Juraj Vavro

Magnetically aligned single wall carbon nanotube films: preferred orientation and anisotropic transport properties

Thick films of single wall carbon nanotubes (SWNT) exhibiting in-plane preferred orientation have been produced by filter deposition from suspension in strong magnetic fields. We characterize the field-induced alignment with x-ray fiber diagrams and polarized Raman scattering, using a model which includes a completely unaligned fraction. We correlate the texture parameters with resistivity and thermal conductivity measured parallel and perpendicular to the alignment direction. Results obtained with 7 and 26 T fields are compared. We find no significant field dependence of the distribution width, while the aligned fraction is slightly greater at the higher field. Anisotropy in both transport properties is modest, with ratios in the range 5–9, consistent with the measured texture parameters assuming a simple model of rigid rod conductors. We suggest that further enhancements in anisotropic properties will require optimizing the filter deposition process rather than larger magnetic fields. We show that both ...

Single wall carbon nanotube fibers extruded from super-acid suspensions: Preferred orientation, electrical, and thermal transport

Fibers of single wall carbon nanotubes extruded from super-acid suspensions exhibit preferred orientation along their axes. We characterize the alignment by x-ray fiber diagrams and polarized Raman scattering, using a model which allows for a completely unaligned fraction. This fraction ranges from 0.17 to 0.05±0.02 for three fibers extruded under different conditions, with corresponding Gaussian full widths at half maximum (FWHM) from 64° to 44°±2°. FWHM, aligned fraction, electrical, and thermal transport all improve with decreasing extrusion orifice diameter. Resistivity, thermoelectric power, and resonant-enhanced Raman scattering indicate that the neat fibers are strongly p doped; the lowest observed ρ is 0.25 mΩ cm at 300 K. High temperature annealing increases ρ by more than 1 order of magnitude and restores the Raman resonance associated with low-energy van Hove transitions, without affecting the nanotube alignment.

Correlation of properties with preferred orientation in coagulated and stretch-aligned single-wall carbon nanotubes

We report structure-property correlations in single-wall carbon nanotube (SWNT) fibers, among electrical, thermal, and chemical parameters with respect to stretch-induced preferential SWNT alignment along the fiber axis. Purified HiPco (high-pressure CO) conversion tubes are dispersed with the aid of an anionic surfactant and coagulated in the co-flowing stream of an adsorbing polymer. The fibers are then dried, rewetted under tensile load, and redried to improve the alignment. Complete removal of the polymer was assured by annealing in hydrogen at 1000°C. The degree of alignment was determined by x-ray scattering from individual fibers using a two-dimensional detector. The half width at half maximum describing the axially symmetric distribution of SWNT axes decreases linearly from 27.5° in the initial extruded fiber to 14.5° after stretching by 80%. The electrical resistivity ρ at 300K decreases overall by a factor ∼4 with stretching, for both as-spun composite and polymer-free annealed fibers. However, ...

Exact solution for the lattice gas model in one dimension.

A simple method to obtain a canonical partition function for a one-dimensional lattice gas model is presented. The simplification is based upon rewriting a sum over all possible configurations as a sum over all possible numbers of clusters in the system.

Electrical and Thermal Properties of C60‐filled Single Wall Carbon Nanotubes

We measured temperature‐dependent resistivity ρ, thermal conductivity κ and thermopower S of C60@SWNT (“peapods”). C60 filling reduces ρ much less than alkali metal doping at all T, indicating weak charge transfer. κ is enhanced by filling, the enhancement showing interesting structure vs. T for which we give a tentative explanation. S is reduced at all T by C60 filling, which we explain by a) C60@SWNT blocking some of the oxygen doping sites, and b) weak disorder on the 1‐D chain which reduces the mean free path of tube phonons and thus the phonon drag contribution.