Prepared	Before 200°	56	1.5	4.8	3.1 × 10⁴
in 6 M	C. heat treat-
HNO₃	ment
	After 200°	71 ± 5	2.9 ± 0.2	3.4 ± 0.2	2.4 × 10⁴
	C. heat treat-
	ment
Prepared	Before 200°	68	4.5	3.0	1.3 × 10⁴
in 10 M	C. heat treat-
HNO₃	ment
	After 200°	74 ± 2	5.0 ± 0.2	3.0 ± 0.1	1.2 × 10⁴
	C. heat treat-
	ment

EXAMPLE 3

Dynamic mechanical analysis (DMA) as a function of temperature was done on the “as prepared” (200° C. heat-treated) small-diameter carbon nanotube films, prepared in Example 1, at a frequency of 10 Hz and at 0.1% dynamic strain using a RSA III Rheometrics Solids Analyzer. During the dynamic test, the static force adjusted automatically to 40% larger than dynamic force.

The results show a fairly constant E′ storage modulus for the “as prepared” small-diameter carbon nanotube films throughout the temperature range of about 30° C. to about 210° C., with a slight increase in storage modulus above 150° C. The Tan δ values were very low (i.e., about 0.02), indicating that the films are fairly elastic throughout the 30° C. to about 210° C. temperature range. A plot of the dynamic mechanical behavior for the “as prepared” film made with small-diameter carbon nanotubes processed in 6 M nitric acid is shown inFIG. 1.

EXAMPLE 4

The effects of heat treating on the films of small-diameter carbon nanotubes were studied by Scanning Electron Microscopy (SEM). The “as prepared” films made with small-diameter carbon nanotubes treated in nitric acid were compared to the same films after an additional high temperature heat treatment. “High temperature heat-treated” films were prepared from the “as-prepared” films by heating the “as-prepared” films in nitrogen in a TGA to 900° C. at a rate of 20° C./minute and then holding for 2 minutes at 900° C.

Scanning electron micrographs of the “as-prepared” heat-treated films are shown inFIGS. 2A, 2B and2C, which correspond to films prepared from small-diameter carbon nanotubes treated in 3M, 6M and 10M nitric acid, respectively. Scanning electron micrographs of the “high temperature heat treated” films are shown inFIGS. 2D, 2E and2F, made by heat-treating the “as prepared” films shown inFIGS. 2A, 2B and2C, prepared with small-diameter carbon nanotubes processed in 3M, 6M and 10M nitric acid, respectively, at 900° C. as described above.

FIGS. 2A and 2D are SEMs showing nanotube ropes on the surface of the “as prepared” heat-treated and “high temperature heat-treated” films, respectively, in which the nanotubes were processed in 3M nitric acid. Ropes are not seen on the film surfaces of the “as prepared” heat-treated films made with nanotubes processed in 6 M nitric acid (SeeFIG. 2B) and 10 M nitric acid (FIG. 2C). However, after the “as prepared” films were heat-treated at 900° C., the nanotube ropes can be clearly seen on both the surfaces of the “high temperature heat-treated” films.FIG. 2E shows a SEM of the “high temperature heat-treated” film made from the “as prepared” film, shown inFIG. 2B, which was made from nanotubes processed in 6M nitric acid.FIG. 2F shows a SEM of the “high temperature heat-treated” film made from the “as prepared” film, shown inFIG. 2C, which was made from nanotubes processed in 10M nitric acid. Although not meant to be held by theory, the material surrounding the nanotube ropes in films made with 6M and 10M-nitric acid treated nanotubes is attributed, at least in part, to amorphous carbon resulting from acid treatment-induced nanotube decomposition. The 900° C. heat-treatment of the films made with 6M nitric acid- and 10M nitric acid-treated nanotubes appears to have removed the continuous phase attributed to amorphous carbon from the surface of the films so that the nanotube bundles can be clearly observed. These observations support the theory that the films prepared from nitric acid-treated small-diameter carbon nanotubes have a composite structure comprising nanotube ropes embedded in a continuous matrix phase attributed to amorphous carbon and polycyclic aromatic material, which was expected to have formed, at least in part, from the decomposition of small-diameter carbon nanotubes. The appearance of an amorphous phase is consistent with the disappearance of small diameter nanotubes as the concentration of nitric acid treatment was increased.

EXAMPLE 5

Raman studies were done on the “as prepared” and “high temperature heat-treated” films comprising with small-diameter carbon nanotubes processed in various concentrations of nitric acid, as prepared in Example 1. “High temperature heat-treated” films were prepared by heating the “as-prepared” films in nitrogen in a thermogravimetric analyzer (TGA) at rate of 20° C./minute to 900° C. and holding at 900° C. for two minutes.

