atom is connected to three nitrogen atoms by strong covalent bonds and vice versa; the B-N distance is 1.44 Å. Between the layers, every boron interacts with a nitrogen atom through a Van der Waals force [237, 240]. Therefore, the strong B-N bond makes an h-BN atomic layer a mechanically strong material, quite analogous to graphene, and individual BN layers could be isolated from bulk h-BN crystals [15, 108, 229]. As an insulating material with very high thermal conductivity [241], h-BN surpasses other nanofillers and is an attractive material for high thermal transport and electrically insulating composites [15, 133, 236, 242]. Nevertheless, theoretical studies indicate that high thermal conductivities can only be achieved from the (002) planes (up to hundreds to thousands of W/m K) [50, 243]; through a synthesis process of wet exfoliation, h-BN can give maximum exposure to these (002) lattice planes. Meantime, some thermal management systems need electrically conducting fillers for static electric charge dissipation. Graphene-based systems are good for these applications where they can do both thermal and electrical management. This shows how h-BN will be specially applicable and useful in thermal management applications, where electrical insulation is also required. It is also important to mention that BN is environmentally and skin friendly; proof of this is that BN is widely used as the main ingredient for cosmetics. According to the Cosmetic Ingredient Review (CIR) report from 2012, BN is used in 483 cosmetic formulations. Products containing BN are reported to be used on baby skin (in a lotion, oil, powder, or cream), eye area, or mucous membranes. BN is reported to be used at up to 25% in eye product formulations, at 2% in lipstick formulations, up to 16% in powders, and at up to 0.9% in fragrance preparations [244]. Hence, since little research have been conducted on 2D nanostructures, and particularly on boron nitride, exfoliated h-BN, having a few layers in thickness has a remarkable opportunity to overcome the material of the future in the thermal management field for diverse applica‐ tions, including electrical/electronic, since its nature allows it to be a thermal conductor but an

The present work offers a general overview of the recent research and development on preparation and characterization of nanofluids for thermal management and tribological applications, with emphasis on experimental data, variables and features, as well as proposed mechanisms responsible for nanofluids improvement performance. Nowadays, many technologies search for the highest efficiency, mainly and more than ever before, on the cooling and anti-wear challenges within machines, devices and components. Several efforts have been made trying to homogeneously disperse nanostructures (oxides, metals, ceramics, CNTs, 2Dnanosheets and others) within conventional fluids to improve their properties, such as thermal transport, viscosity, lubrication, electrical behavior, among others. Nanofillers size has positive effect on conventional fluids performance, that is compared to larger dispersed solid particles making flow through small channels much more easier, also since diverse parameters are critical for devices performance, such as morphology and stability of dispersed nanostructures within conventional fluids, fluids composition, viscosity, fast sedimentation, channels clogging, wear or erosion, among others, which are often very serious for systems consisting of small channels. Furthermore, properties improvement is also dependent of various factors, such as filler fraction, temperature, chemical, and interfacial properties on the nanostructurefluid interfaces. Some nanofluids are currently expensive, partly due to the difficulty in manufacturing. Hence, mass production of nanostructures could further decrease the cost, also low filler fraction is necessary to make nanofluids more affordable before they will see widespread applications.
