*3.2.5 The neutron absorption*

*Nanostructures*

*3.2.1 Optical properties of BNNSs*

predicted by theoretical calculus (6.0 eV) [66].

main values are somewhat less than these values.

*3.2.2 Thermal conductivity*

ture > 600 W/mK) [68].

*3.2.3 Mechanical properties*

*3.2.4 Lubricant properties*

BNNSs have not any absorption in the visible region but have absorption spectroscopy in the ultraviolet region [62]. Its commercial powders are white and its single crystal is transparent. Thin films obtained from chemical exfoliation or CVD also have high transparency [39, 40]. BNNS dispersion is often transparent at low concentrations and shows the Tyndall effect (the path of visible light inside the dispersion with laser light); at higher concentrations, the laser light is diffracted; and the dispersion is seen milky because of the lateral dimensions of nanosheets that are larger than the wavelength of laser light. Due to diffraction, there is any peak in the visible area. The measured extinction coefficient for absorption and diffraction of nanosheets is much smaller than that of graphene. The smaller lateral dimensions and more defects often influence on the optical bandgap, which is attributed to the absorption of the small distribution bandgap at the fermi surfaces and is produced due to the presence of defects [57]. However, hBN has two peaks in the 4 and 5.7 eVs: the first one related to the bandgap energy of BN, as a direct-gap semiconductor, and the latter is related to impurities and vacancy defects [63]. The lateral dimensions and number of layers affect the bandgap energy of nanosheets. For example, Rafiei-Sarmazdeh et al. [58] reported the absorption spectra of as-obtained BNNSs (2-nm-thick layer) at 204 nm (6.08 eV) that are related to the intrinsic excitement of BN structure and which is consistent with the reported results in other previous literatures [64, 65] and is also close to the bandgap energy

The thermal conductivity for BNNSs is in range 300–2000 W/mK, which is comparable to graphene (1500–2500 W/mK). The difference in the conductivity may be due to the soft phonon modes of carbon sheets and the mass difference between boron and nitrogen [67]. Single BN layers have higher conductivity than multilayers, as the number of layer decreases and the phonon diffraction between layers reduces. As the number of layers increases, the conductivity decreases and converges to the conductivity of hBN. Although hBN has high conductivity and thermal capacity, recent studies have shown that its strong phonon diffraction leads to lower thermal conductivity than graphite. Therefore, the reduction of diffraction in BNNS leads to a significant increase in the conductivity (at room tempera-

The hardness for BNNS and graphene is 267 and 335 TPa, respectively [69]. Hence, BNNSs can be used as reinforcement for polymer composites. It has been shown that modulus and tensile strength for nanosheets (thickness of 1–2 nm) are in the range of 220–510 and 8–16 TPa [39]. For multilayer, it is expected that the

Another interesting case with BN materials is frictional properties. hBN and graphite are used as lubricants for many years. The lubricating properties result from the application of the external shear force on the weak forces between the layers and sliding. At the level of atomic layers, friction force microscopy (FFM) studies show that the friction properties of these nanosheets depend on their thickness.

**28**

Boron is introduced as one of the most important neutron absorbers due to its high neutron absorption cross section. The compounds containing of boron are good neutron absorber. In the middle, hBN and, of course, BNNSs are better absorbers due to layer structure and larger surface area that is exposed to neutron beam than other BN structures (such as nanotube, nanoparticle, etc.). They are used in nuclear shielding [71] and boron neutron therapy [72].
