(Nanowerk Spotlight) Carbon nanotubes, with their remarkable electrical conductivity and impressive mechanical strength, have long been hailed as a revolutionary material with the potential to transform industries. These one-dimensional nanostructures, consisting of cylindrical carbon molecules, can theoretically achieve electrical conductivity of up to 106 S/cm and tensile strength exceeding 100 GPa. Such extraordinary properties make carbon nanotubes highly promising building blocks for advanced fiber materials with unparalleled capabilities.
Despite their immense potential, the road to translating the superlative properties of individual carbon nanotubes into high-performance macroscopic fibers and yarns has been fraught with challenges. Researchers have grappled with the inherent difficulty of dispersing and aligning carbon nanotubes, which tend to clump together due to strong van der Waals forces. Moreover, the high production costs, environmental and health concerns, and limited scalability of many synthesis techniques have hindered progress towards widespread commercial adoption.
Over the years, scientists have explored various approaches to fabricating carbon nanotube yarns, including wet spinning from liquid crystal dope or dispersions, dry spinning from vertically aligned nanotube arrays, and direct spinning from aerogels. While each method has yielded incremental improvements, achieving the full spectrum of desirable attributes – such as high packing density, excellent nanotube alignment, superior mechanical and electrical properties, and efficient large-scale production – has remained an elusive goal.
Now, a team of researchers from the Chinese Academy of Tropical Agricultural Sciences and Deakin University in Australia has developed a groundbreaking wet spinning technique that could finally unlock the true potential of carbon nanotube yarns. As reported in the journal Advanced Functional Materials (“Wet Twisting in Spinning for Rapid and Cost-Effective Fabrication of Superior Carbon Nanotube Yarns”), their rapid and continuous “wet twisting” process can produce highly densified carbon nanotube yarns with remarkable properties in mere minutes.
Wet-spinning and formation mechanism of neat CNT yarns. a) Schematic of the fabrication process of wet spun CNT yarns. The SWCNTs were dispersed in water with STDOC as surfactants. The resulting solution was then extruded into acetic acid (coagulation bath) through a syringe to produce continuous CNT yarns. Finally, the yarn was stretched and twisted to make it quickly dehydrated and then collected by winding onto a spool. b) Scanning electron microscope (SEM) images of the surface of the CNT yarns. c) The magnified SEM image of the marked section in b) showing the neat arrangement of SWCNTs. d) Representative SEM image of tightly intertwined CNT yarns showing great flexibility and even appearance. e) Photographs of a single CNT yarn of over 20 m long wrapped on a spool without breaking. (Reprinted with permission by Wiley-VCH Verlag)
The key innovation lies in the combination of strategic choices at each stage of the spinning process. First, the researchers effectively dispersed single-walled carbon nanotubes in water using sodium taurodeoxycholate as a surfactant. This environmentally friendly approach ensures uniform dispersion while preserving the nanotubes’ length, a crucial factor for achieving optimal properties in the final yarn.
Next, the well-dispersed nanotube solution is extruded through a syringe into an acetic acid coagulation bath. The acetic acid not only enables rapid solidification of the yarn but also effectively removes excess surfactant, yielding a pure and densely packed nanotube assembly. This judicious selection of the coagulation medium maintains the alignment of the nanotubes and prevents the introduction of impurities that could compromise the yarn’s conductivity.
The final and most critical step is the application of a wet twisting treatment to the freshly formed nanotube yarn. By twisting the yarn while it is still in a gel-like state, the researchers achieved a dramatic increase in packing density and nanotube alignment. This ingenious technique efficiently squeezes out residual solvent, resulting in a compact yarn with a well-defined spiraling texture.
The impact of this wet twisting treatment on the properties of the carbon nanotube yarn is nothing short of remarkable. Compared to untreated yarns, the twisted yarns exhibit a tensile strength of up to 600 MPa, representing a nearly three-fold improvement. The Young’s modulus, a measure of the material’s stiffness, reaches an impressive 40 GPa. Simultaneously, the electrical conductivity soars to 8990 S/cm, far surpassing previous wet-spun carbon nanotube yarns produced without the use of harsh acids.
These exceptional mechanical and electrical properties place the twisted carbon nanotube yarns on par with some of the best-performing yarns fabricated using more complex and hazardous methods, such as liquid crystal spinning with chlorosulfonic acid. Notably, the wet twisting approach offers the advantages of simplicity, safety, and efficiency, making it a more attractive option for large-scale production.
To demonstrate the practical utility of their twisted carbon nanotube yarns, the researchers constructed a high-performance yarn heater. Remarkably, this heater showcases an ultrafast electrothermal response, achieving heating rates of over 1000 °C/s at a low operating voltage of just 5 V. Such rapid and efficient heating capabilities open up exciting possibilities for applications in thermal management, wearable devices, and beyond.
Moreover, the mechanical properties of the twisted carbon nanotube yarns exhibit excellent stability across a wide temperature range. Even when heated to 150 °C, the yarns retain their impressive strength and modulus, a crucial factor for their reliable performance in real-world applications.
While this study represents a significant advancement in the fabrication of high-performance carbon nanotube yarns, the researchers acknowledge that there is still room for further improvement. The mechanical strength of the twisted yarns, although greatly enhanced compared to untreated yarns, remains lower than some recently reported dry-spun carbon nanotube yarns. The authors suggest that optimizing the initial yarn alignment prior to twisting and exploring additional post-treatment methods, such as bonding molecular solutions to enhance inter-nanotube interactions, could potentially boost the mechanical properties even further.
Another important consideration is the environmental and economic impact of this novel wet spinning process. The use of acetic acid as the coagulation bath offers a more environmentally friendly alternative to the harsh acids commonly employed in other methods. Moreover, the low cost and excellent prospect of large-scale production associated with wet spinning techniques make this approach particularly attractive for industrial adoption. As the demand for sustainable and cost-effective materials grows, the development of green and scalable fabrication methods becomes increasingly crucial.
The development of this rapid wet spinning and twisting process for fabricating superior carbon nanotube yarns marks a significant milestone in the field of advanced fiber materials. By addressing the long-standing challenges of nanotube dispersion, alignment, and densification, while prioritizing scalability and environmental friendliness, this innovative approach paves the way for the large-scale production and widespread adoption of high-performance carbon nanotube yarns.
As the demand for lightweight, strong, and multifunctional materials continues to grow across industries, from aerospace and automotive to wearable electronics and energy storage, carbon nanotube yarns stand poised to revolutionize the landscape. With their unparalleled combination of mechanical robustness, electrical conductivity, and thermal responsiveness, these yarns offer tremendous potential for pushing the boundaries of material performance and enabling novel applications.
The simplicity and efficiency of the wet spinning and twisting technique developed by the Chinese and Australian researchers bring us one step closer to realizing the full promise of carbon nanotubes. As further optimizations and scaled-up production methods are explored, we can anticipate a future where carbon nanotube yarns become a ubiquitous and indispensable material, driving innovation and transforming industries worldwide. However, continued research efforts are necessary to fully unlock the potential of these remarkable materials and overcome the remaining challenges on the path to commercialization.
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