Raman spectra were collected using a Holoprobe Research 785 Raman Microscope made by Kaiser Optical System, Inc. with an incident laser wavelength of 785 nm. Diameter determinations were made using the Raman radial breathing modes (RBM) peaks in the range of about of 150 to 350 cm⁻¹that correlate with carbon nanotube diameters. The nanotube diameters were calculated based on the RBM band peak position of the control film using the empirical equation ω_RBM=238/d^0.93. Metallic carbon nanotube configurations cannot be observed using a 785-nm laser wavelength, thus the present Raman observations and analyses are limited to semiconducting tubes that are observable with the 785-nm incident wavelength.

No RBM peaks in the Raman spectra were observed for “as produced” films.

Raman RBM peak spectra for the 900° C. “high temperature heat-treated” films are shown in FIGS.3A-D.FIG. 3A shows a dominant RBM peak at about 267 cm⁻¹for the control film, while the dominant RBM peak shifts to lower frequency for samples made with increasing nitric acid concentrations, consistent with the destruction of smaller diameter tubes and survival of larger diameter tubes. The stress-induced curvature around the circumference of the nanotubes makes small-diameter nanotubes more reactive and liable to chemical attack than larger diameter nanotubes.

The changes in the diameter distribution of the nanotubes resulting from the oxidative nitric acid treatment were quantified by resolving the spectra in FIGS.3A-D, using a Lorentz line shape fitting routine. Using a 785-nm incident laser wavelength, six different diameters in the range of 0.88-1.19 nm were identified in these films. The RBM peaks for carbon nanotube films made with nitric acid-treated nanotubes were also up-shifted by 1-3 cm⁻¹compared to the corresponding peaks for the control film processed without nitric acid.

The relative fraction of a particular diameter of nanotube was determined by taking the ratio of the area of the corresponding peak to the sum of the area of all the RBM peaks.FIGS. 4A, 4B,4C,4D,4E and4F show plots of the relative fractions of small-diameter carbon nanotubes having diameters of 1.19 nm, 1.11 nm, 1.07 nm, 1.02 nm, 0.89 nm and 0.88 nm, respectively, in “high temperature heat-treated” small-diameter carbon nanotubes films as a function of nitric acid concentration.FIGS. 4E and 4F show that the relative fractions of 0.88-nm and 0.89-nm small-diameter nanotubes, respectively, significantly decreased with increasing nitric acid concentration. That the percentage of these small-diameter nanotubes decreased from 70% in the control film to less than 20% in samples processed in 10 M HNO₃, supports the theory of selective degradation of the small-diameter carbon nanotubes by nitric acid. The destruction of small-diameter nanotubes, corresponds to an increase of the relative fraction of the 1.19-nm and 1.11-nm diameter small-diameter carbon nanotubes, the amount of which increased, generally monotonically, with increasing nitric acid concentration.

The chemical reactivity of small-diameter carbon nanotubes with respect to their diameter is further demonstrated by the relative population changes of 1.02 and 1.07-nm diameter nanotubes. The relative fraction of 1.07-nm diameter nanotubes was fairly constant when processed in 6 M and 10 M HNO₃, however, the 1.02-nm diameter fraction decreased when processed in 6 M nitric acid and further decreased when processed in 10 M nitric acid. This suggests that the 1.07-nm diameter nanotubes have higher resistance to oxidative degradation than the 1.02-nm diameter nanotubes. Although the diameter difference between these two tubes is only 5%, there is a measurable difference between their oxidative resistances. In FIGS.4A-F, the circle symbol in the plots represents the relative fraction of small-diameter carbon nanotubes in a film sample where the nanotubes were processed in 6 M nitric acid and the film was subjected to a more severe heat-treatment of 900° C. for 30 minutes and 700° C. for 4.5 hrs in N₂. The similarity in the relative populations between the high temperature treatment (squares) and the more severe heat treatment (circles) suggests that little or no additional nanotube degradation occurred by subjecting the nanotubes to the more severe heat treatment. These results are consistent with the higher propensity for the smaller diameter carbon nanotubes to be attacked and damaged first by the nitric acid.

EXAMPLE 6

The “as-prepared” films made with small-diameter carbon nanotubes processed in 3M, 6M and 10M nitric acid as described in Example 1 were analyzed by thermogravimetric analysis (TGA) using a TA Instruments TGA2950. The samples were heated to 900° C. at 10° C./minute in nitrogen. Ash residues were 80, 65, 57, and 52 wt % for the small-diameter nanotube control, and the nanotubes processed in 3M, 6M, and 10M nitric acid, respectively. The results are consistent with the production of more amorphous carbon with increasing nitric acid concentration.

All of the compositions and methods disclosed and claimed herein can be made and executed without undue experimentation in light of the present disclosure. While the compositions and methods of this invention have been described in terms of preferred embodiments, it will be apparent to those of skill in the art that variations may be applied to the compositions and methods and in the steps or in the sequence of steps of the method described herein without departing from the concept, spirit and scope of the invention. More specifically, it will be apparent that certain agents which are chemically related may be substituted for the agents described herein while the same or similar results would be achieved. All such similar substitutes and modifications apparent to those skilled in the art are deemed to be within the spirit, scope and concept of the invention as defined by the appended claims.