|Ph.D Student||Olga Kleinerman|
|Subject||Carbon Nanotubes in Superacid and Organic Solvents:|
Phase Behavior and Applications
|Department||Department of Chemical Engineering||Supervisors||Professor Emeritus Talmon Yeshayahu|
|Full Professor Cohen Yachin|
|Full Thesis text|
In recent years it has been demonstrated that carbon nanotubes (CNTs) spontaneously dissolve in chlorosulfonic acid (CSA), and at high concentrations form a liquid crystalline nematic phase. Actually, CSA is the only known solvent for CNTs, to produce a thermodynamically stable solution, from which CNTs may be made into their applicative form via wet processing. The transition between the isotropic and the liquid crystalline phase, and formation of intermediate meta-stable structures in solutions, depends strongly on the CNT type, concentration, and solvent strength.
These transitions were studied in this work by cryogenic transmission- and cryogenic scanning-electron microscopy (cryo-TEM and cryo-SEM), applying unique cryo-EM specimen preparation and imaging methodologies, suitable for strong acid systems, we have developed. Those techniques preserve the native nanostructure in the system, while not harming expensive equipment and the operator. By direct imaging of several CNT batches, having different diameter, aspect ratio, and varying in the content of defective CNTs and impurities, we were able to follow phase transitions of CNTs dissolved in CSA, starting from very dilute solutions, through the concentrated isotropic phase and bi-phasic regime, up to the fully liquid crystalline phases. We succeeded to capture morphological changes of different CNTs as the nanotube concentration increases and isotropic-to-nemaitic phase transition occurs. By direct imaging of solutions, we have proved that most of studied nanotubes behave in superacid in accordance to rigid rod polymer theory.
The cryo-SEM study was extended to direct imaging of CNT fiber formation by spinning the CNT dope solution directly into liquid nitrogen. We also succeeded in imaging of the nanometer-size internal structure of CNT tactoids, formed in aqueous dispersions, stabilized with surfactant.
Combined with supplementary analysis techniques of CNT films deposited on glass, and spun fibers, the study of nanotube behavior in superacid solution in a wide concentration range, provides information important to understanding of factors affecting the final fiber properties, and has strong impact on the film casting and fiber spinning scale-up towards production. For example, we found that CNT film and fiber surface morphology, as captured by high-resolution SEM, is in good correlation with measured electrical conductivity of the same films and fibers. In CNT fibers, visual observation of CNT alignment along the long fiber axis supports the quantified degree of alignment by wide angle x-ray diffraction measurement.
Although CSA has proven its efficiency in molecularly dissolving CNTs, this solvent is a powerful acid, highly corrosive and reactive. It decomposes exothermally upon contact with water into gaseous hydrochloric and sulfuric acids. Therefore, storage and work with CSA require highly controlled and dry conditions. Here we suggest a new idea of superacid exchange with more common solvents, by preserving the dispersion of CNTs in the liquid state.