Characterization of Boron Carbide Ceramics with Nuclear Magnetic Resonance Spectroscopy

The lattice sites of carbon in B sub 12 C sub 3 and B sub 9 C are examined in terms of their boron-carbon nuclear dipolar couplings and chemical shielding anisotropies as revealed by sup 13 C nuclear magnetic resonance spectroscopy. High-resolution sup 13 C spectra obtained by spinning samples at the 'magic angle' reveal a single isotropic shift at -13 ppm in B sub 9 C and two isotropic shifts (-70 ppm and -9 ppm) in B sub 12 C sub 3. In a stationary sample of B sub 9 C, the sup 13 C NMR spectrum is a Gaussian of halfwidth 6.0 kHz (120 ppm) centered at 13 ppm. In a stationary sample of B sub 12 C sub 3, the spectrum is a superposition of two broad components.