Broader applications of carbon nanotubes to real-world problems have largely gone unfulfilled because of difficult material synthesis and laborious processing. We report high-performance multifunctional carbon nanotube (CNT) fibers that combine the specific strength, stiffness, and thermal conductivity of carbon fibers with the specific electrical conductivity of metals. These fibers consist of bulk-grown CNTs and are produced by high-throughput wet spinning, the same process used to produce high-performance industrial fibers. These scalable CNT fibers are positioned for high-value applications, such as aerospace electronics and field emission, and can evolve into engineered materials with broad long-term impact, from consumer electronics to long-range power transmission.O n the molecular level, carbon nanotubes (CNTs) have an outstanding combination of mechanical strength and stiffness, electrical and thermal conductivity, and low density, making them ideal multifunctional materials that combine the best properties of polymers, carbon fibers, and metals (1). However, such outstanding properties have remained elusive on a macroscopic scale. Handling CNTs with sufficient length, stiffness, and chemical inertness introduces major challenges in material processing. Here we report lightweight fibers that approach the high specific strength of polymeric and carbon fibers, while also achieving the high specific electrical conductivity of metals and the specific thermal conductivity of graphite fibers.Two distinct routes have been developed for manufacturing neat CNT fibers (2). One route employs a solid-state process wherein CNTs are either directly spun into a fiber from the synthesis reaction zone (3, 4) or from a CNT forest grown on a solid substrate (5). This approach does not lend itself to the typical easy scale-up of chemical processes, as it combines multiple steps into a single one, limiting the options for process and material optimization. Indeed, solidstate fibers have low packing and poor orientation, and include impurities within their structure (6). Despite these shortcomings, solid-state CNT fibers have delivered the best properties so far (3, 4, 7-9). The reason for this relative success is the length of the CNTs that constitute these fibers-1 mm or more (2). Longer CNTs reduce the number of CNT ends in a fiber, yielding greater strength (10) and reducing CNT junctions, which increases electrical and thermal conductivity (11). The alternate fiber production route-wet spinning-was the first method for producing CNT fibers (12). In this process, premade CNTs are dissolved or dispersed in a fluid, extruded out of a spinneret, and coagulated into a solid fiber by extracting the dispersant. Wet spinning is easily scaled to industrial levels and is indeed the route by which highperformance fibers are manufactured (including ballistic fibers such as Kevlar and Twaron and structural fibers such as Toho Tenax and Thornel carbon fibers) (13). Decoupling the synthesis of CNTs from the spinning of the fibers allo...

We demonstrate that the length of carbon nanotubes (CNTs) can be determined simply and accurately from extensional viscosity measurements of semidilute CNT solutions. The method is based on measuring the extensional viscosity of CNT solutions in chlorosulfonic acid with a customized capillary thinning rheometer and determining CNT aspect ratio from the theoretical relation between extensional viscosity and aspect ratio in semidilute solutions of rigid rods. We measure CNT diameter d by transmission electron microscopy (TEM) and arrive at CNT length L. By studying samples grown by different methods, we show that the method works well for CNT lengths ranging from 0.4 to at least 20 μm, a wider range than for previous techniques. Moreover, we measure the isotropic-to-nematic transition concentration (i.e., isotropic cloud point) φ iso of CNT solutions and show that this transition follows Onsager-like scaling φ iso ∼ d/L. We characterize the length distributions of CNT samples by combining the measurements of extensional viscosity and transition concentration and show that the resulting length distributions closely match distributions obtained by cryo-TEM measurements. Interestingly, CNTs appear to have relatively low polydispersity compared to polymers and high polydispersity compared to colloidal particles.

At the microscopic scale, carbon nanotubes (CNTs) combine impressive tensile strength and electrical conductivity; however, their macroscopic counterparts have not met expectations. The reasons are variously attributed to inherent CNT sample properties (diameter and helicity polydispersity, high defect density, insufficient length) and manufacturing shortcomings (inadequate ordering and packing), which can lead to poor transmission of stress and current. To efficiently investigate the disparity between microscopic and macroscopic properties, a new method is introduced for processing microgram quantities of CNTs into highly oriented and well-packed fibers. CNTs are dissolved into chlorosulfonic acid and processed into aligned films; each film can be peeled and twisted into multiple discrete fibers. Fibers fabricated by this method and solution-spinning are directly compared to determine the impact of alignment, twist, packing density, and length. Surprisingly, these discrete fibers can be twice as strong as their solution-spun counterparts despite a lower degree of alignment. Strength appears to be more sensitive to internal twist and packing density, while fiber conductivity is essentially equivalent among the two sets of samples. Importantly, this rapid fiber manufacturing method uses three orders of magnitude less material than solution spinning, expanding the experimental parameter space and enabling the exploration of unique CNT sources.

Boron nitride nanotubes (BNNT) are poised to fill an electrically insulating, high-temperature, highstrength niche. Despite significant progress over the past two decades, BNNTs are not yet synthesized in high enough quantity and quality to permit their use in engineering applications. The next necessary step to make BNNTs accessible for research and applications is to improve the availability of high-quality BNNTs. Here, we present a scalable bulk purification technique that yields high-purity BNNTs. Bulk synthesized material is introduced to a wet oxygen environment at elevated temperatures to remove elemental boron and hexagonal boron nitride impurities with a final yield of purified BNNTs near 10 wt %. This process shows full removal of impurities, as observed by scanning electron microscopy (SEM), cryogenic transmission electron microscopy (TEM), and high-resolution TEM. X-ray photoelectron spectroscopy (XPS) and infrared spectroscopy show minimal BNNT functionalization, while high-resolution TEM shows damage to large-diameter BNNTs.