Our goal is to develop new methods to rapidly screen bulk carbon nanotubes for chemical purity and homogeneity. The different synthesis routes used to produce these materials generate different mixtures of tube geometries, along with varying amounts of carbonaceous and metallic impurities. New inspection techniques are needed to enable material benchmarking, process optimization, and quality control. Ensuring material quality is the first step toward widespread commercialization of these materials.
Thermogravimetric analysis (TGA) is widely used to gather data on nanotube chemistry. By monitoring weight loss as a function of temperature, one can determine decomposition kinetics and use this data to closely approximate the distribution of impurities present in a few milligrams of material. Oxidative stability provides an indirect measure of the types of carbons present. The residual mass provides an estimate of the metal fraction, which primarily consists of the catalyst material.
One disadvantage of TGA, however, is the need for relatively large specimen sizes, which is particularly problematic for highly purified materials (where process yields are low). As an alternative to TGA, we developed an elevated temperature quartz crystal microbalance (QCM) technique that interrogates samples on the order of 1 microgram or less. A variety of coating techniques can be used to deposit the nanotube material, including drop casting, spin coating, and spray deposition.
We validated our elevated temperature QCM method using several calibration materials, as well different commercial-grade carbon nanotube materials with different degrees of chemical purity. We first characterized the bulk materials by TGA, identifying their oxidation temperatures. We used these temperatures as a guide in identifying temperatures for QCM measurements. After heating to each temperature, we monitored the resonance frequency of the quartz crystal using an impedance analyzer.
During coating deposition, the resonance frequency decreases, with the shift in frequency directly proportional to the change in mass. On heating, the coating mass decreases due to oxidation of the carbon material, resulting in an increase in resonance frequency. These changes in coating mass correlated well with mass losses identified during TGA measurements for all materials tested.
In the case of certain carbon nanotube materials, where the metal catalyst content was relatively high, higher variability was observed in the coatings using the elevated temperature QCM approach as compared to the bulk materials by TGA. These results confirm the need to monitor material quality at these smaller scales.
- Sales of carbon nanotubes are projected to exceed $1.9 billion by 2010, with applications in lightweight composites, microelectronics, and biomedical products. Highest growth is projected for single-walled carbon nanotubes, where performance is directly related to chemical purity.
- Characterizing the purity of carbon nanotubes currently requires measurements via multiple analytical techniques. As production volumes increase, screening tools will become essential for quickly identifying batch-to-batch inconsistency, promoting quality control.
- Subtle changes in nanotube chemistry can have a dramatic impact on device reliability. For example, the presence of residual catalyst particles can alter the current-carrying capacity of transparent conductive electrode. Understanding raw material quality is the first step to predicting performance.