C. Bower

Alignment of carbon nanotubes in a polymer matrix by mechanical stretching

We report a method to fabricate polymer-based composites with aligned carbon nanotubes, and a procedure to determine the nanotube orientation and the degree of alignment. The composites were fabricated by casting a suspension of carbon nanotubes in a solution of a thermoplastic polymer and chloroform. They were uniaxially stretched at 100u2009°C and were found to remain elongated after removal of the load at room temperature. The orientation and the degree of alignment were determined by x-ray diffraction. The dispersion and the alignment of the nanotubes were also studied by transmission electron microscopy.

Large current density from carbon nanotube field emitters

Field emitters made from carbon nanotubes exhibit excellent macroscopic emission properties; they can operate at a very large current density, as high as 4 A/cm/sup 2/. At electric fields as low as 4-7 V//spl mu/m, they emit technologically useful current densities of 10 mA/cm/sup 2/. The emission originates from nanotube ends with a characteristic structured ring pattern.

Deformation of carbon nanotubes in nanotube–polymer composites

Composites of uniaxially oriented multiwalled carbon nanotubes embedded in polymer matrices were fabricated and investigated by transmission electron microscopy. In strained composite films, buckling was ubiquitously observed in bent nanotubes with large curvatures. By analyses of a large number of bent nanotubes, the onset buckling strain and fracture strain were estimated to be ≈5% and ⩾18%, respectively. The buckling wavelengths are proportional to the dimensions of the nanotubes. Examination of the fracture surface showed adherence of the polymer to the nanotubes.

Electrochemical intercalation of single-walled carbon nanotubes with lithium

Abstract Single-walled carbon nanotubes (SWNT) synthesized by laser ablation were electrochemically intercalated with lithium. As-grown SWNTs showed a reversible saturation composition of Li 1.2 C 6 (450 mAh g −1 ). After removing the impurity phases by filtration, the reversible saturation composition increased to Li 1.6 C 6 (600 mAh g −1 ), significantly higher than the ideal value of LiC 6 (372 mAh g −1 ) for graphite. All the SWNT materials showed large irreversible capacities and voltage hysteresis. Upon processing the nanotubes by ball-milling, the reversible Li capacity increased to 1000 mAh g −1 (Li 2.7 C 6 ) while the irreversible capacity decreased to 650 mAh g −1 .

Enhanced saturation lithium composition in ball-milled single-walled carbon nanotubes

Abstract The effects of processing on the structure and morphology of single-walled carbon nanotubes (SWNT) and their electrochemical intercalation with lithium were investigated. Purified SWNTs were processed by impact ball-milling and were electrochemically intercalated with lithium. The reversible saturation Li composition increased from Li 1.7 C 6 in purified SWNTs to Li 2.7 C 6 after 10 min of milling. The irreversible capacity decreased from Li 3.2 C 6 to Li 1.3 C 6 . Electron microscopy, Raman and X-ray diffraction measurements indicated that ball-milling induced disorder within the bundles and fractured the nanotubes.

Intercalation and partial exfoliation of single-walled carbon nanotubes by nitric acid

Abstract Single-walled carbon nanotubes (SWNTs) were reacted with HNO 3 solution. An expansion of the inter-nanotube spacing and increase in the amount of hydrogen in the material were observed by X-ray diffraction and proton NMR measurements after the SWNTs were immersed into the HNO 3 solution for less than 2 hours. The change in diffraction pattern was reversible, indicating intercalation of HNO 3 into the SWNT bundles. When the SWNTs were immersed in the HNO 3 solution for a longer period of time, the bundles became disordered and partially exfoliated. Isolated SWNTs and large nano-particles were often observed.

In-situ TEM and EELS studies of alkali–metal intercalation with single-walled carbon nanotubes

Abstract Cesium (Cs) or potassium (K) was deposited on single-walled carbon nanotube bundles in vacuum at room temperature. The deposited bundles were analyzed in-situ by transmission electron microscopy and electron energy loss spectroscopy techniques. The results indicate that both Cs and K can be reversibly intercalated with the bundles. The intercalants reside in-between the individual nanotubes within the bundles. Intercalation caused structural disorder to the two-dimensional lattice of the pristine nanotube bundles. The chemical compositions of the nanotube bundles intercalated with K and Cs were found to be about KC 24 and CsC 24 to CsC 8 .

Fabrication and Field Emission Properties of Carbon Nanotube Cathodes

A variety of carbon nanotube films have been fabricated and tested as cold cathodes. A spray deposition technique was developed for processing as-grown bulk nanotubes, both single-walled and multi-walled, into films of randomly oriented nanotubes. Films of randomly oriented multi-walled nanotubes were grown using thermal chemical vapor deposition, and arrays of well-aligned multi-walled nanotubes have been fabricated using a microwave plasma enhanced chemical vapor deposition technique. The emission current-voltage (I-V) characteristics of these nanotube cathodes have been measured. Both multi-walled (random and aligned) and single-walled carbon nanotubes exhibit low turn-on fields (∼ 2 V/μm to generate 1 nA) and threshold fields ( 2 ). Significantly, these cathodes were capable of operation at very large current densities (> 1A/cm 2 ), making them candidates for application in a variety of vacuum microelectronic devices.

Energy distribution of field emitted electrons from carbon nanotubes

Abstract Field emission properties of single-walled carbon nanotubes (SWNTs), deposited on n-type Si substrates, were investigated by a combination of classical I–V characterization and field emission energy distribution (FEED) measurements. I–V characterization showed that current densities on the order of 1xa0mA/cm2 could be obtained at nominal electric fields as low as 3xa0V/μm. A current density of up to 25xa0mA/cm2 was observed, and long-term current stability and reproducibility were excellent. FEED measurements revealed that at low current densities, the field emitted electrons originated from energy levels close to the Fermi level of the Si substrate. At larger applied voltages and larger current densities, I–V characterization of the field emitters showed primarily an ohmic behavior; FEED data taken under the same experimental conditions showed a shift of the main spectral peak toward lower kinetic energies with increasing applied voltages. This energy shift was found to be linear with the applied voltage and emission current over a wide range. Both FEED and I–V data thus indicated that the field emission current was primarily limited by the ohmic resistance of the nanotubes and/or the contact resistance between the nanotubes and the Si substrate; typical measured values for the cathode surface resistances were 100xa0kΩxa0cm2.

Extremely small diffusion constant of Cs in multiwalled carbon nanotubes

The Cs intercalation process in multiwalled carbon nanotubes (MWNTs) was studied by cross-sectional scanning photoemission microscopy. Cs atoms initially deposited on the tips of aligned nanotubes diffused toward their roots. The Cs diffusion constant for the MWNTs at room temperature was evaluated from the Cs distribution measured along the axes of the tubes. The value of 2×10−12u2009cm2/s obtained is seven orders of magnitude smaller than that in graphite, although the local atomic structure of an intercalated MWNT is very similar to that of intercalated graphite.