Characterization of Purity Levels

It is at the secondary and tertiary structure levels that we begin to see, measure, and discuss impurities. (At the primary structure level, there are no “impurities,” only “defects.”) Some impurities, such as Thermal Gravimetric Analysis (“TGA”) residuals (ash) are easy to measure with high accuracy. An example is shown in Figure 10. We typically perform the TGA test in triplicate, as illustrated in Figure 10. The test is normally quite reproducible. A small sample is placed in a balanced pan and subjected to a programmed temperature increase in the presence of air. We use the range and median values of residual (weight %) and peak burn-off temperature (oC) to monitor quality of some grades. For the example shown, the values are 1.8 weight % residual and 558 to 570 oC respectively.

Figure 10.
Example of TGA Test Performed in Triplicate.

Other impurities, such as non-tubular carbon (“NTC”), are harder to measure quantitatively. However, semi-quantitative and qualitative assessments readily show differences in rope carbon/NTC levels. Figure 11 below shows a SEM based view of this characterization approach. The examples in Figure 11 range from a high impurity low cost grade on the upper left to a high purity DWNT grade on the lower right.

Figure 9.
Amorphous (Non-Crystalline Rope) Containing SWNT and DWNT
with Polydisperse Diamter Distribution.

Figure 11.
Range of Tubular Carbon Purity Level for Some Fullerene Nanotube
Grades as Measured by SEM.