11B nuclear magnetic resonance study of boron nitride nanotubes prepared by mechano-thermal method

We reported ¹¹B nuclear magnetic resonance studies of boron nitride (BN) nanotubes prepared by mechano-thermal route. The NMR lineshape obtained at 192.493 MHz (14.7 T) was fitted with two Gaussian functions, and the ¹¹B nuclear magnetization relaxations were satisfied with the stretched–exponential function, exp[-(<i>tl</i>T₁)^(D+1)^/6] (<i>D</i>: space dimension) at all temperatures. In addition, the temperature dependence of spin–lattice relaxation rates was well described by <b><i>T</i>_i^-1 <i>= aT</i></b> (<i>a</i>: constant, <i>T</i>: temperature) and could be understood in terms of direct phonon process. All the <i>11</i>BNMR results were explained by considering the inhomogeneous distribution of the paramagnetic metal catalysts, such as α-Fe, Fe–N, and Fe₂ B, that were incorporated during the process of high-energy ball milling of boron powder and be synthesized during subsequent thermal annealing. X-ray powder diffraction as well as electron paramagnetic resonance (EPR) on BN nanotubes were also conducted and the results obtained supported these conclusions.<br />