Carbon nanotubes (CNTs) are low density, flexible, electrically conductive materials, with individual tubes having relatively high tensile strength. Nanocomp Technologies, Inc. produces carbon nanotubes in the form of sheets, tapes, powders, dispersions, and yarns. Their products are used for aerospace, aviation, armor, and flame-resistant applications.
Nanocomp’s CNTs have tremendous aspect ratios; thousands of times greater than other commercially available carbon nanotubes.
Part of producing the carbon nanotube powders and slurries involves a proprietary, surfactant-free dispersion process. The result of this process is bundles of CNTs that are ~100 um in width and ~1000 um in length. That’s where Joe Johnson, Ph.D., the Principal Scientist at Nanocomp comes in. He oversees the dispersion and particle labs, as well as related applications like paints, composites and polymers. He’s been with the company for three years, but his background includes over 20 years of experience working with anything involving particles including synthesis, modification, product development, applications, and characterization.
Dr. Johnson wanted an instrument that would help with determining the grinding quantification of carbon nanotubes. Nanocomp’s CNT production procedure results in non-woven sheets being made, with the CNTs held together mainly by van der Waal’s forces. An analogy, although using vastly different forces and scale, would be having many Velcro® tapes being pushed together forming a sheet. A grinding device was previously identified to “rip” the nanotubes from each other resulting in fibers, or bundles of CNTs. Previous experiments identified process conditions (e.g., CNT concentration and instrument conditions). The time of grind to achieve small, uniform fibers was unknown, so a study was planned that would measure particle size vs. grind time. Dr. Johnson needed a quantifying instrument that could accurately measure the width, length, aspect ratio, ESD (equivalent size diameter), and their distributions for dispersed carbon nanotubes.
He did some research to learn more about particle size analyzers, and after comparing the available instruments he knew the FlowCAM® dynamic imaging particle analysis system met his requirements. With its VisualSpreadsheet® software, the FlowCAM can record over 30 different measurements per particle and can capture particle images at up to 22 frames per second which would enable Dr. Johnson to quickly get the answers he needed. “I have used many types of particle sizing instruments in the past and almost everyone makes assumptions which are not valid in many respects,” explains Dr. Johnson. “They assume that the particle is spherical, and that’s not always the case, especially for high aspect ratio materials. Additionally, laser or light scattering devices ignore light absorption and scattering effects in their analysis. Acoustic devices have many assumptions including no particle-solvent interactions, uniform particle size and shape. I needed an instrument for measuring real-world particles and fibers. With the FlowCAM, you can visually see the particles as well as their dimensions; no assumptions needed.”
Dr. Johnson ran over a dozen batches using the same process and conditions. “After dispersion we use the FlowCAM to characterize the carbon nanotube's ESD,” he explains. “And we use ESD to determine how the product decreased with dispersion energy and time.” The quantification involved measuring 3,000 CNT fibers 3x, taking only a few minutes per test. That’s 9,000 particles per run condition.
The testing involved dispersing CNTs of a given concentration and set dispersion conditions. Small aliquots of dispersed CNTs were removed from the disperser over set times and analyzed using the FlowCAM instrument. As a result, particle size was plotted versus time of dispersion with a typical graph shown in Fig 1.
Note that for the first few points (under 30 minutes), estimates were “averaged” due to large fluctuations. Also, other studies showed no decrease in size >200 minutes.
Figure 2 has typical FlowCAM images showing diminishing carbon nanotube size with increased dispersion time.
For the studies, Dr. Johnson and his team expanded their work to include many standard and experimental sheets, different CNT concentrations, and differing process conditions. Particle characterization with the FlowCAM helps the Nanocomp team understand their products’ properties, and deliver uniform products to their customers.