Phase transitions of carbon nanotube bundles under non-proportional triaxial compressions

Abstract

Lateral compressions of (n, n) and (n, 0) single-walled carbon nanotube (SWCNT) bundles are simulated by density functional theory. Feasible transition pathways are verified by the body-centered tetragonal C4 (bct-C4) phase from the (4, 4) bundle and the carbon-centered orthorhombic C8 (Cco-C8) phase from (6, 6) and (8, 8) bundles. Three new phases, the sp3-hybridized phases Cco-C16 from the (4, 0) bundle and Cco-C32 from the (8, 0) bundle, the hexagonal phase Hex-C24 composed of sp2- and sp3-hybridized carbons from the (6, 0) bundle, under non-proportional triaxial loading are predicted. Measured hardness values for Cco-C16/C32 exceed 40 GPa, indicating super-hardness. Electronic band structures of Cco-C16/C32 exhibit a 3.64 eV bandgap, while Hex-C24 exhibits metallic carbon properties. Our results provide the potential phase transition pathways of SWCNT bundles under non-proportional compression.