Electron spin resonance and raman scattering spectroscopy of multi-walled carbon nanotubes: a function of acid treatment.

We compare the fundamental transport mechanism in multi-walled carbon nanotubes (MWNTs) by means of electron spin resonance (ESR) and Raman spectroscopy as a function of acid treatment. The ESR and Raman results show that the acid treatment reduces the density of states at the Fermi level. Defects introduced through the acid treatment move the Fermi level closer to the K points in the valence band, and consequently conduction is reduced. These defects are identified as Stone-Wales type from the Raman results.

129Xe and 131Xe NMR of gas adsorption on single- and multi-walled carbon nanotubes.

129Xe and 131Xe nuclear magnetic resonance (NMR) spectroscopy was used to study the adsorption of xenon gas on as-produced single-walled and multi-walled carbon nanotubes. Overall, the adsorption was weak, with slightly stronger interaction between xenon and multi-walled nanotubes. Temperature-dependent spectra, relaxation times, line widths, and signal intensities provide evidence that xenon forms a multilayer bulk phase rather than a homogeneous surface coating. The estimated adsorption energy of 1.6 kJ/mol is significantly lower than 23 kJ/mol predicted for monolayer adsorption but is in keeping with the Xe-Xe attractive potential. Xenon preferentially adsorbs on metallic particles in single-walled tubes, while defects are the nucleation sites for the stronger adsorption on multi-walled tubes.

Characterization of partially sintered ceramic powder compacts using fluorinated gas NMR imaging.

We use nuclear magnetic resonance (NMR) imaging of C2F6 gas to characterize porosity, mean pore size, and permeability of partially sintered ceramic (Y-TZP Yttria-stabilized tetragonal-zirconia polycrystal) samples. Conventional measurements of these parameters gave porosity values from 0.18 to 0.4, mean pore sizes from 10 nm to 40 nm, and permeability from 4 nm(2) to 25 nm(2). The NMR methods are based on relaxation time measurements (T(1)) and the time dependent diffusion coefficient D(Delta). The relaxation time of C2F6 gas is longer in pores than in bulk gas and it increases as the pore sizes decrease. NMR yielded accurate porosity values after correcting for surface adsorption effects. A model for T(1) dependence on pore size that accounts for collisions between gas molecules and walls as well as surface adsorption effects is proposed. The model fits the experimental data well. Finally, the long time limit of D(Delta)/D(o), where D(o) is the bulk gas diffusion coefficient is useful for measuring tortuosity, while the short time limit was not achieved experimentally and could not be used for calculating surface-area to volume (S/V) ratios.