Exploration into Interstitial Crosslinks Capable of Strengthening Boron Nitride Nanotube Bundles

Boron nitride nanotubes (BNNTs) are promising strong, lightweight, and piezoelectric nanomaterials that can be used in applications ranging from vibration sensing to resilient aerospace materials with radiation shielding capabilities [1]. However, the superior mechanical properties of individual nanotubes [2] and ultrahigh interlayer friction within multi-walled BNNTs [3] have yet to be manifested in practical macroscale BNNT bundles: despite their partially ionic B-N bonds, synthesized BNNT bundles [4] have not exhibited strengths substantially higher than carbon nanotube bundles.<br/><br/>Here, we use <i>ab initio</i> calculations to search for interstitial species that can strengthen BNNT bundles via the formation of interlayer crosslinks. Density-functional theory (DFT) calculations indicate that small species present in the laser ablation synthesis of BNNTs [5,6], such as BN dimers, form stable links between h-BN bilayers, with binding energies greater than 2 eV. Additional ab initio molecular dynamics (AIMD) and DFT calculations suggest that larger species found during BNNT purification [7] (such as deprotonated forms of boric acid), can form covalent bridges within small-diameter BNNT bundles. Full enumeration of low-energy interstitial configurations within various h-BN bilayer stacking modes reveals that some interstitials prefer to bridge the layers in their ground states. The results demonstrate not only the recoverability and stability of these links under shear, but also the potential for high friction [8] as bound interstitials interact or transition between their ground states. Moreover, this suggests that links can strengthen BNNT bundles by orders of magnitude, thereby bolstering BNNT fibers up to the long-sought intrinsic strength of individual tubes, in the range of tens of GPa [2].<br/><br/>This work was supported by the NASA grant 80NSSC21K1299.<br/><b>References</b><br/>[1] J. H. Kang et al., ACS Nano 9, 11942–11950 (2015).<br/>[2] X. Wei et al., Advanced Materials 22, 4895–4899 (2010).<br/>[3] A. Niguès et al., Nature Materials 13, 688–693 (2014).<br/>[4] C. J. Simonsen Ginestra et al., Nature Communications 13, 3136 (2022).<br/>[5] J. H. Kim et al., Scientific Reports 9, (2019).<br/>[6] S. Yatom et al., Physical Chemistry Chemical Physics 22, 20837–20850 (2020).<br/>[7] D. M. Marincel et al., Chemistry of Materials 31, 1520–1527 (2019).<br/>[8] N. Gupta, J.M. Alred, E.S. Penev, B.I. Yakobson, ACS Nano 15, 1342–1350 (2021).