Ideal conditions for producing high performance CNT fibers

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A preproof published recently in the journal Carbon investigates changes in fiber morphology, particularly inter- and intra-bundle voids, resulting from solution spinning. This in-depth look at fiber creation, the authors say, offers valuable insights into the synthesis of versatile, high-performance carbon nanotube (CNT) fibers.

Study: Fiber optic sensors for the detection of sodium plating in sodium-ion batteries. Image Credit: Yurchanka Siarhei/Shutterstock.com

What distinguishes carbon nanotubes?

Carbon nanotubes (CNTs) have great appeal due to their potential use in sensing, actuation, military hardware, robotic systems, and energy storage equipment due to their unique combination of functionality. These include exceptional electrical, mechanical and thermal capabilities. Due to their excellent elasticity and low densities, CNTs are expected to become a next-generation fiber of choice.

While several efforts have recognized the characteristics of isolated CNTs as the characteristics of bulk fiber, their macroscopic qualities depend on the type, quality, and interconnection of the CNT bundles.

Figure 1. Schematic illustrations of (a) hierarchical structure of CNT fiber and (b) unit operations for solution spinning. (A color version of this figure can be viewed online.)

Inter-tube interactivity is essential for high-performance carbon nanotubes

Optimizing the interactivity of neighboring carbon nanotubes is essential for performing carbon nanotube fibers from a micromechanical point of view.

From a materials perspective, the characteristics of carbon nanotubes, including the aspect ratio (L/d, where L represents the length and d represents the diameter of the carbon nanotube) and the purity, can strengthen the connection between the tubes in the fiber, leading to a considerable link with the characteristics of the fiber. .

Higher purity can reduce the influence of residual contaminants with tube alignment and inter-tube interactivity. Additionally, the importance of aspect ratio for fiber characteristics has often been noted.

Mechanically, inter-tube friction is a crucial element in defining the tensile strength of carbon nanotube fiber, and a larger aspect ratio creates a stronger frictional force at carbon nanotube interfaces. Manufacturing or refinement procedures significantly influence the quality and type of CNT.

Direct Spinning Vs. Solution Spinning – Which Synthesis Method Is Better?

Direct spinning and solution spinning are two widely recognized means of producing high performance carbon nanotube fibers.

CNT fibers and sheets formed by direct spinning have excellent tensile strength and fairly long component nanotubes (∼1 mm) but have limited electrical conductance due to poor alignment and low density of packaging.

Fibers produced by solution spinning, on the other hand, comprise relatively shorter nanotubes (∼12 μm) but possess high tensile strength and electrical conductance.

To improve macroscopic characteristics of carbon nanotube fibers such as fiber orientation and packing, pre- and post-treatment of nanotube fibers are some of the reported strategies.

The solution spinning process is a continuous complicated process in which liquid crystal (LC) solution flows inside the spinneret while coagulation and extensional deformation take place outside the spinneret.

Rheological properties of LC dopes and aspect ratio of CNTs.  (a) Polarized microscopy images for DX-2, Tuball and XBC2340 as a function of concentration.  Scale bars are 100 µm.  (b) Phase transition concentration from biphase to nematic phase determined from stable shear viscosity at 52 s-1.  (c) The evolution of the diameter with time by the dropwise rheometry on the substrate for the solutions DX-2 (0.012 vol%), Tuball (0.035 vol%) and XBC2340 (0.03 vol% ).  Extensional viscosity was extracted through linear fits of hair thinning curves.  (d) Specific strength of CNT (DX-2) fibers as a function of concentration.  (e) Dependence of aspect ratio on specific strength of CNT fibers.  (A color version of this figure can be viewed online.)

Figure 2. Rheological properties of LC dopes and aspect ratio of CNTs. (a) Polarized microscopy images for DX-2, Tuball and XBC2340 as a function of concentration. Scale bars are 100 μm. (b) Phase transition concentration from biphase to nematic phase determined from stable shear viscosity at 52 s−1. (c) The evolution of the diameter with time by the dropwise rheometry on the substrate for the solutions DX-2 (0.012 vol%), Tuball (0.035 vol%) and XBC2340 (0.03 vol% ). Extensional viscosity was extracted through linear fits of hair thinning curves. (d) Specific strength of CNT (DX-2) fibers as a function of concentration. (e) Dependence of aspect ratio on specific strength of CNT fibers. (A color version of this figure can be viewed online.)

Improving the fiber properties of carbon nanotubes

There are many structural ways to improve the characteristics of carbon nanotube fibers. The CNT fiber has a hierarchical structure, with several nanotubes forming a bundle and many bundles forming the CNT fiber.

As a result, CNT fibers possess multiple voids and defects on the inter and intra-bundle interfaces that are created in fiber synthesis. The macroscopic characteristics of the resulting fiber strongly depend on the number and size of existing defects.

Therefore, it is essential to optimize packing density by reducing voids and defects in carbon nanotube fibers. Optimizing fiber alignment can also be a viable approach used to increase interactions between tubes by reducing the number of voids in the fiber.

Flow fields depending on nozzle design.  (a) Schematic illustration of flow fields for LC dope in straight and converging nozzles.  (b) Changes in initial orientation factor (S0) by shear and extension rates.  The experiments were carried out with a nematic LC solution (DX-2, 0.43 vol%).  (c) Flow models in the converging nozzle and the straight nozzle by Comsol Multiphysics.  (A color version of this figure can be viewed online.)

Picture 3. Flow fields depending on nozzle design. (a) Schematic illustration of flow fields for LC dope in straight and converging nozzles. (b) Changes in initial orientation factor (S0) by shear and extension rates. The experiments were carried out with a nematic LC solution (DX-2, 0.43 vol%). (c) Flow models in the converging nozzle and the straight nozzle by Comsol Multiphysics. (A color version of this figure can be viewed online.)

Study Highlights

In this study, researchers investigated the development of structural alignment and interior voids of CNT fibers for each solution spinning unit process, relating them to the macroscopic characteristics of the fibers.

To acquire the best fiber qualities, precision ϕnematic should be determined while the liquid crystal solution is being centrifuged, and then the centrifugation operation should be performed above this value ϕnematic.

Alignment optimization was found to significantly reduce intra-bundle and inter-bundle voids in the fiber microstructure. During solidification, large aspect ratio, higher concentration, and smaller D-values ​​allowed fabrication of fibers with circular cross-sections, further minimizing interior voids in the fiber.

A significant link was observed between the intra-fiber voids and the macroscopic characteristics of the fiber. The tensile modulus and electrical conductance of the carbon nanotube fiber were improved by increasing the packing density, with the finished version including a minimal number of void defects (0.08% by volume).

The fiber has demonstrated exceptional tensile strength, heat conduction and knot efficiency, showing its viability for use as an all-purpose fiber that outperforms traditional carbon fibers.

Source

Kim, SG, Choi, GM et al. (2022). Hierarchical structure control in solution spinning for strong and multifunctional carbon nanotube fibers. Carbon. Available at: https://www.sciencedirect.com/science/article/pii/S0008622322003189?via%3Dihub

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